Elodie Brisset
email: elodie.brisset@gmail.com

Francesc Burjachs

Bruno José

Ballesteros Navarro

Javier Fernández-López De Pablo

Iphes

Socio-ecological adaptation to Early-Holocene sea-level rise in the western Mediterranean

Keywords:

Consecutively to the global sea-level rise, coastal landscapes significantly changed during the Early Holocene. We explored how coastal mutations have affected ancient human systems in the western Mediterranean. The Pego-Oliva basin (Spain) provides a thick sediment sequence located in proximity to two well-studied Mesolithic and Neolithic archaeological sites. Based on 16 sediment cores, sedimentological analyses, 14 C dates, and integration of previous works, we reconstructed the architecture of the sedimentary facies and the chronology of deposition for the last 9500 years. From 9500 to 8200 cal. BP, the stepwise inland migration of the coastline is characterised by the deposition of peat layers at distinct depths indicating four phases of barrier-lagoon systems development. The marine transgression caused the disappearance of the lagoon at 8200 cal. BP, and reached a maximum inland position at 7300 cal. BP. Since 5800 cal. BP, sea-level stabilisation induced coastal progradation and barrier-lagoon system construction. Five marine flooding phases dated at 9500, 9100-9000, 8750-8650, 8450-8200, and 8100-7300 cal. BP match Atlantic meltwater pulses. Comparison to archaeology shows that the long-lasting Mesolithic occupation corresponds to the presence of coastal offering easily exploitable local intertidal resources. In contrast, during the Late Mesolithic, we observe a lagoon contraction and the reduction of lagoon bivalves size arguing for a decrease in resource productivity. This changes in the lagoon ecosystems negatively impacted to Late Mesolithic populations leading to a less intense occupation of this area. Therefore, the 700 years' temporal hiatus between the last hunter-gatherers and the first farmers occupying this area is explained by the disappearance of the formerly exploited biotopes because of sea-level rise, a situation that may have been aggravated by coincidence of the 8200 cal. BP climate event. The maximal sea transgression corresponded to a shift from intertidal to infralittoral species between the Impressa/Precardial and the Postcardial Neolithic, indicating a good adaptation to changing environments at that time. However, comparison between Late Mesolithic to Early Neolithic suggests that communities with different cultural baggage have had a different perception of the same environment. By the past perspective, it emphasises that perception of the ecosystem value as a benefit or constraint, and of the risks associated with sea-level rise, relies first on the local and regional scales adaptive capacity of society to global changes.

Introduction

Coupled paleoenvironmental and archaeological studies offer longterm perspectives of the resilience of societies to current global changes [START_REF] Van De Noort | Conceptualising climate change archaeology[END_REF]. However, understanding how past humans interrelate to the environment further temporal comparisons between climatic, environmental and archaeological archives need to be implemented, by accessing the cascading set of socio-ecosystem responses from the global to the local scale. Following the end of the Last Glaciation, the coastal configuration profoundly changed resulting in the global sea-level rise flooding of the Late Pleistocene littoral plains [START_REF] Shennan | Modelling western North Sea palaeogeographies and tidal changes during the Holocene[END_REF][START_REF] Jakobsson | Post-glacial flooding of the Bering Land Bridge dated to 11 cal ka BP based on new geophysical and sediment records[END_REF]. Sea-level rise was translated into the loss of human settlement areas, reduction of hunting territories, and modification of coastal ecosystems. Opposite views on whether coastal plains have been effectively attractive for prehistoric communities persist [START_REF] Bailey | World prehistory from the margins: the role of coastlines in human evolution[END_REF]. While some authors have proposed that Holocene coasts consisted of marginal areas, compared to the hinterland, others have argued for coastal landscapes as an interface between land and sea being places of countless opportunities for early humans [START_REF] Dupont | Harvesting the seashores in the late mesolithic of northwestern Europe: a view from Brittany[END_REF][START_REF] Walsh | The archaeology of Mediterranean landscapes[END_REF][START_REF] Benjamin | Late Quaternary sea-level changes and early human societies in the central and eastern Mediterranean Basin: an interdisciplinary review[END_REF]. In the Mediterranean, marine resources have represented a low but significant part of the human dietary needs during the Early Holocene [START_REF] Costa | Early settlement on Tyrrhenian islands (8th millennium cal. BC): Mesolithic adaptation to local resources in Corsica and Northern Sardinia[END_REF][START_REF] Goude | New insights into mesolithic human diet in the mediterranean from stable isotope analysis: the sites of Campu Stefanu and Torre d'Aquila, Corsica[END_REF]. However, the interaction between humans and their environments were not restricted to consumption purposes. Geomorphological heterogeneity and the associated biotic richness and diversity [START_REF] Burnett | The influence of geomorphological heterogeneity on biodiversity I. A patch-scale perspective[END_REF] may have rendered these areas favourable for a large range of activities including shell gathering, fishing, hunting, agriculture and they well as offered easily accessible corridors for human movements [START_REF] Erlandson | The archaeology of aquatic adaptations: paradigms for a new millennium[END_REF]. The issue is to determine to what extent Early to Mid-Holocene changes on coastal biotopes associated to sea-level rise have affected patterns of human exploitation and occupation [START_REF] Colonese | Marine mollusc exploitation in Mediterranean prehistory: an overview[END_REF][START_REF] Mannino | Stuck to the shore? Investigating prehistoric hunter-gatherer subsistence, mobility and territoriality in a Mediterranean coastal landscape through isotope analyses on marine mollusc shell carbonates and human bone collagen[END_REF][START_REF] Colonese | Late Pleistocene-Holocene coastal adaptation in central Mediterranean: snapshots from Grotta d'Oriente (NW Sicily)[END_REF].

This work focuses on the paleogeographic reconstruction of coastal and human systems in the course of the Mesolithic and the Neolithic in the Pego-Oliva basin (Valencia, Spain). In the Western Mediterranean, recent studies have investigated human-environmental interactions during the prehistoric period (e.g. [START_REF] Dolez | Holocene palaeoenvironments of the Bages-Sigean lagoon (France)[END_REF][START_REF] Currás | Reconstructing past landscapes of the eastern plain of Corsica (NW Mediterranean) during the last 6000 years based on molluscan, sedimentological and palynological analyses[END_REF][START_REF] Fontana | Lagoonal settlements and relative sea level during bronze age in northern Adriatic: Geoarchaeological evidence and paleogeographic constraints[END_REF][START_REF] Vacchi | New insights into the sea-level evolution in Corsica (NW Mediterranean) since the late Neolithic[END_REF][START_REF] Melis | 8000 years of coastal changes on a western Mediterranean island: a multiproxy approach from the Posada plain of Sardinia[END_REF]. However, the low preservation of Early Holocene coastal sites as well as the paucity of integrated research programs correlating archaeological and palaeoenvironmental records, have prevented the answering of this question. Pego-Oliva area is a very well-adapted case study site for investigating the relationship between environmental changes and prehistoric socio-economic dynamics. This area provides sedimentary records to identify the coastal evolution from the Middle Pleistocene to the Holocene [START_REF] Torres | The long Pleistocene record from the Pego-Oliva marshland (Alicante-Valencia, Spain)[END_REF]. Previous studies based on sedimentary and paleontological analyses have established the main trend of the morphogenetic evolution [START_REF] Dupré | Quaternary evolution of the Pego coastal lagoon (Southern Valencia, Spain). Palaeogeogr. Palaeoclimatol[END_REF][START_REF] Viñals | Aportación al conocimiento de las facies lagunares y litorales de la marjal de Oliva-Pego (Valencia)[END_REF]Fumanal et al., 1993aFumanal et al., , 1993b;;[START_REF] Viñals | Secuencias estatigráficas y evolución morfológica del extremo meridional del Golfo de Valencia (Cullera-Dénia)[END_REF][START_REF] Viñals | Quaternary development and evolution of the sedimentary environments in the Central Mediterranean Spanish coast[END_REF][START_REF] Navarro | Zonas húmedas mediterráneas y acuíferos asociados. Condicionantes hidrogeológicos del Marjal de Pego-Oliva (Alicante-Valencia)[END_REF], but are insufficient to be compared and contrasted to the human record given the low chronological resolution. In addition, the interest of this area lies in its proximity to two major archaeological sites. A renewed record has been produced, that includes dated Mesolithic human remains [START_REF] Gibaja | The emergence of mesolithic cemeteries in SW Europe: insights from the El Collado (Oliva, Valencia, Spain) radiocarbon record[END_REF] and shell midden deposits (Fernández-López de Pablo, 2016), isotopic and zooarchaeological reconstructions of dietary patterns [START_REF] García Guixé | Palaeodiets of humans and fauna at the Spanish mesolithic site of El Collado[END_REF][START_REF] De Pablo | El Collado shell midden and the exploitation patterns of littoral resources during the Mesolithic in the Eastern Iberian Peninsula[END_REF] and the discovery of Early Neolithic occupations containing rich mollusc assemblages [START_REF] Esquembre-Bebia | El yacimiento neolítico del Barranquet de Oliva (Valencia)[END_REF][START_REF] Bernabeu | The first agricultural groups in the Iberian peninsula[END_REF], representing the introduction of farming dispersal episodes by seafaring pioneering groups. In this study, we first reconstruct the coastal morphogenetic evolution during the Early and Middle Holocene; then we compare the sedimentary evolution at Pego-Oliva with other Mediterranean sedimentary records to evaluate the driving forces (e.g. climate change) behind the Early Holocene relative sea-level changes, and finally, we explore how coastal mutation affected prehistoric settlement and subsistence patterns.

