PhD Aneta Gorczynska

Ph.D Pierre Stéphan
email: pierre.stephan@univ-brest.fr

Yvan Pailler

Clement Nicolas

Aurélie Pénaud

Ophélie David

Muriel Vidal

Bernard Le Gall

PhD Clément Yvan Pailler

PhD Aurélie Nicolas

PhD Ophélie Penaud

Muriel David

PhD Bernard Vidal

PhD Le Gall

PhD Marie Hélène Ruz
email: ruz@univ-littoral.fr

Professor, Derek W T Jackson
email: d.jackson@ulster.ac.uk

Professor, Ian K Bailiff
email: ian.bailiff@dur.ac.uk

Clément Nicolas

Aurélie Penaud

Holocene evolution of coastal dunes in western France: regional reconstruction from archaeological and historical data

Keywords: Coastal dune, Holocene, western France, archaeology, chronology, sand-drift event Coastal dune, Holocene, western France, archaeology, chronology, sand-drift event

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The formation, stabilization, and mobility of coastal sand-dune systems are controlled by a combination of several driving mechanisms such as coastal sediment supply and distribution, relative sea level (RSL) changes, wind speed, storm-induced erosion and vegetation cover stability [START_REF] Hesp | Dune Coasts[END_REF], as well as by anthropogenic pressures on these drivers. Numerous morpho-stratigraphic and geochronological studies have attempted to define the respective contribution of these factors to the morphological behaviour of coastal dunes at different spatial and temporal scales. Along the Atlantic coasts of Europe, periods of coastal dune mobility have been well reconstructed for the Holocene period in Portugal [START_REF] Clarke | Effects of storminess, sand supply and the North Atlantic Oscillation on sand invasion and coastal dune accretion in western Portugal[END_REF][START_REF] Costas | Sand invasion along the Portuguese coast forced by westerly shifts during cold climate events[END_REF], Ireland (e.g. [START_REF] Wilson | Mid-to late-Holocene coastal dune event stratigraphy for the north coast of Northern Ireland[END_REF], and the Great Britain (e.g. [START_REF] Wilson | Late-Holocene (post-4000 years BP) coastal dune development in Northumberland, northeast England[END_REF][START_REF] Sommerville | Optically stimulated luminescence (OSL) dating of coastal aeolian sand accumulation in Sanday, Orkney Islands, Scotland[END_REF]. Such reconstructions remained limited about the French Atlantic coasts, as being restricted to geochronological investigations of dune systems in the Aquitaine region (SW France, e.g. [START_REF] Clarke | Late-Holocene sand invasion and North Atlantic storminess along the Aquitaine Coast, southwest France[END_REF][START_REF] Bertran | Mapping and chronology of coversands and dunes from the Aquitaine basin, southwest France[END_REF][START_REF] Stéphan | Holocene coastal changes along the Gironde estuary (SW France): new insights from the North Médoc peninsula beach/dune system. Quaternaire[END_REF].

Most of these studies have demonstrated that the morphodynamic of Holocene coastal dunes is characterized by a succession of sand-drift events associated to the deposition of aeolian sand inland alternating with periods of dune stabilization that led to the formation of organic sandy soils. Periods of dune stability have been reconstructed by the radiocarbon dating of dune paleosoils interbedded within aeolian sand deposits (e.g. [START_REF] Wilson | Late-Holocene (post-4000 years BP) coastal dune development in Northumberland, northeast England[END_REF][START_REF] Wilson | Mid-to late-Holocene coastal dune event stratigraphy for the north coast of Northern Ireland[END_REF][START_REF] Clemmensen | The evolution of Holocene coastal dunefields, Jutland, Denmark: A record of climate change over the past 5000 years[END_REF], while Optically Stimulated Luminescence (OSL) dating has been used on sandy material to date sand-drift events (e.g. [START_REF] Sommerville | Optically stimulated luminescence (OSL) dating of coastal aeolian sand accumulation in Sanday, Orkney Islands, Scotland[END_REF][START_REF] Costas | Sand invasion along the Portuguese coast forced by westerly shifts during cold climate events[END_REF]. A few works have also relied on the analysis of archaeological remains and available historical data to consolidate the local chronologies of coastal dune morphological changes (e.g. [START_REF] Sommerville | Optically stimulated luminescence (OSL) dating of coastal aeolian sand accumulation in Sanday, Orkney Islands, Scotland[END_REF][START_REF] Bampton | Little Ice Age catastrophic storms and the destruction of a Shetland Island community[END_REF].

Along the western coasts of France, the dune sedimentary sequences still stay to be accurately dated despite some dating works have been attempted over the last decades (Regnault et al., 1995;[START_REF] Haslett | Geomorphologic and palaeoenvironmental development of Holocene perched coastal dune systems in Brittany, France[END_REF][START_REF] Meurisse-Fort | Enregistrement haute résolution des massifs dunaires. Manche, mer du Nord et Atlantique. Le rôle des tempêtes[END_REF]Van Vliet-Lanoë et al., 2014a, 2014b[START_REF] Nicolas | Flèches de pouvoir à l'aube de la métallurgie, de la Bretagne au Danemark[END_REF].

The absence of well-developed palaeosols within aeolian sand deposits limited the use of radiocarbon dating, except for several interbedded organic-rich humic layers, while no OSL dating was attempted on aeolian sand deposits.

Since the end of the 19th century, numerous archaeological remains have been discovered, excavated and documented within the coastal sand-dune complexes of Brittany. These remains attest to a long-lasted human occupation of coastal sandy environments covering cultural periods that range in age from the Neolithic to the Modern periods. Consequently, the analysis of archaeological remains as chrono-stratigraphic markers appears as an alternative method to identify the stability/mobility phases of dune complexes in this area. Furthermore, a great number of historical sources indicate coastal sand inundations during the last centuries are welldocumented. Numerous detailed descriptions of sand-drift events inundating villages, churches and cultivated fields during the 18th century are available for the northern coast of Brittany [START_REF] Charpentier | Le littoral et les hommes : espaces et sociétés des côtes nord de la Bretagne au XVIIIe siècle[END_REF]. [START_REF] Lageat | Délits d'eau" et "volements de sables", les "offensives éoliennes" sur les côtes bretonnes au XVIIIème siècle[END_REF] also present a review of all archived catastrophic flooding and sand-blow events that occurred in the region during the 18th century.

The aim of this paper is to establish the accurate chronology of coastal dune mobility periods based on the re-evaluation of available archaeological and historical data in Brittany. First, a geoarchaeological database (GAD) was constructed in order to get a complete inventory of archaeological discoveries about the coastal sand-dune systems. The archaeological remains considered in this study concern the different cultural periods over the last 7,000 years, starting from the beginning of the Neolithic up to the present. Second, the archaeological observations previously made during the excavation operations have been systematically reviewed to extract relevant stratigraphic and chronological data. Third, the Harris diagram was used to estimate the periods of sand deposition on a large number of dune complexes. This approach was complemented by a review of historical data and furthermore by a series of radiocarbon dates devoted to beach/dune systems. Finally, all these data were compiled to reconstruct the main periods of coastal dune mobility in western France during the mid-to late-Holocene.

Environmental setting of the study area

Coastal dune morphology

The geological context in Brittany, West Armorican Massif, is dominated by Proterozoic and Paleozoic series that recorded the Variscan orogenic events [START_REF] Le Corre | Le Massif Armoricain / The Armorican Massif[END_REF][START_REF] Ballevre | Histoire Géologique du massif Armoricain : Actualité de la recherche[END_REF]. The diversity of the rocks largely accounts for the varying coastal morphology with alternating hard cliffs on rocky headlands, sandy or gravelly beach/dune systems in bays, and estuarine mouths in the lower parts of incised river valleys. As a result, a great number of sedimentary cells is present and display a large diversity in terms of lithology, morphology, hydrodynamic and sedimentology (Stéphan et al., 2019a).

Brittany coastal dunes stretch over 310 km long, i.e. 12.6% of the present-day coastline (Guilcher and Hallegouet, 1991;Stéphan et al., 2019a) (Fig. 1a). In northern Brittany, dune complexes are less numerous and they occupy small-sized bays sheltered between rocky headlands [START_REF] Meur | Géomorphologie, protection et gestion des dunes de Bretagne septentrionale : éléments de comparaison avec d'autres régions de la Manche occidentale : Cotentin[END_REF]. Normally, the morphology of coastal dunes corresponds to a single sandy ridge isolating lagoons and/or salt marshes from the sea. The dunes culminate between 5 and 6 m above the highest astronomical tide level (HAT). The mean grain size ranges from 200 to 315 µm (Stéphan et al., 2019a). In the southern and western parts of Brittany, dune massifs cover larger areas, usually stretching over several kilometres long. The topography of the largest dune field is characterized by transgressive dunes that are now completely stabilized by vegetation. For example, in the Audierne Bay (Fig. 1a), a >11 km-long dune barrier stretches along the coastline whilst sandy deposits extend up to 2 km inland [START_REF] Hallégouët | Un littoral fragilisé: le sud de la baie d'Audierne, in: Le Pays Bigouden. A la croisée des chemins[END_REF]. In SW Brittany, several perched dune complexes are now disconnected from current beaches and occupy coastal slopes up to 25-30 m above sea level [START_REF] Haslett | Geomorphologic and palaeoenvironmental development of Holocene perched coastal dune systems in Brittany, France[END_REF]. During the 20th century, the intensification of human activities along the coasts led to a large range of morphogenetic impacts on dune barriers. Farmers massively exploited calcareous marine sand on beaches for soil amendment [START_REF] Guilcher | Coastal dunes in Brittany and their management[END_REF][START_REF] Hallégouët | L'engraissement des plages de l'anse de Dinan à l'ouest de la presqu'île de Crozon en Bretagne occidentale[END_REF][START_REF] Yoni | Extractions d'amendements marins et recul de la ligne de rivage en baie de Goulven (Finistère). Les paradoxes de la gestion d'un site[END_REF][START_REF] Blanken | Traezh: sables marins et voiliers de travail autrefois en Bretagne, Skol Vreizh[END_REF]. Coastal sands were also exploited in many quarries for industry purposes. During the 1970s and 1980s, the rapid extension of urbanized areas along the coasts led to the installation of residential housing and its infrastructures and facilities close to the coastal dunes [START_REF] Le Berre | Moderation effect of planning on housing development along the French Atlantic coast: findings from an event history hazard model[END_REF]. Coastal urbanisation and sand extraction led to the incidental discovery of many archaeological sites.

Between the 1960s and the 1980s, the rapidly increasing number of tourist on beaches led to a strong degradation of the dune vegetation and a remobilization of the sand, caused by trampling, camping, riding, driving, motorcycling, etc.

Besides, over the last few decades, the vast majority of coastal dunes have experienced a shoreline retreat in Brittany (Stéphan et al., 2019a), especially when severe storm events are combined with high spring tide levels [START_REF] Suanez | Empirical Parameterization of Wave Runup and Dune Erosion during Storm Conditions on a Natural Macrotidal Beach[END_REF]Stéphan et al., 2019a). The erosion trend of many beach/dune systems reflects a scarcity of coastal sand supplies that have been aggravated by human actions during the last three centuries. Due to the high elevations of the dune ridge, erosional processes led to the formation of scarps on the dune front, in turn favouring the discoveries of buried archaeological remains.

From the 1980s onward, the creation of the Coastal Conservancy (Conservatoire du Littoral) in 1975, conducted after the National Trust of Britain, facilitated the practice of "soft management" of dune environments. This public agency either bought or received coastal lands and delegated their management to public councils or private nature conservation societies [START_REF] Deboudt | Recent developments in coastal zone management in France: The transition towards integrated coastal zone management (1973-2007)[END_REF]. Marram grass plantations and wooden fences were installed on the most degraded dunes. These soft solutions were very efficient in preventing aeolian erosion and blowouts and in channeling pedestrian flows through the coastal dunes [START_REF] Guilcher | Coastal dunes in Brittany and their management[END_REF]. Where legislation protects the coastal environment, smaller number of archaeological artefacts are discovered. However, a few archaeological sites have been the subject of specific scientific research programs and have provided fruitful data (e.g. Beg ar Loued site, Molène Island, W Brittany: [START_REF] Pailler | Une maison sous les dunes : Beg ar Loued, Île Molène, Finistère. Identité et adaptation des groupes humains en mer d'Iroise entre les IIIe et IIe millénaires avant notre ère[END_REF]Le Douet site, Hoëdic Island, S Brittany: Large, 2014).

Currently, many coastal dunes are a part of the natural protected areas in Brittany. The dunes are stabilized by dense vegetation cover and only the foredunes are partly mobile [START_REF] Meur | Géomorphologie, protection et gestion des dunes de Bretagne septentrionale : éléments de comparaison avec d'autres régions de la Manche occidentale : Cotentin[END_REF][START_REF] Suanez | Dune recovery after storm erosion on a high-energy beach: Vougot Beach, Brittany (France)[END_REF]. During the summer period, the upper beaches are colonized by annual halonitrophilous species (e.g. Atriplex laciniata, Cakile maritima, Salsola kali). The vegetation of the foredunes is mainly composed of psammophilous herbaceous species such as Ammophila arenaria or Agropyrum junceum [START_REF] Meur | Géomorphologie, protection et gestion des dunes de Bretagne septentrionale : éléments de comparaison avec d'autres régions de la Manche occidentale : Cotentin[END_REF][START_REF] Favennec | Guide de la flore des dunes littorales non boisées: de la Bretagne au sud des Landes[END_REF]. In the inner parts of the dune complexes are colonized by dense grasslands with a wide variety of plant species. The more sheltered and partially decalcified sectors are colonized by vegetation dominated by eagle ferns (Pteridium aquilinum), gorse (Ulex spp.), and heather (Erica spp., Calluna vulgaris). Finally, some dunes have a wooded cover mainly composed of coniferous trees such as maritime pine (Pinus pinaster) and Lambert cypress (Cupressus macrocarpa).

