Holocene palaeoenvironmental evolution of the Médoc peninsula (SW France): insights from the sedimentological study of the "Lède du Gurp" archaeological site

High coastal erosion rates along the beaches of the Medoc peninsula urge for the study of the Pleistocene and Holocene deposits buried under the modern dune. Those deposits are known to be exceptionally rich in archaeological remains (from the Mesolithic at least). The aggressive predation of these fossil deposits by the sea explains the urgency of exploring their content and their sedimentological history. Within these formations, the "Lede du Gurp" archaeological site located on the shore, on the administrative boundary of Soulac-sur-Mer and Grayan-et-l'Hopital, is of tremendous value for palaeogeographical and palaeoenvironmental reconstructions of the Medoc peninsula. This site has preserved remains of dense anthropic activity but also evidence of the history of the estuary's migration before our contemporary era (i.e. recent centuries). Here we present results of new sedimentological investigations conducted as part of the LITAQ project on and around the site. Continuous and undisturbed sections several metres thick (up to 3.5 m) have been obtained by sampling (sediment slides) and coring operations in the field and studied for their lithostratigraphy (using classic and X-ray imaging coupled with spectrocolorimetry, X-ray fluorescence and grain-size analyses together with radiocarbon dating) along with simultaneous archaeological investigations. Our study refines the palaeoenvironmental evolution of the site over at least the Neolithic period and supports previous interpretations of the origin of these atypical lithofacies, i.e. the deposition of a thick Holocene sequence of clays and peats in a depression associated with karstification processes. Our new datings and observations suggest the possible occurrence of a thermokarst system during the coldest phases of the last deglaciation.