Study site

The study area is located in the south of the Valencian gulf in Western Mediterranean (Fig. 1A). This area is characterised by a succession of flat coastal plains occupied by lagoons (e.g. Albufera de Valencia) and marshes, interrupted to the south by sea cliffs [START_REF] Viñals | Quaternary development and evolution of the sedimentary environments in the Central Mediterranean Spanish coast[END_REF]. The saltmarsh of Pego-Oliva is separated from sea by an 8 km long and 1.5 km wide sand barrier system parallel to the coastline. Several washover fans passing through the barrier are developed above the marsh. The saltmarsh is fed by two intermittent rivers fed by runoff, karstic springs and the ground-water nappe. The area is artificially drained for agricultural purposes and it is subjected to saline water intrusions resulting in groundwater exploitation [START_REF] Navarro | Zonas húmedas mediterráneas y acuíferos asociados. Condicionantes hidrogeológicos del Marjal de Pego-Oliva (Alicante-Valencia)[END_REF]. Colluvial glacis (Fig. 1B) developed in the mountain feet are attributed to the Pleistocene [START_REF] Viñals | Quaternary development and evolution of the sedimentary environments in the Central Mediterranean Spanish coast[END_REF]. A lower level of colluvial fans is partly developed on the marsh. The catchment is oriented SW-NE, dominated by Upper Cretaceous limestone mountains reaching 700 m a.s.l. The basin floor consists of Miocene formations (Fig. 1B). A thick Pleistocene sediment sequence is deposited below the sea level [START_REF] Torres | The long Pleistocene record from the Pego-Oliva marshland (Alicante-Valencia, Spain)[END_REF]. Beachrocks attributed to the Early Holocene have been identified between -25 and 30 m (Fig. 1B) according to the previous stratigraphic correlation based on sediment cores and underwater seismic profiles [START_REF] Somoza | El sector de plataforma en el tramo meridional del Golfo de Valencia[END_REF]Fumanal et al., 1993b;[START_REF] Rey | The Valencian coast (western mediterranean): neotectonics and geomorphology[END_REF].

The study area has been successively occupied by Mesolithic and Neolithic human communities during the Early to Mid-Holocene. El

Collado is an open-air site that has uncovered a Mesolithic cemetery composed of 14 burials associated with a stratigraphic sequence containing lithic, faunal and shellfish assemblages. Several studies have been published on the stratigraphic sequence (Fernández-López de Pablo, 2016), lithic typology [START_REF] Aparicio | El Collado, in: Pleistocene and Holocene Hunter Gatherers in Iberia and the Gibraltar Strait: The Current Archaeological Record[END_REF], chronology of the funerary practices [START_REF] Gibaja | The emergence of mesolithic cemeteries in SW Europe: insights from the El Collado (Oliva, Valencia, Spain) radiocarbon record[END_REF], human diet [START_REF] García Guixé | Palaeodiets of humans and fauna at the Spanish mesolithic site of El Collado[END_REF], and exploitation of littoral resources [START_REF] De Pablo | El Collado shell midden and the exploitation patterns of littoral resources during the Mesolithic in the Eastern Iberian Peninsula[END_REF]. The chronological model of El Collado (Fernández-López de Pablo, 2016) integrates 14 C dates on human burials with dates from previous and posterior occupation horizons. This site includes two Early Mesolithic phases (level IV: 9830-9550 cal. BP and level II: 9435-8475 cal. BP) and one Late Mesolithic phase (level I: 8510-8390 cal. BP to 8500-8060 cal. BP). Despite an extensive program of dating, there is no evidence of Late Mesolithic occupation during and after the chronological span of the 8200 cal. BP event, suggesting the end of the human occupations at this time. El Barranquet is an open-air Early Neolithic site comprising two occupational phases [START_REF] Esquembre-Bebia | El yacimiento neolítico del Barranquet de Oliva (Valencia)[END_REF]. The first phase, characterised by Impressa/Precardial pottery, has been dated at 7550-7320 cal. BP. Both the ceramic record and the chronology show strong cultural affinities with the impressa Neolithic facies documented in the Ligurian-Provencal arc, associated with the first spread of farming in the western Mediterranean [START_REF] Bernabeu | La cerámica impresa mediterránea en el origen del Neolítico de la península ibérica[END_REF]. The second phase (UE 74 and 71) corresponds to the Postcardial Neolithic which is dated in the region at 6800-6000 cal. BP by its decorative style. Preliminary works have provided paleodietary information from malacological assemblages (Luján Navas, 2016).

Material and methods

Six new boreholes were carried out in the Pego-Oliva basin in 2017 using a geotechnical piston corer comprising core segments of 50 cm and a 10 cm core catcher. The core segments were then stored in 60 cmwidth boxes in a cold room (2 °C). The borehole locations were acquired by GPS and altitude is reported according to LIDAR data (altimetric accuracy of 0.3 cm) of the Institut Cartogràphic Valencià with mean sea-level as a benchmark (P17-2: N38.87124, W0.04462, 0.89 m; P17-3: N38.86138, W0.05557, 0.67 m; P17-4: N38.85476, W0.06613, 1.86 m; P17-5: N38.85556, W0.07360, 3 m; P17-7: N38.85790, W0.06142, 0.67 m; P17-8: N38.87569, W0.08774, 1.8 m). The lithostratigraphy has been described based on colour, texture, grain-size, and presence of macro-remains (plant debris, shells, gravels).

Thirty-nine volumetric samples were obtained from the different sedimentary facies from three representing cores (P17-3, P17-5, and P17-7) and homogenised. A volume of 5 cm 3 was dedicated to dry density and loss on ignition analyses. These samples were weighed, dried at 60 °C for 7 days, and weighed again to estimate the dry density. These samples were then ignited at 550 °C for 5 h [START_REF] Meyers | Sediment organic matter[END_REF] and weighed to measure the Total Organic Carbon (TOC), and then ignited again at 950 °C for 1 h to estimate the Total Inorganic Carbon (TIC). The remaining ignited fraction corresponds to the Non-Carbonate Inorganic Residue (NCIR). The remaining sample volume was dried and sieved at meshes of 2 mm, 150 μm, and 50 μm for grainsize analysis. Mesh residues were weighed to estimate the percentage of gravel, sand, silt, and clays.

Terrestrial vegetal macro-remains (wood, charcoal, peat stem and leaves) were manually picked under a binocular microscope and 14 C dated (Radiocarbon Laboratory of Poznan). The 14 C ages were calibrated in calendar years Before Present (cal. BP) using the Clam R package [START_REF] Blaauw | Methods and code for "classical" age-modelling of radiocarbon sequences[END_REF]) that includes the IntCal13 calibration curves Fig. [START_REF] Navarro | Zonas húmedas mediterráneas y acuíferos asociados. Condicionantes hidrogeológicos del Marjal de Pego-Oliva (Alicante-Valencia)[END_REF]and blue: Dupré et al., 1988;[START_REF] Viñals | Aportación al conocimiento de las facies lagunares y litorales de la marjal de Oliva-Pego (Valencia)[END_REF]Fumanal et al., 1993aFumanal et al., , 1993b) ) and, of the transects discussed in the manuscript. The seismic profile carried out by [START_REF] Viñals | Quaternary development and evolution of the sedimentary environments in the Central Mediterranean Spanish coast[END_REF] is approximately located on the a'-b' profile. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) [START_REF] Reimer | IntCal13 and Marine13 radiocarbon age calibration curves 0-50,000 years cal BP[END_REF]. Two models have been computed that takes into account the probability density function of the 14 C ages and a confidence interval of 2σ (10,000 iterations). The age-depth model, corresponding to the vertical axis, is calculated using a smooth spline function (Supplementary material 1). The age-distance model, corresponding to the horizontal axis, is calculated using a linear function (Supplementary material 2). Ten boreholes previously published have been integrated to this study [START_REF] Dupré | Quaternary evolution of the Pego coastal lagoon (Southern Valencia, Spain). Palaeogeogr. Palaeoclimatol[END_REF][START_REF] Viñals | Aportación al conocimiento de las facies lagunares y litorales de la marjal de Oliva-Pego (Valencia)[END_REF]Fumanal et al., 1993b;[START_REF] Viñals | Quaternary development and evolution of the sedimentary environments in the Central Mediterranean Spanish coast[END_REF][START_REF] Navarro | Zonas húmedas mediterráneas y acuíferos asociados. Condicionantes hidrogeológicos del Marjal de Pego-Oliva (Alicante-Valencia)[END_REF]. Sedimentological descriptions have been harmonised on the basis of the analysis carried out for this study, and the consultation of the cores PDGEST-2 and PDGEST-7 at the IGME geological samples library (Peñarroya, Spain). Three samples were taken in PDGEST-2 for 14 C dating. The radiocarbon ages have been re-calibrated using IntCal13 and Marine13. For the marine sample (Glycimeris shell, 6130 ± 100 years BP), an isotopic fractionation (δ 13 C = 1.14‰) and an average marine reservoir effect (ΔR) of 94 ± 61 years have been accounted for in the study site based on the closest ΔR estimates, i.e. Banyuls in France and Castiglione in Algeria [START_REF] Siani | Radiocarbon reservoir ages in the Mediterranean Sea and Black Sea[END_REF][START_REF] Reimer | Marine radiocarbon reservoir corrections for the mid to late Holocene in the eastern subpolar North Atlantic[END_REF].