Holocene coastal changes

The post-glacial RSL changes were recently reconstructed along the western coast of France, based on a reappraisal of 66 sea level index points [START_REF] Stéphan | Évolution du niveau marin relatif à l'Holocène le long des côtes françaises de l'Atlantique et de la Manche : réactualisation des données par la méthode des « sea-level index points[END_REF][START_REF] García-Artola | Holocene sea-level database from the Atlantic coast of Europe[END_REF]. Slight differences in the rates of RSL rise were attributed to glacio-isostatic adjustments and reflect the migration of the European peripheral forebulge from the northeast to the northwest of Brittany during the Holocene [START_REF] Goslin | A new Holocene relative sea-level curve for western Brittany (France): Insights on isostatic dynamics along the Atlantic coasts of north-western Europe[END_REF][START_REF] García-Artola | Holocene sea-level database from the Atlantic coast of Europe[END_REF]. A similar pattern of RSL change was depicted from spatio-temporal statistical model predictions for north, west and south Brittany. The rate of marine transgression slowed down at 7,000 cal BP, with RSL positions at -6.50 ±1.1 m, at -7.35 ±0.9 m, at -5.75 ±1 m in north, west and south Brittany, respectively [START_REF] García-Artola | Holocene sea-level database from the Atlantic coast of Europe[END_REF]. In response to the RSL rise slowdown, modern coastline and costal barriers formed, with basal Phragmites peats occupying the base of the coastal sequences. In Brittany, these deposits have been dated in the range 7,000-4,500 cal BP and suggest a large extension of swampy environments behind relatively stable coastal barriers [START_REF] Morzadec-Kerfourn | Variations de la ligne de rivage armoricaine au Quaternaire : analyses polliniques de dépôts organiques littoraux[END_REF][START_REF] Goslin | Holocene relative sea-level changes in western Brittany (France) between 7600 and 4000 cal. BP: Reconstitution from basal-peat deposits[END_REF][START_REF] Stéphan | Holocene salt-marsh sedimentary infilling and relative sea-level changes in West Brittany (France) using foraminifera-based transfer functions[END_REF].

Estuaries infilled with tidal flat and salt-marsh sediments [START_REF] Menier | The Holocene transgression as recorded by incised-valley infilling in a rocky coast context with low sediment supply (southern Brittany, western France)[END_REF]Penaud et al., 2020). With the stabilization of the RSL in the mid-to late-Holocene, alongshore sediment transport favoured the development of coastal spits and associated salt-marshes in low-lying coastal areas [START_REF] Lespez | Middle to Late Holocene landscape changes and geoarchaeological implications in the marshes of the Dives estuary (NW France)[END_REF][START_REF] Stéphan | Mise au point sur l'évolution géomorphologique et le devenir des flèches de galets du Loc'h de Landévennec (Bretagne, France)[END_REF][START_REF] Stéphan | Holocene salt-marsh sedimentary infilling and relative sea-level changes in West Brittany (France) using foraminifera-based transfer functions[END_REF]. In some embayed beaches, sand supply locally favoured the coastal sand-dune movement on surrounding slopes, as evidenced by the perched dunes dated to ca. 5,280 cal BP and ca. 4,350 cal BP at Pen Hat beach [START_REF] Meurisse-Fort | Enregistrement haute résolution des massifs dunaires. Manche, mer du Nord et Atlantique. Le rôle des tempêtes[END_REF] and Gwendrez beach [START_REF] Haslett | Geomorphologic and palaeoenvironmental development of Holocene perched coastal dune systems in Brittany, France[END_REF], respectively. Between 2,900 and 2,700 cal BP, subtidal sedimentary prisms and coastal barriers recorded deep morphosedimentary changes. Sharp erosional contacts and/or sedimentary hiatuses were systematically observed in the coastal sedimentary sequences, indicating increased hydrodynamics and barrier breaching [START_REF] Fernane | Coastal palaeoenvironmental record of the last 7 kyr BP in NW France: Submillennial climatic and anthropic Holocene signals[END_REF][START_REF] Goslin | Holocene relative sea-level changes in western Brittany (France) between 7600 and 4000 cal. BP: Reconstitution from basal-peat deposits[END_REF][START_REF] Goslin | A new Holocene relative sea-level curve for western Brittany (France): Insights on isostatic dynamics along the Atlantic coasts of north-western Europe[END_REF][START_REF] Stéphan | Holocene salt-marsh sedimentary infilling and relative sea-level changes in West Brittany (France) using foraminifera-based transfer functions[END_REF][START_REF] Ehrhold | Fossil maerl beds as coastal indicators of late Holocene palaeo-environmental evolution in the Bay of Brest (Western France)[END_REF]). After ca. 2,700 cal BP, marsh sedimentary sequences record continuous accretion to the present day, suggesting a new period of coastal barrier stabilization [START_REF] Stéphan | Colmatage sédimentaire des marais maritimes et variations relatives du niveau marin au cours des 6 000 dernières années en rade de Brest (finistère)[END_REF][START_REF] Stéphan | Holocene salt-marsh sedimentary infilling and relative sea-level changes in West Brittany (France) using foraminifera-based transfer functions[END_REF]. However, coastal sand-dune systems appear to have experienced several phases of destabilization over the past 3,000 years. Based on the analysis of 18 sediment cores distributed along the coast of Brittany, [START_REF] Vliet-Lanoë | Holocene formation and evolution of coastal dunes ridges, Brittany (France)[END_REF] recently identified four periods of aeolian sand movement during the following periods: ca. 3,050-2,300 cal BP (Phase 1), ca. 1,600-1,150 cal BP (Phase 2), ca. 1,150-750 cal BP (Phase 3) and from ca. 600 cal BP (Phase 4).

Wind, wave and tide conditions

In western France, the coastal hydrodynamic pattern jointly depends on tides, wind and wave conditions. The climate of Brittany is characterized by temperate oceanic conditions with generally mild, wet winters and cool summers. Westerly-and south-westerly-directed winds are dominant (Fig. 1b). Wind speeds are strongly seasonally-modulated, with winter wind speeds almost 50% higher than summer values offshore [START_REF] Bentamy | Spatial and temporal characteristics of wind and wind power off the coasts of Brittany[END_REF].

The mean wind speed values are about 9 m/s and 5.50 m/s during the winter and summer seasons, respectively. During severe winter storm events, wind speeds can reach 50 to 60 m/s. On average, winds are mostly westerly-directed in winter. In spring, dominant winds are northwesterly-and south-westerly-directed in S and N Brittany, respectively, whereas they are mostly north-westerly-directed over the area during summer and fall [START_REF] Bentamy | Spatial and temporal characteristics of wind and wind power off the coasts of Brittany[END_REF].

As similarly displayed by wind conditions, the wave climate is energetic and strongly seasonally-modulated, especially in western Brittany where high-energy winter swells and storm waves come from the W-NW with significant wave height frequently exceeding 5 m.

Wave refraction and diffraction are significant near the coast due to the presence of numerous islands and reefs.

The coast is typically a meso-to macro-tidal environment with a highly variable tidal range from the NE to the W (Fig. 1a). The maximum tidal range reaches up to 14 m during spring tides in the Mont-Saint-Michel Bay and gradually decreases westward with a maximum ranging from 6.9 m to 7.6 m in the Iroise Sea. Along the southern coast of Brittany, the tidal range goes from 6.1 m in the Bay of Penerf to a minimum value of 3.3 m in the Gulf of Morbihan (Fig. 1a).

The Geoarchaeological Database (GAD) : methodology

General presentation

A geoarchaeological database (GAD) (see supplementary table 1) grouping all archaeological remains discovered in the coastal dunes of western France has been created from three main sources of information which are: (i) the national archaeological geodatabase (called "carte archéologique nationale") that has inventoried archaeological discoveries in France since the 1975s [START_REF] Chaillou | L'application Patriarche. Inventaire informatique de la carte archéologique nationale[END_REF][START_REF] Lorho | Patrimoine archéologique et système d'information géographique[END_REF]; (ii) archaeological operation archives (reports and notes taken in the field during excavations); and (iii) scientific papers published in local and national journals from the end of the 19th century. Three main criteria were utilised:

(i) the location of the archaeological remains, (ii) their stratigraphic position within the aeolian sand deposits, and (iii) their age or chronological attribution (Fig. 2a and2b).

Site location

In this study, we have only considered the archaeological finds located within the Holocene coastal sandy formations, whose spatial extension inland is provided by the 1:50,000 geological maps edited by the BRGM (Geological and Mining Research Institute). Most of the locations of discoveries made before the 1980s are poorly described and the geographical position of the archaeological sites was deduced from iconographic sources available in publications (maps, plans, photographs) and from local place names. For the most recent discoveries, the location of archaeological remains is based on more precise GPS or DPGS measurements. The local geographic coordinates have been converted into the international WGS84 system. The GAD includes 221 archaeological sites covering cultural periods from the Neolithic to present. These remains are located within 78 dune complexes, evenly distributed along the northern, western and southern coasts of Brittany (Fig. 3, Tab. 1).

Fig. 3.

Location map of the studied coastal sand-dune systems. The numbers refer to sites listed in Table 1.

Stratigraphic position within the sandy sequences

Stratigraphic positions of the archaeological remains within the aeolian sand deposits were systematically integrated into the database according to three main configurations which are below, interbedded between and above (an) aeolian sand deposit(s) (Fig. 2b). Only archaeological sites with a clearly established stratigraphic context were included. Sites without significant sandy deposits (thickness < 0.2 m) were not taken into account.

Chronological attribution

Ages of the remains were determined from a reappraisal of available archaeological data (absolute dating, architectural elements, type of monuments, and presence of ceramic, lithic and/or metallic materials). Based on the quality of these data, two degrees of chronological precision were attributed. The first precision degree characterizes poorly documented remains whose ages can only be related to the major cultural periods recognized in Brittany (Fig. 4). In this case, chronological attributions show large uncertainties, especially for the Neolithic (±1,500 years) and Bronze Age (±700 years) periods. The second precision degree characterizes well-documented remains, with detailed archaeological information and, in some cases, absolute dating data (AMS-14 C and OSL dates, archaeomagnetism). Ages of the remains were then related to different subdivisions of the major cultural periods in Brittany (Fig. 4). This second temporal division offers chronological attributions of a three centuries or less. Chronological attributions of remains are subdivided into two precision degrees corresponding to major cultural periods (first degree) and to subdivisions of the latter (second degree).

Additional data

Three types of additional data were integrated into our GAD: (i) archaeological, (ii) sedimentological, and (iii) historical (see suplementary table 1 and2). First, some archaeological discoveries carried out on the beaches of the studied beach/dune systems were taken into account as the remains systematically corresponded to ancient terrestrial sites (habitats, burial sites) that were buried under the present-day coastal dunes. Therefore, they constitute temporal markers that pre-date the aeolian sand deposits and which are used for calibrating the first phases of coastal dunes mobility. Additional chrono-stratigraphic markers are supplied by accurately dated (radiocarbon) intertidal organic-rich peat deposits outcropping on the beaches of the studied beach/dune systems where they are commonly associated with the development of back-barrier marshes [START_REF] Morzadec-Kerfourn | Variations de la ligne de rivage au cours du Post-glaciaire le long de la côte nord du Finistère. Analyses polliniques de tourbes et de dépôts organiques littoraux[END_REF][START_REF] Morzadec-Kerfourn | Variations de la ligne de rivage armoricaine au Quaternaire : analyses polliniques de dépôts organiques littoraux[END_REF][START_REF] Visset | Évolution du littoral et du paysage, de la presqu'île de Rhuys à la rivière d'Étel (Massif armoricain -France), du Néolithique au Moyen Âge[END_REF][START_REF] Goslin | Reconstitution de l'évolution du niveau marin relatif holocène dans le Finistère (Bretagne, France): dynamiques régionales, réponses locales[END_REF]. In direct contact with the bedrock, these deposits form the base of coastal sedimentary sequences, pre-dating the overlying aeolian sand deposits. Third, historical data (archives, old maps, historical photos) available from the last three centuries have been used to chronologically constrain the most recent phases of dune mobility. The Cassini map (produced between 1683 and 1744) and the Etat-Major map (edited between 1820 and 1866) were used to identify the extension of the dunes during this period. However, with regards to the criteria above, they are considered to be 1st degree chronological markers (not very precise) (Fig. 4) because the dune morphologies are not represented with sufficient detail as to distinguish between mobile and fixed dunes. For the last few decades, aerial photos produced by the Institut Géographique National (IGN) have been examined to complete the observations. Locally, images of coastal landscapes represented on old postcards were also studied. Lastly, we used recent historical syntheses that provide an in-depth analysis of coastal dune dynamics over the last centuries in Brittany [START_REF] Charpentier | Le littoral et les hommes : espaces et sociétés des côtes nord de la Bretagne au XVIIIe siècle[END_REF][START_REF] Lageat | Délits d'eau" et "volements de sables", les "offensives éoliennes" sur les côtes bretonnes au XVIIIème siècle[END_REF].

Radiocarbon dates

AMS-14 C dates available in literature were included in the GAD because they provide information on the chronology of archaeological sites (92 dates) and on the age of peat deposits (14 dates) located in the intertidal zone (see supplementary table 3). About organic sediments, conventional radiocarbon ages were calibrated with OxCal v.4.4.4 software [START_REF] Ramsey | Summer Temperature Variations in the European Alps, a.d. 755-2004[END_REF] using the IntCal20 calibration curve [START_REF] Heaton | Marine20-The Marine Radiocarbon Age Calibration Curve (0-55,000 cal BP)[END_REF]. Three dates obtained on marine shells in the Hoedic Island site were calibrated using the Marine20 curve [START_REF] Heaton | Marine20-The Marine Radiocarbon Age Calibration Curve (0-55,000 cal BP)[END_REF].

Besides, the marine reservoir correction database was used (Reimer and Reimer, 2001) to determine the average reservoir age along the coasts of Brittany. An error-weighted mean of five ΔR values from sites located within ca. 150 km of the archaeological sites concerned yielded an average ΔR of -95±62 years (Tisnérat-Laborde et al., 2010). Calibrated radiocarbon ages were reported in calendar years before present (cal BP) relative to 1950 AD with an uncertainty of 2 sigma. Despite a large number of dates (n=106), no statistical analysis of the chronological distribution of ages was attempted due to the over-representation of some archaeological sites documented by a large number of AMS-14 C dates (e.g. 36 AMS-14 C dates for Beg ar Loued site on Molène Island, 21 AMS-14 C dates for Groah Denn site on Hoedic Island).

