FREQUENCY OF EXCEPTIONAL NILE FLOOD EVENTS AS AN INDICATOR OF HOLOCENE HYDRO-CLIMATIC CHANGES IN THE ETHIOPIAN HIGHLANDS

Keywords: African Humid Period, Human-Climate co-evolution, Nile Deep Sea Fan, Horn of Africa, East African Rift System, Lake Abhe

During the last fourteen thousand years, East Africa was the locus of significant climatic and cultural changes, which highly affected landscape evolution and prehistoric ways of life. Enhanced monsoonal precipitations over the tropics, mainly driven by Earth's precessional and eccentricity orbital parameters, were the primary causes of the African Humid Period (AHP; ∼14 -∼6 ka). This humid period led to the transgression of continental lacustrine waterbodies, the increase of fluvial discharge and the spread of vegetation cover over the present-day desertic regions. The onset and the end of this wetter phase were not homogeneous across the continent, and the AHP was punctuated by centennial-scale episodes of hyper-aridity which varied in time and in space. This highly variable environmental scenario was the theatre of one of the most important socio-economic changes in human history, that of the transition between the last hunters-gatherers and the first food production societies. The concomitance of such cultural transformations and climatic disruptions raises the question of the role of changing environmental conditions on the distribution patterns of new socio-economic practices, namely herding.

The Ethiopian Highlands (EH) represent a major receiver of monsoonal precipitations in East Africa. During the AHP, substantial variations of the monsoon regime affected the functioning of the hydro-systems originating in the EH. This included the Blue Nile River and Lake Abhe, the endorheic receptacle of the Awash River waters, and located in the northern East African Rift System (EARS). Additionally, the Nile valley and the EARS, constituted the main corridors for prehistoric human trajectories across the Horn of Africa. With the aim to better understand human-climate coevolution during this crucial period a multiscale and multidisciplinary approach is proposed.

First, I decipher Holocene climatic variations from the functioning of the two main EH-originated hydro-systems by: a) tracking exceptional Blue Nile paleo-flood frequencies using a high resolution study of a laminated sequence from the Nile Deep Sea Fan deposits, and b) reconstructing the hydrological fluctuations of paleo-Lake Abhe (Ethiopia, Djibouti), from the study of wave-dominated clastic paleo-shoreline sequences. Second, I characterize the impact of hydro-climatic changes on littoral paleolandscape evolution along the Lake Abhe basin. And finally, I estimate the interactions between climaticinduced landform dynamics and human occupations with respect to the main socio-economic shifts that occurred over the same lacustrine basin.

This study improves our comprehension of the spatial-temporal patterns of Holocene hydro-climate in East Africa: a strong and stable monsoonal regime has been evidenced between ∼11 and ∼9 ka BP, followed by a stepwise ardification until the end of the AHP. Moreover, belated Mid-Holocene humid conditions in the Northern Rift suggest that the Congo Air Boundary had a significant role on rainfall controls in this area. Additionally, evaluating the interplay between climatic-, geomorphic-and tectonic-induced sedimentary transfer mechanisms over the investigated basins allowed us to localise some hydro-sedimentary processes decoupled from climatic forcing and to propose a diachronic palaeolandscape reconstitution of the Lake Abhe basin for this period.

Early Holocene Lake Abhe waters and environments were strongly exploited by hunter-fisher-gatherers until the last lake drop around ∼4.5 ka BP, when herding practices were integrated in this region. Associations between prehistoric settlements and Holocene hydrological variations demonstrate how humans were extremely reactive to fluctuating lake environments and how new socio-economic practices were often adapted to specific local to regional environmental trajectories.

Résumé

Au cours du Quaternaire, l'Afrique de l'Est a été le théâtre d'importants changements climatiques et culturels. L'augmentation des précipitations, principalement liée aux paramètres orbitaux de la Terre, a été responsable de la dernière Période Humide Africaine (PHA ; ∼14 -∼6 ka). Une période caractérisée par la transgression des grands bassins lacustres, par l'activation des systèmes fluviatiles et par la diffusion de la couverture végétale sur des zones aujourd'hui désertiques. Cependant, le début et la fin de cette période humide n'ont pas été synchrones sur tout le continent et de courts épisodes d'aridité l'ont ponctuée. A ce jour, les mécanismes de ces variabilités millénaire et centennale restent encore fortement débattus. A partir de 14 mil ans, l'augmentation d'intensité de mousson associée aux masses d'air venant de l'océan Atlantique modulée par celle venant de l'océan Indien, semble avoir été le principal moteur de la distribution des précipitations au niveau du Hauts Plateaux Ethiopiens. Ceci a fortement affecté le régime hydrologique des deux grands hydro-systèmes d'Afrique de l'Est : les bassins du Nil et du Lac Abhé. Dans ce scenario de forte variabilité environnementale, un des plus importants changements socio-économiques de l'histoire de l'Homme a eu lieu, dans la Corne de l'Afrique : la disparition des dernières populations de chasseurs-cueilleurs et l'émergence des premières sociétés de production. La concomitance de ces bouleversements climatiques et culturels pose donc la question du rôle des changements environnementaux dans la diffusion des nouvelles formes de modèles socio-économiques.

L'objectif de ce travail de thèse est de mieux comprendre la coévolution Homme-Climat pendant la Période Humide Africaine à partir d'une étude multi-échelle et multidisciplinaire. Premièrement, une reconstitution des variations hydroclimatiques est proposée sur la base d'une comparaison entre: a) les reconstitutions centennales d'intensité et de fréquence des crues du Nil à l'Holocène à partir de l'étude d'une séquence sédimentaire prélevée au sein du Delta Profond du Nil ; b) la reconstitution des fluctuations hydrologiques du paléo-Lac Abhé (Ethiopie, Djibouti) à partir de la documentation et de la datation des anciens paléo-rivages lacustres. Deuxièmement, la caractérisation de l'impact de ces changements hydrologiques sur l'évolution des paléo-paysages littoraux autour du bassin du Lac Abhé a été étudiée. Enfin, l'étude des changements paysagers et morpho-sédimentaires en lien avec les dynamiques chrono-spatiales des occupations humaines dans ce même bassin est abordé.

Ce travail de thèse a permis de préciser la compréhension de la distribution spatio-temporelle des changements hydroclimatiques en Afrique de l'Est, caractérisée par une forte activité de la mousson entre ∼11 et ∼9 ka BP, suivi par une aridification progressive jusqu'à la fin de la PHA. Ensuite, la persistance de conditions humides dans la partie Nord du Rift à l'Holocène Moyen suggère une influence de la Zone de Convergence du Congo dans le contrôle des précipitations dans cette région. La détection des facteurs du contrôle géomorphologiques et tectoniques sur les transferts hydro-sédimentaires qui affectent l'évolution de ces bassins à permit aussi de mieux contraindre les effets de ces changements climatiques sur large et petite échelle.

A partir de l'estimation de la réactivité des paléo-paysages sous le forçage de ces changements hydrologiques, nous démontrons une forte mobilité des populations en lien avec les fluctuations hydrologiques du paléolac Abhe. D'abord avec une forte exploitation des ressources aquatiques par des sociétés de chasseurs-pécheurs les et ensuite par l'intégration de l'élevage en concomitance avec l'installation de conditions plus arides. Cette étude a permis de souligner l'évidence d'une adaptation des systèmes socio-économiques Holocènes aux changements hydro-climatiques à l'échelle régionale mais aussi locale.

Preface

Brief context of the origin of this multidisciplinary thesis

This manuscript contains the results of three years (and four months) of Ph.D research at the Géoazur and CEPAM laboratories (University of Côte d'Azur, CNRS-UMR7329 and CNRS-UMR7264 respectively), that included 6 fieldwork seasons in the Central Afar Region, both in Ethiopia and the Republic of Djibouti, for a total of around 170 days spent in East Africa. This work has been possible thanks to the intensive multi-disciplinary collaboration between M. Revel and L. Khalidi, my supervisors, who have combined mutual interests at the boundary between the Earth Sciences and Archaeology. The subject of this study was woven together with my supervisors around a common objective: understanding the impact of climate change on prehistoric settlement and mobility during the period of the last humid to arid transition in the Horn of Africa (the last ∼14 ka). This thesis was financed by a UCA-IDEX JEDI doctoral grant and was possible thanks to the support of my host laboratories. Moreover, the VAPOR-Afar project (Ethiopia; L. Khalidi director) allowed me to concretize this doctoral project with a strong fieldwork perspective.

The particularity of the hydro-climatic patterns which characterize the Ethiopian Highlands at the source of the Blue Nile and the Awash Rivers sparked my interest in the region. When I was introduced to the exceptional paleo-ecological work that Françoise Gasse carried out on the Lake Abhe basin in the 1970s, it further stimulated my interest in the Awash River and the northern branch of the East African Rift System, and their importance in paleoclimatological and geological research. The Lake Abhe basin also represents an area rich in archaeological sites and research projects, which thanks to my integration in three field programs, provided me with new and unpublished data which are presented in this work.

I started my Ph.D with a background in geoarchaeology and paleoenvironmental sciences, and I had previously worked on the environmental significance of sedimentary formation processes of Middle Palaeolithic sites in Portugal and in the South of France (under the supervision of D. E. Angelucci and L. Purdue respectively). Meetings with my thesis supervisors during my Masters studies allowed me to expand these issues and to explore new research interests in Africa.

I was first introduced to paleoclimatology by M. Revel through the study of Nile Deep Sea Fan cores at Géoazur laboratory and through the detailed reading of previous works of F. Gasse. This experience allowed me to explore deep questions related to climate evolution in continental Africa, and to develop a network with the paleoclimatological community in France and abroad. Many discussions with L. Khalidi about recent research in African Prehistory also stimulated my interest in the links between African Prehistoric cultures and climatic changes which occurred over this continent during the last twenty thousand years.

Fieldwork carried out in the Central Afar Region (Lake Abhe basin) laid the groudwork for this doctoral research through the my integration to the VAPOR-Afar (L. Khalidi director), TelluS-Rift (C. Doubre director) and PSPCA (J. Cauliez director) programs. The geomorphological aspects of my thesis were principally developed during these fieldwork experiences, under the mentorship, guidance of L. Bruxelles and M. Schuster. During this time, I got a chance to also learn about structural geology, and geophysical concepts applied to the Afar region thanks to exchange with researchers belonging to these fieldwork programs. Moreover, during my Ph.D I had the opportunity to learn some laboratory methods applications (geochemistry, electron microscopy, sampling strategies), and to develop some numerical tools (radiocarbon age modelling, spatial GIS analyses) for data processing.

Because this work was based on a multidisciplinary approach that required analysing objects and areas that had never been previously investigated, the research questions were updated and redirected several times. Continuous experimenting and testing of the field data, as well as the complications due to non-exploitable study materials (inaccessible areas, not yet dated lake cores), were the reasons behind the continuous reorganization of this Ph.D.

The results presented in this manuscript therefore represent one part of my own as well as communal observations, sampling and documentation of geological and geomorphological features in the Central Afar Region collected during the last three and a half years. I thus consider this work to be a preliminary look at human-climate coevolution in the Horn of Africa during the Late Quaternary, which deserves to be refined and improved in the following years.

General Introduction

During the last fourteen thousand years, intertropical East Africa was the locus of important climatic disruptions which caused the extension and the regression of large waterbodies, of vegetation cover and which drove the retreat of the last tropical glaciers over the highlands. Enhanced monsoonal precipitation, mainly controlled by precession and eccentricity orbital parameters, are at the origin of the African Humid Period (AHP: ∼14 -∼6 ka BP). However, the onset and the end of this wetter phase are not homogeneous over the continent and were punctuated by centennial-scale episodes of hyper-aridity, linked to Heinrich, Younger Dryas, 8.2 ka and 4.2 ka North Hemisphere climatic events. In this highly variable environmental scenario, many cultural changes and technological innovations characterized African prehistoric societies, of which the most important are those of the adoption of herding and the emergence of food production. In the Horn of Africa, such crucial socio-cultural transformations occurred disjointedly with respect to northern Africa, showing a lapse of a few millennia between the apparition of Neolithic practices in the Sahel and the East African Rift. The concomitance between prominent climatic changes and the diffusion of food production societies over the continent raises questions on the role of changing environmental conditions on the distribution patterns of these new socio-economic practices.

The investigation of past human-climate co-evolution is normally approached through a comparison between continuous regional or sub-continental climatic records, and regional archaeological sequences. However, human adaptation could have been influenced differently by specific local to regional environmental and climatic feedbacks which differ from regional or sub-continental trends. These divergences can be caused by the differential distribution of monsoonal rainfall due to specific internal climatic forcing, or by the reactivity of specific hydro-systems due to anomalies of surface processes, or to tectonic control. Consequently, a study of the distribution of regional hydro-climate trajectories combined with the detailed study of the reactivity of morpho-sedimentary systems -which constitute prehistoric inhabited environments -to these hydrological changes is required.

The aim of this manuscript is to propose an approach which combines the reconstitution of hydroclimate oscillations in East Africa, the impact of these changes on the geomorphic evolution in a specific basin and their association with the sedimentary processes which governed the formation of archaeological sequences in this same basin. This approach involves regional to local points of view, using a multi-scalar and multi-proxy perspective that aims towards an understanding of the interactions between these different scales. To this end, I have focused my research on two East African basins: that of the Nile River and its offshore deep Delta, and that of Lake Abhe (Central Afar Region, Ethiopia and Djibouti).

The choice of these two basins was made for geographic, scientific and practical reasons. From a geographic point of view, these two basins are the final receptacles of the hydro-sedimentary discharge that originated under the forcing of East African monsoons in the Horn of Africa. Indeed, the Ethiopian Highlands, the principal receiver of moist air masses originating both from the Atlantic and the Indian Ocean, are the source of both the Blue Nile and the Awash Rivers. Together these two basins represent exceptional case studies for the investigation of hydro-climatic variation in East Africa. From a scien-

•0 •0 GENERAL INTRODUCTION
tific point of view, the complementary study of these basins (one that extends from the highlands to the Sahara and the Mediterranean region and the other located in the East African Rift System) allows us to estimate their hydrological, morphological and environmental reactivity to similar climate forcing. Indeed, their comparison makes it possible to identify local geomorphic incongruencies that are unconnected to wider hydro-climatic variations. From a practical point of view, the study of the Nile delta was possible because I was granted access by M. Revel to Nile deltaic cores from previous offshore drilling during oceanic campaigns conducted by the Géoazur laboratory (VANIL cruise [START_REF] Mascle | The Nile Continental Margin: A Compendium Compiled by Jean Mascle[END_REF][START_REF] Migeon | Lobe construction and sand/mud segregation by turbidity currents and debris flows on the western Nile deep-sea fan (Eastern Mediterranean)[END_REF]. The Lake Abhe basin was chosen because of the ongoing work of two multidisciplinary archaeological programs along its paleoshores, namely the VAPOR-Afar program in Ethiopia (L. Khalidi director) and the PSPCA program in Djibouti (J. Cauliez director).

The directors of these programs identified a need to renew previous environmental studies conducted by F. Gasse in the 1970s [START_REF] Gasse | L'évolution des lacs de l'Afar Central (Ethiopie et T.F.A.I.) du Plio-Pléistocène à l'Actuel. Recostitution des paléomilieux lacustres à partir de l'étude des Diatomées[END_REF][START_REF] Gasse | Evolution of Lake Abhe[END_REF] on this lake basin, and to correlate this work with recent archaeological discoveries. These fieldwork projects have identified the largest number of Holocene period human occupation sites in the Horn of Africa, outside of the Lake Turkana basin. Moreover, this doctoral work was initiated synchronously with the first coring attempts in Lakes Gamari and Afambo (Lake Abhe basin) that were conducted in 2018 in the framework of the VAPOR-Afar project and in collaboration with Géoazur (CLIMAFAR UCA-IDEX program, L. Khalidi and M. Revel directors). During my Ph.D, I participated in the coring fieldwork and began the study of these short cores (Sup. Mat. BII.2). However, without a confident age model, I was unfortunately not able to include these preliminary results in this thesis work.

The access to materials from these complementary basins have permitted me to carry out an ambitious study on a large region of Northeast Africa using a multi-proxy and multi-scalar approach which is ongoing and still needs to be improved. This work was carried out both in the field in Ethiopia and Djibouti and in the laboratory, in collaboration with a number of institutions and laboratories (ABTC, AR-CCH, CCMA, CEPAM, CERD, CEREGE, CFEE, EDYTEM, GEOAZUR, IPGS, LIENSs, TRACES).

Manuscript organisation

This work is subdivided into two Volumes: Volume-A, contains the issues to be addressed in the study, the state of the art, the methods, the questions, the results and the general discussion and conclusion; and Volume-B, which includes all of the annexes, the supplementary materials, the glossary and all the materials which are referred to in Volume-A. Summarised below are the structure and the principal contents of Volume-A, which is composed of 8 chapters. From the presentation of the study contexts and objectives to the results and discussion chapters, this manuscript aims to present the reader with various issues and reflections on paleoclimatic, geomorphological and geoarcheological research in East Africa during the Holocene period.

Chapter-I (Present climate and the context of the study) is dedicated to the presentation of the climatic, geological and geographical context of East Africa with a specific focus on the Ethiopian Highlands region. Hosting combined air moisture masses transported from both the Indian and Atlantic oceans, the Ethiopian Highlands (EH) represent a major receptacle for monsoonal precipitations in East Africa. During the Holocene, the variations in the intensity of the African monsoon affected the functioning of the main hydro-systems originating in the EH, namely those of the Blue Nile and the Awash Rivers which are the focus of this thesis. Specific chapter sections are thus dedicated to the description of these two basins, including the Lake Abhe basin, the endorheic receptacle of the Awash River catchment.

In Chapter-II (Late Quaternary Paleosciences research in East Africa and problematics of the study), the state of the art of paleoclimatological, geomorphological and archaeological research out-Although these results are discussed independently and in detail in each of the previous sections, Chapter-VII (General Discussion) correlates the research results of the different scales of analyses presented in this manuscript: the regional changes in climate during the Holocene, the impact of these changes on Lake Abhe's littoral morphosedimentary features and the adaptation of prehistoric populations on the climate-induced landscape evolution in this same basin. Moreover, on the basis of these preliminary result outcomes, new research perspectives borne of the different scales of analysis approached in this manuscript are proposed.

Chapter-VIII (Conclusion) provides a general synthesis of the interpretations that resulted from this study.

I.1 General context

The aim of this section is to introduce the general climatological, geographical and geological context of Eastern Africa and of two specific study areas studied in this manuscript: The Nile and the Awash River catchments, including the Central Afar Region.

I.1.1 Global climate and the Tropical African Monsoon

Earth's climate is characterized by an average temperature of 15°C, with a median solar irradiation flux of 342 W/m 2 . The solar energy gradient received between high (∼150 W/m 2 ) and equatorial (∼400 W/m 2 ) latitudes is due to the Earth's sphericity, which mainly controls the temperature and the precipitation gradients over the globe (https://earthobservatory.nasa.gov/features/EnergyBalance/page3.php).

Additionally, an internal redistribution of temperature and air moisture masses towards the high latitudes occurs because of the atmospheric and oceanic masses distribution. Strong heating near the thermal equator triggers ascending air which spreads northward, and which descends after into the sub-tropics along the high-pressure belt. This process is named the 'Hadley cell convection', and is mainly responsible for the rising of air moisture over the Eatrh's surface (Fig. I.1). The redistribution of air masses under the influence of the Hadley circulation is the basis for the establishment of tropical forests and the great deserts such as the Sahara in North Africa. The tropics play a fundamental climatic role where the abundant surplus of temperature and humidity is redistributed towards the subtropical latitudes (30 °north and south). The inflow of low-level moisture air masses to the hinterlands mainly results from land-ocean thermal contrast during the summer, because of the insolation energy over the surface, together with the cross-equatorial transport energy due to the Hadley circulation. However, this process is modulated by a more complex regional-to-local land-ocean-atmosphere system interaction. The land-ocean surface contrast of specific heat is the more relevant factor and the source which feeds monsoons. Additionally, plant evapotranspiration may represent a non-negligible moisture source via local water recycling. Even aerosols may affect the land-sea thermal contrast influencing the transfer of solar energy through the atmosphere (Fig. The seasonal displacement of moist air masses and the precipitation distribution over the tropics is driven by the migration of the dividing line between the north and south hemispherical Hadley cells, which is called the Intertropical Convergence Zone (ITCZ; Fig. I.1). Characterized by low pressure and high heating, the ITCZ migrates following the seasonal insolation patterns: during summer, strong landsurface heating between the equator and the north hemisphere extra-tropics is responsible for a major temperature and pressure gradient between ocean (high pressure) and land (low pressure), which induce the ITCZ northward migration over the continent; inversely, during the winter season, a less thermal and ocean-land gradient concentrates air moisture masses over oceans following the insolation winter peak localised in the southern hemisphere extra-tropic low latitudes. This is the main mechanism under which the African monsoonal rainfalls are concentrated during the summer between 0°and 20°N. The role of oceanic circulation is also important. In particular, in the Atlantic Ocean, the northern flowing of warm and saline upper waters join the higher northern latitudes, where colder waters drop in depth and inversely flow to the tropical latitudes [START_REF] Broecker | Unpleasant surprises in the greenhouse?[END_REF][START_REF] Bryden | Slowing of the Atlantic meridional overturning circulation at 25°N[END_REF]. This heat transfer pattern in the Atlantic ocean is called Atlantic Meridional Overturning Circulation (AMOC) and its slowing or acceleration play a fundamental role in atmospheric circulation dynamics, impacting the climate over large regions [START_REF] Zhang | Multidecadal North Atlantic sea surface temperature and Atlantic meridional overturning circulation variability in CMIP5 historical simulations: Amo and Amoc Simulations in CMIP5[END_REF].

I.1.2 The East African Monsoon and the Ethiopian Highlands

This study is focused on East African climate dynamics for several reasons. Indeed, the particular climatology of this region is a result of a complex interaction between land topography and latitudinal and longitudinal variations of displacement dynamics of air masses, in which the North and South Hadley Cells meet. Here, the tropical climate is completely dependent on summer monsoonal precipitation activity, which is concentrated during summer between the equator and ∼20°N. However, longitudinal and latitudinal precipitation rates over the rain belt may be modulated by specific local parameters, including the topography, the geographical placement against the surrounding oceans and the vegetation cover. In Africa, during the summer season, the monsoon precipitations are mainly concentrated over the West-Central Congo Basin and over the Ethiopian Highlands. West-Central African summer precipitations are principally originated from the air moisture masses generated in the Atlantic Ocean, where a high pressure and low temperature gradient against the continent enhance this process [START_REF] Chiang | Extratropical Cooling, Interhemispheric Thermal Gradients, and Tropical Climate Change[END_REF]. Differently, the generation and the transfer processes of moist air in East Africa appears quite complex, which deserves to be discussed in detail. Its geographical location, embedded between the Indian Ocean in the South, the Arabian Sea in the East, the Red/Mediterranean Seas in the North and the Gulf of Guinea/Congo basin in the West, provide a large variety of air moisture masses, which make this region particularly sensitive to wider monsoonal tropical activity (Fig. I.3a). These geographical patterns combined with high-altitude plateaux promote low pressure conditions and then a major convection formation zone over the highlands. Additionally, the strong elevation contrast between the highlands and Rift valleys morphology, may favour the intrusion of air masses in the East African Rift System (EARS) and the consequent enhanced rainfalls on the highlands.

Recent studies have tried to quantify (in % and kg/day) the air moisture content which comes from the respective sources (Viste and Sorteberg, 2013, Fig. I.3b). The air masses contribution over the Ethiopian Highlands flows from the North and from the East (arrows 2, 3 and 4 in Fig. I.3a), showing how East Africa seems less influenced by the Atlantic source. The specific geographical and topographical configuration of the Ethiopian Highlands permits it to receive a high variety of monsoonal sources. It thus represents a unique receptacle of air moisture mass dynamics originating from large water surfaces spanning ∼60°in latitude and longitude [START_REF] Levin | Isotopic composition of waters from Ethiopia and Kenya: Insights into moisture sources for eastern Africa[END_REF].

I.1.3 The Ethiopian Highlands: at the source of the Nile and Awash Rivers

Because the Ethiopian Highlands are characterized by the particular climatological patterns illustrated above, this study focuses on this region, which is the major receiver of monsoonal precipitation in East Africa and which represents the source area of two of the largest hydro-systems of East-Africa: The Nile and the Awash Rivers. Receiving great amounts of precipitations, East Africa developed a large hydrological system which drains the highland headwaters to the continental margins or to the endorheic basins of the Rift. Several lakes in the EARS and over the Sahelian margin are supplied with perennial waters. The strong precipitation gradient between the high and lowlands permit the development of a large variety of ecosystems. Indeed, the complex East African physiography combined with the marked elevation gradient result in a variety of climates, from hyper-arid to very humid. The highest annual rainfall is recorded in the Western Highlands at ∼2000 m a.s.l. with 2101 mm/yr, while the lowest values are recorded in the Central Afar region (Elidar; 153 mm/yr) and over the Eritrean coast (Asab) with an average of only 45 mm/yr (Fig. I.4).

Two of the longest East African rivers take source in the Ethiopian Highlands: The Blue Nile, which flows though the northern highlands margin to the Sahelian zone, and the Awash river, which descends the plateaux in the Main Ethiopian Rift (MER) towards the north-east (Fig. I.4). The Blue Nile and Awash freshwaters, as well as the lowland hydro-systems, are thus completely dependent on the summer monsoonal supplies. In fact, both rivers represent the principal water supply of two of the most arid regions in the world, the Eastern Sahara (Sudan, Egypt) and the Afar Triangle (Ethiopia, Djibouti).

I.2 The study areas: geology, geomorphology and climatology

The understanding of Quaternary hydro-climatic variations in East Africa is not only decipherable through the study of changes in air mass displacements, which trigger the intensification and the reduction of monsoonal activity over the continent. The hydrological patterns of this region are the result of the complex interaction between the climate, geological history, continental erosion, and tectonic activity. A holistic perspective on these three aspects is thus required. In this Section, the major geological events which characterize East Africa are presented (Section I.2.1). Subsequently, in Sections I.2.2 and I.2.3 the geological, geographical and climatological characteristics of the two basin studied in this work will be illustrated: The Nile and the Awash River basins, with a specific focus on the Central Afar region and the Lake Abhe basin. dataset [START_REF] Harris | Updated high-resolution grids of monthly climatic observations -the CRU TS3.10 Dataset: UPDATED HIGH-RESOLUTION GRIDS OF MONTHLY CLIMATIC OB-SERVATIONS[END_REF] downscaled with WorldClim 2.1 [START_REF] Fick | WorldClim 2: New 1km spatial resolution climate surfaces for global land areas[END_REF], in which the Blue Nile and Awash River tracks (black lines), the Ethiopian Highlands (red dotted lines), and the highest (blue dot) and lowest (red dots) annual average precipitation zones in East Africa are indicated.

I.2.1 The East African geological setting

The geological diversity of East Africa can be summarized by the occurrence of some main lithological groups of assorted ages and formation processes.

The oldest geological formation in East Africa is represented by the Congo and Tanzania Cratons which are composed of metamorphosed Paleoproterozoic and Archean gneissic-granulitic complexes dated between 3.3 and 2.8 Ga [START_REF] Link | Continuous cratonic crust between the Congo and Tanzania blocks in western Uganda[END_REF]. These Archean units are exposed at the sources of the White Nile, around Lake Victoria and at the eastern watershed heads of the Congo basin (Fig. I.5;[START_REF] Garzanti | The modern Nile sediment system: Processes and products[END_REF]. In East Africa, the pre-Neoproterozoic continental crust, dated between 1.4 and 1.6 Ga, presents some particular characteristics. Indeed, this formation was heavily rejuvenated during the following Pan-African orogeny and the whole basement is thus part of the so-called "Saharan Metacraton" (Fig. I.5;[START_REF] Abdelsalam | The Saharan Metacraton[END_REF]. Covering a large part of the north and central African continent, the "Saharan Metacraton" extends over 5 000 000 km 2 . Successively, the emplacement of the Neoproterozoic crystalline basement ("Pan-African" basement) is the result of the second main geological event in East Africa. Dated from about 900 to 500 Ma, the Pan-African basement is the crustal block of East Africa, principally outcropping at the western margin of the Ethiopian Highlands and in the north-eastern head of the Congo Basin. It is related to the East African Orogen process (Fig. I.6;[START_REF] Fritz | Orogen styles in the East African Orogen: A review of the Neoproterozoic to Cambrian tectonic evolution[END_REF] produced by the collision of West and East Gondwana, along the structure connecting the Middle East to Madagascar. This crystalline basement is mainly composed of deformed metamorphic and igneous rocks. During a following phase, this crystalline basement was heavily abraded by the great Paleozoic glaciation, as evident in rock striations, roches moutonnées and grooves (Fig. I.5;[START_REF] Dow | Palaeozoic glacial rocks recently discovered in northern Ethiopia[END_REF][START_REF] Sacchi | On the Palaeozoic Tillite of the Adigrat Group (Tigrai, Ethiopia)[END_REF]. These glacial formations are also observable thanks to the fluvial-glacial sandstones preserved in North Ethiopia, in Eritrea [START_REF] Bussert | Palynological evidence for a latest Carboniferous-Early Permian glaciation in Northern Ethiopia[END_REF], and in the Blue Nile gorges [START_REF] Russo | Sedimentary evolution of the Abay River (Blue Nile) Basin, Ethiopia[END_REF]. The opening of the Tethys Ocean (started in the Permian-Trias) was the principal process at the basis of the Mesozoic limestone and sandstone formations in East Africa. These carbonate formations, correspond to a thick sedimentary sequence deposited in shallow fluvial-marine basins between 300 Ma and 70 Ma (Fig. I.5). This kind of sedimentary rock formation is concentrated over the Somalian plate and along the Blue Nile gorges.

Following the Tethys ocean closure during the Eocene (∼40 Ma), a tectonic-volcanic activity characterized the geology of East Africa, more specifically that of the Ethiopian Highlands and the EARS. Initially, during the Oligocene, a series of thick basaltic traps and lava flows covered the ancient geological formations (Fig. I.5). This basaltic sequence flowed approximately around 30 Ma, over a period of about 1 Ma [START_REF] Hofmann | Timing of the Ethiopian flood basalt event and implications for plume birth and global change[END_REF] and covered a large portion of East Africa, including the almost entirety of the EARS and the Ethiopian Highlands, where both the Blue Nile and the Awash Rivers take their sources. For example, just in Ethiopia it has been estimated that the basaltic trap sequence has covered an area of about 750 000 km 2 , equivalent to ∼70% of the country's surface [START_REF] Mohr | Ethiopian flood basalt province[END_REF]. This characteristic extrusive sequence of basaltic lava flows is the result of the emplacement of a large mantle plume prior to the onset of rifting [START_REF] White | Magmatism at rift zones: The generation of volcanic continental margins and flood basalts[END_REF]. It is from this moment that deep mantle processes start to trigger the uplift of the Ethiopian-Yemen plateaux, against the subsidence of the Levantine Sea and northern Egypt. This process is at the base of the evolution of the current topographical patterns, which controlled the installation of the Nile River hydrographic network since 30 Ma [START_REF] Faccenna | Role of dynamic topography in sustaining the Nile River over 30 million years[END_REF].

East-African continental rifting processes began at ∼28 Ma after the emplacement of the basaltic Traps, contributing to the upwelling of the Ethiopian and Yemeni high plateaux [START_REF] Hofmann | Timing of the Ethiopian flood basalt event and implications for plume birth and global change[END_REF][START_REF] Macgregor | History of the development of the East African Rift System: A series of interpreted maps through time[END_REF][START_REF] Tiercelin | A 300 Million Years History of Rift Lakes in Central and East Africa: An Updated Broad Review[END_REF]. Since 25 and 20 Ma, the continental tectonic extension is responsible for the formation of the large escarpments bounding the Ethiopian Rift and the Afar Depression. This process triggered the development of the large endorheic depression basins which have been filled by sediment since the Miocene [START_REF] Gasse | Tectonic and Climatic Controls on Lake Distribution and Environments in Afar from Miocene to Present[END_REF][START_REF] Wolfenden | Evolution of a volcanic rifted margin: Southern Red Sea, Ethiopia[END_REF]. Indeed, the main phase of continental rifting seems to be attributed to the Miocene, when the major fault activity in the EARS was recorded in association with subsidence events and highland uplift between 9 to 2 Ma [START_REF] Macgregor | History of the development of the East African Rift System: A series of interpreted maps through time[END_REF]. The most recent rift-related effusive magmatic activity is localized in the northern EARS, which is represented by the Plio-Pleistocene lava flows of the basaltic series covering the totality of the Afar Triangle [START_REF] Barberi | Volcanism of Afar: Smallscale plate tectonics implications[END_REF][START_REF] Varet | Geology of Afar (East Africa)[END_REF] at the apex of the Afar plume. There, the 1.5 km-thick "Stratoid" series was dated from 4.4 Ma to 0.4 Ma [START_REF] Varet | Geology of Afar (East Africa)[END_REF]. Its tectonic deformation likely began around 1.8 Ma, forming the complex network of NNE/ NW normal faults, and several NNW sub-rifts, horsts and grabens characteristic of the Afar region [START_REF] Abbate | Strike-slip faults in a rift area: A transect in the Afar Triangle, East Africa[END_REF].

Figure I.6:

Simplified geological timescale showing the main geological events and corresponding rock formations in East Africa [START_REF] Williams | Understanding Ethiopia[END_REF].

I.2.2 The Nile basin

The Nile is the world's longest river (6700 km long), representing an exceptional case-study for understanding the geological/geomorphological processes and the evolution of climate and environmental regimes of East Africa. The Nile watershed covers around 3 254 555 km 2 or 10.7% of the African Continent. Over latitudes between the Equator (4°S) and the Tropic of Cancer (31°N), the Nile runs through a wide range of environments, from the intertropical lands in the south, to the Sahara desert in the North, crossing 10 different countries (Tanzania, Uganda, the Democratic Republic of the Congo,Kenya,South Sudan,Central African Republic,Ethiopia,Eritrea,Sudan,and Egypt;Fig. I.7a). The Main Nile is the result of the confluence of the White and the Blue Nile in Karthoum, in Sudan (Fig. I.7a). The Blue Nile originates in the Ethiopian Highlands around Lake Tana, and the White Nile in the Tanzanian and Congo Cratons around Lakes Albert and Victoria.

From the lower White and Blue Nile and continuing along the Main Nile track after the Khartoum confluence, the river seems to follow the geological boundary between the Pan-African Orogen basement and the Saharan Metacraton [START_REF] Johnson | Late Cryogenian-Ediacaran history of the Arabian-Nubian Shield: A review of depositional, plutonic, structural, and tectonic events in the closing stages of the northern East African Orogen[END_REF]. However, the establishment of the Nile drainage patterns seems to be principally linked to the tilt of the Sudan/Egypt crustal block due to the uplift to the south [START_REF] Moucha | Changes in African topography driven by mantle convection[END_REF]. More particularly, the mantle dynamics between the convective upwelling under the Ethiopian Highlands and the downwelling under the Mediterranean Sea are the principal factors responsible for the topographic gradient which has controlled Nile drainage since ∼30 Ma [START_REF] Faccenna | Role of dynamic topography in sustaining the Nile River over 30 million years[END_REF].

I.2.2.a The White Nile

The White Nile takes its sources in the Equatorial lakes region, mainly characterized by the Tanzania Craton and by the Archean Gneiss Granulite complex lithology (Figs. I.7a, I.5; [START_REF] Link | Continuous cratonic crust between the Congo and Tanzania blocks in western Uganda[END_REF]. The White Nile sources are composed of the freshwater outflow of three principal lakes: Victoria, Kyoga and Albert. Despite the high freshwater influx from the monsoon-filled lakes, the majority solid discharge is trapped in the Sudd marshes basin (Fig. I.7a): a natural physical filter for the coarser particles. These geomorphological patterns, combined with the low elevation contrast of the river drainage against the upwelled Blue Nile drainage, strongly influence the contribution of the White Nile in the Main Nile total discharge, represented by 30% of the mean annual water flux and only ∼3% of the mean annual sediment flux (Fig. I.7b;[START_REF] Garzanti | The modern Nile sediment system: Processes and products[END_REF][START_REF] Sutcliffe | The Hydrology of the Nile[END_REF]. After the Sudd marshes, the White Nile discharge is supplied by water and sediment from the Sobat River, which is characterized by a Proterozoic and Archean cortege. Finally, the White Nile joins the Blue Nile through an intricate low-gradient channel system [START_REF] Williams | Late Pleistocene and Holocene environments in the Nile basin[END_REF][START_REF] Williams | Late Quaternary Environments in the Nile Basin[END_REF]. The White Nile solid flux is principally characterized by a quartz, amphibole, epidote and garnet mineralogical assemblage and by illite and kaolinite clay minerals [START_REF] Garzanti | The modern Nile sediment system: Processes and products[END_REF].

I.2.2.b The Blue Nile and the Atbara Rivers

The Nile Ethiopian Highlands sources are represented by the Blue Nile and Atbara Rivers (Fig. I.7a). They contribute to the Nile total discharge with a 70% mean annual water flux and ∼96% of the mean annual sediment flux (Fig. I.7b). This high flux discharge is strongly influenced by the tectonic-induced upwelling gradient topography with elevations concentrated between 2000 and 3000 m a.s.l. with several peaks above 4000 m (>2 km elevation gradient) and by the intense strong summer monsoonal precipitations (up to 300 mm/month). As explained in Section I.1.3, rainfall over the Ethiopian Highlands is governed by seasonal migration of the ITCZ strongly influencing the different fluvial regimes through the latitudes. The Ethiopian Highlands are the major receiver of monsoonal air moisture discharge in East Africa (Fig. I.4). This exceptional rainfall regime produces a fluvial water discharge of 5000 to 6000 m3/s (for the Blue Nile River) and 2000 m 3 /s (for the Atbara River) during the summer, with an 85% of the flow input in the total hydrological budget of the Nile River between July and October.

Contrarily, during the winter season the water discharge decreases to 100 m 3 /s for the Blue Nile, and to 1 m3/s for the Atbara River. This strong seasonal water-flow contrast is the main factor responsible for the enhanced erosional rate in East African mountain areas [START_REF] Billi | Landscapes and Landforms of Ethiopia[END_REF]. The Blue Nile flows along a restricted geological lithology, draining principally the basaltic Traps and the alkaline rhyolites around the Lake Tana watershed (Fig. I.5). Descending the deep gorges, the river crosses the Mesozoic sedimentary outcrops and the Neoprotozoic Pan-African basement [START_REF] Tadesse | Geology and mjneral potential of Ethiopia: A note on geology and mineral map of Ethiopia[END_REF]. The mineralogical assemblage is principally composed of pyroxene, amphibole, quartz and feldspaths, although the clay minerals are mainly represented by smectites [START_REF] Garzanti | The modern Nile sediment system: Processes and products[END_REF][START_REF] Stanley | Sediment Transport on the Coast and Shelf Between the Nile Delta and Israeli Margin as Determined by Heavy Minerals[END_REF]. Because of the strong physical disaggregation of mica-bearing bedrock occurring between the Equatorial crystalline basement and the Sudd basin, resulting in illite formation, the mica minerals reaching the Main Nile likely have their origins in Dabus and Diesda mica-bearing basement-derived sediments (Fig. I.7).

I.2.2.c The Main Nile

After the Khartoum confluence, the Main Nile receives no additionally relevant water supplies until the Nile delta (Fig. I.7a). The only tributaries are represented by seasonal local dry valleys (called "awdiyan") located along the hyper-arid Saharan region, which is supplied by <50 mm/yr of rainfall. The Main Nile crosses the Saharan Metacraton and finally the Mesozoic Nubian Sandstones. Some aeolian inputs may reach the river course, however they are included in the eroded Saharan Metacraton cortege.

I.2.2.d The Nile Deep-Sea Fan (NDSF)

A part of this Ph.D work is focused on the study of a long piston core retrieved in the NDSF Western province (MD04-2726) and on its comparison with two other piston cores from the same zone Fig. I.8b). For this reason, a stratigraphical and morphological description of the NDSF and of the respective submarine province that composes it is proposed here, with a specific focus on the submarine geomorphology of the Western branch.

After crossing the Sudanese and Egyptian desert track, the Nile reaches Egypt's capital city, Cairo, where it separates into two main branches: The Rosetta branch to the East and the Damietta branch to the West (Fig. I.8a). Between the two river mouth branches, the delta measures 250 km long and 150 km wide covering a surface of about 24 000 km 2 . The Nile fluvial water and sedimentary fluxes flow into the Mediterranean Levantine basin, forming the complex Nile Deep-Sea Fan system. ∼600 km wide and ∼320 km long and reaching depths between 1600 and 2800 m, the Nile Deep-Sea Fan (NDSF) is the largest sedimentary accumulation in the Eastern Mediterranean. The fan sedimentation began in the Miocene, after the onset of the upwelling of the East African highlands (increase of sediment delivery; [START_REF] Faccenna | Role of dynamic topography in sustaining the Nile River over 30 million years[END_REF] and after the discontinuity of the Messinian evaporites [START_REF] Gaullier | Salt tectonics in and around the Nile deep-sea fan: Insights from the PRISMED II cruise[END_REF]. The NDSF is composed of four morpho-sedimentary zones linked with four chronological phases and processes of sediment delivery (Fig. I.8a): the Western Province is characterized by a complex channel-levee network [START_REF] Garziglia | Mass-transport deposits on the Rosetta province (NW Nile deep-sea turbidite system, Egyptian margin): Characteristics, distribution, and potential causal processes[END_REF] representing the best preserved and the more recent channel system, which is the more active today; the Central Province is characterized by large growth faults and related slope instability and by inactive sedimentary delivery processes [START_REF] Bayon | Multi-disciplinary investigation of fluid seepage on an unstable margin: The case of the Central Nile deep sea fan[END_REF]; the Eastern Province hosts several NNW-SSE oriented faults, and sediments are mainly trapped in the respective semi-grabens [START_REF] Loncke | The Nile deep-sea fan: An example of interacting sedimentation, salt tectonics, and inherited subsalt paleotopographic features[END_REF]; finally the Far Eastern Province is characterized by several long channels. The Central and the Eastern provinces are characterized by complex meandering deep-sea channels, however, high tectonic activity produces instability of sedimentary mass movements. Over the Western province the continental shelf is about 30 km wide, and the head of the Rosetta Canyon (C10) lies on the outer continental shelf at a water depth of about 700 m (Fig. I.8b). It is about 25 km long and 200-250 m deep, and is straight following the main continental slope angle. In cross section, the Rosetta Canyon is characterized by U-to poorly-developed V-shaped profiles where a large number of small-scale failures affected the canyon flanks (Fig. I.8c). The Rosetta Canyon connects to a meandering channel-levee system named DSF6, which was initiated during Marine Isotopic Stage 3 (MIS 3, ∼57 -∼29 ka BP; [START_REF] Ducassou | Evolution of the Nile deep-sea turbidite system during the Late Quaternary: Influence of climate change on fan sedimentation[END_REF] and potentially recorded the Holocene activity of Nile flood-induced activity. The upper stratigraphic complex incised by the C10 canyon (S4, S3, SB, in yellow) corresponds to the sedimentary aggradation started around 130 ka BP (Fig. I.8c). [START_REF] Revel | 20,000 years of Nile River dynamics and environmental changes in the Nile catchment area as inferred from Nile upper continental slope sediments[END_REF], and the P362-2-33 core (Blanchet et al., 2013, black dot); the grey line indicates the seismic profile of Fig. I.8c. c) Interpreted seismic profile along the edge of the Rosetta system platform, with canyons (red lines), the sedimentary aggradation that started around 130 ka BP (yellow area) until the Plio-Pleistocene transition (blue and green area), after Rouillard (2010) (modified from [START_REF] Mascle | The Nile Continental Margin: A Compendium Compiled by Jean Mascle[END_REF] .

I.2.3 The Awash basin and the Afar depression

The Awash River takes its sources in the Ethiopian Highlands, near Addis Ababa, at an altitude between 2000 and 2500 m a.s.l. on the steep northern escarpment of the Main Ethiopian Rift (MER; Fig. I.9). Descending the Highlands for 1200 km towards the north-east, the river follows the topographical patterns of the Rift until it reaches the Abhe endorheic basin in the Afar triangle at 230 m a.s.l. The Awash basin can be divided into four hydrological zones: the Upper Awash (above 1500 m a.s.l.), the Middle Awash (between 1500 and 1000 m a.s.l.), the Lower Awash (< 1000 m a.s.l.), and the eastern catchment (< 500 m a.s.l.) (Fig. I.9). The Upper/Middle and Lower zones belong to the MER, while the eastern catchment belongs to the Afar rift system coinciding with the Lake Abhe basin.

I.2.3.a The Awash drainage basin: geology

Contrary to the Nile drainage system, the Awash River crosses recent geological formations that are subsequent to the Oligocene Ethiopian Traps flows and that belong to the sin-rift extrusive dynamics (see Section I.2.1). However, due to strong rift-related volcano-tectonic activity, the geological outcrops of the Awash valley present quite complex intra-variability. The Upper Awash watershed is dominated by the upper Miocene-Pleistocene Magdala group which includes rhyolites, trachytes, tuffs, ignimbrites, agglomerates and basalt. The Upper north-western and eastern portions of the basin are mainly composed of the Ashangi group, represented by basalts, tuffs and intrusive gabbro, although on the eastern side of the watershed some outcrops of the Lower Cretaceous Amba Aradom group are composed of clay, silt, sandstone and conglomerate carbonate lithology. Along the western margin of the Middle-Lower Awash track, the river carves the thick Late Miocene to Holocene sedimentary deposits. The Middle Awash sedimentary sequences (Adu-Asa, Sagantole, Haradaso, Aramis, Gawto and other formations) have been largely studied because of the discovery of high interest sites for paleontological research on the earliest hominids (Fig. I.10a; [START_REF] Renne | Chronostratigraphy of the Miocene-Pliocene Sagantole Formation, Middle Awash Valley, Afar rift, Ethiopia[END_REF][START_REF] Woldegabriel | Geology and palaeontology of the Late Miocene Middle Awash valley, Afar rift, Ethiopia[END_REF]. These sequences are dated between ∼5.7 and ∼5.5 Ma (Fig. I.10a, b). Descending the Awash valley in the Lower eastern sector, the river crosses the Afar group mainly composed of Stratoid basalts formed between 4 and 0.4 Ma [START_REF] Barberi | Volcanism of Afar: Smallscale plate tectonics implications[END_REF][START_REF] Varet | Geology of Afar (East Africa)[END_REF] and intra-graben sedimentary accumulations, until the Afar rift system, which still belongs to the Stratoid series group (see detailed description in Section I.2.3.c).

I.2.3.b The Awash drainage basin: hydrology

Originating in the Ethiopian Highlands and ending in the Afar hyper-arid region, the Awash River crosses a large variety of environments which strongly influence its hydrological characteristics. Primarily, the freshwater flow, represented by 17 136 x 10 6 m 3 /yr, depends principally on the monsoonal rainfall regime over the Highlands. With a drainage basin surface of over 7 656 km 2 , the Awash receives 39 845 mm 3 of rainfall/yr, whereas 72% of it is lost to evaporation in the lowlands [START_REF] Taddese | THE WATER OF THE AWASH RIVER BASIN A FUTURE CHAL-LENGE TO ETHIOPIA[END_REF]. During the monsoonal summer season, the river water-heads receive ∼200 mm/month, although in the Central Afar region summer rainfalls are almost absent, with a peak of 153 mm/yr. These are conditions which keep Lake Abhe filled, even though it lies at the end of the Awash River track.

I.2.3.c The Central Afar region: geological and volcano-tectonic setting

The Central Afar region is mainly represented by the Tendaho-Gobaad graben, the widest and longest basin within the Afar triangle, where the Awash River track ends (Fig. I.11a). The extensional tectonic movements which are taking place in the Afar are associated with the triple junction of the divergent boundaries between the Somalian, Arabian and Nubian plates (Fig. I.11b). Over its 1.8-Ma-long development, the Tendaho-Gobaad graben has been the locus of interactions between the three extensional systems related to the EARS, the Red Sea and the Aden ridges. The Tendaho-Gobaad graben is considered the southwestern tip of the Red Sea propagating on land (Nubia-Arabian boundary) towards the Aden Ridge (Somalia-Arabian boundary), whereas the extensional structures related to the northern tip of the EARS are observed in its southern margin along the Main Ethiopian Rift (MER; Fig. I.11a;[START_REF] Acton | Block rotation and continental extension in Afar: A comparison to oceanic microplate systems[END_REF].

The Stratoid series, a 1.5 km thick basaltic sequence which covers most of the Afar region, was dated from 4.4 Ma to 0.4 Ma (Varet, 2018, Middle extrusive complex, Fig. I.12). From 1.8 Ma, the continental extension in the Central Afar is focused across several NE-trending active rift segments, concentrating the tectonic deformation and, in most cases, the magmatic activity as well. These rift segments formed a complex network of 130°N normal faults and several sub-rifts, horsts and grabens, to which the Tendaho-Gobaad graben belongs [START_REF] Abbate | Strike-slip faults in a rift area: A transect in the Afar Triangle, East Africa[END_REF]. Since the Plio-Early Pleistocene [START_REF] Boucaurt | Impact des événements tectono-volcaniques Plio-Pléistocenes sur la sédimentation en République de Djibouti (Afar Central)[END_REF][START_REF] Gasse | Evolution tectonique et climatique de l'Afar central d'apres les sediments plio-pleistocenes[END_REF], the Central Afar grabens are filled, forming several sedimentary basins such as those of Asal,Immino,Dobi,Gaggadé,and Hanlé basins (Fig. I.11a). The Tendaho-Gobaad graben contains the Lake Abhe basin, the widest sedimentary basin in the Central Afar region (Fig. I.11a,Fig. I.12). Many recent fissural volcanic flows occurred between ∼2 and ∼0.5 Ma related to the emplacement of the Stratoid series, evidencing the link between the outflows of the relevant volumes of Stratoid magma and the collapse of the Tendaho-Gobaad graben axial valley (Acocella, 2010, Upper extrusive complex, Fig. I.12). Holocene rifting activity is evidenced by some volcanic cones and domes associated with NNW-trending sub-rifts (Fig. I.12; [START_REF] Barberi | Volcanism of Afar: Smallscale plate tectonics implications[END_REF][START_REF] Manighetti | Propagation of rifting along the Arabia-Somalia Plate Boundary: Into Afar[END_REF], and by the probable NE-tilting of the basin plain proposed by [START_REF] Gasse | L'évolution des lacs de l'Afar Central (Ethiopie et T.F.A.I.) du Plio-Pléistocène à l'Actuel. Recostitution des paléomilieux lacustres à partir de l'étude des Diatomées[END_REF]. This recent subsidence movement would have induced the capture of the Awash River [START_REF] Gasse | L'évolution des lacs de l'Afar Central (Ethiopie et T.F.A.I.) du Plio-Pléistocène à l'Actuel. Recostitution des paléomilieux lacustres à partir de l'étude des Diatomées[END_REF] leading to the current hydrological basin patterns. From recent geodetic studies [START_REF] Doubre | Current deformation in Central Afar and triple junction kinematics deduced from GPS and InSAR measurements[END_REF], the current extensional deformation, which is mainly SW-NE oriented and perpendicular to the graben margins, is distributed over the width of the basin and does not exceed 7 mm/yr, when the subsidence velocity of the inner floor with respect to its margins remains unclear. other Central Afar sedimentary basins (light yellow), present natural waterbodies (blue), the Awash River main lower track (blue line), recent lava flows (light red), and active (red lines) and ancient (dashed red lines) rift segments (modified after [START_REF] Barberi | Volcanism of Afar: Smallscale plate tectonics implications[END_REF] The dynamics information of past climate between Atlantic/Indian oceans are stored in East African sedimentary archives. The lake /oceanic sedimentary records show that during precession minima, when summer solar insolation was maximal in the northern hemisphere, the ITCZ strengthened and extended northward [START_REF] Demenocal | Abrupt onset and termination of the African Humid Period: Rapid climate responses to gradual insolation forcing[END_REF][START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF]. During the Quaternary, the variability of monsoon intensity is best illustrated by the repeated cycles of enhanced (humid periods) and reduced precipitation (arid periods) over tropical Africa (Fig. II.1). Changes in Earth obliquity (the tilt of the earth's axis), in Earth axial and apsidal precessions (axis oscillation and rotation along the elliptical orbit), and in the shape of Earth's orbit around the sun (eccentricity) vary cyclically (Fig. II.1b). These three orbital parameters modify the quantity and distribution of insolation on the Earth's surface, which in turn, alters climate [START_REF] Berger | The Milankovitch astronomical theory of paleoclimates: A modern review[END_REF][START_REF] Berger | Insolation values for the climate of the last 10 million years[END_REF][START_REF] Milankovitch | Kanon der Erdbestrahlungen und seine Anwendung auf das Eiszeitenproblem[END_REF].

It has been proven that these precession-driven insolation changes, are the main factors responsible for changes in interhemispheric monsoon intensities and in latitudinal displacements of the ITCZ around the tropical belt (Fig. II.1a;[START_REF] Mohtadi | Palaeoclimatic insights into forcing and response of monsoon rainfall[END_REF][START_REF] Schneider | Migrations and dynamics of the intertropical convergence zone[END_REF]. Accordingly, monsoon-induced humid-arid cycles in Africa follow the effects of the precessional cycle, occurring with a seasonal rhythm with a duration ranging between 23 and 19 ka (Fig. II.1a, b;[START_REF] Berger | The Milankovitch astronomical theory of paleoclimates: A modern review[END_REF][START_REF] Kutzbach | Monsoon Climate of the Early Holocene: Climate Experiment with the Earth's Orbital Parameters for 9000 Years Ago[END_REF][START_REF] Kutzbach | Milankovitch forcing of fluctuations in the level of tropical lakes from 18 to 0 kyr BP[END_REF][START_REF] Mohtadi | Palaeoclimatic insights into forcing and response of monsoon rainfall[END_REF].

Figure II.1:

Variations in paleo-precipitation from a.1) Northern Hemisphere in Africa [START_REF] Tjallingii | Coherent high-and low-latitude control of the northwest African hydrological balance[END_REF] and a.2) Southern Hemisphere in Africa [START_REF] Partridge | Orbital forcing of climate over South Africa: A 200,000-year rainfall record from the pretoria saltpan[END_REF] in relation to variations in their respective summerseason insolation (grey dashed line [START_REF] Berger | Insolation values for the climate of the last 10 million years[END_REF]; (modified after [START_REF] Singarayer | Interhemispheric dynamics of the African rainbelt during the late Quaternary[END_REF]. b) Schematic and simplified representation of Earth's precession and eccentricity against time; blue stars and orange circles correspond respectively to high and low rainfall periods (modified after [START_REF] Bastian | Abrupt response of chemical weathering to Late Quaternary hydroclimate changes in northeast Africa[END_REF] As explained in Section I.1.1, the modulation of monsoonal rainfall intensities and distribution over the tropics are the result of complex ocean-atmospheric interactions. Here, we succinctly present the main mechanisms which governed the long-term orbital-induced African monsoon activity during the last 130 ka. Tropical Africa's hydrological variations are principally linked to North Hemispheric (NH) solar irradiation [START_REF] Schefuß | Climatic controls on central African hydrology during the past 20,000 years[END_REF][START_REF] Tierney | Northern Hemisphere Controls on Tropical Southeast African Climate During the Past 60,000 Years[END_REF][START_REF] Tjallingii | Coherent high-and low-latitude control of the northwest African hydrological balance[END_REF], resulting from the AMOC and Hadley cell (see Section I.1.1) north-south latitude connections [START_REF] Chiang | Influence of high latitude ice cover on the marine Intertropical Convergence Zone[END_REF][START_REF] Rahmstorf | Ocean circulation and climate during the past 120,000 years[END_REF]. Through the southern masses transport via the Hadley circulation [START_REF] Muri | The Climate of the MIS-13 Interglacial according to HadCM3[END_REF], changes in tropical interhemispheric thermal distribution are linked to the variation in the AMOC and in the Sea Surface Temperature (SST) [START_REF] Schneider | Migrations and dynamics of the intertropical convergence zone[END_REF][START_REF] Zhang | Spatial fingerprint and magnitude of changes in the Atlantic meridional overturning circulation during marine isotope stage 3: Footprint and change in the AMOC in MIS3[END_REF]. From climatic modelling, [START_REF] Braconnot | Monsoon response to changes in Earth's orbital parameters: Comparisons between simulations of the Eemian and of the Holocene[END_REF] shows that continental Africa is particularly sensitive to high summer SST values, which enhance inland transport of air moisture masses from the Atlantic to the African monsoon domains. As already presented above (Section I.1.1), East Africa is even particularly influenced by the combined intrusion of air mass moisture originated in the Mediterranean Sea and Atlantic and Indian Oceans, emphasizing this process. Consequently, variations in ocean-atmospheric energy balance strongly influenced the magnitude and the spatial distribution of rainfall over eastern continental Africa.

During the Quaternary, the variation of NH insolation magnitude combined with SST oscillations seasonally produced the symmetrical interhemispheric intensity/expansion and the reduction of rainfalls following precessional pacing [START_REF] Collins | Interhemispheric symmetry of the tropical African rainbelt over the past 23,000 years[END_REF][START_REF] Singarayer | Interhemispheric dynamics of the African rainbelt during the late Quaternary[END_REF]. Contemporarily, following the shifts of the atmospheric energy balance described above, the high cross-equatorial interhemispheric temperature contrast produced a displacement of the air mass convection zones. These patterns resulted in the latitudinal oscillation of the continental and marine ITCZ (Fig. II.2; [START_REF] Chiang | Extratropical Cooling, Interhemispheric Thermal Gradients, and Tropical Climate Change[END_REF][START_REF] Schneider | Migrations and dynamics of the intertropical convergence zone[END_REF]. During minimal precession, high solar irradiation occurred and the ITCZ migrated northward, resulting in the increase of the SST gradient in the northern extratropical waters. Contrarily, during the maximal precession, the ITCZ migrated to the south. Several paleo-records have attested to the correlation between summer insolation, monsoonal activity and the position of the ITCZ over continental Africa [START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF][START_REF] Mcintyre | Surface water response of the equatorial Atlantic Ocean to orbital forcing[END_REF][START_REF] Partridge | Orbital forcing of climate over South Africa: A 200,000-year rainfall record from the pretoria saltpan[END_REF][START_REF] Shanahan | The time-transgressive termination of the African Humid Period[END_REF][START_REF] Verschuren | Half-precessional dynamics of monsoon rainfall near the East African Equator[END_REF]. This general mechanism has produced firstly an increase of precipitations and secondly a diminution in rainfall in the northern tropics (Fig. II.1b).

Recent paleo-climatic simulations suggest that the seasonal position of the ITCZ is not the only factor affecting the geographical distribution of rainfall over continental Africa, arguing installation of a more complex scenario including more specific intra-regional forcings (Mohtadi et al., 2016, see Sections II.1.4.b and II.1.4.c for details).

II.1.2 The 'African Humid Period' or 'Green Sahara'

Long-term orbitally-induced humid-arid cycles are the main motor of hydrologic changes over the tropics. The last of these cycles is the more known and the more studied. The humid phase covering the Pleistocene-Holocene transition is widely known as the 'African Humid Period' (AHP;[START_REF] Demenocal | Abrupt onset and termination of the African Humid Period: Rapid climate responses to gradual insolation forcing[END_REF] or 'Green Sahara' [START_REF] Tierney | Rainfall regimes of the Green Sahara[END_REF]. The AHP generally ranges between ∼14.8 and ∼5.5 ka BP (Fig. II.3;[START_REF] Demenocal | Abrupt onset and termination of the African Humid Period: Rapid climate responses to gradual insolation forcing[END_REF][START_REF] Shanahan | The time-transgressive termination of the African Humid Period[END_REF], even if a chronological intracontinental variability is evidenced latitudinally and geographically (see Section II.1.4.a for details). The AHP follows quite well the precessional control, which was evidenced for the first time by [START_REF] Rossignol-Strick | After the deluge: Mediterranean stagnation and sapropel formation[END_REF] thanks to the study of sapropelic events in the eastern Mediterranean region [START_REF] Emeis | Eastern Mediterranean surface water temperatures and δ 18 O composition during deposition of sapropels in the late Quaternary: SST AND δ 18 O OF MEDITER-RANEAN SAPROPELS[END_REF].

Several combined internal mechanisms are responsible for the enhanced rainfall over continental Africa during this period. During the maximal boreal-summer insolation an increased SST gradient in the NH was recorded. This factor, combined with strong wind-evaporation feedbacks over the Atlantic, have led to massive air moisture mass transport from the oceans to the lands [START_REF] Zhao | Simulated changes in the relationship between tropical ocean temperatures and the western African monsoon during the mid-Holocene[END_REF]. Contemporarily, the warming of the SST in the northern extratropics have induced the northward migration of the ITCZ particularly influencing North African hyper-arid regions.

AHP enhanced monsoonal activity has huge consequences on African environments, for example [START_REF] Wolff | Millennial-scale variability during the last glacial: The ice core record[END_REF]; b) reflectance of Cariaco basin sediments interpreted as the latitudinal excursion variations of the ITCZ during boreal summer [START_REF] Deplazes | Links between tropical rainfall and North Atlantic climate during the last glacial period[END_REF]; c) δ18O cave stalagmites from Borneo as a proxy of rainfall in the equatorial western Pacific [START_REF] Carolin | Varied Response of Western Pacific Hydrology to Climate Forcings over the Last Glacial Period[END_REF] ; d) reflectance of Arabian Sea sediments interpreted as a runoff proxy triggered by Indian monsoon activity [START_REF] Deplazes | Links between tropical rainfall and North Atlantic climate during the last glacial period[END_REF]. Cold intervals HE6-1, including YD, are indicated with grey bands, (after [START_REF] Schneider | Migrations and dynamics of the intertropical convergence zone[END_REF][START_REF] Gottschalk | Abrupt changes in the southern extent of North Atlantic Deep Water during Dansgaard-Oeschger events[END_REF] the increase of rock silicate weathering [START_REF] Bastian | Abrupt response of chemical weathering to Late Quaternary hydroclimate changes in northeast Africa[END_REF], the strong activation of fluvial systems [START_REF] Revel | 20,000 years of Nile River dynamics and environmental changes in the Nile catchment area as inferred from Nile upper continental slope sediments[END_REF][START_REF] Skonieczny | African humid periods triggered the reactivation of a large river system in Western Sahara[END_REF][START_REF] Weldeab | The pace of East African monsoon evolution during the Holocene: WELDEAB ET. AL.; EAST AFRICAN MON-SOON EVOLUTION[END_REF] and the extension of continental waterbody surfaces [START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF][START_REF] Junginger | The effects of solar irradiation changes on the migration of the Congo Air Boundary and water levels of paleo-Lake Suguta, Northern Kenya Rift, during the African Humid Period (15-5ka BP)[END_REF], even over the arid regions of the Sahel and the Sahara (Fig. II.3;[START_REF] Fontes | Freshwater to marine-like environments from Holocene lakes in northern Sahara[END_REF][START_REF] Hoelzmann | Environmental change and archaeology: Lake evolution and human occupation in the Eastern Sahara during the Holocene[END_REF][START_REF] Holmes | The Late Pleistocene-Holocene African Humid Period as Evident in Lakes[END_REF][START_REF] Lézine | Late Holocene plant and climate evolution at Lake Yoa, northern Chad: Pollen data and climate simulations[END_REF][START_REF] Marks | Holocene lake sediments from the Faiyum Oasis in Egypt: A record of environmental and climate change[END_REF][START_REF] Schuster | Holocene Lake Mega-Chad palaeoshorelines from space[END_REF]. Finally, these hydrological conditions led to a significant spread of vegetation cover over the Sahara desert with a shift of the Sahelian belt ∼800 km further north and with rainfall rates ∼10 times higher than the present-day arid Sahara [START_REF] Claussen | The Greening of the Sahara during the Mid-Holocene: Results of an Interactive Atmosphere-Biome Model[END_REF][START_REF] Hopcroft | Multi vegetation model evaluation of the Green Sahara climate regime: RAINFALL SUPPORT-ING A GREEN SAHARA[END_REF][START_REF] Prentice | Mid-Holocene and glacialmaximum vegetation geography of the northern continents and Africa[END_REF].

Two abrupt arid events (∼200 to ∼400 yr) likely linked to the North Atlantic cooling stadials punctuated the AHP: the YD [START_REF] Alley | The Younger Dryas cold interval as viewed from central Greenland[END_REF] and the 8.2 ka events [START_REF] Matero | The 8.2 ka cooling event caused by Laurentide ice saddle collapse[END_REF][START_REF] Rohling | Centennial-scale climate cooling with a sudden cold event around 8,200 years ago[END_REF]. These arid events are recognised over intertropical Africa from lacustrine fluctuations [START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF][START_REF] Roberts | Timing of the Younger Dryas event in East Africa from lake-level changes[END_REF], from paleo-precipitation proxies [START_REF] Tierney | Rainfall regimes of the Green Sahara[END_REF], and from continental margin sedimentation [START_REF] Blanchet | Runoff and precipitation dynamics in the Blue and White Nile catchments during the mid-Holocene: A data-model comparison[END_REF][START_REF] Liu | Two phases of the Holocene East African Humid Period: Inferred from a high-resolution geochemical record off Tanzania[END_REF][START_REF] Ménot | Timing and stepwise transitions of the African Humid Period from geochemical proxies in the Nile deep-sea fan sediments[END_REF][START_REF] Revel | 20,000 years of Nile River dynamics and environmental changes in the Nile catchment area as inferred from Nile upper continental slope sediments[END_REF][START_REF] Wu | North-African paleodrainage discharges to the central Mediterranean during the last 18,000 years: A multiproxy characterization[END_REF]. The general mechanisms which govern these Late Quaternary abrupt climatic events are presented in the following Section II.1.3. 20.8°N (deMenocal et al., 2000); δDwax from c) the Senegal River at 15.5°N [START_REF] Niedermeyer | Orbital-and millennial-scale changes in the hydrologic cycle and vegetation in the western African Sahel: Insights from individual plant wax δD and δ13C[END_REF], d) Lake Bosumtwi at 6.5°N [START_REF] Shanahan | The time-transgressive termination of the African Humid Period[END_REF] and from the Congo Basin at 5.5°S [START_REF] Schefuß | Hydrologic control of carbon cycling and aged carbon discharge in the Congo River basin[END_REF]. Grey bands indicate short-term arid events; the red arrow indicates the delayed southward shift of the ITCZ, (modified after [START_REF] Shanahan | The time-transgressive termination of the African Humid Period[END_REF]

II.1.3 Internal forcing and abrupt climatic events

We illustrated how, during the Late Quaternary, the principal cyclic alternation of humid and arid periods has been the result of the orbital pacing occurring every 19-to-21 ka, always modulated between high and low latitudes by the ocean-atmospheric cross-equatorial energy balance. However, these longterm cycles were punctuated by other millennial to sub-millennial climatic events which have strongly influenced continental African hydrological patterns.

These millennial climatic shifts are widely known under the name of Dansgaard-Oeschger events (DO), which are composed of cold and warm phases (called stadial and interstadial respectively; [START_REF] Dansgaard | Evidence for general instability of past climate from a 250-kyr ice-core record[END_REF][START_REF] Heinrich | Origin and Consequences of Cyclic Ice Rafting in the Northeast Atlantic Ocean During the Past 130,000 Years[END_REF][START_REF] Voelker | Global distribution of centennial-scale records for Marine Isotope Stage (MIS) 3: A database[END_REF]. Even if the strongest temperature anomalies linked to the DO are mainly recorded in the North Atlantic [START_REF] Rahmstorf | Ocean circulation and climate during the past 120,000 years[END_REF][START_REF] Zhang | Spatial fingerprint and magnitude of changes in the Atlantic meridional overturning circulation during marine isotope stage 3: Footprint and change in the AMOC in MIS3[END_REF], in the tropics, these millennial climatic events are consistently recorded as changes in the global monsoonal activity [START_REF] Wang | The global monsoon across timescales: Coherent variability of regional monsoons[END_REF][START_REF] Zhisheng | Global Monsoon Dynamics and Climate Change[END_REF]. Particularly, some of the DO events are characterized by extremely low temperatures in the NH, which are called Heinrich Events, corresponding to a systematic diminution of SST over the Atlantic (HE; Fig. II.2;[START_REF] Sanchez Goñi | Millennialscale climate variability and vegetation changes during the Last Glacial: Concepts and terminology[END_REF]. These events are the result of the ice rafting originated in the North Atlantic ice cover, which was recognised in the Ice Rafted Detritus (IRD) recorded in several North Atlantic cores [START_REF] Heinrich | Origin and Consequences of Cyclic Ice Rafting in the Northeast Atlantic Ocean During the Past 130,000 Years[END_REF]. During the last Glacial Cycle (60 ka to 11.7 ka) 17 DO and 6 HE events were counted [START_REF] Gottschalk | Abrupt changes in the southern extent of North Atlantic Deep Water during Dansgaard-Oeschger events[END_REF][START_REF] Rahmstorf | Ocean circulation and climate during the past 120,000 years[END_REF] including the Younger Dryas cold interval (YD; Fig. II.2;[START_REF] Alley | The Younger Dryas cold interval as viewed from central Greenland[END_REF]. Similar climatic events occurred more recently in the Holocene period at 8.2 and 4.2 ka, which was presented in Section II.1.2 and will be detailed in Section II.1.4.a. However, their origin is still debated (see Section II.1.4.c for details). During the Late Quaternary, HEs have been crucial for the evolution of tropical African climate because they represent a strong freshwater-input perturbation of the AMOC and a rapid SST temperature drought which affected cross-equatorial meridional heat transport (Fig. II.4;[START_REF] Broecker | A salt oscillator in the glacial Atlantic? 1. The concept[END_REF].

In low latitudes, the HEs correspond to severe aridity events recorded from western to eastern tropical Africa [START_REF] Mulitza | Sahel megadroughts triggered by glacial slowdowns of Atlantic meridional overturning: SAHEL DROUGHT AND ATLANTIC OVERTURNING[END_REF] triggered by a drastic reduction in precipitation [START_REF] Castañeda | Hydroclimate variability in the Nile River Basin during the past 28,000 years[END_REF]. Consequences over continental Africa are characterized by a massive increase in dust transport [START_REF] Collins | Abrupt shifts of the Sahara-Sahel boundary during Heinrich stadials[END_REF], by a reduction of silicate weathering [START_REF] Bastian | Abrupt response of chemical weathering to Late Quaternary hydroclimate changes in northeast Africa[END_REF] and by lacustrine drying episodes [START_REF] Foerster | Climatic change recorded in the sediments of the Chew Bahir basin, southern Ethiopia, during the last 45,000 years[END_REF] sometimes associated with enhanced lacustrine biological productivity [START_REF] Scholz | Paleolimnology of Lake Tanganyika, East Africa, over the past 100 kyr[END_REF]. Finally, NH cooling events seem to have impacted the low latitudes with a southern and short-term shift of the ITCZ position.

We have illustrated how the primary factor of HE break-out is represented by the North Atlantic ice-sea interactions mediated by the AMOC through the low latitudes. However, the variability in intensity and in centennial chrono-spatial distribution of these climatic droughts over continental Africa could be linked to specific regional land-atmospheric interactions (see Section II.1.4 for specific information). For this reason, the hydrological response to climate-change appears quite complex and little is known over continental Africa today, in particular regarding the evolution of current monsoonal dynamics [START_REF] Masson-Delmotte | par rapport aux niveaux préindustriels et les trajectoires associées d'émissions mondiales de gaz à effet de serre, dans le contexte du renforcement de la parade mondiale au changement climatique, du développement durable et de la lutte contre la pauvreté[END_REF]. The multiplication of studies of decennial resolution on new and un-investigated environmental archives is needed today for the improvement of numerical simulations of the climates of the future.

II.1.4 Open questions

II.1.4.a Timing, dynamics and magnitude of the AHP

Some studies focused on the AHP have evidenced a chronological offset of the installation and demise of humid conditions between equatorial and north extratropical latitudes [START_REF] Tierney | Rainfall regimes of the Green Sahara[END_REF] resulting in relatively restricted high precipitation rate periods over higher latitudes against the longer humid periods in the south (Fig. II.4). The same problem arises for the irregular spatio-temporal patterning of hydrological regimes between East and West Africa (Fig. II.5;[START_REF] Shanahan | The time-transgressive termination of the African Humid Period[END_REF][START_REF] Tierney | Late Quaternary behavior of the East African monsoon and the importance of the Congo Air Boundary[END_REF][START_REF] Lézine | Across an early Holocene humid phase in western Sahara: Pollen and isotope stratigraphy[END_REF]. Another critical and debated issue about the dynamics of the AHP concern the abruptness or the gradualness of the arid/humid switches [START_REF] Demenocal | Abrupt onset and termination of the African Humid Period: Rapid climate responses to gradual insolation forcing[END_REF][START_REF] Ménot | Timing and stepwise transitions of the African Humid Period from geochemical proxies in the Nile deep-sea fan sediments[END_REF][START_REF] Shanahan | The time-transgressive termination of the African Humid Period[END_REF], particularly regarding the aridification events that occurred around 8 and 6 to 4 ka. Understanding and explaining this variability represent the new challenges of Holocene paleoclimate research in Africa.

The response of the hydrological cycle to the increase of summer insolation is modulated by ocean, atmosphere and surface interactions and retroactivities, showing the complexity of the processes which govern the environmental changes over the continent. The spatio-temporal hydrologic variability of the African Holocene could be related to the displacement of air moisture masses over the continent. This is linked to the migration of convergence zone boundaries, but also to the topographic patterns of the Earth's surface, as well as the altitudinal gradient produced by the East African rift-controlled relief and the respective watershed network. Furthermore, this kind of reasoning should take into account the non-negligible reactivity of specific geomorphic systems to hydrological change. Indeed, the specific reactivity patterns of continental geomorphic systems to past climatic changes remain little investigated, raising doubts about an autogenic control of continuous sedimentary records exploited for paleoclimatic reconstructions. Indeed, some erosional, residence or reservoir effects of sediments along rivers and lakes could be caused by some independent geomorphological controls, disconnected from climatic forcing. This aspect will be discussed in detail in Section II.2. In tropical Africa there are two main convergence zones which control the spatial distribution of precipitations: the ITCZ (see Section I.1.1 for details) and the CAB (Fig. II.6;Congo Air Boundary). The CAB delimits the confluence of air moisture masses derived from both the Atlantic and the Indian Oceans. Located along the East African Rift System, which represents the main African orographic separation, the CAB is northeast-southwest-oriented. While the past north-south ITCZ oscillation ranges are well known, the Holocene northwest-southeast competence zone displacements of the CAB are less defined. As illustrated in Section II.1.1, the main hydrological variations are governed by the ITCZ latitudinal oscillation, however, the CAB represents an important rainfall supplier over the tropics [START_REF] Levin | Isotopic composition of waters from Ethiopia and Kenya: Insights into moisture sources for eastern Africa[END_REF].

Modelling experience studies indicate that during the AHP the enhanced atmospheric pressure gradient between the East African Monsoon and the Indian Summer Monsoon (ISM) would have triggered an Eastern migration of the Congo Air Boundary [START_REF] Marzin | The role of the ocean feedback on Asian and African monsoon variations at 6kyr and 9.5kyr BP[END_REF]. It has been argued that these patterns would have induced an incursion of the West African Summer Monsoon (WAM) and the ISM air masses along the East African Rift System (EARS), causing a non-linear response of the hydrosedimentary systems to the ITCZ north-south oscillations [START_REF] Costa | Isotopic reconstruction of the African Humid Period and Congo Air Boundary migration at Lake Tana, Ethiopia[END_REF][START_REF] Junginger | The effects of solar irradiation changes on the migration of the Congo Air Boundary and water levels of paleo-Lake Suguta, Northern Kenya Rift, during the African Humid Period (15-5ka BP)[END_REF][START_REF] Tierney | Late Quaternary behavior of the East African monsoon and the importance of the Congo Air Boundary[END_REF]. In particular, the East African highlands may have been significantly subjected to the ISM moisture influence because of its particular eastern position [START_REF] Camberlin | Rainfall Anomalies in the Source Region of the Nile and Their Connection with the Indian Summer Monsoon[END_REF][START_REF] Tierney | Abrupt Shifts in Horn of Africa Hydroclimate Since the Last Glacial Maximum[END_REF][START_REF] Viste | Moisture transport into the Ethiopian highlands: MOISTURE TRANSPORT INTO THE ETHIOPIAN HIGHLANDS[END_REF].

