Taux d'accumulation de sédiments récents et bilan de la matière particulaire dans le Léman (Suisse - France)

Dated lake sediments are excellent archives of continental paleoenvironments. Thus, a good chronology of the sediments is essential to reconstruct the evolution of lakes and their catchments. In the last 40 years two radioisotopic methods (137Cs and 210Pb) have been frequently used to date sediments for the last 50 to 100 years. A review of the literature revealed the existence of 76 dated sediment cores that have been collected in Lake Geneva between 1970 and 2010. A compilation of these data allow us to map the sediment accumulation rates (SAR) in Lake Geneva and to estimate a yearly deposition of sediment of ~1.7 x106 metric tons. The total sediment accumulation rate is strongly influenced by the upper Rhône River particulate load, decreasing logarithmically from the river mouth to the central plain, and remaining constant (~0.1 g cm-2 y-1) in the other portion of the lake. The comparison of sediment accumulation rates estimated from the two 137Cs peaks for the periods 1964-1986 and post-1986, as well as those obtained from 210Pb profiles does not show any changes for the last decades. This is despite major changes occurring in the lake and its catchment (dam construction, reforestation, retreat of glaciers). This relative sediment accumulation stability can result from opposed processes in the watershed, some promoting and others reducing sediment availability and transport. Although the lake sediment budget is balanced, several uncertainties remain due to i) inaccurate evaluation of the Rhone sediment load ; and ii) insufficient number of measurements of sediment accumulation rates in some delta areas (Rhône, Dranse). The detrital inputs account for 90% of the sediment load to Lake Geneva