Results

Geometry of the sedimentary bodies

The sediment cores are characterised by six sedimentary units (Fig. 2). Unit 1, corresponds to the pre-Holocene infilling basin (Fumanal et al., 1993a;[START_REF] Torres | The long Pleistocene record from the Pego-Oliva marshland (Alicante-Valencia, Spain)[END_REF]. It consists of colluvial facies made of a red-brown clay matrix with heterometric coarse gravel that laterally corresponds to a palustrine facies made of light beige silt-clays dominated by calcium carbonates. In discontinuity to Unit 1, the Holocene sedimentation starts in Unit 2. Unit 2 consists of decimetre-thick homogeneous black peat facies (Fig. 2) rich in organic matter (TOC = 45%) and Non-Carbonate Inorganic Residues (NCIRs = 43%), having a relatively low density (0.5 g/cm 3 ). Benthic foraminifera are present (Ammonia beccarii) together with Cerastoderma glaucum bivalves that are characteristic of lagoonal environmental conditions influenced by marine waters [START_REF] Dupré | Quaternary evolution of the Pego coastal lagoon (Southern Valencia, Spain). Palaeogeogr. Palaeoclimatol[END_REF][START_REF] Torres | The long Pleistocene record from the Pego-Oliva marshland (Alicante-Valencia, Spain)[END_REF]. Unit 3 is decametre-thick, dominated by a calcium carbonate sand fraction (mean sand value = 60%, Total Inorganic Carbon = 13.5% in a greyblue clay matrix (29%). TOC is minimal (2%). From bottom to top, increasing proportions in benthic foraminifera (Haynesina germanica and Ammonia beccarii compared to Elphidium excavatum and Ammonia perlucida) suggest a decrease in salinity over this littoral facies, from open-sea to semi-enclosed lagoon environments [START_REF] Dupré | Quaternary evolution of the Pego coastal lagoon (Southern Valencia, Spain). Palaeogeogr. Palaeoclimatol[END_REF]. Unit 4 consists of a decametre-thick homogeneous to fibrous black peat sediments (TOC = 55%) that are developing up to the core surface of several boreholes. Microfossil assemblages are dominated by benthic foraminifera A. beccarii reflecting a lagoon environment, mostly influenced by brackish waters subjected to marine intrusion [START_REF] Dupré | Quaternary evolution of the Pego coastal lagoon (Southern Valencia, Spain). Palaeogeogr. Palaeoclimatol[END_REF]. In one borehole, the sedimentary sequence is capped by Unit 5, characterised by dense (1.8 g/cm 3 ) minerogenic clays (NCIR = 88%, clays = 92%). Isolated angular gravels occurred in this unit (maximum of 7%). This facies corresponds to a terrestrial detrital environment, characteristic of colluvial fan activity (Fig. 1B). This unit classification (Unit 1 to 5), based on sedimentological and biological content, is applied to all the boreholes taken in the basin, including those from previous studies (Fig. 3).

Fig. 1C shows the position of the boreholes: two transects (Fig. 3A andB) across the basin perpendicularly to the shoreline, and a parallel one (Fig. 3C) located on the beach barrier. All the boreholes except P17-2 reach the Pleistocene deposits of Unit 1 [START_REF] Torres | The long Pleistocene record from the Pego-Oliva marshland (Alicante-Valencia, Spain)[END_REF]. The thickness of the Holocene infilling increases from the land toward the sea (1.5 m in P17-5 to 22 m in P1) and from the South-east of the basin toward the central part (5.5 m in PDGEST-4, 10 m in PDGEST-3 and 22 m in P1 and PDGEST-1). Unit 2 (lagoon facies) occurs in 8 boreholes at four depths (Fig. 3): -20 m in P1-7, -17.5 m in PDGEST-2 and PDGEST-7, -15.5 m in P3, -11 m in P4 and P17-3, P6 and P7. Unit 2 is deeper toward the sea and shallower landward, reflecting a backstepping retrogradational sequence associated with a transgressive system tract. Unit 2 is always capped by the shoreface sand facies (Unit 3) reflecting a marine transgression that reached its maximum of Asterisks indicate references to this study (*), [START_REF] Dupré | Quaternary evolution of the Pego coastal lagoon (Southern Valencia, Spain). Palaeogeogr. Palaeoclimatol[END_REF][START_REF] Viñals | Quaternary development and evolution of the sedimentary environments in the Central Mediterranean Spanish coast[END_REF]Fumanal et al., 1993aFumanal et al., , 1993b landward migration 4 km to the modern shoreline. Unit 3 reaches the modern surface (0 m) at the position of the sand barrier (PDGEST-7 and PDGEST-2). In the marsh, the contact between the Unit 3 and Unit 4 occurs at -2.5 m in P17-3 and -2 m in P17-7, P7, PDGEST-6, P6, P4, and P3. Unit 4 reflects a highstand system tract. Colluvial deposits of Unit 5 are intercalated below Unit 4 (-2 m in PDGEST-8 and P17-4) and above (P17-5).

Chronostratigraphy

Nineteen radiocarbon ages were obtained, comprising nine new ones in this study (Table 1). Compared to previous work the particular methodological attention paid to the new samples (i.e. picking up small, up to 200 μm in size, identified terrestrial material one by one under a binocular loupe to avoid old carbon contamination) allows for greatly improving the chronological constrain (e.g. reducing error to a maximum of 50 years BP). All samples except one (R-2013) are from terrestrial macroremains (peat leave, charcoal, wood) from Unit 2 and Unit 4. The 14 C ages are classified following the methodology of [START_REF] Vacchi | Multiproxy assessment of Holocene relative sea-level changes in the western Mediterranean: sea-level variability and improvements in the definition of the isostatic signal[END_REF] estimating the Relative Sea Level (RSL) elevation based on environmental indicators (Table 1). Most of the 14 C ages (18 ages) were collected from lagoon environments influenced by marine water, which means a RSL elevation of about -0.5 m of the sample depth. One sample from a shoreface facies (Table 1) provides a minimum RSL depth position. The cumulative altimetric error associated to the RSL estimation includes the local tide (10 cm), levelling (10 cm), deformation due to coring (20 cm), and angle deviation (10 cm for sample having depth superior to 2 m). An error of 40 cm has been applied for coring/sampling deformation for the previous studies. The RSL vertical errors are comprised between ± 0.2 and ± 0.4 m.

Results indicate that sedimentary infilling took place between 9430 and 9060 cal. BP (2σ) and Present (Fig. 4B). The pre-marine transgression lagoon environment (Unit 4) has provided seven 14 C ages ranging from 12,830-10,520 cal. BP at -20 m, to 8290-8010 cal. BP at -11 m. The sample UBAR-77, considered with caution because of the large uncertainty (460 years BP), is discarded according to the result of the age-depth model. The modelled age at -20.5 m is 9430-9060 cal. BP (Fig. 4B) is significantly different from the age obtained at -11.5 m (8250-8060 cal. BP). The three samples measured at -18, -18.5 and -19 m in the same borehole (PDGEST-2) have similar ages ranging between 8675 and 9050 cal. BP (2σ). The maximum deposition duration of this peat layer is of 375 years, indicating that the sedimentation rate cannot be inferior to 20 m/year. The maximal sea inland extension (4 km from the modern position) is dated at 7280-7420 cal. BP (-4 m in core P17-8). According to this result, the sea-level rise has been high up to that date with a mean value of 9 mm/year (Fig. 4B). The second lagoonal phase (Unit 4) developed on the marine sediments of Unit 3 has been dated at 6090-5760 cal. BP in core P17-3, 6180-5760 cal. BP in P7, and 6280-5930 cal. BP in P6. Overlapping ages (2σ) indicate that lagoon development was synchronic at the scale of the basin. By that time, the shoreline position was closed to the modern one, characterised by a sand barrier probably closing, at least partially, the bay of Pego-Oliva. Moreover, age continuity over the Unit 4 indicates that no significant hiatus of sedimentation or abrupt environmental changes occurred from that time. Finally, several colluvial layers (Unit 5) occurred before 7420-7280 cal. BP (P17-8), before 5890-5620 cal. BP (P17-4), and after 1170-930 cal. BP (P17-5). The spatial extent of the fans has never reached Unit 3 indicating that catchment sediment supply has been modest during the Holocene.

Based on these results, the RSL position has been reconstructed for the six key-dates of the morphogenetic evolution at 9030, 8840, 8555, 8095, 7330, 5760 cal. BP (Fig. 5A). The shoreline displacement is 7 km inland in line with the wetland, and 5 km inland in the northern sector that corresponds respectively to 4 and 2 km inland compared to the present shoreline. The absolute horizontal shoreline displacement is characterised by a linear relationship and a displacement rate of 180 m/ 50 years (Supplementary material 2).

Discussion

Significance of the early to mid-Holocene reconstructions

At a global scale, observation and model indicate that the RSL rising rate was almost uniform from 11,650 to 7000 cal. BP [START_REF] Lambeck | Sea level and global ice volumes from the Last Glacial Maximum to the Holocene[END_REF]. A rising rate of 8 mm/year has been documented in the Mediterranean [START_REF] Vacchi | Multiproxy assessment of Holocene relative sea-level changes in the western Mediterranean: sea-level variability and improvements in the definition of the isostatic signal[END_REF], that is in good agreement with the estimated rate at Pego-Oliva of 9 mm/year (Fig. 4B). Besides, we have shown that peat layer deposition between -18.5 and -17.5 m spanned a maximum of 375 years that also fits a rising rate of 9 mm/year. A sudden slowdown occurred at 7000 cal. BP (the best modelled age for the breakpoint is 7150 cal. BP, Supplementary Material 1) with a mean value of 0.6 mm/yr. In general, the ICE 5G-VM2 isostatic model in

Table 1

Radiocarbon ages in their sedimentation context in relation with the sea-level position. Relative Sea-Level (RSL) inferred depth, vertical error, and type of index point [START_REF] Vacchi | Multiproxy assessment of Holocene relative sea-level changes in the western Mediterranean: sea-level variability and improvements in the definition of the isostatic signal[END_REF]. Numbering refers to 1) this study; 2) [START_REF] Dupré | Quaternary evolution of the Pego coastal lagoon (Southern Valencia, Spain). Palaeogeogr. Palaeoclimatol[END_REF]; 3) [START_REF] Viñals | Quaternary development and evolution of the sedimentary environments in the Central Mediterranean Spanish coast[END_REF]4) Fumanal et al., 1993a4) Fumanal et al., , 1993b Central Spain [START_REF] Vacchi | Multiproxy assessment of Holocene relative sea-level changes in the western Mediterranean: sea-level variability and improvements in the definition of the isostatic signal[END_REF]; Fig. 4B) and the observations in Pego-Oliva are in good agreement in the Early and Mid-Holocene, while there is a misfit in the Late Holocene (Fig. 4B). This offset is likely to be explained by subsidence [START_REF] Vacchi | Multiproxy assessment of Holocene relative sea-level changes in the western Mediterranean: sea-level variability and improvements in the definition of the isostatic signal[END_REF], that may be an effect of tectonic and compaction [START_REF] Marriner | Nile Delta's sinking past: quantifiable links with Holocene compaction and climate-driven changes in sediment supply?[END_REF]. Indeed, the Miocene basement of Pego-Oliva presents normal fault-limited blocks have been responsible for subsidence during the Quaternary at rates of 4 to 7 mm/ year, making conditions for deposition of a 140 m-thick sediment pile [START_REF] Torres | The long Pleistocene record from the Pego-Oliva marshland (Alicante-Valencia, Spain)[END_REF]. Besides, during the Holocene, the newly acquired 14 C ages having analytical errors greatly reduced compared to previous works (Table 1), indicates that peat layers deposited at -18 m and -11 m have significant different ages (respectively 8594-8977 cal. BP and 8011-8292 cal. BP) indicating that during the Early Holocene, varying depths of these peat layers cannot be explained by subsidence alone.