II.1.4.c Low and High Latitude climatic interconnections

As explained above in Sections II.1.1 and II.1.2, the changes in precession-driven solar irradiation in NH during the boreal summer represents the main forcing of long-term global climate change. Influencing the AMOC and the SST energy balance, the ocean heat transport to tropical latitudes is considered the process which mainly affects air moisture evaporation and transfer of monsoon activity to the hinterlands. The same transfer process is valid for the changing in ocean-atmosphere energy gradient caused by the HE ice sheet/rafting from the North Atlantic and resulting in severe arid droughts (see Section II.1.3 for details). However, recent studies indicate that an obliquity-driven changing in insolation gradient over low latitudes can directly affect the monsoon rainfalls [START_REF] Bosmans | Obliquity forcing of low-latitude climate[END_REF][START_REF] Mantsis | The Response of Large-Scale Circulation to Obliquity-Induced Changes in Meridional Heating Gradients[END_REF]. In other words, the direct solar irradiation on the tropics could have a large contribution in air moisture generation over the oceans without the influence of the mediated and indirect energy-balance inputs from high-latitude forcing and/or from NH ice-sea interactions (Bosmans et al., 2015b,a;[START_REF] Clement | The Atlantic Multidecadal Oscillation without a role for ocean circula-tion[END_REF]. In addition, it has been suggested that the change in orbitally-driven tropical SST gradient would have had a global impact and a principal role in abrupt climatic cooling events through ocean circulation retroactivity [START_REF] Clement | An Orbitally Driven Tropical Source for Abrupt Climate Change[END_REF]. Accordingly, it has even been argued that the tropical land-atmosphere teleconnection can have impacted the NH climate during the Holocene, caused by the albedo increase of the Sahara desert [START_REF] Davies | The impact of Sahara desertification on Arctic cooling during the Holocene[END_REF]. All of these issues bring new considerations to the role of the African climate on the NH dynamics and to the development of a new point of view In Section II.1 we illustrated the principal mechanisms which governed East African hydro-climate variability with a specific focus on the Pleistocene-Holocene transition and related current and past research issues. Paleoclimatic interpretations often concern precipitation-temperature variation on a wider subcontinental scale. However, as shown above, the complexity of the chronological and spatial pattern variations of East-African climate require a more specific comprehension of the hydro-sedimentary system feedbacks and of land-surface retroactions. It has been estimated that between ∼14 and ∼10 ka BP, precipitation rates over East Africa increased by < 26 % [START_REF] Junginger | The effects of solar irradiation changes on the migration of the Congo Air Boundary and water levels of paleo-Lake Suguta, Northern Kenya Rift, during the African Humid Period (15-5ka BP)[END_REF], while over the Saharan and Sahelian regions it has been estimated that rainfall rates were ∼10 times higher than the presentday [START_REF] Claussen | The Greening of the Sahara during the Mid-Holocene: Results of an Interactive Atmosphere-Biome Model[END_REF][START_REF] Hopcroft | Multi vegetation model evaluation of the Green Sahara climate regime: RAINFALL SUPPORT-ING A GREEN SAHARA[END_REF][START_REF] Prentice | Mid-Holocene and glacialmaximum vegetation geography of the northern continents and Africa[END_REF]. Such hydrological variations impacted the fluvial, fluvio-glacial, lacustrine and soil systems, causing a prominent modification of the paleolandscapes and of the continental geomorphic systems through time. Study of the Late Pleistocene-Holocene geomorphological evolution of East African landforms is thus crucial for the understanding of spatio-temporal distribution of climate change. Additionally, a holistic interpretation of continental sedimentary system mechanisms will allow us to localise some possible geomorphic failures responsible for the non-linear1 response of sedimentary recording.

II.2.1.a Fluvial systems

There is little comprehensive literature about the geomorphology and hydrology of present and subactual East-African rivers [START_REF] Billi | Landscapes and Landforms of Ethiopia[END_REF]. Moreover, studies on paleo-hydro-sedimentary dynamics of ancient rivers are even more uncommon.

Over the Eastern Saharan and Sahelian belts, the majority of geomorphological studies address the evolution of the Nile fluvial system. Some studies that focus on Late Pleistocene-Holocene periods have evidenced the main aggradation and erosional phases along the lower White and Blue Nile Rivers [START_REF] Lario | Palaeoenvironmental evolution of the blue Nile (Central Sudan) during the early and mid-holocene (Mesolithic-Neolithic transition)[END_REF][START_REF] Wendorf | Desert and the Nile: Prehistory of the Nile Basin and the Sahara: Papers in Honour of Fred Wendorf[END_REF][START_REF] Williams | The Nile Basin: Quaternary Geology, Geomorphology and Prehistoric Environments[END_REF][START_REF] Williams | Late Quaternary Environments in the Nile Basin[END_REF][START_REF] Williams | Late Quaternary environments and prehistoric occupation in the lower White Nile valley, central Sudan[END_REF][START_REF] Williams | Late Quaternary floods and droughts in the Nile valley, Sudan: New evidence from optically stimulated luminescence and AMS radiocarbon dating[END_REF]Woodward et al., 2015a,b). In the same studies, data interpretation has led to the definition of their flood periods, often coinciding with AHP rainfall pacing: high water-levels linked to the formation of ephemeral lakes started around 14 ka BP. Accordingly, the White and Blue Nile Rivers were in high energy 'flood periods' together during the Early Holocene (9.7 and 9 ka BP), followed by shorter flooding events at 8.6 and between 7.8 and 6 ka BP. Other studies have focused on the upper Nile course and on more recent periods [START_REF] Ghilardi | Nile River evolution in Upper Egypt during the Holocene: Palaeoenvironmental implications for the Pharaonic sites of Karnak and Coptos. Géomorphologie : relief, processus[END_REF][START_REF] Toonen | Holocene fluvial history of the Nile's west bank at ancient Thebes, Luxor, Egypt, and its relation with cultural dynamics and basin-wide hydroclimatic variability[END_REF].

In the East-African Highlands, there are few studies of Late Pleistocene-Holocene fluvial sequences [START_REF] Blond | Terrasses alluviales et terrasses agricoles. Première approche des comblements sédimentaires et de leurs aménagements agricoles depuis 5000 av. n. è. à Wakarida ([END_REF][START_REF] Coltorti | The Geomorphological Map of Mt. Amba Aradam Southern Slope (Tigray, Ethiopia)[END_REF][START_REF] Machado | Paleoenvironmental changes during the last 4000 yr in the Tigray, Northern Ethiopia[END_REF][START_REF] Moeyersons | Age and backfill/overfill stratigraphy of two tufa dams, Tigray Highlands, Ethiopia: Evidence for Late Pleistocene and Holocene wet conditions[END_REF], despite the fact that they may represent a powerful archive for climate-related earth surface processes study as shown by [START_REF] Garcin | Short-lived increase in erosion during the African Humid Period: Evidence from the northern Kenya Rift[END_REF]. The majority of research is thus focused on actual or sub-actual fluvial processes, specifically on hydrological modelling to predict flow regime and sediment transport or to estimate the impact of current climate change, land use and human activities [START_REF] Ayenew | Environmental isotopes and hydrochemical study applied to surface water and groundwater interaction in the Awash River basin[END_REF][START_REF] Billi | Landscapes and Landforms of Ethiopia[END_REF][START_REF] Gebre | Flood Hazard Assessment and Mapping of Flood Inundation Area of the Awash River Basin in Ethiopia using GIS and HEC-GeoRAS/HEC-RAS Model[END_REF][START_REF] Moeyersons | Mass movement mapping for geomorphological understanding and sustainable development: Tigray, Ethiopia[END_REF][START_REF] Müller | Towards an optimal integrated reservoir system management for the Awash River Basin, Ethiopia[END_REF]. Some researchers have focused on more ancient periods, for example on the formation contexts of the Plio-Pleistocene formations of the Omo valley in Ethiopia [START_REF] Butzer | The lower Omo Basin: Geology, fauna and hominids of Plio-Pleistocene formations[END_REF].

More pertinent to the periods investigated in this manuscript is recent research conducted on the Pleistocene-Holocene hydro-sedimentary evolution of the Ziway-Shala basin and the Bulbula River that are based on prior studies by several researchers [START_REF] Gasse | Late Quaternary lake-level fluctuations and environments of the northern Rift Valley and Afar region (Ethiopia and Djibouti)[END_REF][START_REF] Gillespie | Postglacial arid episodes in Ethiopia have implications for climate prediction[END_REF]. More recent work in this basin evidences the formation of mega and macro Ziway-Shala paleo-lakes between ∼100 and ∼5 ka BP and the intercalated phases of lake lowstands and valley incision [START_REF] Benvenuti | The Ziway-Shala lake basin (main Ethiopian rift, Ethiopia): A revision of basin evolution with special reference to the Late Quaternary[END_REF][START_REF] Bon | Archéologie préhistorique de la partie centrale du Main Ethiopian Rift (bassin lacustre de Ziway-Shala) : contribution à l'établissement de la séquence Late Stone Age d'Afrique orientale[END_REF][START_REF] Carnicelli | Dynamics and driving factors of late Holocene gullying in the Main Ethiopian Rift (MER)[END_REF][START_REF] Sagri | Latest Pleistocene and Holocene river network evolution in the Ethiopian Lakes Region[END_REF]. Supported by significant stratigraphic work and numerous radiocarbon dates, this geomorphological work gives a complete view on the processes (geomorphologic and tectonic) which govern the evolution of this basin. Finally, more complete information on ancient hydrological regimes of the large East-African rivers can only be found in literature studies on lacustrine sequences or on continental deltaic margins [START_REF] Ducassou | Nile floods recorded in deep Mediterranean sediments[END_REF][START_REF] Revel | 20,000 years of Nile River dynamics and environmental changes in the Nile catchment area as inferred from Nile upper continental slope sediments[END_REF][START_REF] Schefuß | Hydrologic control of carbon cycling and aged carbon discharge in the Congo River basin[END_REF][START_REF] Skonieczny | African humid periods triggered the reactivation of a large river system in Western Sahara[END_REF][START_REF] Wagner | Holocene rainfall runoff in the central Ethiopian highlands and evolution of the River Nile drainage system as revealed from a sediment record from Lake Dendi[END_REF].

II.2.1.b Lacustrine systems

The EARS intra-rift faulting mainly occurred between 9 and 2 Ma [START_REF] Macgregor | History of the development of the East African Rift System: A series of interpreted maps through time[END_REF], see Section I.2.1), resulting in the formation of tectonic structural depressions and is the main process responsible for the filling of several lakes. It is for this reason that the paleoclimatic significance of the East-African lakes has interested naturalists since the beginning of the last century. Across the Ethiopian Highlands and in the EARS, the first geomorphological perspectives on climate-driven landforms were proposed by [START_REF] Nilsson | Ancient Changes of Climate in British East Africa and Abyssinia. A Study of Ancient Lakes and Glaciers[END_REF], with several observations on ancient lake shorelines, fluvial morphologies, erosional dynamics and on afro-alpine glacier expansion-retreat dynamics. Specific rift-controlled morphology and the high contrast between high rainfall rates in high altitude source zones and high evaporation rates in lowlands, produce the specific morphological characteristics of amplifier lakes [START_REF] Maslin | East African climate pulses and early human evolution[END_REF][START_REF] Trauth | Human evolution in a variable environment: The amplifier lakes of Eastern Africa[END_REF]. Amplifier lake settings have the particularity of responding rapidly to regional hydrological change. The ancient large waterbody surface area of these great lakes has the potential to leave an imprint of major hydrological changes on landscape morphologies [START_REF] Schuster | Holocene Lake Mega-Chad palaeoshorelines from space[END_REF][START_REF] Schuster | Lacustrine wavedominated clastic shorelines: Modern to ancient littoral landforms and deposits from the Lake Turkana Basin (East African Rift System, Kenya)[END_REF], highly influencing sedimentary processes and representing a powerful tool for the comprehension of climate-induced landform dynamics and feedbacks. Geomorphic features linked to East-African ancient lacustrine shorelines are often littoral notches, wave abrasion platforms, beach ridges composed of rounded cobbles or pebbles, Gilbert-type deltas and overflow-sill morphologies. These features have been largely studied and interpreted at Lake Turkana and at palaeo-Lake Suguta basins [START_REF] Garcin | East African mid-Holocene wet-dry transition recorded in palaeo-shorelines of Lake Turkana, northern Kenya Rift[END_REF][START_REF] Garcin | Late Pleistocene-Holocene rise and collapse of Lake Suguta, northern Kenya Rift[END_REF][START_REF] Schuster | Lacustrine wavedominated clastic shorelines: Modern to ancient littoral landforms and deposits from the Lake Turkana Basin (East African Rift System, Kenya)[END_REF] in Kenya. The results have allowed precise reconstruction of the ancient hydrological fluctuations of these lakes giving important information on the distribution patterns of climate change along the EARS.

Concerning the Late Pleistocene to Holocene periods, several studies are focused on the imprint of the last orbitally-driven humid-arid hydrological cycle on lacustrine morphologies in East Africa thanks to geomorphological field survey and supported by radiocarbon dates on lake paleoshorelines. This kind of research has been conducted in the lacustrine basins from the equatorial to the northern part of the EARS (Fig. II.7), for example at Lake Manyara [START_REF] Bachofer | The Delineation of Paleo-Shorelines in the Lake Manyara Basin Using TerraSAR-X Data[END_REF]Casanova and Hillaire-Marcel, 1992a), Magadi-Natron (Hillaire-Marcel and Casanova, 1987), Nakuru-Elementeita, Naivasha [START_REF] Bergner | Paleoprecipitation estimates for the Lake Naivasha basin (Kenya) during the last 175 k.y. using a lake-balance model[END_REF][START_REF] Hastenrath | Paieoclimatic Estimates from Water and Energy Budgets of East African Lakes[END_REF][START_REF] Richardson | Paleolimnology of mid-elevation lakes in the Kenya Rift Valley[END_REF], Baringo-Bogoria [START_REF] Tiercelin | Le Demi-Graben de Baringo-Bogoria, Rift Gregory, Kenya, 30,000 Ans d'Histoire Hydrologique et Sédimentaire[END_REF], Suguta [START_REF] Garcin | Late Pleistocene-Holocene rise and collapse of Lake Suguta, northern Kenya Rift[END_REF][START_REF] Junginger | Hydrological constraints of paleo-Lake Suguta in the Northern Kenya Rift during the African Humid Period (15-5kaBP)[END_REF], Tanganyika [START_REF] Cohen | Lake level and paleoenvironmental history of Lake Tanganyika, Africa, as inferred from late Holocene and modern stromatolites[END_REF][START_REF] Gasse | Water-lake fluctuations of lake Tanganiyka in phase with oceanic changes during the last glaciation and deglaciation[END_REF], Turkana [START_REF] Bloszies | Water level history for Lake Turkana, Kenya in the past 15,000years and a variable transition from the African Humid Period to Holocene aridity[END_REF][START_REF] Forman | Variations in water level for Lake Turkana in the past 8500 years near Mt. Porr, Kenya and the transition from the African Humid Period to Holocene aridity[END_REF][START_REF] Garcin | East African mid-Holocene wet-dry transition recorded in palaeo-shorelines of Lake Turkana, northern Kenya Rift[END_REF][START_REF] Hastenrath | Paieoclimatic Estimates from Water and Energy Budgets of East African Lakes[END_REF][START_REF] Schuster | Lacustrine wavedominated clastic shorelines: Modern to ancient littoral landforms and deposits from the Lake Turkana Basin (East African Rift System, Kenya)[END_REF], Awassa and ChewBahir (Foerster et al., 2012;[START_REF] Grove | Former Lake Levels and Climatic Change in the Rift Valley of Southern Ethiopia[END_REF][START_REF] Grove | Late Quaternary Lake Levels in the Rift Valley of Southern Ethiopia and Elsewhere in Tropical Africa[END_REF]; Ziway-Shala [START_REF] Benvenuti | The Ziway-Shala lake basin (main Ethiopian rift, Ethiopia): A revision of basin evolution with special reference to the Late Quaternary[END_REF][START_REF] Carnicelli | Dynamics and driving factors of late Holocene gullying in the Main Ethiopian Rift (MER)[END_REF][START_REF] Gasse | Late Quaternary lake-level fluctuations and environments of the northern Rift Valley and Afar region (Ethiopia and Djibouti)[END_REF][START_REF] Gillespie | Postglacial arid episodes in Ethiopia have implications for climate prediction[END_REF][START_REF] Grove | Late Quaternary Lake Levels in the Rift Valley of Southern Ethiopia and Elsewhere in Tropical Africa[END_REF][START_REF] Nilsson | Ancient Changes of Climate in British East Africa and Abyssinia. A Study of Ancient Lakes and Glaciers[END_REF] and Abhe [START_REF] Gasse | L'évolution des lacs de l'Afar Central (Ethiopie et T.F.A.I.) du Plio-Pléistocène à l'Actuel. Recostitution des paléomilieux lacustres à partir de l'étude des Diatomées[END_REF][START_REF] Gasse | Evolution of Lake Abhe[END_REF][START_REF] Gasse | Variations hydrologiques et extension des lacs holocenes du Desert Danakil[END_REF][START_REF] Gasse | Tectonic and Climatic Controls on Lake Distribution and Environments in Afar from Miocene to Present[END_REF]. In particular, the pioneering work of F. Gasse on the Lake Abhe basin, led to the definition of ancient lacustrine levels on the basis of the altitude and ages of littoral lacustrine carbonates (nearshore deposits and carbonate crusts) over the last ∼70 ka BP [START_REF] Gasse | Evolution of Lake Abhe[END_REF]. Outside the Rift, fewer studies are focused on this topic, for example at Lake Hayk [START_REF] Ghinassi | Shoreline fluctuations of Lake Hayk (northern Ethiopia) during the last 3500years: Geomorphological, sedimentary, and isotope records[END_REF] and at Lake Tana [START_REF] Nilsson | Ancient Changes of Climate in British East Africa and Abyssinia. A Study of Ancient Lakes and Glaciers[END_REF]. Normally, stromatolites are the most used datable markers of lacustrine paleo-shorelines in Eastern Africa [START_REF] Casanova | Stromatolites et hauts niveaux lacustres pléistocènes du bassin Natron-Magadi (Tanzanie-Kenya). Pleistocene stromatolites and high lake levels of the Natron-Magadi basin (Tanzania-Kenya)[END_REF][START_REF] Cohen | Lake level and paleoenvironmental history of Lake Tanganyika, Africa, as inferred from late Holocene and modern stromatolites[END_REF][START_REF] Garcin | East African mid-Holocene wet-dry transition recorded in palaeo-shorelines of Lake Turkana, northern Kenya Rift[END_REF][START_REF] Gasse | Evolution of Lake Abhe[END_REF][START_REF] Gasse | Late Quaternary lake-level fluctuations and environments of the northern Rift Valley and Afar region (Ethiopia and Djibouti)[END_REF][START_REF] Gillespie | Postglacial arid episodes in Ethiopia have implications for climate prediction[END_REF]Hillaire-Marcel et al., 1986). Stromatolite structures are the result of the bioaccumulation and precipitation of micritic/sparitic beds from cyanobacteria microbial activity [START_REF] Casanova | Biosedimentology of Quaternary stromatolites in intertropical Africa[END_REF]. Their structure, mineralogy and isotopic composition provide precise information about their littoral depositional environment, chemical paleo-conditions of the waters, and paleo-hydrological variations [START_REF] Abell | Plio-pleistocene lacustrine stromatolites from lake Turkana, Kenya: Morphology, stratigraphy and stable isotopes[END_REF][START_REF] Casanova | Late holocene hydrological history of Lake Tanganyika, East Africa, from isotopic data on fossil stromatolites[END_REF][START_REF] Ghinassi | Shoreline fluctuations of Lake Hayk (northern Ethiopia) during the last 3500years: Geomorphological, sedimentary, and isotope records[END_REF]. However, only few studies have focused on the specific characteristics of these formation, which could be variable between the different lake basins. The most complete work on stromatolites in the EARS is represented by the Ph.D thesis of [START_REF] Casanova | Les Stromatolites Continentaux: Paleoecologie, Paleohydrologie, Paleoclimatologie[END_REF]. In this work he uses multi-disciplinary perspectives to analyse the stromatolite formations of the Natron-Magadi Lakes (Fig. In general, the totality of these studies on the EARS lake systems demonstrate how on the basis of the documentation and dating of ancient paleoshoreline geomorphic features, it is possible to provide a solid interpretation of the hydrological variations that occurred in the EARS basins during the Late Pleistocene and the Early Holocene. Summarily, these results show an onset of humid conditions around ∼14 ka BP until ∼4 ka BP with interruptions (lake level droughts) at ∼12.4, ∼8.2, ∼6.6 and ∼4 ka BP (Fig. II.8;[START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF][START_REF] Holmes | The Late Pleistocene-Holocene African Humid Period as Evident in Lakes[END_REF].

Figure II.8:

Comparison of water levels in East African lakes over the past 16,000 years: Lake Bosumtwi [START_REF] Shanahan | The time-transgressive termination of the African Humid Period[END_REF], Lake Abhe [START_REF] Gasse | Evolution of Lake Abhe[END_REF], Lakes Ziway-Shala [START_REF] Gillespie | Postglacial arid episodes in Ethiopia have implications for climate prediction[END_REF], Lake Turkana (Garcin et al., 2012, filled curve), Paleo-Lake Suguta [START_REF] Junginger | The effects of solar irradiation changes on the migration of the Congo Air Boundary and water levels of paleo-Lake Suguta, Northern Kenya Rift, during the African Humid Period (15-5ka BP)[END_REF], Lake Nakuru [START_REF] Richardson | Paleolimnology of mid-elevation lakes in the Kenya Rift Valley[END_REF], with the potassium record interpreted as an aridity index from the Chew Bahir basin [START_REF] Foerster | Climatic change recorded in the sediments of the Chew Bahir basin, southern Ethiopia, during the last 45,000 years[END_REF]. Dotted white lines indicate water level estimations from discontinuous records. Light grey area indicates the AHP, red dashed bands indicate evident arid short-term arid events from lake level data (YD at ∼12.4 ka BP, 8.2, 6.6 and 4.2 ka BP events). Modified from [START_REF] Junginger | The effects of solar irradiation changes on the migration of the Congo Air Boundary and water levels of paleo-Lake Suguta, Northern Kenya Rift, during the African Humid Period (15-5ka BP)[END_REF].

However, these types of studies rarely provide a high resolution recording of hydrological changes because of the temporal formation of plaeoshoreline geomorphic features. A study which combines geomorphological data with multiproxy analyses on lacustrine core sediments allows us to refine the interpretation of lake fluctuation models. Another way to refine and consolidate interpretations on climatedriven ancient lake oscillations is the integration of geomorphological data in numerical modelling. Only few recent studies propose a modelled evaporation-rainfall balance of Holocene East African lakes combining actual climatic parameters and lake level reconstitutions [START_REF] Fischer | Determining the Pace and Magnitude of Lake Level Changes in Southern Ethiopia Over the Last 20,000 Years Using Lake Balance Modeling and SEBAL[END_REF][START_REF] Junginger | The effects of solar irradiation changes on the migration of the Congo Air Boundary and water levels of paleo-Lake Suguta, Northern Kenya Rift, during the African Humid Period (15-5ka BP)[END_REF][START_REF] Junginger | Hydrological constraints of paleo-Lake Suguta in the Northern Kenya Rift during the African Humid Period (15-5kaBP)[END_REF]. Their results allow us to more properly discuss the changes in hydrologic lacustrine variations driven by monsoonal activity. However, until today, the best continental data for discussions of past hydro-climatic changes in East Africa remain the combined paleolimnological, palynological, and geochemical approaches (major elements and isotopic, core micro-XRF analysis, [START_REF] Croudace | Micro-XRF Studies of Sediment Cores, volume 17 of Developments in Paleoenvironmental Research[END_REF], of lacustrine sedimentary sequences because of their temporal high-resolution and the continuity of the records. These combined or separate methods have been widely applied in several lakes: Lake Tana [START_REF] Costa | Isotopic reconstruction of the African Humid Period and Congo Air Boundary migration at Lake Tana, Ethiopia[END_REF][START_REF] Marshall | Late Pleistocene and Holocene drought events at Lake Tana, the source of the Blue Nile[END_REF], Lake Abiyata [START_REF] Chalié | Late Glacial Holocene diatom record of water chemistry and lake level change from the tropical East African Rift Lake Abiyata (Ethiopia)[END_REF], Lake Ashenge [START_REF] Marshall | Climatic change in northern Ethiopia during the past 17,000 years: A diatom and stable isotope record from Lake Ashenge[END_REF], Lake Awassa [START_REF] Lamb | Climatic and non-climatic ef-fects on the δ18O and δ13C compositions of Lake Awassa, Ethiopia, during the last 6.5ka[END_REF], Lake Challa [START_REF] Tierney | Late Quaternary behavior of the East African monsoon and the importance of the Congo Air Boundary[END_REF][START_REF] Verschuren | Half-precessional dynamics of monsoon rainfall near the East African Equator[END_REF], Lake Chew Bahir [START_REF] Foerster | 46 000 years of alternating wet and dry phases on decadal to orbital timescales in the cradle of modern humans: The Chew Bahir project, southern Ethiopia[END_REF][START_REF] Foerster | Climatic change recorded in the sediments of the Chew Bahir basin, southern Ethiopia, during the last 45,000 years[END_REF], Lake Dendi [START_REF] Wagner | Holocene rainfall runoff in the central Ethiopian highlands and evolution of the River Nile drainage system as revealed from a sediment record from Lake Dendi[END_REF][START_REF] Jaeschke | Holocene Hydroclimate Variability and Vegetation Response in the Ethiopian Highlands (Lake Dendi)[END_REF], Lake Garba Gouracha [START_REF] Bittner | Revisiting afro-alpine Lake Garba Guracha in the Bale Mountains of Ethiopia: Rationale, chronology, geochemistry, and paleoenvironmental implications[END_REF][START_REF] Tiercelin | High-resolution sedimentary record of the last deglaciation from a highaltitude lake in Ethiopia[END_REF][START_REF] Umer | Late Pleistocene and Holocene vegetation history of the Bale Mountains, Ethiopia[END_REF], Lake Hayk [START_REF] Lamb | Oxygen and carbon isotope composition of authigenic carbonate from an Ethiopian lake: A climate record of the last 2000 years[END_REF][START_REF] Loakes | Late Quaternary climate change in the north-eastern highlands of Ethiopia: A high resolution 15,600 year diatom and pigment record from Lake Hayk[END_REF], Lake Tanganyika [START_REF] Burnett | Tropical East African climate change and its relation to global climate: A record from Lake Tanganyika, Tropical East Africa, over the past 90+kyr. Palaeogeography[END_REF][START_REF] Scholz | Paleolimnology of Lake Tanganyika, East Africa, over the past 100 kyr[END_REF], Lake Tilo [START_REF] Lamb | Multi-proxy records of Holocene climate and vegetation change from Ethiopian crater lakes[END_REF], Lake Abhe [START_REF] Gasse | Evolution of Lake Abhe[END_REF], Lake Turkana (van der [START_REF] Van Der Lubbe | Gradual or abrupt? Changes in water source of Lake Turkana (Kenya) during the African Humid Period inferred from Sr isotope ratios[END_REF], Lake Victoria (Berke et al., 2012a), Lakes Nivasha-Nakuru [START_REF] Richardson | Paleolimnology of mid-elevation lakes in the Kenya Rift Valley[END_REF], and Lake Edward [START_REF] Ivory | Lowland forest collapse and early human impacts at the end of the African Humid Period at Lake Edward, equatorial East Africa[END_REF].

In this domain, fluvio-lacustrine geomorphological studies may represent a powerful tool for highresolution paleoclimatic research, which allow identification of sedimentary anomalies linked to sedimentary processes of the Earth's surface (due to residence, connectivity and transient sedimentary dynamics, [START_REF] Hoffmann | Sediment residence time and connectivity in non-equilibrium and transient geomorphic systems[END_REF] or to tectonic activity (see specific discussion in Section II.2.2.a).

II.2.1.c Dune and soil systems

Soil systems can represent a powerful complementary tool for paleoclimatic and paleoenvironmental data, especially in arid or hyper-arid regions [START_REF] Sheldon | Quantitative paleoenvironmental and paleoclimatic reconstruction using paleosols[END_REF]. Unfortunately, in East Africa they have been rarely utilised to define local-to-regional past hydrological variations. Few studies in the Sahelian and Saharan belts interpret pedological development as a marker of enhanced monsoonal activity [START_REF] Cremaschi | Holocene Climatic Changes and Cultural Dynamics in the Libyan Sahara[END_REF][START_REF] Cremaschi | The palaeoclimatic significance of paleosols in Southern Fezzan žLibyan Sahara/: Morphological and micromorphological aspects[END_REF][START_REF] Felix-Henningsen | Paleosols on Pleistocene dunes as indicators of paleo-monsoon events in the Sahara of East Niger[END_REF][START_REF] Mauz | Palaeosols in Saharan and Sahelian dunes of Chad: Archives of Holocene North African climate changes[END_REF][START_REF] Dal Sasso | Radiocarbon dating reveals the timing of formation and development of pedogenic calcium carbonate concretions in Central Sudan during the Holocene[END_REF][START_REF] Williams | Climate Change in Deserts: Past, Present and Future[END_REF][START_REF] Zerboni | Micromorphological approach to polycyclic pedogenesis on the Messak Settafet plateau (central Sahara): Formative processes and palaeoenvironmental significance[END_REF]. These studies evidence occurrence of humid events between 9 and 4 ka BP . Despite the fact that the East African Highlands host a very large variety of soil types, research has been focused on their paleoclimatic signification. Especially in Ethiopia, 60 soil types were classified covering the majority of regions [START_REF] Berhanu | GIS-based hydrological zones and soil geo-database of Ethiopia[END_REF]. Most of these studies are principally employed for current soil use and agricultural scopes. However, geoarchaeological perspectives were used to study ancient soils around the city of Aksum with the aim to understand the agricultural societies of the pre-Axumite and Axumite kingdom starting around 2400 yr BP [START_REF] Butzer | Rise and Fall of Axum, Ethiopia: A Geo-Archaeological Interpretation[END_REF][START_REF] Terwilliger | Environmental changes and the rise and fall of civilizations in the northern Horn of Africa: An approach combining δD analyses of land-plant derived fatty acids with multiple proxies in soil[END_REF]. Another recent study has focused on climatic and anthropic controls on soil formation since ∼8 ka BP in the Tigray region [START_REF] Blond | Terrasses alluviales et terrasses agricoles. Première approche des comblements sédimentaires et de leurs aménagements agricoles depuis 5000 av. n. è. à Wakarida ([END_REF]. The few research works illustrated in this section are an example of the utility of soil studies for a better understanding of the environmental conditions produced by past climatic oscillations.

II.2.1.d Glacial and periglacial systems

During the Last Glacial Maximum (LGM) only four mountain areas in Africa were ice-covered: the Drakensberg of South Africa, the Atlas Mountains of northern Morocco and Algeria, and the Ethiopian Highlands. Nowadays, glaciers occupy mountain areas with an elevation > 5000 m a.s.l., namely Rwenzori, Mount Kenya and Kilimanjaro. From Mount Kilimanjaro, six roughly dated cores were retrieved from the glacier lobes, giving a paleo-climatic reconstitution over the last ∼11.7 ka [START_REF] Thompson | Kilimanjaro Ice Core Records: Evidence of Holocene Climate Change in Tropical Africa[END_REF].

In East Africa great paleo-glaciers were present in the Simien Mountains (4 350 m a.s.l.; northern Ethiopia; (Hendrickx et al., 2015a,b;[START_REF] Hurni | Late Quaternary of Simen and other mountains in Ethiopia[END_REF][START_REF] Hurni | Paleoglaciated Landscapes in Simen and Other High-Mountain Areas of Ethiopia[END_REF][START_REF] Williams | Fossil Periglacial Deposits in the Semien Highlands, Ethiopia[END_REF] and in the Bale mountains (Fig. II.9; 4 100 m a.s.l.; [START_REF] Osmaston | Quaternary glaciation of the Bale Mountains, Ethiopia[END_REF]. Ancient glaciers are responsible for the imprint of some geomorphic and morpho-sedimentary markers of past climate change, which evidence the past advancement and retreat of glacier lobes, ice-gravity periglacial related processes, as well as the formation of intramorenic peat basins. However, until today few studies have been focalised on climate reconstitutions from mountains landforms and sedimentary archives (Hendrickx et al., 2015b,a). Indeed, proglacial and intramorenic peat bog or sedimentary accumulations may provide a high resolution climatic record in association with glacier retreat.

The last evidence of glaciers in the Horn of Africa was observed in the Bale mountains from a moraine dated with a Surface Exposure Dating technique ( 37 Cl/ 35 Cl measures) to around 15.3 ± 1.2 ka BP [START_REF] Osmaston | Quaternary glaciation of the Bale Mountains, Ethiopia[END_REF][START_REF] Ossendorf | Middle Stone Age foragers resided in high elevations of the glaciated Bale Mountains, Ethiopia[END_REF]. Additionally, palynological studies on peat bog sequences indicated that the glacial-tardiglacial transition ranges between 13 and 10 ka BP [START_REF] Mohammed | A late Glacialrlate Holocene pollen record from a highland peat at Tamsaa, Bale Mountains, south Ethiopia[END_REF][START_REF] Umer | Late Pleistocene and Holocene vegetation history of the Bale Mountains, Ethiopia[END_REF].

Interest in ancient afro-alpine glaciers isn't just about paleoclimatic landform interpretations which could be used to better define the magnitude of humid-arid periods and their chronological boundaries, but may represent a solid dataset for wide paleoclimate reconstitutions, and for the understanding of some source-to-sink interconnection processes during prominent hydro-climatic shifts. A glacier mass modelling could contribute to an estimation of the impact of ice melting during warm periods on the hydrological balance of East African lowland lakes during Late Pleistocene-Holocene environmental changes.

Figure II.9: Ethiopian mountains above 3 000 m, with the location of Pleistocene glacial and periglacial features (Hendrickx et al., 2015a)

II.2.2 Open questions

II.2.2.a The role of geomorphological studies in East African paleoclimate research

In Section II.2.1 we summarised the significance of geomorphological studies for palaeo-climate research in East Africa. Many studies are focused on present or sub-actual processes, while more limited research is concentrated on the Pleistocene to Holocene period, often without paleoclimatic interpretation. However, the study of Earth surface processes, may represent a powerful tool for the understanding of climatic and palaeoenvironmental reconstructions for several reasons.

First, geomorphic features can represent a robust marker of regional to global hydro-climate change. They have the potential to decipher detailed spatial and chronological data, in particular where highresolution studies were not possible because of the lack of appropriate continuous archive sequences. As evidenced in Section II.1.4, the multiplication of regionally specific case studies is needed to reconstruct the spatio-temporal patterns of climatic variations and to enrich the data used for climate modelling. A new frontier of paleoclimatic research is thus represented by the detailed comprehension of hydrological changes in specific basins and regions.

The second aspect concerns the investigation of Earth surface processes controls on sedimentary accumulation or recording. Paleoclimatology problematics are often originated from the disconnected climatic interpretation of sedimentary anomalies probably linked to the nonlinearity of local-to-regional geomorphic processes [START_REF] Phillips | Sources of nonlinearity and complexity in geomorphic systems[END_REF]. In Section II.1.1, how changes in solar irradiation can drive cyclic variation in precipitations, in erosion patterns and in sediment supply is explained, and we generally assume that these processes are directly recorded in sedimentary basins or in the landscape without any filtering or erosional activity. However, tectonic activity or residence, connectivity and transient geomorphic processes [START_REF] Hoffmann | Sediment residence time and connectivity in non-equilibrium and transient geomorphic systems[END_REF] can heavily distort the paleoclimatic signal even in a continuous and sedimentary sequence. For example, the combined geomorphological study of the evolution of both the White and Blue Nile showed how the confluence of the two rivers during humid seasons can induce a non-linear response of the hydro-sedimentary system to monsoonal activity. Specifically, the river with the greatest discharge (Blue Nile) operated a temporary dam barrier to the river of lesser discharge (White Nile), inducing the formation of a seasonal lake at the mouth of the second one [START_REF] Williams | The Nile Basin: Quaternary Geology, Geomorphology and Prehistoric Environments[END_REF]. This process probably influenced the sediment recording at the mouth of the fluvial system.

The understanding of how sedimentary systems respond to long-term or abrupt external forcing is then crucial for palaeoclimatic interpretation of data results from continuous sedimentary sequences. If we cannot interpret the reaction and the retroaction of a geomorphic system to the wider hydrological, temperature and tectonic forcing, the respective paleoenvironmental or paleoclimatic information risks being misunderstood.

Finally, the geomorphological approach, coupled with high resolution paleoclimatic studies, could represent the missing link for a more holistic understanding of climatic processes/controls and to assure or discredit their reliability.

II.3 Advances in Late Quaternary Prehistory of East Africa

II.3.1 Introduction to East African Prehistory

Since the 1970s, the celebrity of the East African Rift region in research on human prehistory has been mainly due to the discovery of Hominin specimens (namely Australopithecus afarensis, 'Lucy'; [START_REF] Johanson | A new species of the genus Australopithecus (Primates: Hominidae) from the Pliocene of eastern Africa[END_REF][START_REF] Kimbel | Was Australopithecus anamensis ancestral to A. afarensis? A case of anagenesis in the hominin fossil record[END_REF] contemporary with Plio-Pleistocene sedimentary formations spanning ∼3.7 to ∼1.5 Ma [START_REF] Johanson | Pliocene hominids from the Hadar formation, Ethiopia (1973-1977): Stratigraphic, chronologic, and paleoenvironmental contexts, with notes on hominid morphology and systematics[END_REF]Kimbel and Delezene, 2009, see Section I.2.3;Fig. I.10). The exceptional preservation of these sequences is due to the particular morpho-tectonic patterns of the East African Rift System which made this area a privileged spot for research on early humankind. Several projects have focused on the investigation of these thick sedimentary formations, from the Middle Awash valley, to the Kenyan Rift and the Omo valley [START_REF] Asfaw | Remains of Homo erectus from Bouri, Middle Awash, Ethiopia[END_REF][START_REF] Boisserie | A new species of Hippopotamidae (Mammalia, Artiodactyla) from the Sagantole Formation, Middle Awash, Ethiopia[END_REF][START_REF] Sahle | Human emergence: Perspectives from Herto, Afar rift, Ethiopia[END_REF]. Even today this region continues to provide new results on some of the earliest evidence of hominins, with an increase in paleontological research on the early history of the genus Homo [START_REF] Sano | A 1.4million-year-old bone handaxe from Konso, Ethiopia, shows advanced tool technology in the early Acheulean[END_REF][START_REF] Souron | A new species of the suid genus Kolpochoerus from Ethiopia[END_REF]. Studies are increasingly focused on the 'Out of Africa' theories concerning the spread of Homo sapiens from the EARS to the rest of the world between 200 and 70 ka BP [START_REF] Beyin | Upper Pleistocene Human Dispersals out of Africa: A Review of the Current State of the Debate[END_REF][START_REF] Grove | Climatic variability, plasticity, and dispersal: A case study from Lake Tana, Ethiopia[END_REF][START_REF] Richter | Contextual areas" of early Homo sapiens and their significance for human dispersal from Africa into Eurasia between 200 ka and 70 ka[END_REF]. However, interest in more recent human prehistory has been less investigated, in particular for the period spanning the last ∼40 ka BP which include the human occupations linked to the Later Stone Age (LSA) [START_REF] Bon | Archéologie préhistorique de la partie centrale du Main Ethiopian Rift (bassin lacustre de Ziway-Shala) : contribution à l'établissement de la séquence Late Stone Age d'Afrique orientale[END_REF][START_REF] Brandt | The Upper Pleistocene and early Holocene prehistory of the Horn of Africa[END_REF][START_REF] Kusimba | Hunter-gatherer land use patterns in Later Stone Age East Africa[END_REF] and the subsequent emergence of the first herding and food production societies [START_REF] Ambrose | Chronology of the Later Stone Age and food production in East Africa[END_REF][START_REF] Gutherz | New insights on the first Neolithic societies in the Horn of Africa: The site of Wakrita, Djibouti[END_REF][START_REF] Marshall | Cattle Before Crops: The Beginnings of Food Production in Africa[END_REF]. In this section, the main cultural and technological aspects of these societies, as well as their chronology and spatial distribution in the Horn of Africa are presented with the aim to provide a framework and to describe some of the issues approached in this manuscript.

II.3.2 The Later Stone Age (LSA)

The term Late(r) Stone Age (LSA) was introduced in the 1920s in South Africa by [START_REF] Goodwin | The Stone Age Cultures of South Africa[END_REF] with the aim to separate three major regional lithic techno-complexes: Earlier Stone Age, Middle Stone Age and Later Stone Age. Since the 1950s this separation has been employed over the rest of the African continent [START_REF] Clark | The Prehistoric Cultures of the Horn of Africa[END_REF] with the aim to mark the discontinuity with the European prehistoric tradition [START_REF] Schlanger | Making the past for South Africa's future: The prehistory of Field-Marshal Smuts (1920s-1940s)[END_REF]. Currently, the term LSA is used to define the lithic technologies that were present from around ∼40 ka BP until the onset of herding [START_REF] Ambrose | Chronology of the Later Stone Age and food production in East Africa[END_REF][START_REF] Mitchell | Stone Age Sub-Saharan Africa[END_REF], corresponding to the last hunter-gatherer societies of Africa. The LSA is difficult to define because it has high spatio-temporal variability and because of its diffusion over the whole continent: from the West [START_REF] Scerri | The Stone Age Archaeology of West Africa[END_REF], to the East [START_REF] Brandt | The Upper Pleistocene and early Holocene prehistory of the Horn of Africa[END_REF] and to the south [START_REF] Bousman | The emergence, spread, and termination of the Early Later Stone Age event in South Africa and southern Namibia[END_REF]. In this section, a synthesis of LSA characteristics and findings in the Horn of Africa are presented.

The first focused research on the prehistory of the Horn of Africa was conducted by J. D. Clark [START_REF] Clark | The Prehistoric Cultures of the Horn of Africa[END_REF]. Since the 1970s, several studies focused on the Horn of Africa were improved to better characterize and understand LSA patterns in this region [START_REF] Brandt | The Upper Pleistocene and early Holocene prehistory of the Horn of Africa[END_REF][START_REF] Clark | Recent Archaeological Research in Southeastern Ethiopia. 1974 -1975[END_REF][START_REF] Phillipson | The Later Stone Age in sub-Saharan Africa[END_REF]. Very recent studies on LSA stone tool technology suggest that starting around 30 ka BP, the principal characteristics of these populations are the lithic laminar debitage produced with a core-on-flake technique, but also by some markers of microlithism [START_REF] Leplongeon | Microliths in the Middle and Later Stone Age of eastern Africa: New data from Porc-Epic and Goda Buticha cave sites, Ethiopia[END_REF][START_REF] Ménard | Late Stone Age variability in the Main Ethiopian Rift: New data from the Bulbula River, Ziway-Shala basin[END_REF]. Generally, these populations have been defined as riparian forager groups with economic subsistence based on fishing and hunting. It is one of the reasons why in East Africa these populations were previously called 'aqualithic' [START_REF] Sutton | The Aquatic Civilization of Middle Africa[END_REF], characterized by bone harpoon use over a territory that extended from the Sahel to the Kenyan Rift [START_REF] Beyin | New radiocarbon dates for terminal Pleistocene and early Holocene settlements in West Turkana, northern Kenya[END_REF][START_REF] Goldstein | New archaeological investigations at the Lothagam harpoon site at Lake Turkana[END_REF][START_REF] Sereno | Lakeside Cemeteries in the Sahara: 5000 Years of Holocene Population and Environmental Change[END_REF][START_REF] Wright | Lakeside View: Sociocultural Responses to Changing Water Levels of Lake Turkana, Kenya[END_REF]. However, recently, more complex variability and specific characteristics of these LSA societies has been evidenced along the Northern East African Rift.

Despite an increase of research on this topic, a relatively low number of LSA sites have been documented in this region (Fig. II.10). In the Horn of Africa, the main LSA reference areas/sites are: Goda Buticha [START_REF] Leplongeon | Microliths in the Middle and Later Stone Age of eastern Africa: New data from Porc-Epic and Goda Buticha cave sites, Ethiopia[END_REF][START_REF] Pleurdeau | Cultural change or continuity in the late MSA/Early LSA of southeastern Ethiopia? The site of Goda Buticha, Dire Dawa area[END_REF], Porc-Epic [START_REF] Clark | A Middle Stone Age occupation site at Porc Epic Cave, Dire Dawa (east-central Ethiopia)[END_REF][START_REF] Pleurdeau | Human Technical Behavior in the African Middle Stone Age: The Lithic Assemblage of Porc-Epic Cave (Dire Dawa, Ethiopia)[END_REF], Midishi 2 [START_REF] Gresham | An Investigation of an Upper Pleistocene Archaeological Site in Northern Somalia[END_REF], Laas Geel 7 [START_REF] Gutherz | The Hargeisan revisited: Lithic industries from shelter 7 of Laas Geel, Somaliland and the transition between the Middle and Late Stone Age in the Horn of Africa[END_REF], Lake Besaka [START_REF] Brandt | The Upper Pleistocene and early Holocene prehistory of the Horn of Africa[END_REF][START_REF] Brandt | A Late Quaternary Cultural/Environmental Sequence from Lake Besaka, Southern Afar, Ethiopia[END_REF], the Ziway-Shala sites [START_REF] Ménard | Late Stone Age variability in the Main Ethiopian Rift: New data from the Bulbula River, Ziway-Shala basin[END_REF], Mochena Borago [START_REF] Brandt | Early MIS 3 occupation of Mochena Borago Rockshelter, Southwest Ethiopian Highlands: Implications for Late Pleistocene archaeology, paleoenvironments and modern human dispersals[END_REF][START_REF] Fisher | Late Pleistocene Technological Change And Hunter Gatherer Behavior At Moche Borago Rockshelter, Sodo-Wolayta, Ethiopia: Flaked Stone Artifacts From The Early Ois[END_REF] and Lake Turkana and surrounding sites: Koobi Fora, [START_REF] Barthelme | Fisher-Hunters and Neolithic Pastoralists in East Turkana, Kenya[END_REF], Lothagam-Napadet, [START_REF] Robbins | Archeology in the Turkana District, Kenya[END_REF][START_REF] Robbins | The Lothagam Site. East Lansing: Anthropological Series[END_REF], Kalodir-Kapua, [START_REF] Beyin | Upper Pleistocene Human Dispersals out of Africa: A Review of the Current State of the Debate[END_REF], Omo River Delta/Kibish, [START_REF] Brown | BARBED BONE POINTS FROM THE LOWER OMO VALLEY, ETHIOPIA[END_REF][START_REF] Butzer | The Holocene Lake Plain of North Rudolph, East Africa[END_REF]; see [START_REF] Wright | Lakeside View: Sociocultural Responses to Changing Water Levels of Lake Turkana, Kenya[END_REF] for a review. As illustrated in Fig. II.10, the context of these settlements is split between rockshelters and open air lake-side sites along the Kenyan and Ethiopian Rift, with very rare findings concerning the Highlands and the Central Afar Region (Lake Abhe basin; [START_REF] Diaz | Les Industries Lithiques Des Sites d'habitat Des Premiers Éleveurs Du Bassin Du Gobaad En République de Djibouti : Apport de La Technologie Lithique à La Caractérisation Des Cultures Des Premières Sociétés de Production[END_REF]. From all acquired datasets, it seems that along the Rift, LSA populations preferred open air settlements located in ancient lake or river littoral environments. If we focus especially on these 'lake-side' sites along the northern EARS, the Ziway-Shala, the Lake Besaka and the Lake Turkana archaeological assemblages are comparable. These populations occupied the Kenyan and the Ethiopian Rift between ∼30 and ∼6 ka BP (Fig. II.11;[START_REF] Ménard | Hiatus et lacune. Occupation du Rift éthiopien à la fin du Pleistocene et au debut de l'Holocene[END_REF]. However, chronological offsets and highly variable techno-cultural characteristics, make the characterization of this culture difficult. Indeed, even if all of these basins were occupied by populations characterized by fishing and hunting economies, divergences between them suggest large techno-cultural variability. For example, some 'Ethiopian Blade Tool Tradition' aspects defined on the basis of the archaeological assemblages from Lake Besaka [START_REF] Brandt | The Upper Pleistocene and early Holocene prehistory of the Horn of Africa[END_REF][START_REF] Brandt | A Late Quaternary Cultural/Environmental Sequence from Lake Besaka, Southern Afar, Ethiopia[END_REF] differ from the features recognised along the neighbouring Ziway-Shala basin sites [START_REF] Ménard | Late Stone Age variability in the Main Ethiopian Rift: New data from the Bulbula River, Ziway-Shala basin[END_REF]. Similarly, along the Kenyan Rift, barbed points or bone harpoons are characteristic of the LSA traditions of this region [START_REF] Wright | Lakeside View: Sociocultural Responses to Changing Water Levels of Lake Turkana, Kenya[END_REF], while such features are completely absent at Ziway-Shala Lakes or at Lake Besaka. Additionally, the development of impressed pottery in the Turkana basin as early as ∼9 ka BP [START_REF] Wright | Lakeside View: Sociocultural Responses to Changing Water Levels of Lake Turkana, Kenya[END_REF] make this region culturally different from the northern Rift, where the pottery appears only after ∼4.5 ka BP with the introduction of herding and food production [START_REF] Gutherz | New insights on the first Neolithic societies in the Horn of Africa: The site of Wakrita, Djibouti[END_REF][START_REF] Lesur | The advent of herding in the Horn of Africa: New data from Ethiopia, Djibouti and Somaliland[END_REF]. Interestingly, the spatial-temporal variability of the technological practices of LSA populations is not only between the different basins which compose the Rift. Indeed, [START_REF] Ménard | Late Stone Age variability in the Main Ethiopian Rift: New data from the Bulbula River, Ziway-Shala basin[END_REF] have demonstrated that some technological discrepancies can be identified between human settlements that are really close in space and in time along the same basin.

In summary, the lithic industries dated to the Late Pleistocene-Early Holocene period along the Rift present some substantial differences. All of these elements indicate that the LSA in the Horn of Africa is characterized by high local variability and that it is not simple to define specific central elements that characterize a single culture. Some authors indicate that these patterns could be the result of economic specialization linked to the variability of local geographic and environmental factors [START_REF] Bon | Archéologie préhistorique de la partie centrale du Main Ethiopian Rift (bassin lacustre de Ziway-Shala) : contribution à l'établissement de la séquence Late Stone Age d'Afrique orientale[END_REF][START_REF] Ménard | Late Stone Age variability in the Main Ethiopian Rift: New data from the Bulbula River, Ziway-Shala basin[END_REF][START_REF] Ménard | Hiatus et lacune. Occupation du Rift éthiopien à la fin du Pleistocene et au debut de l'Holocene[END_REF][START_REF] Pleurdeau | Cultural change or continuity in the late MSA/Early LSA of southeastern Ethiopia? The site of Goda Buticha, Dire Dawa area[END_REF]. [START_REF] Ménard | Late Stone Age variability in the Main Ethiopian Rift: New data from the Bulbula River, Ziway-Shala basin[END_REF] have demonstrated how during the beginning of the AHP, a technological specialisation focused on fishing practices occurred, showing the strong adaptability of humans to local-to-regional hydrological changes. Considering that the period at the transition between the Late Pleistocene and the Early Holocene is characterized by strong hydro-climatic shifts, the link between the distribution of precipitation, paleolandscape reactivity and human adaptation is not negligible. From this point of view, the climatic-induced local variability of the northern EARS environments in concomitance with the highly variable LSA technocultural patterns, could represent a powerful case-study tool to estimate human adaptability in relation to past hydro-climatic forcing. Some studies have approached the question of the link between climatic changes and human occupations around Lake Turkana [START_REF] Foerster | Environmental change and human occupation of southern Ethiopia and northern Kenya during the last 20,000 years[END_REF][START_REF] Wright | Lakeside View: Sociocultural Responses to Changing Water Levels of Lake Turkana, Kenya[END_REF], but without any link between the environmental significance of local site sedimentary formation processes, the regional hydrological changes and the wider sub-continental hydro-climatic variations, making the correlation quite general.

An additional element which complicates the understanding of the spatial-temporal distribution of LSA societies along the northern Rift is the hiatus in the archaeological record. Indeed, an important gap in the archaeological sequences of the Horn of Africa is recoded during Marine Isotopic Stage 2 (MIS2) and the local Last Glacial Maximum between ∼29 and ∼14 ka BP, but also around ∼12 ka BP (Fig. II.11). A regional sedimentation hiatus also occurred in some lacustrine sequences between ∼20 to ∼12 ka BP [START_REF] Beck | Onset of the African Humid Period by 13.9 kyr BP at Kabua Gorge, Turkana Basin, Kenya[END_REF][START_REF] Gasse | Evolution of Lake Abhe[END_REF][START_REF] Gasse | Late Quaternary lake-level fluctuations and environments of the northern Rift Valley and Afar region (Ethiopia and Djibouti)[END_REF][START_REF] Lamb | 150,000-year palaeoclimate record from northern Ethiopia supports early, multiple dispersals of modern humans from Africa[END_REF][START_REF] Wagner | Holocene rainfall runoff in the central Ethiopian highlands and evolution of the River Nile drainage system as revealed from a sediment record from Lake Dendi[END_REF], indicating that these phenomena could be related to a larger environmental/geomorphological issue. In effect, this period is characterized by prominent environmental shifts, oscillating between arid and humid periods in association with long-term and short-term hydro-climatic variations. More precisely, the impact of the MIS2 (∼29 -∼18 ka BP) and the HE1 (∼16 -∼15 ka BP) dry periods is that they would have produced less sedimentary accumulations. In contrast, the beginning of the AHP (∼14 ka BP), characterized by strong enhanced monsoonal precipitations, could have been at the origin of some erosional processes which effaced the sedimentary records. Finally, the impact of the Younger Dryas event (∼13 -∼11.5 ka BP) would have produced environmental and paeolandscape instability which is likely the origin of the lack of clear sediment layers in archaeological sequences.

In presenting previous archaeological work on the LSA in the Horn of Africa, the gaps and issues in the record stand out, namely the high spatial and temporal heterogeneity and the difficulty in defining a single LSA techno-cultural tradition, and the hiatus in the LSA sedimentary sequences in the Horn of Africa. Geomorphological and environmental research concentrated on specific case studies over the norther EARS basins have the potential to better clarify the role of climatic change on LSA population technological adaptability and specialisation to specific local environmental conditions. A geomorphological/paleoclimatic approach may also help to explain the sedimentary gap which affected the archaeological sequences, and which was probably due to the contrasted alternation between arid and humid periods. In this study, I propose a new approach which considers the local landscapes/environments and human settlements in a specific lacustrine basin (Lake Abhe basin), the hydrological oscillations recorded in the same basin and their link to the wider hydro-climatic regional forcing in order to better understand these issues.

II.3.3 The Neolithic 2 and prehistoric mobility

Because of its high geographical variability and because of its complex and arrhythmic apparition over the continent, the definition of the African Neolithic remains controversial today. Neolithic are those societies which developed an economic subsistence based on domesticates and the production and elaboration of food products. For most Neolithic societies, hunting and gathering are not the principal ways of acquiring food. Indeed, these economic systems can include farming and/or herding and are often associated with advanced systems of food processing and storage that are represented by ceramic production among other things.

African societies developed a Neolithic quite different from their European and Near-Eastern neighbours. For example, in the Sahara, the apparition of pottery occurred before the earliest forms of herding [START_REF] Huysecom | The emergence of pottery in Africa during the tenth millennium cal BC: New evidence from Ounjougou (Mali)[END_REF], with the eventual diffusion of food production practices by mobile groups of herders and not by farmers [START_REF] Lesur | The advent of herding in the Horn of Africa: New data from Ethiopia, Djibouti and Somaliland[END_REF], or even with the development of multiple economies including hunting, herding, gathering and fishing as complementary ways of supporting populations [START_REF] Marshall | Cattle Before Crops: The Beginnings of Food Production in Africa[END_REF]. The diffusion of herding in East Africa took several millennia, developing a coexistence with previous hunter-gatherer societies [START_REF] Lesur | The advent of herding in the Horn of Africa: New data from Ethiopia, Djibouti and Somaliland[END_REF]. This section provides a synthesis of the characteristics of these societies and their chronological distribution in the Horn of Africa.

The transition from LSA fisher-hunter-gatherers to herding practices appears to have been quite complex and extremely variable across Africa. Out of East Africa, the earliest evidence of herding is dated to between 9 and 8 ka BP in South Arabia [START_REF] Fedele | Wādī at̲ -T ̲ ayyilah 3, a Neolithic and Pre-Neolithic occupation on the eastern Yemen Plateau, and its archaeofaunal information[END_REF][START_REF] Khalidi | Obsidian sources in highland Yemen and their relevance to archaeological research in the Red Sea region[END_REF][START_REF] Martin | Early Arabian pastoralism at Manayzah in Wādī Ṣanā, Ḥaḍramawt[END_REF] and between 9 and 6 ka BP in the Eastern Sahara and in the Lower Blue and White Nile valleys, with the apparition of the first ceramic traditions (Fig. II.12;[START_REF] Hoelzmann | Environmental change and archaeology: Lake evolution and human occupation in the Eastern Sahara during the Holocene[END_REF][START_REF] Honegger | La Nubie et le Soudan: Un bilan des vingt dernières années de recherche sur la pré et protohistoire[END_REF][START_REF] Linseele | The earliest phase of introduction of Southwest Asian domesticated animals into Africa. New evidence from the Fayum Oasis in Egypt and its implications[END_REF][START_REF] Williams | Late Quaternary environments and prehistoric occupation in the lower White Nile valley, central Sudan[END_REF]. The apparition of impressed ceramics in the Lake Turkana basin around ∼9 ka BP represent the first spot in the EARS were these kinds of traditions occurred without herding [START_REF] Grillo | Nderit Ware" and the origins of pastoralist pottery in eastern Africa[END_REF][START_REF] Wright | Lakeside View: Sociocultural Responses to Changing Water Levels of Lake Turkana, Kenya[END_REF]. Southward and further inland, the emergence of the first Neolithic societies appears with a large delay with respect to adjacent regions. In the EARS, the first herders appeared around ∼4.9 ka BP at Lake Turkana [START_REF] Chritz | Climate, ecology, and the spread of herding in eastern Africa[END_REF][START_REF] Foerster | Environmental change and human occupation of southern Ethiopia and northern Kenya during the last 20,000 years[END_REF][START_REF] Grillo | Nderit Ware" and the origins of pastoralist pottery in eastern Africa[END_REF], recognised at the sites Lothagam-Napadet [START_REF] Robbins | The Lothagam Site. East Lansing: Anthropological Series[END_REF][START_REF] Robbins | Archeology in the Turkana District, Kenya[END_REF] and Dongodien [START_REF] Ashley | Paleoenvironmental Reconstruction of Dongodien, Lake Turkana, Kenya and OSL Dating of Site Occupation During Late Holocene Climate Change[END_REF][START_REF] Marshall | Early domestic stock at Dongodien in Northern Kenya[END_REF]. Some authors have suggested that the Kenyan Rift represented a corridor for the spread of the Neolithic to the southern EARS and to South Africa, passing through the White Nile and Lake Turkana basins [START_REF] Ashley | Paleoenvironmental Reconstruction of Dongodien, Lake Turkana, Kenya and OSL Dating of Site Occupation During Late Holocene Climate Change[END_REF]. Indeed, in the southern Kenyan Rift or around the Lake Victoria regions, herding appeared quite late, around ∼3 ka BP (Fig. II.12;[START_REF] Ambrose | Chronology of the Later Stone Age and food production in East Africa[END_REF][START_REF] Gifford-Gonzalez | Animal Disease Challenges to the Emergence of Pastoralism in Sub-Saharan Africa[END_REF][START_REF] Hildebrand | The Holocene Archaeology of Southwest Ethiopia: New Insights from the Kafa Archaeological Project[END_REF][START_REF] Prendergast | Kansyore fisher-foragers and transitions to food production in East Africa: The view from Wadh Lang'o, Nyanza Province, Western Kenya[END_REF]. Similarly, in the Northern Rift, the Neolithic also occurs late at the sites of Lake Besaka [START_REF] Brandt | The Upper Pleistocene and early Holocene prehistory of the Horn of Africa[END_REF][START_REF] Brandt | A Late Quaternary Cultural/Environmental Sequence from Lake Besaka, Southern Afar, Ethiopia[END_REF][START_REF] Endalew | Archaeological survey around Lake Beseka (Northern Main Ethiopian Rift)[END_REF] and Laga Oda [START_REF] Clark | Possible FunctionaI Interpretations[END_REF][START_REF] Clark | Recent Archaeological Research in Southeastern Ethiopia. 1974 -1975[END_REF]. In the Afar Region, the first evidence of herding was documented around ∼4.7 ka BP, but was consolidated only around 3.4 ka BP at the sites of Asa Koma [START_REF] Coudert | New archaeozoological results from Asa Koma (Djibouti): Contributing to the understanding of faunal exploitation during the 3rd millennium BC in the Horn of Africa[END_REF] and Wakarita [START_REF] Gutherz | New insights on the first Neolithic societies in the Horn of Africa: The site of Wakrita, Djibouti[END_REF]. This late spread of herding practicies in the Horn of Africa is even attested in the Eastern Ethiopian Highlands at the Lake Tana sites of Danei Kawolos [START_REF] Marshall | Early hunters and herders of northern Ethiopia: The fauna from Danei Kawlos and Baati Ataro rockshelters[END_REF] and Gobedra (Phillipson, 1977, Fig. II.12). It has been proposed that the complex topographical and environmental network of mountain and rift valleys of the Ethiopian Highlands played a principal role in the diffusion of the Neolithic combined with difficulties of hunter-gatherer groups to integrate food production into their economic systems [START_REF] Lesur | The advent of herding in the Horn of Africa: New data from Ethiopia, Djibouti and Somaliland[END_REF]. Towards the south documented sites are rare, with only the sites of Laas Geel 7 [START_REF] Gutherz | The Hargeisan revisited: Lithic industries from shelter 7 of Laas Geel, Somaliland and the transition between the Middle and Late Stone Age in the Horn of Africa[END_REF] and Gogoshiis Qabe [START_REF] Brandt | The Upper Pleistocene and early Holocene prehistory of the Horn of Africa[END_REF][START_REF] Jones | Huntergatherer reliance on inselbergs, big game, and dwarf antelope at the Rifle Range Site, Buur Hakaba[END_REF] attesting to the spread of Neolithic practices in Somalia and Somaliland. The last pulse of the spread of herding in the Horn of Africa is attested at Kumali [START_REF] Hildebrand | The Holocene Archaeology of Southwest Ethiopia: New Insights from the Kafa Archaeological Project[END_REF] after ∼3 ka BP.

Despite the rarity of sites in the Horn of Africa, some studies have demonstrated intensive Neolithic mobility and raw material exchange along the Ethiopian Rift region. On the basis of geochemical characterisation of the obsidian used for stone tool production, these studies have determined long-distance obsidian procurement strategies [START_REF] Khalidi | Late prehistoric oasitic niches along the southern Red Sea (Yemen and Horn of Africa). In From Refugia to Oases: Living in Arid Environments from Prehistoric Times to the Present Day[END_REF][START_REF] Oppenheimer | Risk and reward: Explosive eruptions and obsidian lithic resource at Nabro volcano (Eritrea)[END_REF] and it has even been argued that the amount and size of artefacts decreased with distance from the sources [START_REF] Negash | Varieties and Sources of Artefactual Obsidian in the Middle Stone Age of the Middle Awash, Ethiopia*[END_REF][START_REF] Negash | Geochemical sources for the terminal Pleistocene/early Holocene obsidian artifacts of the site of Beseka, central Ethiopia[END_REF]. In addition, geochemical characterisation of obsidian Neolithic tools and debitage from sites in the Tihamah coastal region in Yemen, as well as from the highlands, Rift region, and coastal areas Figure II.12: East African elevation base map (GMTED2010, SRTM), with a schematic representation of the earliest attestation phases of herding in the Horn of Africa between 9-8 ka cal. BP (red), 8-6 ka cal. BP (green), 5-3 ka cal. BP (blue) and 3-1 ka cal. BP (yellow). High and Low accuracy of archaeological site chronologies are indicated with white and black dots respectively. Capital towns are indicated with black stars.

of the Horn of Africa have demonstrated matches to obsidian sources in the Horn of Africa, namely those of the Ethiopian Rift, the Afar and coastal Eritrea [START_REF] Khalidi | Holocene Obsidian Exchange in the Red Sea Region[END_REF][START_REF] Khalidi | Late prehistoric oasitic niches along the southern Red Sea (Yemen and Horn of Africa). In From Refugia to Oases: Living in Arid Environments from Prehistoric Times to the Present Day[END_REF][START_REF] Khalidi | Considering the Arabian Neolithic through a reconstitution of interregional obsidian distribution patterns in the region[END_REF][START_REF] Khalidi | The growth of early social networks: New geochemical results of obsidian from the Ubaid to Chalcolithic Period in Syria, Iraq and the Gulf[END_REF][START_REF] Oppenheimer | Risk and reward: Explosive eruptions and obsidian lithic resource at Nabro volcano (Eritrea)[END_REF]. These results confirm long-distance mobility through extreme mountainous landscapes and across the Red Sea during the Neolithic period. In the northern Rift region (Afar Region), food production societies are characterized by the coupling of herding, the production of impressed pottery [START_REF] Cauliez | Première caractérisation des faciès céramiques néolithiques de la région du Gobaad en République de Djibouti. Les sites d'Hara-Idé2 et d'Asa-Koma (As-Eyla, district de Dikhil)[END_REF][START_REF] Lesur | The advent of herding in the Horn of Africa: New data from Ethiopia, Djibouti and Somaliland[END_REF], the production of microlithic backed tools and the building of monumental collective burials [START_REF] Cauliez | La plateforme circulaire d'Antakari 3 en République de Djibouti (région d'As Eyla, District de Dikhil). Monument funéraire et premières sociétés de production dans la Corne de l'Afrique[END_REF], practices which have also been documented at Lake Turkana (Hildebrand et al., 2018b;[START_REF] Marshall | Cattle Before Crops: The Beginnings of Food Production in Africa[END_REF]. Interestingly, in the Ziway-Shala basin, Neolithic sites have not been attested [START_REF] Ménard | Hiatus et lacune. Occupation du Rift éthiopien à la fin du Pleistocene et au debut de l'Holocene[END_REF].

It has been proposed that the aridification at the end of the AHP (from 6 ka BP) influenced the spread of herding into the Horn of Africa, inducing pastoral groups to move from desiccated plains in the Sahel into the highlands and into the Rift, which was still supplied by monsoonal rainfall until the Late Holocene [START_REF] Lesur | The advent of herding in the Horn of Africa: New data from Ethiopia, Djibouti and Somaliland[END_REF]. In this scenario, the particular topographic patterns of the Rift where monsoonal precipitation supplied large waterbodies until the end of the Middle Holocene (∼5 -∼4 ka BP) would have served as corridors of herding diffusion. It has been argued that paleolake fluctuations played an important role in human trajectories in the northern EARS. This may have been particularly true during the Holocene when the exceptional sensitivity of lacustrine basin systems along the East African Rift produced large water surfaces, known as amplifier lakes (see Section II.2.1). It has been hypothesized that these climate-induced patterns of East African lakes influenced the migration, the isolation and the dispersal and mixing of human groups [START_REF] Maslin | East African climate pulses and early human evolution[END_REF][START_REF] Trauth | Human evolution in a variable environment: The amplifier lakes of Eastern Africa[END_REF]. However, these lakes not only represented a physical barrier for human diffusion, but also supported human subsistence systems. Indeed, herding, microlithism and ceramic traditions are not the only characteristics of food production societies in the Horn of Africa. It has been demonstrated that fishing practices occupied a large place in Neolithic nutrition [START_REF] Coudert | La Place de La Pêche Pendant l'Holocène de La Vallée Du Rift Au Sahel Occidental[END_REF][START_REF] Coudert | New archaeozoological results from Asa Koma (Djibouti): Contributing to the understanding of faunal exploitation during the 3rd millennium BC in the Horn of Africa[END_REF][START_REF] Coudert | The importance of the fishing in northeast Africa: The example of the sites around Lake Abbé (Djibouti, 3-2 millenium BC)[END_REF]. These societies are thus characterized by a mixed and complex economic system of groups of herders with ceramic traditions, and with intensive fishing that persisted since the last LSA occupations. In this scenario, the hydrological oscillations of the Middle Holocene northern EARS lacustrine systems could have represented an important factor in the evolution of food production societies in the Horn of Africa, and provided an environment in which new techno-cultural traditions could be adopted alongside ancient ones.

All these indications suggest that hunter-gathering traditions were replaced by domestication following different rhythms, modalities and in different ways across East Africa, probably influenced by regional to local climatic, environmental, topographical or cultural factors. From this point of view, human-landscape interaction probably played a crucial role in the diffusion, introduction or exchange of different ways of life from the Middle to the Late Holocene. Investigation of the reactivity of specific Rift lacustrine basins to regional hydro-climatic forcing, coupled with the study of human adaptability to highly variable lacustrine environments may represent one way to better understand the modalities of the adoption of food production in the Horn of Africa between the Middle and the Late Holocene. In this manuscript I propose to apply this approach to the Lake Abhe basin, located in the Central Afar region, with the aim to clarify the link between regional hydro-climate oscillations, hydrological/hydrosedimentary reactivity of this specific basin and the evolution of prehistoric inhabited environments in this lacustrine landscape.

II.3.4 Open questions

II.3.4.a Sedimentological lacunae, lack of occupation or climate-induced settlement abandon?

In the Horn of Africa, for both the LSA and Neolithic periods, there are few archaeological sites and they are culturally heterogeneous and chronologically and spatially dispersed, giving insufficient correlations for the building of a robust regional chronology. Millennial scale archaeological lacunae combined with several gaps in regional Late Pleistocene and Holocene continental sedimentary sequences represent an obstacle to a full comprehension of diachronic prehistoric settlement distribution and their relation to environmental change. However, the lack of archaeological data could also be related to peopling dynamics and to the rupture and continuity of Late Pleistocene to Holocene settlement.

In this context, the comprehension of Earth surface processes which govern the formation and the erosion of sedimentary archaeological sequences, as well as their relationship to regional to global hydro-climatic forcing, is of crucial importance for a proper definition of past human occupation phases. This approach will allow us to estimate the integrity of archaeological sequences, and to distinguish between occupation hiatuses due to a) lack (absence) or to the loss (erosive processes) of sedimentation in association with prominent climatic events; or b) independent human choices related to site occupation or abandonment.

II.3.4.b The complexity of the Neolithic emergence in the northern EARS

The delayed chronological apparition of a Neolithic way of life in the Horn of Africa in relation to the Sahara, the Arabian Peninsula and the Middle East has been evidenced by several authors (see Section II.3.3). Here, the Neolithic developed arrhythmically and in different and independent regional or local ways, probably influenced by specific local topographical/environmental patterns. The Saharan Neolithic occurred during a period of strong climatic stress at the end of the AHP, and several authors have suggested that Saharan prehistoric settlements were strongly influenced by Middle Holocene aridification [START_REF] Hoelzmann | Environmental change and archaeology: Lake evolution and human occupation in the Eastern Sahara during the Holocene[END_REF][START_REF] Kuper | Climate-controlled Holocene occupation in the Sahara: Motor of Africa's evolution[END_REF] and that the diffusion of herding followed the installation of arid conditions throughout the Horn of Africa [START_REF] Lesur | The advent of herding in the Horn of Africa: New data from Ethiopia, Djibouti and Somaliland[END_REF]. However, some additional questions arise concerning the importance of specific human reactivity and adaptation to changing environments in the northern EARS. For example, why were there different adaptations of LSA societies to the introduction of new herding practices across tropical Africa, despite the southward migration of the rainbelt from ∼6 ka BP. It is certain that the substitution of socio-economic practices didn't occur abruptly, but that they were modulated by the integration and modification of different ways of life over millennia, showing the complexity of Neolithic transformations in Africa. In Section II.3.3 we demonstrated how in the northern Rift the adoption of food production appeared to have been independent in specific lacustrine basins. This raises questions about the cause of differential integration or substitution of economic practices in the Horn of Africa during the second half of the Holocene. To begin to answer these questions, a specific case-study focused on the reconstitution of the hydrological and hydro-sedimentary reactivity of one of these lacustrine basin in relation to local human inhabited environments through combined approaches in landscape archaeology, geomorphology and paleoclimatology is proposed in this study.

II.4 Synthesis of the research issues and the objectives of the thesis

Section II.1.4 is focused on how the timing and magnitude of past African hydro-climate changes, as well as their spatial distribution, could be influenced by several regionally specific processes, and how they are still not fully understood.

Section II.2.2 summarised previous geomorphological studies that focused on climatic effects on the evolution of landscapes between the Late Pleistocene and the Holocene in the Horn of Africa. In most of these studies, either the understanding of Earth surface processes is disconnected from climatic interpretations or palaeoclimatic studies of continuous sedimentary sequences fail to explore the role of geomorphic processes on the recording of the climatic signal. I propose that a geomorphological approach could represent a powerful tool to trace local to regional hydro-climate change, to decipher the impact of these changes on palaeoenvironments and on specific geomorphic systems, and to assure the correct interpretation of continuous environmental archives largely exploited in paleoclimatic research in East Africa.

Finally, Section II.3.4 discussed the strong temporal and spatial variability of LSA prehistoric occupations along the northern Rift between the Late Pleistocene and the Early Holocene (∼30 to ∼6 ka BP) and how this variability is linked to specific lacustrine basins. Similarly, food production societies emergence at different periods across the Rift at the end of the Middle Holocene (∼5 to ∼3 ka BP) integrating new (herding, pottery traditions) and retaining ancient (intensive fishing, backed microliths) economic and technological practices. It has also been demonstrated how the interpretation of the rupture and/or continuity of archaeological chrono-sequences could be strongly dependent on Earth surface processes, and how landscape evolution is a crucial factor for prehistoric mobility and economic choices, representing a non-negligible element that should be considered in the development of human-climate coevolution theories.

As illustrated in Section II.3, many questions are still open on the causes of the socio-cultural choices made by populations during the Holocene in the Horn of Africa. Several hypotheses have been proposed, most of them converging on the role of environmental and climatic change on human adaptation and on changes in subsistence economies. However, despite the increasing number of high-resolution studies measuring climate change over the globe, our knowledge of hydro-climatic dynamics and their specific distribution over continental Africa remains limited. Indeed, the majority of studies are based on the direct comparison of regional large-scale climatic data with regional or punctual archaeological chrono-cultural sequences, without any interpretation of the local geomorphological patterns [START_REF] Clarke | Climatic changes and social transformations in the Near East and North Africa during the 'long'4th millennium BC: A comparative study of environmental and archaeological evidence[END_REF][START_REF] Gatto | Holocene Supra-Regional Environmental Changes as Trigger for Major Socio-Cultural Processes in Northeastern Africa and the Sahara[END_REF][START_REF] Kuper | Climate-controlled Holocene occupation in the Sahara: Motor of Africa's evolution[END_REF][START_REF] Manning | The demographic response to Holocene climate change in the Sahara[END_REF][START_REF] Maslin | East African climate pulses and early human evolution[END_REF][START_REF] Weiss | Global megadrought, societal collapse and resilience at 4.2-3.9 ka BP across the Mediterranean and west Asia[END_REF]. Following this approach, it is hazardous to make a proper link between wider climate change and prehistoric human settlement and subsistence because of the complexity of the reactivity of each specific hydro-sedimentary system. Assuming that the hydro-climate cannot directly affect humans but considering it is always mediated by specific palaeoenvironmental/geomorphological feedbacks, the methodological approach required must combine and link different scales of analysis, from the study of the regional hydro-climate dynamics, to the geomorphological evaluation of basin mechanisms, to the local sedimentary context of archaeological sites. A few studies have employed this approach with fulfilling results [START_REF] Ashley | Paleoenvironmental Reconstruction of Dongodien, Lake Turkana, Kenya and OSL Dating of Site Occupation During Late Holocene Climate Change[END_REF][START_REF] Foerster | Environmental change and human occupation of southern Ethiopia and northern Kenya during the last 20,000 years[END_REF][START_REF] Hoelzmann | Environmental change and archaeology: Lake evolution and human occupation in the Eastern Sahara during the Holocene[END_REF][START_REF] Lézine | Climate change and human occupation in the Southern Arabian lowlands during the last deglaciation and the Holocene[END_REF][START_REF] Williams | Late Quaternary environments and prehistoric occupation in the lower White Nile valley, central Sudan[END_REF]. Indeed, they interpreted not only the chronology of human occupation in relation to regional climatic changes, but they also tried to decipher prehistoric mobility and cultural changes relying on local hydrological and sedimentary processes.

In the context of my Ph.D, I developed a combined multiscalar approach focused on East Africa for the period concerning the last ∼13 ka, and based on three research axes (Fig. II.13):

The first research axis is focused on the study of a continuous sedimentary sequence. This approach The second axis aims to investigate the reactivity of continental hydro-systems to these climatic oscillations. Focused on the detailed understanding of the evolution of lacustrine littoral morphologies and of monsoonal-induced lake level oscillation of a specific large scale basin, I applied this approach to the Lake Abhe basin, located in the Central Afar region (Chapter V). This lake represents an exceptional receptacle of monsoonal-induced water output originating in the Ethiopian Highlands.

Finally, the third axis of this study focuses on the geoarchaeology of specific prehistoric occupation sequences across this same basin, with the aim to decipher the formation processes from each archaeological site and to identify the links between climate-induced site formation processes, lacustrine environments and wider regional hydrological oscillations (Chapter VI).

Chapter III

Materials and methods

III.1 Presentation of study areas and materials

III.1 Presentation of study areas and materials

In this chapter, materials and methods of the two studied sites are introduced. The Nile and Awash rivers take sources in the Ethiopian Highlands ( The aim of Chapter III (Materials and Methods) is to introduce the study sites and present the totality of the geological/sedimentological objects studied and the related methods employed in this study Because the manuscript's structure is built on the basis of published or submitted articles, to avoid repetitions, the detailed study samples and descriptions of the methodology employed are presented in each results section (article format; Sections IV, V, VI) and in the respective supplementary materials (Section BI) Section. However, a synthesis of methods employed and the respective sample is presented at the end of this chapter (see Section III.4).

III.1.1 The Nile Deep Sea Fan and the MD04-2726 core

The first study area of this study is the Nile Deep Sea Fan. The investigation of the core MD04-2726 responds to issues corresponding to the reconstitution of hydro-climatic variations in East Africa during the Holocene (see Section II.1.4 for open questions), and their impact on the mechanisms of the Nile fluvial system (see Sections II.2.1 and II.2.2 for open questions).