Archaeological-based dune stratigraphic reconstruction: Harris matrix

A Harris matrix [START_REF] Harris | Principles of archaeological stratigraphy[END_REF] has been applied to each dune complex under study in order to reconstruct the local chronology of dune mobility periods (Fig. 2b). Conceptualized in the 1970s, the Harris matrix is widely used in archaeological research to document and represent stratigraphic relationships during excavations (e.g. [START_REF] Kobyliński | Polish medieval excavations and the Harris Matrix: applications and developments[END_REF][START_REF] Trócoli | The contribution of the Harris Matrix to the development of Catalan archaeology[END_REF][START_REF] Dye | Archaeological sequence diagrams and Bayesian chronological models[END_REF]. The Harris matrix is a sequence diagram that represents the relative temporal succession of all stratification units observed within an archaeological site (e.g. sedimentary layers, constructions, removal or erosional surfaces). In this paper, we consider each coastal dune as an archaeological site, i.e. as a coherent set of long-lasted deposits resulting from various natural and anthropogenic events. The archaeological remains are considered as uniform stratigraphic features in relation to the bedrock and to the lower and/or upper sand dune layers. Three main stratigraphic relationships are represented (Fig. 2b): (i) superposition of layers observed in the field; (ii) lateral correlation between layers of the same nature (especially dune sand layers), and (iii) stratigraphic relationships not observed in the field, but inferred from the nature and location of the stratigraphic units (e.g. foreshore sites pre-dating the formation of coastal dunes). Chronological data (age of archaeological sites and AMS-14 C dates) were integrated into the Harris matrix to infer the maximum age of each aeolian sand layer. The beginning of a sand-drift event was defined from the age of the archaeological remains subsequently buried by the corresponding aeolian sand deposit. The end of a sand-drift event corresponds to the human settlements at the top of the corresponding aeolian sand deposit.

Results

This section describes the information provided by several of the archaeological discoveries in the coastal dunes of Brittany according to cultural periods (Fig. 4 and5). Then, the Harris matrix applied to several dune complexes is described to illustrate the approach used to reconstruct the dune chrono-stratigraphy. Finally, an evolutionary temporal template of dune mobility phases is proposed based on the compilation of the corresponding Harris matrices. 1.

Coastal dune archaeological remains

Neolithic period (ca. 6,850-4,100 cal BP)

The Neolithic archaeological remains correspond either to megalithic monuments (n=28) found at the base of dune sedimentary sequences, or to lithic and ceramic material found within the underlying clayey-silty palaeosols (n=12). Because of their basal stratigraphic positions, the Neolithic remains can be considered only as terminus post-quem that provide information on the earliest aeolian sand-drift events. The Neolithic megaliths discovered in the coastal dunes of Brittany correspond to either (i) burial monuments of the mound/cairn type (n=17), or (ii) a series of standing stones (n=8). The available archaeological data suggest that these monuments are associated with long periods of human occupation and multiple reuses subsequent to their construction. Without numerous AMS-14 C dates, ages of the megalithic monuments are difficult to precisely establish from the archaeological data.

The Bronze Age period (ca. 4,100-2,750 cal BP)

Most of the Bronze Age sites date to the Early Bronze Age (ca. 4,100 to ca. 3,550 cal BP) and were found at the base of aeolian sand deposit sequences (n=33) within silty-clayey paleosoils.

They represent terminus post-quem to the overlying aeolian sand deposits. These remains mainly correspond to stone cists (individual burials [START_REF] Briard | Les Tumulus d'Armorique, L'âge du bronze en France[END_REF][START_REF] Blanchet | Le Campaniforme et la genèse de l'âge du Bronze ancien en Bretagne : vers une nouvelle donne[END_REF] discovered along the dune fronts affected by marine erosion. At Port-Blanc (Penvénan, N Brittany, site no.11 on Fig. 5b), after episodes of shoreline retreat, numerous cists forming part of a necropolis have been discovered since the 1930s [START_REF] Merlet | Découvertes néolithiques à Roc'has en Penvénan[END_REF][START_REF] Giot | Informations archéologiques[END_REF][START_REF] Le Roux | Circonscription de Bretagne[END_REF]. These burials were dug in paleosoil covered by a ca. 2 m-thick aeolian sandy deposit (Fig. 6a and6b). At Port-Mélite (Groix Island, S Brittany, site no.63 on Fig. 5b), a similar burial cist contained a well preserved female skeleton which yielded a radiocarbon age of 3,888-3,692 cal BP [START_REF] Le Roux | Ile de Groix (56). Découverte d'une tombe en coffre à port-mélitte[END_REF][START_REF] Nicolas | L'artisanat à l'âge du Bronze ancien en Bretagne[END_REF][START_REF] Nicolas | Flèches de pouvoir à l'aube de la métallurgie, de la Bretagne au Danemark[END_REF]. The burial was dug into silty paleosoil in turn overlaid by a 0.5 m-thick aeolian sand deposit.

Four discovered archaeological sites are interbedded within the aeolian sand deposits. On the Kerminihy dunes (Erdeven, S Brittany, site no.65 on Fig. 5b), numerous scatters of coastal flint debitage have been found in an extensive sand quarry [START_REF] Chapuy | Erdeven (56[END_REF][START_REF] Chapuy | Les petits ateliers de taille de la carrière de Kerminihy[END_REF]. Large quantities of lithic material and ceramic fragments indicated temporary settlements between the Final Neolithic and the Early Bronze Age (4,850-3,550 cal BP; [START_REF] Chapuy | Erdeven (56), Kerminihy. Rapport de sondage[END_REF]. This archaeological level is interbedded with two aeolian sand deposits. As the remains are not associated with a humic horizon, these data suggest a human presence during a phase of aeolian sand deposition.

On the Saint-Nicolas Island dune (Glénan archipelago, SW Brittany, site no.58 on Fig. 5b), the remains of a wall dating from the Early Bronze Age are interbedded with two sand-dune deposits, indicating perennial occupation during dune stabilization (see section 4.2). On the Béniguet Island (Molène archipelago, SW Brittany, site no.41 on Fig. 5b), a shell-midden dated to the Early Bronze Age (4,140-3,908 cal BP) is also interbedded within aeolian sand deposits (see section 4.2). Its upper part clearly shows a layer of humic sand, which also indicates stabilization of the dune massif during this period [START_REF] Cudennec | Les coquilles de Patella sp. issues d'amas coquilliers anthropiques, archives culturelles et environnementales des environnements côtiers[END_REF][START_REF] Pailler | Première campagne de fouille programmée sur le site multi-périodes de Porz ar Puns, Ile de Béniguet (Le Conquet, Finistère)[END_REF].

The Iron Age period (ca. 2,750-2,000 cal BP)

A total of 54 archaeological sites attributed to the Iron Age have been inventoried within 35 coastal dune complexes in Brittany (Fig. 5c). A large number of these remains are interbedded within aeolian sand deposits (n=20) and were mainly dated at the Late Iron Age (ca.

2,400-2,000 cal BP). The sea salt industry developed along the Brittany coasts during this period is documented by remains, called "briquetages", containing large quantities of terracotta waste, containers and characteristic oven remains [START_REF] Daire | Le sel des Gaulois, Collection des Hespérides[END_REF]. For example, the Moulin de la Rive site (Locquirec, W Brittany, site no.14 on Fig. 5c), corresponds to a vast complex of habitats and sea salt production workshops attributed to the end of the Late Iron Age (2,100-2,000 cal BP) [START_REF] Giot | L'Habitat protohistorique du Moulinde-la-Rive en Locquirec (Finistère)[END_REF][START_REF] Giot | Un habitat protohistorique le Moulin de la Rive en Locquirec (29), Travaux du laboratoire « Anthropologie, Préhistoire, Protohistoire, Quaternaire Armoricains[END_REF]. The archaeological levels are interbedded with two layers of aeolian sand, reaching 2.5 m-and 1.5 m-thick for the lower and upper deposit, respectively.

Gallo-Roman period (ca. 2,000-1,470 cal BP)

For the Gallo-Roman period, the most characteristic archaeological remains discovered in the coastal dunes correspond to villae maritimae dated to the High Roman Empire (between ca.

1,970 and 1,660 cal BP). On the dunes of Sables-d'Or-les-Pins (Fréhel, NE Brittany, site no.5 on Fig. 5d), such a villa associated to a thermal complex was dated at ca. 1,775-1,755 cal BP (Fig. 6c). The abandonment appears to have been caused by a fire dated to ca. 1,680 cal BP [START_REF] Langouët | Villa romaine de Sables d'Or -les Pins[END_REF]. Archaeological excavations shown that the residences were built on a paleosoil covered by a 2.3 m-thick aeolian sand deposit [START_REF] Langouët | Villa romaine de Sables d'Or -les Pins[END_REF][START_REF] Bizien-Jaglin | Carte archéologique de la Gaule: Côtesd'Armor (22)[END_REF][START_REF] Bot | Les thermes gallo-romains en Armorique[END_REF]. At the Légenèse site (Carnac, S Brittany, site no. 72 on Fig. 5d), a similar thermal complex dated between 1,850 and 1,650 cal BP was discovered after an episode of shoreline retreat [START_REF] Le Rouzic | Carnac. Fouilles faites dans la Région. Ruines gallo-romaines de Légénèse[END_REF]. The foundations of this villa overlaid aeolian sand deposits.

The site was also abandoned after a fire and the remains were later buried under an aeolian sand deposit about 2.5 m-thick [START_REF] Bot | Les thermes gallo-romains en Armorique[END_REF].

The medieval period (ca. 1,470-450 cal BP)

Only 12 medieval archaeological sites distributed within 8 dune complexes have been discovered on the coastal dunes of Brittany (Fig. 5e). Two medieval habitats have been particularly well documented. The site of Pen-er-Malo (Guidel, S Brittany, site no.61 on Fig. 5e) shows a group of 6 dry-stone houses built on a basal paleosoil overlaid by aeolian sand deposits [START_REF] Bertrand | Le site médiéval de Pen er Malo en Guidel. Le bâtiment A : étudie du matériel[END_REF][START_REF] Bertrand | Un village côtier du XIIe siècle en Bretagne : Pen-er-Malo en Guidel (Morbihan)[END_REF]. The archaeological remains were radiocarbon-dated between 972 and 567 cal BP. The abandonment of the houses has been attributed to a sand inundation that buried the entire hamlet [START_REF] Bertrand | Le site médiéval de Pen er Malo en Guidel. Le bâtiment A : étudie du matériel[END_REF][START_REF] Bertrand | Un village côtier du XIIe siècle en Bretagne : Pen-er-Malo en Guidel (Morbihan)[END_REF]. At the Tronoën site (Plomeur, SW Brittany, site no.50 on Fig. 4) two houses dates at ca. 950 to 750 cal BP were buried beneath aeolian sand deposits (see section 4.2.5). The most accurate data for this period comes from a necropolis located at Saint-Urnel (Plomeur, SW Brittany, site no.50 on Fig. 5e) where three levels of burials are interbedded in an aeolian sand deposit (see section 4.2.5).

Modern and contemporary periods (ca. 450-0 cal BP to present)

Between the 16th and 17th centuries, the destruction of arable land, houses and churches by aeolian-sand inundation is reported in many archival documents. In the village of Tréménac'h (Plouguerneau, NW Brittany, site no.31 on Fig. 5f), the first invasions of aeolian sand occurred during the mid-16th century [START_REF] Lageat | Délits d'eau" et "volements de sables", les "offensives éoliennes" sur les côtes bretonnes au XVIIIème siècle[END_REF]. In 1722, nearly two-thirds of the area was inundated by sand dunes [START_REF] Lageat | Délits d'eau" et "volements de sables", les "offensives éoliennes" sur les côtes bretonnes au XVIIIème siècle[END_REF]. Historical sources indicate that the church was abandoned in 1728 after being buried under 3 m-thick deposit of wind-blown sand (Perennec, 2004;[START_REF] Charpentier | Le littoral et les hommes : espaces et sociétés des côtes nord de la Bretagne au XVIIIe siècle[END_REF]Reille-Taillefert, 2010). On the Batz Island (NW Brittany, site no.15 on Fig. 5f), the church of Sainte-Anne, built around the 10th century, was severely damaged under the weight of dune sands and abandoned in the 17th century [START_REF] Moret | Prospection-inventaire archéologique et historique de l'Ile de Batz[END_REF][START_REF] Charpentier | Le littoral et les hommes : espaces et sociétés des côtes nord de la Bretagne au XVIIIe siècle[END_REF][START_REF] Lageat | Délits d'eau" et "volements de sables", les "offensives éoliennes" sur les côtes bretonnes au XVIIIème siècle[END_REF]. Similarly, the chapel of Saint-Guevroc (Tréflez, NW Brittany, site no.22 on Fig. 5f), built between the 15th and 16th centuries, was quickly abandoned because of its burial under sand deposits. In 1869, the church was almost invisible under the sandy cover [START_REF] Madeleneau | Ruines de la chapelle de l'abbaye de Guévroc[END_REF]. Other localities in western Brittany (e.g. Landeda, Santec) also experienced sand inundation during the 18th century [START_REF] Charpentier | Le littoral et les hommes : espaces et sociétés des côtes nord de la Bretagne au XVIIIe siècle[END_REF]. In the village of Lampaul-Plouarzel (NW Brittany, site no.37 on Fig. 5f), historical documents dating from the 18th century mention the frequent destruction of agricultural crops due to the deposition of aeolian sands [START_REF] Charpentier | Le littoral et les hommes : espaces et sociétés des côtes nord de la Bretagne au XVIIIe siècle[END_REF]. The migration of the mobile coastal dunes reached the church of the village in 1760. The religious authorities then decided to move the services to another church because the building may have collapsed under the weight of the sand. From the 19th century, numerous historical archives indicate a progressive stabilization of coastal dunes. In 1838, a new church was built on the ruins of the old church of Lampaul-Plouarzel (Thomas, 1889) (Fig. 6d). In the area of Tréflez, the church of Saint-Guevroc was unearthed in 1872 and rebuilt from 1895 to 1898 once the dunes were stabilized [START_REF] Menant | La Bretagne[END_REF]. The stabilization of the dunes from the 19th century is also indicated by the presence of numerous seaweed ovens built on the surface of the coastal dunes to produce soda [START_REF] Arzel | Les goémoniers[END_REF] (Fig. 6e and6f). In this study, 47 seaweed ovens were inventoried and provide evidence of local stability of coastal dunes since the 19th century.

