Holocene glacial activity and climatic variations in the Swiss Alps: reconstructing a continuous record from proglacial lake sediments

Cores recovered from proglacial Lake Silvaplana (Swiss Alps) were investigated to examine the influence of Holocene glaciers on lacustrine sedimentation. During times of glacial activity in the catchment the sedimentation in the lake is dominated by the input of silt and clay-sized material delivered by glacial meltwaters. The deposition of fine-grained material and the formation of varves reflect the existence of glaciers in the catchment and can therefore be used for the reconstruction of glacial history. Sedimentation rates are up to nine times higher in the varved than in the non-varved sediments, indicating significantly higher erosion rates under glaciated catchment conditions. Glacial varves were deposited in Lake Silvaplana during glacial retreat in the latest Pleistocene and early Holocene until 9400BP. Non-varved deposits having a higher organic content and coarser grain- sizes accumulated at lower rates when glacial activity was absent or negligible within the catchment between 9400 and 3300 BP. The continuous deposition of varves after 3300 BP indicates that Neoglacia tion provided glacial cover for part of the catchment since then. Long-term variation in varve thickness is interpreted as being controlled by the size of the glaciers in the catchment. Maximal varve thicknesses are observed from AD 1790 to 1870. Since the changes in glacial activity for this area seem to be mainly controlled by temperature rather than precipitation, we suggest that the entire lacustrine sediment record of Lake Silvaplana reflects relative changes of summer air temperature during the Holocene.