The MD04-2726 has been retrieved during the Marion Dufrense oceanographic cruise VANIL in 2004, one of the closest cores ever collected from the active Nile Rosetta branch margin [START_REF] Migeon | Lobe construction and sand/mud segregation by turbidity currents and debris flows on the western Nile deep-sea fan (Eastern Mediterranean)[END_REF]. The VANIL cruise belongs to a large research program started in 1998 and composed of 9 oceanographic campaigns of the GDR (Group de Recherche) "Marges", under the direction of Jean Mascle (Géoazur laboratory, CNRS-UMR7329). This project has included geological and geophysical investigations of the NDSF, with the employment of multi-track seismic-lines and the EM12 and EM300 multi-receptors. These studies permitted to reconstitute the morphosedimentary architecture of the NDSF with the objective to localise the ideal sites for more than 70 coring operations (Calypso piston corer; [START_REF] Mascle | The Nile Continental Margin: A Compendium Compiled by Jean Mascle[END_REF]. The results of more than 10 years of interdisciplinary researches on this study area are presented in the [START_REF] Mascle | The Nile Continental Margin: A Compendium Compiled by Jean Mascle[END_REF] compendium. Today, the scientific operations conducted by an international team of researchers, in which GEOAZUR-UMR7329 laboratory belongs, continue with the SEAGAL cruise campaign (planned for 2021; S. Migeon director), and other related lab research. The MD04-2726 core is located in the upper part of the Rosetta Turbidite System, [START_REF] Ducassou | Evolution of the Nile deep-sea turbidite system during the Late Quaternary: Influence of climate change on fan sedimentation[END_REF][START_REF] Mascle | Morphostructure of the Egyptian Continental Margin: Insights from Swath Bathymetry Surveys[END_REF], see Sections I.2.2.d and IV.2 for details), off the Rosetta branch of the Nile subaerial delta (Fig. IV.1a). Thanks to this particular location, recent geochemical and organic analyses have demonstrated that sediment on the Rosetta system is ideal to monitor the Nile flood activity since 130.000 years (see Section I.2.2.d for details). A first AMS 14 C datings on the MD04-2726 (based on 21 ages from foraminifera shells) and sedimentological analyses on the first 1.5 m of sediment were reported by [START_REF] Revel | 20,000 years of Nile River dynamics and environmental changes in the Nile catchment area as inferred from Nile upper continental slope sediments[END_REF]. In this Ph.D work, the investigation have been extended on the first 5 sections, on the upper 732 cm laminated/turbiditic sediment of the core (Fig. III.2).

While this study responds to some of the issues presented in this manuscript, it also contributes to NDSF data-set production in the continuity of previous studies focused on morpho-sedimentary and morpho-dynamic patterns of the NDSF [START_REF] Bastian | Abrupt response of chemical weathering to Late Quaternary hydroclimate changes in northeast Africa[END_REF][START_REF] Bastian | Investigating Li Isotope Composition of Nile Deltaic Sediments as Paleotracer of Continental Alteration[END_REF][START_REF] Ducassou | Evolution of the Nile deep-sea turbidite system during the Late Quaternary: Influence of climate change on fan sedimentation[END_REF][START_REF] Ducassou | Nile floods recorded in deep Mediterranean sediments[END_REF][START_REF] Ducassou | Multiproxy Late Quaternary stratigraphy of the Nile deep-sea turbidite system -Towards a chronology of deep-sea terrigeneous systems[END_REF][START_REF] Mascle | The Nile Continental Margin: A Compendium Compiled by Jean Mascle[END_REF][START_REF] Mascle | Morphostructure of the Egyptian Continental Margin: Insights from Swath Bathymetry Surveys[END_REF][START_REF] Migeon | Lobe construction and sand/mud segregation by turbidity currents and debris flows on the western Nile deep-sea fan (Eastern Mediterranean)[END_REF][START_REF] Revel | 20,000 years of Nile River dynamics and environmental changes in the Nile catchment area as inferred from Nile upper continental slope sediments[END_REF][START_REF] Revel | 21,000 Years of Ethiopian African monsoon variability recorded in sediments of the western Nile deep-sea fan[END_REF][START_REF] Revel | 100,000 Years of African monsoon variability recorded in sediments of the Nile margin[END_REF].

The detailed core description and the respective study result are presented in Chapter IV.

III.1.2 The Lake Abhe basin (VAPOR-Afar and PSPCA research programs)

The Lake Abhe basin represents the second study area of this study. It is located in the Central Afar region, between Ethiopia and the Republic of Djibouti (Fig. III.1). The detailed geological, tectonic and hydrological context descriptions of this basin, as well as the larger Awash River basin, are presented in Section I.2.3, although a synthesis of previous literature on this lake is presented in Section II.2.1.b. After an intense period of geological and paleoenvironmental research in the 1970s by a French-Italian team (Barberi andVaret, 1977, 1972;[START_REF] Gasse | Evolution of Lake Abhe[END_REF][START_REF] Varet | Geology of Central and Southern Afar (Ethiopia and Djibouti Republic)[END_REF], no studies have focused on the Quaternary sedimentary basins in this region until today. A great deal of research on Plio-Pleistocene morphosedimentary evolution of the Lake Abhe basin was carried out by F. Gasse and collaborators [START_REF] Faure | Les formations lacustres Holocènes et l'industrie épipaléolithique de la région de Loggia (bassin du lac Abhé Ethiopie)[END_REF][START_REF] Fontes | Les cheminées du lac Abhé (T.F.A.I.)[END_REF][START_REF] Fontes | Freshwater to marine-like environments from Holocene lakes in northern Sahara[END_REF][START_REF] Gasse | Evolution tectonique et climatique de l'Afar central d'apres les sediments plio-pleistocenes[END_REF][START_REF] Gasse | Evolution of Lake Abhe[END_REF][START_REF] Gasse | L'évolution des lacs de l'Afar Central (Ethiopie et T.F.A.I.) du Plio-Pléistocène à l'Actuel. Recostitution des paléomilieux lacustres à partir de l'étude des Diatomées[END_REF][START_REF] Gasse | Evolution tectonique et climatique de l'Afar central d'apres les sediments plio-pleistocenes[END_REF][START_REF] Gasse | Variations hydrologiques et extension des lacs holocenes du Desert Danakil[END_REF][START_REF] Gasse | Late Quaternary lake-level fluctuations and environments of the northern Rift Valley and Afar region (Ethiopia and Djibouti)[END_REF][START_REF] Rognon | Le désert de l'Afar (Ethiopie)[END_REF][START_REF] Rognon | Depots lacustres quaternaires de la basse vallée de l'Awash (Afar, Ethiopie): Leurs rapports avec la techtonique et le volcanisme sousaquatique[END_REF]. Since, the Central Afar region has mainly interested geophysicists and seismologists, but without any regard for paleo-climatic and Quaternary geomorphology issues [START_REF] Doubre | Current deformation in Central Afar and triple junction kinematics deduced from GPS and InSAR measurements[END_REF][START_REF] Manighetti | Strain transfer between disconnected, propagating rifts in Afar[END_REF][START_REF] Manighetti | Propagation of rifting along the Arabia-Somalia Plate Boundary: Into Afar[END_REF]. Archaeological research along the Lake Abhe basin was first initiated in the Republic of Djibouti through a series of field campaigns between the 80s and the 90s under the direction of R. Joussaume and X. Gutherz [START_REF] Gutherz | Le site d'Asa Koma (République de Djibouti) et les premiers producteurs dans la Corne de l'Afrique[END_REF][START_REF] Joussaume | Les premières sociétés de production. In Tiya, l'Éthiopie Des Mégalithes[END_REF], but without any detailed regard for the geomorphic dynamics and sedimentary formation processes of the archaeological sites and landscapes investigated.

Only recently, has a new wave of archaeological research been initiated, this time with the inclusion of a geoscience perspective. Since 2014 multidisciplinary and international research program was inaugurated: the "Volcanological and Archaeological Program for Obsidian Research -Afar" (VAPOR-Afar, in Ethiopia; Fig. III.4; field reports in Sup. Mat. BII.1,BII.2) directed by L. Khalidi (initially as AIYS and Fyssen Foundation grants), combined to "Premières Sociétés de Production dans la Corne de l'Afrique" program (PSPCA,in Djibouti;Fig. III.4; ; field reports in Sup. Mat. BII.3,BII.4) directed by J. Cauliez and financed by the French Ministry of Europe and Foreign Affairs. In just a few years, the success of these research programs permitted multidisciplinary collaborations with the ANR Big Dry (F. Bon) and the TelluS-Rift AGXIM project. In addition, the VAPOR-Afar program integrated a paleoclimate perspective through lake coring into its fieldwork with the CLIMAFAR program (L. Khalidi, M. Revel). The datasets presented in this work are the result of 6 archaeological, geomorphological, geophysical and coring operations during VAPOR-Afar and PSPCA fieldwork campaigns that occurred between January 2018 and March 2020 during the duration of my Ph.D studies at the University of Côte d'Azur, for a total of 170 days spent doing fieldwork in the Horn of Africa. Indeed, the fieldwork investigation, observation and sampling represent the basis of the unpublished work presented in this manuscript. Contrary MD04-2726 marine sediment sequence (Section III.1.1), the investigation on the Lake Abhe basin is based on a large variety of geological and sedimentological materials collected during fieldwork. In the next sections are presented each type of geomorphic, sedimentary and geological object analysed in this study. Although, the detailed description of samples is presented in the respective results Sections IV, V and VI.

III.1.2.a Lake Abhe basin study materials

Hydrological fluctuations of the Lake Abhe basin have been identified by the documentation of littoral clastic geomorphic features, such as coastal wave notches, wave-cut abrasion terraces, cobble/pebble accumulations and overflow sill surfaces (Fig. III.4). More than one hundred of these features was documented. A detailed description and interpretation of each littoral geomorphic feature is presented in Section V.1.

Secondly, Nearshore, Foreshore and Offshore lacustrine deposits have been sampled and documented as evidence of an extended Holocene Lake Abhe. In this study, dated nearshore deposits are used as a complementary tool to reconstruct the Holocene fluctuation of Lake Abhe presented in Section V.1. Another evidence of lake shoreline deposits are microbialites/stromatolite deposits. A detailed description and interpretation of Lake Abhe stromatolites is presented in Section V.1. During three years of fieldwork, a total of 51 stromatolite specimens were documented and sampled (Fig. III.6c;Sup. Mat. BI.6). The need to understand the holistic evolution of past lacustrine and perilacustrine environments of Lake Abhe led me to the documentation and the study of 38 pedological horizons dispatched over 15 paleosoil sequences (Fig. III.4). Detailed description, study and interpretation of these paleosoils are presented in Section V.2.

Finally, fieldwork survey and explorations over the Lake Abhe basin allowed the discovery of a large number of archaeological sites with specific pedosedimentary sequences (Fig. III.4). More than 50 archaeological sites were documented over the Lake Abhe basin (Fig. III.4). With the aim respond to the issues of this study, 8 archaeological sequences were dated and selected for supplementary and specific analyses. The detailed localisation, sedimentary description, investigation and interpretation of these sequences is presented in Chapter VI. Identifying geomorphological and pedosedimentary elements during fieldwork and mapping them represents the first study approach employed at the Lake Abhe basin study site. During the 2018-2020 field campaigns, survey over the entire Abhe basin was focused on the identification and GPS measurement of littoral morpho-sedimentary and geomorphic elements. Additionally, DGPS survey was performed by G. Davtian (CEPAM) and myself using the Trimble Geo 7X differential GPS, with a double frequency receptor (L1 = 1575.46 MHz, L1 = 1227.60 MHz; Fig.

III.2 Fieldwork methods

III.2.1 Geomorphological survey and GPS treatment

III.5).

The vertical standard deviation of our measurements ranged between 1 and 2 cm, which is in accord with the analytical Trimble value error. The Geographical Information System analyses were conducted using the Esri ArcGIS10.3.1® program. All sample locations and maps presented here are expressed in a geographic coordinate system (WGS84).

For the production of multi-data maps and for the spatial analyses two Digital Elevation Models (DEM) were employed: for low resolution imaging a SRTM® DEM (www2.jpl.nasa.gov/srtm; 30 m resolution), and for high resolution imaging a TanDEM-X (https://tandemx-science.dlr.de; [START_REF] Rizzoli | Generation and performance assessment of the global TanDEM-X digital elevation model[END_REF] DEM. The accuracy of our DGPS measurements was tested with respect to both DEMs, showing accurate elevations and permitting a homogeneous DEM measurement calibration (Fig. III.4; see Section V.1 for details).

III.2.2 Archaeological survey and excavation methods

A combination of remote sensing, archaeological survey was carried out from the lowlands to the fault margin slopes over the totality of the Lake Abhe basin. Archaeological test pits ranging from 1 to 1.3 m 2 area were excavated during fieldwork. The richest archaeological levels were investigated over more extensive surfaces dimensionally adapted to each site following a 1 m grid system (Fig. III.6a). Fine excavation methods were used and all sediment was sieved using a 2 mm mesh. Levels with archaeological materials were sieved in their totality. All archaeological artifacts >2 cm were geolocalized in well controlled excavated squares and assigned a unique number.

III.2.3 Pedo-sedimentary lithological descriptions and sampling

Sedimentogenetic investigation (application and interpretation of laboratory analytical methods presented in Section III.3) was made on different types of sedimentary sections obtained from manual sounding (1 x 1 m area or manual coring), exposed sedimentary sections or archaeological excavations. The fieldwork operations on these sections included pedostratigraphic descriptions of the deposits. The aim of these macro-to meso-scopic descriptions is to define the characteristic of each layer (called Sedimentary Units = SU) and their general patterns including their colour, texture, sorting, structure, allochtonous inclusions, secondary mineralogical inclusions and the anthropic materials, as well as the sedimentary layer morphologies (where possible) and the topographical contexts (Fig. III.6b,c). Each SU from the sequences observed in the field was attributed a number. Sedimentary Unit numbering generally followed the layers order from bottom to top. However, to adapt the numbering strategies to the archaeological excavation operations techniques (numbering in the order of SUs discover) a combined SU numbering order between sedimentological and archaeological technique is often provided. Each SU from all investigated sequences was systematically sampled (bulk sediment), however, some selective sampling strategies was employed for specific laboratory analyses (radiocarbon dating materials, micromorphological samples; see details in next Sections).

III.3 Laboratory analytical methods

III.3.1 Radiocarbon dating, calibration and modelling

III.3.1.a Type of samples and dating labs

AMS radiocarbon measurements presented in this study have been obtained on a large variety of materials like a Foraminifers (bulk planktonic and G. ruber alba), bulk organic carbon in sediments, charcoals, ostracods (Melanoides tuberculata) and bivalve shells, bones and ostrich egg shells (see Section III.4 for a summary). Chemical sample pre-treatment (acid-alkali-acid treatment), carbon dioxide production and the graphitisation, as well as the AMS (Accelerator Mass Spectrometry) 14 C measures was made directly in the laboratories where the samples were analysed, namely 'Poznan Radiocarbon Laboratory' (POZ), 'LMC14 National Platform' (SAC), 'Beta Analytical Laboratory' (BETA), '14Chrono Centre' (UBA) and 'CEDAD' (LTL). The lab code is indicated in each age reference in the results sections.

III.3.1.b Calibration, Age modelling and Age reservoir effect

The calibration of radiocarbon ages was performed using the OxCal 4.3 program (Bronk [START_REF] Bronk Ramsey | Development of the Radiocarbon Calibration Program[END_REF]. The ages were considered here using 95.4% ranges and are reported in cal. BP or cal. ka BP (calibrated kilo annum before 1950). Application of specific calibration curves (IntCal and Marine13) and reservoir age effect corrections [START_REF] Reimer | IntCal13 and Marine13 Radiocarbon Age Calibration Curves 0-50,000 Years cal BP[END_REF][START_REF] Reimer | Marine radiocarbon reservoir corrections for the midto late Holocene in the eastern subpolar North Atlantic[END_REF][START_REF] Reimer | An Online Application for ∆R Calculation[END_REF] was applied differently for each type of dated material. Age model building, as well as sedimentation rate calculations of continuous sedimentary sequences (ex: MD04-2726 core) was provided using the Bacon software (v3.6.3, [START_REF] Blaauw | Flexible paleoclimate age-depth models using an autoregressive gamma process[END_REF]. The subdivisions of the Holocene that are used follow those formally ratified by the International Union of Geological Sciences [START_REF] Walker | Formal ratification of the subdivision of the Holocene Series/Epoch (Quaternary System/Period) : Two new Global Boundary Stratotype Sections and Points (GSSPs) and three new stages/subseries[END_REF].

Because of the large variety of dated geological and sedimentological material from different contexts, the problem of the reservoir age effect was recurrent. Indeed, the atmospheric carbon measured on terrestrial materials (charcoals, plants, organic matter) differs from than measured on marine or fluvial/lacustrine materials (lacustrine shells, stromatolites, foraminifera) which is dissolved in waters. While the reservoir age effect on marine biogenic carbonates in the Mediterranean Sea is the more and more controlled [START_REF] Siani | Mediterranean Sea Surface Radiocarbon Reservoir Age Changes Since the Last Glacial Maximum[END_REF], that of the East Africa Rift lacustrine basins is almost unknown. Indeed, it can vary in space (for each basin) and in time, because of the different residence times of the dissolved Carbon in waters. The only way to coerce the reservoir effect is to measure two contemporaneous terrestrial and aquatic carbonate materials. The age difference between two samples allow to correct the reservoir in age. In this study, two ages permitted to estimate the reservoir age over the Lake Abhe basin for the Early Holocene on lacustrine carbonate materials (∼100 yrs; for details see Section VI.1.3.c). Because the dissolved Carbon in waters could vary through the time, one result value will not reliable for the correction of the age reservoir effect on the whole Holocene period. However, considering that this measured age reservoir effect not exceed the 100 years, and because no more data are available for the moment, any correction was applied on the biogenic carbonate material coming from the Lake Abhe basin.

In contrast, for the MD04-2726 core, because of the variability of the age reservoir of the dated organic matter (originated from the organic matter residence time along the river basin) an age reservoir correction has been applied on terrestrial material. In this case, an inversed correction has been proposed (see Section IV.3.1 for details). Paired measurements of AMS 14 C of planktonic foraminifera and bulk organic matter on the same depth interval allows the estimation of the age reservoir correction of the continental organic matter. Inversely to classical methods to determine the local age reservoir for marine sediments [START_REF] Dewar | Late-Holocene marine radiocarbon reservoir correction (∆R) for the west coast of South Africa[END_REF][START_REF] Reimer | Marine radiocarbon reservoir corrections for the midto late Holocene in the eastern subpolar North Atlantic[END_REF][START_REF] Reimer | An Online Application for ∆R Calculation[END_REF], we used the calibrated age of planktonic foraminifera to determine the apparent age of contemporaneous organic matter.

The specific information and discussion on the choice of the applied calibration curve and reservoir age effect calibration are presented in respective result Sections of chapters IV, V and VI.

III.3.2 Light and Electron Microscopy Analyses

III.3.2.a Micromorphology of soils and sediments

Micromorphological block sampling from investigated sections during field operations followed a selective strategy [START_REF] Bullock | Handbook for Soil Thin Section Description[END_REF] to respond to specific questions on sedimentary formation and postdepositional processes (Fig. Soil thin sections were prepared following the standard procedure described in [START_REF] Guillore | Méthode de Fabrication Mécanique et En Série Des Lames Minces[END_REF]. Thin sections were scanned in plane-polarized and crossedpolarized light following the method described in [START_REF] Angelucci | Automatic segmentation of thin sections with GRASS GIS: First experiments[END_REF]. Microscopic observations were made using a Zeiss Axioskop 40 Pol/ 40 A Pol polarizing microscope at a magnification between 25x and 1000x using plane-polarized (PPL), crossed-polarized (XPL), oblique incident (OIL), and fluorescence light (UV). Thin sections are described following the guidelines proposed by [START_REF] Bullock | Handbook for Soil Thin Section Description[END_REF] and [START_REF] Stoops | Guidelines for Analysis and Description of Soil and Regolith Thin Sections[END_REF]. Semi-quantitative counts were performed using the JMicrovision® software.

With the aim to facilitate the reading of this manuscript, micromorphological descriptions have been summarized in Tables in the text following the description guidelines provided in [START_REF] Stoops | Guidelines for Analysis and Description of Soil and Regolith Thin Sections[END_REF].

III.3.2.b Scanning electron microscopy (SEM)

Qualitative and semi-quantitative geochemical analysis on uncovered thin sections or polished rock sections was performed using scanning electron microscopy (SEM), thanks to a collaboration with the Centre Commun de Microscopie Appliquée (CCMA) of the University of Côte d'Azur (coll. F. Orange). SEM observations and Energy Dispersive X-Ray spectroscopy (EDX) analyses were carried out with a Tescan Vega3 XMU scanning electron microscope (TESCAN FRANCE, Fuveau, France) equipped with an Oxford X-MaxN 50 EDX detector (Oxford Instruments, Abingdon, U.K.). SEM was also used for the surface textural analyses of quartz grains, following [START_REF] Vos | Surface textural analysis of quartz grains by scanning electron microscopy (SEM): From sample preparation to environmental interpretation[END_REF] sampling processing and analysis settings. Thin sections were observed and analysed with no prior metal or carbon coating, using the low vacuum mode (N2 pressure ∼35 Pa) and at a 20 kV accelerating voltage. Imaging was performed using backscattered electrons. EDX spectra were processed with the Aztec software (version 3.1, Oxford Instruments). Surface textural analyses of quartz grains were performed on 10 to 20 grains for each of the beds identified in thin sections, with a grain-size range between 100 μm to 1 mm, randomly selected under a binocular microscope and then analysed using the SEM. Quartz grains were coated with platinum (3 nm) prior to SEM observations at a 5kV accelerating voltage.

III.3.3 Sedimentological analyses

III.3.3.a Grain-size laser diffraction

Grain-size analyses were performed using a Coulter-LS2000 at GEOAZUR-UMR7329 laboratory. All samples presented in this manuscript has been previously decarbonated (HCl, 1 M). For the MD04-2726 study (Chapter IV) the organic matter was maintained (no H 2 O 2 dissolution), and considered as a grain-size marker of river-induced instantaneous deposits. Contrarily, for all other samples presented in Chapters V and VI, organic matter was removed using H 2 O 2 solution (30%) at 60°C. To avoid clay flocculation and to dissociate mineral particles before the grain-size laser diffraction measure, we used sodium hexametaphosphate (NaHMP) and ultrasonic treatment. Grain-size laser diffraction raw data has been subsequently analysed to determine the grain-size of the intercepts for 10%, 50% and 90% of the cumulative grain-size curves (named Q90, Q50 and Q10 values; [START_REF] Folk | Brazos River Bar: A Study in the Significance of Grain Size Parameters[END_REF].

III.3.3.b Loss on ignition

Loss On Ignition (LOI) was performed on 2 g of crushed sediment for each analysed sample at the CEPAM-UMR7264 laboratory. Samples has been heated for 24 h at 100°C with the aim to determine the residual water and gypsum content. Subsequently samples has been heated for 4 h at 550°C with the aim to determine total content measures of organic carbon [START_REF] Santisteban | Loss on ignition: A qualitative or quantitative method for organic matter and carbonate mineral content in sediments[END_REF]. LOI is expressed in percentage (%).

III.3.4 Geochemistry and isotopic analyses

III.3.4.a X-ray fluorescence core scanner on soft sediment

X-Ray fluorescence core scanning (XRF-CS) was performed on an Avaatech core scanner at the EDYTEM laboratory (CNRS-Université de Savoie Mont-Blanc). The XRF-CS analyses were performed on I-II-III-IV-V sections of the core MD04-2726 (see Chapter IV) with a 1 mm sampling step and with a 10 kV voltage (Al, Si, S, K, Ca, Ti, Mn, Fe) for sections I-II [START_REF] Revel | 20,000 years of Nile River dynamics and environmental changes in the Nile catchment area as inferred from Nile upper continental slope sediments[END_REF], and with a 10 KV and 30 kV voltage (Al, Si, S, K, Ca, Ti, Mn, Fe; Cu, Zn, Br, Sr, Rb, Zr, Pb) for sections III-IV-V (this study). Each individual power spectrum was transformed by deconvolution into relative contents (intensities) expressed in counts per seconds (cps). In this study we principally used the logs of elemental ratios, following a well-established methodology to circumvent problems associated with the closed-sum effect [START_REF] Croudace | Micro-XRF Studies of Sediment Cores, volume 17 of Developments in Paleoenvironmental Research[END_REF][START_REF] Croudace | ITRAX: Description and evaluation of a new multifunction X-ray core scanner[END_REF][START_REF] Weltje | Calibration of XRF core scanners for quantitative geochemical logging of sediment cores: Theory and application[END_REF].

III.3.4.b Handheld X-ray fluorescence on sediment (P-XRF)

Chemical elemental analyses on 38 bulk sediment samples (see Section V.2, Tab. BI.9) was obtained with the Olympus Vanta™ handheld XRF machine at the GEOAZUR-UMR7329 laboratory. Bulk sediments were previously crushed and measured at stable room temperature. The power source of the Olympus Vanta™ is composed of 4 Watt X-ray tube with anode materials (Rh, Ag and W) with an 8 -50 kV capacity voltage. Measurements were obtained with 10 kV voltage for Al, Si, S, K, Ca, Ti, Mn, Fe elements, and 30 kV voltage for Cu, Zn, Br, Sr, Rb, Zr, Pb elements. Calibration of our measures has been made from the standard deviation calculation from the result analyses of soils, lacustrine sediments and rocks certificated samples.

III.3.4.c Nd radiogenic isotope analyses

Isotope ratios were measured on carbonate-free <63 µm particle size sediments of the MD04-2726 core, using 100 mg of the dried aluminosilicate fraction (see Chapter IV, Tab. BI.3). Chemical extractions for Nd isotopes were carried out at the Laboratory of Geoscience Montpellier (D. Bosch expertise). Samples were leached for 1 hour with 1N HCl to dissolve biogenic carbonates. The Fe-Mn oxyhydroxide fraction and organic carbon were removed using 10 ml 1M Hydroxylamine hydrochloride and 10 ml 5% H 2 O 2 , respectively [START_REF] Bayon | An improved method for extracting marine sediment fractions and its application to Sr and Nd isotopic analysis[END_REF]. After leaching, the samples were centrifuged and the supernatant discarded. The remaining sediment was washed three times with ultra-pure water, discarding the supernatant each time to remove the biogenic components and to eliminate any marine Nd isotope signal adsorbed on to the silt-clay fraction. The chemical separation of Nd included a first separation step using AG50W-X-8 cation exchange resin to collect rare earth elements followed by purification of Nd using HDEHP conditioned Teflon columns. Nd isotopes were measured using a Thermo Scientific Neptune Plus MC-ICP-MS from the AETE-ISO geochemical platform (OSU-OREME, Montpellier University). The 143 Nd/ 144 Nd isotopic compositions were corrected for internal mass bias using an exponential law and a value of 0.7219 for the 146 Nd/ 144 Nd ratio. The external mass bias was corrected using the standard bracketing method using AMES-Rennes and analysing each of the 2 unknowns. During the course of the study AMES-Rennes standards yielded an average 0.511946±6 (2σ, n=10) for the 143 Nd/ 144 Nd ratio [START_REF] Chauvel | A hafnium isotope and trace element perspective on melting of the depleted mantle[END_REF]. The Nd procedural blank measured during the course of this study was 25 pg. Epsilon Nd values ( Nd) were calculated using 143 Nd/ 144 Nd = 0.512638 [START_REF] Bouvier | The Lu-Hf and Sm-Nd isotopic composition of CHUR: Constraints from unequilibrated chondrites and implications for the bulk composition of terrestrial planets[END_REF]. Nd results of 23 samples are presented in Section BI.1 of 'VOLUME B' of this manuscript (Tab. BI.3).

III.4 General plan of laboratory methods

IV.6 Conclusion Key points:

This chapter corresponds to the article entitled "Frequency of exceptional Nile flood events as an indicator of Holocene hydro-climatic changes in the Ethiopian Highlands" [START_REF] Mologni | Frequency of exceptional Nile flood events as an indicator of Holocene hydro-climatic changes in the Ethiopian Highlands[END_REF] published the 2 September 2020 in the journal Quaternary Science Reviews.

The projections of future rainfall in the Sahel latitude have low confidence. It is therefore crucial to study monsoon evolution beyond the instrumental record to better understand forcing mechanisms of monsoon rainfall. The Nile watershed is chosen for its exclusively silicate lithology, with contrasting basaltic (Ethiopian Traps) and granitic (Central African craton) outcrops (Fig. IV.1b). Thus, because the Ethiopian highlands source region has also specific climatic conditions (rainfall is concentrated in the summer months over the Ethiopian Highlands), the changes in monsoon intensity have been finely recorded in the sedimentary archives of the Nile Delta. Previous studies [START_REF] Bastian | Abrupt response of chemical weathering to Late Quaternary hydroclimate changes in northeast Africa[END_REF][START_REF] Blanchet | A global database of radiogenic Nd and Sr isotopes in marine and terrestrial samples[END_REF][START_REF] Blanchet | High-and low-latitude forcing of the Nile River regime during the Holocene inferred from laminated sediments of the Nile deep-sea fan[END_REF][START_REF] Ménot | Timing and stepwise transitions of the African Humid Period from geochemical proxies in the Nile deep-sea fan sediments[END_REF][START_REF] Revel | 20,000 years of Nile River dynamics and environmental changes in the Nile catchment area as inferred from Nile upper continental slope sediments[END_REF][START_REF] Revel | 100,000 Years of African monsoon variability recorded in sediments of the Nile margin[END_REF] focused on analysis of neodymium isotopes, have confirmed that most of the clastic inputs of sediment transported and deposited in the Nile Delta came from the erosion of the Ethiopian traps.

In this chapter, we report grain size, Nd isotope and element concentrations, as well some sedimentological and microscopic analyses results, of the MD04-2726 core, which was recovered in the upper Rosetta turbidite-levee system, located in the Western branch of the Nile Deep Sea Fan. Differently from previous studies, this work has demonstrated the flood-Induced origin continuous sequence of turbiditic deposits contained in the MD04-2726 core spanning the Early to the Middle Holocene. On the basis of the construction of a frequency and intensity curve of Nile floods, we were able to track the hydrological variations of the Nile River in correlation with the monsoonal activity. The results will add some elements to the discussion of the issues presented in Section II.1.4, concerning the distribution patterns of hydro-climatic variations in East Africa during the Holocene and to investigate some problematics illustrated in Section II.2.2 about the role of geomorphic processes on the earth surface sedimentary transfer processes along the Nile River basin.

The salient research results of this work are:

• The construction of the first Nile exceptional paleo-flood frequency from flood-induced layers recorded in the Nile deep sea fan.

• High hydrological activity of the Nile River between 10 and 9 ka BP, in phase with high East African monsoonal activity.

• A shift in the hydro-climatic regime as early as 9 ka BP, with lowering in flood frequency and magnitude.

• The hydrological shift around ∼9 ka BP brings into question the linkages between low and high latitudes around the 8.2 ka BP NH cooling event.

• An unstable Ethiopian Nile hydro-system from 7.8 ka BP evident in river sediment delivery decoupled from climatic forcing suggest the occurrence of autogenic sedimentation processes.

INTRODUCTION

IV.1 Introduction

Monsoons are the dominant seasonal mode of climate variability in the tropics, acting as the main driver of climate worldwide. At present, the tropics concentrate the maximum amount of energy and redistribute it in the form of air and moisture masses to subtropical latitudes between 30°N and 30°S. Hydroclimatic conditions in the current Sahel area (Fig. IV.1b) have varied in the past following a precessional pacing (i.e., every 21,000 years; [START_REF] Mohtadi | Palaeoclimatic insights into forcing and response of monsoon rainfall[END_REF]. Periods that were more humid in the past compared with present-day conditions have been attributed to the northward migration of the rainfall belt associated with the Inter Tropical Convergence Zone (ITCZ; [START_REF] Mohtadi | Palaeoclimatic insights into forcing and response of monsoon rainfall[END_REF]. This migration is governed by precession-driven insolation changes and modulated by the eccentricity [START_REF] Demenocal | Abrupt onset and termination of the African Humid Period: Rapid climate responses to gradual insolation forcing[END_REF][START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF][START_REF] Rossignol-Strick | After the deluge: Mediterranean stagnation and sapropel formation[END_REF][START_REF] Skonieczny | Monsoon-driven Saharan dust variability over the past 240,000 years[END_REF]. Past humid periods were characterized by enhanced discharge and sediment export from the large African river systems to surrounding ocean margins. The most recent period of more intense rainfall, the African Humid Period (AHP: from ∼14 to ∼6 ka BP;[START_REF] Demenocal | Abrupt onset and termination of the African Humid Period: Rapid climate responses to gradual insolation forcing[END_REF][START_REF] Shanahan | The time-transgressive termination of the African Humid Period[END_REF], led to the expansion of continental water bodies [START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF], the development of vegetation cover over the present-day Sahelian-Saharan desert [START_REF] Hopcroft | Multi vegetation model evaluation of the Green Sahara climate regime: RAINFALL SUPPORT-ING A GREEN SAHARA[END_REF] and very high riverine-induced terrigenous inputs. Many recent studies conducted at high temporal resolution (100 years) in lake and deltaic sediment records across the East African Rift System and over the north African continental margins suggest that gradual long-term monsoon oscillations (controlled by external orbital forcing) were often punctuated by millennial-scale episodes of hyper aridity [START_REF] Bastian | Abrupt response of chemical weathering to Late Quaternary hydroclimate changes in northeast Africa[END_REF][START_REF] Chalié | Late Glacial Holocene diatom record of water chemistry and lake level change from the tropical East African Rift Lake Abiyata (Ethiopia)[END_REF][START_REF] Costa | Isotopic reconstruction of the African Humid Period and Congo Air Boundary migration at Lake Tana, Ethiopia[END_REF][START_REF] Foerster | Climatic change recorded in the sediments of the Chew Bahir basin, southern Ethiopia, during the last 45,000 years[END_REF][START_REF] Tierney | Abrupt Shifts in Horn of Africa Hydroclimate Since the Last Glacial Maximum[END_REF][START_REF] Verschuren | Half-precessional dynamics of monsoon rainfall near the East African Equator[END_REF].

The most recent arid episodes occurred during the AHP and at the onset of the AHP (8.2 and 12.8 ka BP), coinciding with the melting of icebergs from the Laurentide ice sheet into the Arctic [START_REF] Collins | Rapid termination of the African Humid Period triggered by northern high-latitude cooling[END_REF][START_REF] Collins | Interhemispheric symmetry of the tropical African rainbelt over the past 23,000 years[END_REF].It has been proposed that the reduction of Atlantic thermohaline circulation could have induced a reduction of African monsoon activity [START_REF] Mulitza | Sahel megadroughts triggered by glacial slowdowns of Atlantic meridional overturning: SAHEL DROUGHT AND ATLANTIC OVERTURNING[END_REF]. In order to understand the mechanisms linked to these abrupt climate changes between low and high latitudes, it is crucial to work on natural archives with a centennial to decadal temporal resolution.

The main goal of this work is to investigate Nile river discharge over the last ∼10.3 thousand years at a centennial time resolution, with the aim of documenting the internal dynamics of the AHP in East Africa. A particular focus will be set on the 8.2 ka BP arid event, to better understand the timing, mechanism and feedback of African monsoon activity in the Ethiopian-Nile basin.

Sediment deposition in deltas is dominated by terrigenous material delivered from flooding rivers and is highly sensitive to changes in precipitation rate and the extension of vegetation cover in drainage basins. The sediment records preserved on the Nile Deep-Sea Fan (NDSF) provide suitable archives for reconstructing past continuous climate variations [START_REF] Ducassou | Evolution of the Nile deep-sea turbidite system during the Late Quaternary: Influence of climate change on fan sedimentation[END_REF]. Previous studies on the NDSF have demonstrated that past humid periods were systematically accompanied by higher deposition rates and enhanced terrigenous supply, reflecting strong physical erosion and transport processes from the Ethiopian Highlands related to African monsoon activity [START_REF] Blanchet | Asynchronous Changes in Vegetation, Runoff and Erosion in the Nile River Watershed during the Holocene[END_REF][START_REF] Ducassou | Nile floods recorded in deep Mediterranean sediments[END_REF][START_REF] Krom | Nile River sediment fluctuations over the past 7000 yr and their key role in sapropel development[END_REF][START_REF] Revel | 20,000 years of Nile River dynamics and environmental changes in the Nile catchment area as inferred from Nile upper continental slope sediments[END_REF][START_REF] Revel | 100,000 Years of African monsoon variability recorded in sediments of the Nile margin[END_REF]. In addition, the provenance of particles exported to the NDSF has been inferred from the application of Nd and Sr radiogenic isotopes in sediment cores. Over the NDSF, the variability of lithological facies includes pelagites/hemipelagites, turbidites, debrites, slide/slump deposits and clastic muds [START_REF] Ducassou | Nile floods recorded in deep Mediterranean sediments[END_REF][START_REF] Ducassou | Evolution of the Nile deep-sea turbidite system during the Late Quaternary: Influence of climate change on fan sedimentation[END_REF][START_REF] Migeon | Lobe construction and sand/mud segregation by turbidity currents and debris flows on the western Nile deep-sea fan (Eastern Mediterranean)[END_REF]. During strong flood events, fine-to coarse-grained beds named hyperpycnites can be deposited at ocean and sea margins [START_REF] Mulder | Marine hyperpycnal flows: Initiation, behavior and related deposits. A review[END_REF](Mulder et al., , 2001;;[START_REF] Parsons | Hyperpycnal plume formation from riverine outflows with small sediment concentrations[END_REF]. However, in the NDSF, Nile flood events seem to be recorded through the emplacement of the so-called "clastic-mud beds" [START_REF] Ducassou | Nile floods recorded in deep Mediterranean sediments[END_REF][START_REF] Revel | 20,000 years of Nile River dynamics and environmental changes in the Nile catchment area as inferred from Nile upper continental slope sediments[END_REF][START_REF] Blanchet | High-and low-latitude forcing of the Nile River regime during the Holocene inferred from laminated sediments of the Nile deep-sea fan[END_REF]. The deposition of these beds is explained by the reconcentration of surface plumes originating from terrigenous river discharge by convective processes leading to the formation of hyperpycnal flows at depth [START_REF] Ducassou | Nile floods recorded in deep Mediterranean sediments[END_REF][START_REF] Parsons | Hyperpycnal plume formation from riverine outflows with small sediment concentrations[END_REF].

In this study, we focus on well-preserved turbidite beds located in the Western Province of the NDSF (MD04-2726 core, Fig. IV.1), that we interpret to be a potential instantaneous and direct record of the Nile-river floods during the AHP, related to African paleo-monsoon activity. Integrating multiproxy sedimentological and geochemical (major elemental and Nd isotopes) data, we address four challenges:

• distinguishing between beds deposited by hyperpycnal flows and those induced by failure-triggered gravity flows or by density convective processes (clastic-mud beds) along the Rosetta Canyon and the continental slope;

• confirming the coherency of these hyperpycnal-flow deposits over the western NDSF by comparing them with similar laminated and well-dated records in the Rosetta System (MS27PT and P3622-33 cores;[START_REF] Demenocal | Abrupt onset and termination of the African Humid Period: Rapid climate responses to gradual insolation forcing[END_REF]Shanahan et al., 2015, respectively);

• characterising the frequency and magnitude of Nile paleo-floods over the last 10.1 kyr;

• finally, discussing the meaning of Nile-flood frequencies at centennial resolution and their links with paleo-monsoon activity during the AHP, with a particular focus on the Early to Mid-Holocene and on the the 8.2 ka BP hyper-arid event [START_REF] Rohling | Centennial-scale climate cooling with a sudden cold event around 8,200 years ago[END_REF].

These results will be of interest to better constrain identification of hyperpycnal-flow deposits within mud-prone turbidite systems similar to the Nile Deep-Sea Fan and to better constrain African paleomonsoon dynamics for integration within broader climate modelling.

IV.2 Regional settings

IV.2.1 The NDSF Western Province morphology

Core MD04-2726 was collected in the upper part of the Rosetta Turbidite System, located on the western NDSF [START_REF] Ducassou | Evolution of the Nile deep-sea turbidite system during the Late Quaternary: Influence of climate change on fan sedimentation[END_REF][START_REF] Mascle | Morphostructure of the Egyptian Continental Margin: Insights from Swath Bathymetry Surveys[END_REF], off the Rosetta branch of the Nile subaerial delta (Fig. IV.1a). Here, the continental shelf is about 30 km wide, and the head of the Rosetta Canyon lies on the outer continental shelf at a water depth of about 70 m (Fig. IV.1a). It is about 25 km long and 200-250 m deep, and is straight following the main slope angle of the continental slope. In cross section, the Rosetta Canyon is characterized by U-to poorly-developed V-shaped profiles where a large number of small-scale failures affected the canyon flanks (Fig. IV.1a). The Rosetta Canyon connects to a meandering channel-levee system named DSF6, which was initiated during Marine Isotopic Stage 3 (MIS 3) [START_REF] Ducassou | Evolution of the Nile deep-sea turbidite system during the Late Quaternary: Influence of climate change on fan sedimentation[END_REF] and potentially recorded the Holocene activity of the Nile flood-induced inputs.

IV.2.2 The NDSF Western Province sediment sources

The sedimentary provenance of terrigenous material in the western NDSF can be characterized through its neodymium isotopic signature ( Nd) [START_REF] Bastian | Abrupt response of chemical weathering to Late Quaternary hydroclimate changes in northeast Africa[END_REF][START_REF] Blanchet | Asynchronous Changes in Vegetation, Runoff and Erosion in the Nile River Watershed during the Holocene[END_REF][START_REF] Blanchet | A global database of radiogenic Nd and Sr isotopes in marine and terrestrial samples[END_REF][START_REF] Weldeab | The pace of East African monsoon evolution during the Holocene: WELDEAB ET. AL.; EAST AFRICAN MON-SOON EVOLUTION[END_REF][START_REF] Revel | 100,000 Years of African monsoon variability recorded in sediments of the Nile margin[END_REF]. Since the Nile basin encompasses the Precambrian African basement as well as the Cenozoic Ethiopian traps, which have contrasting Nd isotopic compositions, geochemical studies on its sediment loads allow us to distinguish between the sources of the material (Fig. IV.1b). 96% of the particles deposited in the NDSF today are estimated to originate from the Blue Nile and Atbara rivers and only 4% from the White Nile (Be 'eri-Shlevin et al., 2018;[START_REF] Garzanti | The modern Nile sediment system: Processes and products[END_REF][START_REF] Padoan | Tracing Nile sediment sources by Sr and Nd isotope signatures (Uganda, Ethiopia, Sudan)[END_REF]. The Bahr el Jebel and Victoria-Albert Nile-derived fluvial muds are characterized by Nd(0) ≈-25 [START_REF] Padoan | Tracing Nile sediment sources by Sr and Nd isotope signatures (Uganda, Ethiopia, Sudan)[END_REF], whereas the White Nile mud reaching Khartoum could have a Saharan Metacraton imprint with Nd around -13 (Fig. IV.1b Skonieczny et al., 2011). Aeolian dust from the western to central Saharan region are characterized by relatively constant values of Nd ranging between -10 to -15 [START_REF] Grousset | Tracing dust sources and transport patterns using Sr, Nd and Pb isotopes[END_REF][START_REF] Scheuvens | Bulk composition of northern African dust and its source sediments -A compilation[END_REF]. Thus, temporal changes in both Nd signature and sedimentation rate from sedimentary records in the NDSF allow us to track past temporal changes in relative proportions of detrital sediment originating from the Ethiopian Traps located in tropical latitudes between 5°N to 15 °N ( Nd > 0), the Central Africa Craton located in (Blanchet et al., 2013, black dot). b) Hydrological context of North Africa and the present-day active Nile watershed (modified from [START_REF] Skonieczny | African humid periods triggered the reactivation of a large river system in Western Sahara[END_REF]. Three main sources of suspended sediment load are identified in the Nile Deep-Sea Fan: the basaltic rocks (purple) of the Ethiopian Traps (Highlands), which are drained by the Blue Nile, the Atbara and Sobat rivers located at tropical latitude (around 5 to 15°N); the Precambrian metamorphic rocks (green) of the Central African Craton located in the equatorial latitude of Lakes Albert and Victoria in the Ugandan headwaters region of the White Nile; the Saharan Metacraton [START_REF] Abdelsalam | The Saharan Metacraton[END_REF] which produce aeolian dust representing the source region of most deflation documented by [START_REF] Scheuvens | Bulk composition of northern African dust and its source sediments -A compilation[END_REF]. The outcrops of the Ethiopian Traps, Precambrian Craton and Saharan Metacraton provide sediments characterized by Nd values of > 0, > -30 and from -10 to -14 respectively [START_REF] Garzanti | The modern Nile sediment system: Processes and products[END_REF][START_REF] Grousset | Tracing dust sources and transport patterns using Sr, Nd and Pb isotopes[END_REF][START_REF] Skonieczny | The 7-13 March 2006 major Saharan outbreak: Multiproxy characterization of mineral dust deposited on the West African margin[END_REF]. The seasonal positions of the Intertropical Convergence Zone (ITCZ; red dotted line, [START_REF] Viste | Moisture transport into the Ethiopian highlands: MOISTURE TRANSPORT INTO THE ETHIOPIAN HIGHLANDS[END_REF] as well as of the Sahel zone (orange area) are shown, as well as the Nile catchment mica sources (Dabus and Diesda rivers, orange dots; [START_REF] Garzanti | The modern Nile sediment system: Processes and products[END_REF].

equatorial latitude ( Nd ≈ -30), and the Saharan Metacraton located in the present Sahara [START_REF] Abdelsalam | The Saharan Metacraton[END_REF] and which is an important aeolian dust producer ( Nd ≈ -10 to -15).

IV.3 Materials and methods

The present study is based on the analysis of the MD04-2726 core (N 31°51.00', E 29°47.49') collected during the VANIL campaign ( 2004). The total MD04-2726 piston core length measures 26.5 m and is composed of 18 sections. Because of the good preservation of the hyperpycnite sequence and according to the aim of this and previous studies [START_REF] Revel | 20,000 years of Nile River dynamics and environmental changes in the Nile catchment area as inferred from Nile upper continental slope sediments[END_REF] we focused our investigation on the upper five sections (732 cm; Fig. IV.2e). The core was extracted on the right-hand levee of the upper Rosetta Channel at a water depth of 1058 m (Fig. IV.1a), and represents the closest core from the coastline to have been collected on the Nile margin, i.e. <60 km away from the Rosetta-river mouth. This core is characterized by high accumulation rates and thus provides high-resolution temporal information on the links between hydro-climate and erosion processes on continents. Identification of individual depositional beds was based on the integration of various methods including visual description at millimetre scale, grain-size, XRF-CS, Nd isotopic composition and thin-section analyses that are described below.

IV.3.1 AMS 14 C measurements

The age model of the MD04-2726 sequence was constructed using thirty Accelerator Mass Spectrometry (AMS) 14 C dates (Tab. IV.1). In order to improve the age model for core MD04-2726 [START_REF] Revel | 20,000 years of Nile River dynamics and environmental changes in the Nile catchment area as inferred from Nile upper continental slope sediments[END_REF], a set of 18 additional radiocarbon ages was obtained (Tab. IV.1, Fig. IV.2a). The scarcity of planktonic foraminifera shells in sections III-IV-V precludes the addition of new radiocarbon measurement on biogenic carbonates. We therefore obtained seven new AMS 14 C measurements on bulk organic matter (bulk OM) for this part of the core and 11 new AMS 14 C measurements of planktonic foraminifera shells in sections I-II-III (Fig. IV.2, Tab. IV.1). Calibrated ages were obtained with the OxCal software (v4.3.2, Bronk Ramsey, 2017), while the age-depth modelling was performed using the Bacon software (v3.6.3, [START_REF] Blaauw | Flexible paleoclimate age-depth models using an autoregressive gamma process[END_REF]. Conventional ages on foraminifera assemblages were calibrated using the MARINE13 calibration curve, which includes a mean global reservoir correction of 400 yrs [START_REF] Reimer | IntCal13 and Marine13 Radiocarbon Age Calibration Curves 0-50,000 Years cal BP[END_REF][START_REF] Siani | Mediterranean Sea Surface Radiocarbon Reservoir Age Changes Since the Last Glacial Maximum[END_REF]. Paired measurements of AMS 14 C of planktonic foraminifera and bulk OM on the same depth interval (658 cm, section V; Tab. IV.1) allows us to estimate the age reservoir correction of the continental OM. Inversely to classical methods to determine the local age reservoir for marine sediments [START_REF] Dewar | Late-Holocene marine radiocarbon reservoir correction (∆R) for the west coast of South Africa[END_REF][START_REF] Reimer | Marine radiocarbon reservoir corrections for the midto late Holocene in the eastern subpolar North Atlantic[END_REF][START_REF] Reimer | An Online Application for ∆R Calculation[END_REF], we used the calibrated age of planktonic foraminifera (to MARINE13, 9736 cal. yr BP) to determine the apparent age of contemporaneous organic matter (from IntCal13, 8766 cal. yr BP) and calculated the difference to the radiocarbon age of bulk OM (10620 yr BP). The calculated offset of 1854 yr was used as a constant reservoir age to correct all radiocarbon ages on organic matter (Tab. IV.1). Owing to the dominantly continental origin of bulk OM in sediments from the NDSF deposited during the African Humid Period [START_REF] Blanchet | Asynchronous Changes in Vegetation, Runoff and Erosion in the Nile River Watershed during the Holocene[END_REF][START_REF] Ménot | Timing and stepwise transitions of the African Humid Period from geochemical proxies in the Nile deep-sea fan sediments[END_REF], the ages on bulk OM and on organic macro-remains were calibrated using the IntCal13 curve [START_REF] Reimer | IntCal13 and Marine13 Radiocarbon Age Calibration Curves 0-50,000 Years cal BP[END_REF]. The reliability of the age model and inherent limitations will be further discussed in Section IV.4.1.

IV.3.2 XRF core scanner on soft sediment

X-Ray fluorescence core scanning (XRF-CS) was performed on an Avaatech core scanner at the EDYTEM laboratory (CNRS-Université de Savoie Mont-Blanc). The XRF-CS analyses were performed on core MD04-2726 with a 1 mm sampling step and with a 10 kV voltage (Al, Si, S, K, Ca, Ti, Mn, Fe) for sections I-II , and with a 10 KV and 30 kV voltage (Al, Si, S, K, Ca, Ti, Mn, Fe; Cu, Zn, Br, Sr, Rb, Zr, Pb) for sections III-IV-V. Each individual power spectrum was transformed by deconvolution into relative contents (intensities) expressed in counts per seconds (cps). In this study we principally used the logs of elemental ratios, following a well-established methodology to circumvent problems associated with the closed-sum constrains [START_REF] Croudace | Micro-XRF Studies of Sediment Cores, volume 17 of Developments in Paleoenvironmental Research[END_REF][START_REF] Weltje | Calibration of XRF core scanners for quantitative geochemical logging of sediment cores: Theory and application[END_REF]. A principal component analysis (PCA) was performed on geochemical results using the R® software. This statistical procedure uses an orthogonal transformation to convert a set of observations of possibly correlated variables (our elements) into a set of values of linearly uncorrelated variables called principal components (dimensions). This analysis has been thus applied to XRF results in order to decipher sedimentary processes controlling MD04-2726 geochemical variations (Fig. BI.1).

IV.3.3 Particle grain size analysis and LOI

Grain-size analyses were performed using a Coulter-LS2000 following two sampling strategies: a) a centimetre-scale sampling step over the total core length (733 cm); b) a millimetre-scale sampling step (2 mm) in selected individual beds identified using the mm-XRF major elemental variation and centimetreresolution grain-size distribution data. To avoid clay flocculation and to dissociate mineral particles, we used sodium hexametaphosphate (NaHMP) and ultrasonic treatment. Sediment was previously decarbonated (HCl, 1 mol), while the organic matter was maintained (no H 2 O 2 dissolution), considered as a grain-size marker of river-induced instantaneous deposits. SEM and light microscopic qualitative analyses on this section and on bulk decarbonised sediment (see Section III.3.2 for methodology) showed the near-absence of diatom frustules, which are thus considered as a negligible factor for grain-size measurements and interpretations. We determined the grain-size of the intercepts for 10%, 50% and 90% of the cumulative grain-size curves (named Q90, Q50 and Q10 values;[START_REF] Folk | Brazos River Bar: A Study in the Significance of Grain Size Parameters[END_REF]. The ratio Q90/Q10 is used to identify the coarser-grained beds (hyperpycnites) along the studied interval, while the Q90/Q50 ratio and Gaussian curves are used to discriminate between the bed types and to propose a hydrodynamic interpretation. Loss On Ignition (LOI) was performed on 2 g sediment samples by heating for 24 h at 100°C for residual water and gypsum content and then 4 h at 550°C for organic carbon total content measures [START_REF] Santisteban | Loss on ignition: A qualitative or quantitative method for organic matter and carbonate mineral content in sediments[END_REF]. v3.6.3, Blaauw and Christen, 2011) based on Marine13 and IntCal13 calibration curves [START_REF] Reimer | IntCal13 and Marine13 Radiocarbon Age Calibration Curves 0-50,000 Years cal BP[END_REF]. Bulk OC = Bulk Organic Carbon; * = new ages (this study).

IV.3.4 Light and Electron Microscopy Analyses

Microscopic analyses were performed on 11 thin sections sampled in selected individual beds (Fig.

IV.3). Thin sections were analysed using the Zeiss Axioskop 40 Pol/ 40 petrographic microscope with UV light used to detect phosphate materials. Pictures of total thin sections presented in this article were obtained from high-resolution transmission scanning in crossed (XPL) and direct polarized light (PPL). Qualitative and semi-quantitative geochemical analysis on uncovered thin sections was performed using scanning electron microscopy (SEM). SEM was also used for the surface textural analyses of quartz grains, following [START_REF] Vos | Surface textural analysis of quartz grains by scanning electron microscopy (SEM): From sample preparation to environmental interpretation[END_REF] sampling processing and analysis settings. SEM observations and Energy Dispersive X-Ray spectroscopy (EDX) analyses were carried out with a Tescan Vega3 XMU scanning electron microscope (TESCAN FRANCE, Fuveau, France) equipped with an Oxford X-MaxN 50 EDX detector (Oxford Instruments, Abingdon, U.K.). Thin sections were observed and analysed with no prior metal or carbon coating, using the low vacuum mode (N2 pressure ∼35 Pa) and at a 20 kV accelerating voltage. Imaging was performed using backscattered electrons. EDX spectra were processed with the Aztec software (version 3.1, Oxford Instruments). Surface textural analyses of quartz grains were performed on 10 to 20 grains for each of the beds identified in thin sections, with a grain-size range between 100 µm to 1 mm, randomly selected under a binocular microscope and then analysed using the SEM. Quartz grains were coated with platinum (3 nm) prior to SEM observations at a 5kV accelerating voltage.

IV.3.5 Nd radiogenic isotope analyses

Isotope ratios were measured on carbonate-free <63 µm particle size both on hemipelagic and turbidite deposits, using 100 mg of the dried aluminosilicate fraction. Chemical extractions for Nd isotopes were carried out at the Laboratory of Geoscience Montpellier. Samples were leached for 1 hour with 1N HCl to dissolve biogenic carbonates. The Fe-Mn oxyhydroxide fraction and organic carbon were removed using 10 ml 1M Hydroxylamine hydrochloride and 10 ml 5% H 2 O 2 , respectively [START_REF] Bayon | An improved method for extracting marine sediment fractions and its application to Sr and Nd isotopic analysis[END_REF]. After leaching, the samples were centrifuged and the supernatant discarded. The remaining sediment was washed three times with ultra-pure water, discarding the supernatant each time to remove the biogenic components and to eliminate any marine Nd isotope signal adsorbed on to the silt-clay fraction. The chemical separation of Nd included a first separation step using AG50W-X-8 cation exchange resin to collect rare earth elements followed by purification of Nd using HDEHP conditioned Teflon columns. Nd isotopes were measured using a Thermo Scientific Neptune Plus MC-ICP-MS from the AETE-ISO geochemical platform (OSU-OREME, Montpellier University). The 143 Nd/ 144 Nd isotopic compositions were corrected for internal mass bias using an exponential law and a value of 0.7219. The external mass bias was corrected using the standard bracketing method using AMES-Rennes and analysing each of the 2 unknowns. During the course of the study AMES-Rennes standards yielded an average 0.511946±6 (2σ, n=10) for the 143 Nd/ 144 Nd ratio [START_REF] Chauvel | A hafnium isotope and trace element perspective on melting of the depleted mantle[END_REF]. The Nd procedural blank measured during the course of this study was 25 pg. Epsilon Nd values ( Nd) were calculated using 143 Nd/ 144 Nd = 0.512638 [START_REF] Bouvier | The Lu-Hf and Sm-Nd isotopic composition of CHUR: Constraints from unequilibrated chondrites and implications for the bulk composition of terrestrial planets[END_REF].

IV.4 Results

IV.4.1 Age model, lithology and sedimentation rates

According to the age model, the upper 7.3 m of core MD04-2726 span the last 10 kyr, covering the Holocene period (Fig. IV.2a). Both the range (between 0.2 and 3.5 mm/yr) and the timing of changes in sedimentation rates are coherent with previous reports from nearby archives [START_REF] Blanchet | High-and low-latitude forcing of the Nile River regime during the Holocene inferred from laminated sediments of the Nile deep-sea fan[END_REF][START_REF] Revel | 21,000 Years of Ethiopian African monsoon variability recorded in sediments of the western Nile deep-sea fan[END_REF][START_REF] Revel | 20,000 years of Nile River dynamics and environmental changes in the Nile catchment area as inferred from Nile upper continental slope sediments[END_REF][START_REF] Hennekam | Integral view of Holocene precipitation and vegetation changes in the Nile catchment area as inferred from its delta sediments[END_REF]. A large variability in radiocarbon ages (including several reversals) is observed in sections III-IV-V, where the highest sedimentation rates are computed (>1 mm/yr). This is probably related to unaccounted changes in the reservoir age of bulk OM through time, due to varying proportion of terrestrial and marine OM in the sediments as well as to the incorporation of terrestrial OM of varying ages. Complex interplay between storage and erosion processes in fluvial basins can indeed lead to the release of pre-aged OM as old as 2000 yrs, as shown by [START_REF] Schefuß | Hydrologic control of carbon cycling and aged carbon discharge in the Congo River basin[END_REF] for the Congo River. A reservoir age of ∼1200 yr has been obtained for a layer dated at ca. 9 ka BP in nearby core P362/2-33 (unpublished results, C. Blanchet), which is 650 yrs younger than the reservoir age of ∼1850 yrs obtained for core MD04-2726 at ca. 9.8 ka BP. We therefore consider that uncertainties on radiocarbon ages obtained on bulk OM associated with likely variations in reservoir age through time are in the order of ∼500 yrs. For all other ages, uncertainties range between ∼30 and ∼100 yr, see Tab. IV.1). Accounting for these inherent limitations and for the generally lower temporal resolution of regional records, we limit our interpretations to centennial-scale changes.

Based on the lithology, four sedimentological intervals were identified in core MD04-2627, depicted here from bottom to top. The first interval lies between 732 and 700 cm and is dated between ∼10.3 to ∼10.1 ka BP (Fig. IV.2a). It is composed of muddy non-laminated sediment (Fig. IV.2e). The second interval ranges from 700 to 132 cm and is dated between ∼10.1 and ∼7.2 ka BP. It is characterized by sandy-silt to silty-mud turbidites and laminated hemipelagic muddy beds, with sedimentation rates between ∼1 and ∼3.5 mm/yr. The total carbonate content never exceeds 10%. The whole interval corresponds to the Sapropel S1, formed under anoxic bottom-waters conditions which circumventing the bioturbation [START_REF] Thomson | Review of recent advances in the interpretation of eastern Mediterranean sapropel S1 from geochemical evidence[END_REF]. Thus we observe a high organic content (8-9%; ). It represents the transition from sapropelic to pelagic conditions with no turbidite beds, corresponding to the increase of deep and intermediate water ventilation in the Eastern Mediterranean during the termination of the AHP. The fourth interval corresponds to the upper 70 cm and spans the last 5.8 kyr. A reduction of S/Cl [START_REF] Revel | 20,000 years of Nile River dynamics and environmental changes in the Nile catchment area as inferred from Nile upper continental slope sediments[END_REF] and two peaks of Mn/Al ratio at 6.8 and 4.3 ka BP mark the reestablishment of Mid-Holocene oxic conditions (ig 2d; De [START_REF] De Lange | Synchronous basin-wide formation and redox-controlled preservation of a Mediterranean sapropel[END_REF]. This interval has a sedimentation rate of ∼0.15 mm/yr (Fig. IV.2a) and consists of yellowish-brown bioturbated pelagic mud devoid of turbidite beds with a great number of planktonic foraminifera.

IV.4.2 Nd isotopic composition

In core MD04-2726, Nd values measured on the <63µm silicate fraction range between -2 and -6 and three isotopic trends are distinguished (Fig. IV.2b). Values of Nd between -2.5 and -4 are observed between 700 and 400 cm, indicating a larger proportion of particles derived from the Ethiopian Traps. These are followed by values of Nd between -4 and -5 observed between 400 and 70 cm.

IV.4.3 Description of T1 and T2 event-layers

The integration of sedimentological and geochemical data allows the identification at millimetre-scale of 102 coarser grain-size beds within the sapropelic interval (∼7.2 to ∼10.1 ka BP; Together with an inverse grading, Facies I exhibits a gradual upward increase of the Ti/K ratio (Fig. IV.4a). Facies II is characterized by a sharp basal contact, the disappearance of clay and fine-silt fractions and an increase in grain-size (Fig. IV.4a). Horizontal parallel laminations are present, with a coarsening-up pattern. Facies II is also characterized by high peaks in Ti/K and Mn/Al ratios, associated with higher Q90/Q10 values. The transition between Facies II and III is gradual. Facies III is the coarsest interval of the T1-type beds. It consists of laminated and cross-laminated intervals (Fig.

IV.4.3.b T2 event-layer description

The beds of type T2 (0.2 to 2 cm thick) are either massive or laminated silty to very-fine sand intervals (Fig. IV.5a,b,c), with a sharp basal contact, and are not composed of a superposition of subsequent facies. The clay fraction is abundant, while the coarse-grained fraction is well sorted. In contrast to T1 event-layers, the few organic-matter fragments follow random organisation and orientation patterns, while rare foraminifera and coccolithophore shells are fragmented (Fig. IV.5c). Within the whole sapropelic laminated interval (∼7 to 10.5 ka BP), beds of type T2 are characterized by more or less pronounced Q90/Q10, Ti/K peaks and by the lack of or very low Mn/Al peaks (Figs. IV.3b, IV.6b). All these characteristics suggest a mixed material probably caused by reworked sediments. The plot of Q90 versus Q50 is mainly comprised between 60 µm (Q90max) and 40 µm (Q50; Fig. IV.6b, c) with a grain-size distribution similar to T1α beds (bimodal with a main mode around 20-30 µm).

IV.5 Interpretations and Discussion

Recurrent Nile exceptional floods trigger high detrital inputs in the Rosetta channel-levee system, resulting in the frequent occurrence of turbidites during the AHP (Fig. IV.3). Using a multi-proxy method, we are able to distinguish between deposits resulting from "classical" landslide-triggered turbidity currents and hyperpycnal flows (Sec 5.1). We present an estimation of the frequency of T1 hyperpycnal eventlayers in core MD04-2726 which is compared with two well-dated sediment intervals already identified in cores from the Rosetta System (Sec. IV.5.2). Finally, from this comparison, we validate the reliability of the Nile flood frequency reconstruction at the scale of the Rosetta System and we propose a robust index of the exceptional flood frequency of the Nile River during the AHP (IV.5.3). We then discuss the timing and pattern of this flood frequency in comparison with hydro-climatic data from the Nile basin and East Africa.

IV.5.1 T1 and T2 event-layers: interpretation and formation processes

The full T1 event-layers consist of the superposition of five intervals with different types of sedimentary structure and an inverse then normal grading (Fig. IV.4). In a turbidite system, superposition of an inverse then a normal grading reflects deposition by a waxing (accelerating) then waning (decelerating) flow. First, waxing flows can be generated both by an increasing slope angle and/or by an increasing entrainment of seafloor sediment. Because T1 event-layers were recovered from an almost-flat levee (Figs. IV.1a, IV.7b) on which overflow energy should be low, these two hypotheses of waxing flow formation processes are unlikely. Second, waxing flows can be generated at a river mouth during severe floods, such as hyperpycnal flows (Mulder et al., 2001). In this case, the mixed grading together with occurrence of sedimentary structures should record the rapid succession over time of a depletive waxing flow generated during river rise, and a depletive waning flow generated during river fall [START_REF] Migeon | Detailed seismic-re¯ection and sedimentary study of turbidite sediment waves on the Var Sedimentary Ridge (SE France): Signi®cance for sediment transport and deposition and for the mechanisms of sediment-wave construction[END_REF]Mulder et al., 2001). T1 event-layers are similar in both grading and sedimentary structures to hyperpycnites first described in the Var Turbidite System [START_REF] Migeon | Detailed seismic-re¯ection and sedimentary study of turbidite sediment waves on the Var Sedimentary Ridge (SE France): Signi®cance for sediment transport and deposition and for the mechanisms of sediment-wave construction[END_REF]Mulder et al., 2001).

Additional observations reinforce the flood-related hypothesis for T1 layers. The fluvial origin of T1 layers is supported by both sedimentological and geochemical tracers. First, quartz grains show small to large fresh conchoidal microfractures and V-shaped percussion cracks, typical of fluvial transfer processes (Fig. IV.5c;[START_REF] Vos | Surface textural analysis of quartz grains by scanning electron microscopy (SEM): From sample preparation to environmental interpretation[END_REF]. Furthermore, Nd values from T1 event-layers (-3 to -2) are systematically higher than corresponding hemipelagic sediments (Fig. et al., 2020). Higher Ti/K ratios in T1 layers also support a higher contribution of basaltic rocks from the Ethiopian Traps, with primary minerals enriched in Ti, while secondary clay minerals are enriched in K [START_REF] Croudace | Micro-XRF Studies of Sediment Cores, volume 17 of Developments in Paleoenvironmental Research[END_REF]. The good correspondence between Ti/K and Q90/Q10 ratios suggests that the Ti/K ratio traces the silt versus clay fraction (Fig. IV.3b). In addition to coarser volcanic-derived particles, T1 event-layers contain large non-weathered mica minerals (> 150 µm, Fig. IV.4b,c), which suggests a rapid terrigenous transfer from Blue Nile sources, potentially from the Dabus, Didesa (Blue Nile Basin) and Gash (Atbara basin) rivers (Fig. IV.1b;[START_REF] Garzanti | The modern Nile sediment system: Processes and products[END_REF].

Owing to this converging array of evidence, we propose that T1 event-layers represent the successive deposition of hyperpycnal flows overflowing on the right-hand levee of DSF6 (Fig. IV.7) during exceptional and near-instantaneous flooding events, which rapidly transferred sediments eroded in the Ethiopian Highlands to the western NDSF. This depositional model can even explain the difference observed between T1α and T1β layers (Figs. IV.6, IV.6): during the low energy flood-induced phase, thin hyperpycnal flows are constrained at the channel floor, producing a graded-suspended sedimentation restricted to the levee crest (T1α). During the highest magnitude flood-induced flow phase, thick hyperpycnal flows expand within the whole channel conduit and overflow the levee crest, generating tractive processes that bring the coarsest grains over the levee area (T1β; Fig. IV.7). In T1 event-layers, an increase of Mn/Al was systematically detected (Figs. IV.3,IV.4a,IV.6b). This enrichment in Mn might be related to the delivery of oxygen to the bottom waters by hyperpycnal flows (oxic freshwater front) that could be trapped in the porosity of the T1 beds. Mn would then be oxidized and precipitated in the form of Mn-oxyhydroxides (e.g. [START_REF] Deflandre | Early diagenetic processes in coastal marine sediments disturbed by a catastrophic sedimentation event[END_REF][START_REF] Torres | Early diagenetic processes generate iron and manganese oxide layers in the sediments of Lake Baikal, Siberia[END_REF][START_REF] Wilhelm | Frequency and intensity of palaeofloods at the interface of Atlantic and Mediterranean climate domains[END_REF]. Thus, we use here Mn/Al peaks as a marker of flood-induced deposit.

Regarding the T2 deposits, the presence of non-oriented organic matter, the more or less pronounced Ti/K ratios and the Q90/Q10 peaks suggest a continental origin, but the systematic absence of Mn/Al peaks (Figs. IV.3, IV.5a, IV.6b) also indicate a lack or lower amounts of fresh-water input. We propose that normally-graded T2 layers result from low-energy turbidity currents generated by landslides triggered at the Rosetta-Canyon head that reworked terrigenous and biogenic deposits from the outer continental shelf (Fig. IV.5c). This hypothesis seems to be confirmed by Q90/Q50 values in T2, which are consistent with uniform suspension processes resulting from a mixing of suspended material (Fig.

IV.6b, c). T2 beds will thereafter be excluded from the climatic interpretations.

IV.5.2 Coherence of Nile flood records over the upper slope of the Rosetta System

The frequency and the Nd isotopic signature of the hyperpycnal deposits (T1 event-layers recorded in MD04-2726 core) and clastic-mud beds (recorded in MS27PT core; upper slope. A similarity in elemental composition (increase of Ti/K and Mn/Al; Fig. BI.3) a temporal coherence can be observed between the T1 event-layers and the clastic-mud deposits recorded in core MS27PT (Fig. IV.8d, f;[START_REF] Revel | 20,000 years of Nile River dynamics and environmental changes in the Nile catchment area as inferred from Nile upper continental slope sediments[END_REF]. During strong floods, two types of processes can be generated synchronously: a low-concentrated hypopycnal plume at the sea surface and a high-concentrated hyperpycnal flow plunging at the bottom [START_REF] Mulder | Marine hyperpycnal flows: Initiation, behavior and related deposits. A review[END_REF][START_REF] Mulder | The physical character of subaqueous sedimentary density ¯ows and their deposits[END_REF]. During exceptional Nile floods, we therefore suggest that the resultant hyperpycnal flows are trapped within the Rosetta Canyon and channels and form T1 event-layers on levees (MD04-2726 core site). Low-density hypopycnal plumes spread at the sea surface while their fine-grained suspended particles are affected by decantation and convection processes along the water column, resulting in the deposition of clasticmud beds over more than 30 km on the continental slope (MS27PT and PS3622-33core sites; Fig. IV.7;[START_REF] Ducassou | Evolution of the Nile deep-sea turbidite system during the Late Quaternary: Influence of climate change on fan sedimentation[END_REF]. The exceptional nature of the T1α and T1β floods, occurring at an average of 1 to 7 events every 100 yr (Fig. IV.8b), suggests an intermittent high-energy hydrological activation. As described in detail in paragraph IV.5.1, T1β could indicate higher-energy floods than T1α event-layers.

IV.5.3 Holocene hydro-climatic shifts over the Blue Nile basin

Our results document the frequency of exceptional (high-energy) Ethiopian Nile flood events during the Early and Middle Holocene ( 10.1 to 7.3 ka BP; Fig. IV.9e). This multi-centennial paleo-flood chronicle seems to be uniformly recorded in two piston-cores as hyperpycnal-flow deposits and clastic-mud beds over the proximal Rosetta System (western NDSF; Fig. IV.8). We compared this flood frequency record with the precipitation-induced runoff activity recorded at Lake Tana, as derived from the Ti concentration profile (Figs. IV.1b, IV.9g; [START_REF] Marshall | Late Pleistocene and Holocene drought events at Lake Tana, the source of the Blue Nile[END_REF], and with the K and Ti events recorded at Lake Dendi (Figs. IV.1b, IV.9f;[START_REF] Wagner | Holocene rainfall runoff in the central Ethiopian highlands and evolution of the River Nile drainage system as revealed from a sediment record from Lake Dendi[END_REF] interpreted as an intense erosion and rainfall runoff at the head of the Blue Nile Basin. Precise correlation between single runoff and flood events would be hazardous because of the chronological uncertainty when comparing regional archives. However, general trends of the selected proxies seem to be coherent at centennial-scale.

Fluctuations of radiogenic Nd values in both T1 layers and hemipelagic sediments in all cores from the western NDSF (Fig. IV.9c, d, for MD04-2726 andMS27PT and[START_REF] Blanchet | Asynchronous Changes in Vegetation, Runoff and Erosion in the Nile River Watershed during the Holocene[END_REF], for P362/2-33)reflect changes in the proportion of eroded material from Ethiopian Traps. Similar source-to-sink flood/runoff trends, combined with the high radiogenic Nd signature of T1 event-layers and clastic mud deposits (Fig. IV.9c, IV.9d) demonstrate the capacity of the Nile drainage system to rapidly connect the Ethiopian Traps source to its deep delta, and discredit the hypothesis of autogenic sediment transport anomalies (non-linear geomorphic system; [START_REF] Jerolmack | Shredding of environmental signals by sediment transport: SIGNAL SHRED-DING[END_REF]. Decrease in Nd values could be explained by a higher contribution from the Equatorial-White Nile (African craton Nd = -30) or from the lower-course White/Blue Nile sources (Saharan Metacraton imprint; [START_REF] Abdelsalam | The Saharan Metacraton[END_REF][START_REF] Grousset | Tracing dust sources and transport patterns using Sr, Nd and Pb isotopes[END_REF][START_REF] Skonieczny | The 7-13 March 2006 major Saharan outbreak: Multiproxy characterization of mineral dust deposited on the West African margin[END_REF]. However, the Early to Mid-Holocene relative elevated εNd values of the silt fraction show that the proportion of Ethiopian Traps material remained high even during more arid periods (such as the 8.2 ka event; Nd = -4; This temporal coherency in runoff activity along the Nile River drainage basin suggests strong erosion dynamics from the source to the sink of the fluvial system. This strong erosional activity during the Early Holocene occurs during an interval of intense monsoonal precipitations recorded from 10 ka BP onwards by low and constant δDwax values at Lake Tana (Fig. IV.9b;[START_REF] Costa | Isotopic reconstruction of the African Humid Period and Congo Air Boundary migration at Lake Tana, Ethiopia[END_REF],Lake Victoria (Berke et al., 2012a) and by stable high water-levels of East African lakes [START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF][START_REF] Junginger | The effects of solar irradiation changes on the migration of the Congo Air Boundary and water levels of paleo-Lake Suguta, Northern Kenya Rift, during the African Humid Period (15-5ka BP)[END_REF]. We therefore propose the occurrence of enhanced erosional activity within the whole drainage basin of the Nile River during the Early Holocene, largely related to high and stable monsoonal rainfall on the Ethiopian Highlands. Ménot et al., 2020, this study); e) Q90/Q50 and frequency (100-yr running average, brown line) of exceptional flood deposits (T1α in yellow and T1β in green) of the MD04-2726 core; f) erosion runoff-induced K and Ti event layers from Lake Dendi [START_REF] Wagner | Holocene rainfall runoff in the central Ethiopian highlands and evolution of the River Nile drainage system as revealed from a sediment record from Lake Dendi[END_REF]; g) Ti (mg/g) at Lake Tana [START_REF] Marshall | Late Pleistocene and Holocene drought events at Lake Tana, the source of the Blue Nile[END_REF]; h) Blue (in blue) and White (in yellow) lower Nile course flood periods recorded over the Karthoum region [START_REF] Williams | Late Pleistocene and Holocene environments in the Nile basin[END_REF][START_REF] Williams | Late Quaternary environments and prehistoric occupation in the lower White Nile valley, central Sudan[END_REF]. Light grey bands indicate the lower flood frequency recorded in MD04-2726 core.

IV.5.3.b Stepwise Early Holocene hydro-climatic degradation from 9 ka BP

From 9 ka BP onwards, Nd values of hemipelagic sediment and Ti layers from cores MD04-2726 and MS27PT start to decrease following the progressive decrease in frequency of T1 event-layers and of sedimentation rates in cores MS27PT and P362/2-33 (Fig. . A lower frequency of T1α layers, rare T1β event-layers and less radiogenic Nd values indicate that Nile hydrologic activity had reduced very gradually from 9 until 7.8 ka BP, with an emphasis of reduction between 8.2 and 7.8 ka BP (Fig.

IV.9c-e).

After a short-lived drop in flood frequency at 9 ka BP, the thickness of T1 event-layers and clasticmud beds slightly decrease, indicating a reduction of flood particle discharge and/or shorter-lasting floods (Fig. IV.8a) while the higher Q90/Q50 ratio of T1 event-layers points to reduced flow energy (Fig. IV.8b). The combination of reduced flood particle discharge, weakened transport energy and decreasing flood frequency suggests a switch of the Nile River hydrological regime from episodic highenergy flood discharge dynamics (between 10 and 9 ka BP) to low-energy and shorter-lasting flood discharge dynamics (after 9 ka BP). This progressive shutdown of flood frequency and intensity, with more negative Nd, culminates between 8.2 and 7.8 ka BP, suggesting an 800 year-long stepwise Early Holocene aridification interval (Fig. IV.9).

