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Holocene coastal changes along the Gironde estuary (SW France): new insights from the North Médoc peninsula beach/dune system

Changements côtiers holocènes le long de l’estuaire de la Gironde : nouvelles données à partir du système plage/dune de la péninsule du nord‑Médoc
Pierre Stéphan, Florence Verdin, Gilles Arnaud‑Fassetta, Frédéric Bertrand, Frédérique Eynaud, Ane García‑Artola, Mathieu Bosq, Camille Culioli, Serge Suanez, Clément Coutelier, Pascal Bertran et Stéphane Costa
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Résumés

Le long du littoral de Soulac, à l’embouchure de la Gironde, l’érosion marine provoquée par les récentes tempêtes hivernales a mis au jour un ensemble de dépôts sédimentaires à caractère estuarien et dunaire contenant de nombreux vestiges archéologiques. Des relevés topographiques réalisés au DGPS et TLS durant les printemps 2014 et 2015 ont été effectués le long des 2 km de la plage de l’Amélie dans le but de reconstituer la stratigraphie de ces dépôts. Des analyses sédimentologiques (granulométrie, micro-faune) ont été réalisées afin de caractériser les unités lithologiques et reconstituer le cadre paléoenvironnemental. L’étude du mobilier archéologique et plusieurs datations par le radiocarbone ont permis de proposer un cadre chronologique cohérent pour cette séquence sédimentaire. Dix principaux lithofaciès ont été distingués depuis le bas de plage jusqu’au sommet de la dune. La base de la séquence se compose de formations pléistocènes datée du MIS 9 au MIS 2 (Bosq et al., 2019), lesquelles montrent une topographie déprimée dans la partie centrale de la plage de l’Amélie. Le colmatage sédimentaire de cette dépression débute vers 5000 BC par le dépôt d’un sable grossier marin suggérant la présence d’une passe tidale. De part et d’autre de ce vaste chenal de marée, des vestiges archéologiques datant du Néolithique ancien jusqu’à l’âge du Bronze témoignent d’une occupation humaine du secteur. De 5000 à 3500 a cal. BP environ, un dépôt caractéristique des vasières intertidales indique la transformation du milieu en marais estuarien protégé par un cordon dunaire. Entre 3500 et 3000 a cal. BP, ce cordon enregistre une phase d’érosion qui se solde par l’ouverture du marais sur la mer. De 2800 à 1650 a cal. BP, une seconde phase de sédimentation estuarienne est reconnue, signalant le retour à des conditions d’abri. Cette période se caractérise par une forte occupation humaine et la présence de nombreux vestiges archéologiques de l’âge du Fer et de l’Antiquité associés à l’exploitation d’un environnement intertidal à saumâtre. De 1650 à 1250 a cal. BP, le marais est progressivement colmaté par les dépôts dunaires qui progradent en direction du nord. Après 1250 a cal. BP, le marais est entièrement recouvert par les sables éoliens. Ces résultats sont cohérents avec les données publiées antérieurement et permettent de détailler localement la séquence des changements paléogéographiques de la péninsule du nord-médoc. En appliquant la méthode des « sea-level index points » (Hijma et al., 2015), l’analyse des niveaux datés par le radiocarbone apporte également de nouvelles données sur la remontée du niveau marin relatif dans cette partie du littoral atlantique français. Enfin, les phases d’activité et de stabilité des dunes reconnues le long de la plage de l’Amélie montre un déphasage par rapport aux évolutions dunaires du reste de la côte aquitaine et suggère une dynamique sédimentaire influencée par la migration vers la côte des bancs de sables intertidaux et subtidaux distribués à l’embouchure de la Gironde.

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This study has been carried out with financial support from the French National Research Agency (ANR) in the framework of the Investments for the future Programme, within the Cluster of Excellence COTE (ANR-10-LABX-45) and within the Cluster of Excellence LaScArBx (ANR-10-LABX-45). Romain Cancouët and Ronan Autret are thanked for their valuable help in the field. The authors are very grateful to Eric Chaumillon and Marc P. Hijma for their constructive comments.

1 - Introduction

1During the Holocene period, most estuaries in the world recorded significant environmental and palaeogeographical changes in response to (1) transgression (induced by post-glacial sea-level rise) of irregular-shaped Pleistocene fluvial-incised valleys that controlled accommodation space, (2) changes in regional and global climate and human activities that affected sediment supply to the estuaries and, (3) tectonics (e.g. Fletcher et al., 1990 ; Zaitlin et al., 1994 ; Ruiz et al., 2004 ; Boyd et al., 2006 ; Anderson et al., 2008 ; Chaumillon et al., 2010 ; Schneider et al., 2010 ; Durand et al., 2016 ; Clement et al., 2017). Studies of the Holocene sedimentary fill of estuaries show that the nature and organization of facies within incised-valley estuaries was controlled by the interplay between marine processes (waves and tides), which generally decrease in intensity up-estuary, and fluvial processes, which decrease in strength down-estuary (Dalrymple et al., 1992).

2Located on the SW Atlantic coast of France, the Gironde is a tide-dominated estuary, with a 20 km wide tidal mouth open to the Atlantic Ocean (fig. 1) where wave and aeolian processes have led to the construction of coastal dune barriers that are affected by episodic storm-induced erosion. Seismic stratigraphic investigations along the inner and outer parts of the Gironde estuary show a sequence comprising a diverse assemblage of lithofacies that can be grouped into three main systems tracts (lowstand, transgressive and highstand systems tracts, fig. 2) (Allen & Posamentier, 1993; Lericolais et al., 1998; Féniès et al., 2010). The lowstand systems tract comprises a continuous unit of fluvial gravel and coarse sand deposited in the thalweg of the incised valley during the periods of low eustatic sea‑level stand. The transgressive systems tract comprises the bulk of the incised valley fill and forms a landward-thinning wedge of tidal-estuarine sands and muds deposited during the post-glacial marine transgression (before 7000 cal. a BP). In the estuary mouth, these deposits are replaced by a thick unit of coarse-grained, estuary-mouth tidal-inlet and tidal-delta sands. At the same time, the adjacent shorelines were starved of sediment and eroded by waves. The highstand systems tract, initiated at about 7000-6000 cal. a BP, comprises seaward-prograding, tide-dominated estuarine bayhead delta deposits that have gradually filled the estuary since the late-Holocene stillstand (Allen, 1991; Allen and Posamentier, 1993; Féniès and Tastet, 1998; Chaumillon et al., 2013).

3Studies carried out in the southern coastal marshes of the Gironde estuary suggest that, as relative sea-level rose to the elevation of the relatively flat fluvial terraces, these areas were flooded rapidly, resulting in rapid expansions of swamps and salt marshes in lateral bay areas (Diot & Tastet, 1995; Pontee et al., 1998; Diot et al., 2001). The development of these coastal marshes was favoured by the presence of a large dune barrier elongated to the north, forming the north-Médoc peninsula. Although the Holocene evolution of Aquitaine dune complexes has been studied in detail by many authors (Marionnaud & Dubreuilh, 1972; Tastet, 1998; Clavé, 2001; Clarke et al. 1999, 2002), the construction phases of the north-Médoc dune barrier are poorly known. Furthermore, very little attempt has been made to correlate either marsh development -on the western bank of the Gironde estuary- or dune development - on the ocean side of the peninsula - with changes in configuration at the estuary mouth, notably in the vicinity of lateral bays which exhibited more or less wide openings onto the estuary and have yielded archaeological evidence of early human presence as around the ‘Anse du Gurp’, which is assumed to be a former mouth of the estuary.

4During the winter of 2013-2014, significant erosion of the beaches and the dune front escarpment along the Aquitaine shoreline (Bulteau et al., 2014; Castelle et al., 2014, 2015; Baumann et al., 2017) uncovered numerous sedimentary deposits containing a high density of archaeological remains along L’Amélie beach (Verdin et al., 2019). Due to the long coastal human occupation, the beaches of the North-Médoc are occasionally the subject of archaeological discoveries (Marambat, 1992; Diot et al., 2001; Clavé, 2001) that testify to a complex palaeogeographic evolution of this coastal area, characterized by high shoreline mobility at the mouth of the Gironde during the Holocene period (Allen et al., 1974; Pontee et al., 1998). From March 2014, several campaigns of post-storm geomorphological and archaeological prospection were simultaneously undertaken on the intertidal zone of L’Amélie beach in the framework of an interdisciplinary research programme (LITAQ programme). Thus the specific objectives of the present study are (1) to detail the stratigraphy and evolution of the North-Médoc dune barrier in the light of recent archaeological finds, (2) to date both archaeological and sedimentary material, (3) to identify the major morphological changes that occurred on the south side of the Gironde estuary inlet in relation to both sea-level rise (SLR) and climatic variability, and (4) to define the effects of dune formation on coastline changes and human settlement of the North-Médoc peninsula over the last seven millennia.

Fig. 1: Location maps of the study site.

Fig. 1: Location maps of the study site.

(A) Location of the North Médoc peninsula and the Gironde estuary along the Atlantic coast of France. (B) Topo-bathymetric digital elevation model of the mouth of the Gironde estuary showing the numerous sandbanks in the intertidal and subtidal areas. Black arrows indicate the main direction of the longshore drift and the value corresponds to the volume of sediment transfer. (C) “Anchored tidal ravinement process” and associated ribbon-shaped reservoir geometry (from Féniès et al., 2010).

Fig. 2: Sequence of sedimentary fill of the Gironde estuary during the Holocene

Fig. 2: Sequence of sedimentary fill of the Gironde estuary during the Holocene

(A) Schematic illustration of the incised-valley of Gironde in the Early Holocene. (B) Schematic illustration of the initiation of post-glacial marine transgression into the valley. (C) Schematic illustration of the maximum marine flooding around 7000-6000 BP resulting in estuarine mud deposited in the funnel zone and landward migration of the tidal inlet. (D) Schematic illustration of the geological evolution of the Gironde estuary over the last 7000 years. A regressive tidal-estuarine bay-head delta has prograded into the upper estuary, gradually filling it with sediment. (E) Internal stratigraphy of the Gironde incised-valley fill.

modified from Lericolais et al. (2001), Féniès and Lericolais (2005) and Féniès et al. (2010)

2 - Study area

2.1 - The Gironde estuary mouth

5The open sandy beach of L’Amélie lies south of the Gironde estuary inlet (fig. 1), which has shifted significantly landward - by an estimated 1 to 2 km / 1000 years (LCHF, 1979, 1982, 1987) over the last 3000 years - under bedrock control of the shoreline retreat. Unlike the Leyre estuary, located a hundred kilometres south of the Gironde mouth, which has shifted laterally to the south (Féniès & Lericolais, 2005), the Gironde tidal inlet has remained largely in a fixed location along a NW-SE-trending structural axis (Féniès et al., 2010). The presence of resistant bedrock headlands near the inlet of the Gironde estuary (composed of Eocene carbonates, see fig. 1c) has prevented its lateral migration to the south under the influence of the prevailing southward littoral drift. As a consequence, the northern coastal barrier has shifted cross-shore over a distance of a few kilometres whereas the southern coastal barrier has migrated alongshore from the N-S-trending linear Aquitaine coastline to a SW-NE-oriented spit - the Pointe de Grave - which divides the Gironde mouth into an inner estuary and an outer estuary. Thus the peculiar northwest orientation of this part of the Aquitaine coast must be linked to its specific location and recent morphological evolution within the inlet of the Gironde estuary.