Chrono-stratigraphical reconstructions of coastal dune complexes

Dunes of the Molène Island

The recent excavation of a Bronze Age habitat in the coastal dunes of the Molène Island (W Brittany, site no.39 on Figs. 3 and7) provided numerous precise archaeological and stratigraphic data. The different phases of human occupation have been reconstructed based on 36 AMS-14 C dates [START_REF] Pailler | Une maison sous les dunes : Beg ar Loued, Île Molène, Finistère. Identité et adaptation des groupes humains en mer d'Iroise entre les IIIe et IIe millénaires avant notre ère[END_REF]. The earliest evidence of occupation was attributed to the Early Neolithic, but the most abundant remains are associated with two oval houses composed of dry stone walls (Fig. 7b) dated to the Early Bronze Age. The oldest house was occupied between ca. 4,150 and ca. 3,900 cal BP and the youngest one between ca. 3,950 and ca. 3,700 cal BP [START_REF] Pailler | Une maison sous les dunes : Beg ar Loued, Île Molène, Finistère. Identité et adaptation des groupes humains en mer d'Iroise entre les IIIe et IIe millénaires avant notre ère[END_REF]. After abandonment, the ruins of these houses were used as sheep barns between ca. 3,800 and ca. 3,250 cal BP. The stratigraphic data reveal that much of the site was subsequently buried by a 1 m-thick aeolian sand deposit. Several contemporary remains associated with hunting, seaweed harvesting, and agricultural activities have been found at the top of these sandy sequences (Fig. 7e). A well-preserved human skeleton dated from the 18th to 19th centuries was also discovered in a pit dug into the dune and the buried soil. From these data, a single sand-drift event was identified between ca. 3,250 and 200 cal BP (Fig. 7d). [START_REF] Hyvert | Quiberon (56). Habitat de Goulvars. Rapport sauvetage urgent[END_REF]; (e) Simplified stratigraphic section of the Goulvars II site, after [START_REF] Hyvert | Quiberon (56). Habitat de Goulvars. Rapport sauvetage urgent[END_REF].

Dunes of Goulvars

A late Iron Age settlement (Goulvars II site; ca. 2,060-2,000 cal BP) was discovered interbedded within sandy sediments indicating the stabilisation of the dune complex at this period. Several dry-stone houses whose foundations were dug in underlying aeolian sand deposits occur beneath two layers of white aeolian sands (Figs 8c, 8d and 8e). The latter are 2.5 m-thick and are separated by an undated humic horizon (Fig. 8e). From these records, the first sand-drift event is identified between the Early Bronze Age and the Late Iron Age, i.e. in the range ca. 3,550 and 2,060 cal BP. A further Roman-age settlement (Goulvars I site) has been discovered at the base of the aeolian stratigraphic sequence. This is the most recent site positioned below the sand deposit and it represents terminus post-quem for the second sanddrift event. At the top of the Goulvars dunes, 4 kelp kilns indicate the stabilisation of the massif during the 19th century. From these data, a second sand-drift event has been identified between the Roman and the Contemporary period ca. 1,450-120 cal BP.

Dunes of the Béniguet Island

On the Béniguet Island (Le Conquet, W Brittany, site no.41 on Fig. 3), a loessic paleosoil is overlaid by 3 shell-middens interbedded within aeolian sand deposits [START_REF] Cudennec | Les coquilles de Patella sp. issues d'amas coquilliers anthropiques, archives culturelles et environnementales des environnements côtiers[END_REF][START_REF] Pailler | Première campagne de fouille programmée sur le site multi-périodes de Porz ar Puns, Ile de Béniguet (Le Conquet, Finistère)[END_REF]Fig. 9). The basal shell-midden has been radiocarbon-dated from 4,404-4,150 cal BP, i.e. the end of the Neolithic period (bell beaker culture). This anthropogenic deposit is overlaid by a 0.3 m-thick aeolian sand layer, at the top of which an intermediate shell-midden has been dated to the Early Bronze Age, from 4,140 to 3,908 cal BP. This succession of sedimentary deposits suggests a sand-drift episode between ca. 4,250 and ca. 4,000 cal BP (Fig. 9c). A humic aeolian sand deposit, about 0.2 m-thick and without archaeological remains, overlies this second shelly level. Its upper part is composed of a 1.05 m-thick layer of white aeolian sand, and an upper shell-midden dated between 1,286 and 1,123 cal BP, indicating a second sand-drift event between ca. 4,000 and ca.1,200 cal BP (Fig. 9c).

Finally, the top of the sedimentary sequence is formed by a layer of humic aeolian sand.

Dunes of the Saint-Nicolas Island

The stratigraphic sequence of the aeolian sand deposits of the Saint-Nicolas Island (Fouesnant, W Brittany, site no.58 on Fig. 3) was reconstructed from the analysis of 6 archaeological sites named Saint-Nicolas I, II, III, IV, V and VI, respectively (Fig. 10). The most detailed data has been obtained from the Saint-Nicolas IV,V, VI sites [START_REF] Hamon | Sondages sur l'île Saint-Nicolas[END_REF][START_REF] Hamon | Sondages sur l'île Saint-Nicolas[END_REF], where a basal paleosoil is overlaid by 4 successive sand dune deposits with interbedded humic horizons containing a high number of archaeological materials (Fig. 10c). The paleosoil situated at the base of the sequence contained numerous ceramic and lithic remains attributed to the Recent Neolithic (ca. 5,750-5,050 cal BP). A Late Neolithic settlement has been also discovered in the basal palaeosol of Saint-Nicolas III site [START_REF] Hamon | Sondages sur l'île Saint-Nicolas[END_REF]. This deposit is overlaid by a 0.15 m-thick aeolian sand layer, at the top of which the first humic horizon contains a dry stone wall and ceramics characteristic of the Early Bronze Age (ca. 4,100-3,550 cal BP), both indicating a stabilisation of the dune complex during this period. This succession of sedimentary deposits indicates that the first sand-drift episode occurred between ca. 5,050 and ca. 4,100 cal BP (between Late Neolithic and Early Bronze Age). This sequence is overlaid by a 0.18 m-thick aeolian sand layer at the top of which a second humic horizon that contains fragments of ceramics dating from the Late Iron Age (ca. 2,400-2,000 cal BP). Two other settlements located at the north of the island have been attributed to the same period (Saint-Nicolas I and II sites). One of which has been discovered in a humic horizon interbedded in the dune sands. These data indicate a second period of dune stabilization during the Late Iron Age.

A second sand-drift event is consequently identified between ca. 3,550 and 2,100 cal BP (between Early Bronze Age and Late Iron Age). These remains are covered by two successive aeolian deposits with an interbedded humic horizon. A stone construction associated with fragments of Pecten maximus shells is present within this humic layer, but no dating was carried out. Finally, the upper humic horizon corresponds to the current dune soil. Four kelp kilns observed on this island indicate the stabilization of the dune complex in the 19th century. Since the dating of the interbedded humic horizon is not yet available, only one sand-drift event can be identified between the Late Iron Age and the 19th century (ca. 2,000 and 120 cal BP).

Dunes of the Audierne Bay

Among all the studied dune complexes, those of the Audierne Bay (Plomeur, SW Brittany, site no.50 on Figs 3 and 11) contain the largest number of archaeological remains (n=15). Neolithic megalithic monuments as well as Bronze Age stone cist has been discovered in a clayey paleosoil forming the base of the aeolian sequences (Fig. 11c). There, 4 Iron Age settlements have been identified, two of which were interbedded in the aeolian deposits. The oldest welldated remains are a Late Iron Age necropolis (La Torche, Plomeur: [START_REF] Kayser | Plomeur (Finistère), Beg-an-Dorchenn. Fouille de sauvetage, campagne[END_REF][START_REF] Kayser | Plomeur (Finistère), Beg-an-Dorchenn. Fouille de sauvetage, campagne[END_REF][START_REF] Kayser | Plomeur (Finistère), Beg-an-Dorchenn. Fouille de sauvetage, campagne[END_REF], which indicates that the dune was stabilised between 2,400 and 2,000 cal BP (Fig. 11d). From these data, the first sand-drift event at this site is identified as occurring between 3,550 and 2,400 cal BP, i.e. between the end of the Early Bronze Age and the beginning of the Late Iron

Age. An extensive medieval necropolis has been excavated in the Saint-Urnel site (Plomeur locality, Fig. 11e and11f). The sedimentary sequence shows aeolian sand deposits with three interbedded levels of burials (levels I to III in Fig. 11e) [START_REF] Giot | Le cimetière des anciens Bretons de Saint-Urnel ou Saint-Saturnin en Plomeur (Finistère)[END_REF]. Level I is associated with remains of wooden and dry stone constructions built on a ca. 0.9 m-thick sand layer. A humic horizon at the top of this sandy deposit indicates that the dune was stabilised by vegetative cover from 1,708 to 979 cal BP. Level II is observed in a sedimentary deposit ca. 0.4 m-thick that encloses alternating layers of humic horizons and white sand, indicating an episodic invasion of aeolian sand. A AMS-14 C date on bones provided ages ranging from 1,065 to 691 cal BP. Level II is overlaid by a ca. 0.7 m-thick homogeneous aeolian sand deposit. Finally, the level III occuring at the top of this sandy layer yields ages between 1,059 and 692 cal BP. AMS-14 C dates from levels II and III are quite similar, suggesting the continuous use of this necropolis despite the increasing invasion of dune sands. Consequently, a second sanddrift event has been identified during this period between ca. 1,050 and 700 cal BP. Level III of the Saint-Urnel necropolis is overlaid by a layer of aeolian sand about 0.4 m-thick. Another medieval settlement has also been investigated at the Tronoën site where two dry stone houses dated from ca. 950 to 750 cal BP are buried beneath a sandy cover ca. 0.5 m-thick [START_REF] Villard | Un habitat médiéval des Xe-XIIe siècles à Tronoen en Saint-Jean-Trolimon (Finistère)[END_REF]. At the top of this last aeolian deposit, i.e. at the present-day dune soil, 2 kelp kilns have been identified, indicating the stabilisation of the complex during the 19th century. A third sand-drift event is then defined between 700 and 120 cal BP.

Regional chronology of coastal dunes mobility

Archaeological and historical data supplied by most of coastal dunes in Brittany (n=60) are too limited to identify more than one sand-drift event at a regional scale. Two sand-drift events have been accurately documented at only 14 studied sites, whereas three coastal dune sites are sufficiently well-documented to highlight three successive chronologically well-constrained sand-drift events (Tab. 1). Although these events cover a wide time-period ranging from 6,700 to 50 cal BP, the compilation of all these data clearly shows 4 main periods of coastal dune mobility (Fig. 12).

A first period is identified at the end of the Neolithic, between ca. 4,250 and ca. 4,100 cal BP, although being evidenced by a limited number of archaeological sites. The beginning of this period is in the time-range 4,404-4,150 cal BP from one AMS-14 C date obtained on a paleosoil at the base of the aeolian sand deposit sequence on the Béniguet Island site (W Brittany). The end of this period of dune mobility is supported by Early Bronze Age (ca. 4,100 to ca. 3,550 cal BP) archaeological remains interbedded within aeolian sand deposits on the Saint-Nicolas

Island (SW Brittany).

A second period of coastal dune mobility is recognized between the end of the Bronze Age and the beginning of the Iron Age, between ca. 3,250 and ca. 2,400 cal BP. Numerous Early Bronze Age sites have been discovered at the base of dune deposit sequences (e.g. Port Blanc I site, Penvénan) or correspond to the final use of Neolithic megalithic tombs (e.g. Conguel I site, Quiberon). The most recent date for the beginning of this period comes from the Beg-ar-Loued habitat site (Molène Island) and it ranges 3,446 to 3,251 cal BP. This radiocarbon dating is used to define the beginning of this mobility phase. The end of this period takes place during the Late Iron Age (ca. 2,400-2,000 cal BP), as suggested by the numerous remains of settlements discovered on top of the sandy deposits of this period (e.g. Mesperleuch site, Plouhinec; La Torche site, Plomeur).

A third phase of dune mobility is highlighted during the Middle Ages, between ca. 1050 and ca. 700 cal BP. However, this phase is only identified on the coastal dune of the Audierne Bay where burial levels II and III of the Saint-Urnel necropolis provide dates for the beginning and end of this period, respectively.

A fourth period of dune mobility is identified between ca. 350 and ca. 110 cal BP from historical data. The beginning of this period is represented by the abandonment of the church of St-Anne (Batz Island, no. 15 on Fig. 3) in the 17th century caused by the invasion of the coastal dune sands. For this last period, the end is defined by the reconstruction of the chapel of Lampaul-Plouarzel in 1838. Fig. 12. Compiled data about sand-drift events obtained for each dune massif (for numbers refer to Tab. 1 and Fig. 3). Yellow bands represent major sand-drift periods identified at a regional scale.

Discussion

Methodological issues

The geoarchaeological approach used in this study is based on the analysis and synthesis of abundant scientific litterature devoted to archaeological sites and further completed by an extensive historical dataset. Such an approach is only made possible in countries, which benefit from a long-lasted tradition of archaeological and historical researchs that permit a large amount of data to be re-examined in order to question regional-scale environmental changes. Along the UK coastlines, for instance, numerous excavations of multi-period archaeological sites in machair and coastal dunes areas were coupled with abundant historical references to wheather events in chronicles, hagiographies and laws in order to finely reconstruct the dynamics of coastal sand-dune systems in Holocene times (e.g. [START_REF] Wilson | Development and age structure of Holocene coastal sand dunes at Horn Head, near Dunfanaghy, Co Donegal, Ireland[END_REF][START_REF] Wintle | Luminescence dating of recent dunes on Inch Spit, Dingle Bay, southwest Ireland[END_REF][START_REF] Gilbertson | Sand-drift and Soil Formation Along an Exposed North Atlantic Coastline: 14,000 Years of Diverse Geomorphological, Climatic and Human Impacts[END_REF][START_REF] Wilson | Late-Holocene (post-4000 years BP) coastal dune development in Northumberland, northeast England[END_REF][START_REF] Wilson | Mid-to late-Holocene coastal dune event stratigraphy for the north coast of Northern Ireland[END_REF][START_REF] Sommerville | Optically stimulated luminescence dating of large storm events in Northern Scotland[END_REF][START_REF] Sommerville | Optically stimulated luminescence (OSL) dating of coastal aeolian sand accumulation in Sanday, Orkney Islands, Scotland[END_REF][START_REF] Griffiths | Medieval coastal sand inundation in Britain and Ireland[END_REF].

One of the difficulties in analysing archaeological and historical data is the great heterogeneity of the available information, which concerns diverse cultural periods and various types of corresponding material (e.g., temporary camps, perennial habitats, cult and funerary sites, sites related to the exploitation of marine resources). For archaeological purposes, chronological attributions are primarily based on the nature of the remains which makes it possible to associate objects with material cultures. Absolute dating (AMS-14 C, dendrochronology, archaeometry, etc.) is only used to confirm initial interpretations and to precise the chronological attribution.

