"The Alps with little ice" : new evidences from a distant lake sediment record : Lake Bourget (NW French Alps)

International audienceIt is a common practice to reconstruct glacier activity from lake sediment records. Such an approach is generally applied to proglacial lakes. However in some cases, terrigenous inputs from glaciated areas are dominating enough to make distant sedimentary basins sensitive to glacier fluctuations. This is the case in Lake Bourget, located on the course of the French higher Rhône, 250km downstream the Mont Blanc Massif. Two Holocene-long sediment cores were studied, in order to reconstruct past terrigenous fluxes. We coupled a high resolution geochemical approach to a source-to-sink monitoring (isotope geochemistry and clay mineralogy) of sediment provenance in order to prove the main influence of glacier fluctuations upon terrigenous input. The obtained reconstructions are in agreement with recent discoveries from Swiss Alps arguing for a highly retreated position of glaciers during most of the Holocene period. However, an increase in sediment flux, which we attribute to more erosive conditions in the Mont Blanc massif, occurred since 2.7 ka cal. BP. Since then, glaciers seem to have been most of the time, more advanced than they are today. The precession orbital forcing appears to have been the main factor of glacier fluctuations at a pluri-millenial time scale. As a consequence, nowadays glaciers "should not" be as retreated as they are. This last remark contradicts a common misinterpretation of Holocene glacier fluctuations by climato-skeptics: even if Alpine glaciers have been more retreated in the early to mid-Holocene, accordingly to known climate forcing, they "should not" be as retreated as they are today. Our method is far to be able to accurately reconstruct past glacier fluctuations, but presents the interest to be continuous and to have preserved a pluri-millenial trend. To go a step further, ongoing studies aim at reconstructing terrigenous inputs in a proglacial lake close to the Mont Blanc Massif