6During the mid to late Holocene, the North-Médoc peninsula exhibited morpho-sedimentary evidence of significant coastal changes near the mouth of the Gironde due to tidal channel migration and dune ridge breaching (Pontee et al., 1998). Based on the exploitation of 360 geological cores, Pontee et al. (1998) proposed a set of three palaeogeographic maps to illustrate these coastal changes (fig. 3). The general principles of these changes were primarily established by the characterization of two generations of tidal marshes (Allard et al., 1974) separated by a shelly ridge (the ‘Cordon de Richard’) that began to form ca. 2500 cala BP (Diot & Tastet, 1995; Massé et al., 2001) (fig. 4). Sedimentation within the estuary reduced the considerable available accommodation space produced by earlier post-glacial relative sea-level (RSL) rise largely accounting for the extent of the present-day Médoc peninsula deposits (Pontee et al., 1998).

7Coastal changes in the area were also influenced by dune field formation and migration landwards across the Atlantic coastal zone, depositing massive amounts of sand within the transgressive systems that have predominantly filled the Gironde incised-valley. The stratigraphy and a relative chronology of the dune barrier complex of the coastal zone of North-Médoc were first based upon the morphology of the dunes (Marionnaud & Dubreuilh, 1972) and later refined based on fieldwork, new chronological data and a reanalysis of historical maps (Tastet, 1998; Clarke et al. 1999, 2002). The North-Médoc coastal dunes consist of two major transgressive dune fields (fig. 5). The primary dunes, invariably compound climbing parabolic dunes, were mobile during two periods (between 5000-3500 cala BP and between 3000-2300 cal. a BP) and then fixed by the natural development of pine forest covering the whole coastal zone until its invasion by the modern dune system. The modern dunes consist of a variety of types of crescentic dunes. Three main periods of sand drift were recognized by authors (Tastet, 1998; Clarke et al. 1999, 2002): (i) around 3000-4000 cal. a BP, (ii) around 900-1300 cal. a BP and (iii) around 250-550 cal. a BP during the ‘Little Ice Age’. From the beginning of the nineteenth century, the dune ridges were stabilized by the planting of maritime pines (Buffault, 1942). In the absence of any detailed chronology of dune development and of an accurate RSL history for the region, the evolution of the Médoc peninsula has been interpreted as having formed under the influence of a series of positive and negative sea-level trends, possibly coupled with storm activity and vegetation growth (Pontee et al., 1998; Clavé, 2001).

8

Fig. 3: Palaeogeographic reconstructions of the North Médoc peninsula from ca. 6000 to 1500 cal. a BP

Fig. 3: Palaeogeographic reconstructions of the North Médoc peninsula from ca. 6000 to 1500 cal. a BP

from Pontee et al., 1998

Fig. 4: Late Quaternary coastal formations of the North Médoc peninsula.

Fig. 4: Late Quaternary coastal formations of the North Médoc peninsula.

(A) Simplified geological map. (B) Schematic geological west–east cross-section

from Pontee et al., 1998

Fig. 5: Classifications of the north-Médoc coastal dune system.

Fig. 5: Classifications of the north-Médoc coastal dune system.

(A) Classification from Marionnaud (1972) and Dubreuilh & Marionnaud (1973). (B) Revised classification from Tastet (1998) and Tastet & Pontee (1998).

2.2 - L’Amélie beach

9The north-Médoc coast is macro-tidal with a maximal tidal range recorded at the Pointe de Grave of 5.4 m (SHOM, 2013). The wave climate is energetic, strongly seasonally modulated, with a dominant W to NW incidence (Butel et al., 2002; Bertin et al., 2008). High-energy winter swells and storm waves are typically from the W-NW with significant wave height sometimes exceeding 8 m, while the smaller and shorter summer waves generally arrive from the NW (Castelle et al., 2014; 2015). Tidal currents are small compared with wave-induced currents, except in the close vicinity of the Soulac area where bottom tidal currents exceed 0.5 m.s-1 in the Gironde inlet, but they decline rapidly on the inner shelf with values ranging from 0.10 to 0.15 m.s-1 (Lesueur & Tastet, 1994).

10The prevailing winds on the Médoc peninsula are WNW to WSW, representing 50 % of the total winds (Michel & Howa, 1999; Favennec et al., 2002). The strongest winds reaching 26 m.s-1 are recorded in winter and have an average direction between SW and NW. From sand trapping measurements, Favennec et al. (2002) showed around 70 % of aeolian sand volumes are transported in autumn and early spring along the Aquitaine coast. Moreover, sands of grain sizes up to 300 µm can be blown to the inner part of the dune ridge during the strongest wind events.

11L’Amélie beach is a sandy beach some 2 km long and about 100 m wide, south of the Soulac-sur-Mer urban area. Like most of the beaches along the Gironde coast, L’Amélie beach is characterized by the presence of rip-dominated intertidal sandbars (e.g. Bruneau et al., 2011). The sandbars are predominantly rip-channelled with rip spacing of about 300 m. The rip-channels are favourable points for observing the underlying sediment outcrops along the beach.

12The dune front exhibits more than 10 m high erosion scarps indicative of the present-day shoreline retreat. Since the early eighteenth century, the shoreline of the North-Médoc peninsula has been retreating, but with high alongshore variability. For the period between 1707 and 1966, Froidefond (1982) calculated shoreline migration for different locations along the coast: Pointe de Grave (‑1500 ± 370 m), Soulac-sur-Mer (-300 ± 75 m), L’Amélie beach (+100 ± 25 m), Anse du Gurp (‑500 ± 125 m), Montalivet (-50 ± 25 m). For the last decades, the shoreline sections north and south of La Négade cape (fig. 1) have displayed different migration patterns (Aubié & Tastet, 2000). The southern section is dominated by southward longshore drift and the maximum erosion rate is about 7 m/yr in its northern part, decreasing southward to about 1-2 m/yr at Lacanau (fig. 1). The section north of La Négade cape on the other hand is dominated by northward longshore drift and recorded shoreline retreat rates were about 5-10 m/yr from the 1960s to the 1990s (Aubié & Tastet, 2000). More recently, Castelle et al. (2018) have calculated multidecadal shoreline changes along the Aquitaine coast from a set of orthorectified aerial photographs and highlighted a higher sensitivity to erosion for beaches located close to the estuarine mouths. German bunkers located about 100 m seaward from the present-day shoreline are indicative of this historical coastal erosion. The eroded coastal sediments are transported by longshore drift to subtidal areas of the Gironde mouth where the sand supply is estimated to be 1.8 106 m3/yr (Allen et al., 1981). Idier et al. (2013) highlighted the strong interannual variability of the shoreline dynamics of the Aquitaine coast. Storm periods appear to be responsible for an acceleration of the retreat rates. The example of the severe storms of winter 2013-2014 is illustrative, with mean values of retreat around 20 m along the coast of Soulac-sur-Mer and maximum retreat values reaching 40 m on L’Amélie beach (Bulteau et al., 2014). In response to the recent coastal erosion, several defence structures have been installed along the coast in front of urbanized areas.

2.3 - Coastal archaeological remains of human settlements

13Along the northern coast of the Médoc, erosion of Holocene estuarine silty-sediment intertidal deposits has uncovered plentiful evidence of past human settlements dating from the Mesolithic (10600‑8000 cal. a BP) to the end of Antiquity (1450-1350 cal. a BP) (Marambat, 1992; Clavé, 2001). The high density of archaeological remains in coastal areas is attested by the numerous field discoveries reported by non-professional archaeologists. The lack of any real archaeological research programme has led to some confusion about the data acquired in this area. These findings were re-examined on the basis of the knowledge gained from the LITAQ programme in order to better understand the dynamics of settlement and the nature of the activities practised in the estuarine marshes (Verdin et al., 2019). In the Mesolithic and early Neolithic periods (8000-6500 cal. a BP), human settlements were located essentially around the sites of La Négade cape (Lède du Gurp) and Le Gurp Bay (fig. 1b). From the middle Neolithic and the late Neolithic (6500-4200 cal. a BP), the coastal area located between the localities of Soulac-sur-Mer and Montalivet displays a high density of archaeological sites, associated with a large scale practice of salt mining. The middle Bronze Age (3600-3400 cal. a BP) saw intense frequentation of the estuarine marshes for a purpose that is not yet well defined. Salt production is likely, but it is not truly evidenced by archaeological data. Numerous deposits of axes testify to a flourishing metallurgical industry and the coast of North Médoc perhaps lay at the crossroads of maritime and terrestrial communication routes for the trading of metals. The final Bronze period (3400‑2800 cal. a BP) corresponds to a break in the human occupation of the Médoc coastal zone. In the Iron Age (2500-1950 cal. a BP), the archaeological evidence of past activities resumed with numerous signs of production of ignigenous salt. Finally, in Antiquity (1950-1450 cal. a BP), the human settlements are associated with shellfish deposits, especially oysters, the harvesting of which could explain, among other things, the regular frequentation of these locations.

3 - Materials and methods

3.1 - Topography

3.1.1 - DGPS Surveys

14During the field campaigns, Differential Global Positioning System (DGPS) surveys were systematically conducted to obtain homogenously spread information about geomorphological and archaeological discoveries (contours of sediment outcrops on the beach, archaeological remains, dune palaeosols, sediment samples, position of beach cores, excavation by mechanical digger, etc.). Surveys were carried out using (i) a Trimble 5700/5800 Differential GPS in March and June 2014, and (ii) a Topcon Hyper V Differential GPS in March 2015. Data points described by three coordinate values (x, y, z) were collected in Real Time Kinematic (RTK) mode with an accuracy of, respectively, 4-5 cm (x and y) and 1 cm (z) (Suanez et al., 2012).

15Each DGPS measurement was converted into the French Lambert 93 coordinate system and elevations were linked to the French national datum NGF (Nivellement Général Français). Data were converted with respect to a benchmark installed on the jetty of L’Amélie beach during the first field campaigns in March 2014. The absolute geographical coordinates of the benchmark were obtained from a post-processing procedure using GrafNet 8.3 software. The raw GPS base data were stored and processed against raw GPS files provided by the permanent GPS network GNSS (IGN) for fixed stations close to the study site. The accuracy of the post-processing procedure is estimated to be within 1 cm.

