Holocene evolution of a montane lake catchment inferred from multiproxy sediment analysis: climatic and anthropic impacts in french prealps

International audienceake La Thuile in the Massif des Bauges (874 m a.s.l. French Alps) provides a 18 meters sedimentary sequence.Due to its mid-altitude position, this lake is one of the first to be formed through the glacial retreat and documentsthe evolution of its catchment since the Late Glacial Maximum. The first 6 meters of the core cover the last 12000 years, and allowed to study human/climate/environment interactions in a carbonated environment. This studyis the first one to investigate a mid-altitude lake in the French Alps for paleoenvironment reconstruction fromlake sediment archive. Its altitudinal position presents the advantage to be very accessible to human activitiesand allows more developed agriculture than in higher altitude. This study aims to determined how and when isexpressed the erosive response of such an environment to human settlement.High resolution multiproxy analysis of the first 6 meters including sedimentological, palynological andgeochemical data associated to a well-constrained chronology over the Holocene period allows us to understandthe respective impacts of both climate and human on the evolution of Lake La Thuile environment.Five major phases of evolution have been highlighted over this period. From 12 000 to 10 000 yr cal. BP,the vegetation is developing with the onset of hardwood species and the disappearance of Pinus. From 10 000to 4500 yr cal. BP the warmer climatic conditions of the middle of the Holocene allows the forest to densifyand the very low sedimentation rate indicates that the forest stabilizes slopes and prevents from the erosionon the watershed. The climate cooling of the Neoglacial period triggers a first erosive phase with a decreasingof the forest around 3300 cal. BP. Human settlements are suggested at La Thuile from 2500 yr cal. BP bypalynological evidence of anthropic taxa. The triggered clearing is accompanied by a second erosive phase relatedto anthropic activities during the Roman period. Erosion intensified from 1600 cal. BP in a third erosive phase cor-responding to agriculture intensification during the Middle Ages. At the end of the Middle Ages, human footprintseems to decrease but the reason of this change is not resolved between social determination or/and climate forcing.These three erosive phases may have completely changed the catchment behavior until the agriculturalabandonment in the middle of the 20th century. Lake La Thuile sedimentary infill shows a landscape evolutionfirst controlled by climate and progressively, since 2500 years ago, human activities came to superimpose. Therole of human seems to be more important with abrupt and maybe irreversible modifications of the landscap