A distinct interruption of the Blue and White Nile flooding events was also recorded around 9 ka BP in the Khartoum region (Fig. IV.9h;[START_REF] Williams | Late Pleistocene and Holocene environments in the Nile basin[END_REF], in phase with an interruption of rainfall-induced runoff events at Lake Dendi (Fig. IV.9f;[START_REF] Wagner | Holocene rainfall runoff in the central Ethiopian highlands and evolution of the River Nile drainage system as revealed from a sediment record from Lake Dendi[END_REF]. Similarly, at Lake Tana a slight reduction in precipitations is evidenced by δDwax values around 9 ka BP (Fig. IV.9b;[START_REF] Costa | Isotopic reconstruction of the African Humid Period and Congo Air Boundary migration at Lake Tana, Ethiopia[END_REF] interpreted as reflecting either a southward retreat of the monsoonal rain belt or an eastward shift of the Congo Air Boundary. Changes in erosion and monsoonal dynamics as early as 8.7 ka BP have already been pointed out by previous studies [START_REF] Blanchet | Asynchronous Changes in Vegetation, Runoff and Erosion in the Nile River Watershed during the Holocene[END_REF][START_REF] Ménot | Timing and stepwise transitions of the African Humid Period from geochemical proxies in the Nile deep-sea fan sediments[END_REF]. Subsequently, around 8.2 to 8 ka BP, a period of marked aridification was reported elsewhere in North East Africa as a single event [START_REF] Costa | Isotopic reconstruction of the African Humid Period and Congo Air Boundary migration at Lake Tana, Ethiopia[END_REF][START_REF] Garcin | East African mid-Holocene wet-dry transition recorded in palaeo-shorelines of Lake Turkana, northern Kenya Rift[END_REF][START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF][START_REF] Marshall | Late Pleistocene and Holocene drought events at Lake Tana, the source of the Blue Nile[END_REF]. A classical scenario suggests that high latitudes fully drove low-latitude hydrology at millennial timescales [START_REF] Collins | Rapid termination of the African Humid Period triggered by northern high-latitude cooling[END_REF]. The onset of aridification in the Nile catchment would then result from the release of freshwater in the North Atlantic during the 8.2 ka BP event, leading to a slowdown of the Atlantic Meridional Overturning Circulation (AMOC) and/or a decrease in arctic air temperatures which induces a decrease on the Tropical Easterly Jet [START_REF] Collins | Rapid termination of the African Humid Period triggered by northern high-latitude cooling[END_REF]. These processes, together with a reduction in summer insolation, might have led to drier conditions in tropical monsoonal areas. However, this scenario does not account for a destabilization of the monsoonal system as early as 9 ka BP, as seen in our record. The observed time lag between the beginning of the hydrologic shift in East Africa at 9 ka BP and the North Atlantic 8.2 ka BP cooling event casts doubt on the mechanisms linking highand low-latitude hydrological changes [START_REF] Collins | Rapid termination of the African Humid Period triggered by northern high-latitude cooling[END_REF][START_REF] Mohtadi | Palaeoclimatic insights into forcing and response of monsoon rainfall[END_REF]. We propose here that an early shift of the main climatic boundaries above the Ethiopian Highlands [START_REF] Costa | Isotopic reconstruction of the African Humid Period and Congo Air Boundary migration at Lake Tana, Ethiopia[END_REF] linked with the gradual decrease in summer insolation might have been sufficient to initiate a drier period in the tropics between 9 and 8 ka BP. Accordingly, recent modelling experiments suggest that an obliquity-driven change in insolation gradient at low latitudes might have directly enhanced monsoonal rainfall [START_REF] Bosmans | Obliquity forcing of low-latitude climate[END_REF][START_REF] Mantsis | The Response of Large-Scale Circulation to Obliquity-Induced Changes in Meridional Heating Gradients[END_REF]. Consequently, the insolation on the tropics could have had largely contributed in generating air moisture masses over the oceans without the influence of the indirect energy-balance inputs from high-latitude internal forcing and from North Atlantic icesea interactions (Bosmans et al., 2015b,a;[START_REF] Clement | The Atlantic Multidecadal Oscillation without a role for ocean circula-tion[END_REF]. Finally, has been even argued that the high-latitude cooling events would be further enhanced by the decrease in African precipitation and by Saharan aridification, which may have contributed to a tipping point behaviour between low and high latitudes [START_REF] Clement | An Orbitally Driven Tropical Source for Abrupt Climate Change[END_REF][START_REF] Davies | The impact of Sahara desertification on Arctic cooling during the Holocene[END_REF].

IV.5.3.c Unstable and anomalous Nile hydrologic conditions from 7.8 ka BP

The low Nd values observed from 7.8 ka BP in both T1 layers and hemipelagic sediments in all cores from the western NDSF (Fig. IV.9c, d;for MD04-2726 andMS27PT and[START_REF] Blanchet | Asynchronous Changes in Vegetation, Runoff and Erosion in the Nile River Watershed during the Holocene[END_REF], for P362/2-33) indicate a lower contribution of Ethiopian Trap detrital particles suggesting a White Nile or lower-course White and Blue Nile source contributions, characterized by a Saharan aeolian Metacraton imprint ( Nd from -10 to -14; [START_REF] Abdelsalam | The Saharan Metacraton[END_REF][START_REF] Grousset | Tracing dust sources and transport patterns using Sr, Nd and Pb isotopes[END_REF][START_REF] Scheuvens | Bulk composition of northern African dust and its source sediments -A compilation[END_REF]. After low flood activity around 8 ka BP, T1 event-layers resume from 7.8 ka BP onwards, with a frequency of five event-layers/100 yr, thereby characterizing higher particle discharge (increase in T1 thickness; [START_REF] Costa | Isotopic reconstruction of the African Humid Period and Congo Air Boundary migration at Lake Tana, Ethiopia[END_REF]. The occurrence of higher flood frequency and higher particle discharge during a lower rainfall period shows a decoupling between erosive activity and climatic forcing, suggesting the development of an "out of equilibrium" geomorphic system (i.e., the sedimentary input into the system is not equal to the system output, [START_REF] Ahnert | Equilibrium, scale and inheritance in geomorphology[END_REF][START_REF] Hoffmann | Sediment residence time and connectivity in non-equilibrium and transient geomorphic systems[END_REF]. This anomaly could be explained by a process during the entire 8.2-7.8 ka BP arid period whereby low precipitation favoured the fluvial riverbed aggradation (i.e., storage effect; [START_REF] Phillips | Sources of nonlinearity and complexity in geomorphic systems[END_REF]. The subsequent slight resumption of precipitation post-7.8 ka BP (reduced δD W AX at Lake Tana, Fig. IV.9b, f) led to a rapid erosion of the previously deposited sediment with low radiogenic Nd values along the Nile course, producing a dense plume at the river mouth and the large amount of T1 event-layers. This is supported by other geomorphological data, which indicate the resumption of White Nile floods in the Khartoum region from 7.8 ka BP, in phase with the numerous low radiogenic T1 event-layers (Fig. IV.9h, e;[START_REF] Williams | Late Pleistocene and Holocene environments in the Nile basin[END_REF][START_REF] Williams | Late Quaternary environments and prehistoric occupation in the lower White Nile valley, central Sudan[END_REF]. Following this scenario, drastic reduction in vegetation cover over the Saharan Nile and the Sahelian belt [START_REF] Hopcroft | Multi vegetation model evaluation of the Green Sahara climate regime: RAINFALL SUPPORT-ING A GREEN SAHARA[END_REF] during the 8.2 ka BP event could have accentuated this process with an enhanced soil erosion. Our results therefore show how at the end of abrupt arid climatic events, such as those occurring around 8.2-7.8 ka BP, an "out of equilibrium" fluvial effect may occur, characterized by a disproportionate sediment delivery to the continental margins and resulting in an autogenic sedimentation process relatively decoupled from the climatic forcing. From 7 ka BP, re-oxygenation of bottom waters in the Levantine area mark the end of S1 [START_REF] De Lange | Synchronous basin-wide formation and redox-controlled preservation of a Mediterranean sapropel[END_REF] and the end of the hyperpycnite-prone interval due to resumed bioturbation in MD04-2726 and MS27PT cores (Fig. IV.1d, 1e). The input of Blue Nile suspended material, as well as the flood energy, decreased progressively until 4 ka BP, thus preventing the formation of high dense plume capable to induce hyperpycnal flows. Accordingly, the lowest Nd and Ti/K values were measured from hemipelagic sediment Fig. IV.1b,c), corresponding to the 4.2 ka BP drought event [START_REF] Bini | The 4.2\,ka\,BP Event in the Mediterranean region: An overview[END_REF].

IV.6 Conclusion

This study presents the first continuous reconstruction of exceptional Nile floods at centennial resolution from a well-preserved sedimentary interval from the Nile Deep-Sea Fan during the Early to the Middle Holocene. A multiproxy approach using sedimentological, geochemical (elemental contents and Nd isotopes) and microscopic methods have allowed us to identify instantaneous deposits (as hyperpycnites and clastic-mud beds) related to exceptional floods from the Blue Nile recorded in two cores from the upper slope of the Rosetta System. Similar trends between the occurrence of flood layers at centennial scale and changes in terrigenous input at Lake Tana (the source of the Blue Nile under direct influ-ence of the African summer monsoon), as well their Nd signature, suggest their direct relationship to high-intensity and exceptional monsoon-related erosive and runoff events in the Ethiopian Highlands.

Our results enable us to draw some hydro-climatic inferences, in particular: a) the occurrence of recurrent (5 to 7/100 yr) high energy instantaneous floods between 10.1 and 9 ka BP during the height of the African Humid Period; b) a noticeable shift in the hydro-climatic regime around 9 ka BP characterized by a progressive reduction of Blue Nile input and by lower frequency and magnitude of floods until 8.2 ka BP; c) a low precipitation regime from 8.2 ka BP expressed in very low river runoff; d) the installation of an unstable and reduced Ethiopian-Nile hydrological regime from 7.8 ka BP where erosive activity and sediment transport is decoupled from climatic forcing, followed by a gradual reduction in precipitation intensity until 4 ka BP.

In conclusion, we highlight the occurrence of a stepwise aridification process during the Early Holocene in the Blue Nile basin that questions the climatic linkages and feedbacks between low and high latitudes around the 8.2 ka BP North Atlantic cooling event. We also show how hyper-arid climatic episodes such as the 8.2 ka BP event might produce a destabilisation of the fluvial geomorphic system, inducing an anomaly of sediment transfer and the consequent autogenic sediment recording over the continental margin basins.

Chapter V Impact of Holocene hydro-climatic changes on continental geomorphic systems: the case of the Lake Abhe Basin, Central Afar region (Ethiopia & Djibouti)

V

Key points

The first part of this work (Section V.1) was submitted to the journal Geomorphology (Mologni et al., reviewed in December 20201 ), with the title "Holocene East African monsoonal variations recorded in wave-dominated clastic paleo-shorelines of Lake Abhe, Central Afar region (Ethiopia and Djibouti)". The results of this chapter have been carried out from 6 fieldwork missions along the lake Abhe basin, in the Central Afar Region (Ethiopian and Djibouti) between 2018 and 2020. The interest and the choice of this study area have been based on the previous research on the Lake Abhe area provided by F. Gasse and collaborators during the 1970s. This chapter documents and describes more than one hundred geomorphic littoral lacustrine features in the Lake Abhe basin. A first comprehensive characterization of these features and their specificity within the Lake Abhe basin have allowed to define a wave-dominated clastic paleoshoreline sequence. The paleo-lake highstands were chronologically controlled with new and previous 51 radiocarbon ages on stromatolites and lacustrine nearshore deposits (radiocarbon ages on gastropods) linked to these paleo-shorelines sequences, which have allowed to propose a revised curve of Lake Abhe fluctuations during the Holocene. The lake attained its Maximum Holocene Highstand Shoreline between ∼10.5 and ∼9 ka BP followed by a progressive stepwise regression linked to two lake overflow-paleo-sills incision. The Lake Abhe dropped two times during the Early to Middle Holocene, between ∼8.4 and ∼7.7 ka cal. BP and finally at ∼4.5 ka cal. BP. The tectonic control of these fluctuating lake levels has been taken into account with an estimation of the subsidence rate of the Lake Abhe graben during the Holocene. Similarly, fluvial transient connectivity processes with other Lake Abhe headwater basins has been constrained, evidencing the role of the earth surface processes on the paleo-lake hydro-balance variations.

Comparison with other paleo-hydrological data from the East African Rift lacustrine systems have allowed to discuss the regional climatic variations the over the last 11 ka. Additionally, the characterization and interpretation of littoral paleosoils have permitted to evaluate the impact of monsoonal-induced hydrological oscillation on the Lake Abhe pedological systems during the last ∼13 ka (Section V.2). These results improve our understanding of the Late Pleistocene to Holocene littoral environments on this area (to respond to issues presented in Section II.1.4) and permit to discuss the spatio-temporal patterns of the hydro-climatic changes in East Africa during the Holocene (to respond to issues presented in Section II.2.2).

V.1 Holocene East African monsoonal variations recorded in wave-dominated clastic paleo-shorelines of Lake Abhe, Central Afar region (Ethiopia & Djibouti)

Abstract

During the Late Quaternary, tropical Africa was characterized by strong hydro-climatic fluctuations principally driven by precessional forcing. The 'African Humid Period' (AHP; ∼14.8 -∼5.5 ka BP) was the last cycle of enhanced precipitation punctuated by dramatic short-term arid events before being replaced by drier conditions. The timing and the magnitude of the AHP hydrological dynamics are not yet fully understood due to the extreme variability in African geomorphic patterns and the complex network of past air convergence boundary shifts. Lake Abhe is an endorheic basin situated in the Afar Depression at the northern extremity of the East African Rift System (EARS). Located at the boundary of the Red Sea and the Indian and Atlantic Oceans' air moisture mass fluxes and deriving its source in the Ethiopian Highlands, the Lake Abhe watershed represents a system sensitive to changes in Northeast African hydro-climate. Its sedimentary archives have allowed us to reconstruct these changes and to better understand the mechanisms of regional internal hydro-climatic forcing responsible for past and present droughts. Across Lake Abhe's ∼200 km-wide basin, more than one hundred littoral lacustrine geomorphic features were defined and documented for the first time and chronologically controlled with 52 related radiocarbon ages measured on stromatolites and nearshore deposits. By analysing data we define the main Holocene lake highstands and propose a detailed paleo-shoreline record in order to more precisely reconstruct Lake Abe fluctuations during the duration of the Holocene. The first rise in water level is dated to ∼11.1 ka cal. BP, while the Maximal Holocene Highstand Shoreline (∼420 m a.s.l.) was rapidly reached around ∼10 ka cal. BP. Evidence of the impact of the 8.2 ka North Atlantic cooling event on tropical hydrographic systems is demonstrated with slow lake level regression to ∼375 m a.s.l. until ∼8.3 ka cal. BP and an abrupt drop to ∼250 m a.s.l. at ∼8 ka cal. BP. The resurgence of humid conditions was recorded from ∼7.7 ka cal. BP (∼380-385 m a.s.l. water level), until ∼4.6 ka cal. BP, were the absence of littoral deposits suggests a drastic drop. Finally, the lake level rose gently from 2.8 ka cal. BP until the maximal Late Holocene level (330 m a.s.l.) at ∼1.5 ka cal. BP. Radiocarbon dates on littoral materials have permitted us to propose a tectonic-controlled Holocene subsidence rate of the Abhe basin axial valley, and a more solid model of lake hydrographic fluctuations. Our results show a delayed Mid-Holocene aridification of the Lake Abhe system relative to other southern EARS lakes, providing evidence for its incoherence with the southward migration of the rain belt scenario. The persistence of humid conditions in the northern EARS could be explained by the high atmospheric pressure gradient between the East African Monsoon and the Indian Summer Monsoon (ISM) which would have led to an enhanced influence of the Congo Air Boundary (CAB) moisture masses into the Rift region. Belated Mid-Holocene precipitation in the Ethiopian Highlands would have ensured lake highstands and vegetation cover in the Afar region and in the northern Rift lowlands. Finally, comparison of our data with recent archaeological findings from the Lake Abhe basin allows us to suggest that these environmental conditions would have favored the persistence of fisher-hunter-gatherer subsistence practices over the emergence of pastoralism in the Horn of Africa in the second half of the Holocene.

Keywords

East African Rift System, Central Afar Region, Lake Abhe, Palaeo-shorelines, African Humid Period, Holocene, 8.2 ka event, Tectonic deformation

V.1.1 Introduction

Today, the tropics concentrate the maximum amount of solar energy and redistribute it in the form of air and moisture masses to latitudes between 30°N and 30°S by atmospheric and oceanic circulation. The intertropical solar-irradiation-induced precipitation/evaporation balance is at the base of Earth's atmospheric convection, and is thus considered one of the main driving processes of global climate evolution [START_REF] Mohtadi | Palaeoclimatic insights into forcing and response of monsoon rainfall[END_REF].

From the Late Pleistocene to the Mid-Holocene periods, Africa was the theatre of significant changes in hydro-climatic conditions, which mainly followed precessional pacing (i.e., every 21,000 years; [START_REF] Mohtadi | Palaeoclimatic insights into forcing and response of monsoon rainfall[END_REF][START_REF] Rossignol-Strick | After the deluge: Mediterranean stagnation and sapropel formation[END_REF][START_REF] Skonieczny | Monsoon-driven Saharan dust variability over the past 240,000 years[END_REF]. From ca. 14.8 to 5.5 ka, the precession-driven increase of the northern hemisphere insolation magnitude seems at the origin of the northward migration of the rainfall belt associated with the Intertropical Convergence Zone (ITCZ de-Menocal et al., 2000;[START_REF] Haug | Southward Migration of the Intertropical Convergence Zone Through the Holocene[END_REF][START_REF] Viste | Moisture transport into the Ethiopian highlands: MOISTURE TRANSPORT INTO THE ETHIOPIAN HIGHLANDS[END_REF]. This period, referred to as the African Humid Period (AHP Tierney et al., 2011a), is characterized by enhanced precipitation in the tropics, the northward spread of vegetation cover [START_REF] Hopcroft | Multi vegetation model evaluation of the Green Sahara climate regime: RAINFALL SUPPORT-ING A GREEN SAHARA[END_REF] and the massive reactivation of fluvial systems [START_REF] Mologni | Frequency of exceptional Nile flood events as an indicator of Holocene hydro-climatic changes in the Ethiopian Highlands[END_REF][START_REF] Revel | 20,000 years of Nile River dynamics and environmental changes in the Nile catchment area as inferred from Nile upper continental slope sediments[END_REF][START_REF] Schefuß | Hydrologic control of carbon cycling and aged carbon discharge in the Congo River basin[END_REF] leading to the development of numerous lakes [START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF][START_REF] Holmes | The Late Pleistocene-Holocene African Humid Period as Evident in Lakes[END_REF][START_REF] Kutzbach | Milankovitch forcing of fluctuations in the level of tropical lakes from 18 to 0 kyr BP[END_REF]. However, the general timing (onset and termination) and timing offsets of the AHP between Western and Eastern Africa, as well as between the Sahelian and Equatorial latitudes, need to be fully understood [START_REF] Collins | Rapid termination of the African Humid Period triggered by northern high-latitude cooling[END_REF][START_REF] Demenocal | Abrupt onset and termination of the African Humid Period: Rapid climate responses to gradual insolation forcing[END_REF][START_REF] Shanahan | The time-transgressive termination of the African Humid Period[END_REF]. The establishment of short-term (200-400 yr) African arid episodes, likely synchronous with the North Atlantic cooling events (HE1, YD, 8.2, 4.2 ka events [START_REF] Collins | Abrupt shifts of the Sahara-Sahel boundary during Heinrich stadials[END_REF][START_REF] Rasmussen | A stratigraphic framework for abrupt climatic changes during the Last Glacial period based on three synchronized Greenland ice-core records: Refining and extending the INTIMATE event stratigraphy[END_REF][START_REF] Rohling | Centennial-scale climate cooling with a sudden cold event around 8,200 years ago[END_REF], bring to question the atmospheric/oceanic causal link between high and low latitudes, the role of the tropics in these cooling episodes [START_REF] Clement | An Orbitally Driven Tropical Source for Abrupt Climate Change[END_REF][START_REF] Davies | The impact of Sahara desertification on Arctic cooling during the Holocene[END_REF], the mechanisms of the AHP [START_REF] Shanahan | The time-transgressive termination of the African Humid Period[END_REF], and the potential local non-linear response of African depositional basins to global climatic trends. The numerous lakes of the 3000 km-long and N-S-tending East African Rift System (EARS) have proven to be excellent recorders of past climate evolution, providing both long and high-resolution archives [START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF]. However, the extensional tectonics [START_REF] Déprez | Seismic and aseismic deformation along the East African Rift System from a reanalysis of the GPS velocity field of Africa[END_REF][START_REF] Stamps | Current kinematics and dynamics of Africa and the East African Rift System[END_REF][START_REF] Stamps | Redefining East African Rift System kinematics[END_REF] coupled with the extreme contrast between high precipitation rates in the highland catchment zones and high evaporation in the lowlands ('amplifier lake' patterns [START_REF] Trauth | Human evolution in a variable environment: The amplifier lakes of Eastern Africa[END_REF] may induce a modulated response of lacustrine hydro-sedimentary systems to atmospheric inputs from continental climatic change. During the AHP, the enhanced atmospheric pressure gradient between the East African Monsoon and the Indian Summer Monsoon (ISM) would have led to an eastward migration of the Congo Air Boundary (CAB [START_REF] Braconnot | Monsoon response to changes in Earth's orbital parameters: Comparisons between simulations of the Eemian and of the Holocene[END_REF][START_REF] Camberlin | Rainfall Anomalies in the Source Region of the Nile and Their Connection with the Indian Summer Monsoon[END_REF]. These patterns induced an incursion of the West African Summer Monsoon (WAM) air masses along the EARS, causing a non-linear response of the hydro-sedimentary systems to ITCZ north-south oscillations (Fig. V.1a Costa et al., 2014;[START_REF] Junginger | The effects of solar irradiation changes on the migration of the Congo Air Boundary and water levels of paleo-Lake Suguta, Northern Kenya Rift, during the African Humid Period (15-5ka BP)[END_REF][START_REF] Tierney | Late Quaternary behavior of the East African monsoon and the importance of the Congo Air Boundary[END_REF]. The acquisition of paleo-hydrological data from the southern to the northern EARS ranges adds relevant information to our understanding of moisture mass convergence zone dynamics during the AHP, which would have impacted, sometimes over the course of a few hundred years, the equilibrium and paleoenvironmental stability of ancient ecosystems.

The Abhe Lake basin, located at the northern extremity of the EARS (∼12°N), is the widest and longest rift-controlled sedimentary basin of the Afar Depression. It derives its source in the Ethiopian Highlands (Fig. V.1b, V.1c). Currently reduced to a small (∼350 km 2 ) and shallow (max. depth <30 m) waterbody, Lake Abhe experienced a highstand resulting in a large freshwater lake during the Early Holocene (∼7000 km 2 and ∼230 m max. depth [START_REF] Gasse | L'évolution des lacs de l'Afar Central (Ethiopie et T.F.A.I.) du Plio-Pléistocène à l'Actuel. Recostitution des paléomilieux lacustres à partir de l'étude des Diatomées[END_REF][START_REF] Khalidi | 9000 years of human lakeside adaptation in the Ethiopian Afar: Fisherforagers and the first pastoralists in the Lake Abhe basin during the African Humid Period[END_REF]. The fluctuations of Lake Abhe's water level seem to follow the main AHP dry-wet oscillations [START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF], producing some very marked littoral geomorphic features. Although they represent a reliable indicator of the paleo-hydrological evolution history of the basin, these littoral geomorphic features have never been documented or investigated in the Abhe region prior to this study. Deciphering the precise control of geomorphic systems over both hydro-climatic and geodynamic forcing is essential for a more robust interpretation of past climate-change data. [START_REF] Viste | Moisture transport into the Ethiopian highlands: MOISTURE TRANSPORT INTO THE ETHIOPIAN HIGHLANDS[END_REF]; and the wind directions of the Western African Summer Monsoon (WAM) and the Indian Summer Monsoon (ISM). b) Simplified morpho-tectonic patterns of the Afar Triangle depression (light gray), enclosed by the Arabian, Nubian and Somalian plates (pre-rift groups, dark gray) with the location of the present Lake Abhe (red star). c) Structural map of the Afar depression (modified from [START_REF] Doubre | Current deformation in Central Afar and triple junction kinematics deduced from GPS and InSAR measurements[END_REF] in which the present Abhe Lake (red star), the extension of the Abhe Lake sedimentary system (light green), other Central Afar sedimentary basins (light yellow), the present natural waterbodies (blue), the ancient Abhe basin overflow sills (blue arrows), recent lava flows (light red), active (red lines) and ancient (dashed red lines) rift segments (modified from [START_REF] Barberi | Volcanism of Afar: Smallscale plate tectonics implications[END_REF] are indicated. The dashed dark green line indicates the Tendaho-Gobaad discontinuity, while the large and small faults are indicated by bold and thin lines, respectively.

In this study, we present the first hydro-climatic record established using multiple paleo-shoreline archives spanning the last ∼11 ka of Holocene Lake Abhe. Our record is age-controlled by 52 new and existing dates on foreshore/nearshore carbonate deposits such as stromatolites and shells of Melanoides tuberculata. With the aim of determining the precise moisture-related water-level variation history of Holocene Lake Abhe, our results are combined with tectonic deformation kinematics data in order to take neo-tectonic control and deformation of the investigated geomorphic markers into account, and with archaeological and perilacustrine sedimentary sequence data [START_REF] Khalidi | 9000 years of human lakeside adaptation in the Ethiopian Afar: Fisherforagers and the first pastoralists in the Lake Abhe basin during the African Humid Period[END_REF]. Our study is compared with other regional lacustrine paleo-hydrological records with the aim of estimating the influence of the monsoonal variations on a N-S transect of the EARS during the Holocene period. Finally, we discuss the role of the CAB and the ITCZ on Northeast African lacustrine highstands during the Mid-Holocene, and the potential impact of these hydrological changes on regional prehistoric peopling.

V.1.2 Regional Settings

V.1.2.a Volcano-Tectonic control of the Abhe basin formation

The Lake Abhe is located in the southeastern part of the Tendaho-Gobaad graben, which is the widest and longest basin within the Afar Depression. The extensional tectonics occurring in the Afar are associated with the triple junction of the divergent boundaries between the Somalian, Arabian and Nubian plates (Fig. V.1b). Over its 1.8 Myr-long development, the Tendaho-Gobaad graben has been the locus of interactions between three extensional systems related to the EARS, the Red Sea and the Aden ridges [START_REF] Audin | Palaeomagnetism and K-Ar and 40 Ar/ 39 Ar ages in the Ali Sabieh area (Republic of Djibouti and Ethiopia): Constraints on the mechanism of Aden ridge propagation into southeastern Afar during the last 10 Myr[END_REF][START_REF] Manighetti | Strain transfer between disconnected, propagating rifts in Afar[END_REF]. It is considered the southwestern tip of the Red Sea propagating on land (Nubia-Arabian boundary), overlapping with the Aden Ridge (Somalia-Arabian boundary), while the extensional structures related to the northern tip of the EARS are observed in its southern margin along the Main Ethiopian Rift (MER; Fig. V.1c; [START_REF] Acton | Block rotation and continental extension in Afar: A comparison to oceanic microplate systems[END_REF].

East African continental rifting began between ∼31 and ∼28 Ma following the emplacement of the flood basalts traps forming the Ethiopian and Yemeni high plateaus (current highlands; [START_REF] Hofmann | Timing of the Ethiopian flood basalt event and implications for plume birth and global change[END_REF][START_REF] Macgregor | History of the development of the East African Rift System: A series of interpreted maps through time[END_REF][START_REF] Tiercelin | A 300 Million Years History of Rift Lakes in Central and East Africa: An Updated Broad Review[END_REF]. Soon after, starting around ∼25 to ∼20 Ma, continental extension formed the large escarpments bounding the Afar Depression and developed the filling of large Miocene basins. The Stratoid series, a 1.5-km-thick basaltic sequence which covers most of the Afar region, was dated between 4.4 Ma and 0.4 Ma [START_REF] Varet | Geology of Central and Southern Afar (Ethiopia and Djibouti Republic)[END_REF][START_REF] Kidane | New paleomagnetic and geochronologic results from Ethiopian Afar: Block rotations linked to rift overlap and propagation and determination of a ∼2 Ma reference pole for stable Africa: NEW PALEOMAGNETIC AND GEOCHRONOLOGIC RESULTS[END_REF]. From 1.8 Ma, continental extension in the Central Afar was focused across several NNW to NW-trending active rift segments, concentrating tectonic deformation and, in most cases, magmatic activity. They formed a complex network of normal faults and several sub-rifts, horsts and grabens, to which the Tendaho-Gobaad graben belongs [START_REF] Abbate | Strike-slip faults in a rift area: A transect in the Afar Triangle, East Africa[END_REF]. Many fissural volcanic flows related to the emplacement of the Stratoid series occurred between ∼2 and ∼0.5 Ma across the Tendaho-Gobaad graben, evidenc-ing the link between the outflows of the relevant volumes of Stratoid magma and the collapse of the Tendaho-Gobaad graben [START_REF] Acocella | Coupling volcanism and tectonics along divergent plate boundaries: Collapsed rifts from central Afar, Ethiopia[END_REF]. Holocene rifting activity is evidenced by some volcanic cones and domes associated with NNW-trending sub-rifts in its northern part (Fig. V.1c; [START_REF] Barberi | Volcanism of Afar: Smallscale plate tectonics implications[END_REF][START_REF] Manighetti | Propagation of rifting along the Arabia-Somalia Plate Boundary: Into Afar[END_REF], and by the probable NE tilting of the basin plain proposed by [START_REF] Gasse | L'évolution des lacs de l'Afar Central (Ethiopie et T.F.A.I.) du Plio-Pléistocène à l'Actuel. Recostitution des paléomilieux lacustres à partir de l'étude des Diatomées[END_REF]. This recent surface subsidence would have induced the capture of the Awash River (Gasse, 1975, Fig. V.1) leading to the current hydrological basin patterns. From recent geodetic studies [START_REF] Doubre | Current deformation in Central Afar and triple junction kinematics deduced from GPS and InSAR measurements[END_REF], the current NE-trending extension across the Tendaho-Gobaad graben does not exceed 7 mm/yr. The subsidence velocity of the inner floor cannot be easily estimated and no clear current slip on the bounding normal faults of the graben is detected from long InSAR time series [START_REF] Doubre | Current deformation in Central Afar and triple junction kinematics deduced from GPS and InSAR measurements[END_REF][START_REF] Pagli | Current plate boundary deformation of the Afar rift from a 3-D velocity field inversion of InSAR and GPS: Current Afar plate boundary deformation[END_REF]. [START_REF] Temtime | Spatial and temporal patterns of deformation at the Tendaho geothermal prospect, Ethiopia[END_REF] identified an elongated NE-trending zone of present day subsidence reaching ∼4 cm/yr over the 2004-2010 period from InSAR data, which they attributed to magmatic and hydrothermal activity below the basin. These data indicate how tectonic control represents the primary driver of the large-scale landform-formation-processes of the Abhe Lake basin. The strong extensional deformation of the crustal layer would have affected the sedimentogenetic processes and the organization of the lacustrine-induced morphological elements investigated in this study. However, the strong impact of past hydrological dynamics on the sedimentary East African systems produced a considerable imprint on landscape morphologies [START_REF] Garcin | East African mid-Holocene wet-dry transition recorded in palaeo-shorelines of Lake Turkana, northern Kenya Rift[END_REF][START_REF] Nilsson | Ancient Changes of Climate in British East Africa and Abyssinia. A Study of Ancient Lakes and Glaciers[END_REF].

V.1.2.b Hydro-climatic control of the Abhe basin and previous paleo-lacustrine studies

The Abhe basin sedimentary system receives its water catchment from the Awash River which derives its source in the region of highland Ethiopia south-west of Addis Ababa. Freshwater supplies are mainly generated by African monsoon precipitation on the Ethiopian Highlands during the rainy season from June to September (200 to 300 mm/month). Air moisture flows moving into northern Ethiopia originate from the Gulf of Guinea, the Indian Ocean, the Red/Mediterranean Seas and the Arabian Peninsula [START_REF] Viste | Moisture transport into the Ethiopian highlands: MOISTURE TRANSPORT INTO THE ETHIOPIAN HIGHLANDS[END_REF].

Descending the highlands, the Awash River is joined by many affluent rivers until it meets the Lower Awash Plain (4000 km 2 ), which coincides with the northwestern part of the Tendaho-Gobaad graben.

Here, the river course continues, without any perennial freshwater supply, to Gamari and Afambo Lakes (339 m a.s.l.) and to its final destination, endorheic Lake Abhe (Fig. V.1c; 240 m a.s.l.). Over the course of one year, the Abhe basin is seasonally supplied by smaller local catchments at the extremities of the graben, namely the Gobaad (South-East) and the Logiya (North-West) Rivers. Presently, the Central Afar region is characterized by a hyper-arid climate with 200 mm/yr precipitation and with an evaporation rate of 3500 mm/yr [START_REF] Gasse | Tectonic and Climatic Controls on Lake Distribution and Environments in Afar from Miocene to Present[END_REF][START_REF] Rognon | Depots lacustres quaternaires de la basse vallée de l'Awash (Afar, Ethiopie): Leurs rapports avec la techtonique et le volcanisme sousaquatique[END_REF]. At the tropics, monsoonal rainfall magnitude variations over the last 1 Ma have been attributed to the northward migration of the rainfall belt associated with the ITCZ together with precession-driven insolation changes [START_REF] Demenocal | Abrupt onset and termination of the African Humid Period: Rapid climate responses to gradual insolation forcing[END_REF][START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF][START_REF] Lamb | 150,000-year palaeoclimate record from northern Ethiopia supports early, multiple dispersals of modern humans from Africa[END_REF][START_REF] Mohtadi | Palaeoclimatic insights into forcing and response of monsoon rainfall[END_REF][START_REF] Tuenter | The response of the African summer monsoon to remote and local forcing due to precession and obliquity[END_REF]. The large offset between the local precipitation and evaporation rates indicates that the orbital-induced monsoon activity in the Ethiopian highlands was primarily responsible for the hydro-sedimentary balance which generated the Late Pleistocene-Holocene synthems of the Abhe basin. Except for a few Mio-Pliocene sequences located along the margins of the Afar Depression [START_REF] Gasse | Tectonic and Climatic Controls on Lake Distribution and Environments in Afar from Miocene to Present[END_REF][START_REF] Tiercelin | Rifts Contintntaux. Tectonique, Climats, Sédiments. Exemples: La Sédimentation Dans Le Nord Du Rift Gregory (Kenya) et Dans Le Rift de l'Afar (Ethiopie) Depuis Le Miocéne[END_REF], the lacustrine sedimentary accumulation in the Central Afar region began during the Early Pleistocene [START_REF] Boucaurt | Impact des événements tectono-volcaniques Plio-Pléistocenes sur la sédimentation en République de Djibouti (Afar Central)[END_REF][START_REF] Gasse | Evolution tectonique et climatique de l'Afar central d'apres les sediments plio-pleistocenes[END_REF], which seems synchronous with the Tendaho-Gobaad graben formation. However, sedimentary deposits developed within a topography similar to the current one started around 130 ka BP [START_REF] Fontes | Les cheminées du lac Abhé (T.F.A.I.)[END_REF][START_REF] Gasse | L'évolution des lacs de l'Afar Central (Ethiopie et T.F.A.I.) du Plio-Pléistocène à l'Actuel. Recostitution des paléomilieux lacustres à partir de l'étude des Diatomées[END_REF].

The first research conducted on Abhe basin sedimentary formations occurred in the 1970s and was focused on paleoecological reconstruction from deep (offshore) lacustrine deposits [START_REF] Faure | Les formations lacustres Holocènes et l'industrie épipaléolithique de la région de Loggia (bassin du lac Abhé Ethiopie)[END_REF][START_REF] Gasse | Evolution of Lake Abhe[END_REF][START_REF] Rognon | Depots lacustres quaternaires de la basse vallée de l'Awash (Afar, Ethiopie): Leurs rapports avec la techtonique et le volcanisme sousaquatique[END_REF][START_REF] Gasse | L'évolution des lacs de l'Afar Central (Ethiopie et T.F.A.I.) du Plio-Pléistocène à l'Actuel. Recostitution des paléomilieux lacustres à partir de l'étude des Diatomées[END_REF]. Three main lacustrine sedimentary sequences spanning 100 to 3 ka BP, were identified by based on study of exposed sections and offshore sediment cores: ∼100-70 ka BP (Abhe I), 60-18 ka BP (Abhe II -Abhe III), and 10-4 ka BP (Abhe IV -Abhe V; Fig. V.1c) corresponding to the AHP (∼14.8 and ∼5.5 ka; [START_REF] Shanahan | The time-transgressive termination of the African Humid Period[END_REF]. [START_REF] Gasse | L'évolution des lacs de l'Afar Central (Ethiopie et T.F.A.I.) du Plio-Pléistocène à l'Actuel. Recostitution des paléomilieux lacustres à partir de l'étude des Diatomées[END_REF][START_REF] Gasse | Evolution of Lake Abhe[END_REF] reconstructed past Lake Abhe highstands on the basis of conventional radiocarbon ages and elevations from nearshore to offshore carbonate deposits, with little link to paleo-shoreline morpho-sedimentary structures and dynamics. A recent revision of these previous data with the integration of new calibrated ages performed on littoral carbonates and archeological assemblages has led to the definition of a new water-level variation curve for Holocene Lake Abhe [START_REF] Khalidi | 9000 years of human lakeside adaptation in the Ethiopian Afar: Fisherforagers and the first pastoralists in the Lake Abhe basin during the African Humid Period[END_REF]. In this study, we have further developed this approach with the aim of reconstructing Holocene East-African monsoon-induced paleohydrological variations in the Lake Abhe basin with higher precision. During three survey campaigns (2018-2020), we documented different littoral morpho-sedimentary features with specific regard for a wide range of littoral-related geomorphic and depositional environments such as wave abrasion platforms/notches, beach ridges, Gilbert-deltas, pedological horizons, gravity-induced deposits and fluvial or fluvio-lacustrine deposits. This approach, combined with a chronology assured by more than fifty radiocarbon ages on littoral carbonates, allows us to indicate the precise paleolake outlines and elevations and their relation to volcano-tectonic rifting-related deformation.

V.1.3 Methodology

V.1.3.a Differential GPS (DGPS) survey and Digital Elevation Model (DEM) calibration

During the 2018-2020 field campaigns, a large survey over the entire Abhe basin was focused on the identification and GPS measurement of the littoral morpho-sedimentary and geomorphic elements of the Abhe basin. The detailed description and definition of the precise geomorphic elements considered as paleo-shoreline markers are provided in the results Section V.1.4.b and in Sup. Mat. BI.2. Additionally, in order to provide more accurate elevations from new GPS plots, and with the aim of testing and validating our previous measurements and calibrating the elevation data from [START_REF] Gasse | L'évolution des lacs de l'Afar Central (Ethiopie et T.F.A.I.) du Plio-Pléistocène à l'Actuel. Recostitution des paléomilieux lacustres à partir de l'étude des Diatomées[END_REF], a systematic DGPS survey was performed using the Trimble Geo 7X differential GPS, with a double frequency receptor (L1 = 1575.46 MHz, L1 = 1227.60 MHz). The vertical standard deviation of our measurements range between 1 and 2 cm. This analytical value error is similar to the Trimble error value. Due to the quasi-absence of geodetic reference points in the investigated region and with the aim to validate our elevation measurements, we compared our DGPS measurements with elevations from 2 DEMs: a total of 54 DGPS measurements were compared with SRTM® (www2.jpl.nasa.gov/srtm) and TanDEM-X (https://tandemx-science.dlr.de; [START_REF] Rizzoli | Generation and performance assessment of the global TanDEM-X digital elevation model[END_REF] DEMs (Fig. V.2). Results show generally accurate elevations from the SRTM® (30 m resolution), however, they do not follow the homogeneity of DGPS measurements (Fig. V.2). In contrast, elevations extracted from the TanDEM-X data (12m resolution; [START_REF] Rizzoli | Generation and performance assessment of the global TanDEM-X digital elevation model[END_REF] are more coherent with the DGPS data but with a systematic offset of -10 m (Fig. V.2). The elevation differences between our DGPS and the TanDEM-X data (with a +10 m offset applied) never exceed ∼5 m (1.7 m average), while the differences with the STRM data processing sometimes reach ∼10 m (2.3 m average). Taking into account the precision required for the elevation measurements, as well as the coarse sampling resolution of the SRTM DEM, the TanDEMx (+10 m offset) was preferred for the calibration of the GPS measurements obtained from three years of field campaigns and from the previous location data provided in [START_REF] Gasse | L'évolution des lacs de l'Afar Central (Ethiopie et T.F.A.I.) du Plio-Pléistocène à l'Actuel. Recostitution des paléomilieux lacustres à partir de l'étude des Diatomées[END_REF]. For each surveyed site, a topographic profile was generated with the location of geomorphic features (Supp. Mat. BI.2), providing support for each paleo-shoreline level definition (see V.1.4.b).

V.1.3.b Age Control

With the aim of proposing a robust age model of the main paleo-shorelines recorded across the Lake Abhe basin, a calibration was performed using 17 AMS 14 C ages on stromatolites, 9 on nearshore deposits and 26 selected ages published in [START_REF] Gasse | L'évolution des lacs de l'Afar Central (Ethiopie et T.F.A.I.) du Plio-Pléistocène à l'Actuel. Recostitution des paléomilieux lacustres à partir de l'étude des Diatomées[END_REF]. The revision of previous Holocene Abhe paleolake water level data is based on the calibration and the DEM re-calculated elevations of 14 C ages on nearshore carbonate deposits published by [START_REF] Gasse | L'évolution des lacs de l'Afar Central (Ethiopie et T.F.A.I.) du Plio-Pléistocène à l'Actuel. Recostitution des paléomilieux lacustres à partir de l'étude des Diatomées[END_REF]. The 26 published ages were selected according to the following criteria: a) only ages from littoral lacustrine (nearshore) material from the Abhe basin were considered; b) using stratigraphic information provided by [START_REF] Gasse | L'évolution des lacs de l'Afar Central (Ethiopie et T.F.A.I.) du Plio-Pléistocène à l'Actuel. Recostitution des paléomilieux lacustres à partir de l'étude des Diatomées[END_REF], inversed ages or ages from reworked sediments were not considered for the age model building; c) ages from paleo-shores of residual lakes were not considered (e.g., Gamari or Gargori residual lakes, Fig. V.4a); d) ages with incomplete geographic location (incomplete coordinates) do not allow a proper correlation with the DEM processing data and were not used for the model building; e) ages on anthropic material were not considered. The selected results are presented in Tab. V.1. The age calibration (Tab. V.1) was performed using the OxCal 4.3 program (Bronk [START_REF] Bronk Ramsey | Development of the Radiocarbon Calibration Program[END_REF]. For the conventional ages on littoral lacustrine carbonates, the calibration parameters were selected from the IntCal13 atmospheric calibration curve [START_REF] Reimer | IntCal13 and Marine13 Radiocarbon Age Calibration Curves 0-50,000 Years cal BP[END_REF]. Considering that the estimated reservoir age effect for Early Holocene biogenic lacustrine carbonates is <100 yrs [START_REF] Khalidi | 9000 years of human lakeside adaptation in the Ethiopian Afar: Fisherforagers and the first pastoralists in the Lake Abhe basin during the African Humid Period[END_REF], which remains lower than the majority of the age standard deviations, no reservoir correction was applied. The ages considered here used 95.4% ranges and are reported in median ka cal. BP (calibrated kilo annum before 1950).

V.1.4 Results

V.1.4.a Paleo-shoreline geomorphic features of the Lake Abhe basin

Over the ∼200-km-wide Abhe basin, more than one hundred littoral lacustrine geomorphic features were identified and mapped during the survey campaigns (2018-2020; Sup. Mat. BI.2). The detection of more or less pronounced wave-cut abrasion features, typical beach pebble band concentrations, and continuous paleo-beach ridges, combined with other shore-related morpho-sedimentary facies (Gilberttype deltas), ascribes the Abhe paleolake to the wave-dominated clastic shoreline lake-type as defined by [START_REF] Schuster | Lacustrine wavedominated clastic shorelines: Modern to ancient littoral landforms and deposits from the Lake Turkana Basin (East African Rift System, Kenya)[END_REF]. In this section, we first describe and define the geomorphic marker types observed in the field. We then present each surveyed site together with the list of shorelines. Finally, we summarize the dataset of the paleo-shorelines recognized across the Abhe basin.

Wave-cut and overflow abrasion platforms

Wave energy enhanced by wind direction and magnitude can produce more or less developed shore platforms, abrasion ramps or cliff notches [START_REF] Schuster | Coastal conglomerates around the Hadjer el Khamis inselbergs (western Chad, central Africa): New evidence for Lake Mega-Chad episodes[END_REF][START_REF] Trenhaile | Coastal notches: Their morphology, formation, and function[END_REF]. The maximal estimated extension of the Holocene Lake Abhe waterbody (7000 km 2 Gasse, 1975) presents the conditions to produce well-defined littoral abrasion features on clastic supports. Throughout the Abhe basin, abrasion platforms and notches vary in relation to lateral changes in rock resistance. Wave-cut abrasion platforms that developed in acidic volcanic contexts such as rhyolitic domes and associated eruptive material (ignimbrite and pyroclastic rocks) are more pronounced, horizontally shaped, and frequently associated with well-developed cliff notches (Fig. V.3.a). Most wave-cut abrasion platforms that developed on basaltic stratoid series outcrops present a sub-horizontal or seaward-sloping shore morphology (gently sloping abrasion platforms as defined by [START_REF] Sunamura | Geomorphology of Rocky Coasts[END_REF] without any cliff notch features; Fig.

V.3.b)

. The width of sub-horizontal abrasion platforms on rock outcrops can vary between ∼10 and ∼200 m, while when wave-cut incision occurs on pre-existing sedimentary deposits, the platform can achieve up to 1 km in width (Fig. V.3.c). Abrasion platform shape and dimension are even determined by long-term exposure to wave erosion, lake waterbody amplitude, which coupled with wind action influences the magnitude of waves, and by local topographical morphology. For example, a short-term wave-cut abrasion on a plunging cliff could produce a slight incision observable in a slope failure feature (1 to 3 m high). We refer to this kind of slight pronounced abrasion feature with the term 'abrasion terrace'. Abrasion platforms on overflow sill ridges represent important additional geomorphic paleoshoreline markers which are likely produced by the mixing of wave abrasion and traction current action in the overflow direction. Because of the homogeneous composition of the basaltic stratoid series layers, vertical differential erosion is reduced. Consequently, shore platforms as described in [START_REF] Bird | Coastal Geomorphology: An Introduction[END_REF] are almost absent across the Abhe basin. The great number of normal faults which characterize the Tendaho-Gobaad semi-graben could highly affect the recognition and the preservation of continuous morphologic elements of paleo-shorelines. For this reason, during surveys, sites with clear structural patterns were preferred over sites where faulting has probably been involved in sedimentary/erosion dynamics. The high extensional deformation kinematics of the Tendaho-Gobaad semi-graben does not constitute the only obstacle to abrasion platform recognition. The Late Holocene to current hyper-arid environmental conditions in the Central Afar region, which combine high silicate watering, wind action, thermoclastism and pronounced slope erosion due to soil/vegetation cover denudation, can also deform the original abrasion features. For this reason, coupling the study of wave-cut and abrasion platforms with supplementary geomorphic and morpho-sedimentary markers is needed for the definition of paleo-shorelines.

Flat-pebble/cobble beaches

Beach ridges, with their related clastic components, are commonly used as large-scale wave-related shoreline geomorphic features [START_REF] Hesp | Sand Beach Ridges: Definitions and Re-Definition[END_REF]. Across tropical Africa, these continuous, mound-shaped, and linear accumulations of clastic sediments parallel to coastlines are widely used as markers of regressive paleo-shorelines at the end of the AHP [START_REF] Forman | Variations in water level for Lake Turkana in the past 8500 years near Mt. Porr, Kenya and the transition from the African Humid Period to Holocene aridity[END_REF][START_REF] Garcin | East African mid-Holocene wet-dry transition recorded in palaeo-shorelines of Lake Turkana, northern Kenya Rift[END_REF][START_REF] Schuster | Lacustrine wavedominated clastic shorelines: Modern to ancient littoral landforms and deposits from the Lake Turkana Basin (East African Rift System, Kenya)[END_REF]. In the Central Afar region, beach ridge features are sporadic due to the adverse climatic conditions which prevent their preservation. The total absence of vegetation and soil cover on the basin margins can induce high-energy rill erosion during the short-lived summer rainfalls, causing the vertical displacement of beach ridge clastic components. In addition, heavy silicate weathering and enhanced thermoclastism can produce a high amount of fresh detrital material both from outcrops and from inherited littoral clastic material even on sub-horizontal gentle slope contexts. This pronounced formation process of in situ clastic material can lead to some uncertainty in the recognition of ancient continuous beach ridges. Thus, only the coarser components, namely isolated and continuous beach ridges, were considered as paleo-shoreline markers (Sup. Mat. BI.2). Flat-pebble/cobble beaches are often associated with wave-cut and/or abrasion platform surfaces confirming the pertinence of these clastic geomorphic elements as wave-related shoreline features across the Lake Abhe basin. In this study, the majority of the paleo-shore/foreshore zones are defined by the detection of pebble (4-64 mm diameter) and cobble (64-256 mm diameter; [START_REF] Bird | Coastal Geomorphology: An Introduction[END_REF] concentration bands (Sup. Mat. BI.2). These can be found as occasional (a few meters long), intermittent or continuous bands. The roundness, flat morphology and smooth surface of clasts allow them to be distinguished from fluvial or slope detrital material. Often, identification of the mineralogical cortege allows us to differentiate local beach clastic products from allochthonous fluvial assemblages. Indeed, some beaches supplied with the Awash mineralogical assemblage were documented (see Section V.1.4.b, WK site). The width of the pebble/cobble concentration bands ranges between 2 and 30 meters, and their upper limit is considered a paleo-shoreline elevation marker.

Gilbert-type deltas and foreshore morphologies

Deltas are shore-related morpho-sedimentary features. Their evolution thus directly reflects changes in water level. In particular, Gilbert-type deltas (first described by [START_REF] Gilbert | Topographic Features of Lake Shores[END_REF] are widely considered to be sensitive recorders of short-term water-level changes [START_REF] Breda | Facies and processes in a Gilbert-delta-filled incised valley (Pliocene of Ventimiglia, NW Italy)[END_REF][START_REF] Colella | Coarse-Grained Deltas[END_REF][START_REF] Gobo | Gilbert-type deltas recording short-term base-level changes: Deltabrink morphodynamics and related foreset facies[END_REF][START_REF] Massari | Evolution and types of fan-delta systems in some major tectonic settings[END_REF]. The distinctive tripartite architecture permits their formation context to be recognized: the subhorizontal bottomset corresponds to the prodelta formation processes, the inclined foreset indicates a subaqueous delta-slope progradation, while the horizontal topset suggests a mouth-bar (fluvial) or slack-water deposition context [START_REF] Bridge | Rivers and Floodplains: Forms, Processes, and Sedimentary Record[END_REF][START_REF] Smith | Radar structure of a Gilbert-type delta, Peyto Lake, Banff National Park, Canada[END_REF]. Across the Abhe basin, the Gilbert-type delta is a recurrent landform developed at the mouth of feeder canyons or at the distal end of dejection cones along the basin margins (Fig. V.3d). Gilbert-type delta facies are generally well-defined and easily recognized (Fig. V.3e). These peculiar characteristics allow us to easily recognize this kind of paleo-shoreline morphology preserved in the landscape at distinct elevations. The majority of the tributaries of the lower Awash valley are characterized by a high-energy regime due to the high altitudinal gradient between the marginal horsts and the axial alluvial plain. This gradient is at the origin of the formation processes which give deltas a well-defined shape during lake-level highstands: a pronounced slope failure and well-sorted clastic material between the feeder mouth and the delta topset (Fig. V.3d). In the EARS lakes, deltas are the combined product of fluvial and wave influences [START_REF] Schuster | Lacustrine wavedominated clastic shorelines: Modern to ancient littoral landforms and deposits from the Lake Turkana Basin (East African Rift System, Kenya)[END_REF]. Their shape evolution is driven by wave-induced erosion, and by transport and deposition of river-derived sediment [START_REF] Anthony | Wave influence in the construction, shaping and destruction of river deltas: A review[END_REF]. In the Abhe basin, wave action on delta landforms is evidenced by the frequent association of a flat delta apex associated

V.1.4.b A detailed record of the Holocene paleo-shorelines of Abhe basin

Description of the surveyed sites

Sixteen sites, presenting an assemblage of geomorphic feature types described in Section V.1.4.a, were selected from the survey dataset and are summarized in this section. The spatial location of these sites is provided in Fig. V.4a, while a detailed documentation of each surveyed site is provided in Sup.

Mat. BI.2 (specific site map, photos, and topographic profiles with the location of the inventoried geomorphic markers). Our results demonstrate the existence of multiple paleo-shoreline sequences from 300 to 420 m a.s.l., which are summarized in Fig. V.4b. Here, we summarize the characteristics of the paleo-shorelines recognized at the 16 surveyed sites, which we present in alphabetic order.

The Afbo site ('Afambo'; Fig. BI.5) is a thin ridge located on the foot wall of the stratoid series which separates the Gamari and Afambo lake waterbodies. Survey profiles were carried out on the S-W flank of the hill and on a headland crest that drops into Afambo Lake. Two well pronounced subhorizontal abrasion platforms associated with beach pebble concentrations were recognized at 399 and 380 m a.s.l. A stromatolitic cover was observed at 370 m a.s.l.

The AleK site ('Ara le Koma'; Fig. BI.6) is located on a fault scarp on the south ridge of the Gobaad valley. Two paleo-shoreline levels were recognized: the upper one (402 m a.s.l.) is characterized by a sorted line of beach clastic material (cobbles), which likely forms a beach ridge; the lower (389 m a.s.l.) is clearly marked by a well-developed abrasion platform on lacustrine deposits, and by a beach ridge feature composed of cobbles and sub-rounded blocks. Additionally, one stromatolitic level was documented at 441 m a.s.l., and a second stromatolitic cover at 385 m a.s.l.

The Ayso site ('Aysilo'; Fig. BI.7) is located on the northern flank of the Gobaad valley as a foot wall plateau mainly composed of acidic volcanism products (rhyolitic flows and ejecta) which are intercalated in the stratoid series. Three main paleo-shorelines were recognized. The upper level, at 411 m a.s.l., takes the form of a Gilbert-delta apex deposit associated with an overlying sub-horizontal and well-developed wave-cut platform. The median level is void of any clear upper foreshore geomorphic markers (wave-cut features), however a beach pebble concentration band at 382 m a.s.l. indicates a paleo-shoreline level overlying this elevation. A thick and wide stromatolite cover combined with reworked littoral lacustrine material (379 m a.s.l.) is probably associated with the medial paleo-shoreline level. The lower paleo-shoreline level is characterized by a wave-cut abrasion terrace and by abundant beach pebble/cobble concentration lines at 332 m a.s.l. A thin littoral carbonate crust line (microbialite) at 327 m a.s.l. is clearly associated with the lower paleo-shoreline of the Ayso site.

The BE site ('Borawli East';Fig. BI.8) is situated on the east flank of the Borawli rhyolitic volcano. Because very few faults cut the current topographic surface, Borawli Mountain represents a good landscape element to locate paleo-shoreline morphologic features. The BE surveyed profile crosses the limit between the rhyolitic flows and the stratoid series, in which an elevated sedimentary flat-fan deposit formed (as a plateau). It is delimited by the rhyolitic slopes in the west, north and south, and by a basaltic rampart in the east. The basin is supplied by the mountain's detrital load through two feeder canyons and is drained by two dejection channels towards the plain. The sub-horizontal shape of the fan against the steep mountain slopes suggests a Gilbert-type delta morphology. This hypothesis is confirmed by the horizontal topset layers observed at the medial-proximal part of the fan and by the horizontal shape of the fan apex. Thus, we consider the proximal-apex part of the deltaic fan as a marker of an ancient foreshore-line located at 420 m a.s.l. In addition, delta sediments are heavily incised by the current lateral streams which enable the sequence to be easily observed and suggest a post-lacustrine torrential erosive transport regime, indicating the drop of the water level from the top to the base of the basin, thus confirming our interpretations. Finally, at the BE site, a wave-cut abrasion terrace is documented at 402 m a.s.l. on the eastern stratoid fault scarp, which is clearly associated with a stromatolitic cover-line at 400 m a.s.l.

The BSW site ('Borawli South-West'; Fig. BI.9), like the BE site, benefits from reduced structural faults, which favored the detection of morphologic features. Additionally, the rhyolitic and ignimbritic outcrops of the southwest flank of Borawli Mountain provide more pronounced wave erosion features. For this reason, at this site we counted several well-developed paleo-shorelines. At 415 m a.s.l. some beach cobbles were observed without any clear upper-shore morphologic marker. At 398 m a.s.l. well-developed wave-cut notches and abrasion platforms indicate an ancient shoreline at this elevation. A few meters below (393 m a.s.l.) a concentration-line of beach cobbles associated with an abrasion platform could belong to the low foreshore zone of the overlying paleo-shoreline or indicate an additional paleolake level. Downward, several wave-cut notches associated with abundant beach pebbles/cobbles and large and well-developed abrasion platforms indicate a series of paleo-shorelines at 387, 381, 372 and 367 m a.s.l. Only one stromatolite level is documented, at 354 m a.s.l.

The BW site ('Borawli West'; Fig. BI.10) is characterized by well-preserved Gilbert-type delta deposits located on the west flank of Borawli Mountain (Fig. V.3d). At 391 m a.s.l. a sub-horizontal delta apex, associated with an abrasion terrace feature, indicates a paleo-shoreline at this level. On the delta section the offshore-transition is clearly observable from fine lacustrine deposits, through the bottomset layers to the foreset progradation (Fig. V.3e), suggesting that these deposits and the related paleo-shoreline were probably developed following a regression from the earlier higher lake level. A lower level was also located at 379 m a.s.l., in the form of a well-developed wave-cut abrasion platform on nearshore sediments.

The DDa site ('Deversoir Djibouti A'; Fig. BI.11) is located along the first part of the southern overflow sill channel of the Abhe basin, which drained the lake water towards the Hanle basin (Figs.

V.1c, V.4a). Along the sill, only abrasion platform features were documented, probably due to the high energy of the drainage that occurred through the channel. This is confirmed by the thick fan deposit composed of coarse sediments located at its mouth. On the flanks four overflow abrasion terraces were detected, at 403, 392, 385, and 380 m a.s.l.

The DSa site ('Deversoir Serdo A'; Fig. BI.12) corresponds to the proximal part of the northern overflow sill of the Abhe basin, which drained the lake water towards the Hanle basin (Figs. V.1c, V.4a). The first and the second part of the sill is intercalated by a small alluvial plain. This sill is higher in elevation than the DDa sill, thus it potentially records only the more ancient and higher drainage phases of the paleolake highstands. Our results confirm this hypothesis with the recognition of two overflow abrasion platforms, at 419 and 397 m a.s.l.

The DSb site ('Deversoir Serdo B'; Fig. BI.13) is located at the beginning of the distal part of the northern overflow sill of the Lake Abhe basin (Fig. V.1c, V.4a). The overflow channel is not exploitable for paleo-shoreline reconstruction because of the crossing of several faults. However, at the canyon entrance, three paleo-shorelines were documented: the first (403 m a.s.l.) characterized by a well-developed abrasion platform on the stratoid outcrops associated with abundant beach cobbles/pebbles, and the lower two (390, 387 m a.s.l.) characterized by a well-developed abrasion platform on previous lacustrine sediments associated with beach cobbles/pebbles (Fig.

V.3c).
The GTU-AFI site ("Goma'tu-Afa'si"; Fig. BI.14) is the only site surveyed on the western scarp of the Tendaho-Gobaad graben that provided some paleo-shoreline geomorphic information. The limited data are a result of difficult access to the Magenta massif zone. Nevertheless, two paleo-shorelines were documented. The upper one, recognized at 381 m a.s.l. by some beach ridges and the lower, recognized at 371 m a.s.l. by a large wave-cut abrasion platform on lacustrine sediments. One oncolite level is observed at 369 m a.s.l.

The LAK site ('Lakora'; Fig. BI.15) corresponds to the northern extension of the Borawli massif: a large plateau composed of basaltic stratoid series and crossed by few northwest/southeast faults. One paleo-shoreline level was detected at 385 m a.s.l. in the form of an abrasion platform underlined by a beach pebble concentration (383 m a.s.l.). This site is distinguished by a large stromatolitic shelf positioned on the Lakora plateau and ranging between 376 and 372 m a.s.l.

The MN site ('Manda North'; Fig. BI.16) is located on the east fault scarps of the Abhe basin, on well-defined northwest/southeast faults. This site is distinguished by the continuity of a well-developed wave-cut abrasion platform located at 385 m a.s.l. and extending over more than 10 km. This shoreline platform is clearly discernable from the faults because it is incised on previous sedimentary deposits placed on the scarp margin. A stromatolitic dome level ranging between 380 and 377 m a.s.l. is likely associated with the shoreline level, while a thin and sporadic littoral carbonate crust (microbialite) was detected at 417 m a.s.l.

The Myta site ('Mokoyta'; Fig. BI.17) is located on the northern side of the Gobaad valley and is characterized by a stromatolitic cover level at 376 m a.s.l., while no clear paleo-shoreline was detected. A continuous platform surface is located on topographic profiles around 420 m a.s.l., but no other littoral markers were observed.

The RSO site ('Raso'; Fig. BI.18) corresponds to a semi-enclosed basin delimited by northwest/southeast oriented foot-wall basaltic outcrops, located at the eastern margin of the Abhe basin axial valley. This site is distinguished by a gulf or semi-enclosed bay paleo-morphology, where onshore winds would have maintained stable high-water levels producing well-developed littoral geomorphic markers [START_REF] Bird | Coastal Geomorphology: An Introduction[END_REF]. These favorable morphological factors, coupled with systematic field survey allowed us to recognize multiple ancient paleolake levels. Well-defined wave-cut abrasion platforms combined with beach pebble assemblages observed both on the east and west margins of the semi-enclosed bay indicate paleo-shorelines at 419,410,403,393,389 and 384 m a.s.l. The lower level (384 m a.s.l.) is clearly associated with a thin stromatolitic line located at the same elevation, while two other stromatolitic levels were detected westward at 382 and 376 m a.s.l.

The SD site ('Serdo'; Fig. BI.19) is the northernmost surveyed area which provided some littoral geomorphic markers. The extended plain areas in the north of the basin, which range between 380 and 370 m a.s.l., would have produced large foreshore and shoreline geomorphic markers, making it more difficult to recognize paleo-shore limits. However, in the Serdo area, in the northeastern part of the basin, a continuous abrasion platform and a beach pebble ridge permit the definition of two paleo-shorelines at 400 and 389 m a.s.l.

The SheK site ('Shekaito Koma'; Fig. BI.20) is located on a small basaltic stratoid outcrop at the center of the Abhe basin. The surveyed site offered the same semi-enclosed basin morphological advantages as described for the RSO site, where several paleo-shorelines were documented. The highest level at 418 m a.s.l. is marked by abundant beach pebble/cobble concentrations belonging to the paleoshore/foreshore zone. The underlying levels were detected thanks to abundant clastic beach components and well-developed abrasion platforms at 411, 399, 380 and 369 m a.s.l. The platform at 380 m is incised in foreshore deposits. One stromatolitic line was observed between 369 and 365 m a.s.l.

The WK site ('Wedelli Koma'; Fig. BI.21) is located in the Det Bahari plain on the right bank of the Awash River. It is represented by a hill composed of Tertiary deposits. A wave cut abrasion platform at 418 m a.s.l. is well developed on these deposits and is associated with a large amount of pebbles which indicate an allochthonous origin. The mineralogical allochthonous assemblage could be part of an ancient (Plio-Pleistocene) delta of the Awash River developed at the entrance of the Abhe basin, not far from WK.

Synthesis of the Abhe basin paleo-shoreline sequence

A synthesis of results from more than one hundred geomorphic shoreline markers has allowed us to define the characteristics and elevations of the main Holocene lake highstands (Fig. V.4b). First, we defined the Maximum Holocene Highstand Shoreline (MHHS), corresponding to the maximum elevation of well-pronounced geomorphic features recognized in each surveyed site. The MHHS elevations are similar in the central and eastern part of the basin and range between 420 m a.s.l. (central-northern basin) and 410 m a.s.l. (Gobaad basin branch, Ayso site, Fig. BI.7). However, along the southwestern margin of the basin at the southern margin of the Gobaad valley, we note that the MHSS for the site AleK is located ∼21 m higher. This offset may be due to a heterogeneous Holocene tectonic subsidence rate within the basin which requires specific analysis (see Section V.1.5.a). Secondly, we defined other main paleo-shorelines at [400][401][402][403][380][381][382][383][384][385]368 and 330 m a.s.l. (Fig. V.4b,V.7b). These levels are intercalated by minor intermediate levels probably corresponding to lacustrine regression steps.

V.1.4.c The Lake Abhe stromatolites

Stromatolite structures are the result of the bioaccumulation and precipitation of micritic/sparitic beds from cyanobacteria microbial activity [START_REF] Casanova | Biosedimentology of Quaternary stromatolites in intertropical Africa[END_REF]. They are the most used datable markers of lacustrine paleo-shorelines in Eastern Africa [START_REF] Casanova | Stromatolites et hauts niveaux lacustres pléistocènes du bassin Natron-Magadi (Tanzanie-Kenya). Pleistocene stromatolites and high lake levels of the Natron-Magadi basin (Tanzania-Kenya)[END_REF][START_REF] Cohen | Lake level and paleoenvironmental history of Lake Tanganyika, Africa, as inferred from late Holocene and modern stromatolites[END_REF][START_REF] Garcin | East African mid-Holocene wet-dry transition recorded in palaeo-shorelines of Lake Turkana, northern Kenya Rift[END_REF][START_REF] Gasse | Evolution of Lake Abhe[END_REF][START_REF] Gillespie | Postglacial arid episodes in Ethiopia have implications for climate prediction[END_REF]Hillaire-Marcel et al., 1986). Their structure, mineralogy and isotopic composition provide precise information about their littoral depositional environment, chemical paleo-conditions of the waters, and paleo-hydrological variations [START_REF] Abell | Plio-pleistocene lacustrine stromatolites from lake Turkana, Kenya: Morphology, stratigraphy and stable isotopes[END_REF][START_REF] Casanova | Late holocene hydrological history of Lake Tanganyika, East Africa, from isotopic data on fossil stromatolites[END_REF][START_REF] Ghinassi | Shoreline fluctuations of Lake Hayk (northern Ethiopia) during the last 3500years: Geomorphological, sedimentary, and isotope records[END_REF]. In this study we present for the first time the macro-/meso-/micro-structure of stromatolites from Lake Abhe with the aim of defining their ecological zonation (shallow/deep-waters), which combined with their radiocarbon measurements adds crucial information to the age model of Lake Abhe paleo-shorelines. A preliminary investigation of over fifty stromatolite specimens allows us to define at least two main facies. Facies-A is characterized by a micritic poorly-laminated microstructure, organized in vertical micro-column growing patterns. Between the micro-columns, several vertical-oriented voids are often intercalated, and form a complex porosity network from the base to the top of the stromatolite. These bioherms form a well-defined macro-structure of large, linked domes (Fig. V.5a) which can extend over a few kilometers (LAK site, Fig. BI.14), while the diameter and the thickness of a single dome can reach 1.5 m and 60 cm, respectively. The multiple dome superimposition and their vertical fissured surface form complex morphologies which are attributable to the cerebroïd-like facies as described by Casanova and Hillaire-Marcel (1992b), Casanova and[START_REF] Casanova | Biosédimentologie des stromatolites Holocénes du Lac Tanganyika, Burundi. Implications hydrologiques[END_REF][START_REF] Cohen | Lake level and paleoenvironmental history of Lake Tanganyika, Africa, as inferred from late Holocene and modern stromatolites[END_REF]. The large thickness of the stromatolitic linked-dome macro-structure and their micro-/meso-structure indicates a deep water zonation at ∼20 m depth [START_REF] Cohen | Lake level and paleoenvironmental history of Lake Tanganyika, Africa, as inferred from late Holocene and modern stromatolites[END_REF]. ). The wave-ripple facies, the oncolite morphology, as well as their irregular surface, indicate a shallow water environment [START_REF] Casanova | Les Stromatolites Continentaux: Paleoecologie, Paleohydrologie, Paleoclimatologie[END_REF][START_REF] Schuster | Lacustrine wavedominated clastic shorelines: Modern to ancient littoral landforms and deposits from the Lake Turkana Basin (East African Rift System, Kenya)[END_REF]. Taking into account the silicic-ferromagnesian regional bedrock sources, Si-Mg-rich layers suggest an increase of detrital delivery events in the lake basin enhancing flow currents, while Ca-rich layers probably suggest periods of lacustrine-dominant biogenic productivity or the onset of evaporitic processes.

V.1.4.d Paleo-shoreline age control

During field survey, several stromatolites and near-shore lacustrine carbonate products were inventoried and sampled along the entire Abhe Lake basin. The chronological evolution of lake paleo-shorelines is based on the combination of radiocarbon ages measured on 10 stromatolites of both Facies-A and Facies-B and on 9 new and revised published ages on nearshore deposits (Tab. V.1). Stromatolites were observed at different altitudes ranging from 420 m to 320 m with a concentration between 380 and 360 m. Stromatolites located in the elevation range of ∼381 to ∼375 m a.s.l. (Fig. V.4b, V.7b) are often characterized by a superimposition of the well distinguished Facies A and B: Facies-A as a lower and older stromatolite generation and Facies-B as an upper stromatolite generation (Fig. V.5b, c). The discontinuity between the two facies is marked by a sharp surface of subaerial exposure often filled by a layer of fine sand which can reach 20 cm in thickness as evidenced at the StrIS site (Stromatolis-01 site; [START_REF] Khalidi | 9000 years of human lakeside adaptation in the Ethiopian Afar: Fisherforagers and the first pastoralists in the Lake Abhe basin during the African Humid Period[END_REF]. This facies separation is even characterized by two chronological groups. The more recent facies ranges in date between ∼7.7 and ∼4.7 ka cal. BP and includes only ages from Facies-B (Tab. V.1; 0-2 m water depth), suggesting they are related to wave-induced clastic paleo-shorelines located around 385 m a.s.l. The older chronological group ranges in date between ∼11.1 and ∼8.3 ka cal. BP, and only includes ages from Facies-A (Tab. V.1; ∼20 m water depth), suggesting they are related to the paleo-shorelines located around 400 m a.s.l. A third chronological group was located at the Str20 site (∼369 m a.s.l.), which is characterized by the superimposition of three stromatolitic layers similar to Facies-A, and lying on a palustrine deposit (Str20-SU6). The lower layer (Str20-SU5; 11125 ± 113 ka cal. BP) is separated by a stone-line in the middle (Str20-SU4; 10530 ± 64 ka cal. BP) and the upper (Str20-SU1; 10447 ± 73 ka cal. BP) layers, suggesting the existence of subaerial exposure conditions in between. Stromatolite specimens from ∼440 m (AleK site), ∼420 m (MN site) and ∼330 m (Ayso site) a.s.l. paleo-shorelines (Fig. V.4b; Figs. BI.6, BI.7, BI.16) present characteristics similar to Facies-B, and thus likely belong to the shallow waters ecological zone. The still undated AleK stromatolite could even belong to a previous Pleistocene lake. New and published ages on nearshore deposits (lumachelles, shell-rich deposits, diatomites) follow the stromatolite chronological ranges quite well (Tab. V.1, Fig.

V.7.a). The published ages also allow the definition of a lower lake level between 3 and 2.8 ka cal. BP, which corresponds to the paleo-shoreline observed around 330 m a.s.l. (site Ayso; Fig. BI.7). Only three nearshore deposits are located between 440 and 400 m a.s.l., and are dated between 10.4 and 9.8 ka cal. BP. They are probably contemporaneous with the geomorphic features and stromatolites of the MHHS located at 440 and 420 m a.s.l. (Fig. V.4.b, V.7.b). Following these results and the respective dating standard deviations, we suggest that the Holocene age of the MHHS was in the range of 10 ± 2.5 ka BP. The offsets of dated littoral features suggest the effect of subsidence due to tectonic deformation which will be discussed below. [START_REF] Gasse | L'évolution des lacs de l'Afar Central (Ethiopie et T.F.A.I.) du Plio-Pléistocène à l'Actuel. Recostitution des paléomilieux lacustres à partir de l'étude des Diatomées[END_REF], the ID number refers to the sample number indicated in [START_REF] Gasse | L'évolution des lacs de l'Afar Central (Ethiopie et T.F.A.I.) du Plio-Pléistocène à l'Actuel. Recostitution des paléomilieux lacustres à partir de l'étude des Diatomées[END_REF] V.1.5 Discussion

V.1.5.a Holocene tectonic subsidence of the Abhe basin

Because of the large rates of horizontal extension that characterize the Central Afar region and the rifting-related vertical motion at the surface [START_REF] Doubre | Current deformation in Central Afar and triple junction kinematics deduced from GPS and InSAR measurements[END_REF][START_REF] Manighetti | Strain transfer between disconnected, propagating rifts in Afar[END_REF], we consider it essential to estimate the potential tectonic impact of Holocene rifting on the Lake Abhe paleo-shoreline sequence before discussing its hydro-climatic relevance. Despite the limited number of available data and dates attesting to vertical slipping of the graben, we attempt to find evidence for variations in the rate of subsidence both in space and time.

In the Gobaad branch of the basin (SE, Fig. V.6a) the MHHS elevation, previously estimated to have occurred ∼10 ka cal. BP (see Section V.1.4.d), differs by ∼31 m between two of our sites, Alek and Ayso, located along the margin of the basin and within the sedimentary plain, respectively (Fig. V.4b, Tab. V.2). Between these two sites, several faults are identified and if the relative vertical locations of the two sites are due to tectonics, the fault Holocene activity would have corresponded to a mean vertical slipping rate of ∼3.1 mm/yr cumulated for the faults observed in this area. This value is of the same order of magnitude as the vertical slipping rate of the faults in the Central Afar determined from topographic profiles across fans by [START_REF] Manighetti | Strain transfer between disconnected, propagating rifts in Afar[END_REF]. Over the same period of time, the authors estimated a vertical slipping rate of 1 to 3 mm/yr for faults located in the basins to the north, namely Hanle and Gaggade (Fig. V.1c). Their measurement along the Gobaad fault agrees with a vertical rate of 0.6 mm/yr for the last 1.8 Ma. However, even if the rate over the last 10 ka period could not be measured, it is probably higher by a factor of 2 or 4, as has been demonstrated for most of the faults in the Central Afar [START_REF] Manighetti | Strain transfer between disconnected, propagating rifts in Afar[END_REF].

In the Logiya-Saha branch of the basin (NW, Fig. [START_REF] Abbate | Strike-slip faults in a rift area: A transect in the Afar Triangle, East Africa[END_REF][START_REF] Doubre | Current deformation in Central Afar and triple junction kinematics deduced from GPS and InSAR measurements[END_REF][START_REF] Manighetti | Strain transfer between disconnected, propagating rifts in Afar[END_REF]. The elevation offset is therefore consistent with a mean vertical subsidence rate of 4.7 mm/yr in the central part of the rift with respect to its margin. A high amplitude of subsidence rate is expected in the area, since this rift is affected by regular crustal magma intrusion events within its rift zone, which induce its subsidence, as was the case during the last 2005-2010 dyke sequence event [START_REF] Grandin | Seismicity during lateral dike propagation: Insights from new data in the recent Manda Hararo-Dabbahu rifting episode (Afar, Ethiopia): DIKE-INDUCED SEISMICITY IN AFAR[END_REF][START_REF] Grandin | September 2005 Manda Hararo-Dabbahu rifting event, Afar (Ethiopia): Constraints provided by geodetic data[END_REF]. The vertical offset between the samples Gasse_50 (right bank of the Logiya River valley) and Strom14 (Poz-124783; Magenta ridge; Tab. V.2), both dated around ∼7.7 ka cal. BP, is more complex because they are located ∼60 km apart (Fig. V.6a). However, these dates suggest a mean differential vertical rate reaching ∼9.1 mm/yr over this period. Acocella (2010) mapped the down-sagging of the surface of the Tendaho graben due to the withdrawal of magma from the deep elongated reservoir. This collapse of the surface was particularly important over the period 2 to 0.5 Ma, but may have persevered at a lower rate over the Holocene period. The recent subsidence can be related to the current extension taking place across the graben estimated with GNSS measurements. The rifting across the Tendaho graben occurs with both an opening rate of 7 mm/yr between the southwestern margin of the graben and the Gamari plateau (Fig. 6a, [START_REF] Doubre | Current deformation in Central Afar and triple junction kinematics deduced from GPS and InSAR measurements[END_REF], and a related subsidence of the central part which is not clearly estimated using a geodetic approach. However, because the Tendaho graben is not considered a very tectonically and magnetically active rift segment, our subsidence rate within the central part of the Tendaho graben (∼9.1 mm/yr) is probably overestimated if we compare ties with respect to the stable Nubian plate deduced from GNSS field measurements (black dashed arrows; after [START_REF] Doubre | Current deformation in Central Afar and triple junction kinematics deduced from GPS and InSAR measurements[END_REF], the location of samples used for the calculation of the subsidence rate (black arrows, this study), the MHHS (red line), the location of the ancient overflow sills (blue arrows), of new (green dots) and published (red dots) dated nearshore deposits, of the stromatolitic deposits (blue triangles), the topographic profile represented in Fig. 6b (black line, A-A'), and the littoral geomorphic features (yellow dots) inventoried during the survey. b) Topographic profile A-A' with the main graben faults, and with the location of samples used for the calculation of the subsidence rate (black arrows, this study) and relative offset (red text). it to the subsidence rate of the inner floor of the active volcano-tectonic segment of Asal, reaching ∼8 mm/yr for the last 6 ka, and 8-9 mm/yr for the last 35 ka [START_REF] Stein | Contemporary, Holocene, and Quaternary deformation of the Asal Rift, Djibouti: Implications for the mechanics of slow spreading ridges[END_REF].