3.1.2 - Terrestrial Laser Scanning (TLS) methodology

16Five campaigns of TLS data acquisition were undertaken between March 2014 and March 2015 to cover the studied coastal area (tab. 1). Ground-based laser scanner systems use LiDAR (light detection and ranging) technology to provide very accurate measurements of an object’s range relative to the scanner position (Watt & Donoghue, 2005). This is achieved by recording the time taken for the laser pulse to hit the target and return to the sensor (laser head) (Lichti et al., 2002). Provided that the location and orientation of the sensor head are known at the time of the survey, the scan data can be spatially registered to any given coordinate system. For these reasons, TLS is a useful tool for the acquisition of accurate topographic measurements on morphological features that are difficult to survey such as dune cliffs (fig. 6).

17In this study, scans were recorded using a Riegl VZ400 high-speed laser scanning system linked to a laptop computer. This scanner has a vertical and horizontal scanning range of 100° (Theta angle) and 360° (Phi angle) respectively. The scanning rate can be varied depending on the spatial resolution required, with a maximum data capture rate of up to 120000 points per second. The near-horizontal surfaces of the beaches were scanned with a medium resolution ranging from 0.04° to 0.10° for Theta and Phi angles respectively. Higher resolutions, between 0.02° and 0.06°, were used to scan the vertical face of the dune cliffs and obtain a better picture of the dune section. Data obtained in the form of a point cloud constitute a ‘model space’ with an accuracy of distance from the point location of up to 4 mm and spatial 3D up to 6 mm. A digital camera with a capacity of 5 MP was incorporated in the TLS allowing us to take a spherical photograph of the scanned area during one measurement cycle. The compatibility of distance measurement with the panoramic image enables multi-aspect analysis of the Digital Surface Model and spherical photograph of the surrounding area using RISCANPRO 1.8 software.

18Using TLS for measuring large areas involves integrating data obtained from individual measurement sites (Kociuba et al., 2014). In this study, the TLS data were acquired from 18 different locations spaced 100 m apart at most to ensure an adequate overlap of the separate point clouds along the entire length of the two beaches surveyed. The total area covered by our TLS surveys is around 50 ha and the topographic measurements extend over a distance of about 2270 m along the coast. A target point (TP) method was applied to merge the consecutive point clouds. This method is based on the distribution of a set of reflective targets in the field before TLS acquisition. The reflective targets we used were plastic tubes measuring 10 cm in diameter. The target network was recognized by the scanner after selection of a relevant function of the scanning software. In parallel, the geographical position of each target was measured by means of a differential GPS (Trimble 5700-5800 in 2014, Topcon Hyper V in 2015) allowing georeferencing of individual point clouds. Thus, all the point clouds were replaced in the same geographical coordinate system corresponding to Lambert 93 and elevations were connected to the French altitudinal datum (NGF). The maximum errors of the model came to 0.043 m horizontally and 0.053 m vertically.

19Finally, to facilitate the integration of data in a GIS, measured data in the form of a point cloud were transformed into a Digital Elevation Model (DEM) of the entire scanned area. The DEM was constructed with a resolution of 0.4  0.4 m using a gridding interpolation method.

Tab. 1: Details of TLS surveys made along L’Amélie beach.

Tab. 1: Details of TLS surveys made along L’Amélie beach.

Fig. 6: Topographic surveys along the L’Amélie dune front.

Fig. 6: Topographic surveys along the L’Amélie dune front.

(A) DGPS measurements. (B) TLS measuremnts. (C, D, E) High density point clouds obtained from TLS survey (here in section 23). The elevations of each aeolian sand unit and interbedded palaeosols and/or humic layers were easily obtained. All studied sections were correlated to propose a continuous 2 km alongshore stratigraphic view.

3.2 - Sediment analysis

20Grain-size analyses were performed in two different laboratories. However, the protocol analysis was identical in both cases. The grain-size distribution of 72 sediment samples collected along the dune cliff was measured at the Laboratoire de Géographie Physique de Meudon (LGP, UMR 8591 CNRS) using a Beckmann-Coulter LS230 laser granulometer for the fine fraction (<1 mm) and a sieving procedure for coarse material (>1 mm). A total of 30 sediment samples from the beach outcrops were analysed in the LETG (UMR 6554 CNRS) laboratory using a Malvern Mastersizer 2000 laser analyser. Organic matter was removed for laser-diffraction particle size study with hydrogen peroxide pre-treatment (e.g. Scott-Jackson & Walkington, 2005; Wang et al., 2006). All of the samples were mixed with a dispersing agent (0.3 % sodium hexametaphosphate) in order to disperse the clay particles.

21The formulas proposed by Folk and Ward (1957) were used to calculate the main grain-size characteristics: median (D50), sorting (σg), skewness (SKI) and kurtosis (KG) index values. For micropalaeontological analysis, 17 sediment samples were sieved through a 500 μm and 63 μm mesh and washed to remove clay and silt material. Foraminifera were concentrated by flotation in trichloroethylene as described by Murray (1979). Where possible, a minimum amount of 300 specimens of foraminifera were counted under a stereoscopic binocular microscope. Species identification was mainly based on specific papers showing modern assemblages and their distribution patterns in NE Atlantic estuaries and salt marshes (Allen et al., 1974; Moulinier, 1996; Redois & Debenay, 1996; Goubert, 1997; Armynot du Châtelet et al., 2005; Duchemin et al., 2005; Debenay et al., 2006; Leorri et al., 2010; Rossi et al., 2011; Delaine et al., 2015). Ostracod species were identified by the same approach (Oertli, 1985; Ruiz et al., 2000; Allen et al., 1974; Carbonel, 1980; Clavé et al., 2001).

3.3 - Radiocarbon dating

22A total of 14 carbon-rich sediment samples were collected from the field and radiocarbon dated at the VERA Laboratory (University of Vienna, Austria) and at the Beta-Analytic laboratory (Miami, USA) (tabs. 2 & 3). The conventional radiocarbon dates were calibrated using software Calib 7.0 (Stuiver & Reimer, 1993) and the IntCal13 calibration curve (Reimer et al., 2013). Errors in the inferred radiocarbon dates cannot be excluded, especially on the bulk palaeosol samples, because of the possible contamination of the sediment by older or younger carbon, such as by rootlet penetration or inwashed material. Two marine shells were also radiocarbon dated, taking into account the local reservoir effect. The Marine13 calibration curve was used and a delta R value of -10 (Standard Deviation = 37) was estimated from the Marine Reservoir Correction Database (Reimer et al., 2013) by using the nine nearest measurement points located less than 200 km from our study site (Tisnérat-Laborde et al., 2010). Finally, the radiocarbon results and the chronologically well-defined archaeological remains were used to provide a reliable chronological framework of the Holocene sediment infilling of L’Amélie beach.

Tab. 2: Details of radiocarbon ages obtained from samples collected on the sediment outcrops along L’Amélie beach.

Tab. 2: Details of radiocarbon ages obtained from samples collected on the sediment outcrops along L’Amélie beach.

Geographic coordinates of sampled points are in the French Lambert 93 coordinate system and elevations were attached to the French national datum NGF (Nivellement Général Français) corresponding to the mean sea level. Shaded rows correspond to marine shell samples whose radiocarbon ages were calibrated using the MARINE13 calibration curve with a ΔR of -10 and a standard deviation of 37 (see text for details).

3.4 - Sea-level index points production

23Sedimentological analysis, radiocarbon dating and accurate elevation measurements of coastal deposits and the associated archaeological remains were used to propose some new Holocene RSL data from this part of the French Atlantic coast. Holocene relative sea-levels were reconstructed using the ‘sea-level index point’ (SLIP) methodology (Hijma et al., 2015). The RSL for a SLIP is calculated from:

24RSL = Es - RWLs

25where Es and RWLs are the elevation and reference water level of samples, expressed relative to the same datum, and RSL is relative to local sea level at the time of sampling. The reference water level is the mid-point of the indicative range (IR) considered as the elevational range over which an indicator forms. Table 4 presents all the data used to produce our RSL records. The database is structured in the same format as that proposed for the Atlantic coasts of Europe (García-Artola et al., 2018).

26In this study, the IR was defined for three sediment samples using the palaeoenvironmental information provided by the foraminiferal assemblages. The IR is ranged from (i) mean high water neap tide (MHWNT) to highest astronomical tide (HAT) for assemblages indicative of a high salt marsh depositional environment, (ii) from mean low water spring tide (MLWST) to mean high water spring tide (MHWST) for assemblages dominated by tidal flat species; and (iii) above HAT for archaeological remains associated with human occupation or funeral practices, considered as a limiting point.

27The tide levels were defined by the French Hydrographic and Oceanographic Institute (SHOM, 2016) for the locality of Royan, located 15 km north of the study area. The uncertainty for the vertical position of RSL was quantified by taking into account: (i) the IR, (ii) the sample thickness, (iii) the sampling uncertainty, (iv) the uncertainty associated with the calculation of the elevation from DGPS and TLS measurements. We did not apply any corrections for tidal range variation in the past.

4 - Results

4.1 - Cartography and stratigraphy of intertidal sediment deposits

28Based on field observations, DGPS surveys and sedimentological analyses (grain-size analysis, sedimentary structures, organic and micropalaeontological content, radiocarbon dating) the sediment deposits cropping out along the beaches of L’Amélie and La Glaneuse were subdivided into 10 main lithofacies (U1 to U10, fig. 7) The cartography and the accurate elevation measurements were used to propose a general framework of the stratigraphic succession of deposits for the study area. The interpretation in terms of depositional environments was based on classic models (Reineck & Singh 1980; Allen 2000, 2003), regional studies of estuarine and coastal areas (e.g. Lespez et al. 2010; Tessier et al. 2012; Stéphan et al., 2015) and comparison with modern sediments characterizing coastal environments along the Gironde estuary (e.g. Marionnaud & Dubreuilh, 1972; Diot & Tastet, 1995; Massé et al., 2001). Archaeological remains discovered during field campaigns helped to better interpret the chronostratigraphic context of the sediment successions.

Fig. 7: Archaeological sites and sedimentary outcrops measured along L’Amélie beach.

Fig. 7: Archaeological sites and sedimentary outcrops measured along L’Amélie beach.

(A) Location map. (B) Vertical projection of the alongshore and elevational limits of the sedimentary units offering a stratigraphic view of deposits. The dune sections studied are also located along the dune front.

4.1.1 - Unit 1: Pleistocene deposits

29The oldest sedimentary successions (unit 1) corresponds to Pleistocene deposits represented by a 2 m thick compact dark-green clay (unit 1a) overlain by a 60 cm to 1 m thick gravelly sandstone (unit 1b) (fig. 8a). Unit 1a was only observed in the south of L’Amélie beach and was attributed to an estuarine mudflat sediment dated to MIS9 from the combination of luminescence (OSL, IR-RF) and electron spin resonance (ESR) dating of quartz and feldspar grains (Bosq et al., 2019). The median grain size is 8 µm. These sedimentological characteristics are consistent with results obtained by Marionnaud and Dubreuilh (1972). Unit 1b is a gravelly sandstone, 1.1 m thick at most observed only in the south of L’Amélie beach at the base of the dune front. This deposit is a colluvial material (Grès de l’Amélie) dated to the Weichselian Pleniglacial (MIS 2, Bosq et al., 2019). Numerous archaeological remains were discovered on the top of this deposit (ceramic fragments and flints attributed to the Bronze Age). A pit, dug in the sandstone and containing many wooden poles, was excavated on site AML-N-001 (fig. 9a) and a coal gave a radiocarbon age of 6180-5480 cal. a BP (mid to late Neolithic).