The paleogeography reconstructions proposed in Fig. 5A are built on the step backward peat sequence (Unit 2) interpreted as a lagoonal environments subjected to marine influence. Based on a literature review linking environmental context and RSL position, samples carried out in inner or semi-enclosed lagoon sediments are likely to correspond to a mean RSL position of 0 to -1 m [START_REF] Vacchi | Multiproxy assessment of Holocene relative sea-level changes in the western Mediterranean: sea-level variability and improvements in the definition of the isostatic signal[END_REF]. A lowering of 1 m implies a shoreline displacement of 500 m seaward (Fig. 5A), because projection on the present-day bathymetry implies that bathymetric profile has not changed from that time, an assumption that is easily arguable in context of rocky coast but that is questionable for the soft coasts like Pego-Oliva. Underwater seismic profiles and high-resolution bathymetry carried out in the Gulf of Valencia and at the line with the Pego-Oliva basin have revealed sand wave features at depths ranging from -55 to -80 m, inferring an RSL position between -30 to -50 m (Alcántara- Carrió et al., 2013;[START_REF] Albarracín | Seismic evidence for the preservation of several stacked Pleistocene coastal barrier/lagoon systems on the Gulf of Valencia continental shelf (western Mediterranean)[END_REF][START_REF] Albarracín | Relict sand waves in the continental shelf of the Gulf of Valencia (Western Mediterranean)[END_REF], probably associated with the Younger Dryas (12,900-11,700 cal. BP; [START_REF] Blockley | Synchronisation of palaeoenvironmental records over the last 60,000 years, and an extended INTIMATE event stratigraphy to 48,000 b2k[END_REF]. Besides, several beachrocks are located from -25 to -35 m [START_REF] Rey | The Valencian coast (western mediterranean): neotectonics and geomorphology[END_REF]. Beachrocks, as lithified deposits, are generally formed in intertidal or supratidal zones, but without cement analyses, a conservative approach is to estimate a mean RSL range of +2 to -1 m [START_REF] Vacchi | Multiproxy assessment of Holocene relative sea-level changes in the western Mediterranean: sea-level variability and improvements in the definition of the isostatic signal[END_REF]. Related to these beachrocks, the mean RSL would have been located between -23 and -36 m. This depth interval is in agreement with the development of an inner coastline lagoon at -20 m. Thus, considering the beachrock position (Figs. 1B,4A), the shoreline could not have been more than 1 km seaward, arguing that shoreline projections on the present-day bathymetry are relevant (Fig. 5A).

Influence of climate on the morphogenetic evolution

Three main phases of the coastal evolution have been identified at Pego-Oliva during the Early to Mid-Holocene (Fig. 5B). From 9500 to 8200 cal. BP, the retrogradational architecture is characterised by four steps of shallow inner lagoon deposits. In Iberian Mediterranean, similar isolated ages have been obtained for pre-marine transgression brackish lagoon deposits, dated at 10,555-9665 cal. BP in Xàbia [START_REF] Viñals | Rasgos paleoambientales holocenos de la Bahía de Xábia (Alicante)[END_REF], at 8320-8180 cal. BP and 8530-8280 cal. BP in the Albufera [START_REF] Carmona | Historical morphogenesis of the Turia River coastal flood plain in the Mediterranean littoral of Spain[END_REF][START_REF] Carmona | Environmental evolution and mid-late Holocene climate events in the Valencia lagoon (Mediterranean coast of Spain)[END_REF]. A similar timing of the backstepping sequence has been found in the Rhone delta at 9800-8200 cal. BP (Amorosi et al., 2013a), in Tuscany at 9600-7880 cal. BP (Amorosi et al., 2013b), in the Adriatic Sea at 9200-7700 cal. BP [START_REF] Amorosi | Global sea-level control on local parasequence architecture from the Holocene record of the Po Plain, Italy[END_REF]. Retrogradational coastal sequences are in accordance with the high global RSL rising rate of the end of the last glaciation relative to the Laurentide ice sheet dismantling [START_REF] Ortlieb | Relative sea level in the Western Mediterranean basin: a regional test of the ICE-7G_NA (VM7) model and a constraint on late Holocene Antarctic deglaciation[END_REF]. From 8200 to 7300 cal. BP, the backward shoreline migration reached a maximal inland position leading to the disappearance of the former inner lagoon. The marine transgression inland has been dated in Xàbia, in the Pego vicinity at 7570-6865 cal. BP [START_REF] Viñals | Quaternary development and evolution of the sedimentary environments in the Central Mediterranean Spanish coast[END_REF]. In the Valencian gulf, the maximal inland position of the beach dated at 7500-7200 cal. BP [START_REF] Carmona | Historical morphogenesis of the Turia River coastal flood plain in the Mediterranean littoral of Spain[END_REF] has also been associated with the desiccation of several coastal lagoons [START_REF] Marco-Barba | The influence of climate and sea-level change on the Holocene evolution of a Mediterranean coastal lagoon: evidence from ostracod palaeoecology and geochemistry[END_REF][START_REF] Blázquez | Evidence for Holocene sea level and climate change from Almenara marsh (western Mediterranean)[END_REF]. In very detailed sequences, this evolution has been constrained at 7700-7000 cal. BP in Po delta [START_REF] Amorosi | Global sea-level control on local parasequence architecture from the Holocene record of the Po Plain, Italy[END_REF] and at 7840-7000 cal. BP in Tuscan (Amorosi et al., 2013b). This period corresponds to the complete melting of former ice-sheets that occurred around 8000-7500 cal. BP [START_REF] Ortlieb | Relative sea level in the Western Mediterranean basin: a regional test of the ICE-7G_NA (VM7) model and a constraint on late Holocene Antarctic deglaciation[END_REF]. Following the maximum of inland flooding, barrier construction led to the formation of an inner coastal lagoon since 5800 cal. BP in Pego-Oliva. In the Mediterranean, the RSL global stabilisation has resulted in the formation of highstand system tracts related to seaward progradation of river Fig. 4. A) Interpretative sketch of the sediment infilling reported according to the land to sea transect A-B (Fig. 1C) based on 14 boreholes, seismic profile, current topography, and bathymetry. B) Age-depth model calculated from the probabilities density functions of the 14 C ages of Pego-Oliva compared to the predicted glacioisostatic adjustment ICE5G-VM2 in the area [START_REF] Vacchi | Multiproxy assessment of Holocene relative sea-level changes in the western Mediterranean: sea-level variability and improvements in the definition of the isostatic signal[END_REF].

(caption on next page) deltas [START_REF] Anthony | Human influence and the changing geomorphology of Mediterranean deltas and coasts over the last 6000years: from progradation to destruction phase?[END_REF]Amorosi et al., 2013a), coastal barrier system constructions and large development of lagoon environments [START_REF] Marco-Barba | The influence of climate and sea-level change on the Holocene evolution of a Mediterranean coastal lagoon: evidence from ostracod palaeoecology and geochemistry[END_REF][START_REF] Blázquez | Evidence for Holocene sea level and climate change from Almenara marsh (western Mediterranean)[END_REF][START_REF] Melis | Mid to late Holocene environmental changes along the coast of western Sardinia (Mediterranean Sea)[END_REF][START_REF] Melis | 8000 years of coastal changes on a western Mediterranean island: a multiproxy approach from the Posada plain of Sardinia[END_REF][START_REF] Pascucci | Climate changes and human impact on the Mistras coastal barrier system (W Sardinia, Italy)[END_REF].