In Brittany, the succession of material cultures is now accurately established and the chronological division into cultural periods and sub-periods has become much more precise since the pioneer archaeological work more than a century ago (e.g. [START_REF] Boujot | Le développement des premières architectures funéraires monumentales en France occidentale[END_REF][START_REF] Milcent | Le temps des élites en Gaule atlantique: chronologie (sic) des mobiliers et rythmes de constitution des dépôts métalliques dans le contexte européen, XIIIe-VIIe av[END_REF][START_REF] Menant | La Bretagne[END_REF][START_REF] Blanchard | Néolithique récent de l'Ouest de la France: IVe -IIIe millénaires avant J.-C[END_REF]Stéphan et al. 2017;[START_REF] Nicolas | The introduction of the Bell Beaker culture in Atlantic France: an overview of settlements[END_REF]. Consequently, a systematic reappraisal of the earliest available dataset has been carried out in order to correct potentially erroneous interpretations, especially about previous chronological attributions (e.g. [START_REF] De Closmadeuc | Découverte de stone-cists à Beg-er-Vill (Quiberon)[END_REF][START_REF] Du Chatellier | Les Epoques Préhistoriques et Gauloises dans le Finistère. Inventaire des monuments de ce département des Temps Préhistoriques à la fin de l'Occupation Romaine[END_REF]. Despite these efforts, the age of many remains is still very imprecise, due to their nature or poor state of preservation. This has justified to take into account two degrees of dating precision for all the studied archaeological sites.

About the coastal dunes systems in Brittany, some archaeological remains are clearly associated with organic levels interbedded within the aeolian sand deposits (e.g. sites of Saint-Nicolas Island, Fouesnant). Thus, they represent useful chrono-stratigraphic markers in the absence of absolute dating. However, stratigraphic data are poorly detailed in the archaeological litterature.

Prior to the 2000s, the remains discovered in the coastal dunes of Brittany were most often excavated and analysed without any considerations about their sedimentary and stratigraphic contexts (e.g. [START_REF] Du Chatellier | Une habitation gauloise à Tronoën en Saint-Jean-Trolimon[END_REF]Quinio, 1971;[START_REF] Langouët | Fouille, datation et étude de l'édifice gallo-romain des Sables d'Or. Les dossiers du Centre[END_REF]. Therefore, observations and interpretations were strictly devoted to archaeological issues. The Harris diagram used in the present work aims to partly fill this stratigraphic/chronological gap. Despite its frequent use in archaeological studies (e.g. [START_REF] Kobyliński | Polish medieval excavations and the Harris Matrix: applications and developments[END_REF][START_REF] Trócoli | The contribution of the Harris Matrix to the development of Catalan archaeology[END_REF][START_REF] Dye | Archaeological sequence diagrams and Bayesian chronological models[END_REF], this tool had never been used to reconstruct the chrono-stratigraphy of coastal dunes. From available archaeological and historical information, this method is highly relevant to estimate the age of aeolian sand deposits and then to establish the accurate chronology of sand-drift events. This approach is applied here to coastal sand dune systems in Brittany using archaeological remains as chrono-stratigraphic markers of dune stabilization. We assumed that past human coastal communities favoured fixed dunes for permanent settlement. Therefore, a re-examination of previous archaeological dataset was also carried out to reinterpret the duration of human occupation in the coastal dunes of Brittany. These periods of occupation are very variable from sites to sites, ranging from temporary visitations (e.g. site of Kerminihy, Erdeven) to human settlements covering a few hundred years to several millennia (e.g. site of Goerem, Gâvres). In the case of perennial settlements, it is possible to associate the presence of archaeological remains with periods of dune stabilisation. About temporary settlements, it much less clear whether they fit in time with periods of dune stabilization or mobility.

The adaptive ability (or resilience) of past human communities to manage environmental stress [START_REF] Bradtmöller | Resilience theory in archaeological practice -An annotated review[END_REF] may have induced an abandonment of human settlements long after the initiation of sand dune migration. Data obtained from the Saint-Urnel medieval site clearly show that populations continued to use the necropolis despite sand inundation. Similarly, religious monuments buried by aeolian sands from the 15th and 16th centuries were regularly maintained for several decades before being permanently abandoned. Such a mismatch between the environmental constraint (sand invasion) and the societal response (abandonment) is much more difficult to argue for Prehistoric or Protohistoric periods because attempts to stabilize the coastal dunes have left no known archaeological evidence.

Regional comparison

Between 4,250 and 4,100 cal BP, the oldest phase of dune mobility (Fig. 12) has been identified from a limited number of sites located only on the southern (e.g. sites of Saint-Nicolas Island, Hoëdic Island, Ile aux Chevaux and Kerminihi beaches) and the western coasts of Brittany (Beniguet Island). [START_REF] Bond | Persistent Solar Influence on North Atlantic Climate During the Holocene[END_REF]; (e) δ 18 O curve acquired on Ammonia falsobeccarii benthic foraminifera from CBT-CS11 core, Northern Bay of Biscay, North-East Atlantic Ocean (Penaud et al., 2020), negative anomalies has been established in reference with the average of the considered dataset: blue (more IRD or stronger SPG) ; (f) North Atlantic Oscillation (NAO) index [START_REF] Olsen | Variability of the North Atlantic Oscillation over the past 5,200 years[END_REF] (red : positive NAO-like conditions, blue : negative NAO-like conditions); (g) Strengthened subpolar gyre in Bay of Biscay, North-East Atlantic Ocean (Penaud et al., 2020) and increased river activity of Middle Loire River [START_REF] Arnaud-Fassetta | Fluviatile palaeoenvironments in archaeological context: Geographical position, methodological approach and global change -Hydrological risk issues[END_REF]; (h) RSL model for West Brittany [START_REF] García-Artola | Holocene sea-level database from the Atlantic coast of Europe[END_REF]. This phase of aeolian sand deposition has also been recognized by previsous studies on the western coast of Brittany (Fig. 13a). A AMS-14 C date from the base of a perched dune in the Audierne Bay provided an age between 4,545 and 4,160 cal BP [START_REF] Haslett | Geomorphologic and palaeoenvironmental development of Holocene perched coastal dune systems in Brittany, France[END_REF]. On the northern coast of Brittany, a shell-midden interbedded within aeolian sand deposits was dated between 3,600 and 2,400 cal BP at the Anse de Verger site (Cancale) (Regnault et al., 1995) and attested to dune mobility before 3,600 cal BP. More recently, a phase of aeolian activity has also been documented at around 4,000 cal BP on the Herm Island (English Channel) by [START_REF] Bailiff | Application of luminescence dating and geomorphological analysis to the study of landscape evolution, settlement and climate change on the Channel Island of Herm[END_REF]. Finally, only 7 coastal dune systems recording a period of mobility between 4,600 and 3,600 cal BP have yet been evidenced along the coasts of Brittany. The paucity of such dated aeolian sites likely results from the poor preservation of supratidal sedimentary deposits. Around 4,150 cal BP, the RSL was approximately -3.86 ±0.7 m below the presentday level [START_REF] García-Artola | Holocene sea-level database from the Atlantic coast of Europe[END_REF]Stéphan et al., 2019b) (Fig. 13h). Since this time, it is likely that many coastal dune systems were affected by marine erosion induced by the still rapid RSL rise so that sandy sedimentary material was in turn extensively redistributed during later phases of coastal dune mobility.

The second phase of coastal dune mobility is identified between 3,250-2,400 cal BP, i.e. from the Middle Bronze Age to the end of the First Iron Age (Fig. 12). These results are consistent with previous geomorphological and sedimentological observations along the coasts of Brittany [START_REF] Giot | De l'antiquité des talus et des dunes armoricaines[END_REF][START_REF] Guilcher | Coastal dunes in Brittany and their management[END_REF] (Fig. 13a). Based on the analysis of several sedimentary cores, Van Vliet-Lanoë et al. ( 2016) identified a mobility phase of dune massifs between 3,050 and 2,300 cal BP (Fig. 13a). On the Herm Island, two distinct phases of aeolian activity were recognized around 3,000 and around 2,300 cal BP, respectively [START_REF] Bailiff | Application of luminescence dating and geomorphological analysis to the study of landscape evolution, settlement and climate change on the Channel Island of Herm[END_REF] (Fig. 12). These results suggest several pulses of dune mobility during a ca. 800 yearslong-lasted phase. The thickness of aeolian sand deposited during this second phase is relatively significant, reaching up to 2.5 m at the Moulin de la Rive site. These deposits also cover extensive areas and were observed on archaeological sites located far away onshore from the current coastline. At Saint-Urnel (Plomeur; Fig. 11), the medieval necropolis is located more than 1 km away from the present coastline and lies on aeolian sand deposits attributed to this second period of dune mobility [START_REF] Giot | Le cimetière des anciens Bretons de Saint-Urnel ou Saint-Saturnin en Plomeur (Finistère)[END_REF]. At Kerhilio (Erdeven, S Brittany, site no.66 on Fig. 3) a paleosoil dated at the Late Iron Age and interbedded with 2 aeolian sand deposits was uncovered at about 900 m inland [START_REF] Blanchet | ZAC de Kerhillio[END_REF]. These observations highlight the local extent of sand inundation and suggest the development of transgressive dune fields that migrated over great distances away from the coastline. The 3,000-2,500 cal BP period has long been recognized as a phase of dramatic changes for morpho-sedimentary systems along the coasts of western France. In NW Brittany, erosional surfaces and sedimentary hiatuses recorded in marine and coastal sedimentary sequences reflect enhanced hydrodynamic conditions between 3,000 and 2,500 cal BP [START_REF] Fernane | Coastal palaeoenvironmental record of the last 7 kyr BP in NW France: Submillennial climatic and anthropic Holocene signals[END_REF][START_REF] Stéphan | Holocene salt-marsh sedimentary infilling and relative sea-level changes in West Brittany (France) using foraminifera-based transfer functions[END_REF][START_REF] Ehrhold | Fossil maerl beds as coastal indicators of late Holocene palaeo-environmental evolution in the Bay of Brest (Western France)[END_REF]. Contemporaneous disruptions of coastal sedimentary environments along the English Channel [START_REF] Billeaud | Impacts of late Holocene rapid climate changes as recorded in a macrotidal coastal setting (Mont-Saint-Michel Bay, France)[END_REF][START_REF] Sorrel | Evidence for millennialscale climatic events in the sedimentary infilling of a macrotidal estuarine system, the Seine estuary (NW France)[END_REF][START_REF] Lespez | Middle to Late Holocene landscape changes and geoarchaeological implications in the marshes of the Dives estuary (NW France)[END_REF][START_REF] Tessier | Infilling stratigraphy of macrotidal tide-dominated estuaries. Controlling mechanisms: Sea-level fluctuations, bedrock morphology, sediment supply and climate changes (The examples of the Seine estuary and the Mont-Saint-Michel Bay, English Channel, NW France)[END_REF] and the Atlantic coasts of France [START_REF] Tastet | Morpho-chronology of coastal dunes in Médoc. A new interpretation of Holocene dunes in Southwestern France[END_REF][START_REF] Clavé | Holocene coastal changes and infilling of the La Perroche marsh (French Atlantic coast)[END_REF][START_REF] Stéphan | Holocene coastal changes along the Gironde estuary (SW France): new insights from the North Médoc peninsula beach/dune system. Quaternaire[END_REF][START_REF] Tastet | Morpho-chronology of coastal dunes in Médoc. A new interpretation of Holocene dunes in Southwestern France[END_REF] as regarded as the morphogenic impact of intense storms (Pouzet al., 2018).

The third period of dune mobility recognized in this study between 1,050 and 700 cal BP (Fig. 12) is only argued from the Saint-Urnel site (Plomeur) in the Audierne Bay. Supplementary stratigraphic evidences at Saint-Nicolas Island (Fouesnant, Fig. 10) and Goulvars (Quiberon, Fig. 8) also suggests a destabilization of coastal dunes between 2,000 and 350 cal BP.

Stratigraphical data clearly show a layer of aeolian sand deposited between 2,000 cal BP (end of the Late Iron Age) and 350 cal BP, i.e. the Little Ice Age (LIA) period. Although no absolute dating or chronological markers are available on these both sites, the sand deposition during the medieval period is likely as several aeolian sand deposits have been dated by [START_REF] Vliet-Lanoë | Holocene formation and evolution of coastal dunes ridges, Brittany (France)[END_REF] between 1,150 and 750 cal BP (Fig. 13a). On the Herm Island, aeolian sand deposits dated at around 1,200 cal BP are genetically related to an early dune mobility phase during the LIA period [START_REF] Bailiff | Application of luminescence dating and geomorphological analysis to the study of landscape evolution, settlement and climate change on the Channel Island of Herm[END_REF] (Fig. 13a). On the Picardie coast (northern France), coastal dunes appear to have been reactivated between 1,060 and 720 cal BP [START_REF] Meurisse | Complexes dunaires et tourbeux holocènes du littoral du Nord de la France[END_REF]. Dunes on the Aquitaine coast (SW France) also show an increasing mobility between 1,250 and 850 cal BP [START_REF] Clarke | Late-Holocene sand invasion and North Atlantic storminess along the Aquitaine Coast, southwest France[END_REF] (Fig. 13b). Other dune complexes of the European Atlantic coasts also show a period of increased sand movement around 900 cal BP in Denmark [START_REF] Clemmensen | The evolution of Holocene coastal dunefields, Jutland, Denmark: A record of climate change over the past 5000 years[END_REF] and between 1,100 and 1,000 cal BP in N Ireland [START_REF] Wilson | Coastal dune chronology in the north of Ireland. in: Dunes of the European Coasts[END_REF].

However, the phase of dune mobility recorded between the late Roman and early medieval periods (ca. 1,700-1,150 cal BP) along the European Atlantic coasts [START_REF] Gilbertson | Sand-drift and Soil Formation Along an Exposed North Atlantic Coastline: 14,000 Years of Diverse Geomorphological, Climatic and Human Impacts[END_REF][START_REF] Meurisse | Complexes dunaires et tourbeux holocènes du littoral du Nord de la France[END_REF][START_REF] Clemmensen | The evolution of Holocene coastal dunefields, Jutland, Denmark: A record of climate change over the past 5000 years[END_REF][START_REF] Vliet-Lanoë | Holocene formation and evolution of coastal dunes ridges, Brittany (France)[END_REF] was not clearly identified in this study.