All these results show that the Holocene subsidence taking place within the Tendaho graben varies in time and in space. A striking result is that all the MHHS measurements in the central and the northeastern parts of the Tendaho-Gobaad graben are at the same elevation (∼420 m) which suggests that most of this area was affected by quite a homogeneous subsidence during the Holocene period (Fig.

V.4). This evidence is confirmed by several stromatolites at the same elevation and age throughout the central and northeastern basin (Tab. V.1, Fig. V.7a, c), which indicate a maximum vertical displacement of ∼5 m between them over the last 5-6 ka. However, our results show a relatively higher subsidence rate (∼4.7 mm/yr) in the northern part of the Tendaho graben (Logiya-Saha branch; Fig. V.6a, b) and a lower subsidence rate of ∼3.1 mm/yr in the Gobaad valley (Fig. V.6a, b). This variation in subsidence rate is also consistent with the variations in opening rate measured by GNSS across both ends of the graben [START_REF] Doubre | Current deformation in Central Afar and triple junction kinematics deduced from GPS and InSAR measurements[END_REF]. This is also consistent with the model proposed by [START_REF] Tapponnier | Bookshelf faulting and horizontal block rotations between overlapping rifts in southern Afar[END_REF] and [START_REF] Manighetti | Strain transfer between disconnected, propagating rifts in Afar[END_REF][START_REF] Manighetti | Propagation of rifting along the Arabia-Somalia Plate Boundary: Into Afar[END_REF] where the Tendaho graben corresponds to the tip of the Red Sea ridge propagating southeastward into the Afar, and overlapping with the Aden Ridge. The opening rate and related subsidence rates are larger in the northwest than the southeast, near Lake Abhe (Fig.

V.6a), where the lack of evidence of active deformation matches with smaller vertical displacements.

In light of these results, the low subsidence rates cannot support the hypothesis of a tectonic-driven lacustrine transgression between the Early and the Middle Holocene, since a subsidence rate of the basin that is larger than the 'evaporation rate' cannot be envisaged. Taking into account the paleo-shoreline elevation offsets due to the tectonic vertical slipping between the SW graben margin and the central/NE basin (V.6), we can correct the elevation of samples with the aim of producing a more accurate Holocene lake-level fluctuation model. We consider the subsidence rate measured along the Logiya-Saha transect reliable, and have therefore calibrated elevations of samples from the SW ridge of the basin (see Tab. V.1).

V.1.5.b The last 11 ka of Abhe Lake fluctuations and their hydro-climatic relevance

On the basis of the documentation of 107 littoral geomorphic features and their chronological control obtained from 52 14 C ages on shallow-/deep-water stromatolites and nearshore deposits, we propose a robust Holocene water-level reconstruction model of Lake Abhe. The lake-level variation curve takes paleo-shoreline tectonic deformations into account thanks to the offset correction calculated from Holocene subsidence rates, which are estimated from stromatolite and paleo-shoreline ages. In addition, the integration of archaeological excavation datasets from lake-side prehistoric occupations [START_REF] Khalidi | 9000 years of human lakeside adaptation in the Ethiopian Afar: Fisherforagers and the first pastoralists in the Lake Abhe basin during the African Humid Period[END_REF] enables us to add an altitudinal control to lake-level reconstructions and to improve the paleo-hydrological model (Fig. V.7a).

After the hyperarid episode coinciding with the Younger Dryas NH cooling event (YD, ∼12.9 -∼11.7 ka; [START_REF] Garcin | Abrupt resumption of the African Monsoon at the Younger Dryas-Holocene climatic transition[END_REF], the first evidence for lake-level rise in the Holocene is dated on a stromatolite to ∼11.1 ka cal. BP (Strom20-SU5), after which the Maximal Holocene Highstand Shoreline (MHHS, ∼420 m a.s.l.) reached its peak sometime around 10 ka BP. Following a probable water-level stabilization at ∼400 m a.s.l., dated between ∼9.8 and ∼8.8 ka cal. BP, the lake level regressed slowly to ∼375 m a.s.l. until ∼8.3 ka cal. BP and abruptly dropped to ∼250 m a.s.l. at ∼8 ka cal. BP. This Early Holocene transgression (between ∼11.1 and 8.3 ka cal. BP) generally corresponds to the Abhe-IV lacustrine phase defined by [START_REF] Gasse | Evolution of Lake Abhe[END_REF]. During this phase the overflow sills were certainly active, as evidenced by the DDa, DSa and DSb sites (Fig. V.4 and Figs. BI.11,BI.12,BI.13).

The resurgence of Mid-Holocene humid conditions as revealed by paleo-shoreline markers was recorded at ∼7.7 ka cal. BP, contemporaneous with lakeside human occupations characterized by fishing and hunting of aquatic fauna [START_REF] Khalidi | 9000 years of human lakeside adaptation in the Ethiopian Afar: Fisherforagers and the first pastoralists in the Lake Abhe basin during the African Humid Period[END_REF]. The lake reached the highest Mid-Holocene water level between ∼6 and ∼5.6 ka cal. BP (∼380-385 m a.s.l.). This lacustrine transgression corresponds to the Abhe-IV lacustrine phase defined by [START_REF] Gasse | Evolution of Lake Abhe[END_REF]. After a slow lowering of the waterbody between ∼5.6 and ∼4.6 ka cal. BP, the absence of littoral deposits at lower elevation suggests a drastic and rapid drop in level. Finally, the lake rose gently from 2.8 ka cal. BP until it reached its maximal Late Holocene level (330 m a.s.l.) at ∼1.5 ka cal. BP. Lake-level variations are generally attributed to changes in monsoon precipitation conditions over the catchment area (i.e., the Ethiopian Highlands, [START_REF] Gasse | Evolution of Lake Abhe[END_REF]. Paleoclimatic interpretation of lakelevel changes are rightly based on the analysis and modeling of the hydrological precipitation and evaporation balance [START_REF] Junginger | Hydrological constraints of paleo-Lake Suguta in the Northern Kenya Rift during the African Humid Period (15-5kaBP)[END_REF] coupled with paleolimnological studies. However, changes in lake level and sedimentation can be affected by local geomorphic factors which can mitigate the climatic signal, leading to an underestimation or overestimation of the modeled data. The 420, 400 and 385 m a.s.l. Lake Abhe highstands (Fig. V.7b) are considered to be precipitation-induced lake fillings from Ethiopian Highland summer rainfalls, regulated by overflows towards the Hanle basin (Figs. V.1c, V.6, V.7a). We observe several regression steps over the course of the Holocene that were the result of incision of the overflow sills and subsequent water-level stabilizations, and which are demonstrated by the multi-level abrasion platform sequences documented at the DDa and DSa/DSb overflow sites (Fig. V.4 and Figs. BI.11,BI.12,BI.13). Considering that enhanced silicate chemical weathering occurred throughout the intertropical latitudes during humid periods [START_REF] Bastian | Abrupt response of chemical weathering to Late Quaternary hydroclimate changes in northeast Africa[END_REF], the estimation of overflow incision rates could provide an additional parameter for the calculation of precipitation and evaporation in hydrological models.

Holocene changes in water level are not always the result of balanced source-to-sink mechanisms. A non-linear response of the hydrological system could be conditioned by the interconnections between basin catchments. During the Early/Middle Holocene, the Awash River catchment area was temporarily supplied by freshwater inputs from adjacent basins (Fig. V.8d). The Ziway-Shala paleolake overflow towards the Awash catchment was demonstrated for the first time by [START_REF] Nilsson | Ancient Changes of Climate in British East Africa and Abyssinia. A Study of Ancient Lakes and Glaciers[END_REF] at 1670 m a.s.l., and later revised by [START_REF] Grove | Late Quaternary Lake Levels in the Rift Valley of Southern Ethiopia and Elsewhere in Tropical Africa[END_REF], [START_REF] Gasse | Late Quaternary lake-level fluctuations and environments of the northern Rift Valley and Afar region (Ethiopia and Djibouti)[END_REF] and [START_REF] Gillespie | Postglacial arid episodes in Ethiopia have implications for climate prediction[END_REF] (Fig.

Figure V.7:

Holocene Abhe Lake -level reconstitution. a) Curve of Abhe Lake -level fluctuations (black line) showing the main lake highstands (light blue squares), the dated Facies-A (blue triangles), Facies-B (inverted blue triangles) and undetermined facies (empty blue triangle) stromatolites, the new (black squares) and published (black dots) dated nearshore deposits (Gasse, 1975, revised;), and the main human prehistoric occupations (empty red rhombuses). b) Histogram of the totality of the inventoried paleo-shoreline features (red lines) and stromatolite deposits (blue lines) showing counts against the elevations in ranges of 5 m. c) Longitude against elevation graph of the totality of stromatolites inventoried (empty blue triangles) showing the main Holocene Abhe Lake highstand levels (light blue bands).

V.8c). The possible transient fluvial connection to the Awash River is in phase with the beginning of the Early Holocene Lake Abhe highstand, coinciding with the maximal solar irradiation at 10°N around 11 ka cal. BP (Fig. V.8b, c). Similarly, the Mid-Holocene maximal Abhe Lake level, which occurred between 6 and 5.6 ka cal. BP, is in phase with the observed Mid-Holocene Ziway-Shala Lake overflow, which could have made a significant contribution to the Lake Abhe water-level balance. The Holocene connectivity between these two basins is even supported by the apparition of the Cyprinidae sp. fish in the Lake Abhe basin around ∼10 ka BP probably introduced from the Ziway-Shala basin where it was attested since 13 ka BP [START_REF] Coudert | La Place de La Pêche Pendant l'Holocène de La Vallée Du Rift Au Sahel Occidental[END_REF]. Additionally, [START_REF] Sagri | Latest Pleistocene and Holocene river network evolution in the Ethiopian Lakes Region[END_REF] demonstrated that during the Early Holocene, the Awash River contributed to the water filling of the Ziway-Shala Basin, confirming the mutual strong interconnection between these two basins. After excluding the possibility of tectonicinduced Holocene lake transgressions (Section V.1.4.b) and providing some elements that demonstrate the secondary local geomorphic control of lacustrine fluctuations, we can consider the Holocene Abhe lake-level reconstruction as a reliable record of monsoon-induced hydrologic changes over the Awash basin.

V.1.5.c Implications for Holocene East African moisture masses dynamics

Thanks to Lake Abhe's geographic position, located in the northern mouth of the EARS (Fig. V.1a), our results add an important new element to the discussion of the timing of the AHP transitions in Eastern Africa and more particularly across the EARS basins.

We compared our results with other southernmost lacustrine basins along the rift axis at latitudes between 12°N and the Equator, with the aim of studying Holocene hydrological feedback across a coarse N-S transect (Fig. V.10). From north to south, we included lake-level paleo-data from Abhe (11°-12°N; this study; Fig. V.9c), Ziway-Shala (7.5°-8. 5°N Gillespie et al., 1983, calibrated ). With the aim to constrain the hydro-climatic significance of these water-level changes, we added δDwax data -considered a proxy for precipitation/air-mass dynamics -from the northern and southern extremities of the investigated area (Tana Lake, 11.5°-12.5°N and Challa Lake, 3°S; [START_REF] Costa | Isotopic reconstruction of the African Humid Period and Congo Air Boundary migration at Lake Tana, Ethiopia[END_REF][START_REF] Tierney | Late Quaternary behavior of the East African monsoon and the importance of the Congo Air Boundary[END_REF]Fig. V.9a, g). Our results show a resurgence of humid conditions after the YD cooling event (Younger Dryas; [START_REF] Alley | The Younger Dryas cold interval as viewed from central Greenland[END_REF][START_REF] Rasmussen | A stratigraphic framework for abrupt climatic changes during the Last Glacial period based on three synchronized Greenland ice-core records: Refining and extending the INTIMATE event stratigraphy[END_REF] around ∼11.1 ka cal. BP, which indicate a reduction of the Early Holocene high hydrological regime from ∼8.7 ka cal. BP, and demonstrate a northward installation of Mid-to-Late Holocene aridification with a ∼1000 yr delay with respect to the southern EARS basins (Fig. V.9). Firstly, a shift and reduction in hydrographic regimes as early as ∼8.7 ka cal. BP has already been pointed out by recent studies, and interpreted as a consequence of changes in monsoonal dynamics over the Ethiopian Highlands [START_REF] Blanchet | Asynchronous Changes in Vegetation, Runoff and Erosion in the Nile River Watershed during the Holocene[END_REF][START_REF] Ménot | Timing and stepwise transitions of the African Humid Period from geochemical proxies in the Nile deep-sea fan sediments[END_REF][START_REF] Mologni | Frequency of exceptional Nile flood events as an indicator of Holocene hydro-climatic changes in the Ethiopian Highlands[END_REF].

Secondly, the observed time-latitude delays of the Mid-to-Late Holocene aridification are in conflict with the ITCZ oscillation processes: Mid-Holocene ITCZ southward migration should correspond to a Figure V.9: Paleovariations in a) June insolation at 10°N [START_REF] Berger | Insolation values for the climate of the last 10 million years[END_REF]; b) δDWAX from Lake Tana [START_REF] Costa | Isotopic reconstruction of the African Humid Period and Congo Air Boundary migration at Lake Tana, Ethiopia[END_REF]; c) water level of Abhe Lake (this study); d) human subsistence strategies in the Central Afar region [START_REF] Khalidi | 9000 years of human lakeside adaptation in the Ethiopian Afar: Fisherforagers and the first pastoralists in the Lake Abhe basin during the African Humid Period[END_REF]; and water levels of e) Ziway-Shala Lake [START_REF] Gillespie | Postglacial arid episodes in Ethiopia have implications for climate prediction[END_REF], f) Suguta Lake [START_REF] Junginger | Hydrological constraints of paleo-Lake Suguta in the Northern Kenya Rift during the African Humid Period (15-5kaBP)[END_REF] and g) Nakuru Lake [START_REF] Richardson | Paleolimnology of mid-elevation lakes in the Kenya Rift Valley[END_REF]. Original radiocarbon dates were calibrated and converted into calendar years. Paleovariatons in h) δDWAX from Lake Challa [START_REF] Tierney | Late Quaternary behavior of the East African monsoon and the importance of the Congo Air Boundary[END_REF] and June and December insolation at the equator (Berger and Loutre, 1991, yellow lines;) southward aridification, while we observe the contrary (Fig. V.9). However, changes in precipitation are not only the result of atmospheric convection over the rain belt, but could have also been produced by the migration of other convergence zones. As proposed by [START_REF] Tierney | Late Quaternary behavior of the East African monsoon and the importance of the Congo Air Boundary[END_REF] and [START_REF] Junginger | The effects of solar irradiation changes on the migration of the Congo Air Boundary and water levels of paleo-Lake Suguta, Northern Kenya Rift, during the African Humid Period (15-5ka BP)[END_REF], the incursion of the Congo Air Boundary (CAB) in the specific morpho-tectonic context of the EARS water-sources would have contributed additional moisture to persistent humid conditions, in contrast to the Mid-Holocene southern migration of the ITCZ. This process would have been the result of a high pressure gradient between the CAB and the Indian Summer Monsoon (ISM) air masses, which enhanced the influence of the West African Summer Monsoon (WAM) over Northeast Africa [START_REF] Camberlin | Rainfall Anomalies in the Source Region of the Nile and Their Connection with the Indian Summer Monsoon[END_REF][START_REF] Junginger | The effects of solar irradiation changes on the migration of the Congo Air Boundary and water levels of paleo-Lake Suguta, Northern Kenya Rift, during the African Humid Period (15-5ka BP)[END_REF][START_REF] Marzin | The role of the ocean feedback on Asian and African monsoon variations at 6kyr and 9.5kyr BP[END_REF]. The same hypothesis has been advanced to explain the lower and prolonged leaf-wax values at Tana Lake during the Mid-Holocene period (Costa et al., 2014, Fig. V.10b). The intensified ISM favored an east-shifting of the CAB, fueling humid conditions over the Ethiopian Highlands. In this scenario, thanks to its Eastern Ethiopian Highland sources, Lake Abhe's hydrology would have been particularly subjected to the CAB moisture mass influences from the beginning to the end of the AHP. In contrast, Challa Lake, which lies beyond the CAB boundary ( In summary, the persistence of ISM-CAB pressure regulation combined with the geomorphic patterns of the EARS, enhanced the influence of the CAB moisture in the northeast African Rift until the end of the AHP producing a progressive northward development of the arid Mid-Holocene conditions across the EARS (Fig. V.10). These patterns show an inverse air-moisture dynamic compared with Central and Western intertropical Africa, where the development of Middle-Holocene arid conditions followed the progressive southward retreat of the ITCZ (Shanahan et al., 2015, Fig. V.9h, i).

V.1.5.d Implications for the emergence of herding in the Horn of Africa

As demonstrated by [START_REF] Khalidi | 9000 years of human lakeside adaptation in the Ethiopian Afar: Fisherforagers and the first pastoralists in the Lake Abhe basin during the African Humid Period[END_REF] (SectionVI.1), in the Main Ethiopian Rift (MER) as well as in the Afar Triangle, herding economies appear belatedly around ∼4.8 ka cal. BP, presenting a 3-4 ka delay with the Arabian and North African neighboring regions [START_REF] Chritz | Climate, ecology, and the spread of herding in eastern Africa[END_REF][START_REF] Coudert | La Place de La Pêche Pendant l'Holocène de La Vallée Du Rift Au Sahel Occidental[END_REF][START_REF] Khalidi | 9000 years of human lakeside adaptation in the Ethiopian Afar: Fisherforagers and the first pastoralists in the Lake Abhe basin during the African Humid Period[END_REF]. Adaptability of prehistoric societies to abrupt climate change is demonstrated by some Later Stone Age (LSA; fisher-hunter-gatherers) settlements around Lake Abhe during the ∼8.2 ka arid event (Fig. V.7a, V.9d; [START_REF] Khalidi | 9000 years of human lakeside adaptation in the Ethiopian Afar: Fisherforagers and the first pastoralists in the Lake Abhe basin during the African Humid Period[END_REF], and by the persistence of fishing practices with the introduction of livestock economies during the Mid-Late Holocene [START_REF] Coudert | New archaeozoological results from Asa Koma (Djibouti): Contributing to the understanding of faunal exploitation during the 3rd millennium BC in the Horn of Africa[END_REF][START_REF] Lesur | The advent of herding in the Horn of Africa: New data from Ethiopia, Djibouti and Somaliland[END_REF]. However, prehistoric Mid-to-Late Holocene societal dynamics seem to be particularly linked to the East African climate and more precisely to the influence of lake water oscillations for two reasons: the natural barriers and corridors produced by extreme fluctuations of huge lacustrine waterbody surfaces and the abundance of aquatic resources, both assured by the enhanced monsoonal regime.

We demonstrated that in the northern EARS, humid conditions were assured by highstand lake levels until ∼4.6 ka cal. BP. In contrast, in the Sahelian area, aridification processes seemed to begin earlier, between 6 and 5.5 ka cal. BP [START_REF] Amaral | Palynological evidence for gradual vegetation and climate changes during the African Humid Period termination at 13°N from a Mega-Lake Chad sedimentary sequence[END_REF][START_REF] Cremaschi | Holocene Climatic Changes and Cultural Dynamics in the Libyan Sahara[END_REF][START_REF] Lézine | Late Holocene plant and climate evolution at Lake Yoa, northern Chad: Pollen data and climate simulations[END_REF][START_REF] Lézine | Across an early Holocene humid phase in western Sahara: Pollen and isotope stratigraphy[END_REF], together with the consolidation of herding practices [START_REF] Hoelzmann | Environmental change and archaeology: Lake evolution and human occupation in the Eastern Sahara during the Holocene[END_REF][START_REF] Kuper | Climate-controlled Holocene occupation in the Sahara: Motor of Africa's evolution[END_REF]. We propose that the air moisture mass dynamics which were responsible for the de- [START_REF] Fick | WorldClim 2: New 1km spatial resolution climate surfaces for global land areas[END_REF]. Blue stars indicate the location of lacustrine basins reported in Fig. V.9: b -Lake Tana; c -Lake Abhe; e -Ziway-Shala Lake; f -Lake Suguta; g -Lake Nakuru; and h -Lake Challa. Dashed green line indicates the suggested position of the CAB before 8.5 ka cal. BP, while the dashed red lines indicate the position of the CAB and ITCZ after 8.5 ka cal. BP. Black arrows correspond to the air moisture mass direction of the WAM and of the ISM.

layed Holocene rainfall regime across the northern EARS, helped produce the environmental conditions that were favorable for and assured the persistence of wild resource exploitation by human groups in the region, even while adjacent regions turned to herding.

V.1.6 Conclusion

The documentation, interpretation and dating of more than one hundred littoral geomorphic features have permitted the reconstruction of a detailed paleo-shoreline sequence that elucidates the Holocene lake-level rise and fall of the widest lacustrine system in the Afar region, Lake Abhe. We observe that after the Younger Dryas arid event (∼12.9 to ∼11.7 ka) Lake Abhe began to rise around 11.1 ka cal. BP peaking at ∼9.8 ka cal. BP, in phase with the maximum insolation values over the tropics. A gradual decrease in lake level is recorded from ∼9 ka cal. BP until a drastic drop in level occurred at ∼8 ka cal. BP. A resurgence of humid conditions is confirmed from ∼7.7 ka BP with the establishment of a stable lake highstand until ∼4.6 ka cal. BP when the large waterbody was definitively reduced by a prominent desertification process in the Afar region. Nonetheless, during the Late Holocene, a minor lake transgression was recorded between ∼2.4 and ∼1.4 ka cal. BP, followed by a lake lowstand which persists until the present day. Past Lake Abhe level fluctuations are the result of monsoonal dynamics concentrated in the Ethiopian Highlands. While long-term hydrological changes are mainly triggered by orbitally-driven insolation pacing, our results suggest that short-term events, as well as the variability of humid/arid period durations over continental Africa, may be related to internal/regional forcing mechanisms which are not yet fully understood. Comparison of our results with other lacustrine archives indicates a northward Mid-Holocene aridification along the EARS, which disagrees with the classic scenario of the ITCZ southern retreat as a possible cause of the southern Mid-Holocene aridification over Africa. This evidence could be explained by the incursion of air moisture masses in the EARS induced by a high pressure gradient that formed between the Congo Air Boundary and the Indian Summer Monsoon air masses, resulting in enhanced precipitation over Northeastern Africa. These conditions would have supported the persistence of large waterbodies and their related ecosystems in the northern EARS lowlands until ∼4.6 ka cal. BP. The late adoption of herding and the persistence of hunter-gathering and fishing practices are well documented in the region and correlate well with the continuity of humid environmental patterns in the EARS. In this favorable environmental scenario, populations in the EARS would have benefitted two millennia longer than in surrounding regions like the Sahel, from the rich wild resources provided by ample lakes and rivers.

V.2 Characterisation and significance of the Holocene pedogenic response to East African monsoon dynamics in the Lake Abhe basin

V.2.1 Introduction

Hydro-climatic conditions over intertropical latitudes are the main driver of long-term (millennial) or short-term (centennial) local-to-regional environmental changes. With cycles of 19 and 23 ka, the precession has been the main driver of summer insolation intensity over the tropics, which is at the origin of the deep convection air moisture around the Intertropical Convergence Zone (ITCZ; [START_REF] Mohtadi | Palaeoclimatic insights into forcing and response of monsoon rainfall[END_REF][START_REF] Viste | Moisture transport into the Ethiopian highlands: MOISTURE TRANSPORT INTO THE ETHIOPIAN HIGHLANDS[END_REF]. At the Quaternary scale, following precessional-driven insolation pacing, the rain belt (ITCZ) migrated latitudinally under the cross-equatorial pressure gradients which are associated with the modulation of precipitation intensities, producing an alternation of arid and humid periods. The last interval of intense rainfall falls between 14 and 6 ka BP and is widely known as the 'African Humid Period' (AHP;[START_REF] Demenocal | Abrupt onset and termination of the African Humid Period: Rapid climate responses to gradual insolation forcing[END_REF][START_REF] Shanahan | The time-transgressive termination of the African Humid Period[END_REF]Tierney et al., 2011a). During this period, enhanced moonsonal activity over the East-African Highlands [START_REF] Costa | Isotopic reconstruction of the African Humid Period and Congo Air Boundary migration at Lake Tana, Ethiopia[END_REF] produced a massive extension of continental water surfaces [START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF], the northward spread of vegetation cover [START_REF] Hopcroft | Multi vegetation model evaluation of the Green Sahara climate regime: RAINFALL SUPPORT-ING A GREEN SAHARA[END_REF] and an important reactivation of fluvial systems [START_REF] Mologni | Frequency of exceptional Nile flood events as an indicator of Holocene hydro-climatic changes in the Ethiopian Highlands[END_REF]. Long-term orbital forcing was the main responsible for the timing (onset and termination) of the AHP. However, the amplitude offsets of the AHP between Western and Eastern Africa, as well between the Sahelian and Equatorial zones, may have been subjected to regional internal forcing [START_REF] Demenocal | Abrupt onset and termination of the African Humid Period: Rapid climate responses to gradual insolation forcing[END_REF][START_REF] Shanahan | The time-transgressive termination of the African Humid Period[END_REF]. Additionally, the installation of short-term (200-400 yr) arid episodes, likely synchronous with the North-Atlantic cooling events (Heinrich Event 1 = HE1, Younger Dryas = YD, 8.2, 4.2 ka events; [START_REF] Collins | Abrupt shifts of the Sahara-Sahel boundary during Heinrich stadials[END_REF][START_REF] Rasmussen | A stratigraphic framework for abrupt climatic changes during the Last Glacial period based on three synchronized Greenland ice-core records: Refining and extending the INTIMATE event stratigraphy[END_REF][START_REF] Rohling | Centennial-scale climate cooling with a sudden cold event around 8,200 years ago[END_REF], punctuated the AHP.

The issues regarding the internal variability of the AHP is widely approached with multi-proxy analysis on continuous lacustrine, marine or deltaic archives, providing a solid high-resolution (millennial to centennial) paleoclimatic data [START_REF] Blanchet | High-and low-latitude forcing of the Nile River regime during the Holocene inferred from laminated sediments of the Nile deep-sea fan[END_REF][START_REF] Foerster | 46 000 years of alternating wet and dry phases on decadal to orbital timescales in the cradle of modern humans: The Chew Bahir project, southern Ethiopia[END_REF][START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF][START_REF] Revel | 20,000 years of Nile River dynamics and environmental changes in the Nile catchment area as inferred from Nile upper continental slope sediments[END_REF]Tierney et al., 2011a). However, research on local pedogenic processes could be a complementary powerful tool for the comprehension of hydrological variation through time [START_REF] Sheldon | Quantitative paleoenvironmental and paleoclimatic reconstruction using paleosols[END_REF]. In particular, paleosoil systems in arid regions could provide a useful datasets for the understanding of past monsoonal dynamics [START_REF] Felix-Henningsen | Paleosols on Pleistocene dunes as indicators of paleo-monsoon events in the Sahara of East Niger[END_REF]. This kind of study were able to refine and complement the comprehension of the patterns of installation, internal oscillations and demise of humid periods [START_REF] Pietsch | Holocene soils and sediments around Ma'rib Oasis, Yemen: Further Sabaean treasures?[END_REF][START_REF] Pietsch | Early Holocene paleosols at the southwestern Ramlat As-Sab'atayn desert margin: New climate proxies for southern Arabia[END_REF]. Moreover, pedologicical studies are also important for confirming trends identified using other proxies and understanding the environments that humans occupied.

The evolution of geomorphic/pedogenic systems remains little investigated across intertropical Africa. There have been few studies soil systems over the Sahelian -Saharian region that show the occurrence of humid events between 9 and 4 ka BP [START_REF] Cremaschi | The palaeoclimatic significance of paleosols in Southern Fezzan žLibyan Sahara/: Morphological and micromorphological aspects[END_REF][START_REF] Dal Sasso | Radiocarbon dating reveals the timing of formation and development of pedogenic calcium carbonate concretions in Central Sudan during the Holocene[END_REF][START_REF] Felix-Henningsen | Paleosols on Pleistocene dunes as indicators of paleo-monsoon events in the Sahara of East Niger[END_REF][START_REF] Mauz | Palaeosols in Saharan and Sahelian dunes of Chad: Archives of Holocene North African climate changes[END_REF][START_REF] Williams | Climate Change in Deserts: Past, Present and Future[END_REF][START_REF] Zerboni | Micromorphological approach to polycyclic pedogenesis on the Messak Settafet plateau (central Sahara): Formative processes and palaeoenvironmental significance[END_REF]. Over Ethiopian Highands some authors evidenced the link between Human settlements end the development of favourable environments and soil systems at the end of this Humid phase on the basis of an organic chemistry approach [START_REF] Terwilliger | Environmental changes and the rise and fall of civilizations in the northern Horn of Africa: An approach combining δD analyses of land-plant derived fatty acids with multiple proxies in soil[END_REF]. The specific rift-controlled patterns of the EARS, characterized by extreme contrast between high precipitation rates in the highlands and high evaporation in the lowlands, implies the almost complete dependence of semi-arid distal-catchment hydro-sedimentary systems to the summer monsoon discharge on mountain areas. These are the characteristics of the formation of amplifier-type lakes [START_REF] Trauth | Human evolution in a variable environment: The amplifier lakes of Eastern Africa[END_REF]. In this perspective, the pedological sequences developed around ancient lake shorelines may represent a sensitive proxy of regional scale hydrological changes.

During the Holocene, Lake Abhe experienced strong hydrological variations under the forcing of the East African monsoonal regime. In Section V.1 a reconstruction of Abhe Lake level fluctuations, revealing changes in hydrological variations over this basin was proposed. However, to better constrain the evolution of littoral lacustrine landforms and to better estimate the impact of these hydrological oscillations on the morphosedimentary systems of Lake Abhe, I have developed an approach based on the investigation of Holocene paleosoils. The objectives of this study are:

1. The sedimentological and geochemical (XRF) characterization of the Holocene paleosoils of the Lake Abhe basin. 2. The definition of micromorphological, grain-size distribution and major elemental ratio markers which allows to identify types of paleosoils associated to different environmental formation contexts. 3. The integration of these soil typologies and their environmental significance into a model of Holocene Abhe Lake level fluctuations.

This study will allow us to specify the changes of littoral pedo-sedimentary environments of paleo-Lake Abhe during the Holocene. Additionally, this information will help to precise the sedimentary formation processes of archaeological sequences presented in Chapter VI, with the aim to better understand inhabited prehistoric environments during the LSA -Neolithic transition (see Section VI.2).

V.2.2 Materials and Methods

For this study, 19 pedo-sedimentary sequences, which contain at least one pedological horizon, were documented across the Lake Abhe basin (Fig. V.11, Tab. V.3). A total of 38 pedological horizons from these sequences were sampled and analysed using sedimentological and physico-chemical analytical methods (Tab. BI.9). The majority of paleosoils were sampled along the archaeological sedimentary sequences presented in Section VI. 14 of these pedological horizons were dated with the radiocarbon technique (see Section III.3.1 for methods details and Tab. V.3 for results). PXRF (see Section III.3.4.b), grain-size laser diffraction (see Section III.3.3.a) and Loss on Ignition (see Section III.3.3.b) are the principal analytical methods applied on the majority of the collected samples (Tab. V.3). Additionally, 5 thin sections from five different pedological horizons were analysed under the microscope, following soil micromorphology description and interpretation methodology (see Section III.3.2.a; Tab. V.3).

V.2.3 Results and interpretation

V.2.3.a Field observations

Fieldwork observations give fundamental information on the context of pedo-stratigraphic sequences. They provide the first information needed for the interpretation of paleosoil formation. We thus provide a preliminary classification of pedological horizons observed on the basis of fieldwork macroscopic observation.

Type-1 concerns pedological horizons normally intercalated between paleodune deposits (Fig. V.12a). Paleosoils from aeolian context are thin (2 to 10 cm thick), pale brown, with a well sorted clayeysilty texture, well-developed polyhedral structure and without evident macroscopic inclusions. The intrapedal porosity is usually filled by aeolian silty particles. Type-2 is represented by thick dark brow well sorted clayey accumulations (>1 m thick) with a polyhedral to prismatic structure normally found in the Lake Abhe basin plain (Fig. V.12b). Macroscopic inclusions sometimes include scattered ostrachods and bivalve shells, and gypsum minerals inside the peds blocks or along the intrapedal porosity. Type-3 paleosoils do not exceed ∼1m thickness and are often developed in a slope context with some embedded local detritic clastic components (Fig. V.12c). This kind of pedological horizon presents a pale brown to pale red brown colour, a moderately polyhedral to granular structure, including moderate sandy to gravel loose clastic components.

V.2.3.b Soil Micromorphology description and interpretation

Analysing thin sections selected from each paleosoil type allows us to define the microscopic pedological markers that help to differenciate them, to refine interpretation of their formation processes and to point out their environmental significance. In micromorphological descriptions we principally take into consideration the micromorphological markers given by the grain size, the presence of organic matter, fish bone or shells, as well as the groundmass patterns, microstructure/voids and the pedofeatures. Micromorphological description of each paleosoil type is presented in Tab. V.4, V.5 and V.6, and the extended description and interpretation is presented in the main text. Paleosoil Type-1 facies is characterized by a clayey to silty well sorted (sub-rounded to rounded) coarse fraction, randomly distributed and oriented. C/f related distribution can vary from zonate close Particularly, micritic hypocoatings could indicate plant evapotranspiration during a dry season resulting from rapid precipitation of calcium carbonate due to root metabolism [START_REF] Wieder | Micromorphological fabrics and developmental stages of carbonate nodular forms related to soil characteristics[END_REF]. A typical marker of this soil facies is the fine clayey crust on the soil surface (Fig. V.13a, c), or some fragmented crusts in the groundmass, which is linked to the leaching process of fine particles from overlying aeolian levels and the subsequent aeolian deflation processes during soil formation. Common parallel striated b-fabric (Fig. V.13d) indicate a rapid shift from relative wet to dry seasons, which induce a dynamic clay swelling process [START_REF] Embrechts | Microscopic identification and quantitative determination of microstructure and potentially mobile clay in a soil catena in a humid tropical environment[END_REF][START_REF] Fedoroff | Les vertisols du sud est de la Beauce[END_REF]. This process leads to a planar zone reorientation of clay particles following a face-to-face alignement [START_REF] Wilding | Genesis of Vertisols: Shrink-Swell Phenomena[END_REF]. These planar clay-reorientation zones can correspond to a mechanical process typical of vertic materials [START_REF] Mccormack | Proposed origin of lattisepic fabric[END_REF]Mermut et al., 1996b,a). Vertic features can be due to the rapid shift in water supplies in clayey materials [START_REF] Kovda | Vertic Features[END_REF]. Very well sorted clay with porphyric c/f related distribution patterns organised in angular blocky pedality are the main groundmass characteristics of this kind of pedological horizon. These elements, combined with moderate amounts of fish bone and shells ( As a crystallitic pedofeature, moderate amounts of random and parallel orientated gypsic euhedral crystals were observed, presenting clay impurities and interpreted as crystal intergrowths of pedogenic origin (Fig. V.14g, h). These intergrowths are a characteristic features of arid soils [START_REF] Stoops | Guidelines for Analysis and Description of Soil and Regolith Thin Sections[END_REF]. Zonations of gypsum intergrowths can develop in dry saline lake environments, following an oscillating groundwater table or in waterlogging phenomena [START_REF] Pankova | Forms of gypsic neoformations as a controlling factor affecting the meliorative properties of gypsiferous soils[END_REF][START_REF] Stoops | Gypsum in arid soils, morphology and genesis[END_REF][START_REF] Watson | Structure, chemistry and origins of gypsum crusts in southern Tunisia and the central Namib Desert[END_REF]. In the same micro-unit, gyspum intergrowth crystals are associated with large amounts of redox features such as dendritic or aggregate orthic Fe-Mn nodules, and indicate water saturation processes (Fig. V.14g;[START_REF] Arshad | OCCURRENCE AND CHARACTERISTICS OF FERROMANGANIF-EROUS CONCRETIONS IN SOME SASKATCHEWAN SOILS[END_REF][START_REF] Richardson | Mottling and Iron Distribution in a Glossoboralf-Haplaquoll Hydrosequence on a Glacial Moraine in Northwestern Wisconsin[END_REF][START_REF] Schwertmann | Iron-manganese concretions in a hydrosequence of soil in loess in Bavaria[END_REF]. Additionally, we observe a great deal of crystallitic pedofeatures such as micritic coatings, sparitic nodules and extended micritic crystallitic b-fabric (Fig. V.14f). These pedofeatures likely suggest a transition from wet to dry periods with a temporary water table perched in the lower hori-

Paleosoil Type-1

Paleosoil Type-3

This pedofacies is characterized by a coarser moderately sorted mineral fraction organised in enaulic Figure V.14: Microphotos from paleosoil Type-2 facies : a) well-developed angular blocky microstructure with accommodated planes with double to single spaced prorphyric c/f related distribution patterns (PPL); b) clayey pedorelict with a Fn-Mn dark red coating in channel porosity (PPL); c) pedorelict composed of oriented clays (XPL); d) earthworm bioturbation in angular blocky microstructure evident in loose continuous clayey-silty infilling in channel void (PPL); e) shell fragment (XPL); f) in situ fragmented fish bone with typic anorthic micritic and dendritic orthic Fe-Mn nodules and single crystal gypsum intergrowth, in a single to doube spaced porphyric c/f related distribution patterns (XPL); g) single crystal gypsum intergrowth with hypocoating of dendritic orthic Fe-Mn nodule impregnation (XPL); h) multiple random gypsum crystal intergrowths in plane porosity (XPL).

V.2.3.c Geochemical/sedimentological characteristics and interpretation

The geology of the Central Afar Region and of the northern part of the EARS is mainly composed of basaltic and rhyolitic outcrops (see Section I.2.1 and I.2.3.c), characterized by high Fe, Mg, Si elemental contents (Fig. V.16, Barberi and Varet, 1977).

With the aim to characterize the geochemical variability of the Lake Abhe basin pedological deposits investigated in this study, Si, Ti, Al and K major elements were selected from the PXRF analyses results (Tab. BI.9). We have chosen to exclude Mn, Fe, Ca, Sr elements from our interpretation because they may be the result of oxide-reduction or secondary hydromorphic processes, which can vary strongly in space and in time, and because they don't permit to discriminate clearly source processes. Si, Ti, Al and K major elements variations could track three different processes: the proportion between the primary and secondary mineral fractions, the mineralogy sources, and the intensity of mineral weathering, which are difficult to discriminate. However, coupling geochemical result with microscopic observations, we were able to propose a preliminary interpretation, which deserves to be refined with further analyses. K is considered an alkaline mobile element during the rock weathering process. Contrarily, Al, Si and Ti are less variable during weathering, and thus considered as immobile elements. Is for this reason that K/Al can be used as a tracer of chemical weathering of siliciclastic rocks. Prevalent Ti and Si counts are interpreted as a local inherited siliciclastic material from the erosion and weathering of mineralogical assemblages (such pyroxene and feldspar), mainly represented by basalts. Additionally, in clayey layers, which is the case of the majority of pedogenic deposits investigated here, Ti can indicate the prevalence of primary mineral such a Chlorite [START_REF] Montero-Serrano | Changes in precipitation regimes over North America during the Holocene as recorded by mineralogy and geochemistry of Gulf of Mexico sediments[END_REF][START_REF] Neal | The mineralogy and chemistry of finegrained sediments, Morphou Bay, Cyprus[END_REF] or secondary minerals such as Smectite/Montmorillonite [START_REF] Sterte | Synthesis and Properties of Titanium Oxide Cross-Linked Montmorillonite[END_REF] rather than other terrestrial siliciclastic fine materials. Differently, K element can be associated to primary minerals such Mica, with potassium mica, but also with potassium feldspar (microcline, orthoclase, sanidine) if sediments are coarser (silty-sand fraction). In clayey layers, K reflect secondary minerals mainly associated with terrestrial siliciclastic illite clays [START_REF] Hashemi | Potassium reserves in soils with arid and semi-arid climate in southern Iran: A perspective based on potassium fixation[END_REF][START_REF] Holzwarth | NW African climate variations during the last 47,000years: Evidence from organic-walled dinoflagellate cysts[END_REF][START_REF] Kuhlmann | The transition of the monsoonal and the N Atlantic climate system off NW Africa during the Holocene: HOLOCENE CLIMATE OF NW AFRICA[END_REF][START_REF] Romero | Submillennial-to-millennial variability of diatom production off Mauritania, NW Africa, during the last glacial cycle: DIATOM VARIATIONS IN THE NORTHEAST AT-LANTIC[END_REF]. Additionally, have been demonstrated that K-rich clay (illite) is formed diagenetically from smectite under arid conditions, and particularly in dried out lakes [START_REF] Foerster | Late Quaternary Climate Variability in the Source Region of Homo Sapiens. Dry-Wet Cycles in Chew Bahir, Southern Ethiopia[END_REF][START_REF] Foerster | 46 000 years of alternating wet and dry phases on decadal to orbital timescales in the cradle of modern humans: The Chew Bahir project, southern Ethiopia[END_REF][START_REF] Trauth | Abrupt or gradual? Change point analysis of the late Pleistocene-Holocene climate record from Chew Bahir, southern Ethiopia[END_REF].

PXRF analyses has been made also on actual or sub-actual soils in slope context, with the aim to give a geochemical referential of these kind of horizons (Fig. V.17). All paleosoils of Type-1 (aeolian context) present homogenous geochemical and grain-size (Q90 and Q10) characteristics with relatively high Si and K counts (Fig. V.17a,b,c). From micromorphological observations we determined that the coarse fraction has an aeolian clastic support origin. Well sorted peaks around ∼20 -∼70 µm and at ∼100 µm can thus be considered to be the aeolian material inherited by the paleosoil (Fig. V.18a). Increase in Si/Al is thus interpreted as a mineral fraction resulting from the weathering of the original aeolian castic material from which these soils were developed. From this point of view, high K/Al values could be linked to the allochtonous phyllosilicates developed in arid environments or K-mica aeolian inputs. This facies contains ∼2 to ∼5 % organic matter, confirming its arid formation context (Fig. Considering the majority of clay particles, high K/Al values could be linked to illite materials delivered by Awash River transport into the Lake Abhe basin plain. In these deposits the organic matter is very high, ranging between ∼4 to ∼9 %, which indicates their formation during a humid period (erosion of vegetated watershed), and characterized by the preservation of the organic matter in semi-anoxic conditions assured by water logging processes (Fig. ) confirming the detrital character of this pedo-facies. Similarly, higher Ti/Al ratio and lower K/Al ratios than type 1 et 2 can indicate a higher contribution of primary minerals (feldpath, mica), coarser, eroded from the local basalt. Finally, paleosoils of Type-3, with an LOI ranging between ∼3 to ∼7 %, present a relatively high and variable organic matter content (Fig. V.17d), which probably depends on their period or formation and on specific local topographical/morphosedimentary contexts.

V.2.4 Synthesis and Discussion

Micromorphological, sedimentological and geochemical characterization of 38 pedological horizons sampled across the Lake Abhe basin has permitted us to propose a possible context of their formation. Three paleosoil groups have been determined: the first (Type-1), characteristic of arid or semi-arid environments and developed from an aeolian clastic support; the second (Type-2) includes all pedological horizons which developed from a palustrine context, with strong water supplies and waterlogging dynamics; the third (Type-3) include the paleosoils that developed in a slope context, with high detrital clastic support, moderate water supplies and slight pedogenesis.

These results contribute another element to the comprehension of the morphosedimentary and environmental evolution that occurred around Abhe Lake during the Holocene. Additionally, this information can be used to refine the Holocene Lake Abhe fluctuation curve proposed in Section V.1. These new data from soil studies (type, elevation and age of soils), has been integrated in the Lake Abhe fluctuation curve proposed in Section V (Fig.

V.19).

Figure V.19:

Curve of Abhe Lake-level fluctuations (black line) with dated stromatolites of Facies-A (blue triangles), Facies-B (inverted blue triangles) and undetermined facies (empty blue triangle), the new (black squares) and published (black dots) dated nearshore deposits (Gasse, 1975, revised;), and the dated paleosoil Type-1 (green dots), Type-2 (black cross) and Type-3 (red dots).

The paleosoil sequence chronology generally follows paleo-lake fluctuation (Chapter V.1) and archaeological (Section VI) trends. Moreover, it permits us to add some additional information. Paleosoils of Type-1 are localised around ∼4 -∼3 ka cal. BP and at ∼12.5 ka cal. BP. These two periods are characterised by the occurrence of Lake Abhe low stands in phase with short-term periods of aridity (the 4.2 ka and the Younger Dryas events), confirming our interpretations. Indeed, these soils were developed from aeolian deposit clastic supports. Paleosoils of Type-2 are linked to the Lake Abhe highstands, and occur just below the lake level. Their elevation and chronology indicating the installation of palustrine environments in shallow lacustrine waters confirms our interpretation of the pedological formation processes of this paleosoil type. Finally, paleosoils of Type-3 were localised just above the paleo-lake level highstands. These soils were thus developed during humid conditions on the surrounding slopes of the basin, indicating how the littoral environments of Lake Abhe were affected by pedogenic pro-cesses on the graben horsts in concomitance with high lake water-levels. Moreover, the localisation of a palustrine deposit at >12.5ka cal. BP (Fig. V.19;7 and 11 sites in Tab. V.3 and Fig. V.11) allows us to extend the chronology of the paleo-hydrological reconstitution of the Lake Abhe basin to the Late Pleistocene/Early Holocene transition. During this period, palustrine waterlogged environments located around 360 m a.s.l. allow us to define a lake level signal before the occurrence of the Younger Dryas arid event (∼13.4 -∼11.1 ka cal. BP; see Section VI.2 for other morphosedimentary evidence of the YD event), and to refine the Abhe Lake level fluctuation curve for this period.

Chapter VI

Holocene climatic and paleolandscape changes and the emergence of the first pastoralists in the Lake Abhe basin

Key points

The first part of this Chapter (Section VI.1) was published in the Journal Quaternary Science Reviews [START_REF] Khalidi | 9000 years of human lakeside adaptation in the Ethiopian Afar: Fisherforagers and the first pastoralists in the Lake Abhe basin during the African Humid Period[END_REF] with the title "9000 years of human lakeside adaptation in the Ethiopian Afar: Fisher-foragers and the first pastoralists in the Lake Abhe basin during the African Humid Period". In this section we present the results of three years of combined geoarchaeological, geomorphological and archaeological research over the Lake Abhe basin (Ethiopia and Djibouti). Firstly, we provide the first Holocene chrono-cultural sequence of lakeside human occupations in the Ethiopian Afar in association with fluctuating waters (Section VI.1). Secondly, the sedimentary formation processes of archaeological sequences are described in detail and interpreted from a paleoclimatic perspective (Section VI.2), providing preliminary information on paleolandscape feedbacks of Holocene Lake Abhe under hydro-climate forcing. These results permit us to better discuss the link between Holocene East African hydro-climatic changes and human adaptation during the transition from hunter-gathering to food production societies in the Lake Abhe and wider Horn of Africa region. The results of this section contribute to the discussion of the issues presented in Sections II.2.2.a and II.3.4 about the link between Earth surface process dynamics and prehistoric settlements in a Holocene oscillating lake-level perspective, and about the role of geomorphological/geoarchaeological studies in East African paleoclimate research.

The significant research results of this work are:

• First evidence of Later Stone Age to herding transition in the Ethiopian Central Afar Region.

• Human occupation of lake margins during humid-arid transitions of the African Humid Period.

• Humans responded to strongly fluctuating environments with flexible mixed economies.

• Unstable environments affected the Lake Abhe basin before, during and after the Younger Dryas arid event.

• Belated humid environmental conditions would have favored the persistence of fisher-huntergatherer subsistence practices over the emergence of pastoralism in the Horn of Africa in the second half of the Holocene.

VI.1.1 Introduction

The East African Rift region is renowned for vast exposed sedimentary formations hosting an exceptional hominin record. As a result it has attracted research programs focused on the recovery of our very ancient past at the expense of more recent prehistoric periods. During the Early to early Late Holocene (11.7-3 ka), global climate changes hosted different transitions from hunter-gathering to food production across the world that changed the course of humanity, and yet this period remains largely uninvestigated in the region. Previous studies have tended to suggest correlations between hydroclimatic change and the spread of agro-pastoralism from a center outward to explain these transitions [START_REF] Kuper | Climate-controlled Holocene occupation in the Sahara: Motor of Africa's evolution[END_REF][START_REF] Ashley | Paleoenvironmental Reconstruction of Dongodien, Lake Turkana, Kenya and OSL Dating of Site Occupation During Late Holocene Climate Change[END_REF], rarely taking their inter-and intra-regional arhythmicity or local paleoecological considerations into account. Poor chrono-cultural, hydroclimatological and paleoecological resolution at the inter-regional scale are major obstacles to advancing our understanding of the complexity of the human trajectory -one that is marked by junctures during the Holocene when hundreds of thousands of years of hunting-gathering were replaced by domestication at different rhythms and in different ways [START_REF] Lesur | The advent of herding in the Horn of Africa: New data from Ethiopia, Djibouti and Somaliland[END_REF]. This is certainly the case in the Horn of Africa, a region shaped by a mixture of its tropical latitude and the morpho-tectonic patterns of the East African Rift System into a complex mosaic of steep topography, variable soils, rainfall and vegetation patterns, and the resulting complex hydrography and sedimentation that characterize it. It is within this context that we find extremely varied tempos and types of transition to food production that have likely more to do with local paleoecology than with climate, human migration or cultural influence per se [START_REF] Chritz | Climate, ecology, and the spread of herding in eastern Africa[END_REF].

In Ethiopia more specifically, the few Early to Late Holocene archaeological sites recovered have little in common and are chronologically and spatially dispersed, providing us with insufficient correlations with which to reconstruct a concrete regional Holocene sequence (Fig. VI.1a). Interestingly some of the most comprehensive dated human occupation sequences spanning the Holocene can be found on paleomargins of large lake basins in the Main Ethiopian Rift (MER), namely Lakes Ziway-Shala and Abhe [START_REF] Bon | Archéologie préhistorique de la partie centrale du Main Ethiopian Rift (bassin lacustre de Ziway-Shala) : contribution à l'établissement de la séquence Late Stone Age d'Afrique orientale[END_REF][START_REF] Brandt | A Late Quaternary Cultural/Environmental Sequence from Lake Besaka, Southern Afar, Ethiopia[END_REF][START_REF] Gutherz | Asa Koma : Site Néolithique Dans Le Bassin Du Gobaad (République de Djibouti. Mondes Anciens[END_REF]Ménard and Bon, 2015, Fig. VI.1a). The exceptional sensitivity of water level fluctuations in rift environment lakes such as these to even moderate climate variation was recorded by [START_REF] Gasse | Late Quaternary lake-level fluctuations and environments of the northern Rift Valley and Afar region (Ethiopia and Djibouti)[END_REF] who coined the term amplifier lakes to distinguish them from lakes with less variable surface to volume ratios.

It has been hypothesized that these amplifier lakes may have had a principal role in human evolutionary trajectories in the East African Rift System (EARS) due to their amplified response to climatic variation [START_REF] Maslin | East African climate pulses and early human evolution[END_REF]. This is explained by the geometry and topography of the EARS and the contrast between high rainfall in catchment zones and high evaporation rates in their associated lowland lake basins. Periods of extreme transgression and regression occurring in these lakes are suggested to have isolated populations and deferred migration along or across this natural corridor, while periods of stable lake levels encouraged dispersal and mixing [START_REF] Maslin | East African climate pulses and early human evolution[END_REF][START_REF] Trauth | Human evolution in a variable environment: The amplifier lakes of Eastern Africa[END_REF]. It has also been argued that without analysis of sequence stratigraphy of lacustrine deposits, climate variability can be overstated and its nuances overlooked [START_REF] Nutz | Orbitally-driven evolution of Lake Turkana (Turkana Depression, Kenya, EARS) between 1.95 and 1.72 Ma: A sequence stratigraphy perspective[END_REF].

We often look to climate proxies to understand changes in human behavior, but are less likely to do the inverse. Moreover, studies rarely attempt to gauge the time lag between hydroclimatic change in catchment areas and its local effects (including tipping points) on watersheds making such hypotheses difficult to test. This study aims to demonstrate that correlation of morpho-sedimentary data, welldated human settlements and associated economic strategies can function as an effective multi-proxy for understanding these important subtleties in local paleoecological transformations. This approach can lead to more accurate reconstructions of human and environmental responses to the nuances of climate change. The exceptional sensitivity of amplifier lakes can reflect nuances in climate variability and human adaptation to such fluctuating environments can better explain changes in human behavior than large scale climate-driven population diffusion models.

Figure VI.1: (continued)

Figure VI.1: a) Regional map (after GMTED2010, SRTM) with the location of the VAPOR-Afar project area, sites mentioned in the text, and a schematic representation of the earliest attestations of herding in the Horn of Africa; b) Satellite image (LANDSAT) of the Lower Awash valley in the Ethiopian Afar with the location of VAPOR-Afar archaeological sites and modelling of the Abhe Lake highstand between 10.2-9.8 ka cal. BP; c) Abhe Lake level phases after [START_REF] Gasse | Late Quaternary lake-level fluctuations and environments of the northern Rift Valley and Afar region (Ethiopia and Djibouti)[END_REF].

Major Late Quaternary transgressions and regressions of Lake Abhe, an amplifier lake located at the tripartite junction of the Nubian, Arabian and Somali tectonic plates in the Central Afar depression, were reconstituted and interpreted by [START_REF] Gasse | Evolution of Lake Abhe[END_REF] in the 1970s, mainly through study of diatoms. However, the low resolution of the resulting lake-level model has provided a lacunar framework for understanding the evolution of Abhe lake-level fluctuations (Fig. VI.1c). Until archaeological work was conducted in the linked Gobaad basin in the Republic of Djibouti in the 1980s-90s [START_REF] Gutherz | Le site d'Asa Koma (République de Djibouti) et les premiers producteurs dans la Corne de l'Afrique[END_REF][START_REF] Joussaume | Les premières sociétés de production. In Tiya, l'Éthiopie Des Mégalithes[END_REF], no human occupation sequences were recovered in the context of this research. While archaeological work in Djibouti is ongoing and has provided a large corpus of mainly Middle Holocene sites [START_REF] Gutherz | New insights on the first Neolithic societies in the Horn of Africa: The site of Wakrita, Djibouti[END_REF][START_REF] Gutherz | Asa Koma : Site Néolithique Dans Le Bassin Du Gobaad (République de Djibouti. Mondes Anciens[END_REF], the Lower Awash River valley and the majority of the Abhe Lake basin can be found in Ethiopia, where archaeological work had been lacking prior to this study. Well-developed sedimentary sequences across this basin present an ideal context for the preservation of stratified Holocene sites that cover the period of economic transition from fishing-hunter-gatherers to the first herders. This makes it an exemplary watershed in which to begin to correlate human occupation sequences with Abhe lake-level fluctuations that occurred from the Early to Late Holocene humid to dry transition (corresponding to the AHP). In this study, new approaches are developed for measuring and understanding the reactivity of amplifier lakes and the societies that occupied their margins to African monsoon related hydrological changes.

Archaeological work conducted since 2014 in the Lower Awash River valley and northern Abhe Lake basin in the Ethiopian Afar (Fig. VI.1a,b) has brought to light 9000 years of well-preserved prehistoric human occupation sequences. These have allowed us to construct a local chronology and to begin exploring human, landscape and environmental responses to a fluctuating amplifier lake in one of the most arid parts of the world.

VI.1.2 Regional Settings

VI.1.2.a Geological and tectonic control of Abhe Lake basin formation

The Abhe Lake Basin is located along the axis of the Tendaho-Gobaad graben (NE-SW), corresponding to a subsided floor of basalts from the stratoid series (4.4-2.5 Ma; [START_REF] Varet | Geology of Central and Southern Afar (Ethiopia and Djibouti Republic)[END_REF][START_REF] Varet | Geology of Afar (East Africa)[END_REF] covered by a thick sedimentary series between numerous faults associated with extensional tectonics [START_REF] Barberi | Volcanism of Afar: Smallscale plate tectonics implications[END_REF][START_REF] Manighetti | Propagation of rifting along the Arabia-Somalia Plate Boundary: Into Afar[END_REF][START_REF] Manighetti | Strain transfer between disconnected, propagating rifts in Afar[END_REF]. Plio-Pleistocene basaltic fissure flows and alkaline rhyolitic flows from axial ranges partially cover the stratoid series. This igneous palimpsest represents the base bedrock for the Late Pleistocene to Holocene sedimentary fillings concerning this study. The lacustrine sedimentary sequence accumulation began during the Early Pleistocene [START_REF] Gasse | Tectonic and Climatic Controls on Lake Distribution and Environments in Afar from Miocene to Present[END_REF], while the sedimentary synthem of the Abhe basin developed within a topography similar to that of today starting around 130 ka. The greater part of the sedimentary accumulation seems concentrated in the eastern sector of the Lower Awash plain and rests on the Gamari and Afambo faults which were probably tectonically active until the Middle Holocene [START_REF] Varet | Geology of Central and Southern Afar (Ethiopia and Djibouti Republic)[END_REF][START_REF] Varet | Geology of Afar (East Africa)[END_REF]Gasse, 1975, Fig. VI.1b). The Lower Awash Plain is tectonically tilted toward the eastern graben faults with a 30 m vertical offset over a length of 30 km [START_REF] Acocella | Coupling volcanism and tectonics along divergent plate boundaries: Collapsed rifts from central Afar, Ethiopia[END_REF]. This tectonic activity, coupled with volcanic phenomena localized to the north of the rhyolitic Borawli Mountain, probably caused the deviation of the Awash River and the formations of the Gamari and Afambo fault pits (Fig. VI.1b;[START_REF] Gasse | L'évolution des lacs de l'Afar Central (Ethiopie et T.F.A.I.) du Plio-Pléistocène à l'Actuel. Recostitution des paléomilieux lacustres à partir de l'étude des Diatomées[END_REF]. The final hydrological routing of the Awash River (and morphology of associated residual lakes) is the consequence of Holocene tectonic movements a response to magmatic intrusions below the basin axis [START_REF] Gasse | L'évolution des lacs de l'Afar Central (Ethiopie et T.F.A.I.) du Plio-Pléistocène à l'Actuel. Recostitution des paléomilieux lacustres à partir de l'étude des Diatomées[END_REF]. A similar phenomenon (6 ka) was also detected almost 80 km away from the Lower Awash valley, at the Asal Lake (Afar, Djibouti), with a 70 m tectonic subsidence of the axial valley [START_REF] Ruegg | Mouvements du sol holocènes dans le rift d'Asal à Djibouti[END_REF]. The genesis and evolution of the Abhe Lake basin system is thus the result of ancient (Early Pliocene) to recent (Holocene) volcanotectonic activity, which, coupled with climatic control triggered the current morpho-sedimentary and geomorphological patterns.

VI.1.2.b Hydro-climatological control of Abhe Lake basin formation

The increase in magnitude of precession-driven insolation in the Northern Hemisphere between 14.8 and 5.5 ka appears to be at the origin of the northward migration of the rainfall belt associated with the Inter Tropical Convergence Zone (ITCZ) [START_REF] Collins | Rapid termination of the African Humid Period triggered by northern high-latitude cooling[END_REF][START_REF] Demenocal | Abrupt onset and termination of the African Humid Period: Rapid climate responses to gradual insolation forcing[END_REF][START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF][START_REF] Kutzbach | Milankovitch forcing of fluctuations in the level of tropical lakes from 18 to 0 kyr BP[END_REF][START_REF] Mohtadi | Palaeoclimatic insights into forcing and response of monsoon rainfall[END_REF][START_REF] Skonieczny | Monsoon-driven Saharan dust variability over the past 240,000 years[END_REF]. These changes in convergence zone patterns led to enhanced precipitation in the tropics which expanded symmetrically in both hemispheres [START_REF] Costa | Isotopic reconstruction of the African Humid Period and Congo Air Boundary migration at Lake Tana, Ethiopia[END_REF][START_REF] Tierney | Abrupt Shifts in Horn of Africa Hydroclimate Since the Last Glacial Maximum[END_REF][START_REF] Tierney | Rainfall regimes of the Green Sahara[END_REF]. This can be correlated to a northward spread of vegetation cover [START_REF] Hopcroft | Multi vegetation model evaluation of the Green Sahara climate regime: RAINFALL SUPPORT-ING A GREEN SAHARA[END_REF], and to the massive extension of African lakes [START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF][START_REF] Junginger | The effects of solar irradiation changes on the migration of the Congo Air Boundary and water levels of paleo-Lake Suguta, Northern Kenya Rift, during the African Humid Period (15-5ka BP)[END_REF] such as Lake Abhe. This continental African Late Pleistocene to Holocene pluvial period is known as the 'African Humid Period' (AHP;[START_REF] Demenocal | Abrupt onset and termination of the African Humid Period: Rapid climate responses to gradual insolation forcing[END_REF][START_REF] Shanahan | The time-transgressive termination of the African Humid Period[END_REF].

Three main lacustrine sedimentary sequences covering the period between 130 and 3 ka and corresponding to the Late Quaternary ITCZ oscillations, were first distinguished in the Lake Abhe basin by [START_REF] Gasse | Evolution of Lake Abhe[END_REF]: ∼100-70 ka (Abhe I), 60-18 ka (Abhe II -Abhe III), and 10-4 ka (Abhe IV -Abhe V; Fig.

VI.1c

). During the Holocene, Abhe Lake, currently reduced to a small (∼350 km 2 ) and shallow (<30 m depth) waterbody was part of a large freshwater filling which generated the last highstand phases in response to the AHP hydrological regime: Abhe IV (10-8 ka; Fig. VI.1.c), Abhe V (7.5-4.5 ka) and Abhe VI and VII (0-2.5 ka). Its maximum water surface extent at the height of the AHP was estimated by [START_REF] Gasse | L'évolution des lacs de l'Afar Central (Ethiopie et T.F.A.I.) du Plio-Pléistocène à l'Actuel. Recostitution des paléomilieux lacustres à partir de l'étude des Diatomées[END_REF][START_REF] Gasse | Evolution of Lake Abhe[END_REF] to be around 6000 km 2 .

The Central Afar region receives allochthonous freshwater supplies generated by African monsoon precipitation in the Ethiopian Highlands during the rainy season from June to September (200 mm/month;[START_REF] Tsidu | High-Resolution Monthly Rainfall Database for Ethiopia: Homogenization, Reconstruction, and Gridding[END_REF]. Thus, the African monsoon is responsible for the majority of solid discharge material transported by the Awash River and which constitutes the Late Pleistocene-Holocene sedimentary synthems of the Abhe lake basin. In particular, the Abhe Lake system receives its water catchment from the Awash River (Fig. VI.1a), which is joined by many affluent rivers as it descends the Ethiopian Highlands, until it meets the Lower Awash Plain (4000 km 2 ) where it continues without freshwater supply until it reaches its final destination, endorheic Lake Abhe (Fig. VI.1b; 230 m a.s.l.).

Our study area coincides with this last and arid part of the Awash River course, characterized by 200 mm/yr precipitation, with an evaporation rate of 3500 mm/yr [START_REF] Varet | Geology of Afar (East Africa)[END_REF][START_REF] Gasse | Tectonic and Climatic Controls on Lake Distribution and Environments in Afar from Miocene to Present[END_REF][START_REF] Rognon | Depots lacustres quaternaires de la basse vallée de l'Awash (Afar, Ethiopie): Leurs rapports avec la techtonique et le volcanisme sousaquatique[END_REF]. This plain, including Tendaho (400 m a.s.l.), Gamari and Afambo Lakes (339 m a.s.l.) is limited by the Gamari escarpment to the east (1300 m a.s.l.) and Magenta escarpment to the west (400 m a.s.l.; Fig. VI.1b).

The dynamics of regression and transgression of the Abhe Holocene lakes played a substantial role in paleolandscape evolution, on soil development and on paleo-ecological patterns of the basin surroundings, likely influencing peopling dynamics. In the 1970s, geomorphological research in the Central Afar region was focused on deep (offshore or distal) lacustrine deposits [START_REF] Faure | Les formations lacustres Holocènes et l'industrie épipaléolithique de la région de Loggia (bassin du lac Abhé Ethiopie)[END_REF][START_REF] Gasse | Evolution of Lake Abhe[END_REF][START_REF] Gasse | L'évolution des lacs de l'Afar Central (Ethiopie et T.F.A.I.) du Plio-Pléistocène à l'Actuel. Recostitution des paléomilieux lacustres à partir de l'étude des Diatomées[END_REF][START_REF] Rognon | Depots lacustres quaternaires de la basse vallée de l'Awash (Afar, Ethiopie): Leurs rapports avec la techtonique et le volcanisme sousaquatique[END_REF] with an almost absent regard for a wide range of peri-lacustrine depositional environments and processes (e.g. gilbert-deltas, pedological horizons, gravity-induced deposits and fluvial or fluvio-lacustrine deposits).

Since 2014, our current work on Late Quaternary sedimentary systems of the Abhe Lake paleobasin is focused on pedo-stratigraphical and geochronological investigation of new archaeological sequences linked to past transgression-regression lake dynamics. This approach aims to correlate sedimentary formation processes of human occupation layers with paleolandscape evolution to arrive at a more holistic understanding of the relationship between climate-change forcing and Holocene human settlement strategies in the Lake Abhe basin.

VI.1.2.c Prior research on Holocene human occupation in the Abhe Lake basin

Since the 1980s archaeological investigations in the Gobaad basin in Djibouti [START_REF] Joussaume | Les premières sociétés de production. In Tiya, l'Éthiopie Des Mégalithes[END_REF] have provided evidence of several late LSA sites characterized mainly by graves with human and wild faunal remains, and several sites with cattle and caprine domestication in association with developed impressed pottery and complex built funerary traditions dated today to as early as 4.8 ka BP [START_REF] Lesur | The advent of herding in the Horn of Africa: New data from Ethiopia, Djibouti and Somaliland[END_REF]. Several Middle-Late Holocene periods are represented and characterized by chronologically distinct impressed pottery traditions, and more generally, grinding and pounding equipment, cobble-lined hearths and a lithic tradition of obsidian backed geometrics associated with pièces esquillées tools made almost exclusively from obsidian [START_REF] Gutherz | New insights on the first Neolithic societies in the Horn of Africa: The site of Wakrita, Djibouti[END_REF][START_REF] Gutherz | Asa Koma : Site Néolithique Dans Le Bassin Du Gobaad (République de Djibouti. Mondes Anciens[END_REF]. The eastern Abhe lakeshore has provided the earliest confirmed dates for the onset of herding in the Horn of Africa region, contemporary with those at the Lothagam North Pillar Site in the region of Lake Turkana [START_REF] Lesur | The advent of herding in the Horn of Africa: New data from Ethiopia, Djibouti and Somaliland[END_REF][START_REF] Coudert | New archaeozoological results from Asa Koma (Djibouti): Contributing to the understanding of faunal exploitation during the 3rd millennium BC in the Horn of Africa[END_REF]Hildebrand et al., 2018a, see Asa Koma and Lothagam Fig. VI.1a).

Aside from geomorphological and geological studies carried out in the 1970s [START_REF] Gasse | Evolution of Lake Abhe[END_REF][START_REF] Varet | Geology of Central and Southern Afar (Ethiopia and Djibouti Republic)[END_REF][START_REF] Varet | Geology of Afar (East Africa)[END_REF], the northern Abhe Lake basin lacked archaeological investigation prior to our work save for a briefly mentioned locality found in the Logiya region in the 1970s [START_REF] Faure | Les formations lacustres Holocènes et l'industrie épipaléolithique de la région de Loggia (bassin du lac Abhé Ethiopie)[END_REF]. Since 2014, the VAPOR-Afar program has investigated the remaining paleoshorelines of Abhe Lake in the Ethiopian Afar and confirmed the presence of an equally large number of prehistoric occupations. Some of these sites present nuanced regional and chronological variation in relation to the eastern shores of the same lake basin in Djibouti and permit a more holistic understanding of the entire basin's formation, evolution and its occupation by humans. While Abhe Lake highstands were recorded in the 1970s, relationships between human occupations and lake fluctuations had never been investigated in this region prior to our work. This makes Abhe Lake an exceptional case-study that allows us to evaluate human adaptation to changing lacustrine landscapes and environments over 9000 years.

The Abhe Lake basin represents an important region for our understanding of the northeast African late prehistoric record, and our current work provides a register of new dated sites that contribute to filling gaps in the local and regional chrono-cultural sequence within the context of hydroclimatic and paleoshoreline fluctuations.

VI.1.3 Methodology and materials

VI.1.3.a Survey, excavation and sampling strategies

Since 2014, a combination of remote sensing, geomorphological and archaeological survey was carried out from the lowlands to the fault margin slopes over the totality of the Lake Abhe basin, with a particular regard for littoral morphologies and sedimentary complexes. The building of a revised lake-level curve permitted us to localize the paleo-shorelines of the Holocene Abhe Lake during fieldwork, and facilitated identification of peri-lacustrine deposits associated with archaeological occupation layers.

18 test pits ranging from 1 to 1.3 m 2 area were excavated during fieldwork. The richest archaeological levels were investigated over more extensive surfaces dimensionally adapted to each site following a 1 m grid system. Fine excavation methods were used and all sediment was sieved using a 2 mm mesh. Levels with archaeological materials were sieved in their totality. All archaeological artifacts >2 cm were geolocalized and assigned a unique number. Field pedo-sedimentary sequence description and systematic bulk sampling were carried out in each Sedimentary Unit (SU), while selective sampling in controlled undisturbed contexts was employed for dating material.

VI.1.3.b Cartography and GIS analysis

The project Geographical Information System was constructed using the Esri ArcGIS10.7.1® and the Clark University TerrSet® programs. All sample locations and maps presented here are expressed in a geographic coordinate system (WGS84).

Using an altimetry survey, a high-resolution (1 m) Digital Terrain Model (DTM) was created for our study region which enabled us to carry out different mapping operations and to create precise 3D (X, Y, Z) geographic information using a Trimble® Differential Geographic Positioning System (DGPS) Geo7XT. With a differential correction, artifact coordinates were mapped with a precision of 1-2 cm. The use of a DGPS not only allowed us to record archaeological data but also to georectify the cartographic database created for the project.

Regional and micro-regional cartographic modelling (eg. lake-level modelling) was carried out using a 12 m resolution DTM (Tandemx) and LandSAT® (28.5 m resolution, https://landsat.usgs.gov) satellite images.

VI.1.3.c 14 C dating and calibration

The age model of the archaeological sequences is based on 35 Accelerator Mass Spectrometry (AMS) 14 C measures obtained on charcoal, bone bioapatite, stromatolites and aquatic/terrestrial mollusk shells (Tab. VI.1, 2). Where organic or biogenic carbonate macroremains were absent, 14 C dating was performed on sediment bulk organic carbon (OC > 1%). Dating on bone bioapatite was performed on two faunal remains from the site of Det Bahari-01 (Betba-1291, Betba-11, Tab. VI.2). Bioapatite gives minimum ages due to the possible exchange with dissolved carbonates [START_REF] Zazzo | Bone and enamel carbonate diagenesis: A radiocarbon prospective[END_REF]. The age shift is usually quite low for samples younger than 5000 BP, especially in arid environments like those found in the Afar.

The 14 C reservoir effect on lacustrine carbonates can be extremely variable in continental contexts depending on the periods and water bodies, notably with regards to the water-depth and the content of dissolved ancient calcium carbonates in the water [START_REF] Geyh | Tem-poral Changes of the 14C Reservoir Effect in Lakes[END_REF][START_REF] Ramsey | Radiocarbon Dating: Revolutions in Understanding[END_REF][START_REF] Cohen | Paleolimnology: The History and Evolution of Lake Systems[END_REF]. The reservoir effect in the Awash-Abhe hydrological system has not been estimated to date. However, two new dates on charcoal and shell samples recovered from the same sedimentary unit (Logiya_1A, Logiya_1B, Tab. VI.1) indicate a ∼100 year reservoir effect. To our knowledge, these are the only dated samples today that allow us to discuss the existence of a reservoir effect in this hydrological system. Although these samples are isolated, they enable us to suppose that the reservoir effect is very low if not non-existent in this area. It is therefore on the basis of this assumption that we do not apply a reservoir correction value on lacustrine carbonate ages and that the data below are presented. New and previous ranges and Median ages were calibrated with the Oxcal 4.3 program (Bronk [START_REF] Bronk Ramsey | Development of the Radiocarbon Calibration Program[END_REF] and IntCal13 curve [START_REF] Reimer | IntCal13 and Marine13 Radiocarbon Age Calibration Curves 0-50,000 Years cal BP[END_REF]. The ages were considered here using 95.4% ranges and are reported in cal. BP or cal. ka BP (calibrated year or kilo annum before 1950). Holocene subdivisions used follow those formally ratified by the International Union of Geological Sciences [START_REF] Walker | Formal ratification of the subdivision of the Holocene Series/Epoch (Quaternary System/Period) : Two new Global Boundary Stratotype Sections and Points (GSSPs) and three new stages/subseries[END_REF].

VI.1.4 Results

VI.1.4.a Revised Age Model of Holocene Abhe paleo-lake level phases

The revision of the age model of Holocene Abhe paleo-lake water levels is based on the calibration and the re-calculated elevation of 14 C ages on littoral lacustrine carbonates published by [START_REF] Gasse | L'évolution des lacs de l'Afar Central (Ethiopie et T.F.A.I.) du Plio-Pléistocène à l'Actuel. Recostitution des paléomilieux lacustres à partir de l'étude des Diatomées[END_REF][START_REF] Gasse | Evolution of Lake Abhe[END_REF], and on new radiocarbon dates on stromatolites (Tab. VI.1).

25 previous ages were selected according to the following criteria (Tab. BI.10): a) only ages from littoral lacustrine material from Abhe basin were considered; b) using stratigraphic information provided by [START_REF] Gasse | L'évolution des lacs de l'Afar Central (Ethiopie et T.F.A.I.) du Plio-Pléistocène à l'Actuel. Recostitution des paléomilieux lacustres à partir de l'étude des Diatomées[END_REF], inversed ages or ages from reworked sediments were rejected ; c) ages from shores of residual lakes were not considered (ex: Gamari or Gargori residual lakes); d) ages with incomplete geographic localization (partial GPS points) do not allow proper correlation with DEM data and were not used for model building; e) ages on anthropic material.

Stromatolites from African lakes are well known markers of littoral and stable lacustrine paleoenvironments [START_REF] Casanova | Biosedimentology of Quaternary stromatolites in intertropical Africa[END_REF]Hillaire-Marcel et al., 1986;[START_REF] Lisker | Stromatolites in caves of the Dead Sea Fault Escarpment: Implications to latest Pleistocene lake levels and tectonic subsidence[END_REF]. These are an accurate proxy of shallow waters and can complement and confirm lake level models. 14 new 14 C ages on stromatolites sampled and mapped during fieldwork were compared with previous revised data with the aim of confirming and refining the paleo-lake level model (for subsequent new age references, refer to Tab.

VI.1, Fig. VI.2).

The first Early Holocene lake highstand is localized at 359 m a.s.l. in the north of the basin. A stromatolite newly dated to 11210-10805 cal. BP (Strom_20-SU5) allows us to push back the onset of this lake transgression (Abhe-IV phase) to ∼1000 years earlier than the data presented in [START_REF] Gasse | Evolution of Lake Abhe[END_REF] original lake-level model (Fig. VI.1c,2). After a slight drop in level (342 m a.s.l.; 11086-10253 cal. BP; Gasse_96, Tab. BI.10), the lake level rose again between 10660-10404 and 10565-10295 cal. BP (Strom_20-SU4, Strom_20-SU1). This phase of lacustrine highstand was maintained until 10.2-9.8 ka cal. BP (Logiya_1A; Gasse_94, 90, Tab. BI.10) with a maximal elevation of ∼420 m a.s.l. and a Table VI.2: Radiocarbon ages and elevations from archaeological sites (VAPOR-Afar project, Ethiopia) and associated palustrine layers (last 4 dates). * rejected ages. The site of Det Bahari-01 (correct local name) was originally named Bet Bahari-01 based on appellation of the area on a map of the region (hence the Betba-01 site label). All samples have retained the old site label.

surface extension of ∼6000 km 2 . From 9.8 ka cal. BP Abhe Lake levels appear to have dropped ∼50 m (∼370 m a.s.l.), persisting at this level until 8.4 ka cal. BP (Gasse_85,82,80,76,64,63,62,Tab. BI.10;Strom_10b,31). This result permits us to add a second lake highstand of phase Abhe-IV, with respect to Gasse's original (1977) model (Fig. VI.2b). At 8 ka cal. BP a rapid decrease of the water table is recorded with a lowstand level of 238-236 m a.s.l. (Gasse_57, Tab. BI.10). The resurgence of rising lake levels is recorded at 7498-6448 cal. BP (Gasse_35, Tab. BI.10; 327 m a.s.l.) followed by lake level stabilization (∼360-370 m a.s.l.) which marks the onset of phase Abhe-V (Fig. VI.2b). Between ∼8 and ∼7 ka cal. BP water-level modelling was not possible due to an absence of data from lacustrine carbonates. However, an in situ archaeological site located at 378.4 m a.s.l. and dated to 7564-7424 cal. BP indicates that during this period the lake could not have attained this elevation (sample SP-01_US2_001, Tab. VI.2). The last high waters of the Middle Holocene lake are dated on a stromatolite to 5712-5591 cal. BP (Stromatolis-SU1), which is followed by a slight regression event until 4874-4297 cal. BP (Gasse_16, Tab. BI.10). Low lake-levels probably occur between ∼4.5 and ∼3 ka cal. BP, however the lack of data does not allow lake level modelling during this period. During the Late Holocene, from 3172-2490 cal. BP (Gasse_13, Tab. BI.10) a new rise in lake levels corresponds to phase Abhe-VI which continued until ∼1.4 ka cal. BP. ), the archaeological sequences presented here are derived from 16 test pits conducted on nine sites. Six of these are stratified and were radiocarbon dated on in situ materials in undisturbed contexts. With the addition of the Det Bahari-01 surface site, seven dated archaeological sites are presented in chronological order, from the oldest to the youngest with a brief pedo-stratigraphic description and diagnostic material culture presentation.

