Mountain Areas facing Global Change: a retrospective study in the vicinity of 'Les 2 Alpes' ski resort

This doctoral thesis aims at a paleo-environmental reconstitution of the Oisans valley based on the natural archive of lake sediments. From a multi-proxy approach, we aim at reconstructing: i) Glacial fluctuation in Oisans valley; ii) human activities evolution in mountain areas and their influences on sediment fluxes especially during extreme events; iii) understanding the relationship between climate change, use of mountain lands and natural hazards. Through this study, we identified processes of global change, comprising natural climate variability and human practices, which affected erosion patterns around Lakes Muzelle and Lauvitel during the last millennia. Main results have shown that erosion in the lake Muzelle watershed was dominated in the past by glacial activity, in particular with the presence of fine detrital particles related to sub-glacial abrasion. They were identified to have a relationship never observed before; during glacial extension their presence is triggering a higher number of flood deposits. At this location, human activities were identified through agro-pastoral activities and more precisely by the coprophilous fungi spore counting, revealing cattle presence since 300 years. Lake Lauvitel is located at lower altitude, at which most of the watershed is now situated in an Integral Reserve. Palynological investigations lead to identification of periods of higher human activities in the valley. Extreme events recorded in Lake Lauvitel sediment are on the one hand torrential floods, which allowed a comparison with other reconstructions highlighting changes in the climatic settings in the Alps. On the other hand, wet avalanches deposits were identified with the use of a CT scan and the development of a new methodology based on relative density differences in the sediment. The counting and the quantification of coarse detrital matter within the lacustrine fine sediment matrix allowed income identification of this hazard though time. We then reconstructed wet avalanches events deposited in Lake Lauvitel during the last 3500 years, the occurrence of which is preferentially during glacial retreats. (author)