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

Abstract

Located at the mouth of the Gironde estuary (southwestern Atlantic coast of France), the Amelie beach was strongly eroded during the recent stormy events, especially in the winter of 2013-2014. The escarpment at the dune front and the truncation of the beach uncovered a set of estuarine and aeolian sediment deposits containing numerous archaeological remains. A campaign of topographic surveys was undertaken with DGPS and TLS materials during the springs of 2014 and 2015 along the 2 km length of the Amelie beach in order to reconstruct the elevation and stratigraphy of these deposits. Sedimentological analyses (grain size, micro-faunal) were performed to better constrain the depositional environment. Archaeological remains and several radiocarbon dates obtained from sediment samples collected in the field were used to propose a consistent chronological framework for this sedimentary sequence. Ten main lithofacies were distinguished from the lower part of the beach to the top of the dune. The base of the sedimentary sequence is composed to of Pleistocene deposits dating from MIS9 (unit 1a) to MIS2 (Unit 1b) (Bosq et al., this issue). The Holocene sedimentary infilling began around 5000 BC with an accumulation of marine coarse sands, which suggests the presence of a tidal inlet. On both sides of this tidal channel, archaeological remains dating from the early Neolithic period up to the Bronze Age testify to a human occupation of the coastal area. From ca. 5000 to ca. 3500 cal. BP, a characteristic deposit of intertidal mudflats indicates the transformation of the tidal inlet into estuarine marshes protected by a coastal dune barrier. Between ca. 3500 and ca. 3000 cal. BP, the dune barrier records a phase of erosion which results in the exposure of the marsh to stronger hydrodynamic conditions. From ca. 2800 to 1650, a second phase of estuarine sedimentation is recognized. This period is characterized by significant human occupation as indicated by the presence of numerous archaeological remains of the Iron Age and Gallo-roman periods, associated with the exploitation of a saltwater-to-brackish environment. From 1650 to 1250 cal. BP, the estuarine saltmarsh is gradually overrun by northward migrating coastal-dune fields. After 1250 cal. BP, the marsh is completely covered by the aeolian sands. These results are consistent with previously published data and allow to locally details the sequence of paleogeographic changes of the north-Medoc peninsula. Using the "sea-level index points" method (Hijma et al., 2015), the analysis of radiocarbon dated levels also provides new relative sea-level records for this part of the French Atlantic coast. Finally, the phases of coastal dune activity/stability recognized along the Amelie beach are synchronous with those in the wider Aquitaine coastal region, suggesting a regional driving factor. We propose that the shared sedimentary dynamics are the consequence of the onshore migration of intertidal and subtidal sand banks distributed at the mouth of the Gironde.