Lakora-03

Sounding 1 (S1) measuring 1.5x2 m 2 was excavated at the site of Lakora-03 (Fig [START_REF] Gasse | L'évolution des lacs de l'Afar Central (Ethiopie et T.F.A.I.) du Plio-Pléistocène à l'Actuel. Recostitution des paléomilieux lacustres à partir de l'étude des Diatomées[END_REF] and modelling of Early Holocene Abhe Lake highstand ∼10.6 ka cal. BP; d) Photogrammetry and point plotted artifacts from the main archaeological layer of the Lakora-03 site.

Over the basaltic bedrock, a saprolitic layer developed (SU7) presenting a whitish-brown siltyclayey matrix packing a large amount of weathered basaltic gravels mixed with reworked calcareous crusts. The following layer (SU6) shows the development of a granular structure of dark-brown well sorted silts containing few terrestrial shells and rare obsidian artifacts. SU5 and 4 represent two sedimentary phases of the main human occupation level characterized by a very dark-brown, well to moderately sorted silt with a loose granular structure, containing a small hearth, charcoal, and a large amount of fish remains and obsidian artifacts (Fig. VI.3b,d). SU5 represents the base of a stone line composed of decametric sub-angular basaltic blocks which are capped at the top by a stromatolitic crust (SU3). Several worked lithics in SU5 were also capped by the stromatolitic crust SU3. Charcoal recovered in SU5 was radiocarbon dated to 11164-10737 cal. BP; Tab. VI.2).

Archaeological materials from SU5 and 4 included abundant faunal remains, namely fish (Tab. VI.3), worked obsidian, charcoal, and a single round flat ostrich eggshell bead recovered from the hearth.

The excavation of S1 provided a relatively rich sample of lithic material (n=132) with one refit (Fig. VI.4). The lithic materials from the sounding were compared with surface collections of materials localized quasi-exclusively within a 40x15 m 2 area along the eastern edge and downslope of the basalt outcrop. Surface collections included both systematic collection of all artifacts in a 1 m 2 area where there was the highest density of materials and a random collection of cores and shaped tools from the entire surface area (Fig. VI.4a-c). Lithic materials from the sounding were comparable to surface collections from a technological and typological point of view and are generally very altered, both mechanically and chemically (Fig. VI.4d-f). The materials collected at the site indicate dominant production of large bladelets from cores belonging to distinct operative schemes. Very few shaped tools were present in both the surface and excavated collections. These are mostly burins and endscrapers (Fig. VI.4f). Interestingly, not a single backed piece was collected or observed at the site.

The faunal material is dominated by fish, represented mainly by catfish remains, but also by one tilapia and one Cyprinidae bone. The rest of the fauna includes remains of large mammals, including bovids. Some shell and ostrich eggshell are also present (Tab. VI.3).

Raso-02

A step Sounding 2 (S2) measuring 1.5x2 m 2 was excavated along an exposed section at the site of Raso-02 (Fig. VI.3a) located at the base of a stratoid basalt ridge on the NW edge of the Manda Gargori volcano-tectonic segment (Fig. VI.1b).

From bottom to top (Fig. VI.3a) the sounding included a saprolitic layer (SU9) that formed over the basaltic bedrock; a dark-brown muddy sediment rich in secondary carbonate nodules (SU8); a siltyclayey horizon (SU7 and 6) with a polyhedral structure where secondary carbonates are reduced and replace the intrapedal porosity as infillings. A channel-morphology erosional surface marks the end of the lower pedogenic complex. The channel bottom is characterized by few sub-angular and rounded basaltic blocks packed in the gravely-clayey SU5d and gravely-sandy SU5c which fill the fluvial channel body. SU5b and 5a represent the main archaeological level which is characterized by common fauna, including a large mammal long bone fragment, charcoal and obsidian artifacts, packed in a gravely-sandy matrix (SU5b) and massive medium sand (SU5a). The top of the archaeological level is marked by an erosional surface and by the installation of massive fine-sand/silty texture patterns (SU4b and 4a), with abundant lacustrine shell inclusions (SU3). An oncolite layer (SU2) is packed in a brown muddy matrix rich in lacustrine in situ shells, while the SU2/1b interface contains few obsidian lithic artifacts, charcoal and fish remains. This is followed by a pedogenic event with well-developed clay granular structure (SU1b), affected in its upper part (SU1a) by desiccation cracks filled by reworked surface material.

Charcoal from the base of SU5a was dated to 10744-10418 cal. BP (Raso-02-Sond2-SU5, Tab. VI.2), while the oncolite layer gives an age range of 5913-5746 cal. BP (Raso-02_Strom, Tab. VI.1). In addition, some faunal remains (mammals and fish; n=9) were recovered from SU5a and abundant fish remains (n=1328) from SU2. Materials collected on the surface of the site include noticeably large blades and bladelet cores (which produced relatively large blanks). The extensive surface collection from 1 m 2 area provides similar kinds of materials but shows heavy mechanic alterations, and particularly double-patina breaks and retouch-like damage. Only a very limited archaeological sample was recovered from the sounding (n=36), where materials are also heavily altered. The few illustrative but undiagnostic artifacts are limited to a distal fragment of a retouched bladelet in SU1, and a large blade and two flake cores (including one Levallois) in SU5. SU5 is richer and could correspond to material found on the surface but a larger sample is needed to confirm this and to propose a proper technological or typological description. Besides these limitations regarding sample size, Raso-02 materials do not seem very different from the Lakora-03 ones.

Det Bahari-01

Det Bahari-01 is an open-air site preserved as a residual artifact-rich surface on a low oval mound located on the eastern edge of the Det Bahari sand plain SW of the town of Dubti (Fig. VI.6.c). The site dated on hippopotamus remains to 7956-7761 and 7674-7523 cal. BP (Betba-1291, Betba-11, Tab. VI.2) overlies a palustrine deposit dated on in situ bivalve shells to 10415-10245 and 10230-10170 cal. BP (Betba01log2A, Betba01log2B, Tab. VI.2).

The good state of preservation of fauna, lithics and basalt groundstone fragments, and evident refits within the residual archaeological level localized in a ∼70x45 m 2 area led us to collect the totality of surface materials (Fig. VI.5a). 1438 artifacts were collected including ∼550 knapped obsidian (90%) and chalcedony (10%) artifacts, and ∼750 other mineral items (mostly igneous rocks). The igneous material can be subdivided in two groups: the largest one is composed of heavily fragmented mafic rocks (mostly vesicular basalt(?)) with no apparent sign of anthropic transformation; the second is represented by several artifacts interpreted as pounding stone fragments (grinding stones?, hammers?) providing functional clues. An important number of refits were made on materials from this group (60 refit groups, of which 12 groups include 3 or more artifacts, and 4 groups between 5 or 6 artifacts, or 103 conjoins) (Fig. VI.5a). If some of these refits correspond to natural fragmentation (taphonomic alteration of the most fragile stones, particularly the vesicular basalts), some refits may indicate intrasite artifact circulation (as an indication, distance between conjoined artifacts are greater than 3 m in 26 cases, and greater than 10 m in 6 cases). A second category corresponds to small size pebbles of various materials. Most of them do not feature any sign of use and could have been present in alluvial deposits in the direct vicinity of the site, or perhaps in a dismantled overlying stratigraphic unit. The association between mafic rocks and cobbles remains uncertain but their overlapping distribution at the site and complete absence in the direct vicinity of the site could be an indication of contemporaneous occupation of a single site of limited extent.

Another component of the lithic material was composed of knapped artifacts (obsidian and chalcedony, both similarly treated) (Fig. VI.7). Systematic attempts to find conjoins between all flake fragments were unfortunately unsuccessful. There was an intense re-use of various products and particularly of cores which were almost systematically fragmented and reduced to very small proportions, often by bipolar percussion. Nevertheless, numerous blades and bladelets (some of them used as blanks for shaped and modified tools) were recovered and are an indirect indication of the importance of these productions (Fig. . It remains unclear what proportion of pieces showing bipolar percussion stigmas are core and intermediary tools (pièces esquillées) but some of them are of very reduced dimensions and could not have been cores for the blanks recovered, at least in their last exploitation stages. Relatively few shaped tools (n=45) were recovered at the site, however, most of those that were are curved backed bladelets (n=27, Fig. VI.7d-f), associated with few endscrapers (Fig. VI.7c), thick truncations, and simple burins (angle burins on break, mostly). In addition to these, numerous blanks show irregular modifications of various extent.

The fauna totaled 98 specimens including hippopotamus and crocodile, middle sized bovids and 33 fish remains. The dominance of aquatic fauna suggests the presence of a lakeside settlement with an economy based on fishing of tilapia and catfish and the hunting of mainly aquatic prey (Tab. VI.3).

Stromatolis

The site of Stromatolis is located between the SE slopes of the late quaternary basalt flows of Kurub volcano (Fig. VI.6c). The archaeological findings consist of a small dry wall circular structure ∼150 cm in diameter constructed from naturally sub-angular basalt blocks (SU2; Fig. VI.5c). The blocks were laid on a continuous formation of columnar structure stromatolite domes (SU5) as well as on two thin overlying levels of a clayey silt with terrestrial shell (SU4) and an aeolian fine to medium crosslaminated sand deposited prior to its construction (SU3) (Fig. VI.5b). The anthropic structure is capped by a second generation of stromatolitic crust (SU1) presenting laminar structure and ripple marks on

MS-01/02

Two soundings (01 and 02) were conducted at the site MS located in an active sand dune field in the Kurub Bahari plain ∼2 km due north of the current Awash River bed (Fig. VI.6c). These soundings reveal alternating aeolian layers and pedogenic horizons to a depth of ∼200 cm (Fig. VI.6b). The MS-01 sequence is composed of 20 SU. From bottom to top, SU1 is composed of clayey-fine silty material with a polyhedral to prismatic structure where several obsidian artifacts were recovered. This first pedogenic horizon is capped by alternating fine-silty and clayey bed layers (SU2) containing a discontinuous horizontally oriented line of Fe-Mg oxide nodules in its uppermost matrix. The archaeological evidence and the similarity of pedological characteristics of SU1 and 2 allow us to propose a stratigraphic correlation with SU3 and 4 of the MS-02 sounding (Fig. VI.6b). Along the upper part of the MS-01/02 sequences three types of alternating layers are defined: thin silty-clayey pedological horizons with prismatic to polyhedric structure (MS-01_SU4, 6, 8, 10, 18; MS-02_SU6, 8), silty horizontally laminated layers (MS-01_SU7, 9, 11, 13, 15, 17; MS-02_SU7, 9), and fine to medium sand cross-laminated layers 5,12,14,16,19,20;5). Discontinuous beds of silicic nodules are commonly present in the silty and sandy layers. Archaeological layer MS-01_SU1/MS-02_SU3 was dated to 8190-8000 cal. BP (MS1_US1_001, Tab. VI.2, Fig. VI.6b).

SP-01

Site SP-01 is located at 380 m a.s.l. on the stratoid basalt outcrop on the NW ridges of the rhyolitic Borawli volcano (Fig. VI.6a). A 1 m 2 test pit revealed preserved sedimentary deposits trapped within the parallel NNW-SSE oriented fissure flows of the stratoid series formation and topographically localized above the Holocene lacustrine wave-cut terrace and littoral lacustrine sedimentary deposits (Fig. VI.6a). From bottom to top, we observed a massive silty-clayey layer with abundant gravel inclusions (SU5), a prismatic structure pedologic horizon containing occasional knapped obsidian artifacts (SU4), and a massive fine-sand ensemble (SU3, 2, 1). A date on bulk organic carbon from the base of SU4 gives an age of 7519-7424 cal. BP (SP-01_US2_001, Tab. VI.2). Obsidian artifacts recovered from the sounding were limited to small undiagnostic chips, however the small and selective surface collection recovered on the adjacent outcrop edge, and probably belonging to the eroded pedologic horizon (SU4) correspond to the production of large bladelets. The only shaped tools collected at the site are curved backed bladelets of relatively large dimensions.

Kurub-07

The site of Kurub-07 was excavated over a 34 m 2 surface area (Fig. VI.8a.3). This open-air site is exposed on the slopes of a large dune complex in the SW of the Kurub Bahari plain and several kilometers north of the current Awash River delta (Fig. VI.1b,Fig. VI.8b). 43 SU were described during three seasons of fieldwork on the basis of excavation and pedo-stratigraphic observations from site sections. We present here only the SU observed on the K_trench section, which is representative of the stratigraphic and site formation processes and of the main archaeological layers ( SU18 represents the continuous sedimentary basement of the sequence characterized by dark-brown prismatic structured clays rich in authigenic gypsum which has been interpreted as a palustrine pedogenised deposit. This layer is dated on bulk organic carbon to 7587-6174 cal. BP (Kur-07-K15-US18-1488, Tab. VI.2). A paleodune complex represented by SU41 and 13 composed of cross-laminated and well sorted fine sand follows. At the interface between SU41 and 13 we find a phase of occupation characterized by few lithic artifacts, faunal remains and small charcoal. The following archaeological level SU15 is dated between 3914-3647 and 3830-3587 cal. BP (Kur-07-K14-US15-1203, Kur-07-L16-US15-1283, Tab. VI.2). SU15 is a silty-clayey polyhedral structured layer rich in secondary carbonate nodules with abundant flat-lying broken ceramics, semi-articulated faunal remains, charcoal and lithics. SU12, marked by a more structured matrix, by the lack of secondary carbonates and the persistence of anthropic material is dated between 3638-3477 and 3568-3407 cal. BP (Kur-07-US12-591, Kur-07-US12-2459, Tab. VI.2). SU9/14 are composed of consolidated or non-consolidated aeolian sand and produced some pottery fragments, wild and domestic fauna, charcoal, lithics and groundstone, namely on or just below the surface, and cap the underlying pedogenic horizons. The most recent anthropic layer is a polymorphous oven structure (ST1, Faunal material from the site is dominated by domesticates (sheep, goat and cattle) which are present in SU15, 12, 10, and the surface of 9/14. Wild fauna includes mainly middle to small sized bovids, among which only the Dorcas gazelle could be identified. The rest of the fauna includes rodents, reptiles and fish remains (Siluriform and tilapia), as well as ostrich eggshell (Tab. VI.3).

At least two ceramic technological traditions corresponding to two phases of occupation can be distinguished. Few artifacts are affiliated with the first phase (SU15 and contemporary SU16-17, In addition, the ongoing study of ceramic raw materials allows us to distinguish between two cultural phases. The older one (SU15) is mostly characterized by ceramic pastes with inclusions made of shell tests (e.g. entire gastropods, e.g. Melanoides), charcoal and bones. The more recent phase appears to be mostly characterized by inclusions made of rounded volcanic sand and by other distinctive features such as vegetal remains, grog and diatoms.

The lithic material is mainly composed of worked obsidian with an inclination towards flake debitage, though irregular blades and bladelets are also present. The majority of cores studied are flake or pièces esquillées cores and were fragmented or reduced to small proportions, often by bipolar percussion on anvil. A number of blanks show irregular modifications of various extent, while very few shaped tools (scrapers, perforators and curved backed pieces) were recovered from excavations. The presence of rounded and rolled neo-cortex on some obsidian lithics attests to a partial selection of small obsidian cobbles by populations. While the study is ongoing, there are similar production tendencies across occupation levels pertaining to potter pastoralist populations (SU15, 12, 10, 9/14), while lithic material from the earlier SU41 was rare and therefore difficult to compare. / simple impression: a line incised with a blunt point under which there is a strip composed of multiple tooth comb impressed oblique lines. b) Refitted vessel with flat rim adorned with oblique lines and chevron pendant followed by pivoting impressions on body. c) Refitted vessel with simple impressions by means of oblique lines in horizontal band. Phase 2 is characterized by decorated flat or externally beveled rims and diverse motifs combined on the same vessel d): Vessel with band of oblique comb impressed lines under which parallel horizontal dotted lines frame comb impressed lozenges decorated with comb impressed motifs or herringbone motifs made with comb impressed oblique lines. e) Body sherd with combination of incision / simple impression and impressed oblique lines applied with a spatula on the rim. f) Two body sherds with roulette impressions. g) Rim sherd with simple comb impressions. h-i) Body sherds with pivoting impressions with a comb or shell. j) Body sherd with decorations that associate two impression techniques: a simple impression with a triangular tipped awl and a pivoting impression with a shell or a comb. k) Body sherd with red slip applied before firing, testifying to the application of colorants on certain pottery vessels.

VI.1.5 Discussion

14 new ages on littoral lacustrine carbonates combined with re-calibrated ages and recalculated elevations taken from [START_REF] Gasse | Evolution of Lake Abhe[END_REF] allow us to estimate fluctuating paleoshoreline levels of Abhe Lake and its relationship with other lakes in Ethiopia over the last 12000 years (Fig. VI.10). 23 radiocarbon dates from 7 excavated sites and associated sedimentary records (Tab. VI.2) provide the most robust chronological sequence to date for late prehistoric occupation in Ethiopia, and are the first major evidence for semi-continuous LSA to potter-pastoralist settlement in the Ethiopian Afar.

Paleo-lake level reconstruction combined with pedo-stratigraphic sounding results allow us to propose a semi-continuous Holocene chronostratigraphy of peri-lacustrine landscapes in relation to changes in human settlement, economy and culture across the Abhe Lake basin (Tab. VI.4) and to discuss these results in relation to pertinent patterns observed across northeast Africa.

VI.1.5.a Early Holocene (∼11.7 to ∼8.2 ka)

The Paleolandscape context

Our new dates and geomorphological results show a lack of sedimentary deposits concerning the Marine isotope stage (MIS)2 to early MIS1 periods (∼29-12 ka) in accordance with the lack of regional lacustrine and continental sedimentation [START_REF] Beck | Onset of the African Humid Period by 13.9 kyr BP at Kabua Gorge, Turkana Basin, Kenya[END_REF][START_REF] Gasse | Evolution of Lake Abhe[END_REF][START_REF] Gasse | Late Quaternary lake-level fluctuations and environments of the northern Rift Valley and Afar region (Ethiopia and Djibouti)[END_REF][START_REF] Gillespie | Postglacial arid episodes in Ethiopia have implications for climate prediction[END_REF][START_REF] Lamb | 150,000-year palaeoclimate record from northern Ethiopia supports early, multiple dispersals of modern humans from Africa[END_REF][START_REF] Wagner | Holocene rainfall runoff in the central Ethiopian highlands and evolution of the River Nile drainage system as revealed from a sediment record from Lake Dendi[END_REF].

At the beginning of the Holocene (∼11.1 and ∼10.6 ka cal. BP; Fig. VI.10d) the water-level of Abhe Lake was relatively low, oscillating between 340-360 m a.s.l. During this period, dated SU from two sequences indicate the development of a pedogenic event at the edges of the Awash alluvial plain in which the oldest preserved anthropic settlements occur (Lakora-03_SU4-5). Soil development is followed by a fluvial event dated to ∼10.5 ka cal. BP (Raso-02_SU5), suggesting the re-activation of the hydrological system leading to the rise of lake-levels at the beginning of the Abhe-IV transgressive phase (Raso-02_SU5). Between ∼10.3 and ∼9.8 ka cal. BP the lake attained its maximal water-level (∼410-420 m a.s.l.) occupying the totality of the Lower Awash plain. This Early Holocene highstand demonstrates the centennial scale reactivity of the Awash hydrological system to monsoon magnitude variations associated with the northward migration of the ITCZ (Fig. VI.10d). This period coincides with the highest Holocene precipitation values recorded in the Ethiopian highlands [START_REF] Bastian | Abrupt response of chemical weathering to Late Quaternary hydroclimate changes in northeast Africa[END_REF][START_REF] Costa | Isotopic reconstruction of the African Humid Period and Congo Air Boundary migration at Lake Tana, Ethiopia[END_REF] and with the maximal insolation values (corresponding to precession minima) occurring at this latitude (∼11°N; Berger and Loutre, 1991, Fig. VI.10a, b).

The Early Holocene period comes to an end with a reduced stable lake-level (∼370-380 m a.s.l.) until ∼8.2 ka (Fig. VI.10d), during which contemporary peri-lacustrine sedimentary deposits likely developed. After this period, lake levels did not rise above 380 m a.s.l. probably as a consequence of channel incision that evacuated water towards the Hanle semi-graben. One channel incision, which is a likely candidate but requires ground verification was identified using remote sensing and DEM modelling around Serdo town (Fig. VI.1b). The absence of sedimentary and archaeological data corresponding to Early Holocene high lake stands is likely a result of an extreme post-lacustrine erosional rate on high basaltic outcrops and by difficult to access elevated regions during fieldwork (above 400 m a.s.l.).

Human adaptation to rising lake levels in the Later Stone Age

The rarity of sedimentary deposits during the YD period (12.9-11.7 ka; [START_REF] Rasmussen | A stratigraphic framework for abrupt climatic changes during the Last Glacial period based on three synchronized Greenland ice-core records: Refining and extending the INTIMATE event stratigraphy[END_REF] is echoed by the lack of sites recovered for this period. Sites recovered immediately after the YD concern lakeside settlements as the lake was rising and had reached ∼ 360 m a.s.l. of the 430 m a.s.l. it would progressively attain one millennium later. The first evidence of Holocene occupation at Lakora-03_SU4-5 and Raso-02_SU5 occurred as climatic conditions became more humid after the YD arid event [START_REF] Foerster | Climatic change recorded in the sediments of the Chew Bahir basin, southern Ethiopia, during the last 45,000 years[END_REF]. SU5a at Raso-02 dated to 10744-10418 cal. BP is slightly younger than SU4-5 at the site of Lakora-03. The Raso-02 uppermost layers (SU 1a-1b) may have been redeposited from eroded older levels that can be found preserved under large stromatolite domes slightly uphill of the studied section. This has been observed from study of the top sedimentary units at this site which appear reshuffled, and from a radiocarbon determination on charcoal from the transition between SU1b-2 which provides an inverted age slightly older and closer in age to the Lakora-03 archaeological level, than the distinctly underlying SU5.

The archaeological levels at these two sites present fish and other faunal remains, lithic material exclusively knapped on obsidian and an anthropogenic feature (hearth at Lakora-03). During this period the lakeshore was located several kilometers from Raso-02 and at the foot of the Lakora-03 site. The abundance of fish remains (including burnt) at Lakora-03 confirms that populations were temporarily settled near the lake edge and that fishing constituted a major activity and the source of an important part of their diet. The obsidian lithic assemblage has few shaped tools and is technologically similar on both sites.

The Lakora-03 settlement indicates intensive fishing from a slightly elevated basalt promontory or islet. The faunal assemblage at Raso-02 includes 3 wild mammal remains and one fish remain and is too small to draw any conclusions. However, comparison with Lakora-03 where fish remains are abundant and dominate the assemblage may reflect the slight age difference between these occupations and their respective proximity to the lake edge (Fig. VI.3c). Judging by the exponential rise in lake levels over one millennium, it is likely that these populations adapted to significant centennial if not generationalscale landscape modifications, following lake edges as they rose all the while benefitting from the rich and readily available lacustrine resources. Lithic materials from both sites have features reminiscent of slightly older sites (∼14-13.5 ka cal. BP) from the Ziway-Shala basin [START_REF] Ménard | Late Stone Age variability in the Main Ethiopian Rift: New data from the Bulbula River, Ziway-Shala basin[END_REF], with dominant production of blades and bladelets, few shaped tools and a possible absence of backed pieces. There is unfortunately no synchronous assemblage in the region to make comparisons with but material collected at Lakora-03 and Raso-02 could indicate a late persistence of a Late Glacial (if not older) tradition.

VI.1.5.b Middle Holocene (∼8.2 to ∼4.2 ka)

The Paleolandscape context

The Middle Holocene onset period is marked around 8.2 ka cal. BP by rapid decrease (centennial scale average) of the Abhe Lake water-level (Fig. VI.10d), which was in phase with an abrupt (300 to 100 year) Holocene cold event at 8.2 ka recorded from Greenland ice cores [START_REF] Thomas | The 8.2ka event from Greenland ice cores[END_REF][START_REF] Matero | The 8.2 ka cooling event caused by Laurentide ice saddle collapse[END_REF] and recently revisited by the General Circulation Model [START_REF] Matero | The 8.2 ka cooling event caused by Laurentide ice saddle collapse[END_REF]. A similarly abrupt transition from D-depleted to D-enriched waxes in Lake Tana is dated around 8.5-8 ka cal. BP and is attributed to a reduction of moisture derived from westerly sources (Congo basin, Atlantic ocean; [START_REF] Costa | Isotopic reconstruction of the African Humid Period and Congo Air Boundary migration at Lake Tana, Ethiopia[END_REF]Marshall et al., 2011, Fig. VI.10b). At the scale of tropical Africa, the first manifestation of less humid conditions which began around 8.4 ka cal. BP is documented almost everywhere [START_REF] Costa | Isotopic reconstruction of the African Humid Period and Congo Air Boundary migration at Lake Tana, Ethiopia[END_REF] and Ti (mg/g; [START_REF] Marshall | Late Pleistocene and Holocene drought events at Lake Tana, the source of the Blue Nile[END_REF], Tana Lake; c) planktonic diatoms (%; [START_REF] Chalié | Late Glacial Holocene diatom record of water chemistry and lake level change from the tropical East African Rift Lake Abiyata (Ethiopia)[END_REF] and lake levels [START_REF] Gillespie | Postglacial arid episodes in Ethiopia have implications for climate prediction[END_REF], Ziway-Shala basin. d) Abhe paleolake level curve calculated from calibrated ages and recalculated elevations taken from [START_REF] Gasse | L'évolution des lacs de l'Afar Central (Ethiopie et T.F.A.I.) du Plio-Pléistocène à l'Actuel. Recostitution des paléomilieux lacustres à partir de l'étude des Diatomées[END_REF] and from new ages on littoral lacustrine carbonates with plotted calibrated dates and elevations of dated archaeological sites of the VAPOR-Afar project sites presented in this study. e) Regional map (after GMTED2010, SRTM) with localization of Tana, Ziway-Shala and Abhe Lakes.

in Africa [START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF] and is particularly well documented and dated from large catchment areas such as the Nile and Congo deltaic-fan sediments [START_REF] Ménot | Timing and stepwise transitions of the African Humid Period from geochemical proxies in the Nile deep-sea fan sediments[END_REF][START_REF] Schefuß | Hydrologic control of carbon cycling and aged carbon discharge in the Congo River basin[END_REF]. This latter paleoclimatic scenario is confirmed by the Stromatolis_SU4 palustrine layer, which covers a stromatolitic formation and indicates a slight lake regression that began around ∼8.5 ka cal. BP. This corresponds to an average drop in lake-levels of 4 cm/yr between ∼8.5 and ∼8.2 ka cal. BP. Lacustrine regression accelerated abruptly between ∼8.2 and ∼8 ka cal. BP with an average drop in lake-levels of ∼70 cm/yr. This is evidenced by a paleosoil that developed at 8190-8000 cal. BP (355 m a.s.l.; MS-01_SU1; A resurgence of wet conditions may be suggested namely by two hippopotamus bones directly dated to around ∼7.8 and ∼7.6 ka cal. BP, and a large aquatic faunal assemblage at the Det Bahari-01 site (370 m a.s.l.). A pedogenic event is observed and dated to 7519-7424 cal. BP (SP-01_SU4; Fig. VI.6a) suggesting the reduction of aeolian sediment transport dynamics and the availability of water supplies. These results are in accordance with rainfall and terrigenous input increase recorded at Lake Tana starting ∼7.7 ka cal. BP [START_REF] Costa | Isotopic reconstruction of the African Humid Period and Congo Air Boundary migration at Lake Tana, Ethiopia[END_REF]Marshall et al., 2011, Fig. VI.10b) and from the Nile deep-sea fan archive [START_REF] Ménot | Timing and stepwise transitions of the African Humid Period from geochemical proxies in the Nile deep-sea fan sediments[END_REF].

Across the Abhe basin, rising lake levels were detected on dated stromatolites at ∼7 ka cal. BP (∼330 m a.s.l.; Fig. VI.10.d), reaching a stable level between ∼6.6 and ∼5.6 ka cal. BP (360-370 m a.s.l.). From ∼5.6-4.6 ka cal. BP we observe a decrease in lake levels at a rate of ∼1.5 cm/yr.

Adaptation of Later Stone Age fisher-hunter-gatherers to a fluctuating lacustrine environment

A window into the period leading up to and after the 8.2 ka event is visible at the site of Stromatolis characterized by the first LSA built structure known to date. The sediments that accumulated during the use of the anthropic circular structure were partially deflated by aeolian action after its abandonment and prior to the lake submerging the area. Given this terminus ante quem for the structure's use, it is, to our knowledge, the oldest possibly non-funerary anthropic structure in the Horn of Africa, and was likely used during the 8.2 ka arid event.

Human occupation at MS-01/MS-02 corresponds to a period of continued low lake levels following the 8.2 ka event, and suggests that human groups were extremely reactive to rapid lake level oscillations during this period, moving along with the lakeshore.

The site of Det Bahari-01, occupied between 7.9 and 7.5 ka cal. BP falls within a period where lake levels are not documented. While the site is a relict surface site, dated faunal remains, lithic refits, and the homogeneity of the assemblage suggest it belongs to a single or to multiple occupation events that occurred over a short time. Directly dated hippopotamus bone, and dominant aquatic fauna suggest the Det Bahari plain was partially submerged at this time and that the site itself was near the water's edge. This would suppose that an undocumented transgressive lacustrine phase occurred during the occupation of Det Bahari-01. Lithic traditions observed at Det Bahari-01 (lamino-lamellar debitage, curved backed pieces) and the presence of some aquatic fauna on the surface of site SP-01 (∼7.5-7.4 ka cal. BP) further support the continuity of shoreline settlement over the course of ∼500 years of oscillating lake-levels.

Our results establish that the Lower Awash plain which makes up the northern shore of the Early to early Middle Holocene Lake Abhe was settled before, during and after the 8.2 ka arid event. Between ∼8.1 and ∼7.4 ka cal. BP another phase of settlement is identified in the area. These archaeological sites and their associated aquatic fauna suggest at least one low lake stand before the Abhe-V stable lake transgression. This period differs markedly from the period prior to 8.2 ka in that while fish remain a staple, there is significantly less than at earlier sites, and more large aquatic fauna such as crocodile and hippopotamus, notably on sites like Det Bahari-01. While fishing remained a common practice, the presence of larger quantities of wild aquatic fauna may suggest new technical innovations in hunting and fishing equipment. Use-wear study on a select number of curved backed pieces show diagnostic impact fractures that suggest their use as composite projectile points (barbs and axial points likely set into wood or bone shafts) to act as arrows, spears or harpoons for hunting and/or fishing. Unfortunately the lack of preserved plant material in the deposits limits our understanding of these populations' re-lationships with wild plants and whether or how they may have been used for shelter, food and craft production. The presence of groundstone implements, and debitage oriented towards the production of bladelet blanks for composite backed pieces are also departures from traditions observed in the previous settlement period identified. VI.1.5.c Early Late Holocene (∼4.2 to ∼3 ka BP)

Paleolandscape context

The Late Holocene period is marked by a decrease of lake levels around ∼4.5 ka cal. BP and the spread of arid conditions across the Abhe basin (Fig. VI.10d). This change in hydrological patterns corresponds to the reduction of the African monsoon rainfall regime, which marks the end of the AHP [START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF][START_REF] Shanahan | The time-transgressive termination of the African Humid Period[END_REF]. This period likely corresponds to the renowned 4.2 ka event [START_REF] Bini | The 4.2\,ka\,BP Event in the Mediterranean region: An overview[END_REF] characterized by a rapid (centennial scale average) decrease in precipitation in tropical Africa that led to the spread of hyperarid conditions. In the Abhe basin, the installation of the first Middle to Late Holocene aeolian accumulations fall in the range between ∼4.5 and ∼3.7 ka cal. BP (Kurub-07_SU41 and SU13). At the Kurub-07 site we observe alternations between paleodune layers and aridisols between ∼3.7 and ∼3.2 ka cal. BP, confirming the persistence of arid conditions. Considering the low altitude of human settlements and the similarity in low precipitation values during the YD and the 4.2 event in the Ethiopian Highlands (Costa et al., 2014, Fig. VI.10b, c), this Late Holocene arid event led to a drastic reduction of Abhe Lake levels like those recorded during the 8.2 ka event.

Multi-resource economies and the transition to herding in the Middle to Late Holocene transition

Lower lake-levels and more arid conditions began at ∼5.5 ka cal. BP and took hold around ∼4.5 ka cal. BP though there is a general lack of lake-level data for this period (Fig. VI.10d). The earliest evidence of herding in the region is attested around 4.8 ka cal. BP on the eastern shores of Lake Abhe [START_REF] Coudert | New archaeozoological results from Asa Koma (Djibouti): Contributing to the understanding of faunal exploitation during the 3rd millennium BC in the Horn of Africa[END_REF][START_REF] Lesur | The advent of herding in the Horn of Africa: New data from Ethiopia, Djibouti and Somaliland[END_REF], while we have yet to date a pottery herding site of this period in the Ethiopian Afar.

The site of Kurub-07 is exceptional for the region for its discontinuous roughly two-millennia long human occupation sequence and for the preservation of remains of the first potter pastoralists who inhabited the area. The site features an older level of human occupation (SU41) thus far devoid of ceramics or animal domesticates. Three radiocarbon dates from this level are inverted and have therefore been rejected (Kur_07_ST6_655, Kur_07_Spit8_661, Kur-07-K15-Spit8-663B, Tab. VI.2). However, we can bracket this archaeological level between dates for the last lake transgression ∼4.5 ka cal. BP and the following anthropic level (SU15) radiocarbon dated to ∼3.9-3.6 ka cal. BP.

After a period of abandonment of the site (SU13), several continuous well-preserved levels of occupation by potter herders that cover the period from ∼3.9 to ∼3.3 ka cal. BP follow. Remains of domesticates including cattle, sheep and goat confirm the presence of a herding economy. Although small, the spectrum attests to the importance of livestock in the diet. However, the large numbers of small wild fauna, particularly suited to open and arid plains, and some fish remains attest to the persistence of hunting and fishing probably carried out in the vicinity of the site while also suggesting the presence of drier conditions (semi-arid environment).

In addition to the introduction of small domestic herds during this period, we observe major changes in technical traditions with the appearance of pottery for storage, built structures such as ovens, and bipolar percussion on anvil on small obsidian nodules mainly for flake and to a lesser degree bladelet production. While few shaped tools were recovered, geometric backed pieces, pièces esquillées and few scrapers and perforators were recovered from excavations. This diverges from LSA lamino-lamellar debitage traditions but the production of curved backed composite tools on bladelet blanks is reminiscent of what was identified at Det and dated to the Middle Holocene.

Particularly unique to the Abhe Lake region are the impressed pottery traditions (Fig. VI.9). The Kurub-07 ceramic assemblage is lacking in the regional sequence and the site presents an important milestone that contributes to our understanding of the socio-cultural landscape of this sector. Ceramics from the first and second phase of occupation of this site show some similarities to well-defined potter traditions east of Abhe Lake, in particular with the Asa Koma / Wakrita facies identified on several sites dating to between ∼4.7 and ∼4 ka cal. BP in the Gobaad Basin in Djibouti [START_REF] Gutherz | New insights on the first Neolithic societies in the Horn of Africa: The site of Wakrita, Djibouti[END_REF]. The analogies are based on the exclusivity of simple forms in the assemblage and medium sized formats, the absence of elements in relief, both decorative and for handling, the airy aspect of the compositions, the diversity of techniques and their combinations, and the place granted to herringbone or diamond compositions and to pivot impressions. These similarities may suggest that ceramic features dated to the earliest attestations of herding on the eastern shores of Lake Abhe may have persisted on the northern and western shores of the same lake between ∼4-3.4 ka cal. BP. However, we see more distinctions than parallels between these traditions, namely with regard to more diverse decorative motifs on eastern shore sites. It is therefore not yet possible to directly associate the two phases of Kurub-07 with this tradition.

A few rare isolated body sherd fragments testify to the practice of applying a red slip to undecorated surfaces on pottery vessel walls before firing in the more recent occupation phase at the site of Kurub-07 as well as on other surface sites in the area (Fig. VI.9k). This practice has also been attested at several sites in the eastern sector of Abhe Lake [START_REF] Cauliez | PSPCA -Programme 320 Premières Sociétés de Production dans la Corne de l'Afrique[END_REF].

VI.1.5.d Situating Holocene settlement in the Abhe paleolake basin in the context of Northeast Africa

From a wider inter-regional perspective, fishing, hunting and foraging of aquatic resources characterize many Early Holocene sites in the Saharan belt and the Sahel, at a time when paleolakes and vegetation flourished in today's arid zones as a result of the mesic conditions of the AHP. In these regions, populations drawn to the abundant aquatic resources available in surrounding lakes probably had low residential mobility, judging by the presence of cemeteries, storage vessels and local raw material exploitation [START_REF] Nash | Obsidian types from Holocene sites around Lake Turkana, and other localities in northern Kenya[END_REF]. These lakeside communities once termed aqualithic [START_REF] Sutton | The Aquatic Civilization of Middle Africa[END_REF] are commonly grouped in association with bone harpoons and developed impressed pottery traditions that can first be found as early as ∼12-10 ka cal. BP in the Sahara and Sahel, and as far south as Lake Turkana in Kenya [START_REF] Beyin | New radiocarbon dates for terminal Pleistocene and early Holocene settlements in West Turkana, northern Kenya[END_REF][START_REF] Goldstein | New archaeological investigations at the Lothagam harpoon site at Lake Turkana[END_REF][START_REF] Sereno | Lakeside Cemeteries in the Sahara: 5000 Years of Holocene Population and Environmental Change[END_REF]Wright et al., 2015, Fig. VI.1a). Elsewhere in the Horn of Africa, namely the Ziway-Shala and Abhe lake basins, aquatic resources played an important role in the economy while pottery and bone harpoon traditions were entirely absent [START_REF] Coudert | New archaeozoological results from Asa Koma (Djibouti): Contributing to the understanding of faunal exploitation during the 3rd millennium BC in the Horn of Africa[END_REF][START_REF] Ménard | Late Stone Age variability in the Main Ethiopian Rift: New data from the Bulbula River, Ziway-Shala basin[END_REF].

Through this wide angle, the transition to herding of Mesolithic or LSA fisher-hunter-gatherers was extremely variable, complementing enduring aquatic economies. The earliest evidence of herding in adjacent regions can be found in South Arabia between 9-8 ka cal. BP [START_REF] Martin | Early Arabian pastoralism at Manayzah in Wādī Ṣanā, Ḥaḍramawt[END_REF][START_REF] Fedele | Wādī at̲ -T ̲ ayyilah 3, a Neolithic and Pre-Neolithic occupation on the eastern Yemen Plateau, and its archaeofaunal information[END_REF], several millennia after its northern Near Eastern neighbors adopted farming. In both cases these Neolithic societies remain aceramic for millennia. In northeast Africa, herding, in association with regionally specific ceramic traditions appeared before farming. This is evidenced first in the Eastern Sahara and Nile Valley [START_REF] Honegger | La Nubie et le Soudan: Un bilan des vingt dernières années de recherche sur la pré et protohistoire[END_REF][START_REF] Linseele | The earliest phase of introduction of Southwest Asian domesticated animals into Africa. New evidence from the Fayum Oasis in Egypt and its implications[END_REF] Turkana and Abhe around 4.7 ka cal. BP [START_REF] Chritz | Climate, ecology, and the spread of herding in eastern Africa[END_REF][START_REF] Coudert | New archaeozoological results from Asa Koma (Djibouti): Contributing to the understanding of faunal exploitation during the 3rd millennium BC in the Horn of Africa[END_REF], only becoming common during or after ∼3 ka cal. BP in Southwest Ethiopia, the Kenyan Rift and Lake Victoria region [START_REF] Ambrose | Chronology of the Later Stone Age and food production in East Africa[END_REF][START_REF] Gifford-Gonzalez | Animal Disease Challenges to the Emergence of Pastoralism in Sub-Saharan Africa[END_REF][START_REF] Hildebrand | The Holocene Archaeology of Southwest Ethiopia: New Insights from the Kafa Archaeological Project[END_REF][START_REF] Lesur | The advent of herding in the Horn of Africa: New data from Ethiopia, Djibouti and Somaliland[END_REF]Prendergast, 2010, Fig. VI.1a).

The arid climatic conditions that took hold at the end of the AHP (∼4.5-4.2 ka) are thought to have had a role in the southern spread of herding into East Africa following the retreating monsoon belt [START_REF] Kuper | Climate-controlled Holocene occupation in the Sahara: Motor of Africa's evolution[END_REF]. It has been argued that mobility associated with herding may have supported its spread, both through interaction and exchange of livestock with foragers, and through social networks that enabled them to thrive faced with growing aridity [START_REF] Marshall | Late Pleistocene and Holocene drought events at Lake Tana, the source of the Blue Nile[END_REF]. It has also been suggested that maritime mobility across the Red Sea, confirmed through obsidian sourcing studies [START_REF] Khalidi | Obsidian sources in highland Yemen and their relevance to archaeological research in the Red Sea region[END_REF][START_REF] Oppenheimer | Risk and reward: Explosive eruptions and obsidian lithic resource at Nabro volcano (Eritrea)[END_REF] may have played a role in the spread of herding by means of coastal fisher-herding communities from southern Arabia [START_REF] Gutherz | Asa Koma : Site Néolithique Dans Le Bassin Du Gobaad (République de Djibouti. Mondes Anciens[END_REF][START_REF] Lesur | The advent of herding in the Horn of Africa: New data from Ethiopia, Djibouti and Somaliland[END_REF]. However, the current study provides new data that suggest traditions in the Abhe Lake basin had few to no parallels with those in surrounding regions across the three periods of settlement documented in the region. Parallels that can be drawn with adjacent regions are limited to certain lithic traditions, though these have a large and persistent chrono-regional scope on the African continent and beyond, and to the domestic sheep and goat populations that were integrated into multi-resource economies sometime between 4.8 and 3.9 ka cal. BP in the Afar, but which are similar in all neighboring Neolithic contexts. The unique Abhe traditions are neither in line with Early Holocene harpoon potters and Mid-Holocene potter herders of the Sahara and Nile corridor, nor with Early Holocene Epipaleolithic hunter-gatherers and Mid-Holocene aceramic herders of Arabia.

Today, the most robust chrono-cultural sequence outside of the Abhe and Ziway-Shala regions can be found in the Lake Turkana and Victoria basins in Kenya [START_REF] Ashley | Paleoenvironmental Reconstruction of Dongodien, Lake Turkana, Kenya and OSL Dating of Site Occupation During Late Holocene Climate Change[END_REF][START_REF] Beyin | New radiocarbon dates for terminal Pleistocene and early Holocene settlements in West Turkana, northern Kenya[END_REF]Hildebrand et al., 2018a,b;[START_REF] Owen | Palaeolimnology and archaeology of Holocene deposits north-east of Lake Turkana, Kenya[END_REF][START_REF] Prendergast | Kansyore fisher-foragers and transitions to food production in East Africa: The view from Wadh Lang'o, Nyanza Province, Western Kenya[END_REF][START_REF] Wright | Lakeside View: Sociocultural Responses to Changing Water Levels of Lake Turkana, Kenya[END_REF][START_REF] Robertshaw | Shell Middens on the Shores of Lake Victoria[END_REF][START_REF] Robertshaw | Gogo Falls[END_REF]. A recent study comparing these two basins has demonstrated very distinct ecological responses to climatic changes in each, as well as establishing the complexity of relations between these responses and early herding [START_REF] Chritz | Climate, ecology, and the spread of herding in eastern Africa[END_REF].

Similarly, correlation with lake-level proxies, human settlement pattern and archaeomaterial data demonstrate that over 9000 years, major socio-economic changes of populations inhabiting Abhe paleolakeshores were distinctive ecological responses to transformations in the local environment and to fluctuating lake levels.

VI.1.6 Conclusion

Through cross-examination of morpho-sedimentary and archaeological datasets we provide a preliminary reconstruction of the evolution of the Abhe paleolandscape and the lifeways of its Holocene populations as the lacustrine environment significantly transformed over 9000 years.

We present the first Holocene chrono-cultural sequence for the Ethiopian Afar including 23 14 C dates on archaeological levels ranging between ∼11 and ∼3.5 ka cal. BP. These cluster into three periods during which we find preserved human settlement along shifting Abhe paleolakeshores. These three periods fall within the AHP, which is interrupted by and terminates with two sedimentologically documented hyperarid events (8.2 and 4.2 ka) hosting human occupations. Although human occupation levels are not continuous, these windows of site preservation offer insight into continuities and ruptures in techno-economic traditions of Abhe societies, which we interpret as ecological responses to local amplifier lake behavior in one of the most arid regions of the world.

Our results provide a revised Early to Late Holocene model of lake-level fluctuations linked to dynamic peri-lacustrine landform evolution. The centennial-scale reactivity of this amplifier lake to climatic forcing is echoed in the variability of settlement location and strategy, and in the flexible multiple resource economies of its populations over time. Most interestingly, our data suggest that the Abhe Lake basin hosted populations with local techno-economic traditions who relied on aquatic resources even with the introduction of domestic herds. As we have yet to find continuity in human occupation over 9000 years, it is possible that different populations settled the area over time, and that stable lake levels attracted more groups to the area, while extreme lake level rise and fall forced groups to move to higher altitudes or towards isolated oases and refugia as has been suggested for amplifier lakescapes [START_REF] Maslin | East African climate pulses and early human evolution[END_REF][START_REF] Trauth | Human evolution in a variable environment: The amplifier lakes of Eastern Africa[END_REF], and more generally for arid environments [START_REF] Purdue | From Refugia to Oases: Living in Arid Environments from Prehistoric Times to the Present Day. Des Refuges Aux Oasis : Vivre En Milieu Aride de La Préhistoire à Aujourd'hui[END_REF]. Throughout the periods represented, our data are consistent with populations adapted to lakeshore fluctuations, and populations with continuity in traditions between hyperarid events. This local multi-scalar and multi-proxy approach allows a preliminary diachronic restitution of human ecological responses to local landscape transformation that would normally have been overlooked and overshadowed by correlations with regional and global climate proxies. These results provide the first data available on Early to Late Holocene human occupations in the Ethiopian Afar and on the transformation and resilience of prehistoric human societies in the context of the dynamic and arid landscape they occupied.

VI.2 Sedimentary formation processes and environmental context of human settlements in the Late Pleistocene -Holocene Lake Abhe basin

VI.2.1 Introduction

The objective of this Section is to investigate the archaeological sequences presented in Section VI.1, with the aim to refine the environmental context of the prehistoric occupations along Lake Abhe's palaeoshores during the Late Pleistocene to Holocene periods. The link between human settlements and inhabited environments is fundamental for understanding the link between wider climatic and landform changes and the mobility and changes in economic strategies of prehistoric groups. Particularly, thanks to the high-resolution studies (microstratigraphy and soil micromorphology) of anthropised sedimentary sequences, we are able to point out specific spatio-temporal information on the distribution of environmental feedbacks to regional hydro-climatic oscillations described in Sections IV and V. For do this, specific sedimentary sequences were selected for study and presented in Section VI.1, including an additional archaeological site not yet presented in this manuscript (Hara-Ide-3 site, Section VI.2.3.a) that completes the Late Pleistocene-Holocene chrono-stratigraphy. Archaeological sequences selected are those which can provide enough of an extended diachronic framework to track sedimentary formation processes over a few millennia. We thus include, from the more ancient to the most recent, the sites of Hara-Ide-3, Raso-02, Lakora-03 and Kurub-07. Methods used to geoarchaeologically investigate these sequences are detailed in Section VI.2.2 and results are presented and interpreted in Section VI.2.3. Finally, a synthesis and a general discussion of environmental-driving processes in relation to climatic forcing and to prehistoric settlement dynamics is presented in Section VI.2.4.

VI.2.2 Methodology

The investigation of specific sedimentary formation processes of archaeological sites presented in this manuscript is principally based on the soil micromorphology analytical method (see Section III.3.2.a). However, the combination of methods including grain size distribution, loss on ignition and X-ray fluorescence analyses (Sections III.3.3.a,III.3.3.b,III.3.4.b respectively) was necessary to complete data acquisition from some specific sequences.

VI.2.3 Results

VI.2.3.a Hara-Ide-3

The Hara-Ide-3 site study is not included in the archaeological sequence presented in Section VI.1. However, because of its importance to the description of sedimentary formation processes of the Pleistocene-Holocene boundary period, we present here its main sedimentogenetic characteristics and their correlations with the chronostratigraphy of the Late Pleistocene-Holocene sedimentary formations of the Lake Abhe basin.

General stratigraphy and preliminary interpretation

The Hara-Ide-3 site is located on the left bank of the Degadle stream (Djibouti branch of Lake Abhe basin), a tributary of the Gobaad river (Fig. VI.11b). The excavations of the Hara-Ide-3 site (Fig. The pedosedimentary description of the sequence is presented in the text and synthetized in Tab. VI.5. Subsequently a specific micromorphological investigation of the anthropic layers is proposed. Finally, the relevance of the sedimentary formation processes of the Hara-Ide-3 sequence is discussed and a chrono-stratigraphic and environmental interpretation is proposed. From the bottom to the top, Phase_A is composed of an offshore biogenic diatomite formation (SU16) followed by a silty diatomite unit (SU15) characterized by pale brownish colour attesting to a lowering in lake water column depth. Phase_B is generally composed of a brownish clayey deposit intercalated with a thin fine sand event of alluvial origin (SU13). At the base of this Phase, SU14 presents a strong prismatic clay aggregation which turns upwards into a coarser clayey unit characterized by a polygonal structure. This sedimentary Phase likely corresponds to the retreat of the lake shoreline correlated with the development of a palustrine deposit (SU14), a slight reactivation of the local hydrographic system (SU13) followed by a pedogenic event (SU12 and 11).

Sedimentary Phase_C starts with an erosive event and by the deposition of grey gravel and sand which vary laterally according to the topography of the studied area (Fig. VI.12.a). Over the archaeo-logical site area, the beginning of this Phase is characterized by a diagenised laminated and well sorted fine/medium sandy unit SU1, followed by a fine sand laminated layer (SU2). This deposit indicates a fluvial deposit of medium to low energy flow, probably corresponding to the activity of a lateral stream of the Degadle river (Fig. VI.12.b). Next to the actual main hydrographic network (Hara-Ide-1 section; Fig. VI.12.b), deposits are coarser and composed of gravels embedded in a coarse sand matrix (SU10) attesting to a higher energy flow. The end of sedimentary Phase_C is characterized by a well sorted fine to medium sand without any evident structure patterns (massive deposit; SU3 and 4). SU3 is coarser with some gravels and calcareous pebble inclusions. It's in these sedimentary units that prehistoric human occupation has been attested (Fig. VI.12.a). Indeed, human bones, charcoal and ostrich egg shell has were found and documented (Sup. Mat. BII.3,BII.4). A more precise description and sedimentogenetic interpretation of SU4 and SU3 has been possible thanks to micromorphological analyses presented in the next Section.

Phase_D is characterized by massive and very well sorted fine sand probably of aeolian origin (SU5 to SU7). Macroscopic evidence which permits us to differentiate these upper layers from the main archaeological units includes the absence of coarse inclusions or of secondary nodules and the pale reddish colour of the matrix. A more recent prehistoric occupation layer is attested by the presence of anthropic hearths which cut the upper levels (ex. SU8a-b).

The Hara-Ide-3 site is sealed by a whitish diatomite deposit attesting to a more recent lacustrine transgression (SU9) corresponding to the sedimentary Phase E. All of the Hara-Ide-3 sequence is traversed by sub-vertical and horizontal calcareous crusts, probably linked to the dissolution and re-precipitation of secondary carbonates percolating from the upper CaCO 3 -rich lacustrine deposits.

Micromorphological description and interpretation

With the aim to better understand the sedimentary formation processes of the upper Phase_C, and thus the sedimentary context of the prehistoric occupation of the Hara-Ide 3 site, three micromorphological blocks were collected from the sections. Here we present the micromorphological description and interpretation of the Sample HI-3_Mic3 (Fig. VI.13) which covers the Sedimentary Units 3, 4 and 7. The micromorphological description is synthesized in Tab. VI.6, and discussed in the text below.

From the bottom to the top, the SU7-4-3 are well recognised in the section ): SU3 is characterized by a subangular coarse fraction moderately sorted with a main range between 20 and 300 µm (Fig. VI.13d), and with a slightly developed inter-grain micro-aggregate microstructure which includes several porous crumbs with a strong micritic b-fabric. The porosity network is organised in a mix of Complex Packing Voids and abundant channels and chambers. Pedofeatures are mainly represented by abundant and coarse micritic disorthic nodules (2mm to 1 cm), however, no calcitic features on voids are observed. SU4 presents a more sorted subangular coarse fraction without fine fraction peds. The porosity acquires a simple packing voids pattern, and the pedofeatures follow the same characteristics of SU3 (Fig. VI.13g,h). With SU7.1 the coarse fraction became very well sorted with rounded to sub-angular grain shape. Porosity is mainly represented by simple packing voids. The specific markers of this unit are the abundance of Fe-Mn dendritic orthic nodules (Fig. VI.13f,d) mainly concentred at the limit between SU 7.1 and SU4, and the pronounced bioturbation (Fig. VI.13e). Finally, SU7.2 presents a slightly smaller and very well sorted rounded to sub-angular coarse fraction without evident pedofeatures.

In all four SU (3, 4, 7.1, 7.2), the predominance of sorted and fine heavy and resistant mineral grains (garnet, quartz) and the low quantities of freshly weathered basalt products (plagioclases, olivine, pyroxenes) exclude the hypothesis of a direct sedimentary transfer linked to gravity-induced or fluvial processes from the proximal basaltic horsts. This mineralogical assemblage rather suggests reworked allochtonous material from a heavily weathered and sorted origin. Additionally, the absence of specific groundmass structure (random distribution and orientation patterns or laminations) excludes the primary deposition of fluvial sedimentation which characterizes the lower Phase_C (SU1 and 2). These mineralogical and groundmass characteristics of well sorted sub-angular and rounded coarse material which rarely exceed the 200 µm can be attributed to aeolian-type sedimentary transfer, which has even been observed in paleo-dune deposits at the Kurub-07 site (see Section VI.2.3.b,Fig. VI.20b). From the bottom to the top of the thin section a slight diminution in grain size together with a complete lack of fine fraction point to the increase of aeolian transfer processes, particularly for the SUs 7.1 and 7.2 (Phase_D). Here, the absence of secondary precipitation of carbonates as coatings or infillings around voids/grains and the absence of micritic nodules, suggest slightly pronounced hydromorphic processes in this part of the sequence. Only Fe-Mn nodules are observed at the beginning of Phase_D. Macroscopically, the precipitation of post-depositional carbonates linked to the installation of upper lacustrine sediment of Phase E are evident as thick carbonate sub-vertical crusts which cross the sediments of Phases_C and_D (Fig. VI.12a). Excluding these crust formations, chemical post-depositional processes seem not to have affected the archaeological deposit. In Phase_C, a post-depositional reworking of sediments is excluded. Only the upper part of Phase_D has been partially affected by actual and sub-actual aeolian deflation.

Chronostratigraphic correlation of the Hara-Ide-3 sequence and its paleoenvironmental significance

This preliminary sedimentological and geoarchaeological investigation of the Hara-Ide-3 site, combined with new radiocarbon ages (Tab. VI.7) have permitted us to propose a chronostratigraphic frame- work of the sequence and to provide a preliminary paleoenvironmental interpretation of the sedimentary formation processes.

The sedimentogenetic and chornostratigraphic interpretation of the Hara-Ide-3 deposits, combined with the paleo-Lake Abhe fluctuation model presented in Chapter V, allowed us to reject certain radio- carbon ages, and to propose a more reliable chronological framework for the site. Indeed, radiocarbon ages on human bone and on enamel from SU3 and SU4 were between ∼10.2 and ∼9.4 ka cal. BP (Tab. VI.7), corresponding to a period of lacustrine overflow over the Hara-Ide-3 site (Lake level > 380 m a.s.l.; Hara-Ide-3 = 363 m a.s.l.). Moreover, deposits from SU3 and SU4 suggest the installation of an arid environment (aeolian activity), which is completely in disagreement with the humid environmental conditions evidenced around 10 ka cal. BP when Lake Abhe was in a highstand phase.

Consequently, following the chronology based on radiocarbon ages measured on OES and charcoal, we are able to propose a solid chronostratigraphy of the Hara-Ide-3 site. The lower clayey and silty diatomites of Phase_A likely correspond to the lacustrine transgression of paleo-Lake Abhe, named Abhe-III by [START_REF] Gasse | Evolution of Lake Abhe[END_REF][START_REF] Gasse | L'évolution des lacs de l'Afar Central (Ethiopie et T.F.A.I.) du Plio-Pléistocène à l'Actuel. Recostitution des paléomilieux lacustres à partir de l'étude des Diatomées[END_REF] and dated between ∼31 and ∼25 ka cal. BP. Clayey deposits of Phase_B are slightly pedogenised attesting to a lacustrine regression accompanied by the development of a palustrine deposit intercalated with a low energy fluvial stream (SU13). Soil development corresponding to SU11 suggests the regression of lake waters over this basin area. Interestingly, an interruption of humid conditions around 20 ka BP is largely attested in East Africa as a strong reduction of hydrological activity sometimes with the drying of large lakes [START_REF] Lamb | 150,000-year palaeoclimate record from northern Ethiopia supports early, multiple dispersals of modern humans from Africa[END_REF], as observed at Hara-Ide-3. Aggradation of fluvial deposits (SU10 and SU11) could be interpreted as a slight resurgence of humid conditions before the installation of an aeolian-type sedimentation in SU3-4-5-6-7, attesting to the installation of a dry environment likely corresponding to the Younger Dryas arid event (YD; ∼13 to 11.6 ka; [START_REF] Alley | The Younger Dryas cold interval as viewed from central Greenland[END_REF][START_REF] Garcin | Abrupt resumption of the African Monsoon at the Younger Dryas-Holocene climatic transition[END_REF]. Radiocarbon ages confirm this hypothesis, with two dates at 13572 ± 71 and 13471 ± 40 yr cal. BP from SU3 (Tab. VI.7). Aeolian-type sedimentation together with prehistoric occupation continues until SU7, which was dated around ∼11 ka cal. BP (Tab. VI.7). The Hara-Ide-3 sequence ends with SU9, a diatomite deposit (offshore and depth water column deposit) which was dated to ∼10 ka cal. BP. Accordingly, around ∼10.5 ka cal. BP, the Lake Abhe paleo-hydrological model (Chapter V) indicates that at the beginning of the first Early Holocene Lake Abhe-IV transgression (∼360 m a.s.l.), the Hara Ide Site would have been under water (∼350 m a.s.l.). Subsequently the lake attained its maximal level around ∼10 ka cal. BP (∼420 m a.s.l.; see Chapter V), producing an offshore lacustrine sedimentation attested here with the formation of the diatomite deposits (SU9).

VI.2.3.b Raso-02

The detailed pedosedimentary description of the Raso-02 site is presented in Section VI.1.4.b. The aim of this section is to present the micromorphological, sedimentological and physico-chemical data analyses of the Raso-02 site and describe in detail the sedimentary formation processes of this sequence (Fig. VI.4). Subsequently, the paleoenvironmental significance of these processes will be discussed in association with the fluctuating lake model presented in Chapter V and with regional hydro-climatic variations.

Physico-chemical results and interpretation

On the basis of elemental composition (pXRF) and physico-chemical (LOI and grain size laser diffraction) analyses on sediments, the sequence of Raso-02 can be devised into four main pedo-stratigraphic complexes. The lower part of the sequence (SU8 and SU7) is characterized by lower and constant number counts in Fe and Si elements (Fig. VI.14b,e), suggesting low amounts of fresh detrital inputs against high weathered mineralogical assemblages (see Section V.2 for geochemical interpretation of soil components). The combination between high amounts of K XRF counts (Fig. VI.14b) and a dominant clayey grain-size (Fig. VI.14d), indicate the presence of clay minerals from volcanic materials [START_REF] Croudace | Micro-XRF Studies of Sediment Cores, volume 17 of Developments in Paleoenvironmental Research[END_REF]. The LOI percentage of these deposits ranges between ∼6 % and ∼7 % (Fig. VI.14f), indicating an organic-type sedimentation in a probable decantation/clay neoformation environment.

From SU5d to SU5a an abrupt shift in sedimentation process patterns is evidenced by the increase of local weathered volcanic mineralogical material (Fig. VI.14b,e). The concomitant increase of Si, Fe and Ca signify a mixing of the mineralogical assemblage including material of volcanic origin with an additional carbonate source, and suggesting significant erosion and transport processes from the surrounding slopes. Accordingly, the drastic reduction of organic matter content and the increase of unsorted grain size indicate the installation of erosional processes. However, strong variations in grain-size (Fig. VI.14d) could reflect the variations of transport energy typical of stream flow activity, composed of separated fine and coarse grain-size beds. No physico-chemical analysis data were provided from SU4 and S3. However, field pedo-stratigraphical observations indicate the installation of a significant biogenic carbonate lacustrine activity (Fig. VI.14a). Finally, with SU1, we observe the return of the same elemental and physico-chemical data patterns described for the base of the sequence.

Micromorphological description and interpretation

With the aim to better understand the sedimentary context of the main Raso-02 prehistoric occupation layer (SU5a), a micromorphological block (Raso-02-Bloc1) was collected from the sequence covering SU6 to SU4 (yellow square, Fig. VI.14). The micromorphological description is presented and interpreted in detail in the text and is synthesized in Tab. VI.8.

At the bottom of the thin section, clay rich SU6 presents well-developed accommodated angular peds characterized by doubled porphyric distribution patterns (Fig. VI.15.g,i). These patterns, combined with fresh fish bones and bivalve shells indicate a palustrine-like sedimentation (Fig. VI.15g; see Section V.2 for the characterization of a palustrine deposit). Accommodated plane voids without micritic coatings or hypocoatings, and rare Fe-Mn coatings on channels indicate the quasi-absence of hydromorphic processes, suggesting the lack of post-depositional surface exposition in a dry environment, or the lack of cycles of waterlogging (Fig. VI.15g,i). However, pedorelicts of oriented clay could indicate the installation of clay vertic processes which normally follow a drying period (Fig. VI.15h). These are the general micromorphological characteristics commonly found in Holocene palustrine deposits around the Lake Abhe basin, which are described in Section V.2.

The sharp and undulated limit between SU6 and SU5d indicates the occurrence of an erosional event (Fig. VI.15.a). The general patterns of SU5, characterized by coarser non-sorted material composed of a fresh basaltic mineralogical assemblage, as well as by parallel distribution and orientation patterns through the current slope topography, support and confirm the hypothesis of the occurrence of an erosional and detrital event probably generated by water flow from a stream. PXRF elemental analyses are in accord with these interpretations. Indeed, SU5 is characterized by a progressive increase of Si (primary minerals) against K (attributed to illite clay minerals or potassium mica; Fig. VI.14.b), or by the increase of Fe (basaltic mineralogical signature; Fig. VI.14.e) and the drastic reduction of organic carbon which characterized the palustran levels SU6, and SU7 (Fig. VI.14.f). Together with eroded basalts, micritic/sparitic coarse sand/gravels were observed in SU5 (Fig. VI.15.d,f) in accordance with the progressive increase of PXRF signal counts of Ca (Fig. VI.14.c). This reworked carbonate assemblage likely originated from the erosion of ancient lacustrine deposits located on the headwaters of the Raso-02 stream, and probably corresponding to a previous lacustrine phase (Abhe-III phase defined by Gasse, 1977, 1975, anddated between ∼31 and ∼25 ka cal. BP). SU5c, characterized by coarser oriented mineral fraction, correspond to the height (maximum energy flow) of this detrital event (Tab. VI.8). SU5a and SU5b marks the re-apparition of fine fraction organized in sub-angular blocky peds, accompanied by finer coarse material (Fig. VI.15c). This SU likely corresponds to the shutdown of the stream flow and to the stabilisation of the topographic surface coinciding with the installation of prehistoric occupation. Presence of some dispatched artefacts in SU5c could be linked to the reworking of previous upstream occupations at the beginning of this hydrological event.

A crystallitic micritic impregnation localised between SU5a and SU4b could indicate the logging of

Chronostratigraphic correlation of the Raso-02 sequence and its paleoenvironmental significance

Thanks to sedimentological, micromorphological and geochemical analysis, several sedimentary formation processes were identified along the Raso-02 sequence and have been interpreted as local paleoenvironmental and paleolandscape evolution dynamics.

At the bottom of the sequence high organic matter content in a well sorted K-rich clayey material with the inclusion of typical lacustrine molluscs (ostrachods and bivalve) and fish bones indicate a palustrine sedimentary environment (SU9-8-7-6). Large detrital carbonate pebbles from SU8 could correspond to ancient offshore biogenic lacustrine material (diatomites) where the palustrine deposit developed following the retreat of lake waters. Accommodated plane voids in the angular blocky peds microstructure of SU6 (Fig. VI.15g,i) combined with the absence of prominent hydromorphic or bioturbation pedofeatures, indicate that this palustrine level had not undergone pedogenic cycles after desiccation, and that it was abruptly replaced by other water supplies. Considering the ages obtained from SU5a and SU5b upper levels, with a range between ∼10.9 and ∼10.5 ka cal. BP, we can advance the hypothesis that this underlying palustrine level belongs to the pre-Holocene humid phase, when humid conditions have been attested but without a very pronounced lacustrine transgression in the Lake Abhe basin (see Section V.2 for the chronological framework). This likely coincided with the beginning of the African Humid Period, which began in East Africa around ∼14 ka cal. BP [START_REF] Beck | Onset of the African Humid Period by 13.9 kyr BP at Kabua Gorge, Turkana Basin, Kenya[END_REF][START_REF] Costa | Isotopic reconstruction of the African Humid Period and Congo Air Boundary migration at Lake Tana, Ethiopia[END_REF]. Chronological information and sedimentogenetic interpretations allow us to propose a chronostratigraphic correlation between this lower part of the Raso-02 sequence with the sedimentary palustrine Phase D of the Hara Ide sequence (Section VI.2.3.a). The palustrine deposit of the Raso-02 sequence is interrupted by an erosive fluvial event (interface between SU6 and SU5d), which probably eroded a part of previous palustrine sediments. This erosive event is likely due to the reactivation of the Abhe basin hydro-system linked to rainfall increase during an arid-to-humid transition. Similar patterns that also attest to the resumption of humid conditions after the YD dry event have been observed in the Suguta valley [START_REF] Garcin | Short-lived increase in erosion during the African Humid Period: Evidence from the northern Kenya Rift[END_REF]. Runoff and erosional dynamics normally increase during a period of landscape stabilisation during a climate optimum. Radiocarbon ages from SU5 indicate that the erosional event occurred just before ∼11 ka cal. BP, at the end of the YD arid event, thus supporting the hypothesis of an arid-to-humid transition period. In summary, this fluvial erosional event seems linked to the climatic instability produced by the YD arid event over the Lake Abhe basin. The lack of deposits attributable to the YD event can be explained by the erosive processes enhanced by the increase in precipitations from ∼11 ka cal. BP, which produced a stratigraphic hiatus in the Raso-02 sequence. From this point of view, the prehistoric occupation localised at the top of the fluvial detrital products (SU5a and SU5b) occurred during a period of reduced water flow energy and of stabilisation of the stream (see micromorphological interpretation). This shutdown of fluvial dynamics is attributable to the rising of lake waters, attested by the following Sedimentary Units and by the Lake Abhe fluctuation model presented in Chapter V.

The lacustrine sedimentary phase of the Raso-02 sequence is represented by SU4, SU3 and SU2. Differences between these Sedimentary Units are evident in biogenic and sedimentological lacustrine products: SU4, characterized by great amounts of bivalve and ostracods in a silty-to-fine sandy matrix suggests a nearshore-to-offshore lacustrine deposits; differently, SU3, characterized by large amounts of oncolites in a medium sand matrix indicates more shallow-water/shore lacustrine environments. Finally, in SU2, oncolites embedded in clayey aggregates attest to the shut-down of the lacustrine shallow water shoreline context and to the return of palustrine conditions. SU1, presenting a clay polyhedral to granular aggregation, and great amounts of organic matter with a poor Ca content, indicate a paleosoil development reworked from the previous palustrine deposit at the end of the last transgression phase. This is attributed to the lake regression from phase Abhe-V dated around ∼4.6 ka cal. BP.

Considering the radiocarbon age from an oncolite sample from SU2, attesting to the permanence of shallow waters around 5.8 ka cal. BP, SU2 and SU3 can be attributed to the lower-level lacustrine Abhe-V phase (∼7.7 -∼4.6 ka cal. BP), although SU4 can be attributed to the Abhe-IV lacustrine phase that has a range between ∼10.6 and ∼8.5 ka cal. BP. Finally, prehistoric occupation of SU5a, SU5b occurred during a period of enhanced humid conditions preceded by an abrupt dry event (YD) and followed by the Lake Abhe Maximum Highstand Holocene Shoreline. Orange-reddish colours of bone fragments indicate the occurrence of combustion materials [START_REF] Canti | The Effects of Fire on Archaeological Soils and Sediments: Temperature and Colour Relationships[END_REF][START_REF] Schiegl | Paleolithic burnt bone horizons from the Swabian Jura: Distinguishing betweenin situ fireplaces and dumping areas[END_REF][START_REF] Stiner | Differential Burning, Recrystallization, and Fragmentation of Archaeological Bone[END_REF].

Chronostratigraphic correlation of the Lakora-03 sequence and its paleoenvironmental significance

The little sedimentary sequence of Lakora-03 site does not permit a precise chonostratigraphical interpretation. However, some environmental considerations and correlations can be advanced on the basis of comparison with the sedimentary formation processes and chronology of the Raso-02 site presented in this chapter (Section VI.2.3.b). Dated at ∼10.9 ka cal. BP (see Section VI.1.4.b for the chronological framework), SU5 of the Lakora-03 site was formed during at the same time as SU5b of the Raso-02 site (∼10.9 -∼10.5 ka cal. BP; (Section VI.2.3.b). In both cases these two SUs are the only anthropized SUs of the sites and are both characterized by a detrital clastic support composed of basaltic fragments and allochtonous carbonate gravels and sand. If the Raso-02 detrital event is characterized by a coarser and oriented material triggered by a stream flow transport, the Lakora-03 detrital event is more fine grained end slightly pedogenised. This difference can be explained by the different geomorphic and topographic patterns of these two sites. The Raso-02 site is located in a discharge zone of a marginal horst stream, and the Lakora-03 site is located in a slope context. The similarity in the sedimentogenetic patterns and in the chronology of both layers allows us to put forward the hypothesis that these are concomitant. The detrital event recognised in both sites is preceded by a clayey sedimentation inherited from ancient palustrine deposits, in which that of Lakora-03 is characterized by more pedogenetic patterns. At the same time, these two sequences terminate with lacustrine sedimentation and the growth of microbialitic formations (stromatolites and oncolites). Similarly to the Raso-02 site, the Lakora-03 prehistoric occupation occurred during a period of paleolandscape and environmental stabilisation, of enhanced precipitation, and just before Lake Abhe's water levels rise during the Abhe-IV lacustrine phase dated to ∼10.6 ka cal. BP (Chapter V).

VI.2.3.d Kurub-07

The field pedostratigraphic description of the Kurub-07 site is presented in Section VI.1.4.b and a complete stratigraphic representation is provided in Fig. VI.18. With the aim to better understand the sedimentary formation process patterns of the main prehistoric occupation layer of this site (SU15), we collected three micromorphological blocks covering a partial part of the site sequence from SU18 to SU 9/14 (samples, 1483, 1482, 1481, Fig. VI.18). The mechanical auger cores acquired from the excavation and surrounding areas (Kurub-07_ST3-K15 and Kurub-07_ST3-K19; Fig. VI.18) have permitted us to contextualise the sedimentary sequence of the Kurub-07 site and to test its extension. Combined with micromorphological investigation, grain size analyses are provided for each SU presented here (micromorphological description synthetized in Tab. VI.10).

SU18 micromorphological description and Interpretation

The single-spaced porphyric c/f related distribution pattern and a dominant angular blocky to vughy microstructure are the main characteristics of SU18. These patterns, combined with the presence of Melanoides tuberculata1 shells mixed in a clayey matrix can suggest a palustrine nature for this SU. In situ crushed shell in intrapedal accommodated planar porosity between prismatic angular blocky peds can be a sign of pedoturbation that occurred under a swelling clay process (Fig. VI.19a). At the same, in situ verticalised fragmented bone is interpreted as reorganized argilloturbation induced by mechanical stress (Fig. VI.19b). However, striated b-fabric is rarely observed and slightly pronounced, wihile micritic crystallitic b-fabric prevails (Fig. VI.19d).

As crystallitic pedofeatures, moderate amounts of random and parallel oriented gypsic euhedral crystals presenting clay impurities are interpreted as crystal intergrowths of pedogenic origin in SU18 (Fig. VI.19b,c,d). These intergrowths are a characteristic features of arid soils [START_REF] Stoops | Guidelines for Analysis and Description of Soil and Regolith Thin Sections[END_REF]. Zonations of gypsum intergrowths can develop in dry saline lake environments, following an oscillating groundwater table or waterlogging phenomena [START_REF] Pankova | Forms of gypsic neoformations as a controlling factor affecting the meliorative properties of gypsiferous soils[END_REF][START_REF] Stoops | Gypsum in arid soils, morphology and genesis[END_REF][START_REF] Watson | Structure, chemistry and origins of gypsum crusts in southern Tunisia and the central Namib Desert[END_REF]. In the same micro-unit, gyspum intergrowth crystals associated with large amounts of redoximorphic features such as dendritic and concentric orthic Fe-Mn nodules, indicate water saturation processes (Fig. VI.19d;[START_REF] Arshad | OCCURRENCE AND CHARACTERISTICS OF FERROMANGANIF-EROUS CONCRETIONS IN SOME SASKATCHEWAN SOILS[END_REF][START_REF] Richardson | Mottling and Iron Distribution in a Glossoboralf-Haplaquoll Hydrosequence on a Glacial Moraine in Northwestern Wisconsin[END_REF][START_REF] Schwertmann | Iron-manganese concretions in a hydrosequence of soil in loess in Bavaria[END_REF]. In accordance with this interpretation, high peaks of very fine silts and clays (2 and 5 µm; Fig.

VI.20a

) observed in the grain size profile reflect higher clay neoformation, and thus moderate to high water supplies in wet periods.

In this SU we observe a large amount of crystallitic pedofeatures such as micritic coatings, sparitic nodules (Fig. VI.21a) and an extended micritic crystallitic b-fabric. These pedofeatures are probably the result of the alternation of dry and wet periods or of a temporary water table perched in the lower horizons, but they are inactive today. Moderate bioturbation is observed as loose continuous infillings of coarse material inside channel porosity (Fig. the vughy porosity suggests that the bioturbation occurred during a more recent period than the other hydromorphic pedofeature formations.

The textural, crystallitic and amorphous hydromorphic pedofeatures described in this SU may indicate a seasonal climatic cycle of alternating wet and dry phases typical of a monsoon climatic regime and characterized by high rainfalls in summer and winter droughts. Fabric and groundmass elements observed in thin section indicate both a water logged-induced origin of this horizon and typical pedogenesis characteristics from an original palustrine deposit.

SU13-41 Micromorphological description and Interpretation

The transition from SU18 to SU41 is characterized by a coarse monic to enaulic c/f related distribution pattern and few textural pedofeatures such as relict crusts and illuviation features which have been displaced by recent bioturbation processes. The top of SU18 is thus characterized by an increase of fine fraction organised in aggregates, making a more enaulic to single grain prophyric c/f distribution pattern and an intergrain microaggregate microstructure. This groundmass variation reflects the gradual transition from a buried paleosoil originated from a palustrine deposit (SU18) to the installation of a paleodune formation (SU41).

SU15 Micromorphological description and interpretation

With a substantial increase of the fine fraction (clay) and well-developed sub-angular blocky peds, SU15 presents the characteristics of a pedogenic event. SU15 represents one of the main archaeological layers of the Kurub-07 site, characterized by in situ fragmented ceramics, remains of domesticates (cattle, sheep and goat) and worked obsidian (Section VI.1).