In the Mediterranean, a large range of literature proposes a causal link between coastal evolution and the sub-millennial North Atlantic Oscillation for the last 7000 years [START_REF] Zazo | The coastal archives of the last 15 ka in the Atlantic-Mediterranean Spanish linkage area: sea level and climate changes[END_REF][START_REF] Amorosi | Global sea-level control on local parasequence architecture from the Holocene record of the Po Plain, Italy[END_REF][START_REF] Pascucci | Climate changes and human impact on the Mistras coastal barrier system (W Sardinia, Italy)[END_REF], because highstand systems tract has allowed for the recording of small-amplitude regressive and transgressive cycles [START_REF] Fruergaard | Stratigraphy, evolution, and controls of a Holocene transgressive-regressive Barrier Island under changing sea level: Danish North Sea coast controls on barrier-island evolution[END_REF]. Fewer data exist during the Early to Mid-Holocene (Amorosi et al., 2013a;[START_REF] Amorosi | Global sea-level control on local parasequence architecture from the Holocene record of the Po Plain, Italy[END_REF]. A cause of RSL variability can be linked to thermal expansion of water masses related to sea temperature [START_REF] González | A new Late Holocene sea-level record from the Mississippi Delta: evidence for a climate/sea level connection? Quat[END_REF]. However, as western Mediterranean Sea surface temperatures are relatively stable over the Early Holocene [START_REF] Jalali | Holocene climate variability in the North-Western Mediterranean Sea (Gulf of Lions)[END_REF], it seems unlikely that thermal expansion/contraction explains the evolution at Pego-Oliva. Another explanation could be related to the alternation of pulse and pause of the RSL. Five transgressive phases have been recorded at Pego-Oliva (Fig. 5B) at 9500 (n°1), 9100-9000 (n°2), 8750-8650 (n°3), 8450-8200 (n°4), and 8100-7300 cal. BP (n°5). The phases 4 and 5 may match with meltwater pulses identified in the North Atlantic [START_REF] Nesje | Were abrupt Lateglacial and early-Holocene climatic changes in northwest Europe linked to freshwater outbursts to the North Atlantic and Arctic Oceans?[END_REF][START_REF] Smith | The early Holocene sea level rise[END_REF] (Fig. 5B). A linkage between North Atlantic has been suggested in the North-East Mediterranean where drops in water salinity were caused by enhancing outflow from the Black-Sea during its high level phases [START_REF] Herrle | Black Sea outflow response to Holocene meltwater events[END_REF]. Correspondence between morphogenetic evolution and meltwater pulses suggests a potential linkage although it remains unclear whether they could have been significant enough to be recorded compared to the massive ice sheets dismantling of the Early Holocene. Finally, sand spit progradation and lagoon formation may be influenced by climate variability by increasing sediment supply entering into the system from rivers and its redistribution along the coast through intensification of longshore drift velocity. Iberian Mediterranean river activity presents an opposite pattern to Pego-Oliva lagoon formation that does not argue for an increase of the sediment supply [START_REF] Thorndycraft | Late Holocene fluvial chronology of Spain: the role of climatic variability and human impact[END_REF]. Besides, a prolonged period of deep-sea current intensification offshore of the Balearic island is recorded between 9500 and 8000 cal. BP [START_REF] Frigola | Holocene climate variability in the western Mediterranean region from a deepwater sediment record[END_REF], concomitant to higher ice-rafted debris in the Northern Atlantic [START_REF] Bond | Persistent solar influence on North Atlantic climate during the holocene[END_REF]. Together, it argues that cold and dry climate events may have locally increased wind intensity, and thus longshore drift velocity. For example, the 8200 cal. BP climate event has been depicted as an aridity crisis in the regional continental archives [START_REF] Morellón | Holocene palaeohydrology and climate variability in northeastern Spain: the sedimentary record of Lake Estanya (Pre-Pyrenean range)[END_REF][START_REF] Jones | A multi-proxy approach to understanding complex responses of saltlake catchments to climate variability and human pressure: a Late Quaternary case study from south-eastern, Spain[END_REF].

Socio-ecological adaptation during the early to mid-Holocene

The area of Pego-Oliva has been occupied by prehistoric communities during the Early and Mid-Holocene (Fumanal et al., 1993b). In order to investigate the influence of coastal evolution on human occupation patterns, we have compared the results of this study with the chronological and occupational evidences of the Mesolithic site of El Collado and the Neolithic site of El Barranquet, both located 5 km north of Pego-Oliva (Fig. 5A andB). The presence of inner coastal lagoons, dated from 9500 to 8200 cal. BP was concomitant to the occupation of El Collado site during both the Early and the Late Mesolithic (Fernández-López de Pablo, 2016). Shallow brackish lagoonal waters in the site vicinity have certainly offered a diversity of resources, including marine mollusc and fish, as it has been reported along the Early Mesolithic levels of El Collado sequence (Fernández-López de Pablo and Gabriel, 2016). Indeed, among marine molluscs the assemblages were noticeably dominated by Cerastoderma glaucum, a marine bivalve inhabiting soft sandy to muddy substrates of brackish lagoon waters and low-energy environment [START_REF] Ivell | The biology and ecology of a brackish lagoon bivalve, C. glaucum, in Lago Lungo, Italy[END_REF]. Marine resources collected in this coastal environment were relevant for the Early Mesolithic communities occupying this area, as independently suggested by the paleodietary isotope analyses on human remains, indicative of an intake of proteins of marine origin of 25% [START_REF] García Guixé | Palaeodiets of humans and fauna at the Spanish mesolithic site of El Collado[END_REF].

It is worth noting that malacological assemblages have shown diachronic differences in the intensity of shellfish exploitation. The two Early Mesolithic phases display a more intense exploitation of shellfish and fish compared to the Late Mesolithic layer in which a significant decrease of the frequency and size of C. glaucum valves are documented at about 8500 cal. BP [START_REF] De Pablo | El Collado shell midden and the exploitation patterns of littoral resources during the Mesolithic in the Eastern Iberian Peninsula[END_REF]. Our results suggest that the reduction of the shellfish productivity was driven by a contraction of the coastal lagoons associated with the sealevel rise. Interestingly, the presence of a short Late Mesolithic occupational interval postdating the chronological span of the funerary activity suggests a change in the settlement pattern probably toward more ephemeral occupations. At this moment, our results date the disappearance of the lagoon biotope right after 8200 cal. BP at Pego-Oliva, one of the shellfish collecting areas exploited over generations during the Mesolithic. Moreover, the absence of pre-transgressive lagoonal deposits in a 20 km radius (Fumanal et al., 1993b) reinforces the interpretation of a major reduction of these habitats in the settlement's vicinity. In addition of loss of biota caused by marine flooding, the backward migration of the shoreline is likely to have also reduced the spatial extension of the colluvial glacis favourable for hunting terrestrial game. Altogether, it is likely that these changes have dramatically reduced the carrying capacity of the ecosystem rendering this area less attractive to forager communities.

The environmental mutation consecutive to the global sea-level rise negatively impacted the Late Mesolithic subsistence and settlement patterns, probably driving to a significantly lower occupational intensity (even depopulation) of this coastal area. Indeed, the end of the occupation at El Collado site coincides with a spatial re-organization of the Late Mesolithic settlement networks and the lack of archaeological sites dated in the adjacent valleys [START_REF] Gómez-Puche | AMS radiocarbon Chronology of Late Mesolithic Sites in the Upper Vinalopo Valley (Eastern Iberia)[END_REF]. The subsequent Final Mesolithic occupations, that have been dated between 7900 and 7700 cal. BP, occurred as inland patches about 60 km away from the coast (Fernández-López de [START_REF] De Pablo | Late Mesolithic burials at casa corona (Villena, Spain): direct radiocarbon and palaeodietary evidence of the last forager populations in Eastern Iberia[END_REF][START_REF] García-Puchol | Cocina cave revisited: Bayesian radiocarbon chronology for the last hunter-gatherers and first farmers in Eastern Iberia[END_REF]. After 700 years (Fig. 5B) from the last dated Mesolithic occupations, the area of Pego-Oliva witnessed the arrival of the first farming communities as represented by the Earliest Neolithic phase of El Barranquet site. This dynamic supports the model of pioneer maritime colonization in Eastern Iberia [START_REF] Zilhão | Radiocarbon evidence for maritime pioneer colonization at the origins of farming in West Mediterranean Europe[END_REF][START_REF] Isern | Modeling the role of voyaging in the coastal spread of the Early Neolithic in the West Mediterranean[END_REF] implying a leap-frog dispersal modality by which the earliest farmers established their settlements in unoccupied areas by Final Mesolithic [START_REF] Bernabeu | The first agricultural groups in the Iberian peninsula[END_REF][START_REF] Bernabeu Aubán | New insights relating to the beginning of the Neolithic in the eastern Spain: Evaluating empirical data and modelled predictions[END_REF]. Preliminary data on the mollusc assemblages of El Barranquet (Luján Navas, 2016) suggests that a major change in the Fig. 5. A) Projected sea-level position at selected key-dates (best age cal. BP) of the morphogenetic evolution on the current elevation contours, that includes the vertical error estimate. The two archaeological sites are shown by stars. The thick black arrows point out the kinematic of shoreline evolution through time. B) Comparison of the coastal morphogenetic evolution at Pego-Oliva with key-dates of the morphogenetic evolution and a simplified representation of transgressive and stillstand phases (this study 1 ), with phases of freshwater meltwater pulses (MWP) from Smith et al. (2011) 3 and Liu et al. (2004) 4 , and with the Ice-Rafted Debris (IRD), a proxy of the North Atlantic oscillation [START_REF] Bond | Persistent solar influence on North Atlantic climate during the holocene[END_REF] 5 . The absolute horizontal displacement of the shoreline at Pego-Oliva referring to the oldest keydate (yellow line in A) is represented according to the best age-distance model (black line) and its 95% confidence (grey envelope). Morphogenetic evolution of the coast is compared to the chronology of the Mesolithic and Neolithic settlements located in the study area ( 6 Fernández-López de Pablo, 2016 -Bayesian chronostratigraphic model of the 14 C ages; 7 Bernabeu et al., 2009individual calibrated 14 C ages from the Precardial pottery phase, 8 Esquembre-Bebia et al., 2008estimated age of the Postcardial pottery phase). Contour colours in A of the selected key-dates refer to colours of the age probability distributions in B. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) relative abundance of marine bivalves collected for consumption occurred between the two Early Neolithic phases. C. glaucum species dominate the Impressa phase whereas the Postcardial layers show a higher representation of Glycimeris glycymeris. This shift in favour of infralittoral species in the archaeological record is in agreement with the environmental reconstruction that shows that the beachfront was getting closer to the site, reaching its proximal position at 7300 cal. BP (Fig. 5A), right after the Precardial phase and during the Postcardial (Fig. 5B). Altogether, these results suggest a lose link between nearby ecosystems and subsistence choices, arguing that the Neolithic communities have adapted species collection to the resource locally available.