Fig. 8: Photographs illustrating the sediment deposits outcropping along L’Amélie beach.

Fig. 8: Photographs illustrating the sediment deposits outcropping along L’Amélie beach.

(A) Green clay (unit 1) uncovered during the winter 2014 in the south of L’Amélie beach (21/03/2015, P. Stéphan). This unit is attributed to the “Argiles du Gurp” Pleistocene formation (Marionnaud & Dubreuilh, 1972; Beauval et al., 1998; Tastet, 1999). (B) Sharp contact between the green clay (unit 3a) and the organic-rich silty-clay (unit 3b) in the north of L’Amélie beach (archaeological site AML‑N-003, 22/03/14, O. Got). (C) Cemented aeolian sand (“alios” or “garluche”) cropping out in the north of L’Amélie beach (21/03/2015, P. Stéphan). (D) Oak post mechanically excavated at the lowest tide level of L’Amélie beach. The post is fixed into a blue silty clay (unit 3b) overlying a coarse sand deposit (unit 3a) (archaeological site AML-N-008, 23/03/2015, P. Stéphan). (E) Silty clay with high proportion of fragmented Scrobicularia shells observed at the low spring tide level in the central part of L’Amélie beach (archaeological site AML-N-008, 23/03/15, P. Stéphan). (F) Silty clay deposit (unit 4) cropping out at the mean tide level in the central part of L’Amélie beach (archaeological site AML-N-007, 18/06/2014, F. Verdin). The surface of the deposits exhibit several tidal channels filled with oyster shells. (G) Organic silty-clay (unit 5) uncovered by marine erosion at the high neap tide level (archaeological site AML-N-002, 17-22/03/2014, F. Verdin). (H) Organic-rich surface of unit 5 showing numerous animal footprints (archaeological site AML-N-002, 17‑22/03/2014, F. Verdin).

Fig. 9: Photographs illustrating the high density of archaeological remains along L’Amélie and La Glaneuse beaches.

Fig. 9: Photographs illustrating the high density of archaeological remains along L’Amélie and La Glaneuse beaches.

(A) Neolithic pit being surveyed (archaeological site AML-N-001) (photo: F. Verdin, 23/03/2015). (B) General view of the pole structure observed at the lowest astronomical tide level (photo: F. Verdin, 23/03/15). (C) Gabion, pit and stakes (archaeological site AML-N-003) (photo: F. Verdin, 09/04/2012). (D) Gabion, pit and stakes being cleared (archaeological site AML-N-004 (photo: O. Got, 22/03/14). (E) Pot dating back to late antiquity discovered in unit U4 (archaeological site AML-N-006) (photo: F. Verdin, 18/06/2014). (F) Animal footprints visible at the top of unit U5 (archaeological site AML-N-002) (photo: Yoann Ceinturet, 08/04/2015).

4.1.2 - Unit 2: Early Neolithic tidal inlet (7410–7180 cal. a BP)

30Unit 2 corresponds to the first stage of the Holocene estuarine sediment infilling of L’Amélie and La Glaneuse beaches. The basal sediment unit (unit 2a) is a very well sorted (σg between 0.62 and 1.08) medium to coarse sand with a mean grain size ranging from 490 µm to 1010 µm (fig. 10). In detail, the mean grain size decreases from the bottom to the top of the deposit, which is characteristic of the filling of a tidal channel. Unit 2 was encountered at an elevation ranging from -3.9 to -2.5 m asl, during a mechanical excavation of the lowest part of L’Amelie beach (fig. 8d). No foraminifera were found within the sediment. A second mechanical excavation on La Glaneuse beach revealed a northward extension of this coarse sand deposit at an elevation ranging from -2.45 to -2.15 m asl. The material is overlain by a silty sand layer containing rootlets (unit 2b) dated to 7420-7180 cala BP (early Neolithic) (tab. 2). This unit is interpreted as the deepest fill of a large tidal channel, recognized in this area by Allen et al. (1974) and named the ‘Soulac channel’. This deposit is thought to be an extension of the tidal ravinement surface recognized in the outer estuary (Allen & Posamentier, 1993; Lericolais et al., 1998; Féniès et al., 2010) that was formed when the rate of the RSL rise slowed down around 7000 cal. a BP.

Fig. 10: Sedimentological analyses of stratigraphic units U1a to U5 obtained from sediment samples collected along the beach.

Fig. 10: Sedimentological analyses of stratigraphic units U1a to U5 obtained from sediment samples collected along the beach.

(A to D) Grain size characteristics. (E and F) Microfaunal content. (G) Coastal environments associated.

4.1.3 - Unit 3: first generation of intertidal estuarine deposits (late Neolithic to the early Iron Age)

31Unit 3 consists of intertidal estuarine sediments that can be subdivided into four main sediment facies.

4.1.3.1 - Unit 3a

32Unit 3a is located at an elevation ranging from -2.5 to -0.9 m asl, close to the lowest astronomical tide level (figs. 78d). Six sediment samples were collected from the base to the top of this unit during mechanical excavation of the archaeological site AML-N-008. The grain-size analysis indicates a gradual change from a sandy-silt at the base of the unit (median grain size from 30 to 40 µm) to a clayey-silt at the top (median grain size from 10 to 13 µm) with bi-modal grain-size distribution curves (fig. 10d). The first mode is between 10 and 20 µm and corresponds to a low-energy depositional environment. The second mode ranges from 200 to 400 µm and could be associated with aeolian sand supplies from a seaward sediment source or stronger tidal influences. The sand fraction decreases from the base to the top of the unit, probably due to the progressive fixation of the dune front or an increase in the distance to the shoreline in a context of dune barrier progradation. The assemblages of foraminifera are dominated by estuarine intertidal mudflat species such as Haynesina germanica and Trochammina inflata (fig. 10e). Two brackish water species of ostracods are also present (Cyprideis torosa and Loxoconcha elliptica). We consider unit 3a to have formed in an intertidal mudflat environment behind a progradational dune barrier. A radiocarbon age obtained from a pit lined with wooden stakes at the top of this unit (archaeological site AML-N-008) gives an age of 2760-2490 cal. a BP and provides a terminus ante quem to the deposit around the first Iron Age.

4.1.3.2 - Unit 3b

33Unit 3b is a dark organic-rich clayey-silt to silty sand (fig. 8b) with a median grain size ranging from 20 to 60 µm (fig. 10). The foraminiferal assemblage is dominated by high salt marsh species (Trochammina inflata and Jadammina macrescens). This deposit crops out only at La Glaneuse beach at an elevation ranging from -0.25 to +0.2 m asl. A wood fragment collected from the base of the unit is dated to 7000-6740 cal. a BP (tab. 2). A pit with oak wood walls was discovered at the top of this deposit (fig. 9c, d) and the wood was radiocarbon dated to 4800-4240 cal. a BP (Late Neolithic, site AML‑N‑004). Several marks of agricultural practices were observed on the surface, but it is possible that they were produced during a later phase of human occupation. Consequently, unit 3b may correspond to a late Neolithic high marsh environment, gradually replaced by an undifferentiated (high or low) salt marsh environment.

4.1.3.3 - Unit 3c

34Unit 3c consists of a humate-impregnated black sandstone cropping out on the northern part of La Glaneuse beach (fig. 8c). The elevation of the sandstone outcrop ranges from +1.2 to +2.2 m asl (fig. 7). The sediment is well-sorted with a median grain size of 450 µm. These sedimentological characteristics are consistent with an aeolian origin (Vincent, 1996). This deposit could be interpreted as an ‘alios’ or as a bog iron (called ‘garluches’ in the Aquitaine area). The ‘alios’ formation is a common feature of podzolic fossil soil profiles in coastal dune deposits and on ocean beaches (Brookfield & Ahlbrandt, 1983; Pye & Tsoar, 2009). A similar indurated, brown-to-black coloured horizon is observed along the present-day dune front of Le Pilat (Suchý et al., 2013). The formation of this particular sandstone is related to rapid (less than 3000 years) cementation and induration processes affecting the aeolian sand deposits underlying a palaeosol within the dune barrier (Suchý et al., 2013). The ‘garluche’ is a 0.3 to 0.4 m thick bog iron hard-pan formed during the Holocene period within the upper Pleistocene ‘Sable des Landes’ sand formation, especially along small coastal streams or in swampy zones surrounding back-barrier freshwater lakes (Gourdon-Platel & Morin, 2004). Consequently, this deposit suggests the presence of a dune barrier in the vicinity of a swamp environment in the sector of La Glaneuse, in parallel with the development of a marsh environment on L’Amélie beach.

4.1.3.4 - Unit 3d

35Unit 3d is a thin layer of reworked lithoclasts (we call ‘alios gravels’) with similar sedimentological characteristics to the alios sandstone (unit 3c). This layer was encountered close to the alios outcrop at an elevation of around +0.2 m asl. Here, the lithoclasts have a mean diameter up to 10 cm. A second layer of alios gravels was also observed in the central part of L’Amélie beach at an elevation of -1.4 m asl. The lithoclasts are smaller (mean diameter of 2 cm) and embedded in a clayey-silt sediment layer the top of which is characterized by a pavement of Scrobicularia shells (fig. 8e) dated to 3434-3070 cal. a BP (recalibrated date from Clavé, 2001). These field observations suggest a significant episode of dune barrier retreat leading to (i) the erosion of the alios formation and the transportation of the lithoclasts into the salt marsh environment by high-energy waves. The pavement of Scrobicularia shells is interpreted as an erosional surface.

4.1.4 - Unit 4: Second generation of estuarine mudflats (ca. 2700 to ca. 1250 cal. a BP)

36Unit 4 is a clayey-silt deposit (mean grain size between 7 and 15 µm) covering a large spatial extent along the beaches of La Glaneuse and L’Amélie (fig. 7). The elevation of the sediment outcrops is between -1 and +1.3 m asl. Sediment samples collected at the base of the unit exhibit unimodal grain-size distribution curves (fig. 10d). The mode is around 25 µm reflecting a low-energy depositional environment. The foraminiferal assemblages are dominated by intertidal mudflat species with Haynesina germanica the dominant species (fig. 10e). These sedimentological characteristics indicate depositional conditions favourable to the formation of back-barrier estuarine mudflats after the erosion phase of unit 3d. The grain-size distribution curve of the sediment sample collected from the top of the unit shows a second mode around 400 µm (fig. 10d). The medium sand fraction represents 15 % of the total sediment volume reflecting episodic aeolian sand supplies from the dune front. This unit yields a high density of archaeological remains, especially wooden poles. On the archaeological site ALM-N008, a construction built on three rows of pine poles, preserved over 24.50 m in length and measuring 3 m in width, is dated to 2700-2495 cal. a BP (fig. 9b). Several tidal channels filled by organic-rich clayey-silt sediment containing a high density of oyster shells were observed in the field (fig. 8f). A radiocarbon age obtained on one of the oyster shells in life position yielded an age of 1900-1510 cal. a BP (tab. 2). Several archaeological remains discovered within this deposit have been attributed to the Iron Age period, Antiquity and the Middle Ages (fig. 9e). The most recent archaeological evidence found in this mudflat sediment is a ceramic whose manufacture dates to the 7th century AD (site AML-N-006).

