First evidence of “in situ” Eemian sediments on the high plateau of Evian (Western Alps, France) : implications for the chronology of the Last Glaciation

International audienceA 2 m profile of organic sediments, interbedded between two tills, was taken from the Evian high plateau at Hameau de Maravant, 860 m a.s.l. (Haute-Savoie, Western Alps, France). Pollen, plant macrofossils and insect remains were used to reconstruct the local palaeoenvironment, which consisted of a moss and Carex mire, surrounded by a dense Abies and Picea mixed forest. Pollen and macrofossil data indicate that this peat layer was deposited at the end of the Eemian interglacial. The lower till should therefore be assigned to the penultimate glaciation and the upper till to the maximum extent of the Rhône glacier during the Würm. A comparison of the results with previous studies carried out in the Geneva area and other alpine valleys has yielded a detailed reconstruction of the past fluctuations of the Rhône glacier: 1. a first Early/Lower Würmian advance, reaching the region around Lyon; 2. a subsequent retreat of ice; 3. a second moderate readvance, restricted to the surroundings of Geneva, followed by a final deglaciation. A continuous lacustrine sequence has been obtained from the nearby Roseire peat-bog (890 m a.s.l). Pollen and radiometric results show the establishment of a former palaeolake during the Upper Pleniglacial, probably before ca. 22 450±850 cal years BP, thus implying that the high plateau was ice-free from that time. These new data appear to suggest that the maximal extension of the Rhône glacier occurred prior to the Last Glacial Maximum (OIS 2) and, according to a well-documented geomorphological context, probably during the Lower Würmian Pleniglacial