Based on the Early to Mid-Holocene geomorphological reconstructions at Pego-Oliva, a vertical sea-level rising rate of 50 cm/50 years has been estimated (Fig. 4B); that once translated into paleogeographical reconstructions (Fig. 5A) allows to estimate a horizontal displacement rate of the shoreline of 180 m/50 years (Fig. 5B). Both vertical and horizontal shoreline displacement rates are rapid enough to be perceptible by human communities, at a level of 1 to 2 generations only, as it has been also suggested during the Early Holocene inundation of North-Western Europe [START_REF] Sturt | New models of North West European Holocene palaeogeography and inundation[END_REF]. More than the vertical rate, the rapidity with which the shoreline came closer may have also increased vulnerability to short high-energy marine events [START_REF] Leary | Perceptions of and responses to the Holocene flooding of the north sea lowlands[END_REF]. Taking into account that Mesolithic occupation of El Collado has spanned 1022-1965years (Fernández-López de Pablo, 2016); this context of highly changing landscapes must have been perceptible by the last hunter-gatherers. [START_REF] Conneller | Rethinking human responses to sea-level rise: the Mesolithic occupation of the channel islands[END_REF] have rather suggested that landscapes "produced and continually reproduced by sea-level rise" may have in fact encouraged exploitation. Nevertheless, the adaptive flexibility of these populations seems to have faced very important environmental changes at 8200 cal. BP, with the extinction of previously exploited ecosystems, and abandonment of the El Collado implantation. The hypothesis of a convergence of environmental events rendering populations more sensitive could be made [START_REF] Leary | Perceptions of and responses to the Holocene flooding of the north sea lowlands[END_REF]. Based on this case-study, it may have resulted in the superimposition of the repercussion of the long-term sea-level rise on the coastal geomorphology and ecosystems, with the abrupt aridity event that has occurred at 8200 cal. BP. Finally, our results show that once the last foragers left, a new population with a different cultural baggage (first Neolithic) settled in the same environment. It clearly comes out that the perception of this same environment has been different for populations with different cultural backgrounds. Thus, this case-study of past Human-environment interactions runs counter to subjective valuations of ecosystems in terms of services or constraints for society (e.g. [START_REF] Costanza | Changes in the global value of ecosystem services[END_REF]. We plead for a more thorough consideration of the local and regional scales of socio-ecological interactions and reorganisations when investigating resilience and adaptive strategies of societies to global changes past and present [START_REF] Redman | Resilience theory in archaeology[END_REF][START_REF] Van De Noort | Conceptualising climate change archaeology[END_REF].

Conclusions

This paper investigated how coastal evolution affected settlement and subsistence patterns during the Mesolithic-Neolithic transition. Based on multiple sediment coring in the Pego-Oliva basin, new sedimentological analyses and 14 C dates combined with previous biological works, we have reconstructed the geometry of the sedimentary facies and the timing of depositions throughout the Holocene. Three main periods have been identified; from 9500 to 8200 cal. BP, the stepwise inland migration of the coastline consecutive to sea-level rise has led to the deposition of four individual peat layers at 20.5, 18.5, 15.5 and 11.5 m below the present sea-level. We interpret this stepwise transgression as a result of periods of acceleration of barrier construction associated with higher drift velocity linked with a drier climate (e.g. at 8200 cal. BP). The transgressive phases, that have been dated at 9500, 9100-9000, 8750-8650, 8450-8200, and 8100-7300 cal. BP, match with some of the phases of meltwater pulses in the North Atlantic. From 8200 to 7200 cal. BP, the pursuit of the backward migration of the coastline provokes the disappearance of the coastal lagoons. The maximum of the marine transgression, 2 km inland, is dated at 7200 cal. BP. This timing is very similar to other Mediterranean records. From 7200 cal. BP to Present: following sea-level stabilisation, sand barrier reconstruction promoted the basin closing, inducing a return to a coastal barrier-lagoon system at 5800 cal. BP, when the shoreline position was similar to the modern's one.

Results have been compared to the archaeological records of the same area. The interdisciplinary approach allowed us to interpret that: 1) the long-lasting occupation of the Mesolithic shell midden of El Collado has corresponded to the period of presence of coastal lagoon, offering a diversity of marine resource available, easily exploited in the inter-tidal shallow and brackish waters; 2) the spatial contraction of the coastal lagoon has corresponded to the sharp drop in Cerastoderma glaucum species during the Late Mesolithic, suggesting that the marine resource depletion primarily occurred because of a decrease in marine resource productivity; 3) the abandonment of the El Collado site and the 700 years' temporal hiatus observed between the Mesolithic and Neolithic implantations is likely to be explained by the consequence of the disappearance of the coastal lagoon and the formerly exploited biotopes at 8200 cal. BP, that may have been together with the aridity crisis occurring at that time factors of depopulation of this area; 4) the maximal inland transgression of the sea at 7200 cal. BP has been contemporary of a shift toward more infralittoral marine shell assemblages during the Postcardial phase at El Barranquet site suggesting that Early Neolithic populations have successfully adapted to rapidly changing environments.

We concluded that prehistoric communities with different cultural baggage have had different perceptions of their environment, questioning subjective valuations of ecosystems in terms of services or costs for society, and emphasising the importance of examining socio-ecological reorganisations at the local and regional scale when investigating adaptive strategy to global changes.

Fig. 2 .

 2 Fig. 2. Sedimentological analysis carried out on three selected representative boreholes. Results are represented by histogram bars plotted at the corresponding depth of the sample. Dry Density (DD), Total Organic Carbon (TOC), Total Inorganic Carbon (TIC) and Non-Carbonate Inorganic Residue (NCIR). Core depth is given below (negative value) or above (positive value) the current mean sea-level.

Fig. 3 .

 3 Fig. 3. Lithostratigraphic description of the sediment cores of the Pego-Oliva wetland according to their respective position on the three transects referring to Fig. 1C: A) southern land to sea transversal profile ab; B) northern land to sea transversal profile a'-b'; C) longitudinal profile of the shoreline c-d. Thick black lines indicate the modern mean sea-level. Simplified chronostratigraphy based is shown beside sedimentary logs (colours refer to units 1 to 5). Radiocarbon ages are reported by the 2 sigma calibration interval.Asterisks indicate references to this study (*),[START_REF] Dupré | Quaternary evolution of the Pego coastal lagoon (Southern Valencia, Spain). Palaeogeogr. Palaeoclimatol[END_REF][START_REF] Viñals | Quaternary development and evolution of the sedimentary environments in the Central Mediterranean Spanish coast[END_REF] Fumanal et al., 1993a Fumanal et al., , 1993b (**) (**) and[START_REF] Navarro | Zonas húmedas mediterráneas y acuíferos asociados. Condicionantes hidrogeológicos del Marjal de Pego-Oliva (Alicante-Valencia)[END_REF].

  Fig. 3. Lithostratigraphic description of the sediment cores of the Pego-Oliva wetland according to their respective position on the three transects referring to Fig. 1C: A) southern land to sea transversal profile ab; B) northern land to sea transversal profile a'-b'; C) longitudinal profile of the shoreline c-d. Thick black lines indicate the modern mean sea-level. Simplified chronostratigraphy based is shown beside sedimentary logs (colours refer to units 1 to 5). Radiocarbon ages are reported by the 2 sigma calibration interval.Asterisks indicate references to this study (*),[START_REF] Dupré | Quaternary evolution of the Pego coastal lagoon (Southern Valencia, Spain). Palaeogeogr. Palaeoclimatol[END_REF][START_REF] Viñals | Quaternary development and evolution of the sedimentary environments in the Central Mediterranean Spanish coast[END_REF] Fumanal et al., 1993a Fumanal et al., , 1993b (**) (**) and[START_REF] Navarro | Zonas húmedas mediterráneas y acuíferos asociados. Condicionantes hidrogeológicos del Marjal de Pego-Oliva (Alicante-Valencia)[END_REF].

  . The 14 C age of the sample R-2013 is reported corrected of its δ 13 C fractionation error of 1.14‰. The ages that have been excluded in the age-depth model are in italics.

	Core	Lat.	Lon.	Depth	Material	Facies	Lab. code	14 C years BP	ΔR (yrs) Minimum	Maximum	Ref RSL (m)	Type of
				(m asl)					age cal. BP	age cal. BP			index
													point
	P17-4	38.854 0.066 0	Charcoals, peat Lagoonal	Poz-93,968	4999 ± 40	5622	5891	1	-0.5 ± 0.2	index
	P17-5	38.855 0.073 0.8	Wood	Lagoonal Poz-93,804	1100 ± 40	929	1171	1	0.3 ± 0.2	index
	P17-7	38.857 0.061 -1.6	Charcoals, peat Lagoonal	Poz-94,010	2970 ± 35	3005	3231	1	-2.1 ± 0.2	index
	P17-3	38.861 0.055 -2.6	Charcoals	Lagoonal Poz-93,967	5180 ± 40	5764	6095	1	-3.1 ± 0.3	index
	P17-8	38.876 0.088 -4	Charcoals	Lagoonal Beta-460,429 6420 ± 30	7280	7421	1	-4.5 ± 0.3	index
	P17-3	38.861 0.055 -11	Charcoals	Lagoonal Poz-93,699	7320 ± 50	8011	8292	1	-11.5 ± 0.3 index
	PDGEST-2 38.882 0.038 -17.5	Charcoals	Lagoonal Poz-93,698	7900 ± 50	8594	8977	1	-18 ± 0.4	index
	PDGEST-2 38.882 0.038 -18	Peat	Lagoonal Poz-93,965	7920 ± 50	8605	8978	1	-18.5 ± 0.4 index
	PDGEST-2 38.882 0.038 -18.5	Charcoals, peat Lagoonal Poz-93,966	7910 ± 50	8598	8977	1	-19 ± 0.4	index
	P-6	38.873 0.064 -0.15	Peat	Lagoonal		1660 ± 50	1415	1696	2	-0.55 ± 0.3 index
	P-6	38.873 0.064 -0.4	Peat	Lagoonal		2400 ± 60	2340	2704	2	-0.9 ± 0.3	index
	P-6	38.873 0.064 -0.65	Peat	Lagoonal		3250 ± 100	3233	3703	2	-1.15 ± 0.3 index
	P-6	38.873 0.064 -0.9	Peat	Lagoonal		3540 ± 80	3619	4080	2	-1.4 ± 0.3	index
	P-7	38.864 0.045 -1.9	Peat	Lagoonal		5200 ± 60	5761	6179	2	-2.3 ± 0.3	index
	P-6	38.873 0.064 -1.8	Peat	Lagoonal		5330 ± 90	5934	6283	2	-2.3 ± 0.3	index
	P-6	38.873 0.064 -2	Shell	Shoreface R -2013	6343 ± 100	94 ± 61 6224	6726	3	-2 ± 0.3	marine
	P-7	38.864 0.045 -10.9	Peat	Lagoonal	UBAR-44	7790 ± 110	8401	8977	2	-11.4 ± 0.4 index
	P-3	38.880 0.048 -15	Peat	Lagoonal UBAR-78	8300 ± 170	8770	9580	2	-15.5 ± 0.4 index
	P-1	38.884 0.040 -20	Peat	Lagoonal	UBAR-77	10,120 ± 460	10,521	12,826	4	-20.5 ± 0.4 index

Acknowledgments

We are grateful to Helena Velayos Mayo and Javier Muñoz of the Litoteca of the Instituto Geológico y Minero de España (IGME) to have kindly facilitated consultation and sampling of the archives. We also thank Malebogo Mvimi (MNHN, IPHES) for her help in the English editing, and two anonymous reviewers for their insightful comments.