4.1.5 - Unit 5: Salt marshes (ca. 2500 to ca. 1070 cala BP)

37Unit 5 is an organic-rich clayey-silt located in the northern and southern parts of L’Amélie beach at an elevation ranging from +0.7 to +1.8 m asl (fig. 7). The mean grain size is about 10 µm. The grain-size distribution is bimodal with a first mode at 8 µm and a second mode at 500 µm (fig. 10d). The sand fraction represents 15 % of the total sediment reflecting significant aeolian or tidal sediment supplies. The southern extent of the deposits shows numerous animal tracks on the surface (fig. 9f) and archaeological materials (lithic and ceramic). The foraminiferal content is dominated by Jadammina macrescens, with a few occurrences of Haynesina germanica and Gavelinopsis praegeri indicating a salt marsh environment. No radiocarbon age was obtained from this deposit, but ceramic fragments found on the archaeological site AML-N-002 were attributed to the Iron Age (ca. 2500-2400 cala BP). Consequently, unit 5 is contemporaneous with unit 4 in this section. The chrono-stratigraphical data from the dune section (see section 4.2) indicate that this deposit was buried by aeolian sand supplies from ca. 1370 cal. a BP onward. On the northern part of L’Amélie beach, the salt marsh deposits display similar sedimentological characteristics. The foraminiferal assemblage is also dominated by a mixed low and high marsh taxa (fig. 10e). A radiocarbon sample obtained at the top of the unit gave an age of 1286-1070 cal. a BP (tab. 2). After this period, the salt marsh was buried by the coastal dune barrier.

4.2 - The dune stratigraphy of l’Amélie beach

38The dune stratigraphy was reconstructed from 28 sections described and topographically surveyed along the dune front of L’Amélie beach (fig. 7). The dune sequence is subdivided into five aeolian sands units (units 6 to 10) from the base to the top, separated by three layers of palaeosols or organic-rich sediment (figs. 1112). The ages of the interbedded humic layers were obtained from seven radiocarbon ages (tab. 3).

Fig. 11: Reconstructed dune stratigraphy along L’Amélie dune front.

Fig. 11: Reconstructed dune stratigraphy along L’Amélie dune front.

The positions of sediment samples collected for sedimentological analyses and radiocarbon dating are also shown. Results of radiocarbon datings correspond to the median probability of calibrated ages.

Fig. 12: Photographs illustrating the aeolian sand deposits along the dune front of L’Amélie beach.

Fig. 12: Photographs illustrating the aeolian sand deposits along the dune front of L’Amélie beach.

(A) Section 18. (B) section 25. (C) section 14. (D) section 22. (E) section 9. (F) section 26.

Tab. 3: Details of radiocarbon ages obtained from samples collected on the dune palaeosols of L’Amélie beach.

Tab. 3: Details of radiocarbon ages obtained from samples collected on the dune palaeosols of L’Amélie beach.

Geographic coordinates of sampled points are in the French coordinate system Lambert 93 and elevations are related to the French national datum NGF (Nivellement Général Français) corresponding to the mean sea level.

4.2.1 - Unit 6: A dune slack environment (ca. 6180-5480 cal. a BP to ca. 1890-1370 cal. a BP)

39Unit 6 is a 1.8 m thick gleyey sand overlying the L’Amélie sandstone (unit 1b) in the south of the L’Amélie dune front (between sections 18 and 28). The unit is encountered at an elevational range from +3.2 to +5 m asl. The material is very well sorted (mean sorting index of 0.42), symmetrically distributed (mean skewness values of -0.01) and very platykurtic (mean kurtosis index values of 0.32) medium sand (mean D50 of 325 µm) (fig. 12). These grain-size characteristics are consistent with an aeolian origin. Stagnogley sands are frequently observed at the base of coastal sand deposits after a long period of dune stability (Packham & Willis, 1997). The process of gleyification suggests the stagnation of fresh water within permeable sediment during a long period of time as confirmed by the archaeological data and the radiocarbon dates obtained. The base of unit 6 is dated to the late Neolithic thanks to the archaeological site AML-N-001 where a pit containing coal residues was excavated. A radiocarbon date obtained from a charcoal indicated an age of 5480-6180 cal. a BP (tab. 2). Within unit 6, numerous lithic and ceramic fragments have been dated to the Bronze Age, indicating that this deposit is contemporaneous with units 3. The top of unit 6 corresponds to a 0.1 to 0.2 m thick peaty palaeosol (palaeosol 0) containing a high proportion of wood fragments which have yielded radiocarbon ages between 2150-1830 cal. a BP and 1890-1370 cal. a BP (tab. 3). This deposit can be interpreted as a dune slack environment where organic material accumulated in low-lying depressions between the dune ridges located several hundred metres seaward.

4.2.2 - Unit 7: Initial phase of dune invasion (1370-1890 cal. a BP to 1100‑1340 cal. a BP)

40Unit 7 is a 1 m-thick medium sand deposit observed between sections 11 and 21 at an elevation ranging from +4 to +5 m asl (fig. 11). To the south, the deposit buries palaeosol 0 indicating that this accumulation is younger than 1370-1890 cal. a BP. To the north, the unit dips northward under the surface of the upper beach. The sandy material is very well sorted (mean sorting index of 0.43), symmetrically distributed (mean skewness values of -0.03) and very platykurtic (mean kurtosis index values of 0.33) with a median grain size of around 333 µm suggesting an aeolian origin (fig. 13). The deposit is characterized by the presence of submillimetre thick horizontal stratification of silty sediment. Unit 7 corresponds to an initial deposit of aeolian sands on the southern edge of the estuarine salt marsh environment represented by units 4 and 5 (L’Amélie beach). Consequently, the silty stratification is interpreted as the episodic deposition of fine material during the highest astronomical tide flooding events.

Fig. 13: Grain-size characteristics of the main stratigraphic units studied along L’Amélie dune front.

Fig. 13: Grain-size characteristics of the main stratigraphic units studied along L’Amélie dune front.

4.2.3 - Unit 8: Massive phase of dune invasion (1100-1340 to 800-1180 cal. a BP)

41Unit 8 is observed along the entire dune front at an elevation ranging from +0.75 to +7 m asl with a thickness between 0.5 m (dune section 20) to 3.5 m (dune section 7, fig. 11). This deposit corresponds to a very well sorted (mean sorting index of 0.44), symmetrically distributed (mean skewness values of 0.01) and very platykurtic (mean kurtosis index values of 0.32) medium sand (326 µm) (fig. 13). No internal stratification was noted because of bioturbation processes. Root penetration and microfaunal activity were evidenced by numerous rust-coloured stains related to redox conditions and attributed to phases of water saturation of aeolian sands. Along the longshore transect, unit 8 displays a slight north–south dip with a slope value of 0.25 %. This suggests northward dune progradation and a gradual burying of salt marsh sediments by aeolian sands. At the top of unit 8, a second 0.1 m thick palaeosol (palaeosol 1) is dated from 1100-1340 to 800-1180 cal. a BP (tab. 3).

4.2.4 - Unit 9: Dune invasion in a context of enhanced wind-forced aeolian activity (post-800-1180 cal. a BP)

42Unit 9 is composed of aeolian medium to coarse sand (mean D50 of 461 µm) that is very well sorted (mean sorting index of 0.41), symmetrically distributed (mean skewness values of 0.06) and very platykurtic (mean kurtosis index values of 0.36) (fig. 13). The elevation ranges from +4.5 to +7 m asl (fig. 11). The deposit thickens northwards. No internal stratification was observed, but we noted a subhorizontal humic horizon at 7 m asl, indicating a short period of dune stability. This sediment unit is characterized by a significant proportion of coarse sands. The size fraction between 500 and 1000 µm accounts for 25 % of the sediment. This is interpreted as an increase in the windiness and/or storminess associated with the cooler climate of the Little Ice Age.

4.2.5 - Unit 10: Modern aeolian sand dune

43Unit 10 is a 5 m thick deposit of medium aeolian sand (290 µm) that is very well sorted (mean sorting index of 0.46), symmetrically distributed (mean skewness values of -0.01) and very platykurtic (mean kurtosis index values of 0.41). Along the dune front, horizontal stratifications were observed (fig. 12). Despite the absence of information about the internal orientation of the strata, these features are similar to those of established foredunes (Hesp, 1988) and are produced by sand deposition within increasing vegetation cover on relatively gentle near-horizontal slopes.

5 - Discussion

5.1 - New sedimentological and archaeological records of the Holocene RSL rise

44The history of the Holocene Relative Sea Level (RSL) along the Aquitaine coast is poorly known. Using a Glacial Isostatic Adjustment (GIA) model, Lambeck (1997) proposed a first RSL curve for the locality of Royan. However, the model used is constrained by very limited RSL data. Pontee et al. (1998) suggest RSL fluctuated around the present-day level in the Gironde area over the last 4000 years in a series of low-amplitude (<1 m) positive and negative trends. Recently, Stephan and Goslin (2014) followed by García-Artola et al. (2018) have used sedimentological and stratigraphic data collected by many authors from the salt marshes of Charentes and Gironde (Gabet, 1973; Ters, 1973; Visset et al., 1989, 1990; Bourgueil 1995, 2005; Diot & Tastet 1995; Tastet et al., 2000a,b; Clavé et al., 2001 ; Wang et al., 2006) to produce a series of 11 index points, six marine limiting ages, and two freshwater limiting ages (tab. 4, fig. 14). Data were then fitted to a spatio-temporal empirical hierarchical model (Cressie & Wikle, 2011; Kopp et al., 2016; Ashe et al., 2019) to propose an RSL curve with 1σ and 2σ confidence interval envelopes (García-Artola et al., 2018) for the SW France coastal area. The oldest freshwater limiting date constrains RSL to below -13.1 ± 2.6 m at ca. 9000 cal. a BP. Two marine limiting dates place RSL above -11.1 ± 2.6 m at ca. 8500 cal. a BP and above -9.5 ± 2.6 m at ca. 8100 cal. a BP. The oldest SLIP indicates RSL was at -5.2 ± 2.9 m at ca. 7300 cal. a BP. RSL rose to -3.5 ± 2.9 m at ca. 7100 cal. a BP. Then, RSL rose from -3.0 ± 2.9 m at ca. 6500 cala BP to -1.5 ± 2.6 m at ca. 4700 cal. a BP at a rate of 0.7 ± 0.5 mm.yr-1. The late Holocene is only constrained by three index points that place RSL at ca. -1.0 ± 2.5 m at ca. 2900 cal. a BP and at 0.5 ± 2.8 m at 600 cal. a BP. SLIPs produced in this paper are consistent with previous RSL records and provide two new RSL records for the last 3000 years.