The most recent phase (phase 4) of dune mobility was evidenced here between ca. 350 and 110 cal BP, i.e. during the LIA period (Fig. 12), is consistent with previous works in Brittany (Guilcher and Hallegouet, 1991;[START_REF] Meur | Géomorphologie, protection et gestion des dunes de Bretagne septentrionale : éléments de comparaison avec d'autres régions de la Manche occidentale : Cotentin[END_REF][START_REF] Vliet-Lanoë | Holocene formation and evolution of coastal dunes ridges, Brittany (France)[END_REF][START_REF] Lageat | Délits d'eau" et "volements de sables", les "offensives éoliennes" sur les côtes bretonnes au XVIIIème siècle[END_REF].

However, the timing of the onset of this mobility phase is still poorly defined with our dataset.

Based on the analysis of several sedimentary cores collected from the coasts of Brittany, Van [START_REF] Vliet-Lanoë | Holocene formation and evolution of coastal dunes ridges, Brittany (France)[END_REF] suggest that dune movement started around 600 cal BP and continued through a series of sand-drift pulses until present (Fig. 13a). One of these pulses appears to be centered on the 18th century, as noted by [START_REF] Lageat | Délits d'eau" et "volements de sables", les "offensives éoliennes" sur les côtes bretonnes au XVIIIème siècle[END_REF] from the analysis of the historical records. On Herm Island, OSL dating revealed a period of sand-drift between ca. 750 and 350 cal BP that has been attributed to an early LIA phase [START_REF] Bailiff | Application of luminescence dating and geomorphological analysis to the study of landscape evolution, settlement and climate change on the Channel Island of Herm[END_REF] (Fig. 13a). Along the Atlantic coasts of France, sand movements dating from this period have also been recorded in Picardie [START_REF] Meurisse | Complexes dunaires et tourbeux holocènes du littoral du Nord de la France[END_REF] and Aquitaine [START_REF] Clarke | Late-Holocene sand invasion and North Atlantic storminess along the Aquitaine Coast, southwest France[END_REF][START_REF] Bertran | Mapping and chronology of coversands and dunes from the Aquitaine basin, southwest France[END_REF] (Fig. 13b). On the coastal sand-dune systems of Brittany, the LIA phase is associated with very thick sand deposits which can reach several meters at the Saint-Egarec church (Lampaul-Plouarzel, Fig. 6) and at the Saint-Guévroc church (Tréflez, site no.22 on Fig. 3). These sandy deposits cover very large areas, as shown by the position of the buried houses and churches relative to the current shoreline. The abovementionned Saint-Egarec church and the Tréménéac'h church (Plouguerneau, NW Brittany, site no.31 on Fig. 3) are located at about 400 m and 350 m from the present coastline, respectively. In the Audierne Bay, the aeolian sand cover extends more than 2 km inland. The initiation and migration of transgressive coastal sand-dune systems during the LIA period have been recognized as the last major phase of aeolian sand mobilization on many western European coasts (e.g. [START_REF] Clarke | Effects of storminess, sand supply and the North Atlantic Oscillation on sand invasion and coastal dune accretion in western Portugal[END_REF][START_REF] Jackson | Large-scale transgressive coastal dune behaviour in Europe during the Little Ice Age[END_REF]. These large-scale transgressive dune systems inherited from the LIA period have also been observed in Denmark [START_REF] Clemmensen | The evolution of Holocene coastal dunefields, Jutland, Denmark: A record of climate change over the past 5000 years[END_REF][START_REF] Clemmensen | The evolution of Holocene coastal dunefields, Jutland, Denmark: A record of climate change over the past 5000 years[END_REF][START_REF] Clemmensen | Episodes of aeolian sand movement on a large spit system (Skagen Odde, Denmark) and North Atlantic storminess during the Little Ice Age[END_REF][START_REF] Clemmensen | The termination of the last major phase of aeolian sand movement, coastal dunefields, Denmark[END_REF][START_REF] Aagaard | Environmental controls on coastal dune formation; Skallingen Spit, Denmark[END_REF], in Britain [START_REF] Gilbertson | Sand-drift and Soil Formation Along an Exposed North Atlantic Coastline: 14,000 Years of Diverse Geomorphological, Climatic and Human Impacts[END_REF][START_REF] Bailey | Sand deposition during the last millennium at Aberffraw, Anglesey, North Wales as determined by OSL dating of quartz[END_REF][START_REF] Wilson | Late-Holocene (post-4000 years BP) coastal dune development in Northumberland, northeast England[END_REF][START_REF] Wilson | Holocene coastal dune development on the South Erradale peninsula, Wester Ross, Scotland[END_REF][START_REF] Bateman | Late-Holocene inland dune activity in the UK: a case study from Breckland, East Anglia[END_REF][START_REF] Sommerville | Optically stimulated luminescence (OSL) dating of coastal aeolian sand accumulation in Sanday, Orkney Islands, Scotland[END_REF][START_REF] Mcilvenny | A 7600-year sedimentary record of climatic instability in Dunnet Bay, North Scotland[END_REF][START_REF] Bampton | Little Ice Age catastrophic storms and the destruction of a Shetland Island community[END_REF], in northern Ireland [START_REF] Wilson | Development and age structure of Holocene coastal sand dunes at Horn Head, near Dunfanaghy, Co Donegal, Ireland[END_REF][START_REF] Wilson | Mid-to late-Holocene coastal dune event stratigraphy for the north coast of Northern Ireland[END_REF]) and in Portugal [START_REF] Clarke | Effects of storminess, sand supply and the North Atlantic Oscillation on sand invasion and coastal dune accretion in western Portugal[END_REF][START_REF] Costas | Sand invasion along the Portuguese coast forced by westerly shifts during cold climate events[END_REF].

Driving mechanisms of coastal dune mobility

According to [START_REF] Hesp | Dune Coasts[END_REF], the development of coastal dunes requires two basic environmental prerequisites: (i) a supply of suitable grain-sized beach sediment from aeolian transport and (ii) wind speeds capable to initating saltation transport of particles towards the supratidal stage. At long time-scales (decades, century to millennia), several intricate driving mechanisms have long been identified about the coastal dune mobility process, such as RSL changes, climate and anthropogenic forcing [START_REF] Christiansen | Coastal dunes in Denmark. Chronology in relation to sea level, in: Dunes of the European Coasts[END_REF][START_REF] Klijn | The chronology of coastal dune development in the United Kingdom, in: Dunes of the European Coasts[END_REF][START_REF] Wilson | Development and age structure of Holocene coastal sand dunes at Horn Head, near Dunfanaghy, Co Donegal, Ireland[END_REF].

Relative sea level changes

RSL changes are a key factor in the morphodynamics of coastal dune systems by controlling sediment supply and redistribution processes [START_REF] Hesp | Dune Coasts[END_REF]. According to previous works, sanddrift events are commonly associated with marine transgressive pulses that could promote high rates of sand supply and/or reworking of pre-existing dunes by marine erosion (e.g. [START_REF] Aagaard | Environmental controls on coastal dune formation; Skallingen Spit, Denmark[END_REF][START_REF] Szkornik | Aeolian sand movement and relative sealevel rise in Ho Bugt, western Denmark, during the "Little Ice Age[END_REF]. Conversely, some other works have shown that a regressive trend in RSL can lead to coastal dune formation and landward sand-drift events by exposing large intertidal sandy areas to wind deflation (e.g. [START_REF] Christiansen | Sea-level changes, coastal dune building and sand drift, North-Western Jutland, Denmark[END_REF][START_REF] Clemmensen | The evolution of Holocene coastal dunefields, Jutland, Denmark: A record of climate change over the past 5000 years[END_REF][START_REF] Wilson | Late-Holocene (post-4000 years BP) coastal dune development in Northumberland, northeast England[END_REF]. In Brittany, the RSL curve produced by [START_REF] Morzadec-Kerfourn | Variations de la ligne de rivage au cours du Post-glaciaire le long de la côte nord du Finistère. Analyses polliniques de tourbes et de dépôts organiques littoraux[END_REF] suggested a significant RSL decrease around 3,000 cal BP. The period of coastal dune mobility identified at the end of the Bronze Age period was initially interpreted as the result of processes linked to this marine regression [START_REF] Giot | De l'antiquité des talus et des dunes armoricaines[END_REF][START_REF] Guilcher | Coastal dunes in Brittany and their management[END_REF]. However, a recent re-examination of RSL index points produced in Brittany clearly demonstrated the absence of a regressive trend in RSL over the last 7,000 years [START_REF] Stéphan | Évolution du niveau marin relatif à l'Holocène le long des côtes françaises de l'Atlantique et de la Manche : réactualisation des données par la méthode des « sea-level index points[END_REF][START_REF] Goslin | A new Holocene relative sea-level curve for western Brittany (France): Insights on isostatic dynamics along the Atlantic coasts of north-western Europe[END_REF][START_REF] Stéphan | Holocene salt-marsh sedimentary infilling and relative sea-level changes in West Brittany (France) using foraminifera-based transfer functions[END_REF]. In addition, statistical modelling performed on RSL records showed that RSL rise rates gradually decreased during the mid-and late-Holocene (García-Artola et al., 2018) (Fig. 13h). The analysis of some back-barrier sediment infillings demonstrated that the RSL rise favoured the landward migration of coastal barriers in NW Brittany [START_REF] Morzadec-Kerfourn | Variations de la ligne de rivage armoricaine au Quaternaire : analyses polliniques de dépôts organiques littoraux[END_REF][START_REF] Stéphan | Holocene salt-marsh sedimentary infilling and relative sea-level changes in West Brittany (France) using foraminifera-based transfer functions[END_REF]. This landward translation of beach/dune systems was associated to a vertical accretion of back-barrier swampy environments that created an accommodation space for aeolian sand deposits and the episodic mobility of transgressive dunes.

Storminess

Because the wind forcing is a primary driving mechanism of coastal dune erosion and mobility [START_REF] Hesp | Dune Coasts[END_REF], the chronology of mid-to late-Holocene sand-drift events was frequently associated with enhanced coastal storminess (e.g. [START_REF] Clarke | Effects of storminess, sand supply and the North Atlantic Oscillation on sand invasion and coastal dune accretion in western Portugal[END_REF][START_REF] Costas | Sand invasion along the Portuguese coast forced by westerly shifts during cold climate events[END_REF]. Five main "storm periods" have been recognized along the north-east Atlantic coasts over the last 7,000 years [START_REF] Sorrel | Persistent non-solar forcing of Holocene storm dynamics in coastal sedimentary archives[END_REF]Van Vliet-Lanoë et al., 2014b;Pouzet et al., 2018) which further coincide with phases I, II, and IV of coastal dune mobility in Brittany (Fig. 13c).

A study of a lake sediments from southwest Greenland [START_REF] Olsen | Variability of the North Atlantic Oscillation over the past 5,200 years[END_REF] (Fig. 13f) suggested a shift from predominantly positive to negative NAO-like conditions over the period 4,500-2,000 cal BP. In particular, a significant strengthening of NAO-conditions was shown first around 4,300 cal BP, and then between 3,400 and 2,000 cal BP, which corresponds to phases I and II of dune mobility, as identified in this study (Fig. 13f). A negative decadal NAO mode is associated with cold and dry winter conditions over northern Europe due to a southward shift of the westerly wind belt (Van Vliet-Lanoë et al., 2014b;[START_REF] Lambert | Striking forest revival at the end of the Roman Period in north-western Europe[END_REF]Penaud et al., 2020). In parallel, storm events are considered more intense due to the abrupt atmospheric front between tropical and Arctic air masses [START_REF] Trouet | North Atlantic storminess and Atlantic Meridional Overturning Circulation during the last Millennium: Reconciling contradictory proxy records of NAO variability[END_REF]Van Vliet-Lanoë et al., 2014b).

A similar negative decadal NAO mode is revealed during the LIA [START_REF] Trouet | North Atlantic storminess and Atlantic Meridional Overturning Circulation during the last Millennium: Reconciling contradictory proxy records of NAO variability[END_REF], also characterized by an increase in storm strength (Fig. 13c). Based on the analysis of a set of historic aerial photographs, [START_REF] Mathew | Evolution of a beach-dune system following a catastrophic storm overwash event: Greenwich Dunes, Prince Edward Island, 1936-2005[END_REF] documented the development of an extensive transgressive dunefield produced by a catastrophic overwash on Greenwich Dunes, Prince Edward Island National Park (Canada). These authors clearly show that the subsequent stabilization of the beach-dune system took place 8 decades after an intense storm event. It is thus assessed here that, moreso than the frequency, the intensity of storms is an essential parameter in the dynamics of dune systems in Brittany.

Climate and vegetation cover interaction

With regards to well-vegetated dunes, [START_REF] Jackson | Large-scale transgressive coastal dune behaviour in Europe during the Little Ice Age[END_REF] point out that the morphological impact of storms alone does not explain the widespread destabilization of coastal dune complexes over several decades or centuries. Massive aeolian sediment transport also implies a significant degradation of the vegetation cover due to climatic and/or anthropogenic factors.