Funding

This work is a contribution to the project MedCoRes (ref. 704822) "Mediterranean Coastal Resources: benefits and constraints for Prehistoric hunters-gatherers" funded by the Marie Sklodowska-Curie Action (European Union) led by E. Brisset (IPHES, URV) and has benefited of the support of the AGAUR agency (SGR 836 Research Group) and the MINECO project Postglacial-Med (HAR2017-88503-P). J. Fernández-López de Pablo (IPHES, URV) is supported by the MINECO Grant IEDI-2017-00889.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.gloplacha.2018.07.016.


A Djarri

A Achour

M A Soussou

N Sobti

S Achour

A Achourb

Characterization of thin films prepared by co-sputtering iron and titanium precursors and thermal oxidation under air atmosphere

Keywords: Fe 2 O 3 -TiO 2, Thin films, XPS, HR-STEM, phase separation, segregation

published or not. The documents may come

Characterization of thin films prepared by co-sputtering

iron and titanium precursors and thermal oxidation under air atmosphere

Introduction

Nanocomposites of metal oxides are useful materials in energy conversion systems.

Extensive earlier experimental findings indicate that inert oxides such as titanium oxide can greatly improve the reactivity of iron oxide, for example, over multiple redox cycles [START_REF] Qin | Nanostructure formation mechanism and ion diffusion in iron-titanium composite materials with chemical looping redox reactions[END_REF]. The promotion or inhibition of solid state reaction in nanocomposite films, such as phase formation, phase separation, segregation, crystallization, etc., plays an important role in ultimate performance of the films. The extraordinary interest in nanocomposite materials is mainly due to the vast range of properties that can arise from the combination of the peculiar characteristics of each component [START_REF] Ingrosso | Colloidal Inorganic Nanocrystal Based Nanocomposites: Functional Materials for Micro and Nanofabrication[END_REF]. TiO 2 has been studied as photo catalyst as well as in many other applications. However, its large band gap which approaches 3.2 eV, depending on the phase structure (rutile: ∼3.0 eV [3], anatase: ∼3.4 eV [4] and brookite: ∼3.3 eV [5]), confines its uses to only the UV region of the electromagnetic spectrum. Hence, lowering the band gap of titania below 3 eV to shift the optical response to the visible light range is a highly active area of research with attention focused on the improvement of the catalytic activities of TiO 2 through doping, metal loading and semiconductor mixing.

Depending on preparation methods, iron oxide can be found in several phases such as magnetite (Fe3O4), wustite (FeO) and trioxide (α-Fe 2 O 3 , β-Fe 2 O 3 , γ-Fe 2 O 3 , and ε-Fe 2 O 3 ) among others. The three most common forms of iron oxides are magnetite (Fe 3 O 4 ), maghemite (γ-Fe 2 O 3 ), and hematite (α-Fe 2 O 3 ). Formation and reactivity of iron oxide at the nanoscale are rather complicated. For instance, Both Fe 2 O 3 and FeO can be detected in 20% iron oxide-silica aerogel calcined at 500 °C [START_REF] Wang | Nanocluster iron oxide-silica aerogel catalysts for methanol partial oxidation[END_REF]. Hematite, which is the main phase detected in the present investigation, is an interesting n-type semiconductor material with a band gap of about 2.2 eV On the other hand, preparation and nanostructuring of mixed oxide thin films where two single oxides of different optical properties are mixed together [START_REF] Peña-Flores | Fe effect on the optical properties of TiO2:Fe2O3 nanostructured composites supported on SiO2 microsphere assemblies[END_REF][START_REF] Mazur | Influence of Material Composition on Structural and Optical Properties of HfO2-TiO2 Mixed Oxide Coatings[END_REF] can be considered as one of the common approaches for tailoring the photonic properties of the materials, such as refractive index and frequency-selective light trapping and absorption. Knowledge of phase and element distribution as well as evolution of the nanoscale morphology of the Fe-Ti thin films during oxidation is thus of considerable importance. The present work is devoted to the structural and optical characterization of thin films prepared by RF magnetron co-sputtering Fe and Ti precursor metallic films on glass and Si substrates and subsequent oxidation at relatively low temperature and under air atmosphere. The surface of these films was found to be composed of iron rich nanospheres with 10 nm diameter or less.

Material and Methods

Films that are mixtures of metallic iron and titanium were deposited on glass substrates by radio frequency magnetron sputtering of pure Ti (99.8 %) target with Fe (99.5%) in the form of circular sheets superimposed on the Ti target (Fig. 1). The ratio of the Fe sputtered area to the

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total sputtered area of Ti was changed, in order to obtain various composites. It was possible to change the concentration of Fe in Ti-Fe thin films by changing the relative area covered with Fe sheets. The base pressure of the system was 3.10-3 Pa and the sputtering Ar gas (99.99) pressure was at 5.10-1 Pa. The RF power was fixed at 10 W/cm2 and the substrate temperature in this case did not exceed 100 °C. After sputtering, the metallic (mixture of Ti and Fe) films were oxidized in air under atmospheric pressure at 520 °C during 8 hours. The preparation conditions and sample designations are summarized in Table 1. The oxidized films were characterized by XRD (Siemens D8000) using Cu Kα radiation at normal incidence in the 2θ range of 20-80° with 40 keV, 35 mA. Raman spectra were recorded with a Bruker Senterra Raman spectrometer equipped with a laser source radiating at wavelength = 532 nm and power = 10 mW. X-ray photoelectron spectroscopy (XPS) depth profiling was carried out on a Kratos Nova system using Al Kα (1486.6 eV) radiation. Both elemental and topographical scanning transmission electron microscopy images were taken in a FEI STEM of type Talos F200A. Focused Ion Beam (FIB) was used to prepare thin cross section of the film for TEM analysis and UV-vis spectrometry (Shumadzu UV 3101 PC spectrometer) was used to record the optical absorbance spectra. Ellipsomertry was performed using a Jobin Yvon UVISEL NIR Spectroscopic Phase Modulated Ellipsometer with an automatic goniometer. The ellipsometric measurements were collected at an angle of incidence of 70° across the spectral range 190 -830 nm. It should be noted that the analysis results of samples S2 and S3 are not very different. So the exposed results will be focused on S1 and S3

Ti Fe

Table1. Sample preparation conditions

except where is useful. . The peaks at 267 and 520 cm -1 (for sample S 1 ) originate from the glass substrate since these peaks disappear when the film thickness increases (S 3 ). The increased intensity of the hematite Raman peaks is related to the increase of iron concentration in the precursor films. The intensities of Raman bands related to TiO 2 are very weak compared to that of iron oxide because the titanium oxide is deeply buried in the film bulk, contrary to the iron oxide which exists near the surface as will be revealed in sections 3.2 and 3.3 below.

Results and Discussion

XRD and Raman spectroscopy

XPS analysis

The XPS depth profiling of the prepared films is shown in Fig. 4. It can be seen that iron diffuses during annealing upward to the film surfaces where it accumulates either as metallic or metal rich oxide, while titanium and oxygen together follow the same profile and accumulate in the bulk, indicating a phase separation. Apparently, the segregation of pure metallic iron or metal rich oxide beneath the film surface is accompanied by titanium inward diffusion to the bulk. Iron outward diffusion seems to be faster than the oxygen inward diffusion as can be seen from the XPS profile shapes. This may be due to the significant size difference between either Fe(II) or Fe(III) which are 0.74 Å and 0.64 Å, respectively and O anion (1.4 Å) [START_REF] Li | Ionic diffusion in the oxidation of iron-effect of support and its implications to chemical looping applications[END_REF]. The outward Fe diffusion and migration proceed, probably along the grain boundaries between nanoparticles. The Formation of titanium oxide in the bulk far below the surface can be evidenced by the oxygen and titanium profile behaviors which at certain depths adopt atomic concentration corresponding almost to TiO 2 (Fig. 4). It should be noted that the binding energy of Ti 2p 3/2 agrees well with the 2p binding energy of Ti 4+ ions (459-458.5 eV) [START_REF] Miao | Photocurrent Enhancement for Ti-Doped Fe2O3 Thin Film Photoanodes by an In Situ Solid-State Reaction Method[END_REF] for the sample S 1 . However, a shift of about 0.8 eV towards lower energies for the sample S 3 (Fig. 5a) is observed, which may be attributed to the existence of reduced titanium oxide or metallic Ti traces in the vicinity of the surface (more probably at about 10 nm, since the measurements were taken on the non eroded freshly prepared surface). Fe 2p, however, remains at the same position and increases only slightly in intensity (Fig. 5b) when the iron concentration increases.

The binding energy of Fe 2p3/2 is located at 709.9 eV, which can be compared to those of metallic Fe (706.8 eV), FeO (709. From this comparison, one can suggest the formation of iron oxide just at the top of the surface where the oxygen concentration is relatively high as is shown, for sample S 3 , in Fig. 4, whereas iron rich clusters precipitate at the subsurface in the form of spherical nanoparticles as can be seen from TEM analysis below. In fact, examining the atomic concentrations (Fig. 4), one can see that the oxygen atomic concentration at a certain depth near the surface can approach almost half of iron, indicating the formation of iron rich oxide which, according to this observation, should be in the form of Fe 2 O or Fe x O where x > 1. The formation and properties of such unusual oxide which results from the interaction of atomic oxygen with iron clusters has been, theoretically and experimentally, studied for small iron clusters [START_REF] Gutsev | Structural and electronic properties of iron monoxide clusters FenO and FnO-(n = 2 -6): A combined photoelectron spectroscopy and density functional theory study[END_REF].