Tab. 4: Excerpt from the RSL database for the Atlantic coasts of Europe (García‑Artola et al., 2018) limited to sub-region 7 corresponding to the coast of Charentes and supplement by RSL data produced in this study.

Tab. 4: Excerpt from the RSL database for the Atlantic coasts of Europe (García‑Artola et al., 2018) limited to sub-region 7 corresponding to the coast of Charentes and supplement by RSL data produced in this study.

45Data are plotted on observations produced by García-Artola et al. (2018) and compared to GIA and spatio-temporal statistical model predictions for the Charentes and Aquitaine coastal regions of France.

5.2 - Palaeogeographic changes of the Gironde mouth

46Most of the sedimentary outcrops identified on the L’Amélie and La Glaneuse beaches correspond to Holocene estuarine mud-flat and salt marsh deposits accumulated within a large topographic depression of Pleistocene age. The fine estuarine sediments (units 3 to 5) cover a deposit of coarse sand (unit 2a) attributed to a tidal channel that could correspond to the ‘Soulac channel’ indicated in several palaeogeographic reconstructions of the Gironde estuarine mouth (Allen et al., 1974; Pontee et al., 1998). In agreement with the palaeogeographical maps of the North Médoc peninsula produced by Pontee et al. (1998), these results provide a more detailed view of Holocene coastal changes with better chronological resolution in the areas of L’Amélie and La Glaneuse beaches. The presence of a coastal marsh open to the east and connected to the mouth of the Gironde starting at 6000 BP is confirmed by this study. According to Pontee et al. (1998), the salt marsh was bordered on its western side by a coastal dune complex with a coastline located further west than at the present-day. The formation of this first coastal dune barrier is not well-dated, but the oldest palaeosol-date in the Médoc (Montalivet) is 6170-5610 cal. a BP (Dubreuilh, 1971). Multiple younger palaeosols are known with ages ranging from 4000 to 3000 cal. a BP along the Médoc coastline (Tastet & Pontee, 1998), as are multiple periods of aeolian activity and dune fixation (Clarke et al., 2002). We assume the dark sandstone (‘alios’ or ‘garluche’) which crops out on La Glaneuse beach is contemporary with one of these two periods and could correspond to the inner part of a first coastal dune complex. Suchý et al. (2013) recently studied the black sandstone horizon forming the base of the Dune du Pilat, 100 km south of the study area. They provided insight into the composition and the formation processes of organic cements and provided information on the rates at which cementation and induration occur in these temperate-climate coastal deposits. An initial aeolian sand deposit was impregnated by an amorphous substance rich in organic matter and of humate composition derived from a palaeosol (palaeosol 1). The humate formed thin coats on the grain surfaces and grain contacts, thus cementing the sediment and causing its penetrative black colouring. The induration of the sand was completed over a relatively short period (less than 3500 years). A similar geochemical process, frequently encountered at the interface between sand dune swamps and stagnant lakes (Lapen & Wang 1999; Gourdon-Platel & Morin 2004), could explain the presence of the dark sandstone on La Glaneuse beach.

47The sedimentary infilling reconstructed along the beaches under study shows two main stages of estuarine deposition, interrupted by a major episode of marine erosion (figs. 1516). This erosional event is characterized by an abrupt change in hydrodynamic conditions inside the estuarine marsh, as proved by the abundant production of blackened sandstone pebbles. These lithoclasts were eroded from the nearby source bed in the north of the study area, rounded by wave action, and transported and incorporated into estuarine sediments further south. In the median part of L’Amélie beach, these sandstone pebbles are interbbeded into a layer of dark clay containing abundant shells of Scrobicularia. The radiocarbon date obtained from one of these shells gave an age of 3070-3435 cal. a BP (Clavé, 2001; tab. 2).

48Again, these results are broadly consistent with the geological surveys conducted in the inner part of the mouth of the Gironde where two main stages of salt marsh sedimentary infilling were recognized. The first generation of marshes is separated from the second by the ‘Cordon de Richard’ shelly ridge, the construction of which was interpreted as the result of a sea-level rise and/or increase in storminess (Pontee et al., 1998; Massé et al., 2001; Clavé, 2001). Radiocarbon ages from Scrobicularia shells collected along the shelly ridge over a long time period ranging from 3175 to 2110 cal. a BP (Massé et al., 2001) while the age obtained on L’Amélie beach indicates a slightly more ancient age for coastal changes of around 3450-3090 cal. a BP. The period around 3000-2700 cal. a BP has been widely recognized as a period of climate deterioration in NW Europe (Van Geel et al., 1996; Barber et al., 2003, 2004; Magny, 2004; Dark 2006; Gandouin et al., 2009; Charman, 2010; Swindles et al., 2013; Tisdall et al., 2013), characterized by colder and wetter climatic conditions with increased rainfall and largely enhanced storminess (Van Geel et al., 1996; Clarke & Rendell, 2009). Major disruptions of coastal sedimentary environments were also identified all along the European coasts, either in the English Channel region (Long & Hughes, 1995; Billeaud et al., 2009; Sorrel et al., 2009; Lespez et al., 2010; Tessier et al., 2012) or along the Atlantic coasts (Pontee et al., 1998; Tastet & Pontee, 1998; Clavé et al., 2001; Moura et al., 2007 ; Sorrel et al., 2009; Allard et al., 2009; Stéphan et al., 2015).

Fig. 15: Mid-Holocene sedimentary filling sequence of L’Amélie beach and interpretation of associated palaeogeographic changes from ca. 7000 to 3000 cal. a BP.

Fig. 15: Mid-Holocene sedimentary filling sequence of L’Amélie beach and interpretation of associated palaeogeographic changes from ca. 7000 to 3000 cal. a BP.

Fig. 16: Mid to late Holocene sedimentary filling sequence of L’Amélie beach/dune system and interpretation of associated palaeogeographic changes from ca. 3000 cal. a BP to the present-day.

Fig. 16: Mid to late Holocene sedimentary filling sequence of L’Amélie beach/dune system and interpretation of associated palaeogeographic changes from ca. 3000 cal. a BP to the present-day.

5.3 - Periods of dune stability and aeolian sand drift episodes

49Many attempts have been made to find a common chronological framework for aeolian sand activity/stability phases along the Aquitaine coast. Two well-differentiated records were used: humic sand units or palaeosols dated using radiocarbon (Froidefond & Legigan, 1985; Tastet, 1998; Dubreuilh, 1971; Tastet & Pontee, 1998) and aeolian sand units dated using luminescence measurements (IRSL) (Tastet, 1998; Clarke et al., 1999, 2002; Sitzia et al., 2015). The two methods appear complementary. The IRSL measurements on sands yield an age at which sand was buried, and underwent no reworking. The measurement therefore provides information about the last phase of wind activity at a given location. Radiocarbon dating of palaeosols provides information about the dune surface fixation over a variable time period and at a given location. Several palaeosols are found in succession in the stratigraphy and may indicate phases of stability separated by phases of sand invasion. However, the organic layers can also be eroded by wind so that the absence of palaeosols within a stratigraphy does not mean that the dunes were never stable. In addition, the ages obtained are only partial and do not provide very accurate information. Indeed, the problems of carbon pollution within organic layers usually require consideration of the total organic material (bulk). The age obtained cannot be used to define the beginning and end of the period of stability. These time limits can only be approached statistically by multiplying the radiocarbon dating of each palaeosol.

50Thanks to the topographic survey of palaeosols and several radiocarbon ages obtained along the 1 km dune front of L’Amélie beach, the chronostratigraphy of the coastal dune system is locally well-reconstructed. If we consider only the median probability of radiocarbon calibrated ages, two main periods of dune stability are recognized: (i) from 1990 to 1640 cal. a BP (palaeosol 0), (ii) from 1260 to 1010 cal. a BP (palaeosol 1). These periods were interrupted by phases of aeolian sand invasion burying the salt marsh deposits dated from 1640 to 1250 cal. a BP (stratigraphic units 7 and 8) and after 1250 cal. a BP (stratigraphic units 9 and 10). A shorter third period of stability is also represented by a thin humic layer between stratigraphic units 9 and 10. Because of the high proportion of coarse sand in the aeolian sediment, unit 9 is interpreted as evidence of stronger winds. In NW Europe, several periods of coastal sand drift were recorded over the last 1000 years especially during the cool ‘Little Ice Age’ (ca. 450-150 a e & Rendell, 2009; Lamb & Frydendahl, 2005; Costas et al., 2012; Danielsen et al., 2012; Van Vliet-Lanoë et al., 2014) in a context of increased storminess. Despite the lack of chronological information, we hypothesize that unit 9 is dated to the LIA period, while unit 10 formed over the last centuries before the dunes were artificially fixed with pine forest (Buffault, 1942).

51In Figure 17 and Table 5, the periods of dune stability recognized along the L’Amélie dune front are compared with data available for the Aquitaine region (Dubreuilh, 1971; Froidefond & Legigan, 1985; Tastet, 1998; Clarke et al., 1999, 2002). While the data on the Médoc dune system and Le Pilat dune reveal numerous common phases of dune stability and aeolian activity, the long-term morphological changes of the L’Amélie dunes present a specific chronology. Along L’Amélie beach, this chronology is limited to the last 2000 years, while the peat layers studied along the Médoc coastal dunes cover a long time period indicating two previous phases of dune fixation, dated at ca. 6000 cal. a BP (Dubreuilh, 1971) and from ca. 4000 to ca. 3000 cal. a BP (Dubreuilh, 1971 ; Tastet & Pontee, 1998). During the last 2000 years, the only common phase of dune stability with the Médoc dune system is represented by palaeosol 0 dated from 1990 to 1650 cal. a BP (median calibrated ages). No synchronicity is observed in the formation of palaeosols between L’Amélie beach and the Dune du Pilat, except for the humic layer observed in the L’Amélie dune sequence that may correspond to palaeosol 3 of the Dune du Pilat dated to 320-157 cal. a BP (Tastet, 1998).