In the North Atlantic Ocean, [START_REF] Bond | Persistent Solar Influence on North Atlantic Climate During the Holocene[END_REF] identified several ice-rafted debris (IRD) events reflecting advections of colder surface waters eastwards from the Labrador Sea and southwards from the northern sea (Fig. 13d). In the Bay of Biscay, IRD events are closely associated with a strengthening of the subpolar Gyre (SPG) dynamic, which in turn results in colder climatic conditions along the Brittany coast (Penaud et al., 2020) (Fig. 13e). These periods of local climate cooling roughly correspond to mobility phases I, II, and IV identified in this study. [START_REF] Jackson | Coastal dune fields in Ireland: rapid regional response to climatic change[END_REF] demonstrated that North Atlantic coastal dune vegetation is highly sensitive to changes in climate conditions. Decreases in temperature and changes in soil salinity both alter the growth of dune plants such as Ammophila arenaria [START_REF] Chergui | The effects of temperature, hydric and saline stress on the seedling growth of marram grass (Ammophila arenaria L.)[END_REF] and reduce the development of the vegetation cover. Such climate changes directly reduce the role of vegetation in dune stabilization. Based on these considerations, [START_REF] Jackson | Large-scale transgressive coastal dune behaviour in Europe during the Little Ice Age[END_REF] proposed a conceptual model of the transgressive behaviour of dune massifs during the LIA period for the western European coasts. According to this model, the vegetation cover of dune systems was progressively destroyed under the combined action of increased storm strength and decreased temperatures. Large quantities of sands thus moved landward from the coastal dunes by aeolian deflation, rapidly leading to the formation of highly mobile transgressive systems. Similar processes may have affected the coastal dunes of Brittany during phases I, II and IV identified in this study. Only phase III identified between 1,060 and 700 yr cal BP is associated with significantly higher temperatures in western Europe which corresponds to the Medieval Warm Period [START_REF] Lamb | The early medieval warm epoch and its sequel[END_REF][START_REF] Bradley | Climate in Medieval Time[END_REF]. However, more recent studies have shown that this period of climate warming was discontinuous and spatially heterogeneous [START_REF] Bradley | Climate in Medieval Time[END_REF][START_REF] Guiot | Growing Season Temperatures in Europe and Climate Forcings Over the Past 1400 Years[END_REF]. Tree ring analysis in the European Alps and Scandinavia has indicated cool conditions between 900 and 800 cal BP (Büntgen et al., 2006;[START_REF] Grudd | Torneträsk tree-ring width and density ad 500-2004: a test of climatic sensitivity and a new 1500-year reconstruction of north Fennoscandian summers[END_REF][START_REF] Corona | Millennium-long summer temperature variations in the European Alps as reconstructed from tree rings[END_REF] with reconstructed temperatures similar to the LIA. Phase III was recorded only at the Saint-Urnel dune in the Audierne Bay and may not be representative of the dynamics of coastal dunes in Brittany as a whole.

Detritism and fluvial sediment input

An increase in detritism has been suggested to explain the development of coastal dunes in Brittany through episodic inputs of fluvial sediments to the coast during the mid-to late-Holocene period [START_REF] Vliet-Lanoë | Holocene formation and evolution of coastal dunes ridges, Brittany (France)[END_REF]. Significant palynological evidences in western France highlighted that the major retreat of mixed temperate forest is recorded between 4,500 and 4,000 cal BP, i.e. at the start of the Bronze Age [START_REF] David | Modélisation de la végétation holocène du Nord-Ouest de la France : reconstruction de la chronologie et de l'évolution du couvert végétal du Bassin parisien et du Massif armoricain[END_REF][START_REF] Fernane | Climate variability and storm impacts as major drivers for human coastal marsh withdrawal over the Neolithic period (Southern Brittany, NW France)[END_REF]Penaud et al., 2020). In Normandy, evidences of detrital sedimentation has been observed since the Bronze Age in many coastal valleys [START_REF] Lespez | Paysages et gestion de l'eau : sept millénaires d'histoire des vallées et des plaines littorales en Basse-Normandie[END_REF]. The landscape opening was then intensified at the Bronze-Iron Age transition and, with more significance, at the start of the Middle Age (Penaud et al., 2020), leading to soil erosion and episodic inputs of river sediments to the coastal waters [START_REF] Marguerie | Evolution de la végétation sous l'impact humain en Armorique du Néolithique aux périodes historiques[END_REF][START_REF] Larue | Les remblaiements de fond de vallons : l'exemple du Royeau dans le bassin de la Sarthe, à l'ouest du Bassin Parisien (France)/Accretion deposits in vale bottoms: the case of the Royeau in the Sarthe Basin, West of the Paris Basin (France)[END_REF][START_REF] Germain-Vallée | L'apport des recherches géomorphologiques et micromorphologiques récentes à l'archéologie des paysages de la Plaine de Caen (Calvados, Basse-Normandie)[END_REF]Penaud et al., 2020). However, fluvial sediment supplies to the coast seems to have been modulated by variations in the hydrological regime of coastal rivers. A synthesis of the fluvial activity in the middle and lower Loire valley highlighted several periods of higher flood frequency and/or magnitude during the Holocene, reflecting wetter conditions over the Loire watersheds [START_REF] Arnaud-Fassetta | Fluviatile palaeoenvironments in archaeological context: Geographical position, methodological approach and global change -Hydrological risk issues[END_REF]. These periods correspond in part to phases of dune mobility identified in this study (Fig. 13g). Strengthened of the Loire River discharge occurred during time intervals characterized by a southward deflection of westerlies (Penaud et al., 2020) (Fig. 13g). Surprisingly, this situation would provide favourable conditions for coastal dune mobility in Brittany.

In Brittany, the size of coastal rivers and the watersheds areas are very limited and not conduce to abrupt detritic events. Furthermore, the incised morphology of the lower parts of these river valleys favoured the trapping of fluvial sediments as shown by the thickness of some Holocene deposits [START_REF] Menier | Basement Control on Shaping and Infilling of Valleys Incised at the Southern Coast of Brittany, France. Incised Valleys in Time and Space[END_REF]. These valleys are connected to the sea by small-size estuaries where large thicknesses of sediment were deposited during the Holocene [START_REF] Jouet | Ys Valley: A submerged palaeoriver pattern in the Bay of Douarnenez (Brittany, France)[END_REF][START_REF] Gregoire | Control factors of Holocene sedimentary infilling in a semi-closed tidal estuarine-like system: the bay of Brest (France)[END_REF][START_REF] Ehrhold | Fossil maerl beds as coastal indicators of late Holocene palaeo-environmental evolution in the Bay of Brest (Western France)[END_REF]. On the foreshore, seismic data show that the valleys extend into the subtidal domain by submarine incised-valleys partially filled by a sequence of fluvial, estuarine, and marine sedimentary deposits (e.g. [START_REF] Sorrel | Evidence for millennialscale climatic events in the sedimentary infilling of a macrotidal estuarine system, the Seine estuary (NW France)[END_REF][START_REF] Sorrel | Sedimentary archives of the French Atlantic coast (inner Bay of Vilaine, south Brittany): Depositional history and late Holocene climatic and environmental signals[END_REF][START_REF] Menier | The Holocene transgression as recorded by incised-valley infilling in a rocky coast context with low sediment supply (southern Brittany, western France)[END_REF][START_REF] Traini | Transgressive systems tract of a ria-type estuary: The Late Holocene Vilaine River drowned valley (France)[END_REF]. Currently, a limited number of lower coastal valleys are completely filled and represent a former corridor for sediment transport along the land-sea continuum in Brittany. Based on a historical analysis of coastline changes, Stéphan et al. (2019a) recently noted that a large majority of beach/dune systems in Brittany have experienced a retreat process during the last decades due to a depletion of sediment supply, with the exception of sites located near some infilled estuarine mouths. In these areas, the foredunes have shown significant progradation reflecting a positive sediment budget. The authors suggested episodic sediment supply associated to the migration of tidal sandbars from the estuarine mouths to the nearby beaches. Such processes have also been inferred to explain the phases of dune mobility recorded at the mouth of the Gironde River over the last 2,000 years [START_REF] Stéphan | Holocene coastal changes along the Gironde estuary (SW France): new insights from the North Médoc peninsula beach/dune system. Quaternaire[END_REF].

Disturbances due to human activities

The archaeological and historical data examined in this study provides little evidence of possible human disturbances on the coastal dunes of Brittany. For the Neolithic and Bronze Age periods, most of the archaeological remains were discovered within paleosoils currently overlain by the dune sand covers. These sites correspond to perennial or temporary occupations initially located far behind the coastline, so that they do not provide any data on a possible use or frequentation of coastal dunes, except for some flintknapping activity as seen at Kerminihy or, not so far from Brittany, at La Caouillerie in Cotentin peninsula [START_REF] Rousseau | Le site Bronze ancien de La Caillouerie à Saint-Lô d'Ourville (50), un site spécialisé en contexte littoral ?[END_REF]. For the Late Iron Age and Roman periods, archaeological data indicate numerous perennial settlements in dune environments, often with long-lasted occupation of several centuries. The stratigraphy of archaeological sites generally shows anthropogenic deposits with a series of interbedded thin sandy layers that reflect episodic aeolian sand-blows during the human occupation period (e.g. Moulin de la Rive, Locquirec, [START_REF] Giot | Un habitat protohistorique le Moulin de la Rive en Locquirec (29), Travaux du laboratoire « Anthropologie, Préhistoire, Protohistoire, Quaternaire Armoricains[END_REF]Goulvars, Quiberon, Hyvert, 1985, 1986, 1987). These sedimentary sequences may reflect the local impact of human pressure on the vegetation cover (e.g. trampling) leading to a spatially limited remobilization of coastal dunes. During the medieval period, few skattered human occupations are suggested along the coast of Brittany [START_REF] Cassard | Chapitre II. La mer oubliée ou l'empaysannement breton[END_REF][START_REF] Tonnerre | Quelques réflexions sur la société rurale dans la Bretagne du XIe siècle[END_REF], quite limiting the potential impact of human activities on dune environments. However, between the 11th and 14th centuries, the significant expansion of the European population resulted in a strong human footprint on local ecosystems (e.g. [START_REF] Mouthon | Le sourire de Prométhée[END_REF]. In western France, this period is associated with significant clearings (e.g. [START_REF] Marguerie | Evolution de la végétation sous l'impact humain en Armorique du Néolithique aux périodes historiques[END_REF][START_REF] Lambert | Striking forest revival at the end of the Roman Period in north-western Europe[END_REF]Penaud et al., 2020) and coincides with the dune mobility phase III identified in this study. From 800 cal BP, a study conducted in the Bay of Brest highlighted the major threshold when anthropogenic-related unoff forcing exceeds natural trends related to precipitation regimes, either above Loire watersheds or Brittant watersheds [START_REF] Lambert | Striking forest revival at the end of the Roman Period in north-western Europe[END_REF]. In the medieval subsistence society, demographic pressure may have led to the increasing use of marginal land such as sand dunes as a resource. Locally, spatial expansion of cultivated areas could have caused a deterioration of dune vegetation cover and consequently the potential for enhanced aeolian impacts. In the Hebrides islands, [START_REF] Sommerville | Optically stimulated luminescence (OSL) dating of coastal aeolian sand accumulation in Sanday, Orkney Islands, Scotland[END_REF] put forward a somewhat similar hypothesis to explain dune mobility as a result of cultivation of marginal dune lands by coastal populations.

From the 18th century onwards, numerous historical sources provide information on the practices responsible for the mobility of the dunes. In 1752, a law voted by the regional authorities forbade the use of coastal dunes as grazing areas, under penalty of fine and imprisonment due to the sand inundations caused by this practice [START_REF] Charpentier | Le littoral et les hommes : espaces et sociétés des côtes nord de la Bretagne au XVIIIe siècle[END_REF][START_REF] Lageat | Délits d'eau" et "volements de sables", les "offensives éoliennes" sur les côtes bretonnes au XVIIIème siècle[END_REF]. In Landeda (NW Brittany), a "dune warden" is appointed each year to enforce this ban [START_REF] Charpentier | Le littoral et les hommes : espaces et sociétés des côtes nord de la Bretagne au XVIIIe siècle[END_REF]. Measures are also taken by local authorities to fix the sand-dune system or slow its landward migration. Santec (NW Brittany) is illustrative of the efforts undertaken by coastal populations to stabilize the dunes. Starting in 1760, rows of fens were installed along the dunes, and regularly raised (Goury Aîné, 1828). From 1807, a "dune police force" was created in order to supervise the activities practiced on the dunes and their surroundings. Behind the dune front, the massif was seeded with maritime pines, gorse, and broom (Goury Aîné, 1828). The circulation paths were delimited and the non-vegetated spaces were covered with seaweed in order to enrich the land and prevent sand-blows.

Pattern of Holocene coastal dune changes

In Fig. 17, we propose an evolutionary temporal sketch, subdivided into five stages, about morphological changes of coastal sand-dune systems and Human settlements during Holocene times along coastal areas in Brittany.

Stage 0 corresponds to an initial state around 7,000 BP during which the slowing RSL rise led to the formation of the first coastal barriers in Brittany [START_REF] Stéphan | Évolution du niveau marin relatif à l'Holocène le long des côtes françaises de l'Atlantique et de la Manche : réactualisation des données par la méthode des « sea-level index points[END_REF][START_REF] García-Artola | Holocene sea-level database from the Atlantic coast of Europe[END_REF]. The Neolithic period (6,850-4,100 cal BP; stage 0; Fig. 14) is represented by numerous archaeological sites distributed along the coast [START_REF] Giot | Préhistoire de la Bretagne[END_REF], but human settlements apprear to have been systematically located behind coastal systems, such as dunes and marshes.

Between 4,250 and 4,100 cal BP (stage 1, Fig. 14), the primary phase of coastal sand-dune mobility is recorded, related to the combined effect of decreasing temperatures and an enhanced storminess in the NE Atlantic. Coastal dunes were stabilized by the vegetative cover from the Early Bronze Age (4,100-3,550 cal BP), as indicated by archaeological evidence associated with humic horizons interbedded within aeolian sand deposits. Between 3,200 and 2,400 cal BP (stage 2; Fig. 14), a destabilization of the coastal dunes results in the establishment of large transgressive systems that affected the entire coast of Brittany.

Archaeological remains dating from the Middle/Final Bronze Age (3,550-2,750 cal BP) and the First Iron Age (2,750-2,400 cal BP) periods are absent in dune environments, reflecting a decline in coastal occupations that can be attributed to a deterioration in climatic conditions (Stéphan et al., 2019b(Stéphan et al., , 2013)), even if we cannot exclude other human factors or archaeological biases. The 3,300-2,800 cal BP interval (Bond event 2; Fig. 13d) characterized by "NAO+, SPG+" (Fig. 13d,e,f) also corresponds to decreased influence of the warm and salty poleward current (Penaud et al., 2020). Penaud et al. (2020) suggested that coastal sociaties might have turned towards land since palynological data south of Brittany reflect major phases of upland clearance on western France watersheds. From the Late Iron Age onwards (2,400-2,000 cal BP), the dunes were stabilized and numerous occupations, mainly related to sea salt extraction, were established.

A third phase of dune mobility is suspected between 1,050 and 700 cal BP (stage 3, Fig. 14), but the corresponding data only concern the Audierne Bay and may reflect local dynamics linked to site-specific processes. During this period, human occupations were very dispersed along the coasts of Brittany and settlements were located far back from the coastal areas.

The last period of dune mobility, dated between 350 and 110 cal BP (stage 4, Fig. 14), corresponds to the LIA climatic deterioration. The destabilization of sand-dune systems concerned all European Atlantic coasts (e.g. [START_REF] Clarke | Effects of storminess, sand supply and the North Atlantic Oscillation on sand invasion and coastal dune accretion in western Portugal[END_REF][START_REF] Jackson | Large-scale transgressive coastal dune behaviour in Europe during the Little Ice Age[END_REF]. In Brittany, the development of large transgressive systems and their migration several kilometres inland led to the inundation of many villages, churches, and cultivated land by aeolian sands.