It should be mentioned that the results of Raman and XPS analysis do not give the same phases for iron oxide phases because hematite is mainly located in the bulk while the iron rich phase is located at the surface as a very thin layer. This can be understood since, contrary to Raman spectroscopy and XRD, the XPS is a very sensitive tool only for surface analysis and the iron rich surface layer is too small to be detected by XRD and Raman analysis. These later two techniques indicate the presence of only the hematite phase which excludes any transformation under Raman laser heating during analysis.

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TEM analysis

Fig. 6 shows the HAADF STEM cross section images of the sample S 3 . The images show a contrast between the volume and the surface of the film where spherical nanoparticles, with

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diameters mainly ranging between 3 and10 nm, can be distinguished and where the bulk seems to be highly porous compared to the surface region. From these images, the thickness of this film can be estimated to be in the order of 140 nm in accordance with ellipsometry measurement (see section 3.5 below). The accumulated spherical nanoparticles at the film surface are identified to be iron rich as can be seen from EDX elemental mapping shown in Fig. 7 (in agreement with the XPS depth profiling results). It should be noted that the EDX oxygen mapping shows clearly a distribution with oxygen poor zones corresponding to the spherical nanoparticle locations. Therefore, these particles should represent an iron riche phase.

What may prohibit the rapid oxidation of iron in the present mixture is, probably, the presence of titanium which would pump oxygen to the detriment of iron, because of the much higher affinity of titanium to oxygen.

Titanium oxide accumulates far below the surface where iron oxide becomes rare. The digital line profiles, shown in Fig. 8, confirm the iron and titanium compound separation in the surface region of the films in accordance with the XPS analysis. The phase separation extends along 20 nm below the surface. A possible explanation of this phenomenon may be as follows:

At the beginning, the oxygen should diffuse from the surface to the bulk and react preferentially with titanium because the affinity of titanium to oxygen is much higher than that of iron. Consequently, iron could easily diffuse to the surface where it can segregate or slowly react with oxygen that is continuously pumped by titanium. Moreover, the accumulation of iron ions beneath the surface may produce an internal electric field which could sweep titanium ions inward. Some of the iron rich spherical nanoparticles can be found at depths greater than 20 nm. It has been shown that not only the diffusion of Fe ions was found to be faster than Ti ions, during Fe-Ti oxidation at 700 °C, but the outward diffusion of Fe ions is more favorable than Ti ions [1], which is in accord with the present results. The observed iron rich nanoparticles were further analyzed using HRTEM which reveals that the biggest particles (about 10 nm diameter) (Fig. 9a), actually, are formed of smaller ones with different orientations as can be seen in Fig. 9b andc. It should be noted that the measured inter reticular distances (Fig. 9b) do not correspond to any of the known iron oxides (FeO,

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Optical properties

The UV-Vis absorbance spectra are presented in Fig. 11. As expected, pure TiO 2 almost has no absorption above its absorption edge rising at about 400 nm. In contrast, the Fe 2 O 3 -TiO 2 films show a strong absorption in the visible range which increases with iron oxide increase.

The enhanced absorption in the visible region can be attributed not only to the increase in the film thicknesses but also to the photosensitization of Fe 2 O 3 [START_REF] Zhang | Preparation of photocatalytic Fe2O3-TiO2 coatings in one step by metal organic chemical vapor deposition[END_REF]. The red shift of the absorption peak as well as the appearance of shoulders at the peaks with increasing iron content may result from the contribution of Fe 2 O 3 to the total absorption as well as from interference effects. The high visibility interference fringes for S 1 , S 2 and S 3 indicate that the films are smooth and relatively dense. It is apparent in these curves that as the amount of Fe in the films increases the amplitude of the oscillations observed in the absorbance curves increases. This behavior seems to be congruent with an increase in the refractive index (n) of the films. Simultaneously, the position of the absorption edge (Eg) shifts to lower energy values, changing from the value of TiO 2 (about 3.2 eV) to that of Fe 2 O 3 (about 2.2 eV). A simple estimate for the optical gap can be given by assuming that its value is near the position of the absorption peak maximum.

Ellipsometry

The nanocomposites feature electromagnetic properties different from those of the constituting materials. They can display high refractive index over a wide range of wavelengths. Higher refractive index (n) is achieved by increasing polarizability and/or increasing density. In Fig. 12, the dependence of n and k (extinction coefficient) on wavelength for S 1 and S 3 films is presented. The refractive index of all films is fairly high but the refractive index of the S 3 film is the highest. The value of the refractive index n increases up to about 3.5 at 550 nm as the amount of Fe 2 O 3 in the films increases. Thereafter, it decreases, slightly, to about 3 and remains constant at higher wavelengths. Above 550 nm, both the refractive index and the coefficient of extinction become constant indicating very low dispersion. Here, the interesting result is that the extinction coefficient rapidly drops to a very low value just above 550 nm, and then remains constant over a large interval of wavelength, while the refractive index also remains constant but at exceptional high value. It should be noted that the average n values of rutile and anatase single crystals are 2.75 and 2.52, respectively [START_REF]CRC Handbook of Chemistry and Physics[END_REF]. Simultaneously, the optical gap defined by the edge of the extinction coefficient k shifts towards longer wavelengths as the amount of Fe 2 O 3 in the films increases. The sharp drop in extinction coefficient indicates the onset of optical absorption in these films which can be estimated to be about 3.2 eV for the sample S1 and 2.1 eV for S 3 and also for S 2 (not shown). Finally, the thickness and roughness, deduced from ellipsometry measurement are given in table 2. In particular, the thickness of S 3 film is comparable with the value deduced from TEM imaging (about 140 nm). From these observations one could, effectively, control the optical properties by controlling the film composition, as can be seen from both ellipsometry and UV-vis spectroscopy, where these properties, considerably, depend on composition.

Conclusions

Thin films of iron oxide and titanium oxide prepared by sputtered metallic iron and titanium mixture and subsequent oxidation were thoroughly characterized using state of the art analysis instruments. Beside TiO 2 /Fe 2 O 3 formation, segregation which leads to phase separation in Fe-Ti composite thin films was observed after annealing under air at 520 °C for 8h. Unusual partially oxidized iron or iron oxide Fe x O y (x > y), in the form of spherical nanoparticles, was found to segregate at the film subsurface while titanium segregates deeply in the bulk and forms TiO 2 . The refractive index of the prepared films can be exceptionally high and the optical absorption could be shifted towards the visible light region. It is foreseen that these results and the preparation method will open new avenues to engineer the structure, optical and electrical properties of metal oxide mixtures, by surface composition modification.

[ 7 ]

 7 photoelectrochemical (PEC) water splitting at Au-modified nanocrystalline TiO 2 films have been studied [10] as alternative materials to TiO 2 and the inverse opal photonic crystals (PC) design has been applied to photocatalytic iron oxides [11] and binary oxides [12]. It has been found that coupling a SOFC (Solid Oxide Fuel Cell) with an external reducer, using Fe-Ti-O as an oxygen carrier is a good candidate for electricity generation [13]. Fe 2 O 3 can be considered as a sensitizer for TiO 2 , since it has a flat band potential of 0.32 V versus NHE (Normal Hydrogen Electrode). Some authors [14] have published a correlation between the enhanced photo

Fig. 1 .

 1 Fig. 1. Schematic of the Ti target (yellow) on which Fe discs (red) are superimposed for the deposition of Ti-Fe mixture by magnetron sputtering

Fig. 2

 2 Fig.2shows the XRD patterns of the films that were deposited on glass substrates. Both anatase (a) which is characterized by the most intense diffraction peak at 25.5° corresponding to the (101) planes and rutile (r) which is characterized by the (110) reflection at 27.6° are visible in the explored region of XRD. It can be seen that no inter metallic FeTi is detected in these films. The other reflections can be assigned to α-Fe 2 O 3 and the intensity of these reflections increased with the increase of the Fe content.

Fig. 2 . 3 . 1 [ 29 , 30 ]

 2312930 Fig. 2. XRD patterns of the prepared films, h: hematite, a: anatase, r: rutile

Fig. 3 .

 3 Fig.3. Raman spectra of the films S1 and S3; h, a, r are for hematite, anatase and rutile, respectively.

3 Fig. 4 .

 34 Fig. 4. XPS depth profiles of O, C, Ti, Fe and Si elements for the films S 1 , S 2 and S 3

Fig. 5 .

 5 Fig. 5. XPS spectra related to Ti 2p (a) and Fe 2p (b) core level binding energies in the samples S1 and S3, measured on the as prepared surfaces.

Fig. 6 .Fig. 7 .

 67 Fig. 6. HAADF STEM cross section image of the S3 film

Fig. 8 .

 8 Fig. 8. EDX analysis of the S3 film, showing quantified (atomic %) digital line profile which starts from the bulk to the surface of the film along the shown arrow on the inset image.

Fe 3 O 4 , 10 .Fig. 9 .Fig. 10 .

 3410910 Fig. 9. HR TEM image of the film surface and selected area diffraction of an iron rich nanoparticle composed of three clusters (Fe x O y ) that are located in the subsurface of the S3 film.

3 Fig. 11 .

 311 Fig. 11. Absorbance spectra of S 1 , S 2 and S 3 with pure TiO 2 film.

3 Fig. 12 .

 312 Fig. 12. Refractive indexes and extinction coefficients as function of wavelength for S1 and S3films.

Table 2 :

 2 Thickness and roughness of the studied films

	Sample	Thickness (nm)	Roughness (nm)
	S 1	100.5	12.6
	S 2	121.8	negligible
	S 3	134.2	negligible

Acknowledgements

The authors are grateful to Dr. Christoph Mitterbauer from FEI Company BV, Eindhoven, The Netherlands, for his valuable help in STEM analysis and M. Kloul from Horiba Jobin Yvon (France) for his help in the ellipsometry measurement. The XPS analysis was done at IMN, Université de Nantes, CNRS 44322 Nantes Cedex 3, France.

Conflict of interest:

The authors declare that this research is not for profit sector and there is no conflict of interest.

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