52Several authors attempted to explain the long-term morphological evolution of coastal dunes of Aquitaine depending on factors such as (i) sediment supply related to coastal erosion by severe storms, (ii) positive or negative variation of RSL during the last 6000 cal. a BP, (iii) changes in vegetation cover, and (iv) increased windiness and storminess. Previous work has favoured dune formation under falling RSL and cooler climatic conditions which, through coastal exposure, supplied sand to the dunes whilst positive RSL tendencies favoured the erosion of the coast and the fixation of the dunes by a more humid climate and a rise in groundwater levels. The evolution of the Médoc peninsula was interpreted as being formed under a series of positive and negative RSL tendencies that affected the area after 6000 cal. a BP (Pontee et al., 1998). However, the RSL records produced in this paper are not reliable enough to confirm periods of significant fall in RSL over the last millennia. In the Dune du Pyla area, Bressolier et al. (1990) also believed that dunes were active during regressive periods, but also considered climatic change to be important. On the dune barrier lining the southern North Sea, Anthony et al. (2010) recently highlighted the role played by repeated storms into the mid-Holocene dune formation. The onshore transfer of tidal-bank sand generates pulses of abundant sand supply and an extremely rapid progradation (up to 1 km over a century) of the sand-flat shore, the surface of which serves as a large aeolian fetch zone for active backshore dune accumulation. Because of the vicinity of the Gironde mouth, the long-term morphological changes of L’Amélie beach/dune system are very sensitive to variations in sediment supplies provided by the estuary through the onshore migration of tidal sandbanks. On a short time scale (decadal), Castelle et al. (2018) highlighted the high spatio-temporal variability of the shoreline at the mouths along the coasts of Charentes and Aquitaine through a very active coastal erosion process over the last decades. Historical records from Roman times, bathymetric maps dating back to the seventeenth century and recent bathymetric surveys also indicate great morphological changes of the sandbanks along the Gironde mouth over the last centuries (Allen et al., 1974; Kapsimalis et al., 2004). Analysis of these bathymetric data indicates the initial construction of sandbanks and high volumes of sediment transfer leading to their migration over the past three centuries. Effects on the long-term evolution of the North Médoc dune system are very difficult to estimate because of the very dynamic nature of these coastal features and the numerous feedbacks between hydrodynamic and morphosedimentary factors (e.g. the modulation of incident storm-wave energy variations alongshore depending on the location and the morphology of sandbanks). Nevertheless, the variability of shoreface sand supply related to wave and wind climate changes seems to provide a template for a better understanding of the late Holocene coastal dune evolution of the storm-dominated Médoc peninsula.

Fig. 17: Synthesis of the data available along the Aquitaine coast related to the chronology of the coastal dunes.

Fig. 17: Synthesis of the data available along the Aquitaine coast related to the chronology of the coastal dunes.

The middle gray bands correspond to the periods of stability of the coastal dunes defined from the radiocarbon dating of the peat layers. The data presented in this paper are compared with previously published data (Froidefond & Legigan, 1985; Tastet, 1998; Dubreuilh, 1971; Tastet & Pontee, 1998) which have been calibrated from the IntCal13 curve (Reimer et al. 2013) using Calib 7.0 software (Stuiver & Reimer, 1993). The lower gray bands correspond to the phases of wind activity and mobility of the sand bodies defined from the IRSL dating performed on the quartz grains (Clarke et al., 2002; Tastet, 1998; Clarke et al. 1999). The radiocarbon data relative to the formation of the “cordon de Richard” shelly ridge (Gironde estuary) are also indicated. These data were calibrated by the procedure described in section 3.3.

Tab. 5: Chronological data available on the Holocene evolution of the Aquitaine coastal dunes.

Tab. 5: Chronological data available on the Holocene evolution of the Aquitaine coastal dunes.

6 - Conclusions

53The recent 2013-2014 erosion events exposed mid- to late-Holocene sedimentary records along L’Amélie beach (locality of Soulac, SW France) that have been intensively studied using topographical surveys and sedimentological analyses of deposits cropping out in the intertidal zone. This paper presents the resulting comprehensive chronostratigraphic and palaeoenvironmental framework that greatly increases our understanding not only of the long-term coastal evolution of this specific part of France, but also -by comparing the established evolution with supra-regional palaeoenvironmental changes- of the variation in driving factors like sea-level change, sediment availability and the wind regime.

54From bottom to top, the Holocene sequence exhibits 10 main lithofacies:

55- Unit 1 corresponds with Pleistocene deposits represented by a 2 m-thick compact dark-green clay dated to MIS 9, overlain by a gravelly sandstone dated to MIS 2;

56- Unit 2 is composed of a very well sorted medium to coarse sand deposit, dated to ca. 7290 cal. a BP and interpreted as the gradual infilling of a large tidal channel;

57- Unit 3 consists of a set of organic to minerogenic silty clay deposits with a sharp erosion surface characterized by the presence of reworked gravelly lithoclasts of humate-impregnated sandstone, locally associated with a pavement of Scrobicularia shells dated to 3434-3070 cal. a BP. These deposits suggest the development of estuarine salt marshes and mud-flats, then their erosion by waves;

58- Units 4 and 5 are formed by a clayey-silt deposit with foraminiferal assemblages dominated by intertidal mudflat species, and by an organic-rich clayey-silt with foraminiferal content indicating a salt marsh environment, respectively. Radiocarbon datings and abundant archaeological evidence indicate the presence of a back-barrier environment along the present-day coastline of L’Amélie beach from ca. 2700 to ca. 1000 cal. a BP;

59- Units 6 to 10 are a succession of aeolian sand deposits separated by three layers of palaeosols or organic-rich sediments dated to ca. 1990 cal. a BP to the present-day.

60The sedimentary sequences are indicative of:

61- a significant Holocene sedimentary sequence composed of coastal material deposited in a large depression located in the central part of L’Amélie beach;

62- the presence of a large tidal channel filled by coarse sand during the first stage of the Holocene sea-level upstand from ca. 7000 to ca. 5000 cal. a BP;

63- fine-grained estuarine sedimentation behind a coastal dune barrier from ca. 5000 to 2700 a calBP, interrupted by an episode of coastal erosion between 3500 and 3000 cal. a BP;

64- gradual burial of the estuarine salt marshes with aeolian sands from ca.1900 to 1200 cal. a BP;

65- two main periods of coastal dune stability dated from 1990 to 1640 cal. a BP (palaeosol 0) and from 1260 to 1010 cal. a BP, respectively.

66Despite the usefulness of these initial results for better interpreting the coastal archaeological contexts of the North Médoc, further research is still required to (i) better constrain the history of palaeogeographic changes over the last ca. 7000 years, (ii) achieve a better understanding of the aeolian activity based on the absolute dating of the aeolian sand material, and (iii) estimate the influence of these coastal changes on human settlement strategies over time.

67The reconstructed sequence of palaeogeographic changes highlights the high mobility of this part of the French Atlantic coastline during the last millennia. At the present-time, the dune barrier of the North Médoc is experiencing significant erosion. The shoreline retreat threatens many urbanized areas and requires consideration of appropriate spatial planning strategies. This context makes it essential to develop integrated approaches combining the monitoring of short-term coastal dynamics and a better knowledge of long-term changes in order to better understand the morphosedimentary processes at work and predict future changes of the coastline.