The dune massifs were stabilized and vegetated during the second half of the 19th century.

Conclusions

In this paper, the dynamics of Holocene coastal dune systems has been reconstructed for western France based on geomorphological, archaeological and historical data. This study shows that in the absence of radiocarbon-datable interbedded paleosoils and OSL dating of sandy deposits, archaeological remains represent useful chronological indicators of coastal dune mobility. From the re-examination of 221 archaeological sites previously studied by archaeologists over the past 150 years, successive phases of sand movement are dated on 78 coastal sand-dune systems in Brittany, by using the Harris diagram method. For the first time, a regional chronological framework is proposed for Brittany based on a representative set of dune systems well distributed along the western coasts of France. Our results emphasize the episodic nature of the major morphological changes leading to the formation of dune complexes. Four periods of major aeolian sand-drifts are identified: 4,250-4,100 cal BP (period 1), 3,250-2,400 cal BP (period 2), 1,050-700 cal BP (period 3), and 350-110 cal BP (period 4).

During periods 1, 2, and 4, sand movements are associated with an enhanced North Atlantic storm regime [START_REF] Sorrel | Sedimentary archives of the French Atlantic coast (inner Bay of Vilaine, south Brittany): Depositional history and late Holocene climatic and environmental signals[END_REF]Pouzet et al., 2018), indicating the primary role of extreme storm events in dune destabilization. These periods also coincide with rapid climatic changes, marked by a decrease in annual temperatures and an increase in precipitation in western France [START_REF] Arnaud-Fassetta | Fluviatile palaeoenvironments in archaeological context: Geographical position, methodological approach and global change -Hydrological risk issues[END_REF]Penaud et al., 2020). These conditions seem to be favourable to the mobility of coastal dunes, especially through a degradation of the vegetation cover.

From a methodological point of view, the use of archaeological remains as chrono-stratigraphic indicators has shown several limitations, notably because of the scarcity of available archaeological data for specific periods, particularly between the end of the Roman period and the Middle Ages. Pionner discoveries of Neolithic and Bronze Age remains rarely document stratigraphic contexts and chronological attributions lack precision because no radiocarbon dating is available. Consequently, the regional chronological framework provided in this study about the spatio-temporal evolution of coastal dune sand systems could be further refined by the use of OSL dating.

Fig. 1 .

 1 Fig. 1. (a) Distribution of the coastal sand-dune systems in western France (source: 1:50 000 geological map, BRGM, http://infoterre.brgm.fr); (b) Wind roses from 30 years of modelled wind data for the north, south and west coasts of Brittany (source: https://www.meteoblue.com). Tide data is obtained from the Service Hydrographique et Océanographique de la Marine, SHOM-Brest.

Fig. 2 .

 2 Fig. 2. Methodological approach applied in the present work. (a) Distribution of archaeological, historical and sedimentological remains associated to a theoretical coastal dune complex; (b) Organization of the geoarchaeological database (GAD); (c) Harris matrix deduced from available stratigraphical data. The numbers refer to newly-discovered remains.

Fig. 4 .

 4 Fig. 4. Chronological boundaries of archaeological and historical periods in western France. Chronological attributions of remains are subdivided into two precision degrees corresponding to major cultural periods (first degree) and to subdivisions of the latter (second degree).

Fig. 5 .

 5 Fig. 5. Location maps of archaeological remains for : (a) the Neolithic period; (b) Bronze Age period; (c) Iron Age period; (d) Roman period; (e) Medieval period; (f) Modern and contemporary periods. The numbers refer to coastal dune sites listed in Table1.

Fig. 6 .

 6 Fig. 6. Examples of archaeological remains discovered in the coastal dunes of Brittany. (a, b) Stone cists discovered around 1923 in the coastal dune of Port-Blanc (Penvénan, N Brittany) (source: Archives du service Régional d'Archéologie, Rennes); (c) Villa maritimma dated to the High Roman Empire during excavation on the dune of Sables d'Or les Pins (Fréhel, N Brittany) (source: Langouët, 1979); (d) Photography of the Saint-Egarec church around 1924 (Lampaul-Plouarzel, W Brittany) (source: Yroise, Bibliothèque numérique patrimoniale de Brest); (e) Kelp kilns and iodine factory at Porspaul in Lampaul-Plouarzel around 1938 (author: Pierre Toulgouat; source: Museum of European and Mediterranean Civilizations -MUCEM); (f) Kelp kiln at Porspaul in Lampaul-Plouarzel in 2016 (author: Yves Gruet; source: Service Régional de l'Inventaire du Patrimoine en Bretagne).

Fig. 7 .

 7 Fig. 7. Coastal dunes of Molène Island (W Brittany, site no.39). (a) Location map of the archaeological remains; (b) Aerial photography of the dry stone oval houses of Beg ar Loued I site during excavation (author : Marine nationale; source: Pailler and Nicolas, 2019); (c) Harris matrix based on observations at the Beg ar Loued I and II archaeological sites; (d) Modern tomb discovered in aeolian sand during excavation of the Beg ar Loued site (source: Pailler and Nicolas, 2019). (e) Stratigraphic section in the northwestern part of the Beg ar Loued site (author: Y. Sparfel; source: Pailler and Nicolas, 2019).

  In the Goulvars site (Quiberon, S Brittany, site no.70 on Figs3 and 8), the stratigraphic sequence was reconstructed from the analysis of 6 archaeological sites. Two Neolithic megalithic sites (Conguel and Roc'h Priol sites) and an Early Bronze Age stone-cists cemetery (Beg er Vil site) are associated with the clayey palaeosol at the base of the aeolian deposits.

Fig. 8 .

 8 Fig. 8. Coastal dunes of Goulvars (Quiberon, S Brittany, site no.70). (a) Location map of the archaeological remains; (b) Harris matrix based on observations of archaeological sites discovered in the dune complex; (c) Photography of the Goulvars II site (house S11) during the excavation stage (source: Hyvert, 1987); (d) Photography of the stratigraphic section of the M3 wall and the marine shell deposits interbedded in the eolian sand at the Goulvars II site (source:[START_REF] Hyvert | Quiberon (56). Habitat de Goulvars. Rapport sauvetage urgent[END_REF]; (e) Simplified stratigraphic section of the Goulvars II site, after[START_REF] Hyvert | Quiberon (56). Habitat de Goulvars. Rapport sauvetage urgent[END_REF].

Fig. 9 .

 9 Fig. 9. Coastal dunes of Béniguet Island (Le Conquet, W Brittany, site no.41). (a) Location map of the archaeological remains; (b) Harris matrix based on observations at the archaeological sites on Béniguet Island; (c) Stratigraphic sections observed in 2014 and 2016, after Cudennec, (2019) and Pailler and Nicolas (2022); (d) Photography of the stratigraphic section observed in 2014, (source: Cudennec, 2019).

Fig. 10 .

 10 Fig. 10. Coastal dunes of Saint-Nicolas Island (Fouesnant, W Brittany, site no.58). (a) Harris matrix based on observations of archaeological sites discovered in the dune complex; (b) Location map of the archaeological remains; (c) Stratigraphic section of the Saint-Nicolas IV, V and VI sites, after Hamon (2007).

Fig. 11 .

 11 Fig. 11. Coastal dunes of the Audierne Bay (Plomeur, W Brittany, site no.50). (a) Harris matrix based on observations of archaeological sites discovered in the dune complex; (b) Location map of the archaeological remains; (c) Photography of the Kerharo standing stone when it was discovered in 1978 (unknown author; source: Service Régional de l'Archéologie Bretagne); (d) Photography of the Iron Age settlement of La Torche during the excavation stage (source: Kayser 1987); (e) Stratigraphic section of the Saint-Urnel site, after[START_REF] Giot | Le cimetière des anciens Bretons de Saint-Urnel ou Saint-Saturnin en Plomeur (Finistère)[END_REF]; (f) Photography of the Saint-Urnel necropolis during the excavation stage (source:[START_REF] Giot | Plomeur, Cimentière de Saint-Saturnin ou Saint-Urnel, campagne[END_REF].

Fig. 13 .

 13 Fig. 13. Aeolian sand-drift events on the Brittany and Atlantic coasts of France and North Atlantic climate records: Yellow and purple bands represent aeolian sand-drift events identified in this study. (a) Aeolian sand-drift events identified in Brittany and the Channel Islands: 1. Verger Bay, Cancale, North Brittany (Regnault et al., 1995) 2. Beach of Guendrez, Plouhinec, West Brittany (Haslett et al., 2000) 3. Dunes building phases in Brittany (Van Vliet-Lanoë et al., 2016) 4. Herm Island, Channel Islands (Bailiff et al., 2014); (b) Aeolian sand-drift events identified on the Atlantic coasts of France : 5. Picardy, North France (Meurisse et al., 2005) 6. Aquitaine, South-West France (Bertran et al., 2020) 7. Aquitaine, South-West France (Clarke et al., 2002); (c) European Atlantic Storm Periods: 8. European Atlantic storm events from Pouzet et al. (2018) 9. Holocene storm periods in northern Europe from Sorrel et al. (2012); (d) Hematite-stained quartz percentages record from stacked North Atlantic Ocean

Fig. 14 .

 14 Fig. 14. Evolutionary temporal sketch of sand-dune systems and Human settlements during the last 7 000 years along the coasts of Brittany.

Figure 14

 14 Figure 14

Table 1 . Chronological boundaries of sand drift events deduced from archaeological, historical and s

 1

		Location
	1 Ébihens Island	North Brittany
	2 Beach of Quatre Vaux	North Brittany
	3 Beach of La Mare	North Brittany
	4 Bay of Croc	North Brittany
	5 Les Sables d'Or	North Brittany
	6 Les Hôpitaux	North Brittany
	7 Beach of Guen	North Brittany
	8 Beach of Caroual	North Brittany
	9 La Ville Berneuf	North Brittany
	10 Beach of Rosaires	North Brittany
	11 Les Dunes	North Brittany
	12 Landrelles	North Brittany
	13 Trozoul	North Brittany
	14 Moulin de la Rive	North-West Brittany
	14 Moulin de la Rive	North-West Brittany
	15 Aod Venn	North-West Brittany
	16 Plage de Pouldu	North-West Brittany
	17 Roc'h Croum	North-West Brittany
	17 Roc'h Croum	North-West Brittany
	18 Mechou Rouz	North-West Brittany
	19 Theven	North-West Brittany
	19 Theven	North-West Brittany
	19 Theven	North-West Brittany
	20 Theven-Kerbat	North-West Brittany
	21 Kernic	North-West Brittany
	22 Keremma	North-West Brittany
	23 Trégueiller	North-West Brittany
	24 Beach of Lividic	North-West Brittany
	25 Le Scluz	North-West Brittany
	26 Chardons Bleu	North-West Brittany
	27 Mentoul	North-West Brittany
	28 La Grève	North-West Brittany
	29 Trerohan	North-West Brittany
	29 Trerohan	North-West Brittany
	30 Le Vougot	North-West Brittany
	31 La Grève Blanche	North-West Brittany
	32 Sainte-Marguerite	North-West Brittany
	33 Corn ar Gazel	West Brittany
	34 Teven	West Brittany
	35 Bec Island	West Brittany
	35 Bec Island	West Brittany
	36 Saint-Gonvel	West Brittany

Name of coastal dunes Coastal dune IDTable 1

 1 Click here to access/download;Table;Table_1.xlsx archeological data

	estern France.	Historicals sources Etat-Major map	2nd 1st	1st
	Houat Island 0	1474 archeological data aerial photography	archeological data 2nd 1st	1st
	Houat Island Hoedic Island Hoedic Island Hoedic Island Arzon Sarzeau age (cal BP) 2400 0 3908 0 1298 0 40 102 102 0 0 102 102 0 0 0 0 40 2100 121 101 0 2400 101 1474 2750 101 121 121 80 0 101 101 101 101 101 2400 101 101 101 121 164 121 0 2250 121 2006 2060 2400 0 2400 1800 0 2750 0 4100 2100 0 2400 2400 84 0 0 1475 0 0	4648 2006 3716 2006 6850 2006 deduced from archeological data archeological data End aerial photography Etat-Major map archeological data Etat-Major map aerial photography archeological data archeological data aerial photography Etat-Major map Etat-Major map postcard Etat-Major map Etat-Major map aerial photography aerial photography Etat-Major map Etat-Major map aerial photography aerial photography aerial photography aerial photography postcard archeological data archeological data Etat-Major map aerial photography archeological data Etat-Major map archeological data archeological data Etat-Major map archeological data archeological data archeological data aerial photography Etat-Major map Etat-Major map Etat-Major map Etat-Major map Etat-Major map archeological data Etat-Major map Etat-Major map Etat-Major map archeological data Cassini map archeological data aerial photography archeological data archeological data archeological data archeological data archeological data Etat-Major map archeological data aerial photography archeological data Etat-Major map archeological data aerial photography Etat-Major map archeological data Etat-Major map Cassini map aerial photography Map of Cassini Etat-Major map Cassini map archeological data archeological data archeological data archeological data aerial photography Cassini map Etat-Major map Etat-Major map Etat-Major map Cassini map Etat-Major map archeological data archeological data archeological data Cassini map Etat-Major map archeological data Etat-Major map Etat-Major map aerial photography aerial photography archeological data aerial photography aerial photography	archeological data archeological data archeological data archeological data archeological data archeological data 2nd 1st 1st 1st accuracy 2nd 2nd level 2nd 2nd 1st 1st 1st 1st 1st 2nd 2nd 2nd 2nd 1st 1st 2nd 2nd 1st 1st 2nd 2nd 1st 1st 1st 2nd 1st 1st 1st 1st 1st 1st 1st 1st 1st 1st 1st 1st 1st 1st 1st 1st 2nd 1st 2nd 2nd 2nd 2nd 1st 2nd 1st 1st 1st 1st 1st 2nd 2nd 1st 1st 1st 1st 1st 2nd 1st 2nd 1st 1st 2nd 2nd 2nd 1st 2nd 1st 2nd 1st 1st 1st 2nd 1st 1st 1st 1st 1st 1st 1st 1st 1st 1st 1st 1st 1st	2nd 2nd 2nd 2nd 1st 1st