Illuviation clay features into the porosity network, crusts and pedorelicts are other additional textural pedofeatures observed in SU15 together with bioturbation features that occur especially in SU15.1 (Fig. VI.23i). SU15.1 shows the highest pedoturbation maturity, with a high amount of fragmented and deformed textural pedofeatures, and a complex peds segregation that may be related to intense water action in wetter seasons (Fig. VI.23h). Bioturbation features are common and are represented by coarse material with open distribution in coated vughs or channels (Fig. VI.23i). Crystalline pedofeatures are represented by micritic typic nodules, void coatings and hypocoatings. Pseudomorphic pedofeatures as dendritic and concentric Fe-Mn nodules are interpreted as pedogenic features associated with the reduction and oxidation of Fe and Mn compounds in the soil after water saturation and desaturation [START_REF] Rabenhorst | Quantifying Soil Hydromorphlogy[END_REF]Soil Science Society of America, 2001) and prevailing wet conditions in the past [START_REF] Kabakchiev | Comparative micromorphological investigation of cernozemsmonitzas and chernozems[END_REF]. A moderate amount of redoximorphic features indicates good water drainage of the horizon [START_REF] Arshad | OCCURRENCE AND CHARACTERISTICS OF FERROMANGANIF-EROUS CONCRETIONS IN SOME SASKATCHEWAN SOILS[END_REF][START_REF] Richardson | Mottling and Iron Distribution in a Glossoboralf-Haplaquoll Hydrosequence on a Glacial Moraine in Northwestern Wisconsin[END_REF][START_REF] Schwertmann | Iron-manganese concretions in a hydrosequence of soil in loess in Bavaria[END_REF]. SU15.2 represents a thin lens in this paleosoil horizon, with an increase of silty-sandy material which is interpreted as a non-pedogenic phase during the SU15 formation process. The presence of fragmented shells, suggest a reworking of previous pedogenic material, or the inclusion of allochthonous material of anthropic origin.

The surface of SU15 (SU15.3) is characterized by a thin non-layered cracked brown clayey crust, developed on an angular blocky peds microstructure (Fig. VI.23d,e). Soil aggregates are slightly vertically displaced and the intrapedal porosity is often filled by the silty material coming from the underlying SU. The good preservation of the pedological crust and the angular blocky microstructure, indicate that the installation of the following paleodune deposit (SU9.1) doesn't lead to a strong aeolian deflation of the archaeological occupation surface. This shallow microstructure and pedoturbation can be associated with a vertic process that rapidly affected the paleosoil after the occurrence of wetter conditions. With a sub-angular blocky peds microstructure and the apparition of striated b-fabric (Fig. VI.23e), dynamic swelling clays are suggested [START_REF] Embrechts | Microscopic identification and quantitative determination of microstructure and potentially mobile clay in a soil catena in a humid tropical environment[END_REF][START_REF] Fedoroff | Les vertisols du sud est de la Beauce[END_REF]. This process leads to a planar zone reorientation of clay particles following a face-to-face alignment [START_REF] Wilding | Genesis of Vertisols: Shrink-Swell Phenomena[END_REF]. These planar clay-reorientation zones can correspond to a mechanical process typical of vertic materials [START_REF] Mccormack | Proposed origin of lattisepic fabric[END_REF]Mermut et al., 1996b,a). Vertic features observed from the top of SU15 can be due to the alternation of moist regimes between water-logging and completely dry conditions [START_REF] Kovda | Vertic Features[END_REF].

Micritic hypocoatings are concentred in the vughs porosity of SU15.3, indicating an evapotranspiration during a dry season resulting from rapid precipitation of calcium carbonate due to root metabolism [START_REF] Wieder | Micromorphological fabrics and developmental stages of carbonate nodular forms related to soil characteristics[END_REF]. Micritic crystallitic b-fabric associated with micrite/sparite coatings can reflect a rapid shift to arid conditions preceded by a wetter phase [START_REF] Khormali | Micromorphology of calcitic features in highly calcareous soils of Fars Province, Southern Iran[END_REF]. Vertic textural features, micritic/sparitic secondary precipitations and redoxymorphic features indicate significant differences in availability of water supplies, and alternation between wet and arid phases [START_REF] Khormali | Micromorphology of calcitic features in highly calcareous soils of Fars Province, Southern Iran[END_REF].

The grain-size analysis profile of SU15 shows two peaks at 2 and 5 µm which may be related to moderate clay neoformation processes (Fig. VI.20c). The peak around 60 µm may represent the inherited coarse mineral fraction of aeolian origin integrated into the pedologenic formation process.

SU9 micromorphological description and interpretation

The SU9 paleodune formation is characterised by a very well sorted coarse fraction presenting subrounded to subangular sharp and smooth surfaces. With a coarse monic (SU9.2) and coarse monic/close fine enaulic (SU9.1) c/f related distribution pattern, SU9 is characterised by a single grain to intergrain microaggregate microstructure. Few bioturbation features are observed as there are coatings composed of fine-grained excreta forming the walls of channels which are filled by coarse material from the fabric (Fig. VI.23b). Few crystallitic features are represented by localised micritic impregnation precipitation probably caused by conditions favouring rapid carbonate precipitation such as frequent desiccation or CO 2 loss from the soil solution (Fig. VI.23c;[START_REF] Sobecki | Formation of Calcic and Argillic Horizons in Selected Soils of the Texas Coast Prairie[END_REF]. Grain size analysis shows VI.2.4 Synthesis and discussion of sedimentary formation processes and environmental context of human settlements in the Late Pleistocene -Holocene Lake Abhe basin

Detailed study of the sedimentary sequences which was first presented in Section VI.1, permitted us to refine our understanding of the processes of sedimentary accumulation in association with local-toregional hydrological and environmental variations. The soil micromorphology analysis tool combined with other complementary methods have permitted us to take a first look on the climate-induced processes which drive archaeological sequence formation. In this section we'll discuss the results of this study following the chronology of the major environmental events: the pre-Holocene humid phase (> 13 ka cal. BP), the Younger Dryas arid event (∼13 -∼11.7 ka cal. BP), the Lake Abhe transgression Phase-IV (∼11.1 -∼8.2 ka cal. BP), the 8.2 ka arid event (∼8.2 -∼7.7 ka cal. BP), the Lake Abhe transgression Phase-V (∼7.7 -∼4.5 ka cal. BP) and the Late Holocene aridification (< 4.5 ka cal. BP).

VI.2.4.a The pre-Holocene humid phase (> 13 ka cal. BP)

Across the Lake Abhe basin the period between the end of the Abhe-III (∼25 ka cal. BP) and the beginning of the Abhe-IV lacustrine phases (∼11 ka cal. BP) was not documented [START_REF] Gasse | Evolution of Lake Abhe[END_REF][START_REF] Gasse | L'évolution des lacs de l'Afar Central (Ethiopie et T.F.A.I.) du Plio-Pléistocène à l'Actuel. Recostitution des paléomilieux lacustres à partir de l'étude des Diatomées[END_REF] representing one of the research questions considered in this manuscript (Section II.3.4.a). Thanks to the study of some sedimentary sequences and their respective significance, we can put forward some hypotheses about the origin of the pre-Holocene sedimentary accumulation across the Lake Abhe basin.

Indeed, the lower deposits of the Hara-Ide-3, Hara-Ide-1, Raso-02 and Lakora-03 sequences, dated > ∼13.4 and > ∼11 ka cal. BP, generally present the same pedo-sedimentary characteristics (Fig. VI.24). These deposits present a fine clayey and organic-rich sedimentation, in some cases indicating often pedogenised palustrine events. These characteristics indicate the occurrence of humid conditions during a period of relative low lake-level (equal or < 367 m a.s.l.). Effectively, in East Africa, a progressive increase of rainfall started ∼14 ka cal. BP [START_REF] Jaeschke | Holocene Hydroclimate Variability and Vegetation Response in the Ethiopian Highlands (Lake Dendi)[END_REF][START_REF] Costa | Isotopic reconstruction of the African Humid Period and Congo Air Boundary migration at Lake Tana, Ethiopia[END_REF]. However, to explain the scarcity of depositional contexts, the geomorphic and environmental patterns of the Awash River headwaters region shall be taken into account.

During this pre-Holocene period, even the Ziway-Shala Lakes remain low (see Section V.1.5.b, Fig.

V.8), not allowing water supplies into the Awash catchment (as documented during the Early and Middle Holocene periods). Contrary to this, it seems that during the pre-Holocene lake filling processes, some of the Awash River headwaters flowed into the adjacent Ziway-Shala basin [START_REF] Sagri | Latest Pleistocene and Holocene river network evolution in the Ethiopian Lakes Region[END_REF], reducing the consistent flow of freshwater into Lake Abhe. Other reasons for low sediment and water inputs in the EARS basins can be attributed to glacial and post-glacial water and sediment retention processes. Indeed, the local East African Last Glacial Maximum has been dated to ∼15 ka BP [START_REF] Ossendorf | Middle Stone Age foragers resided in high elevations of the glaciated Bale Mountains, Ethiopia[END_REF]. Moreover, the beginning of the deglaciation of the Ethiopian Highland mountains has a range between ∼14 and ∼13 ka cal. BP [START_REF] Mohammed | A late Glacialrlate Holocene pollen record from a highland peat at Tamsaa, Bale Mountains, south Ethiopia[END_REF], which probably strongly influenced the distal sedimentation transfer processes to the lowlands.

During this period, no human occupation phase was attested in the sequences documented in the Lake Abhe basin, probably due to the shallow-water/palustrine characteristics of these formations, unsuitable for prehistoric settlement, or due to erosional processes.

VI.2.4.b

The Younger Dryas arid event and the beginning of the Early Holocene Lake Abhe transgression (∼13.4 -∼11.1/∼10.6 ka cal. BP

The second period of human settlements in the Lake Abhe basin is dated with a few radiocarbon ages with ranges between ∼13.4 -∼11.1/∼10.6 ka cal. BP.

The beginning of this period is characterised by strong fluvial erosional processes recognised in the Hara-Ide-3 and Hara-Ide-1 sequences (Fig. VI.24). Along the Hara-Ide sequences this erosive event is likely due to the reactivation of the hydro-system of the Abhe basin probably because of a rapid drop of the lake level which induced incision and erosion of the watershed deposits. Accordingly, this event is followed at Hara-Ide-3 by an aeolian-like sedimentation/remobilisation of sandy-silty material starting around ∼13.4 ka cal. BP and corresponding to the installation of the Younger Dryas North Atlantic arid event (YD [START_REF] Alley | The Younger Dryas cold interval as viewed from central Greenland[END_REF]. In East Africa, some continuous lacustrine sequences and lake level reconstructions have attested to this dry event ∼13.6 ka [START_REF] Beck | Onset of the African Humid Period by 13.9 kyr BP at Kabua Gorge, Turkana Basin, Kenya[END_REF][START_REF] Bittner | Revisiting afro-alpine Lake Garba Guracha in the Bale Mountains of Ethiopia: Rationale, chronology, geochemistry, and paleoenvironmental implications[END_REF][START_REF] Costa | Isotopic reconstruction of the African Humid Period and Congo Air Boundary migration at Lake Tana, Ethiopia[END_REF][START_REF] Fischer | Determining the Pace and Magnitude of Lake Level Changes in Southern Ethiopia Over the Last 20,000 Years Using Lake Balance Modeling and SEBAL[END_REF][START_REF] Foerster | Climatic change recorded in the sediments of the Chew Bahir basin, southern Ethiopia, during the last 45,000 years[END_REF][START_REF] Garcin | East African mid-Holocene wet-dry transition recorded in palaeo-shorelines of Lake Turkana, northern Kenya Rift[END_REF][START_REF] Loakes | Late Quaternary climate change in the north-eastern highlands of Ethiopia: A high resolution 15,600 year diatom and pigment record from Lake Hayk[END_REF][START_REF] Marshall | Climatic change in northern Ethiopia during the past 17,000 years: A diatom and stable isotope record from Lake Ashenge[END_REF][START_REF] Roberts | Timing of the Younger Dryas event in East Africa from lake-level changes[END_REF]. During the Younger Dryas, a human occupation is attested at the Hara-Ide-3 site (∼13.5 to ∼11 ka cal. BP; Tab. VI.7) along the lake Abhe basin, with the occurrence of some burials and stone tool materials.

Subsequently, a fluvial aggradation process took place time as evidenced in the Raso-02 sequence, which has been slightly pedogenised in its upper levels (in correlation with the Lakora-03 site; Fig. VI.24). This phase likely began around 11.1 ka cal. BP, when the Lake Abhe water level started to rise (Fig. VI.24). This fluvial event was likely due to the reactivation of the hydro-system of the Abhe basin probably due to rainfall increase during an arid to humid transition, as has been attested for the resumption of humid conditions after the YD dry event in the Suguta valley (northern Kenyan Rift; [START_REF] Garcin | Short-lived increase in erosion during the African Humid Period: Evidence from the northern Kenya Rift[END_REF]. Runoff and erosional dynamics normally increase during a period of landscape stabilisation towards a climate optimum. Radiocarbon ages indicate that a fluvial erosional event occurred just before ∼11.1 ka cal. BP, at the end of the YD arid event, thus supporting the evidence of an arid to humid transition. The possible lack of deposits at the Raso-02 site is explained by the erosive process enhanced by the increase in precipitations from ∼11.1 ka cal. BP attributable to the end of the YD dry event, which produced a stratigraphic hiatus in the sequence. Subsequently, fluvial aggradation was recorded. The prehistoric occupation localised at the top of the slightly pedogenised fluvial detrital products (SU5a, 5b) occurred during a period of reduced water flow energy and of stabilisation of the stream (∼10.9 -∼10.6 ka cal. BP). This shutdown of fluvial dynamics is attributable to the rising of lake waters, attested by the sedimentological evidence in the following period.

In summary, the ∼13.4 -∼11.1/∼10.6 ka cal. BP period is marked by strong environmental instability in the Lake Abhe basin produced by the effects of the onset and termination of short-term arid conditions linked to the North Atlantic YD event.

VI.2.4.c The Lake Abhe transgression Phase-IV (∼11.1 -∼8.2 ka cal. BP)

Early Holocene humid conditions in association with the enhanced precipitations of the AHP are well attested in lacustrine sediments and from reconstitutions of lake level fluctuations across the Lake Abhe basin [START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF]Gasse and Street, 1978, Fig. VI.24). This prominent lacustrine phase is observed in some sedimentary sequences associated with archaeological sites investigated in this Section, and evident in shallow waters or offshore deposits (Fig. VI.24). The Lake Abhe transgression Phase-IV begins around ∼11.1 ka cal. BP (Fig. VI.24) until ∼8.2 ka cal. BP, and is composed of three sub-phases: the first, when the lake level reached ∼420 m a.s.l. (∼10.2 -∼9.8 ka cal. BP), the second characterized by a lowering in the lake level (∼400 m a.s.l.; ∼9.8 -∼8.7 ka cal. BP) and the third (∼385 m a.s.l.; ∼8.7 -∼8.2 ka cal. BP). As yet explained in Section VI.1.5.a, the absence of archaeological data corresponding VII.1 Insights on East African hydro-climatic dynamics during the Holocene VII.1.1 Comparison of Nile and Lake Abhe basins with some climatic records in Africa

In this Section, we first compare and discuss the main results obtained from the Nile and the Awash/Lake Abhe basins. Secondly, these results are compared and discussed with some climatic records from East Africa, mentioned in Fig. VII.1 and VII.3. The aim of this section is to discuss on the timing dynamics and magnitude of the AHP, the role of the CAB and the ISM, and the low and high latitude connections.

In the publication [START_REF] Mologni | Frequency of exceptional Nile flood events as an indicator of Holocene hydro-climatic changes in the Ethiopian Highlands[END_REF] presented in Section IV, we demonstrated that the timing of high energy flood events originated at the Nile River headwaters is in accordance with previous sediment records collected from the active Rosetta branch of the NDSF for the last 11 ka. Moreover, the frequency and intensity curve of Nile flood events (based on turbidite layers; Fig. VII.1e), coupled with the Nd isotopic composition (measured both on hemipelagic and hyperpycnitic deposits) allow us to demonstrate that for the last 11 ka this flood sediment is mainly originated from the erosion of Ethiopian Traps (Fig. VII.1c,d). In the study of the Lake Abhe basin (Mologni et al., under review; presented in Section V.1), we reconstructed the hydrological fluctuations of Holocene Lake Abhe and distinguished the climatic forcing factors responsible for changes in lake levels from the tectonic and geomorphological ones. The flood-induced characteristics from the Nile delta and their correlation with Lake Abhe, Lake Tana, Lake Dendi and the continental White and Blue Nile terrigenous input signals allow us to demonstrate a coherent hydro-climate evolution at the scale of East Africa from 14 ka BP to present.

VII.1.1.a Stable high monsoon intensity from 10.5 to 9 ka controlled by insolation

High Nd values from 10.5 to 9 ka cal. BP from two Nile delta sediment cores (Fig. VII.1c,d) indicate a stable and strong Blue Nile discharge, suggesting a high monsoonal precipitation regime over the Ethiopian Highlands. It is during this period that the majority of high energy Nile flood events were recorded (Fig. VII.1e). The time of the flood-induced event-layer sequence of the MD04-2726 core, the highest terrigenous inputs in the Lake Tana (Fig. VII.1g) and the decrease of δD wax in Lake Dendi [START_REF] Jaeschke | Holocene Hydroclimate Variability and Vegetation Response in the Ethiopian Highlands (Lake Dendi)[END_REF][START_REF] Wagner | Holocene rainfall runoff in the central Ethiopian highlands and evolution of the River Nile drainage system as revealed from a sediment record from Lake Dendi[END_REF] are in phase with the maximum insolation values at 15°N around 9 ka BP (Fig. VII.1a,b) confirming the orbital control of the Early Holocene hydrological conditions at the sources of the Blue Nile. Accordingly, studies on the lower Blue and White Nile alluvial deposits [START_REF] Williams | Late Pleistocene and Holocene environments in the Nile basin[END_REF][START_REF] Williams | Late Quaternary floods and droughts in the Nile valley, Sudan: New evidence from optically stimulated luminescence and AMS radiocarbon dating[END_REF][START_REF] Williams | Late Quaternary environments and prehistoric occupation in the lower White Nile valley, central Sudan[END_REF] indicate the occurrence of several flood events between 9.7 and 9 ka cal. BP (Fig. VII.1h). These observations are confirmed by the Nile headwaters record of Ti and K events at Lake Dendi [START_REF] Wagner | Holocene rainfall runoff in the central Ethiopian highlands and evolution of the River Nile drainage system as revealed from a sediment record from Lake Dendi[END_REF] Similarly, at Lake Dendi, between 9 and 8.8 ka cal. BP, a strong reduction of Ti and K activity and a small increase in δD wax were recorded [START_REF] Jaeschke | Holocene Hydroclimate Variability and Vegetation Response in the Ethiopian Highlands (Lake Dendi)[END_REF]Wagner et al., 2018, Fig. VII.1a, f). A stepwise aridification from 9 ka cal. BP was also evidenced in the Chew Bahir basin (Foerster et al., at Lake Dendi [START_REF] Jaeschke | Holocene Hydroclimate Variability and Vegetation Response in the Ethiopian Highlands (Lake Dendi)[END_REF]; c) Nd of T1 event-layers (brown line and squares) and hemipelagic sediment (blue line and points) of the MD04-2726 core; d) Nd of hemipelagic sediment (blue line and dots) from core MS27PT (fraction <63 µm) (fraction <63 µm, after [START_REF] Ménot | Timing and stepwise transitions of the African Humid Period from geochemical proxies in the Nile deep-sea fan sediments[END_REF] this study) e) Q90/Q50 and frequency (100yr running average, brown line) of exceptional flood deposits (T1α in yellow and T1β in green) of the MD04-2726 core; f) erosion runoff-induced K and Ti event layers from Lake Dendi [START_REF] Wagner | Holocene rainfall runoff in the central Ethiopian highlands and evolution of the River Nile drainage system as revealed from a sediment record from Lake Dendi[END_REF]; g) Ti (mg/g) at Lake Tana [START_REF] Marshall | Late Pleistocene and Holocene drought events at Lake Tana, the source of the Blue Nile[END_REF]; h) Blue (in blue) and White (in yellow) lower Nile course flood periods recorded over the Khartoum region [START_REF] Williams | Late Pleistocene and Holocene environments in the Nile basin[END_REF][START_REF] Williams | Late Quaternary Environments in the Nile Basin[END_REF][START_REF] Williams | Late Quaternary environments and prehistoric occupation in the lower White Nile valley, central Sudan[END_REF]; i) Curve of Abhe Lake -level fluctuations (black line) with the dated Facies-A (blue triangles), Facies-B (inverted blue triangles) and undetermined facies (empty blue triangle) stromatolites, the new (black squares), published (black dots) dated nearshore deposits (Gasse, 1975, revised), and the dated paleosoils from palustrine (black cross), slope (red squares) and intra-dune (green squares) origins.

2015; Figs. II.8, VII.3f) from K (potassium) XRF counts interpreted as a marker of lower terrigenous inputs into the lake. Other recent studies have attested a reduction in hydrologic regimes as early as ∼8.7 ka cal. BP interpreted as a consequence of changes in monsoonal dynamics over the Ethiopian Highlands [START_REF] Blanchet | Asynchronous Changes in Vegetation, Runoff and Erosion in the Nile River Watershed during the Holocene[END_REF][START_REF] Ménot | Timing and stepwise transitions of the African Humid Period from geochemical proxies in the Nile deep-sea fan sediments[END_REF].

Some climatic records from offshore Somalian [START_REF] Jung | Stepwise Holocene aridification in NE Africa deduced from dust-borne radiogenic isotope records[END_REF] and from the southern Arabian Peninsula [START_REF] Cremaschi | Evidence for an abrupt climatic change at 870014C yr B.P. in rockshelters and caves of Gebel Qara (Dhofar-Oman): Palaeoenvironmental implications[END_REF][START_REF] Fleitmann | Holocene ITCZ and Indian monsoon dynamics recorded in stalagmites from Oman and Yemen (Socotra)[END_REF] show a change in hydrological patterns between ∼9 and ∼8.7 ka cal BP. Southward, and in Central Africa, the beginning of this aridification is documented around 6 ka BP (e.g. δD wax ; [START_REF] Tierney | Northern Hemisphere Controls on Tropical Southeast African Climate During the Past 60,000 Years[END_REF] from Lake Tanganyika, and from lakelevel and vegetation history of Lake Chad [START_REF] Amaral | Palynological evidence for gradual vegetation and climate changes during the African Humid Period termination at 13°N from a Mega-Lake Chad sedimentary sequence[END_REF][START_REF] Armitage | West African monsoon dynamics inferred from abrupt fluctuations of Lake Mega-Chad[END_REF]. The multiple synchronicity of this Early Holocene hydro-climatic shift between East Africa and Southern Arabia and the delay with regards to western precipitation records strongly suggest the influence of Indian Summer Monsoon on the rainfall distribution and intensity over the Ethiopian Highlands.

This stepwise aridification documented as early as ∼9 -∼8.7 ka BP brings into question the role of the tropics on the causes of the 8.2 ka North Atlantic cooling event. Supported by literature, in Chapter IV we discussed how the obliquity-driven change in insolation gradient at low latitudes might have directly impacted monsoonal rainfall at the tropics (Bosmans et al., 2015b,a;[START_REF] Mantsis | The Response of Large-Scale Circulation to Obliquity-Induced Changes in Meridional Heating Gradients[END_REF]. Indeed, the correlation from different archives presented here suggest that an early shift of the main climatic boundaries above the Ethiopian Highlands, in link with the gradual decrease in summer insolation, might have been sufficient to initiate a drier period in the tropics between 9 and 8 ka BP. Furthermore, climate modelling of [START_REF] Clement | An Orbitally Driven Tropical Source for Abrupt Climate Change[END_REF] and [START_REF] Davies | The impact of Sahara desertification on Arctic cooling during the Holocene[END_REF] indicate how high latitude cooling events would be enhanced by the decrease in African precipitation and by Saharan aridification, evidencing the importance of tropical ocean-atmospheric energy balance on North Atlantic temperature variations. While in the Lake Abhe basin the 8.2 ka event is evident with a drop in lake level waters and as dune deposit accumulations (from ∼8.4 ka cal. BP; see Section VI.2), along the MD04-2726 core the 8.2 ka produced low Nd radiogenic values and rare and low energy floods. Interestingly, the resurgence of humid conditions has been recorded homogeneously in both basins since ∼7.7 ka cal. BP (Fig. VII.1).

The second prominent arid shift recorded during the Holocene coincides with the end of the AHP, dated around ∼4.5 ka cal. BP in both the Nile and Awash basins (Fig. VII.1). Comparison of our data with lake level reconstructions and δD wax from five Rift lakes located between 3°S and 12.5°N (Chapter V), permitted us to discuss the spatio-temporal patterns of the Middle-Holocene aridification process in East Africa. We highlighted how, along the EARS, Middle Holocene aridification spread northward, in contrast to the assumed southward migration of the rain belt scenario [START_REF] Shanahan | The time-transgressive termination of the African Humid Period[END_REF]. Some authors suggested how the incursion of the Congo Air Boundary (CAB) in the specific morpho-tectonic context of the EARS water-sources would have contributed additional moisture sup-plies, in contrast to the Mid-Holocene southern migration of the ITCZ [START_REF] Junginger | The effects of solar irradiation changes on the migration of the Congo Air Boundary and water levels of paleo-Lake Suguta, Northern Kenya Rift, during the African Humid Period (15-5ka BP)[END_REF][START_REF] Tierney | Late Quaternary behavior of the East African monsoon and the importance of the Congo Air Boundary[END_REF]. This hypothesis is supported by modelling studies which show how increased moisture patterns over the EARS might be the result of the high pressure gradient between the CAB and the ISM air masses, which enhanced the influence of the WAM over Northeast Africa [START_REF] Camberlin | Rainfall Anomalies in the Source Region of the Nile and Their Connection with the Indian Summer Monsoon[END_REF][START_REF] Marzin | The role of the ocean feedback on Asian and African monsoon variations at 6kyr and 9.5kyr BP[END_REF][START_REF] Junginger | The effects of solar irradiation changes on the migration of the Congo Air Boundary and water levels of paleo-Lake Suguta, Northern Kenya Rift, during the African Humid Period (15-5ka BP)[END_REF]. Finally, these particularly conditions of moisture patterns would have supported the persistence of large waterbodies and their related ecosystems in the northern EARS lowlands until ∼4.5 ka cal. BP, in contrast to earlier aridification recorded in Central Africa.

VII.1.2 Limitations of the study

These Ph.D results have the potential to give an overview of hydrological changes in East Africa from the comparison of two large basins, interpreted as a change in monsoonal dynamics over the Ethiopian Highlands. However, many limitations need to be addressed, especially concerning those due to the types of investigated records (deltaic/lacustrine sediment, stromatolite, paleosoils etc..) and to the combined methods employed to respond to our research issues.

VII.1.2.a Limited Nile flood-induced events record

The first flaw of this study concerns the limited chronological extension of the flood-induced eventlayers sequence (i.e. turbidites) of the MD04-2726 core, which provides a partial record of the hydrological changes along the Nile basin. Indeed, the first flood events were recorded around ∼10.3 ka BP and the last one around ∼7 ka BP. Before this period some Nile floods certainly occurred in phase with the increase of precipitations linked to the beginning of the AHP. However, during this time, anoxic conditions were not yet established on the flat levee were the core was retrieved along the submarine Rosetta Canyon, causing bioturbation and the disruption of these turbidite deposits. After ∼7 ka BP the lack of flood-induced layers is due to the shutdown of the Nile energy flow which kept it from generating enough high-density waterfront to pierce the water column stratification and to produce a hyperpycnal current. Nonetheless, the XRF core scanner and the Nd geochemical data have been applied to the totality of the core section analyses, allowing a more extended (the whole Holocene) interpretation of the variability of terrigenous inputs from the Blue Nile River.

VII.1.2.b The reservoir effect on carbonates and on organic matter

The second limit of the MD04-2726 core study is due to the reliability of the age models. Because of the absence of foraminifera in the hyperpycnite sequence, almost exclusively the organic matter was dated. The calibration of the reservoir effect on organic matter was made based on the comparison of two ages at ∼9.7 ka BP on both foraminifera, shells and organic matter. Consequently, the calibration was applied to the totality of the ages on organic matter. However, I am aware that this reservoir effect likely varied through the time, making our age model uncertain by a few hundred years.

Concerning the results from the Lake Abhe basin, in this study we have taken into consideration some anomalies caused by the controls of the tectonic and geomorphic surface processes, making our paleo-lake fluctuation model as reliable as possible. However, some flaws in the control of the paleoshoreline age need to be mentioned. This concerns the age reservoir effect on the dated stromatolites. Indeed, the almost absence of a reservoir effect on lacustrine carbonates was defined on the basis of a Melanoides tuberculata ostracod shell and not on a stromatolite. Because the paleoshoreline age control is not only based on stromatolites but also on lacustrine shells samples, we consider this age model reliable, but needing to be refined. However, the Abhe paleo-Lake fluctuation reconstruction is quite robust because of the complementarity of several ages obtained on littoral paleosoils and fluvial sedimentary accumulations which fit well with the Holocene lake highstands and lowstands. From this point of view, the pedological, faunal and archaeological features documented in ancient lacustrine littoral environments, have represented a strong complementary tool to better define the spatial-temporal patterning of lake hydrological fluctuations.

VII.2 The impact of the Holocene monsoonal regimes on East African geomorphic systems

This Section summarizes and discusses the results and interpretations of Chapters IV, V and VI and concerns the importance of geomorphological investigations in paleoclimatic studies. Here, we use information archived in the sedimentary formations to better understand the spatial-temporal dynamics of climate change. The aim is to better understand the functioning of geomorphic systems in order to ameliorate paleoclimatic interpretation.

Firstly (Section VII.2.1), in the MD04-2726 core study (Chapter IV; [START_REF] Mologni | Frequency of exceptional Nile flood events as an indicator of Holocene hydro-climatic changes in the Ethiopian Highlands[END_REF], we highlighted how arid events occurring for several hundred years might produce a non-linear sedimentary delivery decoupled from hydroclimate change. Additionally, thanks to comparison with geomorphological studies along the White and Blue Nile branches of the Nile system, we put the accent on geomorphological processes which are not taken into consideration enough in paleoclimatic reconstructions of continuous offshore cores.

Secondly, in Section VII.2.2, we synthetize the results and the interpretations provided in Chapters V and VI of this manuscript, concerning the deciphering of the evolution of landscapes and of littoral morphosedimentary processes around the Lake Abhe basin from the Late Pleistocene to the Holocene, and their link to wider hydro-climatic changes. Our data permits us to illustrate the tectonic and gemorphological patterns which have influenced the hydro-sedimentary balance during this period.

VII.2.1 Allogenic and autogenic controls on Nile River discharge: implications of climatic (mis)interpretation of continuous offshore records

In [START_REF] Mologni | Frequency of exceptional Nile flood events as an indicator of Holocene hydro-climatic changes in the Ethiopian Highlands[END_REF] we used a laminated sequence from the NDFS for track at centennial resolution the hydro-climatic changes occurred in the Ethiopian Highlands during the Early to the Middle Holocene. Similarly, several studies have based their paleoclimatic reconstructions from NDSF continuous sequences: based on the sedimentary provenance sources from Nd/Sr isotopic compositions [START_REF] Bastian | Abrupt response of chemical weathering to Late Quaternary hydroclimate changes in northeast Africa[END_REF][START_REF] Blanchet | Runoff and precipitation dynamics in the Blue and White Nile catchments during the mid-Holocene: A data-model comparison[END_REF][START_REF] Blanchet | High-and low-latitude forcing of the Nile River regime during the Holocene inferred from laminated sediments of the Nile deep-sea fan[END_REF][START_REF] Box | Response of the Nile and its catchment to millennial-scale climatic change since the LGM from Sr isotopes and major elements of East Mediterranean sediments[END_REF][START_REF] Ducassou | Evolution of the Nile deep-sea turbidite system during the Late Quaternary: Influence of climate change on fan sedimentation[END_REF][START_REF] Krom | Nile River sediment fluctuations over the past 7000 yr and their key role in sapropel development[END_REF][START_REF] Ménot | Timing and stepwise transitions of the African Humid Period from geochemical proxies in the Nile deep-sea fan sediments[END_REF][START_REF] Revel | 20,000 years of Nile River dynamics and environmental changes in the Nile catchment area as inferred from Nile upper continental slope sediments[END_REF], on precipitation rate estimation from organic chemistry [START_REF] Castañeda | Hydroclimate variability in the Nile River Basin during the past 28,000 years[END_REF] and on terrigenous input estimation based on major elemental distribution [START_REF] Hennekam | Integral view of Holocene precipitation and vegetation changes in the Nile catchment area as inferred from its delta sediments[END_REF][START_REF] Revel | 100,000 Years of African monsoon variability recorded in sediments of the Nile margin[END_REF][START_REF] Zhao | Variations of the Nile suspended discharges during the last 1.75Myr. Palaeogeography[END_REF]. Some studies advanced the hypothesis of the role of vegetation spread in the Nile catchments as a modulation factor between erosion and sediment transfer processes and as a non-linear response of the environment in river discharges [START_REF] Blanchet | Asynchronous Changes in Vegetation, Runoff and Erosion in the Nile River Watershed during the Holocene[END_REF][START_REF] Foucault | Late Quaternary palaeoclimatic oscillations in East Africa recorded by heavy minerals in the Nile delta[END_REF][START_REF] Krom | Nile River sediment fluctuations over the past 7000 yr and their key role in sapropel development[END_REF][START_REF] Lézine | Timing of the southward retreat of the ITCZ at the end of the Holocene Humid Period in Southern Arabia: Data-model comparison[END_REF]. In addition, a few studies [START_REF] Fielding | A detrital record of the Nile River and its catchment[END_REF] consider the Sudd marches in South Sudan (Fig. VII.3) as a filter for White Nile particles which have induced a retention of terrigenous inputs from the Equatorial Nile, making the Ethiopian traps source signal (mineralogical or isotopic) a proxy of climate-induced hydro-sedimentary variations over the Ethiopian Highlands [START_REF] Blanchet | Runoff and precipitation dynamics in the Blue and White Nile catchments during the mid-Holocene: A data-model comparison[END_REF][START_REF] Revel | 20,000 years of Nile River dynamics and environmental changes in the Nile catchment area as inferred from Nile upper continental slope sediments[END_REF].

Overall, these studies generally consider the Nile hydro-sedimentary system as an exceptional sourceto-sink recorder, characterized by a direct hydro-sedimentary response to hydroclimatic change and by a linear solid discharge transfer process to the basin-floor sinks. This assumption has permitted some authors to build paleoclimatic inferences from the NDSF deposits. However, source-to-sink concepts have to take into consideration all continental factors of routing and storage of sediments through the fluvial system caused by geomorphological processes which are normally neglected in paleoclimate research in the Nile basin. In (Mologni et al., 2020, Chapter IV) we demonstrated the occurrence of an anomalous sedimentary record caused by a non-linear response of the fluvial system to climate forcing. Combining our interpretations with other studies on the continental Nile, we point out some examples of surface processes which could have generated autogenic avulsions in sedimentary signal transfer from source-to-sink along the Nile fluvial system during the Holocene, and thus which can generate a misinterpretation of sedimentary records from the NDSF. Following the 8.2 ka BP arid event, characterized by low precipitation in the highlands (Fig. VII.2b), reduced inputs from the Blue Nile (Fig. VII.2c,d) and flood events (Fig. VII.2e), and a higher flood frequency period between ∼7.8 and ∼7.6 ka cal. BP were recorded. During this time, leaf wax deuterium isotopes at Lake Tana still indicate lower precipitation in accord with low terrigenous inputs from Ethiopian Highlands (Fig. VII.2b, c, d;[START_REF] Costa | Isotopic reconstruction of the African Humid Period and Congo Air Boundary migration at Lake Tana, Ethiopia[END_REF]. Disproportionate higher flood frequency and higher particle discharge during a lower rainfall period shows a decoupling between erosive activity and climatic forcing, suggesting the development of an "out of equilibrium" geomorphic system (Fig. VII.2; i.e., the sedimentary input into the system is not equal to the system output, [START_REF] Ahnert | Equilibrium, scale and inheritance in geomorphology[END_REF][START_REF] Hoffmann | Sediment residence time and connectivity in non-equilibrium and transient geomorphic systems[END_REF]. We explained this anomaly by a process that occurred during the entire 8.2-7.8 ka BP arid period whereby low precipitation favoured fluvial riverbed aggradation (i.e., storage effect; [START_REF] Phillips | Sources of nonlinearity and complexity in geomorphic systems[END_REF]. The subsequent slight resumption of precipitation post-7.8 ka BP (reduced δD wax at Lake Tana, Fig. Lower radiogenic Nd values of these stored and eroded deposits suggest a reduced Ethiopian Traps input in favour of Equatorial Nile supplies (Central African Craton). From a climatic point of view, this evidence can be interpreted as enhanced precipitations over the Lake Victoria (4°S -3°N) basin due to the progressive ITCZ southward migration, with respect to northern Blue Nile sources (9°-15°N; Fig. VII.3). However, because of the possible strong moisture influences of the ISM over the Ethiopian Highlands (see Section VII.1), this shift of Nile solid load sources can be better explained by the geomorphic functioning of the Sudd marshes, located in the lower White Nile valley (Fig. VII.3). [START_REF] Williams | Late Pleistocene and Holocene environments in the Nile basin[END_REF] and [START_REF] Williams | Late Quaternary environments and prehistoric occupation in the lower White Nile valley, central Sudan[END_REF][START_REF] Williams | Late Quaternary floods and droughts in the Nile valley, Sudan: New evidence from optically stimulated luminescence and AMS radiocarbon dating[END_REF] evidenced a strong White Nile fluvial activity during this period. Consistent with our interpretations, the author explained how during low river flow periods the Sudd swamps dried out, taking a few centuries to re-establish during the subsequent humid phase. Only during this period (few centuries), the absence of the filtering swamps effect would have triggered a strong solid load discharge, making the White Nile hydro-system excessively reactive to moonsonal precipitations than during humid periods [START_REF] Williams | The Nile Basin: Quaternary Geology, Geomorphology and Prehistoric Environments[END_REF]. This effect induced an enhanced delivery of coarser sediment with a lower radiogenic Nd signature in the NDSF deposits. Evidence of abrupt disproportionate White Nile solid discharge delivery after a dry period has also been pointed out at the beginning of the AHP, following the LGM and the HE1 aridity [START_REF] Williams | Abrupt return of the summer monsoon 15,000 years ago: New supporting evidence from the lower White Nile valley and Lake Albert[END_REF]. Similarly, [START_REF] Garcin | Abrupt resumption of the African Monsoon at the Younger Dryas-Holocene climatic transition[END_REF] provide a record of a high magnetite-rich clastic input from the Lake Masoko area just after the Younger Dryas dry event. In summary, our study highlights how at the end of abrupt arid climatic events, such as those occurring around 8.2-7.8 ka BP, an "out of equilibrium" fluvial effect may have occurred in the Nile River headwaters. This non-linear functioning of the hydro-system can produce a disproportionate sediment delivery to the continental margins resulting in an autogenic sedimentation process partially decoupled from climatic forcing.

Even during Holocene humid periods some continental geomorphic parameters were able to influence the interpretation of Nile sediment origins. Nd and Sr isotopes are usually used to identify the source provenance of the Blue (Volcanic Traps) and the White (Central African Craton) Nile sediments [START_REF] Blanchet | High-and low-latitude forcing of the Nile River regime during the Holocene inferred from laminated sediments of the Nile deep-sea fan[END_REF]. However, a precise diachronic shift between these sources can be misinterpreted because of some geomorphic processes. For example, during the AHP, the Lake Turkana overflowed at 11.5 -7.8 and at 7.4 -4.3 ka cal. BP, bringing isotopic sediment signature of volcanic Traps into the Lower White Nile catchment through the Sobat river [START_REF] Butzer | The Holocene Lake Plain of North Rudolph, East Africa[END_REF][START_REF] Butzer | Radiocarbon Dating of East African Lake Levels[END_REF][START_REF] Garcin | East African mid-Holocene wet-dry transition recorded in palaeo-shorelines of Lake Turkana, northern Kenya Rift[END_REF][START_REF] Johnson | Lake Turkana and Its Link to the Nile[END_REF][START_REF] Owen | Palaeolimnology and archaeology of Holocene deposits north-east of Lake Turkana, Kenya[END_REF]. During periods when the ITCZ was in a northern position, increase in metasedimentary isotopic signature (against the volcaninc one) can also have origins from the Neoproterozoic basement sediment inputs from the Red Sea hills, from the remobilised [START_REF] Ménot | Timing and stepwise transitions of the African Humid Period from geochemical proxies in the Nile deep-sea fan sediments[END_REF][START_REF] Revel | 20,000 years of Nile River dynamics and environmental changes in the Nile catchment area as inferred from Nile upper continental slope sediments[END_REF][START_REF] Mologni | Frequency of exceptional Nile flood events as an indicator of Holocene hydro-climatic changes in the Ethiopian Highlands[END_REF][START_REF] Blanchet | High-and low-latitude forcing of the Nile River regime during the Holocene inferred from laminated sediments of the Nile deep-sea fan[END_REF]; b) Lake Abhe (this study, Section V.1; and [START_REF] Gasse | Evolution of Lake Abhe[END_REF]; c) Lake Tana [START_REF] Costa | Isotopic reconstruction of the African Humid Period and Congo Air Boundary migration at Lake Tana, Ethiopia[END_REF][START_REF] Marshall | Late Pleistocene and Holocene drought events at Lake Tana, the source of the Blue Nile[END_REF]; d) Lake Dendi [START_REF] Jaeschke | Holocene Hydroclimate Variability and Vegetation Response in the Ethiopian Highlands (Lake Dendi)[END_REF][START_REF] Wagner | Holocene rainfall runoff in the central Ethiopian highlands and evolution of the River Nile drainage system as revealed from a sediment record from Lake Dendi[END_REF]; e) Ziway-Shala Lakes [START_REF] Gillespie | Postglacial arid episodes in Ethiopia have implications for climate prediction[END_REF]; f) Chew Bahir [START_REF] Foerster | Late Quaternary Climate Variability in the Source Region of Homo Sapiens. Dry-Wet Cycles in Chew Bahir, Southern Ethiopia[END_REF]; g) Lake Turkana [START_REF] Garcin | East African mid-Holocene wet-dry transition recorded in palaeo-shorelines of Lake Turkana, northern Kenya Rift[END_REF]; h) Lake Challa [START_REF] Tierney | Late Quaternary behavior of the East African monsoon and the importance of the Congo Air Boundary[END_REF]; Lake Victoria (Berke et al., 2012a); l) Lake Tanganyika (Tierney et al., 2011a).

VII.2.2 Climatic, tectonic and geomorphological controls of Lake Abhe basin environments during the Holocene

VII.2.2.a Impact of the AHP hydro-climatic variations on Lake Abhe littoral lacustrine environments and morphosedimentary systems

As demonstrated in Section V.1 and resumed in Section VII.1, the Holocene East African monsoonal intensity variations left its fingerprints on wave-dominated clastic paleoshorelines all along the borders of the Lake Abhe basin. Well-developed paleoshoreline sequences and littoral morphosedimentary environments have permitted us to reconstruct paleo-hydrological fluctuations of the Awash basin over the last ∼13 ka and to compare them with the regional paleoclimatic record. In Section V.2 we demonstrated that the last lake transgressions strongly influenced soils formation. In Sections V.2 and VI.2 we presented a preliminary look at the sedimentary formation processes which have characterized the evolution of the Lake Abhe basin environments under climatic forcing.

From ∼13.4 to ∼11.1 ka cal. BP the Abhe Lake basin experienced hydrological and landscape instability (Fig. VII.4,grey band). Despite the rarity of data, we argue that a relative low lake level oscillated by ± 30 m, with a probable major drop in level around ∼12.5 ka cal. BP. Because the lake had not yet attained its overflow level, a possible cause of these oscillations are the changes in water inputs from the Awash River. Concerning the local sedimentation patterns, at less two reactivation and shutdown episodes of the hydro-system have been recorded in erosive and aggradation fluvial processes around the lake, likely linked to the drop of the lake level at ∼12.5 ka cal. BP and to the strong water inflow inputs around ∼11.1 ka cal. BP. In between, aeolian processes took place. These changes in morphosedimentary patterns could be attributable to the impact of the Younger Dryas NH cooling event [START_REF] Alley | The Younger Dryas cold interval as viewed from central Greenland[END_REF] on the Lake Abhe basin environments. Accordingly, unstable landscape patterns linked to the YD have also been recorded in East Africa in the Northern Kenyan Rift and at Lake Masoko [START_REF] Garcin | Short-lived increase in erosion during the African Humid Period: Evidence from the northern Kenya Rift[END_REF][START_REF] Garcin | Abrupt resumption of the African Monsoon at the Younger Dryas-Holocene climatic transition[END_REF]. However, along the Lake Abhe basin, improved studies are needed to confirm and better understand the environmental evolution patterns for this period.

The enhanced moonsonal activity at ∼10.5 ka cal. BP in the Ethiopian Highlands was observed in the rapid filling of the Tendaho-Gobaad graben. During this period the lake attained its Maximum Holocene Highstand Shoreline level, overflowing into the Hanle basin (Fig. VII.4). A drop in lake level at 8.7 ka cal. BP is concomitant with the formation of palustrine deposits in the Lower Awash plain. This lake regression culminated around ∼8 ka cal. BP, when a short (∼300 yrs) arid event induced aeolian sediment transfer evident in the accumulation of dune deposits and pedogenesis of the previous palustrine deposits. The timing of this aridification is in phase with a reduction of the Blue Nile flow, linked to the 8.2 ka NH cooling event [START_REF] Matero | The 8.2 ka cooling event caused by Laurentide ice saddle collapse[END_REF], see Section VII.1 for a climatic interpretation). Resurgence of humid conditions since 7.7 ka cal. BP are observed from lake level rises and from the development of pedogenic horizons over the slopes of the graben boarders. At the end of this Middle Holocene lacustrine phase (Phase Abhe-V) the installation of dune-paleosoil sequences (since ∼4 ka cal. BP) all around the basin characterized the Lake Abhe environments indicating an alternation of weak wet periods and dry events in a lightly vegetated environments.

VII.2.2.b Evaluating the tectonic and geomorphologic control of the Holocene Lake Abhe

Thanks to ages measured on littoral lacustrine carbonate formations (stromatolites and microbialites), we were able to estimate the Holocene subsidence rate in the Tendaho-Gobaad basin (Section V.1.5.a). In light of these results, the low subsidence rates cannot support the hypothesis of a tectonic-driven lacustrine transgression between the Early and the Middle Holocene, since a subsidence rate of the basin that is larger than the 'evaporation rate' (actual) cannot be envisaged. Contrarily, thanks to comparison with the hydrological evolution of the Ziway-Shala basin, we demonstrated how the interconnectivity between normally disconnected basins (Zyway-Shala and Awash basins) during the Holocene strongly influenced the hydrological balance of Lake Abhe. Two periods of supplementary water input through the overflowing of the Ziway-Shala megalake into the Awash catchment were identified: between ∼11 and ∼10 ka BP and between ∼6 and ∼5.6 ka BP. The first overflowing coincides with the rapid transgression of Lake Abhe during the Holocene. This additional water supply would have accelerated the rising of lake waters at the beginning of the Holocene. The second period of overflow impacted the water balance of Lake Abhe with an increase of the water-body by ∼15 to ∼20 m. Despite highlighting the role of these tectonic and geomorphic processes on the control of the Lake Abhe hydrological balance, some questions are still open or need to be studied in more detail. For example, a more precise chronology of the functioning of the Lake Abhe overflow sills and their erosion rates during the Holocene as well as the role of the solid and liquid discharge of the Logiya River in the hydrological balance of the lake. In summary, we can argue that the hydrological and sedimentary budget of the Lake Abhe basin results from a complex interaction between climatic, tectonic and geomorphic controls which would have influenced the local environmental morpho-sedimentogenetic littoral patterns.

VII.2.3 Limitations of the study

Thanks to the combined paleo-lake level reconstruction and sedimentary formation processes we were able to propose a preliminary scenario of littoral lacustrine paleolandscape and morpho-sedimentary evolution around the fluctuating Lake Abhe during the last ∼13 ka. However, we are aware that these results represent only a limited window into the complex morphosedimentary variability which characterised the Lake Abhe basin during the Holocene. Indeed, the sequential stratigraphy correlation between all of these sequences is fragmentary showing several chronological and spatial gaps. This incompleteness is due to different reasons, including fieldwork operation constraints, the lack of pedo-sedimentary deposits due to erosional processes on the graben horsts and to the methodological approaches used during fieldwork.

The exploratory nature of the subject of this dissertation, difficulty or impossibility of access to certain regions because of lack of infrastructure, geopolitical and tribal issues, and limited time and resources during the VAPOR-Afar and PSPCA fieldwork projects, meant that the maximum amount of geomorphological observations and soundings were carried out with respect to these factors. However, a complete spatio-temporal geomorphological context of this basin is still missing. As demonstrated in this study, all documented pedosedimentary sequences and archaeological sites are mainly localised in the Lake Abhe plain or on the adjacent slopes of the horsts which surround the basin. The lack of sedimentary sequences from more elevated areas which were formed during the paleo-lake highstands is due to two issues. The first is the high erosional rates which affected these areas and which effaced the deposit accumulations. The second is linked to the difficulty of access to these elevated areas due to logistical, geopolitical and tribal issues.

VII.3

The role of hydro-climatic and environmental changes on human settlements during the last 11 ka in the Horn of Africa

Thanks to the sedimentological, geomorphological and geoarchaeological results presented in this manuscript we are able to reconstruct and to compare the Holocene hydrological variations (mainly linked to the East African monsoon regimes) and the prehistoric peopling dynamics along the Northeastern EARS.

More particularly, the specific study of sedimentary formation processes of archaeological sites permitted us to specify the environmental context of human occupations across the Lake Abhe basin in relation to the main hydrological shifts that took place. In this section we compare human cultural innovations which occurred in East Africa during the Holocene, and try to decipher their relationship with the local paleoenvironment and regional hydroclimate. This comparison concerns the evaluation of the climatic and environmental context of hunter-fishing-gathering practices against that of the first food production societies, which are principally characterized by the introduction of herding and pottery production.

Across the Turkana basin several paleoclimatic and paleoenvironmental reconstructions [START_REF] Beck | Onset of the African Humid Period by 13.9 kyr BP at Kabua Gorge, Turkana Basin, Kenya[END_REF][START_REF] Berke | A mid-Holocene thermal maximum at the end of the African Humid Period[END_REF][START_REF] Bloszies | Water level history for Lake Turkana, Kenya in the past 15,000years and a variable transition from the African Humid Period to Holocene aridity[END_REF][START_REF] Forman | Variations in water level for Lake Turkana in the past 8500 years near Mt. Porr, Kenya and the transition from the African Humid Period to Holocene aridity[END_REF][START_REF] Garcin | East African mid-Holocene wet-dry transition recorded in palaeo-shorelines of Lake Turkana, northern Kenya Rift[END_REF] combined with solid chrono-cultural sequences [START_REF] Barthelme | Fisher-Hunters and Neolithic Pastoralists in East Turkana, Kenya[END_REF][START_REF] Beyin | New radiocarbon dates for terminal Pleistocene and early Holocene settlements in West Turkana, northern Kenya[END_REF]Hildebrand et al., 2018b,a;[START_REF] Nash | Obsidian types from Holocene sites around Lake Turkana, and other localities in northern Kenya[END_REF][START_REF] Wright | Lakeside View: Sociocultural Responses to Changing Water Levels of Lake Turkana, Kenya[END_REF] and with multidisciplinary studies [START_REF] Foerster | Environmental change and human occupation of southern Ethiopia and northern Kenya during the last 20,000 years[END_REF][START_REF] Owen | Palaeolimnology and archaeology of Holocene deposits north-east of Lake Turkana, Kenya[END_REF] make this area ideal for evaluating human-climate co-evolution during the Holocene in the EARS. Thus, to discuss the role of hydroclimatic changes on prehistoric settlements along the Rift, our results and interpretations will be compared with those of the Lake Turkana basin (Figs. VII.5e, VII.3g).

At the beginning of the Holocene, the humans faced enhanced humid conditions in East Africa, with the predominance of fishing practices and the employment of technologies and tools adapted to these needs (Later Stone Age). Indeed, these communities, once called 'aqualitic' [START_REF] Sutton | The Aquatic Civilization of Middle Africa[END_REF], are characterized by the production and use of bone harpoons over a territory that extended from the Sahel to the Kenyan Rift. At Lake Turkana, these societies are also characterized by a developed impressed pottery since 9 ka BP (Fig. VII.5e;[START_REF] Wright | Lakeside View: Sociocultural Responses to Changing Water Levels of Lake Turkana, Kenya[END_REF], indicating the beginning of storage economies in the Rift. Contrary to the Lake Turkana basin, no harpoons and/or pottery traditions were recovered for this period in the Lake Abhe or Ziway-Shala basins [START_REF] Khalidi | Late prehistoric oasitic niches along the southern Red Sea (Yemen and Horn of Africa). In From Refugia to Oases: Living in Arid Environments from Prehistoric Times to the Present Day[END_REF][START_REF] Khalidi | 9000 years of human lakeside adaptation in the Ethiopian Afar: Fisherforagers and the first pastoralists in the Lake Abhe basin during the African Humid Period[END_REF][START_REF] Ménard | Late Stone Age variability in the Main Ethiopian Rift: New data from the Bulbula River, Ziway-Shala basin[END_REF], suggesting a strong cultural divergence between the Kenyan and the Ethiopian Rift (Fig. VII.5). The high techno-cultural variability that characterized LSA populations was probably due to specific adaptations to the numerous lacustrine basin environments along the Rift. Our geoarcheological results suggest that the Lake Abhe settlements between ∼11 and ∼10.5 ka BP were the expression of highly reactive human mobility (few centuries) to fluctuating hydrology after the occurrence of the YD. These sites are characterized by large amounts of burned fish bones, indicating that populations were temporarily settled near the lake or the stream edges and that intensive fishing constituted a major activity and food source. These populations were adapted to centennial Lake Abhe landscape modifications. Additionally, technological similarities with the Ziway-Shala basin (blades and bladelets, few shaped tools and the absence of backed pieces) suggest some connection along this Rift portion (Fig. From ∼9 -∼8.7 ka BP, the beginning of a gradual decrease in precipitation is recorded over the Ethiopian Highlands, culminating with the ∼8.2 -∼7.8 arid episode. In the Lake Abhe basin, a lake regression took place instigating the formation of a temporary semi-arid environment. Populations followed retreating lake waters, displacing their fishing activities from the basaltic outcrops of the graben footwalls to the Awash River plain swamps. The strong environmental reactivity, evident in the dune deposit formations along the Lake Abhe plain after ∼8.4 ka BP, is concomitant with the reactivity of human mobility and practices. Interestingly, the installation of an anthropic circular structure in a new semi-arid environment characterizes this short cultural period in the Lake Abhe basin.

After a few centuries, the resurgence of enhanced monsoonal precipitations induced another less pronounced lacustrine transgression (since ∼7.7 ka BP). A strong increased exploitation of wild aquatic fauna may suggest new technical innovations in hunting and fishing, probably linked to new environmental patterns in the Lake Abhe basin. Indeed, during the previous lacustrine phase (∼10.6 -∼8.2 ka BP), the settlements were located on the basaltic outcrops surrounding the basin in association with deep lake-waters and aquatic fauna (mainly fish). However, during this new lacustrine transgression, humans were obligated to exploit larger swamps and shallow water areas caused by lower lake highstands, which represent favorable environments for wild aquatic fauna. Accordingly, use-wear study on a select number of curved backed pieces show diagnostic impact fractures that suggest their use as composite projectile points (barbs and axial points likely set into wood or bone shafts; [START_REF] Khalidi | 9000 years of human lakeside adaptation in the Ethiopian Afar: Fisherforagers and the first pastoralists in the Lake Abhe basin during the African Humid Period[END_REF] used as arrows, spears or harpoons for hunting.

Herding appeared first in the north-eastern Sahara around ∼8 ka BP [START_REF] Linseele | The earliest phase of introduction of Southwest Asian domesticated animals into Africa. New evidence from the Fayum Oasis in Egypt and its implications[END_REF]. Subsequently, the livestock economy spread to the south, through the Sahel and reached the hunter-gatherer populations of the Highlands of the Horn of Africa [START_REF] Lesur | The advent of herding in the Horn of Africa: New data from Ethiopia, Djibouti and Somaliland[END_REF][START_REF] Marshall | Cattle Before Crops: The Beginnings of Food Production in Africa[END_REF]. The first evidence of herding in East Africa is attested at ∼4.7 ka BP at Lake Turkana [START_REF] Barthelme | Fisher-Hunters and Neolithic Pastoralists in East Turkana, Kenya[END_REF][START_REF] Chritz | Climate, ecology, and the spread of herding in eastern Africa[END_REF]Wright et al., 2015, Fig. VII.5). Southwards, along the Kenyan Rift, the first evidence of pastoralism appears around ∼4 ka BP [START_REF] Ambrose | Chronology of the Later Stone Age and food production in East Africa[END_REF][START_REF] Prendergast | Kansyore fisher-foragers and transitions to food production in East Africa: The view from Wadh Lang'o, Nyanza Province, Western Kenya[END_REF]. In the Ethiopian Rift, information about livestock introduction is fragmentary and highly dispatched through time. However, along the Lake Abhe basin where herding is attested ∼4.7 -∼4.5 ka cal BP, herding does not represent the only technological change for this period. The appearance of pottery for storage, built structures (ovens), bipolar percussion on anvil on small obsidian nodules and a lesser degree of bladelet production indicate significant technical-cultural change. Interestingly, the emergence of herding in East Africa is concomitant with the regressions of the Mid-Holocene Rift Lakes which are linked to the aridification process at the end of the AHP. Some authors have suggested that this regional aridification would have played a key role in the spread of pastoralism [START_REF] Lesur | The advent of herding in the Horn of Africa: New data from Ethiopia, Djibouti and Somaliland[END_REF]. In the eastern Sahara and Sahel, the end of the AHP heavily impacted environments because of the absence of perennial water supplies highly amplified by the occurrence of the 4.2 ka arid event [START_REF] Bini | The 4.2\,ka\,BP Event in the Mediterranean region: An overview[END_REF][START_REF] Weiss | Megadrought and Collapse. From Early Agriculture to Angkor[END_REF][START_REF] Weiss | Global megadrought, societal collapse and resilience at 4.2-3.9 ka BP across the Mediterranean and west Asia[END_REF]. Additionally, several studies aim to demonstrate how this aridification urged pastoral societies to migrate in some humid environmental niches around the Nile River [START_REF] Kuper | Climate-controlled Holocene occupation in the Sahara: Motor of Africa's evolution[END_REF]. Over the East African highlands and along the Rift, a similar pattern of human mobility occurred. Indeed, the Middle-Holocene aridification would have urged pastoralists to move from the desiccating lowland basins to lakes that were still supplied by strong water inputs. Data from the central Afar region, along the Lake Abhe basin, suggest a first occurrence of cattle around ∼4.7 -∼4.5 ka BP (Fig. VII.5), which became common around ∼3.7 ka BP [START_REF] Coudert | New archaeozoological results from Asa Koma (Djibouti): Contributing to the understanding of faunal exploitation during the 3rd millennium BC in the Horn of Africa[END_REF][START_REF] Khalidi | 9000 years of human lakeside adaptation in the Ethiopian Afar: Fisherforagers and the first pastoralists in the Lake Abhe basin during the African Humid Period[END_REF][START_REF] Lesur | The advent of herding in the Horn of Africa: New data from Ethiopia, Djibouti and Somaliland[END_REF]. In this study we showed how at the end of the AHP, the Northern Rift was subjected to belated humid conditions with respect to some Southern Rift basins. The Lake Abhe water-level started to decrease since ∼4.7 ka BP and the first evidence of herding and ceramic traditions appeared around ∼4.7 -∼4.5 ka BP. We put forward the hypothesis that Mid-Holocene persistent water supplies in the Lake Abhe basin favoured the delay of the adoption of herding, accompanied by the persistence of fishing practices until the Late Holocene. In summary, in this manuscript we showed that the extent or retreat of the Abhe Lake basin and fluctuating environmental patterns influenced local human mobility and prehistoric economic behaviour.

However, we also demonstrated that along the EARS some technological choices did not follow local climatic patterns in a linear way, but that they were influenced by preferential ways of knowledge transfer between human groups, modulated by complex cultural assimilations. Finally, we advanced (±) at Lake Tana [START_REF] Costa | Isotopic reconstruction of the African Humid Period and Congo Air Boundary migration at Lake Tana, Ethiopia[END_REF]; c) Lake Abhe water level fluctuations with the respective elevations of dated archaeological sites (coloured dots) and cultural and economic innovations; d) Lake Ziway-Shala water level fluctuations with the respective cultural and economic innovations; e) Lake Turkana water level fluctuations [START_REF] Garcin | East African mid-Holocene wet-dry transition recorded in palaeo-shorelines of Lake Turkana, northern Kenya Rift[END_REF] with the respective cultural and economic innovations. For symbol captions please see the illustration on the right of the figure .   the hypothesis that the emergence of herding and food production follow the process of aridification of the different lacustrine basins of the central and northern East African Rift System. These occurrences were variable because of the differential distribution of monsoonal rainfalls along the Rift due to the interplay between the ITCZ and CAB and because of specific local hydrological reactivity and control patterns. Of course, this technological, economic and cultural switch would have been modulated by the difficulties that hunter-gatherers encountered when attempting to integrate food production into their social and economic systems. It is for this reason that fishing and hunting practices persisted until the Late Holocene. All these aspects suggest strong adaptation and reactivity of prehistoric populations to climate change during the Holocene in the Horn of Africa.

Human-climate co-evolution is thus decipherable through an understanding of the complex interaction between the external/internal climatic forcing on moist air mass displacement over the continent, the local-to-regional reactivity of specific hydro systems, and of the knowledge transfer and the process of cultural transition/assimilation dynamics.

VII.4 Problematics, perspectives and future work overtures

In this section, we propose to develop some points that came up as a result of the research carried out in this thesis work.

VII.4.1 Isotopic characterization of Lake Abhe stromatolites: a paleoclimatic perspective

A total of 48 stromatolite samples were collected during this thesis fieldwork around the Lake Abhe basin, of which half were dated with the 14 C AMS technique. A microscopic study is ongoing with the aim to clarify the first results presented in Section V.1 and to characterize the micro-structure of these microbial formations. The next step will be to focus on the geochemical characterisation of the different stromatolite generations spanning the Early to Middle Holocene.

Quantitative major elemental geochemistry and isotopic analyses on continental stromatolites represent good tools to track past hydroclimatic fluctuations. Stable carbon (δ 13 C) and oxygen (δ 18 O) isotopes will permit clarification of the characteristics of the surrounding waters. The first will give a measure of the carbon sources of the Total Inorganic Dissolved Carbon (TIDC) and the second will provide an account of the water residence time in the lake (H 2 O-CaCO 3 isotopic fractionation; Hillaire-Marcel and [START_REF] Casanova | Stromatolites et hauts niveaux lacustres pléistocènes du bassin Natron-Magadi (Tanzanie-Kenya). Pleistocene stromatolites and high lake levels of the Natron-Magadi basin (Tanzania-Kenya)[END_REF]. Summarily, high δ 13 C and δ 18 O values indicate a good equilibrium between the atmospheric CO 2 and the TIDC assured by long time residence of the lake waters [START_REF] Casanova | Biosedimentology of Quaternary stromatolites in intertropical Africa[END_REF].

However, the Lake Abhe stromatolites also represent an exceptional receptacle of the terrigenous particles delivered from the Awash River or from local volcanic activity. Subsequently other isotopic proxies (such as lithium) may be applied on the allochthonous mineral fraction trapped between the stromatolite laminations or in the carbonate phase. At the same time, these markers will permit us to estimate the shifts in mineral sources and the changes in chemical weathering of the silicate rocks in the Awash basin during paleo-lake highstands. This prospective work would refine our comprehension of the AHP hydro-climatic changes along the Awash River catchment, and thus in the Ethiopian Highlands.

VII.4.2 Research on fluvial-lacustrine dynamics in the Lower Awash valley

In this study, and in continuity with the previous work of [START_REF] Gasse | L'évolution des lacs de l'Afar Central (Ethiopie et T.F.A.I.) du Plio-Pléistocène à l'Actuel. Recostitution des paléomilieux lacustres à partir de l'étude des Diatomées[END_REF], we provided a complementary look at Holocene littoral landscape evolution around Lake Abhe from the sedimentogenetic study of several pedo-sedimentary sequences. The understanding of the evolution of the Lower Awash plain is thus necessary to refine understanding of paleolandscape evolution and to better clarify the morphogenetic characteristics of prehistoric inhabited environments during the last humid-arid transition around Lake Abhe. A systematic coring of the paleo-deltas of the Awash River is suggested. A detailed study combining the analysis of DEMs, electric tomography profiles (acquired data) and paleo-lake extension reconstructions, will permit us to locate coring transects with the aim to provide a complete sequential Holocene chrono-stratigraphy. This approach will not only permit us to clarify geomorphological and paleolandscape evolution of the Lower Awash valley during the Holocene, but will also provide a sedimentary record for analyses of paleoclimatic proxies. Finally, considering that the Holocene filling of the Lake Abhe basin is the result of a complex interaction between regional hydrological transfers and tectonic activity, we consider this work perspective to be a necessary support for interpreting future paleoclimatic studies based on lacustrine deposits. VII.4.3 Coupling numerical modelling of the tectonic and hydro-sedimentary balance during the evolution of the Holocene Tendaho-Gobaad graben

In this work we reconstruct the variations of the Lake Abhe water-level during the Holocene, which represents a good exploitable dataset for hydro-climatic modelling. However, we pointed out how the lake hydro-balance could be modulated by graben tectonic subsidence and extension or by temporary surface process dynamics. Reflections about climatic, tectonic and geomorphic interactions brings up some new questions about the functioning of the Awash and Lake Abhe hydro-systems which must be taken into consideration for future paleoclimatic works. Some studies focused on basins located along the EARS have modelled the hydro-balance of the Holocene lakes on the basis of past lake levels reconstructions with the aim to determine the evolution of paleo-precipitation along the respective basin catchments [START_REF] Bergner | Paleoprecipitation estimates for the Lake Naivasha basin (Kenya) during the last 175 k.y. using a lake-balance model[END_REF][START_REF] Dühnforth | Early Holocene water budget of the Nakuru-Elmenteita basin, Central Kenya Rift[END_REF][START_REF] Fischer | Determining the Pace and Magnitude of Lake Level Changes in Southern Ethiopia Over the Last 20,000 Years Using Lake Balance Modeling and SEBAL[END_REF][START_REF] Junginger | The effects of solar irradiation changes on the migration of the Congo Air Boundary and water levels of paleo-Lake Suguta, Northern Kenya Rift, during the African Humid Period (15-5ka BP)[END_REF][START_REF] Junginger | Hydrological constraints of paleo-Lake Suguta in the Northern Kenya Rift during the African Humid Period (15-5kaBP)[END_REF]. However, they did not take into consideration any tectonic or surfaceprocess anomaly parameters in the lake models, which could distort the final modelled results. From this point of view, thanks to the data provided in this manuscript, the Tendaho-Gobaad graben may represent an experimental laboratory for testing effects of tectonic and hydro-sedimentary interplay on the Holocene evolution of the Lake Abhe basin, with the possibility to provide more precise hydrobalance and paleo-precipitation reconstruction data. Firstly, long-term rifting geodynamic modelling of the Tendaho-Gobaad graben can be carried out using the classic pTatin3D tool [START_REF] May | pTatin3D: High-Performance Methods for Long-Term Lithospheric Dynamics[END_REF] or UW-Geodynamics (Beucher et al., 2019b,a). Secondly, I propose coupling tectonic and solid and liquid surface process transfers using the Badlands LEM tools [START_REF] Salles | Badlands: A parallel basin and landscape dynamics model[END_REF][START_REF] Salles | pyBadlands: A framework to simulate sediment transport, landscape dynamics and basin stratigraphic evolution through space and time[END_REF][START_REF] Salles | Badlands: An opensource, flexible and parallel framework to study landscape dynamics[END_REF]. Finally, I propose integrating the output values on lake-balance models (http://mres.unipotsdam.de/index.php/2017/07/16/lake-balance-modeling-using-matlab/).

This experiment could respond for the first time to some crucial questions about the functioning of a rift basin system under different forcing factors. For example: How much did tectonic subsidence influence the filling lake processes? Did the liquid and solid mass inputs and accumulation accelerate basin subsidence during the AHP? How much did the biogenic productivity influence sedimentary accumulation during humid periods? And finally, could the Lake Abhe sedimentary sequence represent a good record to properly track hydro-climatic variations in East Africa? VII.4.4 Back to the sources: the control of glaciated highlands on the hydro-sedimentary budget of the Rift and the PRISM project

In this manuscript we demonstrated that several studies attested to a high East-African hydro-system variability under climatic forcing during the last 20 thousand years. However, the careful analysis of chrono-stratigraphic datasets from the EARS raise some crucial questions that are still not resolved. Despite the fact that precipitations started to increase at the beginning of the AHP around ∼14 ka BP, most of the EARS lakes started to expand only after ∼11 ka BP. Similarly, most of the NDSF deposits have recorded a disproportionate sedimentation rate only after ∼11 ka BP, during the maximum insolation at tropical latitudes. Additionally, in the northern EARS a lack of sedimentation in lowland lakes or a hiatus in continental archives and in archaeological chrono-sequences has been recorded between ∼25 and ∼13 ka BP. This effect could be exclusively linked to climatic control. However, all these evidences even suggest the installation of a non-linear reactivity or a threshold effect of the East-African hydro-systems to the increase of precipitations. Considering that the East African local LGM is dated between ∼15 and ∼14 ka BP (Hendrickx et al., 2015a;[START_REF] Hurni | Late Quaternary of Simen and other mountains in Ethiopia[END_REF][START_REF] Ossendorf | Middle Stone Age foragers resided in high elevations of the glaciated Bale Mountains, Ethiopia[END_REF][START_REF] Williams | Fossil Periglacial Deposits in the Semien Highlands, Ethiopia[END_REF] and that the activation of high altitude drainage systems is attested only after ∼13 ka BP [START_REF] Mohammed | A late Glacialrlate Holocene pollen record from a highland peat at Tamsaa, Bale Mountains, south Ethiopia[END_REF][START_REF] Tiercelin | High-resolution sedimentary record of the last deglaciation from a highaltitude lake in Ethiopia[END_REF], the role of glacial and periglacial sedimentary and water retention processes is not negligible. Some altitude lakes have been studied [START_REF] Bittner | Revisiting afro-alpine Lake Garba Guracha in the Bale Mountains of Ethiopia: Rationale, chronology, geochemistry, and paleoenvironmental implications[END_REF][START_REF] Costa | Isotopic reconstruction of the African Humid Period and Congo Air Boundary migration at Lake Tana, Ethiopia[END_REF][START_REF] Lamb | Climatic and non-climatic ef-fects on the δ18O and δ13C compositions of Lake Awassa, Ethiopia, during the last 6.5ka[END_REF][START_REF] Marshall | Late Pleistocene and Holocene drought events at Lake Tana, the source of the Blue Nile[END_REF][START_REF] Wagner | Holocene rainfall runoff in the central Ethiopian highlands and evolution of the River Nile drainage system as revealed from a sediment record from Lake Dendi[END_REF], however, there has been an almost absent regard for the climate-driven landscape and geomorphological evolution processes of the surrounding areas [START_REF] Hendrickx | Glacial and periglacial geomorphology and its paleoclimatological significance in three North Ethiopian Mountains, including a detailed geomorphological map[END_REF]. In particular, only few studies have described ancient tropical glaciers and related paleo-environmental archives [START_REF] Hurni | Paleoglaciated Landscapes in Simen and Other High-Mountain Areas of Ethiopia[END_REF][START_REF] Hurni | Late Quaternary of Simen and other mountains in Ethiopia[END_REF][START_REF] Nilsson | Ancient Changes of Climate in British East Africa and Abyssinia. A Study of Ancient Lakes and Glaciers[END_REF][START_REF] Williams | Fossil Periglacial Deposits in the Semien Highlands, Ethiopia[END_REF].

In this context, I have submitted a project application to the Early Career Research Grant of the National Geographic Society entitled "PRISM: Paleoenvironmental Research in the Simien Mountains" (C. Mologni dir.). The project proposes to evaluate the feedback mechanisms of glacier systems to atmospheric forcing variations so as to understand the spatio-temporal patterns of past climate change in East Africa.

During the Quaternary, changes in monsoonal regime heavily impacted the prehistoric inhabited environments in East Africa, oscillating between arid and humid periods. In particular, during the last 14 000 years strong climatic disruptions were concomitant with some crucial techno-cultural innovations and to changes in way of life: the transition from hunter-gatherers' societies to populations characterized by food production economies. The study of the interaction between climate fluctuations, environmental transformations and the human adaptability represent a capital issue for better understand the patterns of this cultural transition.

The understanding of human-climate co-evolution is usually approached by the correlation between continuous regional or sub-continental climatic archives (marine, lacustrine) with the regional archaeological chrono-sequences in which are recorded the principal prehistoric techno-cultural innovations. However, in East Africa, the regional spatial-temporal extent of hydro-climate changes is variable and subject to ongoing debate. Moreover, the regional-to-local palaeoenvironmental evolution is often modulated by the differential reactivity of specific hydro-systems, making the large-scale correlation between prehistoric settlements and hydro-climatic changes superficial. A downsizing of analytical scales is thus required with the aim to better understand the links between regional hydro-climatic changes and the geomorphological factors which have modeled the inhabited prehistoric landscapes. In this manuscript a multi-scalar and multi-proxy approach was proposed, studying and comparing two East African basins whose receive their water supplies from rainfalls localized in the Ethiopian Highlands: those of the Nile Blue and the Lake Abhe.

Firstly, the Holocene monsoon-induced hydrological oscillations has been reconstructed through the study of frequency and intensity of the Nile floods and of the hydrological fluctuations of the Paleo-Lake Abhe (Central Afar Region). Our results highlighted a high monsoonal rainfalls regime between ∼10 and ∼9 ka BP and a stepwise aridification since ∼9 -∼8.7 ka BP culminated in two arid pulses at ∼8 and ∼4.5 ka BP. Some divergences with Central and Western African climatic records evidenced how the beginning of the aridification as soon as ∼9 -∼8.7 ka BP could be linked to the influence of the Indian Summer Monsoon. Moreover, our results show a delayed Mid-Holocene aridification of the Lake Abhe basin in respect to other southern East African Rift System lakes, providing evidence for its incoherence with the southward migration of the rain belt scenario. As already proposed by other authors, we put forward the hypothesis of the role of the Congo Air Boundary and its interplay with the ITCZ oscillation on the control of precipitations in the northern Rift.

Focusing on the Lake Abhe basin, we provided a preliminary look on the reactivity of littoral pedosedimentary systems to this regional hydro-climatic framework. Before ∼10 ka BP the basin was characterized by a high environmental instability resulting from the alternated shutdown and activation of the local fluvial network, in combination with erosional processes and lake level fluctuations. This landscape instability was likely produced by the succession of enhanced precipitations at the beginning of the African Humid Period (∼14 ka BP), the prominent aridification in link to the Younger Dryas North Atlantic event (∼13.4 -∼11.1 ka BP), and by the strong resumption of humid conditions around ∼11 ka BP. Indeed, during the Early Holocene lacustrine water highstands dominated the landscape in link with littoral shoreline morpho-sedimentary formations and the development of pedogenetic processes. In the Lake Abhe basin, the ∼8.2 ka North Atlantic event is recorded by an arid episode resulting in the formation of soils from the previous palustrine deposits and in the establishment of aeolian-dominated landforms during few centuries. Similarly, at the end of the African Humid Period, the desiccated plain of the Lake Abhe was rapidly replaced by sedimentary accumulations alternating dune deposits and thin pedogenic horizons.

The geoarchaeological study of 7 new discovered archaeological sites have furthermore permitted to give a diachronic environmental context of the prehistoric settlements around the Lake Abhe during the Holocene and to put the link between regional hydro-climatic changes, the local paleolandscape evolution and the human behavior. Firstly, we highlighted a strong prehistoric lake-side adaptation re-sulted in settlements mobility whose followed the fluctuating lake waters. These results show how rapid hydrological fluctuations and arid droughts was followed by displacement of human groups close to the lake shorelines. During the Early Holocene, the large lake water body surface heavily assisted the human economic system, almost exclusively organized around the fishing or hunting of aquatic mammals' activities. These strategies, typical of the Late Stone Age societies, persisted even during the ∼8 ka BP arid event and until the end of the Mid-Holocene Lake Abhe highstands, coinciding to the end of the AHP in this region (∼4.5 ka BP). Interestingly, the emergence of the food production societies occurred when the extended lake waters started to be replaced by arid environments. The lapse between the last highstand waters and the first evidence of food production is only of one/two hundred years. Notwithstanding this, the new herding and ceramic production traditions seem to been integrated in the hunter-gatherers techno-cultural network, still suggesting the dependence to reduced Late Holocene lake waters. Similar patterns have been evidenced by other chrono-cultural sequences along the Sahel and the East African Rift. Some authors suggested that this Middle-Holocene regional aridification would have played a key role on the spread of pastoralism. In the Lake Abhe basin, the emergence of herding seems to be delayed of few centuries if compared to the Kenyan Rift, or to the East African Highlands. Here, we showed how at the end of the African Humid Period, the Northern Rift was subjected to belated humid conditions. We thus put forward the hypothesis that Mid-Holocene persistent water supplies of the Lake Abhe play a role in the acquisition of herding in this region.

All these evidences show a strong adaptation of prehistoric populations to the local-to-regional environmental patterns which are dependent to specific hydrological conditions of each intra-Rift basin. Finally, the approach proposed in this work suggest how human-climate co-evolution is decipherable through an understanding of the complex interaction between the external/internal climatic forcing on moist air mass displacement over the continent, the local-to-regional reactivity of specific hydrosystems, and the knowledge transfer and the cultural transition/assimilation processes.

