Eemian palaeoenvironmental evolution in the northern plains of France from the alluvial sequence of Waziers

International audienceIn northern Europe, only few Eemian sedimentary sequences showing the Neanderthal's presence are attested. Therefore, the discovery at Waziers, in the Scarpe River valley, of a lot of flint artefacts within an alluvial sequence is remarkable.The thick alluvial sequence presents from the base to the top: 1) coarse fluvial deposits (lithological unit III); 2) organic silts (lithological unit IV); 3) peaty sediment (lithological unit V) with two archaeological levels (flint artefacts); 4) sand and loess sequence (lithological units VI and VII). The organic silts and peat units IV and V are correlated to the Eemian on the basis of a U/Th date on oogons of Characeae (minimum age: 103 +3.5/-3.4 ka), and the occurrence of the typical Last Glacial (Weichselian) loess sequence overlying fluvial organic deposits.The aim of the poster is to present the main results from the palynological analysis. The pollen record, from a 250 cm sedimentary sequence covering organic silts and peat units, documents palaeoecological changes in northern France from the MIS 6/5 transition to the end of the Eemian. Based on pollen data, three palaeoecological phases have been identified. The first phase corresponds to an open dry grassland driven by a cold climate. The second phase starts with an important expansion of trees indicative of a Betula-Pinus boreal forest development. The third phase is characterized by a warm temperate forest with the successive development of Ulmus, deciduous Quercus, Corylus and finally Carpinus betulus. Palynological data also show changes in the local vegetation, such as aquatic and wetland communities. A nutrient-poor oligotrophic environment with a slow water flow allows aquatic plants to develop. Following the water flow stop, the peat bog is set up then evolves towards an acid peat bog (rise of Sphagnum). Finally, the end of the pollen record shows the colonization by a dominant Alnus swamp forest