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Table des illustrations

Titre Fig. 1: Location maps of the study site.
Légende (A) Location of the North Médoc peninsula and the Gironde estuary along the Atlantic coast of France. (B) Topo-bathymetric digital elevation model of the mouth of the Gironde estuary showing the numerous sandbanks in the intertidal and subtidal areas. Black arrows indicate the main direction of the longshore drift and the value corresponds to the volume of sediment transfer. (C) “Anchored tidal ravinement process” and associated ribbon-shaped reservoir geometry (from Féniès et al., 2010).
URL http://journals.openedition.org/quaternaire/docannexe/image/11172/img-1.jpg
Fichier image/jpeg, 64k
Titre Fig. 2: Sequence of sedimentary fill of the Gironde estuary during the Holocene
Légende (A) Schematic illustration of the incised-valley of Gironde in the Early Holocene. (B) Schematic illustration of the initiation of post-glacial marine transgression into the valley. (C) Schematic illustration of the maximum marine flooding around 7000-6000 BP resulting in estuarine mud deposited in the funnel zone and landward migration of the tidal inlet. (D) Schematic illustration of the geological evolution of the Gironde estuary over the last 7000 years. A regressive tidal-estuarine bay-head delta has prograded into the upper estuary, gradually filling it with sediment. (E) Internal stratigraphy of the Gironde incised-valley fill.
Crédits modified from Lericolais et al. (2001), Féniès and Lericolais (2005) and Féniès et al. (2010)
URL http://journals.openedition.org/quaternaire/docannexe/image/11172/img-2.jpg
Fichier image/jpeg, 68k
Titre Fig. 3: Palaeogeographic reconstructions of the North Médoc peninsula from ca. 6000 to 1500 cal. a BP
Crédits from Pontee et al., 1998
URL http://journals.openedition.org/quaternaire/docannexe/image/11172/img-3.jpg
Fichier image/jpeg, 52k
Titre Fig. 4: Late Quaternary coastal formations of the North Médoc peninsula.
Légende (A) Simplified geological map. (B) Schematic geological west–east cross-section
Crédits from Pontee et al., 1998
URL http://journals.openedition.org/quaternaire/docannexe/image/11172/img-4.jpg
Fichier image/jpeg, 40k
Titre Fig. 5: Classifications of the north-Médoc coastal dune system.
Légende (A) Classification from Marionnaud (1972) and Dubreuilh & Marionnaud (1973). (B) Revised classification from Tastet (1998) and Tastet & Pontee (1998).
URL http://journals.openedition.org/quaternaire/docannexe/image/11172/img-5.jpg
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Titre Tab. 1: Details of TLS surveys made along L’Amélie beach.
URL http://journals.openedition.org/quaternaire/docannexe/image/11172/img-6.jpg
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Titre Fig. 6: Topographic surveys along the L’Amélie dune front.
Légende (A) DGPS measurements. (B) TLS measuremnts. (C, D, E) High density point clouds obtained from TLS survey (here in section 23). The elevations of each aeolian sand unit and interbedded palaeosols and/or humic layers were easily obtained. All studied sections were correlated to propose a continuous 2 km alongshore stratigraphic view.
URL http://journals.openedition.org/quaternaire/docannexe/image/11172/img-7.jpg
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Titre Tab. 2: Details of radiocarbon ages obtained from samples collected on the sediment outcrops along L’Amélie beach.
Légende Geographic coordinates of sampled points are in the French Lambert 93 coordinate system and elevations were attached to the French national datum NGF (Nivellement Général Français) corresponding to the mean sea level. Shaded rows correspond to marine shell samples whose radiocarbon ages were calibrated using the MARINE13 calibration curve with a ΔR of -10 and a standard deviation of 37 (see text for details).
URL http://journals.openedition.org/quaternaire/docannexe/image/11172/img-8.jpg
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Titre Fig. 7: Archaeological sites and sedimentary outcrops measured along L’Amélie beach.
Légende (A) Location map. (B) Vertical projection of the alongshore and elevational limits of the sedimentary units offering a stratigraphic view of deposits. The dune sections studied are also located along the dune front.
URL http://journals.openedition.org/quaternaire/docannexe/image/11172/img-9.jpg
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Titre Fig. 8: Photographs illustrating the sediment deposits outcropping along L’Amélie beach.
Légende (A) Green clay (unit 1) uncovered during the winter 2014 in the south of L’Amélie beach (21/03/2015, P. Stéphan). This unit is attributed to the “Argiles du Gurp” Pleistocene formation (Marionnaud & Dubreuilh, 1972; Beauval et al., 1998; Tastet, 1999). (B) Sharp contact between the green clay (unit 3a) and the organic-rich silty-clay (unit 3b) in the north of L’Amélie beach (archaeological site AML‑N-003, 22/03/14, O. Got). (C) Cemented aeolian sand (“alios” or “garluche”) cropping out in the north of L’Amélie beach (21/03/2015, P. Stéphan). (D) Oak post mechanically excavated at the lowest tide level of L’Amélie beach. The post is fixed into a blue silty clay (unit 3b) overlying a coarse sand deposit (unit 3a) (archaeological site AML-N-008, 23/03/2015, P. Stéphan). (E) Silty clay with high proportion of fragmented Scrobicularia shells observed at the low spring tide level in the central part of L’Amélie beach (archaeological site AML-N-008, 23/03/15, P. Stéphan). (F) Silty clay deposit (unit 4) cropping out at the mean tide level in the central part of L’Amélie beach (archaeological site AML-N-007, 18/06/2014, F. Verdin). The surface of the deposits exhibit several tidal channels filled with oyster shells. (G) Organic silty-clay (unit 5) uncovered by marine erosion at the high neap tide level (archaeological site AML-N-002, 17-22/03/2014, F. Verdin). (H) Organic-rich surface of unit 5 showing numerous animal footprints (archaeological site AML-N-002, 17‑22/03/2014, F. Verdin).
URL http://journals.openedition.org/quaternaire/docannexe/image/11172/img-10.jpg
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Titre Fig. 9: Photographs illustrating the high density of archaeological remains along L’Amélie and La Glaneuse beaches.
Légende (A) Neolithic pit being surveyed (archaeological site AML-N-001) (photo: F. Verdin, 23/03/2015). (B) General view of the pole structure observed at the lowest astronomical tide level (photo: F. Verdin, 23/03/15). (C) Gabion, pit and stakes (archaeological site AML-N-003) (photo: F. Verdin, 09/04/2012). (D) Gabion, pit and stakes being cleared (archaeological site AML-N-004 (photo: O. Got, 22/03/14). (E) Pot dating back to late antiquity discovered in unit U4 (archaeological site AML-N-006) (photo: F. Verdin, 18/06/2014). (F) Animal footprints visible at the top of unit U5 (archaeological site AML-N-002) (photo: Yoann Ceinturet, 08/04/2015).
URL http://journals.openedition.org/quaternaire/docannexe/image/11172/img-11.jpg
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Titre Fig. 10: Sedimentological analyses of stratigraphic units U1a to U5 obtained from sediment samples collected along the beach.
Légende (A to D) Grain size characteristics. (E and F) Microfaunal content. (G) Coastal environments associated.
URL http://journals.openedition.org/quaternaire/docannexe/image/11172/img-12.jpg
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Titre Fig. 11: Reconstructed dune stratigraphy along L’Amélie dune front.
Légende The positions of sediment samples collected for sedimentological analyses and radiocarbon dating are also shown. Results of radiocarbon datings correspond to the median probability of calibrated ages.
URL http://journals.openedition.org/quaternaire/docannexe/image/11172/img-13.jpg
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Titre Fig. 12: Photographs illustrating the aeolian sand deposits along the dune front of L’Amélie beach.
Légende (A) Section 18. (B) section 25. (C) section 14. (D) section 22. (E) section 9. (F) section 26.
URL http://journals.openedition.org/quaternaire/docannexe/image/11172/img-14.jpg
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Titre Tab. 3: Details of radiocarbon ages obtained from samples collected on the dune palaeosols of L’Amélie beach.
Crédits Geographic coordinates of sampled points are in the French coordinate system Lambert 93 and elevations are related to the French national datum NGF (Nivellement Général Français) corresponding to the mean sea level.
URL http://journals.openedition.org/quaternaire/docannexe/image/11172/img-15.jpg
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Titre Fig. 13: Grain-size characteristics of the main stratigraphic units studied along L’Amélie dune front.
URL http://journals.openedition.org/quaternaire/docannexe/image/11172/img-16.jpg
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Titre Tab. 4: Excerpt from the RSL database for the Atlantic coasts of Europe (García‑Artola et al., 2018) limited to sub-region 7 corresponding to the coast of Charentes and supplement by RSL data produced in this study.
URL http://journals.openedition.org/quaternaire/docannexe/image/11172/img-17.jpg
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Titre Fig. 15: Mid-Holocene sedimentary filling sequence of L’Amélie beach and interpretation of associated palaeogeographic changes from ca. 7000 to 3000 cal. a BP.
URL http://journals.openedition.org/quaternaire/docannexe/image/11172/img-18.jpg
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Titre Fig. 16: Mid to late Holocene sedimentary filling sequence of L’Amélie beach/dune system and interpretation of associated palaeogeographic changes from ca. 3000 cal. a BP to the present-day.
URL http://journals.openedition.org/quaternaire/docannexe/image/11172/img-19.jpg
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Titre Fig. 17: Synthesis of the data available along the Aquitaine coast related to the chronology of the coastal dunes.
Légende The middle gray bands correspond to the periods of stability of the coastal dunes defined from the radiocarbon dating of the peat layers. The data presented in this paper are compared with previously published data (Froidefond & Legigan, 1985; Tastet, 1998; Dubreuilh, 1971; Tastet & Pontee, 1998) which have been calibrated from the IntCal13 curve (Reimer et al. 2013) using Calib 7.0 software (Stuiver & Reimer, 1993). The lower gray bands correspond to the phases of wind activity and mobility of the sand bodies defined from the IRSL dating performed on the quartz grains (Clarke et al., 2002; Tastet, 1998; Clarke et al. 1999). The radiocarbon data relative to the formation of the “cordon de Richard” shelly ridge (Gironde estuary) are also indicated. These data were calibrated by the procedure described in section 3.3.
URL http://journals.openedition.org/quaternaire/docannexe/image/11172/img-20.jpg
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Titre Tab. 5: Chronological data available on the Holocene evolution of the Aquitaine coastal dunes.
URL http://journals.openedition.org/quaternaire/docannexe/image/11172/img-21.jpg
Fichier image/jpeg, 243k
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Pierre Stéphan, Florence Verdin, Gilles Arnaud‑Fassetta, Frédéric Bertrand, Frédérique Eynaud, Ane García‑Artola, Mathieu Bosq, Camille Culioli, Serge Suanez, Clément Coutelier, Pascal Bertran et Stéphane Costa, « Holocene coastal changes along the Gironde estuary (SW France): new insights from the North Médoc peninsula beach/dune system », Quaternaire, vol. 30/1 | 2019, 47-75.

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Pierre Stéphan, Florence Verdin, Gilles Arnaud‑Fassetta, Frédéric Bertrand, Frédérique Eynaud, Ane García‑Artola, Mathieu Bosq, Camille Culioli, Serge Suanez, Clément Coutelier, Pascal Bertran et Stéphane Costa, « Holocene coastal changes along the Gironde estuary (SW France): new insights from the North Médoc peninsula beach/dune system », Quaternaire [En ligne], vol. 30/1 | 2019, mis en ligne le 01 janvier 2021, consulté le 18 mai 2022. URL : http://journals.openedition.org/quaternaire/11172 ; DOI : https://doi.org/10.4000/quaternaire.11172

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Pierre Stéphan

 Laboratoire LETG (UMR 6554 CNRS), Université de Bretagne occidentale, Institut Universitaire Européen de la Mer, Place Nicolas Copernic, FR‑29280 PLOUZANÉ. Email: pierre.stephan@univ‑brest.fr ;

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Florence Verdin

 Laboratoire AUSONIUS (UMR 5607 CNRS), Université Bordeaux Montaigne, Maison de l'archéologie, 8, esplanade des Antilles, FR-33607 PESSAC. Email: florence.verdin@u‑bordeaux‑montaigne.fr

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Gilles Arnaud‑Fassetta

 Laboratoire PRODIG (UMR 8586 CNRS), Université Paris-Sorbonne, Institut de Géographie, 191 rue Saint-Jacques, FR-75005 PARIS. Email: gilles.arnaud-fassetta@univ-paris-diderot.fr;

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Frédéric Bertrand

 Laboratoire PRODIG (UMR 8586 CNRS), Université Paris-Sorbonne, Institut de Géographie, 191 rue Saint-Jacques, FR-75005 PARIS. Email: Frederic.Bertrand@paris-sorbonne.fr

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Frédérique Eynaud

 Laboratoire EPOC (UMR 5805 CNRS), Université de Bordeaux, Allée Geoffroy Saint Hilaire, CS 50023, 33615 FR-PESSAC, France. Email: frederique.eynaud@u-bordeaux.fr

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Ane García‑Artola

Departamento de Estratigrafía y Paleontología, Universidad del País Vasco UPV/EHU, ESP-BILBAO. Email: ane.garcia@ehu.eus

Mathieu Bosq

 Laboratoire PACEA (UMR 5199 CNRS), Université de Bordeaux, Allée Geoffroy Saint Hilaire, CS 50023, 33615 FR-PESSAC. Email: mathieu.bosq@gmail.com

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Camille Culioli

 Laboratoire PRODIG (UMR 8586 CNRS), Université Paris-Sorbonne, Institut de Géographie, 191 rue Saint-Jacques, FR-75005 PARIS. Email : culioli.camille2@gmail.com

Serge Suanez

Laboratoire LETG (UMR 6554 CNRS), Université de Bretagne occidentale, Institut Universitaire Européen de la Mer, Place Nicolas Copernic, FR-29280 PLOUZANÉ. Email : serge.suanez@univ‑brest.fr

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Clément Coutelier

 Laboratoire AUSONIUS (UMR 5607 CNRS), Université Bordeaux Montaigne, Maison de l'archéologie, 8, esplanade des Antilles, FR-33607 PESSAC. Email: clement.coutelier@u‑bordeaux‑montaigne.fr

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Pascal Bertran

Laboratoire PACEA (UMR 5199 CNRS), Université de Bordeaux, Allée Geoffroy Saint Hilaire, CS 50023, 33615 FR-PESSAC. Email: pascal.bertran@inrap.fr; INRAP, 140 avenue du Maréchal Leclerc, FR-33130 BÈGLES

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Stéphane Costa

Laboratoire LETG (UMR 6554 CNRS), Université de Bretagne occidentale, Institut Universitaire Européen de la Mer, Place Nicolas Copernic, FR-29280 PLOUZANÉ. Email: stephane.costa@unicaen.fr

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