  Mots clés:Période Humide Africaine • Coévolution Homme-Climat • Delta Profond du Nil • Corne de l'Afrique • Système du Rift Est Africain • Lac Abhé ix hydro-climatic changes in the Ethiopian Highlands IV.1 Introduction xiv IV.2 Regional settings IV.2.1 The NDSF Western Province morphology IV.2.2 The NDSF Western Province sediment sources IV.3 Materials and methods IV.3.1 AMS 14 C measurements IV.3.2 XRF core scanner on soft sediment IV.3.3 Particle grain size analysis and LOI IV.3.4 Light and Electron Microscopy Analyses IV.3.5 Nd radiogenic isotope analyses IV.4 Results IV.4.1 Age model, lithology and sedimentation rates IV.4.2 Nd isotopic composition IV.4.3 Description of T1 and T2 event-layers IV.4.3.a T1 event-layer description IV.4.3.b T2 event-layer description IV.5 Interpretations and Discussion IV.5.1 T1 and T2 event-layers: interpretation and formation processes IV.5.2 Coherence of Nile flood records over the upper slope of the Rosetta System IV.5.3 Holocene hydro-climatic shifts over the Blue Nile basin IV.5.3.a From 10 to 9 ka BP : a strong and stable hydro-climatic activity IV.5.3.b Stepwise Early Holocene hydro-climatic degradation from 9 ka BP IV.5.3.c Unstable and anomalous Nile hydrologic conditions from 7.8 ka BP IV.6 Conclusion

  climate and the Tropical African Monsoon I.1.2 The East African Monsoon and the Ethiopian Highlands I.1.3 The Ethiopian Highlands: at the source of the Nile and Awash Rivers Awash basin and the Afar depression

Figure I. 1 :

 1 Figure I.1: Schematic representation of the Global atmospheric circulation in which the Hadley hemispheric cells, the direction of air mass convection and the location of the Intertropical Convergence Zone are indicated (ITCZ; http://la.climatologie.free.fr).

  I.2).

Figure I. 2 :

 2 Figure I.2: Basic components of a summer monsoon and its driving forces explained in the text, after Mohtadi et al. (2016)

Figure I. 3 :

 3 Figure I.3: a) Trajectories of air parcels reaching the Ethiopian Highlands (red square) in July-August 1998-2008, with arrows indicated with numbers referred to in the text. The map to the right shows the same air trajectories relative to topography (Elevation data: GTOPO30, http://eros.usgs.gov; after Viste and Sorteberg, 2013); b) Mean moisture content (kg/day) of air reaching Ethiopian Highlands (red square) in July-August 1998-2008, after Viste and Sorteberg (2013).

Figure I. 4 :

 4 Figure I.4: Map of East Africa monthly precipitation average (mm/month) in June 2009, from the CRU-TS 4.03

Figure I. 5 :

 5 Figure I.5: Simplified geo-lithological cover map of East Africa with the principal formations cited in the text (USGS, modified African surficial lithology basemap). The watershed boundaries of the Nile and Awash Rivers are indicated with black and yellow lines respectively, including their major hydrographic courses and major lakes. Black arrow indicates the Afar Region.

Figure I. 7 :

 7 Figure I.7: a) The complete Nile River drainage basin system map, in which are indicated in bold text the main affluent rivers (modified from Garzanti et al., 2015); b) water average annual discharge (at the top) and sediment fluxes (below) in the Nile basin, after Sutcliffe and Parks (1999).

Figure I. 8 :

 8 Figure I.8: : Bathymetric map of a) the Nile margin and the Levantine basin, in which the Nile Deep Sea Fan (NDSF) provinces, the Rosetta and Damietta branches and the seismic line (grey line) corresponding to the profile in Fig. I.8c are indicated b) the Western Nile Deep Sea Fan (NDSF; proximal Rosetta Upper slope, C10 Rosetta canyon, DSF6) showing the locations of the studied core MD04-2726, MS27PT (red dots Revel et al., 2015), and the P362-2-33 core (Blanchet et al., 2013, black dot); the grey line indicates the seismic profile of Fig. I.8c. c) Interpreted seismic profile along the edge of the Rosetta system platform, with canyons (red lines), the sedimentary aggradation that started around 130 ka BP (yellow area) until the Plio-Pleistocene transition (blue and green area), after Rouillard (2010) (modified from[START_REF] Mascle | The Nile Continental Margin: A Compendium Compiled by Jean Mascle[END_REF]

Figure I. 9 :

 9 Figure I.9: Elevation map of the Awash catchment area and adjacent basins, in which the Awash River track (bold blue line) the respective hydrological network (slight blue lines), the basin catchment delimitation areas (black lines), the present-day lakes (light blue), and the main Towns (red dots) are indicated.

Figure I. 10 :

 10 Figure I.10: : a) The lithostratigraphic sequences of the Adu-Asa and Sagantole formations along the Middle Awash western rift margin, after WoldeGabriel et al. (2001) ; b) 40Ar/39Ar chronology of the Sagantole sedimentary formation in the Middle Awash River zone, after Renne et al. (1999).

Figure

  Figure I.11: a) Structural map of the Afar depression in which the Abhe Lake sedimentary basin (light green),

  are indicated. The dotted dark green line indicates the Tendaho-Gobaad discontinuity, while the large and small faults are indicated by bold and thin lines respectively (modified after Doubre et al., 2017). b) simplified morpho-tectonic patterns of the Afar Triangle depression (light gray), enclosed by the Arabian, Nubian and Somalian plates (pre-rift groups, dark gray) with the location of the Central Afar region (black dotted line).

Figure I. 12 :

 12 Figure I.12: Map of the geological structures of the Central Afar region and of the Tendaho-Gobaad graben (modified after Abbate et al., 1995).

Figure II. 2 :

 2 Figure II.2: Paleovariations in: a) δ18O from the North Greenland Ice Core Project (NGRIP) as a proxy of changes in arctic temperatures (Wolff et al., 2010); b) reflectance of Cariaco basin sediments interpreted as the latitudinal excursion variations of the ITCZ during boreal summer (Deplazes et al., 2013); c) δ18O cave stalagmites from Borneo as a proxy of rainfall in the equatorial western Pacific[START_REF] Carolin | Varied Response of Western Pacific Hydrology to Climate Forcings over the Last Glacial Period[END_REF] ; d) reflectance of Arabian Sea sediments interpreted as a runoff proxy triggered by Indian monsoon activity[START_REF] Deplazes | Links between tropical rainfall and North Atlantic climate during the last glacial period[END_REF]. Cold intervals HE6-1, including YD, are indicated with grey bands, (after[START_REF] Schneider | Migrations and dynamics of the intertropical convergence zone[END_REF][START_REF] Gottschalk | Abrupt changes in the southern extent of North Atlantic Deep Water during Dansgaard-Oeschger events[END_REF]

Figure II. 3 :

 3 Figure II.3: Paleovariation in: a) June-August insolation at 30°N (Laskar et al., 2004); b) North African dust flux at 20.8°N (deMenocal et al., 2000); δDwax from c) the Senegal River at 15.5°N (Niedermeyer et al., 2010), d) Lake Bosumtwi at 6.5°N[START_REF] Shanahan | The time-transgressive termination of the African Humid Period[END_REF] and from the Congo Basin at 5.5°S[START_REF] Schefuß | Hydrologic control of carbon cycling and aged carbon discharge in the Congo River basin[END_REF]. Grey bands indicate short-term arid events; the red arrow indicates the delayed southward shift of the ITCZ, (modified after[START_REF] Shanahan | The time-transgressive termination of the African Humid Period[END_REF]

Figure II. 4 :

 4 Figure II.4: Areal section of bottom water [ CO 3 2-] reconstruction from B/Ca ratios of benthic foraminifera indicating the flows of northern-sourced deep water to the deep central sub-Antarctic Atlantic Ocean during DO events: grey arrows indicate the flow of North Atlantic Deep Water, white arrows indicate the flow of Antarctic Intermediate Water. Grey and white circles, as well as the white star indicate the cores used in Gottschalk et al. (2015) study.

Figure II. 5 :

 5 Figure II.5: Map of the timing of lacustrine regressions at the end of the AHP, showing the irregular spatiotemporal patterning of the hydrological regimes between North/South and East/West Africa[START_REF] Shanahan | The time-transgressive termination of the African Humid Period[END_REF]

Figure II. 6 :

 6 Figure II.6: Elevation map of Africa with the present summer Intertropical Convergence Zone (ITCZ) and Congo Air Boundary (CAB) positions (black dotted lines), the West African Monsoon (WAM) and Indian Summer Monsoon (ISM) air moisture direction fluxes (white arrows). The red area indicates the central-northern East African Rift System (EARS).

  II.7), combining fieldwork observations, mesoscopic and microscopic analyses and isotopic geochemistry.

Figure

  Figure II.7: a) Map of East Africa with main modern lake and palaeolake basins cited in the text, modified from Maslin et al. (2014) and Junginger et al. (2014); b) Cross section of East Africa showing the main modern lake and palaeolake basins cited in the text, modified from Maslin et al. (2014) and Junginger and Trauth (2013).

Figure II. 10 :

 10 Figure II.10: Localisation of the confidently dated LSA sites/sites-groups in the Horn of Africa. White dots = open air lake-side site; black dot = rockshelter site; black star = capital city. Modified after Ménard and Bon (2015).

Figure II. 11 :

 11 Figure II.11: Density probability of prehistoric settlements through time calculated from radiocarbon ages of Later Stone Age (LSA) and Neolithic sites in the Ethiopian Rift (at the top) and in the Horn of Africa (below). After Ménard and Bon (2015).

Figure II. 13 :

 13 Figure II.13: Schematic representation of the study approach.

  Fig. III.1). Both rivers represent the principal water supply of two of the most arid regions in the world, the Eastern Sahara and the Afar Triangle. It is why in this Ph.D, The Nile Deep Sea Fan (and the associated basin) and the Central Afar region are the two zones which have been chosen to be studied and to be compared. The first case study concerns the study of the MD04-2726 core recovered during the VANIL oceanographic cruise (Mascle, 2014; Migeon et al., 2010, Fig. III.1). The second study area is located in the Central Afar Region (both Ethiopia and Djibouti) and more specifically coinciding with the Lake Abhe basin (concessions of the VAPOR-Afar and PSPCA projects; Fig. III.4). Each site has been approached using various analytical methods adapted to the type of studied geological material.

Figure III. 1 :

 1 Figure III.1: Digital Elevation Model (SRTM) of East Africa with the location of the study sites: Nile Delta (MD04-2726 core, red star) and the Lake Abhe basin (red area).

Figure III. 2 :

 2 Figure III.2: General lithological description of the MD04-2726 core (modified from original CAD by E. Ducassou) in which are indicated the first five sections investigated in this study (red squares).

Figure III. 3 :

 3 Figure III.3: Digital Elevation Model (SRTM) of the Lake Abhe basin with the location of fieldwork projects areas (VAPOR-Afar and PSPCA projects), the surveyed areas and with the respective sampled material used in this study: stromatolites/microbialites (blue triangles), coastal clastic geomorphic features (red triangles), lacustrine nearshore deposits (green squares), paleosoil sequences (brown squares), dated and undated archaeological sites (black stars).

Figure III. 4 :

 4 Figure III.4: Graphic comparison between littoral geomorphic feature elevations measured with the GEO7x differential GPS in the field (red triangles) and the same GPS point elevations processed with a TanDEMx (12.5 m; green dots) and SRTM (30 m; black squares).

Figure III. 5 :

 5 Figure III.5: DGPS measurement operations in the Lower Awash valley using Trimble Geo 7X. From the left: L. Bruxelles, G. Davtian, C. Mologni, A. Mahammad (Ph. L. Khalidi)

Figure III. 6 :

 6 Figure III.6: Fieldwork operations pictures including a) extensive archaeological excavation of the Kurub-07 site (ph. L. Khalidi); b) describing the archaeological site Stromatolis-01 (Ph. E. Soteras Jalil); c) Stromatolite dome sampling (Ph. E. Soteras Jalil); d) describing the sedimentary sequence of Raso-02 (Ph. E. Soteras Jalil).

  III.7).

Figure III. 7 :

 7 Figure III.7: Micromorphological sampling of the Raso-02 sedimentary sequence (Ph. E. Soteras Jalil; in the picture L. Khalidi and C. Mologni)

  and T2 event-layers: interpretation and formation processes IV.5.2 Coherence of Nile flood records over the upper slope of the Rosetta System IV.5.3 Holocene hydro-climatic shifts over the Blue Nile basin

Figure

  Figure IV.1: (continued)

Figure

  Figure IV.1: a) Bathymetric map of the Western Nile Deep Sae Fan (NDSF; proximal Rosetta Upper slope, C10 Rosetta canyon, DSF6) showing the locations of the studied cores MD04-2726 (31°51.00' N, 29°47.49' E, 1058 m water depth) and MS27PT (N31°47.90'; E29°27.700, 1389 m water depth; red dots), the bathymetric profile presented in Fig. IV.7, and the P362-2-33 core(Blanchet et al., 2013, black dot). b) Hydrological context of North Africa and the present-day active Nile watershed (modified from[START_REF] Skonieczny | African humid periods triggered the reactivation of a large river system in Western Sahara[END_REF]. Three main sources of suspended sediment load are identified in the Nile Deep-Sea Fan: the basaltic rocks (purple) of the Ethiopian Traps (Highlands), which are drained by the Blue Nile, the Atbara and Sobat rivers located at tropical latitude (around 5 to 15°N); the Precambrian metamorphic rocks (green) of the Central African Craton located in the equatorial latitude of Lakes Albert and Victoria in the Ugandan headwaters region of the White Nile; the Saharan Metacraton[START_REF] Abdelsalam | The Saharan Metacraton[END_REF] which produce aeolian dust representing the source region of most deflation documented by[START_REF] Scheuvens | Bulk composition of northern African dust and its source sediments -A compilation[END_REF]. The outcrops of the Ethiopian Traps, Precambrian Craton and Saharan Metacraton provide sediments characterized by Nd values of > 0, > -30 and from -10 to -14 respectively[START_REF] Garzanti | The modern Nile sediment system: Processes and products[END_REF][START_REF] Grousset | Tracing dust sources and transport patterns using Sr, Nd and Pb isotopes[END_REF][START_REF] Skonieczny | The 7-13 March 2006 major Saharan outbreak: Multiproxy characterization of mineral dust deposited on the West African margin[END_REF]. The seasonal positions of the Intertropical Convergence Zone (ITCZ; red dotted line,[START_REF] Viste | Moisture transport into the Ethiopian highlands: MOISTURE TRANSPORT INTO THE ETHIOPIAN HIGHLANDS[END_REF] as well as of the Sahel zone (orange area) are shown, as well as the Nile catchment mica sources (Dabus and Diesda rivers, orange dots;[START_REF] Garzanti | The modern Nile sediment system: Processes and products[END_REF].

  age ( 14 C yr BP± 1σ) Carbon content (mg) Reservoir correction (yr) Cal Curve Calibratedage (cal yr BP ± 2σ) Bacon median age (cal. yr BP ± 2σ)

  Fig. IV.4a), a low Mn/Al values (Fig. IV.2d) and the lack of bioturbation features. The third interval lies between 132 and 70 cm and is dated between ∼7.2 and ∼5.8 ka BP (Fig. IV.2a

  Finally, distinctly low Nd values (<-5.5) are measured between 70 to 20 cm in phase with the low values of the Ti/Ca ratio. Nd values from turbiditic beds show a systematic offset by ∼ +0.6 compared to the hemipelagic facies (Fig. IV.2b).

Figure

  Figure IV.2: a) Age model of the MD04-2726 core with new (asterisk, this study), and previous[START_REF] Revel | 20,000 years of Nile River dynamics and environmental changes in the Nile catchment area as inferred from Nile upper continental slope sediments[END_REF] 14 C calibrated ages, indicating ages obtained on foraminifera (blue tie points) and on organic matter (green tie points); sedimentation rate (mm/yr) of the MD04-2726 core. b) Silt (<63 µm) Nd values of T1 event-layers (squares) and hemipelagic sediments (dots) of the MD04-2726 core. c) Log (Ti/Ca) and d) Mn/Al ratios from 1 mm resolution XRF core scanner measurements from the MD04-2726 core. e) Lithology and texture of the MD04-2726 core with indicated section depths and the localisation of new and previous calibrated ages.

  Fig. IV.3a). Two types of beds are distinguished: 83 are characterized by an increase of Ti/K, Mn/Al and Q90/Q10 ratio (T1 event-layers; Figs. IV.3, IV.4, IV.6) and 19 are characterized by an increase of both the Ti/K and Q90/Q10 ratios, without Mn/Al peaks (T2 event-layers; Figs. IV.3, IV.5, IV.6).

Figure

  Figure IV.3: a) Lithology and texture of the turbidity sequence in core MD04-2726 (sections II to V), with the location of the sampled and studied thin sections (orange boxes) and of new and previous calibrated radiocarbon ages (see Fig. IV.2). b) Detail of the lithology for the interval 542 to 624 cm depth with the location of thin section N°7 (orange box, see Fig. IV.4), the Q90/Q10 and Log(Ti/K) and Log(Mn/Al) ratios. The T1α, T1β and T2 event-layers are identified by yellow, green and red highlights, respectively.

Figure

  Figure IV.4: a) Scan in crossed polarized light of thin section N°7 with the location of Fig. IV.4b, 4c and 4d, the T1 Facies (I to V), and the T2 and hemipelagic facies; Q90/Q10 (black line), Log(Ti/K) (blue line) and Log(Mn/Al) (green line); black arrows indicating the fluvial-induced redox facies; LOI (%) over hemipelagic and T1 sedimentary facies. b) SEM image in plane polarized light of T1 Facies III showing the organic matter (OM, black arrows) with horizontal orientation and distribution patterns in climbing ripple sedimentary patterns; c) SEM image in crossed polarized light of T1 Facies III showing a non-weathered mica mineral with the respective EDX analysis spectrum. d) SEM image of quartz grain surface showing small to large conchoidal microfractures and V-shaped percussion cracks (black arrows).

  Figure IV.5: ) Scan in crossed polarized light of thin section N°1 with T2 event-layer facies locations and Log(Ti/K) (blue line), Log(Mn/Al) (green line) and Q90/Q10 (black line). b) Light microscopy and scan images in plane polarized light of T2 facies and localisation of thin section N°10, showing a massive deposition of silty beds. c) Light microscopy and scan images in plane polarized light of T2 facies and location of thin section N°4, showing massive and reworked sediment and a fragmented foraminifer.

Figure IV. 6 :

 6 Figure IV.6: Q90 versus Q50 plot of all the T1 and T2 event-layers from MD04-2726 core, showing the distinction between T1α types suggesting a uniform to graded suspension, and T1β types suggesting a graded suspension to tractive current sedimentation mode and T2 event-layers indicating a uniform to graded suspension sedimentation mode. b) Mn/Al plot (XRF count numbers) of T1α, T1β and T2 event-layers from III-IV-V sections of MD04-2726 core; c) Grain-size distribution of T1α, T1β and T2 event-layers (volume percentage vs. log of grain-size in µm) showing the grain-size sorting effect of the different sedimentation processes.

  IV.2b), indicating a large proportion of particles deriving from the Ethiopian Traps, which have Nd values of 0 to 7 (Fig. IV.8c, e; Ménot

  Fig. IV.1a; Figs. BI.2, BI.3) are compared, with the aim of testing the coherence of these different Nile flood records over the Rosetta

Figure

  Figure IV.7: a) Bathymetric profile of the proximal Rosetta upper slope (A'-A'' , shown in Fig. IV.1a) with the positions of the MD04-2726 and MS27PT cores; the Sapropel Intermediate Water, Intermediate Water, Surface Water limits (after Zirks et al., 2019), and a schematic model of the formation process of the mud clast deposits (after Ducassou et al., 2009). b) Detail of the bathymetric profile of the C10 Rosetta channel (shown in Fig. IV.1a) with the position of the MD04-2726 core; and a schematic model of the formation process of the T1 event-layers (this study).

Figure

  Figure IV.8: a) Thickness of flood-induced deposits (cm; T1α in yellow and T1β in green) of the MD04-2726 core. b) Q90/Q50 ratio and frequency (100-yr running average, brown line) of flood deposits (T1α in yellow and T1β in green) of the MD04-2726 core. c) Nd of T1 event-layers (brown line and squares) and hemipelagic sediment (blue line and dots). d) Sedimentation rate (cm/yr, brown line) and thickness of mud clast deposits (T1α and T1β eventlayers) of the MS27PT core. e) Nd of clastic mud deposits (brown line and squares) and of hemipelagic sediment (blue line and dots) from core MS27PT (fraction <63 µm; after Ménot et al., 2020, this study). f) Sedimentation rate (mm/yr, in brown) and thickness of mud clast deposits from the P3622-33 core.

  Fig. IV.9c, d), hence clearly suggesting persistent inputs from the Ethiopian Traps and/or on-going sediment transport via the Blue Nile and the Sobat/Atbara rivers to the NDSF. Based on correlation with other paleoclimatic and paleoenvironmental regional studies, the centennial time resolution of flood-induced event-layers reveals three trends in Early to Mid-Holocene hydro-climatic dynamics. IV.5.3.a From 10 to 9 ka BP : a strong and stable hydro-climatic activity From 10.1 to 9 ka BP, more radiogenic Nd values of hemipelagic sediment (around -4.5), T1 eventlayers (-3 to -2; MD04-2726 core) and clastic mud beds from MS27PT core (-4 to -3, Fig. IV.9c, d) demonstrate the dominant contribution of Ethiopian Traps material to the sedimentation in the western NDSF. This is accompanied by high sedimentation rates in all three deltaic cores and therefore suggests a large erosional activity in the Nile watershed (Fig. IV.1a, IV.8d, IV.8f). The paleo-flood frequency reconstruction shows a strong increase of the Nile River's hydrological activity from 10.1 ka BP onwards, which is in phase with maximal June insolation at 15°N (Fig. IV.9a). Exceptional Nile flood events culminate at 9.5-9 ka BP with seven high-energy floods every 100 yr (Fig. IV.9e). High precipitation-led runoff at Lake Dendi (Ti and K events, Fig. IV.9f, Wagner et al., 2018) and Lake Tana (Ti content, Fig. IV.9g, Marshall et al., 2011), both located at the source of the Blue Nile, seem to occur synchronously with intervals of the strongest Nile flood events (T1β layers; Fig. IV.9e). Additionally, continental geomorphological data from the Blue and White Nile lower courses (Khartoum region; Williams, 2009; Williams et al., 2015) seem coherent with our results: high energy exceptional T1β event-layers are in phase (within age uncertainty) with enhanced Nile flood periods recorded until 9 ka BP (Fig. IV.9h).

Figure IV. 9 :

 9 Figure IV.9: Paleovariations from 10.5 -5 ka BP of: a) July insolation at 15°N[START_REF] Berger | Insolation values for the climate of the last 10 million years[END_REF]; b) δD-WAX (‰) at Lake Tana[START_REF] Costa | Isotopic reconstruction of the African Humid Period and Congo Air Boundary migration at Lake Tana, Ethiopia[END_REF]; c) Nd of T1 event-layers (brown line and squares) and hemipelagic sediment (blue line and points) of the MD04-2726 core; d) Nd of clastic mud deposits (brown line and squares) and of hemipelagic sediment (blue line and dots) from core MS27PT (fraction <63 µm) (afterMénot et al., 2020, this study); e) Q90/Q50 and frequency (100-yr running average, brown line) of exceptional flood deposits (T1α in yellow and T1β in green) of the MD04-2726 core; f) erosion runoff-induced K and Ti event layers from Lake Dendi[START_REF] Wagner | Holocene rainfall runoff in the central Ethiopian highlands and evolution of the River Nile drainage system as revealed from a sediment record from Lake Dendi[END_REF]; g) Ti (mg/g) at Lake Tana[START_REF] Marshall | Late Pleistocene and Holocene drought events at Lake Tana, the source of the Blue Nile[END_REF]; h) Blue (in blue) and White (in yellow) lower Nile course flood periods recorded over the Karthoum region[START_REF] Williams | Late Pleistocene and Holocene environments in the Nile basin[END_REF][START_REF] Williams | Late Quaternary environments and prehistoric occupation in the lower White Nile valley, central Sudan[END_REF]. Light grey bands indicate the lower flood frequency recorded in MD04-2726 core.

  Fig. IV.8a), reduced flow energy (increase of Q90/Q50 ratio produced by Q50 drop) and a reduction of Ethiopian Traps source inputs (lower Nd values; -4.5, Fig. IV.9c, d). Higher flood frequency is associated with a high amount of T1β layers until 7.6 ka BP, and a reduction of flood frequency to ca. 2.5 event-layers/100 yr is associated with a dominance of T1α layers (Fig. IV.9e). Contemporarily, leaf wax deuterium isotopes at Lake Tana indicate lower precipitation over the Ethiopian Highlands, at the source of the Blue Nile (Fig. IV.9b;

Figure V. 1 :

 1 Figure V.1: (continued)

Figure V. 2 :

 2 Figure V.2: Graphic comparison between littoral geomorphic feature elevations measured with the GEO7x differential GPS in the field (red triangles) and the same GPS point elevations processed with a TanDEMx (12.5 m; green dots) and SRTM (30 m; black squares).

  with abrasion platforms (Fig. V.3d). The specific description of each Gilbert-type delta facies used in this study as a paleo-shoreline marker is provided in the description of the surveyed sites (Section V.1.4.b).

Figure V. 3 :

 3 Figure V.3: Selected photographs of the surveyed littoral geomorphic features described. a) Well-developed wave abrasion notches combined with abrasion platform and beach pebble/cobble concentration features at the BSW site (Fig. BI.9). b) Ridge made on the foot wall of the basalt stratoid series which separates the Gamari and Afambo lake waterbodies with a well-developed seaward-sloping shore morphology (Sunamura, 1992) and the location of the relative paleo-shoreline (dashed black line) at the Afbo site (Fig. BI.5). c) Well-developed abrasion platforms on basaltic bedrock and on sediments at the DSb site (Fig. BI.13). d) Gilbert-type delta morphology at the BE site with black arrows indicating: the feeder canyon, the apex delta and the connected abrasion platform (Fig. BI.8). e) Gilbert-type delta facies at the BE site (Fig. BI.8).

Figure V. 4 :

 4 Figure V.4: Digital Elevation Model of the Central Afar Region base map (SRTM) with the a) location map of surveyed sites over the Abhe Lake basin (black line squares) showing the Maximal Holocene Highstand Shoreline (MHHS, 420 m a.s.l.; red line), location of the ancient overflow sills (black arrows) and the Hanle basin. b) presented from the western to the eastern side of the Abhe Lake basin, a synthesis of the surveyed sites showing the MHHS (bold and dotted blue lines), the paleo-shorelines (black arrows), the stromatolite deposits (bold red lines) and corresponding calibrated median 14 C ages (red text). The dashed black lines indicate the main paleo-shorelines while the dashed red line indicates the connection between stromatolites with similar calendar ages.

Figure V. 5 :

 5 Figure V.5: Selected macro-/meso-/micro-photographs of the Lake Abhe stromatolites. a) Aerial photo of the StrIS site showing the macro-structure of connected large stromatolitic domes, the sampling zone of the sample Strom31 (Poz-113925; Fig. V.1) and the location of the Stromatolis archaeological site (Khalidi et al., 2020), where the dated Stromatolis-SU1 (Poz-113841) was sampled (Fig. V.1). Photo courtesy of E. Soteras b) Photograph showing the superimposition of stromatolitic Facies-A and -B at the site LAK. c) Photograph of a polished section of a stromatolite (Strom-11) showing the meso-structure superimposition of stromatolitic Facies-A and -B. d) SEM-EDX analysis mapping showing the Si-Mg concentration in the stromatolite Facies-B rippled laminations.

  and the Lab ID refers to the Laboratory indicated in Gasse (1975) : LGD = Laboratoire de Géologie Dynamique, Paris; LFR = Laboratoire des Faibles Radioactivités CEA-CNRS, Gif-sur-Yvette; GU= Gakushuin University, Tokyo. Str = Stromatolite undetermined; Str-A = stromatolite Facies-A; Str-B = Stromatolite Facies-B; Mt = Melanoides tuberculata; Ch = Charcoal; hcc = hydrothermal carbonate crust; Sh = Shells mix; U = Unios bivalve shell).

  V.6a), we can compare the littoral carbonate samples of the same age (∼10.4 ka cal. BP, Gasse_94 and Strom20-SU1; Tab. V.2; Fig. V.6), which show an elevation offset of 49.2 m. The sample Gasse_94 is located on the left bank of the Logiya River valley, while Strom20-SU1 was sampled within the Saha plain (Fig. V.6a, b). Between the sites, several normal faults are dissecting the surface and mainly correspond to the southern end of the active Manda-Hararo volcano-tectonic rift segment (Fig. V.6;

Figure

  Figure V.6: (continued)

Figure V. 6 :

 6 Figure V.6: a) Digital Elevation Model of the Central Afar Region (SRTM) showing the current horizontal veloci-

Figure V. 8 :

 8 Figure V.8: Holocene interconnectivity between the Ziway-Shala and Awash River catchments. a) Paleovariations in June insolation at 10°N (after Berger and Loutre, 1991). b) Abhe Lake-level fluctuations (for the legend see Fig. V.7). c) Ziway-Shala Lake-level fluctuations from calibrated ages (after Gillespie et al., 1983). Overflow elevations are indicated with blue lines, while the transient fluvial interconnectivity between the two basin catchments are indicated with light blue bands. d) Map of the Ziway-Shala basin and Awash catchment areas (black bold line) showing the Awash River course (bold blue line), the secondary rivers (light blue lines), the present (light blue) and Early-Mid-Holocene (blue) lake surfaces, and the Early-Mid-Holocene overflows (red arrows).

  to calendar age; Fig. V.9d,), Suguta (1.5°-2.5°N, Garcin et al., 2009, ; Fig. V.9e) and Nakuru basins (0.5°N; Richardson and Dussinger, 1986, calibrated to calendar age; Fig. V.9f

Figure

  Figure V.9: (continued)

  Fig. V.10; Tierney et al., 2011b), presents a short Holocene humid regime and follows the subsequent progressive southern retreat of the rain belt (Fig. V.9g).

Figure V. 10 :

 10 Figure V.10: East Africa map of monthly precipitation average (mm/month) in June, from the CRU-TS 4.03dataset[START_REF] Harris | Updated high-resolution grids of monthly climatic observations -the CRU TS3.10 Dataset: UPDATED HIGH-RESOLUTION GRIDS OF MONTHLY CLIMATIC OB-SERVATIONS[END_REF] downscaled with WorldClim 2.1[START_REF] Fick | WorldClim 2: New 1km spatial resolution climate surfaces for global land areas[END_REF]. Blue stars indicate the location of lacustrine basins reported in Fig.V.9: b -Lake Tana; c -Lake Abhe; e -Ziway-Shala Lake; f -Lake Suguta; g -Lake Nakuru; and h -Lake Challa. Dashed green line indicates the suggested position of the CAB before 8.5 ka cal. BP, while the dashed red lines indicate the position of the CAB and ITCZ after 8.5 ka cal. BP. Black arrows correspond to the air moisture mass direction of the WAM and of the ISM.

Figure V. 11 :

 11 Figure V.11: Digital Elevation Model (SRTM) of the Lake Abhe basin with the location of the pedosedimentary sequences investigated in this study (see Tab. V.3).

Figure V. 12 :

 12 Figure V.12: Photographs of some paleosoil types observed during fieldwork: a) Mine Sable-02 sequence (Type-1); b) Raso-02 sequence (Type-2); c) Paleosoil Masghidlou (Type-3).

Figure V. 13 :

 13 Figure V.13: Microphotos from paleosoil Type-1 facies : a) subangular blocky peds with planes and vesicle voids and clayey crust at the top of the paleosoil (PPL); b) clay and well sorted rounded or sub-rounded silty material in subangular blocky peds with moderately accommodated planes (PPL); c) thin section scanner picture of paleosoil Type-1 facies showing the clayey crusts, the sub-angular blocky peds network and the plane and channels and vesicle voids (PPL); d) zonation between clayey and silty material with parallel striated b-fabric, fine silty infilling into porosity and micritic crystallitic coatings and hypocoatings around voids (XPL).

  Fig. V.14f, e) suggest a sedimentary material inherited from palustrine deposits. Vertically oriented clayey pedorelicts (Fig. V.14a, b) are considered a marker of enhanced pedogenetic evolution of the original deposit.

  zons. Moderate bioturbation is observed as loose continuous infillings of coarse material inside channel porosity (Fig. V.14d).

Figure

  Figure V.14: (continued)

  to porphyric c/f related distribution patterns (Fig. V.15c, d) and often with an oriented distribution and orientation (Fig. V.15e). These patterns indicate the strong role of the detrital components in the formation process of the pedological horizon. The soil structure is mainly organised in crumbs, granules (Fig. V.15b, c) or lenticular plate peds (Fig. V.15a) suggesting a slight development of a pedogenic process probably induced by the high amount of coarse fraction components. This hypothesis is confirmed by the presence of partially in situ weathered mineralogical material (Fig. V.15c). These patterns, combined with the absence of additional inclusions and specific pedofeatures, indicate a paleosoil developed in a slope context, with low amounts of water supply.

Figure

  Figure V.15: a) Thin section scanner photo of paleosoil Type-3 facies, showing subangular blocky peds and lenticular plate microstructure (PPL). Microphotos from paleosoil Type-3 facies: b) granular to sub-angular blocky microstructure (PPL); c) weathered basalt in a granular microstructure (PPL); d) coarse grain of plagioclase in a granular to sub-angular blocky microstructure (XPL). e) Thin section scanner photo of paleosoil Type-3 facies showing an oriented distribution and orientation patterns of basaltic and carbonate coarse fraction in a silty-clay sub-angular blocky microstructure (XPL).

Figure V. 16 :

 16 Figure V.16: Classification of igneous rocks (https://www.tulane.edu/∼sanelson/eens212/igrockclassif.htm)

Figure V. 17 :

 17 Figure V.17: Results of physico-chemical analyses of paleosoil samples investigated in this study: Paleosoil samples of Type-1 (green dot), Type-2 (cross), Type-3 (black and red dots). a) plotted Log(Si/Al) against Log(K/Al) elemental ratios; b) plotted Log(Q10) against Log(Q90) grain-size areas; c) plotted Log(Ti/Al) against Log(K/Al) elemental ratios; d) LOI (%).

Figure V. 18 :

 18 Figure V.18: Grain-size curve profiles on logarithmic (base 10) scale of paleosoils: a) Type-1; b) Type-2 ; c) Type-3.

  V.17d). High quartz and mica content indicates a slight pedogenetic event characterized by a low mineral weathering. Paleosoils of Type-2 (palustrine context) present some homogeneous characteristics which enable us to include them in a specific chemical group (Fig. V.17). Their fine clayey grain-size is evident in the Q10/Q90 plot (Fig. V.17b), as well as in the 1 to 4 µm modes of the grain-size distribution profiles (Fig. V.18b), which is the main granulometric component. A second mode is localised between 10 and 100 µm, probably indicating a silty detrital component (Fig. V.18b).

  V.17d; see micromorphological interpretation). Palosoil Type-3 (clastic slope context) shows a coarser and non-sorted grain-size distribution (Fig. V.17b, Fig. V.18c

Figure

  Figure VI.2: a) Multi-plot of calibrated ages from archaeological and palustrine layers, and littoral lacustrine carbonates (OxCal v4.3.2 and r:5 IntCal13 atmospheric curve Bronk Ramsey, 2017; Reimer et al., 2013, respectively); b) Abhe paleolake level curve calculated from calibrated ages and recalculated elevations taken from[START_REF] Gasse | Evolution of Lake Abhe[END_REF] and from new ages on littoral lacustrine carbonates.

  . VI.3c) located on the slopes of a stratoid series basalt ridge NW of the rhyolitic Borawli volcano (Fig. VI.1b).

Figure

  Figure VI.3: a) Raso-02 archaeological site sequence with calibrated ages; b) Lakora-03 archaeological site sequence with calibrated ages; c) Satellite image (LANDSAT) with locations of Raso-02 and Lakora-03 sites, dated littoral lacustrine carbonates (this study) and dated littoral lacustrine markers[START_REF] Gasse | L'évolution des lacs de l'Afar Central (Ethiopie et T.F.A.I.) du Plio-Pléistocène à l'Actuel. Recostitution des paléomilieux lacustres à partir de l'étude des Diatomées[END_REF] and modelling of Early Holocene Abhe Lake highstand ∼10.6 ka cal. BP; d) Photogrammetry and point plotted artifacts from the main archaeological layer of the Lakora-03 site.

Figure VI. 4 :

 4 Figure VI.4: Lakora-03 obsidian lithic artifacts: a-c) surface collection, d-f) Sounding 1 (S1); a) blade core, b) bladelet core on dorsal surface of truncated blade ("Kostienki knife" core), c) re-oriented bladelet core, d) notched(?) bladelet, e) bladelet, f) endscraper on retouched blade (possibly used as intermediary piece).

Figure

  Figure VI.5: a) DEM (1 m resolution) of the site of Det Bahari-01 with point plotted surface artifacts, calibrated ages, and lithic conjoins; b) Stromatolis archaeological sequence with calibrated ages; c) Photogrammetry of the Stromatolis site.

  their surface. The underlying stromatolite generation (SU5) was dated to 8978-8598 cal. BP, and SU4 to 8593-8419 cal. BP (Stromatolis_SU4, Tab. VI.2, Strom_31, Tab. VI.1, Fig. VI.5b), while the overlying laminar stromatolite generation (SU1) was dated to 5712-5591 cal. BP (Stromatolis_SU1, Tab. VI.1, Fig. VI.5b). No artifact was found in association with the structure.

  Fig. VI.8, Tab. BI.10).

Figure

  Figure VI.6: a) SP-01 archaeological sequence with calibrated ages and a schematic morpho-sedimentary model of the littoral lacustrine profile of Abhe Lake; b) MS-01/02 archaeological sequence with calibrated ages; c) Satellite image (LANDSAT) with locations of SP-01, MS-01/-02, Det Bahari-01 and Stromatolis sites and modelling of Abhe Lake highstand between 9.8 and 8.2 ka cal. BP.

  Fig. VI.8a.2) cutting into SU12, 42, 43 and 18 in the south end of the site (K-L/10-9; Fig. VI.8, Tab. BI.10). The oven, which is the first of its kind for the period and region has been dated on charcoal to between 3600 and 3300 cal. BP (Kur-07-US10-667, Kur-07-US10-1136, Tab. VI.2).

Figure VI. 7 :

 7 Figure VI.7: Det Bahari-01 obsidian lithic artifacts: a) technical blade (neocrest), b) retouched blade fragment, c) double-endscraper on blade, d-f) backed pieces, g-h) splintered pieces.

  Fig. VI.8, Tab. BI.10, Fig. VI.9a-c). These are characterized by at least two well-differentiated and refitted vessels with incised or impressed decorations, simple forms and medium volumetric capacities. Decoration includes the use of a blunt point and comb impressions on one of the pots (Fig. VI.9a), pivoting impressions using a flat spatula on the second (Fig. VI.9b), and simple impressions by means of oblique lines in horizontal bands on a third pot (Fig. VI.9c). The second phase corresponds to the use of the oven feature (ST1, SU10, SU12, Fig. VI.8, Tab. BI.10, Fig. VI.9d-k). From a morpho-metrical standpoint, the containers are of average volumetric capacity and of simple contour like in the preceding phase, but sometimes present rectified edges (Fig. VI.9d). Just as in the previous phase, decoration includes simple impressions with a comb (Fig. VI.9g) or pivoting impressions with a comb or shell (Fig. VI.9h, i). However, this phase also presents few roulette impressions (Fig. VI.9f). The compositions are always very airy and do not cover the entire surface, but are distinguished from the first phase by use of more diverse motifs combined on the same vessel or the association of two decorative techniques (Fig. VI.9e, j). The presence of few sherd fragments with a red slip applied before firing, testify to the application of colorants on certain pottery

Figure

  Figure VI.8: a.1 -Kurub-07 western section of the K-trench archaeological sequence with location of calibrated ages; a.2) Orthophotograph of anthropic structure (ST1, oven) excavated at the Kurub-07 site with locations of calibrated ages; a.3) Aerial kite photograph of excavations at Kurub-07 draped with point-plotted artifacts; b) Satellite image (LANDSAT) with the location of the Kurub-07 site with modelling of the Abhe Lake regression phase between 5.5-4.6 ka cal. BP.

Figure VI. 9 :

 9 Figure VI.9: Decorated ceramics from the site of Kurub-07. Phase 1: a) Refitted vessel with combination incision

Figure VI. 10 :

 10 Figure VI.10: Holocene paleovariations of a) June insolation at 15°N (Berger and Loutre, 1991); b) δD W AX (‰;

  Fig. VI.6b) in the southern part of the Kurub Bahari plain, while the installation of the ensuing drying environmental patterns is observed in a thick paleodune sequence at MS-01 and by aeolian sands in the Stromatolis site sequence (Fig. VI.6b, Fig. VI.5b).

Figure VI. 11 :

 11 Figure VI.11: Map of the archaeological sequences investigated in this study. Red dots with black star = specific study cases of archaeological sequences presented in Section VI.1 (geoarchaeological, micromorphological approach).

Figure

  Figure VI.13: a) PPL scanner picture of HI-03_Mic3 micromorphological sample with the respective Sedimentary Units and the location of the microphotos (white squares); b) coarse monic c/f distribution patterns of randomly oriented and distributed silty material of SU7.1 (PPL); c) coarse monic c/f distribution patterns of randomly oriented and distributed silty material of SU7.1 (XPL); d) Fe-Mn dendritic orthic nodules and rounded well sorted silty material of SU7.2 (PPL); e) Crescent infilling of silty material in a channel void observed in SU7.2 (PPL); f) Fe-Mn dendritic orthic nodules (OIL); g) micritic typic disorthic nodule impregnation of a silty well sorted groundmass of SU3 (PPL); h) channel and chamber voids without coatings in SU3 (PPL); i) moderately sorted coarse monic silty-to-sandy material of SU4 (XPL).

Figure

  Figure VI.14: a) Stratigraphy and radiocarbon ages (red stars) of the Raso-02 site with a field pedo-sedimentary description and the localisation of the micromorphological sample Raso-02-Bloc1 (yellow square). Variations in b) Si/K (blue line), c) Ca/Si (green line), e) Fe/Si (red line) elemental composition pXRF counts, with variations in d) Q90/Q10 (brown line) and f) LOI (%; black line), and g) proposed preliminary sedimentary formation processes of the Raso-02 deposits.

Figure

  Figure VI.15: a) PPL scanner picture of the Raso-02-Bloc1 micromorphological sample, with the respective Sedimentary Units and the location of microphotos (white squares); b) coarse monic c/f distribution patterns of silty material from SU4b with a coarse oriented fragment of organic matter (XPL); c) subangular blocky to intergrain microaggregate microstructure of silty to clayey SU5a deposit (PPL); d) rounded and parallel distributed carbonate (micritic) gravels in a silty-sandy matrix of SU5b (PPL); e) weathered basalt gravel fragment in an intergrain microaggregate microstructure of SU5c (XPL); f) rounded carbonate (micritic) gravels in a silty-sandy matrix of SU5d (PPL); g) double spaced porphyric c/f distribution patterns and well-developed angular blocky microstructure of SU6 with bone fragment; h) pedorelict composed of oriented and high birefringent clays observed in SU6 (XPL); i) accommodated plane voids forming pseudo-lenticular peds and a well-developed angular blocky microstructure of SU6 with fish bones and calcitic-micritic coarse sand inclusions (PPL).

  SU7 presents the characteristics of a paleosoil. Micritic coatings on plane voids indicate cycles of Carich water percolation in the porosity during the development of the pedogenic event (Fig. VI.17h), although the presence of shells could indicate a palustran origin for this pedological horizon (Fig. VI.17i).In SU6, the reduction of organic matter and the apparition of coarse basaltic and carbonate materials suggest the installation of a detrital event (Fig.VI.17e). Coarse carbonate material is mainly composed of a micritic/sparitic Ca crystal aggregate, and is characterized by a well-rounded shape, probably originated from the erosion of previous lacustrine deposits. The clay aggregation of SU6 and SU5, which represents the matrix of this detritic material suggests the stabilisation of erosional processes and the development of a pedogenic event in phase with the first prehistoric occupation at this site (Fig.VI.17c). Finally, SU4 is marked by the increase of anthropogenic material mainly represented by lithics, bone and shell fragments, and coinciding with the main prehistoric occupation layer of the site (Fig.VI.17b).

Figure

  Figure VI.17: a) PPL scanner picture of Lak-03_Bloc1 sample with the respective Sedimentary Unit limits and the location of microphotos (white squares); b) burnt bone in a clay and organic-rich subangular blocky to granular microstructure of SU4 (PPL); c) subangular blocky to vughy microstructure with abundant silty inclusions (olivine, plagioclase and feldspaths) of SU5 (XPL); d) fish bone fragment in a subangular blocky to vughy microstructure of SU5 (XPL); e) subangular blocky to granular microstructure in close porphyric c/f distribution patterns in SU6 (PPL); f) ostracod shell in a subangular blocky to granular microstructure of SU6 (PPL); g) Well-developed subangular blocky microstructure and accommodated to moderately accommodated plane voids of SU7 (XPL); h) shell fragment in a clayey-rich matrix of SU7 with micritic crystallitic coating on a plane void (XPL); i) weathered basalt in a saprolitic clay-rich layer of SU7.1 (XPL).

Figure VI. 18 :

 18 Figure VI.18: Stratigraphy of the Kurub-07 site sequence with a correlation with the Kurub-07 ST3_K19 auger core and a synthesis of the pedostratigraphic description and with the location of micromorphological samples 1481, 1482 and 1483.

  Figure VI.19: SU18, 1483 micromorphological sample: a) in situ crushed shell along the intrapedal planar porosity; b) in situ broken bone associated with a gypsyc intergrowth, micritic disorthic nodule, dendritic Mn-Fe nodule development; c) gypsic intergrowth development along the intrapedal porosity; d) gypsic intergrowth with impurities and associated with a dendritic Mn-Fe nodule in a micritic crystallitic b-fabric.

Figure VI. 20 :

 20 Figure VI.20: Grain size profile of: a) SU18, b) SU41 (black line) and SU13 (grey line), c) SU15, d) SU9

Figure VI. 21 :

 21 Figure VI.21: SU18, 1483 sample: a) sparitic nodule in a single grain porphyric c/f related distribution pattern; b) earthworm bioturbation in angular blocky microstructure.

Figure

  Figure VI.22: a) PPL scanner picture of the 1482 micromorphological sample with the respective Sedimentary Unit limits (SU41, SU13) and the localisation of the microphotos (white squares); b) single grain/intergrain microaggregate microstructure of a very well sorted silty material of SU13; c) clayey fragmented crust which separates SU41 and SU13 with displaced layered coarse material (PPL); d) micritic hypocoatings along the clayey crust between SU41 and 13 (XPL); e) pedorelict of clayey layered fragmented crust in SU41 (XPL); f) micritic nodules and coatings in an intergrain microaggregate microstructure with a coarse monic c/f related distribution pattern in SU41 (XPL); g) well developed micritic hypocoating in SU41 (XPL).

Figure VI. 23 :

 23 Figure VI.23: a) PPL scanner picture of micromorphological sample 1481 with the respective Sedimentary Unit limits and the localisation of the microphotos in this section (white squares); b) fine-grained excreta (bioturbation features) in a coarse monic c/f related distribution pattern in SU9 (PPL); c) micritic orthic nodule in a coarse monic c/f related distribution pattern in SU9 (XPL); d) SU15.3 top pedological crust (PPL); e) parallel-striated bfabric of SU15.3; f) fragmented shell in silty well sorted material of SU15.2 (XPL); g) well-developed angular blocky microstructure with channel filled by loose continuous silty infilling in SU15.1 (PPL); h) pedorelict of fragmented clay layered crust of SU13 (XPL); i) illuviation feature in a chamber with silty loose continuous infilling in SU13.2 (PPL).

  , interpreted as runoff episodes along the lake watershed (Fig. VII.1f). In the Awash basin, after a first humid pulse around ∼11.1 ka cal BP, the Lake Abhe waters rose from ∼10.6 ka cal. BP until the Maximal Holocene Highstand level around ∼10 ka cal. BP (Fig. VII.1i). Even if observed from different proxies, the similarity of the hydrological variability between the Blue Nile and the Awash River systems indicate how the Ethiopian Rift and the Nile sedimentary basins are interdependent on the precipitation regimes over the Ethiopian Highlands. VII.1.1.b First indicator of change as early as 9-8.7 ka and the role of the Indian Ocean and CAB in monsoon dynamics Changes in hydrological regimes have been evidenced around ∼9 ka cal. BP from the MD04-2726 core (Fig. VII.1), as a beginning of a stepwise reduction of Blue Nile flood frequency and intensity until 8.2 ka BP. In the Lake Abhe basin, a drop in water level was recorded around ∼8.7 ka cal. BP (Fig. VII.1i).

Figure

  Figure VII.1: (continued)

Figure VII. 1 :

 1 Figure VII.1: Paleovariations from 14 to 0 ka BP of: a) July insolation at 15°N (Berger and Loutre, 1991); b) δD wax

  VII

  .2) led to a rapid erosion of the previously deposited sediment along the Nile course.

Figure VII. 2 :

 2 Figure VII.2: Palaeovariations from 10.5 -5 ka BP of: a) July insolation at 15°N (Berger and Loutre, 1991); b) δD wax (±) at Lake Tana (Costa et al., 2014); c) Nd of T1 event-layers (brown line and squares) and hemipelagic sediment (blue line and points) of the MD04-2726 core; d) Nd of clastic mud deposits (brown line and squares) and of hemipelagic sediment (blue line and dots) from core MS27PT (fraction <63 µm) (after Ménot et al., 2020, and this study); e) Q90/Q50 and frequency (100-yr running average, brown line) of exceptional flood deposits (T1α in yellow and T1β in green) of the MD04-2726 core. The green dashed band indicates the period of disproportionate exceptional Nile floods in contrast to low precipitations over the highlands, suggesting an out of equilibrium fluvial effect occurred along the Nile River hydro-system.

Figure VII. 3 :

 3 Figure VII.3: Simplified geo-lithological base map of East Africa with the principal formations cited in the text (USGS, African surficial lithology modified basemap). The Nile river watershed boundaries are indicated with a black line, including their major hydrographic courses and major lakes. The black arrow indicates the overflow direction of Turkana Lake during the Holocene. Letters indicate the lake/marine studies cited in the text: a) Nile Deep Sea Fan (MD04-2726 and MS27PT cores; Ménot et al., 2020; Revel et al., 2015; Mologni et al., 2020; Blanchet et al., 2013); b) Lake Abhe (this study, Section V.1; and[START_REF] Gasse | Evolution of Lake Abhe[END_REF]; c) Lake Tana[START_REF] Costa | Isotopic reconstruction of the African Humid Period and Congo Air Boundary migration at Lake Tana, Ethiopia[END_REF][START_REF] Marshall | Late Pleistocene and Holocene drought events at Lake Tana, the source of the Blue Nile[END_REF]; d) Lake Dendi[START_REF] Jaeschke | Holocene Hydroclimate Variability and Vegetation Response in the Ethiopian Highlands (Lake Dendi)[END_REF][START_REF] Wagner | Holocene rainfall runoff in the central Ethiopian highlands and evolution of the River Nile drainage system as revealed from a sediment record from Lake Dendi[END_REF]; e) Ziway-Shala Lakes[START_REF] Gillespie | Postglacial arid episodes in Ethiopia have implications for climate prediction[END_REF]; f) Chew Bahir[START_REF] Foerster | Late Quaternary Climate Variability in the Source Region of Homo Sapiens. Dry-Wet Cycles in Chew Bahir, Southern Ethiopia[END_REF]; g) Lake Turkana[START_REF] Garcin | East African mid-Holocene wet-dry transition recorded in palaeo-shorelines of Lake Turkana, northern Kenya Rift[END_REF]; h) Lake Challa[START_REF] Tierney | Late Quaternary behavior of the East African monsoon and the importance of the Congo Air Boundary[END_REF]; Lake Victoria(Berke et al., 2012a); l) Lake Tanganyika(Tierney et al., 2011a).

Figure

  Figure VII.4: a) Proposed chrono-stratigraphic correlations and interpretation of sedimentary/environmental processes of the sedimentary sequences from all of the archaeological sites presented in Sections VI.1 and VI.2. Red text refers to median radiocarbon ages in yr cal. BP. b) Reconstitution of Abhe paleo-lake level fluctuations over the last 14 ka (Chapter V) with environmental interpretation based on the sedimentary sequences of the archaeological sites

  VII.5). The early introduction of pottery and bone harpoon technology of the Turkana populations with respect to their absence in the northern Ethiopian Rift can be explained by the cultural influence from the Saharian and the Shaelian cultures that was facilitated by the large geographic corridor between the White Nile and the Turkana basin represented by the Sobat River (Fig. VII.3).

Figure VII. 5 :

 5 Figure VII.5: Paleovariations from 16 to 0 ka BP of: a) July insolation at 15°N (Berger and Loutre, 1991); b) δD wax

Further

  work concerning the co-evolution of climate-ecosystem-humans is planned for the future (2021-2025) in the context of the ANR NILAFAR project (financed ANR program; M. Revel PI, L. Khalidi, G. Menot, N. Vigier and L. Bruxelles Co-partners).

2 Advances in East African geomorphological and sedimentolog- ical research on the Late Quaternary period

	Chapter II
	Late Quaternary Paleosciences research
	in East Africa and problematics of the
		study
		II.2.1 The impact of hydro-climatic variation on East African conti-
		nental geomorphic systems
		II.2.2 Open questions
	II.3	Advances in Late Quaternary Prehistory of East Africa
		II.3.1 Introduction to East African Prehistory
		II.3.2 The Later Stone Age (LSA)
		II.3.3 The Neolithic and the prehistoric mobility
		II.3.4 Open questions

II.1 Advances in Late Quaternary East African hydro-climate research II.1.1 Orbital forcing and long-term hydro-climatic variations II.1.2 The 'African Humid Period' or 'Green Sahara' II.1.3 Internal forcing and abrupt climatic events II.1.4 Open questions II.

II.4 Synthesis of the research issues and the objectives of the thesis II

  .1 Advances in Late Quaternary East African hydro-climate research II.1.1 Orbital forcing and long-term hydro-climatic variations

  II.2 Advances in East African geomorphological and sedimentological research on the Late Quaternary period II.2.1 The impact of hydro-climatic variation on East African continental geomorphic systems

III.4 General plan of laboratory methods

	III.1.1 The Nile Deep Sea Fan and the MD04-2726 core
	III.1.2 The Lake Abhe basin (VAPOR-Afar and PSPCA research pro-
	grams)
	III.2 Fieldwork methods
	III.2.1 Geomorphological survey and GPS treatment
	III.2.2 Archaeological survey and excavation methods
	III.2.3 Pedo-sedimentary lithological descriptions and sampling
	III.3 Laboratory analytical methods
	III.3.1 Radiocarbon dating, calibration and modelling
	III.3.2 Light and Electron Microscopy Analyses
	III.3.3 Sedimentological analyses
	III.3.4 Geochemistry and isotopic analyses

Table III . 1 :

 III1 General plan of laboratory methods

	IV.1	Introduction
	IV.2	Regional settings

IV.2.1 The NDSF Western Province morphology IV.2.2 The NDSF Western Province sediment sources

Table IV . 1 :

 IV1 Calibration of new and previous ages on MD04-2726 core, and result from age model building using the Bacon software (

.1 Holocene East African monsoonal variations recorded in wave- dominated clastic paleo-shorelines of Lake Abhe, Central Afar region (Ethiopia & Djibouti)

	V.1.1 Introduction
	V.1.2 Regional Settings
	V.1.3 Methodology
	V.1.4 Results
	V.1.5 Discussion
	V.1.6 Conclusion
	V.2

Characterisation and significance of the Holocene pedogenic re- sponse to East African monsoon dynamics in the Lake Abhe basin

	V.2.1 Introduction
	V.2.2 Materials and Methods
	V.2.3 Results and interpretation
	V.2.4 Synthesis and Discussion

Table V . 1 :

 V1 New and previous 14 C ages calibration and elevation correction: ‡ = Ages on nearshore/offshore deposits; † = ages for reservoir effect estimation; * = Selected and revised ages on nearshore/offshore deposits from Abhe basin, from

  Table of methods applied on pedosedimentary sequences investigated in this study with Latitude and Longitude coordinates, elevations, and radiocarbon ages of each sequence. OM = organic matter, sh = shell, ch = charcoal.

Table V . 4 :

 V4 Micromorphological description of paleosoil Type-1 facies.

	Coarse fraction
	Texture		OM	Fish Bone	Shell
	Silt (10 -80 µm)		*	/		*
	Groundmass
	c/f	distribution	c/f ratio	c/f limit			Sorting	Distribution	Orientation
	pattern
	Close		60/40	10			well sorted	Random	Random
	porphyric/fine
	monic
	Microstructure and voids
	Peds			Separation/	Accommodation	Porosity	%	Microstructure
				pedality
				degree
	Subangular blocky peds	Highly		Moderately	Complex	40	Intergrain	micro-aggregate
				separated		accommodated	packing	microstructure
								voids/planes,
								chambers,
								vesicles
	Pedofeatures
	Type			Localisation		Nature	Colour/birefringence Quantity Figure
	coa�ng/hypocoa�ngs	porosity			micrite	birifrangent	****	Fig V.13.d
	Typic anorthic nodule	groundmass		micrite	high birifrangent	****	Fig V.13.d
	b-fabric		fine frac�on		micri�c	birifrangent	***
	fragmented crust		groundmass		clay	brown	**
	crust			on the soil surface	clay	brown	***	Fig V.13.a, c
	parallel striated b-fabric	groundmass		oriented clay	birifrangent	**	Fig V.13d
	Paleosoil Type-2
	Coarse fraction
	Texture		OM	Fish Bone	Shell
	Clay to fine silt (10-60 µm)	***	** (Fig V.14f)	** (Fig V.14e)
	Groundmass
	c/f	distribution	c/f ratio	c/f limit			Sorting	Distribution	Orientation
	pattern
	Double to single	20/80	10			Very well sorted Random	random
	spaced porphyric
	Microstructure and voids
	Peds			Separation/	Accommodation	Porosity	%	Microstructure
				pedality
				degree
	Angular blocky		Highly		accommodated	Planes	and	30	Angular blocky
				separated				channels
	Pedofeatures
	Type			Localisation		Nature	Colour/birefringence Quantity Figure
	coa�ng		voids			Fe-Mn	Dark red	**	Fig V.14b
	pedorelict		groundmass		(oriented) clay	(birifrangent)	**	Fig V.14a, b
	loose con�nuous infilling	channel			silty material	*	Fig V.14d
	typic anorthic nodule		groundmass		Micri�c	High birifrangent	***	Fig V.14f
	Aggregate/dendri�c	orthic	groundmass		Fe-Mn	Dark red	**	Fig V.14c
	nodule
	intergrowth		groundmass		Gypsic	High birifrangent	**	Fig V.14g, h

Table V.5: Micromorphological description of paleosoil Type-2 facies.

VI.1 9000 years of human lakeside adaptation in the Ethiopian Afar: fisher-foragers and the first pastoralists in the Lake Abhe basin during the African Humid Period

	VI.1.1 Introduction
	VI.1.2 Regional Settings
	VI.1.3 Methodology and materials
	VI.1.4 Results
	VI.1.5 Discussion
	VI.1.6 Conclusion
	VI.2

Sedimentary formation processes and environmental context of human settlements in the Late Pleistocene -Holocene Lake Abhe basin

	VI.2.1 Introduction
	VI.2.2 Methodology
	VI.2.3 Results
	VI.2.4 Synthesis and discussion of sedimentary formation processes
	and environmental context of human settlements in the Late
	Pleistocene -Holocene Lake Abhe basin

Table VI . 1 :

 VI1 Radiocarbon ages and elevations from littoral lacustrine carbonates from Abhe paleolake in Ethiopia.

	Sample Name	Material	Lab Code	Conventional yr BP	yr cal BP 2σ (range, 95.4 %)	Median yr cal BP	Elevation (DEM ± 12 m)
	Strom_20-SU5	Stromatolite	SacA58238	9689 ± 40	11210-10805	11125 ± 113	359
	Strom_20-SU4	Stromatolite	SacA58239	9320 ± 40	10660-10404	10530 ± 64	359
	Strom_20-SU1	Stromatolite	SacA58240	9265 ± 40	10565-10295	10447 ± 73	359
	Logiya_1A	Lacustrine Shell	Beta 446441	9180 ± 30	10477-10245	10241 ± 27	401
	Logiya_1B	Charcoal	Beta 446442	9100 ± 30	10477-10245	10330 ± 55	401
	Strom_10a	Stromatolite	SacA55608	8790 ± 30	10112-9679	9807 ± 81	366.8
	Strom_10b	Stromatolite	SacA55609	8075 ± 35	9123-8785	9008 ± 70	366.8
	Srom_31	Stromatolite	Poz-113925	7910 ± 50	8978-8598	8743 ± 109	371.7
	Strom_13	Stromatolite	SacA55611	5919 ± 30	6829-6666	6738 ± 40	359
	Raso-02_Strom	Stromatolite	Poz-113843	5085 ± 35	5913-5746	5815 ± 51	366
	Strom_7	Stromatolite	SacA55610	5005 ± 30	5889-5653	5732 ± 69	361,5
	Stromatolis-SU1	Stromatolite	Poz-113841	4910 ± 30	5712-5591	5632 ± 29	371.7
	Strom_17A*	Stromatolite	Poz-113842	3310 ± 30	3610-3458	3529 ± 40	368
	Strom_32*	Stromatolite	Poz-116164	1045 ± 30	1050-921	953 ± 31	406

* rejected ages.

Table VI .3: Preliminary

 VI

	Taxa	Kurub-07	Det Bahari-01	Lakora-03
	Large mammals	28	4	67
	Hippopotamus (Hippopotamus amphibius)		12
	Cattle (Bos taurus)	9
	Bovine (Bovini)	9
	Middle-sized bovid	165	8
	Sheep/goat (Ovis aries/Capra hircus)	16
	Gazella cf. dorcas	2
	Unidentified rodent	16
	Unidentified bovid	38		1
	Nile crocodile (Crocodylus niloticus)		6
	Unidentified reptile	7
	Large cyprinid (Cyprinidae)			1
	Clarias sp.			3
	Clariidae			375
	Siluriform	2	17	22
	Tilapia (Haplotilapinii)	4	2	1
	Unidentified fish	5	14	409
	Total identified vertebrate	301	63	879
	Ostrich eggshell	9		2
	Melanoides tuberculata			3
	Unidentified gastropod			3
	Unidentified bivalve			1
	Unidentified shell	6		3
	Unidentified	782	35	1678
	TOTAL	1098	98	2569

faunal spectrum from Kurub-07, Det Bahari-01 and Lakora-03 according to the NISP (Number of Identified Specimens Parts).

Table VI . 4 :

 VI4 around 9-8 ka cal. BP, then in Lakes IN THE LAKE ABHE BASIN DURING THE AFRICAN HUMID PERIOD Aspects of subsistence and technology according to period and Abhe Lake maximum highstand.

	Settlement period	Aspects of subsistence	Aspects of Technology	Representative sites	Lake Abhe highstand (m a.s.l.)
			Blade oriented debitage
	Early Holocene 11.7 -8.2 ka	Fishing, Hunting, Gathering	Personal adornment (ostrich eggshell beads)	Lakora-03 Raso-02	420 -380
			Bone tools?
			Ground stone	Stromatolis
	Middle Holocene 8.2 -4.2 ka	Fishing, Hunting, Gathering	Blade/bladelet oriented debitage/ curved backed pieces	MS-01/02 SP-01	380
			Simple circular structures	Det-Bahari-01
			Ground stone
			Decorated pottery
			Flake oriented debitage/
	early Late Holocene 4.2 -3 ka	Herding, Fishing, Hunting, Gathering Trade?	curved backed pieces Monumental funerary Personal adornment (diverse materials)	Kurub-07	< 350
			traditions
			Complex oven structures
			Long-distance interaction

Table VI . 5 :

 VI5 Field pedosedimentary description of the Hara-Ide-3 sequence, with indicated the respective Sedimentary Units (SU) and Phases discussed in the text.

Table VI . 6 :

 VI6 Synthetic micromorphological description of HI-3_Mic3 sample.

										PLEISTOCENE -HOLOCENE LAKE ABHE BASIN
	Coarse fraction
	Sample SU µm	Qz		Basalt Olivine	Plg	Pyrx	Gr	Ca	OM	Bone	Shell
	HI-	3	20-	**** /	**	*	**	***	***	/	/	/
	3_Mic3		300
		4	10-	**** /	**	*	*	**	***	/	/	/
			200
		7.1 20-	***	/	**	**	**	****	*	/	/	*
			200
		7.2 20-	***	/	**	**	**	*	*	/	/	/
			100
	Groundmass
	Sample SU c/f distribution	c/f ratio c/f limit	Sorting		Distribution		Orientation
			pattern
	HI-	3	coarse			70/30	10		moderately random		random
	3_Mic3		monic/close fine
			enaulic
		4	coarse monic	80/20	7		moderately random		random
		7.1 coarse monic	90/10	10		very well	random		random
		7.2 coarse monic	90/10	15		very well	random		random
	Microstructure and voids
	Sample SU	Peds		Separation/	Accommodation Porosity		%	Microstructure
					pedality degree
	HI-	3	porous		/		/		Complex	packing	30	Inter-grain micro--
	3_Mic3		crumbs						voids/channels/chambers		aggregate
		4	No peds		/		/		Simple		packing	40	single grain
									voids/channels
		7.1 No peds		/		/		compound	packing	40	single grain
									voids/channels/vughs
									planes
		7.2 No peds		/		/		simple packing voids	50	single grain
	Pedofeatures
	Sample SU	Type		Localisation	Nature	Colour/birefringence	Quantity Remarks
	HI-	3	typic		groundmass	micrite	High birifrangent		Fig VI.13i
	3_Mic3		disorthic
			nodules
		4	typic		groundmass	micrite	High birifrangent		***	Fig VI.13g
			disorthic
			nodules
		7.1	Dendri�c	groundmass	Fe-Mn	Red			**	Fig VI.13f
			orthic										Fig VI.13d
			nodules
		7.1	typic		groundmass	micrite	High birifrangent		**
			disorthic
			nodules
		7.2	Crescent	channels	silt		/			*	Fig VI.13e
			infillings										bioturba�on

Table VI . 8 :

 VI8 Synthetized micromorphological description of Raso-02-Bloc1 sample.

										PLEISTOCENE -HOLOCENE LAKE ABHE BASIN
	Coarse fraction
	Sample SU µm	Qz	Basalt Olivine	Plg	Pyrx	Gr	Ca	OM	Bone	Shell
	Raso-	6	30-200	*** *	***		**	*	**	**	**	*	**
	02-	5.d 20-450	*	***	****		****	**	**	***	/	/	/
	Bloc1	5.c 50-3cm	*	***** ***		***	**	***	***	/	/	/
		5.b 40-1cm	*	***	***		***	**	**	***	/	/	/
		5.a 30-1cm	**	**	**		***	*	*	***	**	*	/
		4.b 30-150	*** /	***		***	**	*	*	*	/	/
	Groundmass
	Sample SU c/f distribution pattern	c/f ratio	c/f limit	Sorting	Distribution	Orientation
	Raso-	6	Double		spaced	20/80	10	Moderately	random		random
	02-		porphyric						sorted
	Bloc1	5.d Double spaced equal	40/60	20	Not sorted	parallel		random
			enaulic
		5.c Double spaced equal	70/30	50	Not sorted	parallel		random
			enaulic
		5.b Coarse monic to double	80/20	40	Not sorted	random		random to parallel
			spaced equal enaulic
		5a Double spaced equal	60/40	30	Poorly		random		random
			enaulic to single spaced				sorted
			porphyric
		4b Coarse Monic		90/10	30	Well sorted random	to	random to parallel
											parallel
	Microstructure and voids
	Sample SU	Peds		Separation/	Accommodation Porosity		%	Microstructure
					pedality degree
	Raso-	6	Angular		Highly		Accommodated Planes and channels	30 Angular blocky
	02-		blocky		separated
	Bloc1	5.d Crumbs		Highly		Not		Complex packing voids	30 Intergrain
					separated		accommodated					microaggregate
		5.c Crumbs		Highly		Not		Complex packing voids	40 Intergrain
					separated		accommodated					microaggregate
		5.b Granules		Highly		Not		Complex packing voids	40 Intergrain
					separated		accommodated					microaggregate
		5.a Subangular		Highly		Par�ally	Planes	and	simple	30 Subangular blocky to
			blocky		separated		accommodated	packing voids			intergrain
													microaggregate
		4b	/		/		/		Simple packing voids	50 Single grain
	Pedofeatures
	Sample SU	Type		Localisation	Nature	Colour/birefringence Quantity Remarks
	Raso-	6	coa�ng		voids		Fe-Mn	Dark red	*	Fig VI.15i
	02-Bloc1	6 5.d	pedorelict b-fabric		groundmass fine frac�on	Oriented clay Micri�c				** ***	Fig VI.15g Fig VI.15f
													Fig VI.15d
		5.c	b-fabric		fine frac�on	Micri�c				***
		5.b	Typic		groundmass	Micri�c	High birefringent	***	Fig VI.15e
			anorthic										bioturba�on
			nodule
		5.b	Concentric		groundmass	Fe-Mn	Dark red	*
			disorthic
			nodule
		5b	Hypocoa�ng Fe-Mn nodules Micri�c				*
		5a	b-fabric		micromass	Micrite	High birefringent	*
		4b	Typic orthic	groundmass	Micrite				*
			nodule

Table VI . 9 :

 VI9 Synthetized micromorphological description of Lak-03_Block1 micromorphological sample.

Table VI .10: Synthetized

 VI

							PLEISTOCENE -HOLOCENE LAKE ABHE BASIN
	Pedofeatures
	Samples SU	Type		Localisation	Nature	Colour/birefringence	Quantity Remarks
	1481	18	typic nodules	groundmass	Mn	black	**	allochtonous
	1482	18	dendri�c	orthic	groundmass	Mn/micrite	dark red	***	/
	1483	18	nodules intergrowth	groundmass	gypsic	high birefringence	***
						euhedral
						crystals
		18	coa�ngs		voids	micrite	High birefringence	**	/
		18	infillings		channels	coarse	/	**	bioturba�on
						material
		41	relict	illuvia�on	groundmass	clay	Yellowish brown	**	/
			features/crusts
		13.2	infillings		channels	coarse frac�on black	**	bioturba�on
		13.1	hypocoa�ngs/orthic	vughs/channels	micrite	high	***	plant
			nodules						evapotranspir
									a�on
		13.2	par�ally		top of m-unit	clay and silt	high birefringence clay		bioturba�on
			fragmented crust
		15.3	fragmented crust	on top of peds	clay	brown/low		1:1	clay
							birefringence		mineral
		15.3	coa�ngs/hypocoa�	vughs	micrite	high birefringence	***	/
			ngs
		15.3	aggregate nodules	groundmass	micrite	high birefringence	***	/
		15.3	peds displacement	groundmass	/	/	**	ver�c
									features,
									surficial cracks
		15.2	illuvia�on		porosity	clay	yellowish	*	2:1	clay
							brown/moderate		minerals
							birefringence
		15.2	nodules	and	groundmass	micrite	high birefringence	**	/
			coa�ngs
		15.1	nodules	and	groundmass	micrite/sparite high birefringence	**	/
			coa�ngs
		15.1	pedorelicts/microp	groundmass	clay	/	**	/
			ans
		9.1	typic nodules	groundmass	Mn	black	**	allochtonous
		9.1	loose con�nuous	channels	coarse frac�on		**	bioturba�on
			infillings
		9.2	typic nodules	groundmass	Mn	black	**	allochtonous

micromorphological description of 1481, 1482 and 1483 samples.

The sedimentary input into the system is not equal to the system output. This effect is characterized by a disproportionate sediment delivery to the continental margins and results in an autogenic sedimentation process relatively decoupled from the climatic forcing[START_REF] Ahnert | Equilibrium, scale and inheritance in geomorphology[END_REF][START_REF] Hoffmann | Sediment residence time and connectivity in non-equilibrium and transient geomorphic systems[END_REF]

2015;[START_REF] Phillips | Sources of nonlinearity and complexity in geomorphic systems[END_REF].

With the aim of simplifying the nomenclature and the reader's understanding, we will use the terms 'Neolithic societies' and 'Food Production Societies' synonymously in this manuscript.

This article was accepted on 3 December

with minor-moderate changes which will be integrated following the submission of this thesis work.

YEARS OF HUMAN LAKESIDE ADAPTATION IN THE ETHIOPIAN AFAR: FISHER-FORAGERS AND THE FIRST PASTORALISTS IN THE LAKE ABHE BASIN DURING THE AFRICAN HUMID PERIOD

"Melanoides tuberculata are found in shallow slow running water (0.6-1.2 m in depth), on a substratum consisting of soft mud, or soft mud and sand. But it has also been reported from relatively deep portions of freshwater pools (3 m deep) with substrata composed largely of rocks"[START_REF] Murray | Melanoides tuberculata (Müller), Las Morras Creek, Bracketville[END_REF]

VI•2 •3

VI.2.3.c Lakora-03

The detailed pedosedimentary description of the Lakora-03 site is presented in Section VI.1.4.b or in Sup. Mat. BII.2. The aim of this section is to present the micromorphological and sedimentological analyses of the Lakora-03 site and to describe the sedimentary formation processes of this site in detail. Subsequently, I will discuss the palaeoenvironmental significance of these processes in relation to the paleo-lake fluctuations model and to regional hydro-climatic variations.

Micromorphological description and interpretation

With the aim to better understand the sedimentary context of the main Lakora-03 prehistoric occupation layer (SU4 and SU5), a micromorphological block (Lak-03-Bloc1) was collected from the sequence covering the layers between SU7 and SU4 (Fig. VI.16a). A micromorphological description is presented and interpreted in detail in the text and is synthesized in Tab. VI.9.

At the bottom of the Lakora-03 sequence, SU7.1 presents the characteristics of a weathered saprolitic basaltic layer, principally composed of altered basalts and a fresh mineralogical assemblage of basaltic origin embedded in a silty clayey slightly structured matrix (Fig. VI.17i). With a well-developed pedality (subangular blocky peds) and partially accommodated plane voids of mainly clayey-organic material,

PLEISTOCENE -HOLOCENE LAKE ABHE BASIN

the characteristic peaks of a material of aeolian origin (60 and 100 µm), which is combined with two peaks at 2 and 5 µm (Fig. VI.20d).

The SU9 paleodune deposit did not undergo any alluvial or colluvial re-deposition processes and does not present any significant post-depositional pedogenic process or vegetal cover development, reflecting hyper-arid environmental conditions. This sedimentary unit correspond the last prehistoric occupation of the Kurub-07 site, including domesticated faunal remains. to Early Holocene high lake stands is likely a result of an extreme post-lacustrine erosional rate on high basaltic outcrops and by difficult to access elevated regions during fieldwork (above 400 m a.s.l.).

PLEISTOCENE -HOLOCENE LAKE ABHE BASIN

VI.2.4.d The 8.2 ka arid event ( 8.2 -7.7 ka cal. BP)

From ∼8.2 ka cal. BP Lake Abhe experienced a drastic drop in level (Fig. VI.24). This is linked to the North Hemisphere 8.2 ka cooling episode [START_REF] Matero | The 8.2 ka cooling event caused by Laurentide ice saddle collapse[END_REF][START_REF] Rohling | Centennial-scale climate cooling with a sudden cold event around 8,200 years ago[END_REF], which is even attested in some East African lakes [START_REF] Costa | Isotopic reconstruction of the African Humid Period and Congo Air Boundary migration at Lake Tana, Ethiopia[END_REF][START_REF] Fischer | Determining the Pace and Magnitude of Lake Level Changes in Southern Ethiopia Over the Last 20,000 Years Using Lake Balance Modeling and SEBAL[END_REF][START_REF] Garcin | East African mid-Holocene wet-dry transition recorded in palaeo-shorelines of Lake Turkana, northern Kenya Rift[END_REF]. In the Lake Abhe basin archaeological sequences associated with this arid event are evident as a sand dune deposition concomitant with a human settlement at the Stromatolis-01 site, and at the MS-01/02 site.

VI.2.4.e The Lake Abhe transgression Phase-V ( 7.7 -4.5 ka cal. BP)

After the short 8.2 ka arid event, the resumption of humid conditions are attested at the Det-Bahari-01 site, with the presence of bones of aquatic fauna (fish, crocodiles, hippopotami) in the archaeological assemblage (∼7.8 -∼7.7 ka cal. BP). Accordingly, a lake highstand was documented from ∼7.7 ka cal. BP until ∼4.5 ka cal. BP (Fig. VI.24).

VI.2.4.f Late Holocene aridification (< 4.5 ka cal. BP)

During the end of the Middle Holocene, Lake Abhe experienced its last prominent regression, which induced massive disruption of environmental patterns across the basin. Following the development of palustrine-like deposits, aeolian sedimentation became the main sedimentation mode. During this period the formation of alternating paleodune/paleosoil sequences represent the sedimentary context of the first Neolithic societies in the Afar region (Kurub-07 site; Fig. VI.24). This record suggests alternation between weak humid events and sandy accumulations during dry phases in a weak vegetated environment. A prominent aridification was observed around this period in the majority of continuous continental African archives [START_REF] Gasse | Hydrological changes in the African tropics since the Last Glacial Maximum[END_REF]. However, the alternation of pedogenic horizons and aeolian deposits (as evidenced in the Kurub-07 and MS-01/02 sequences) indicate that the Middle to Late Holocene arid conditions were punctuated by short wetter phases, probably linked to local organisation patterns of the hydrographic network, or by little humid pulses linked to monsoonal activity at the Awash River headwaters.