Varve formation and the climatic record in an Alpine proglacial lake: calibrating annually- laminated sediments against hydrological and meteorological data

Processes leading to the formation of varves in proglacial Oeschinensee (Switzerland) are investigated. The aim is to calibrate annually-laminated sediments of the last three decades against hydrological and meteorological instrumental data. Results from water samples and sediment traps show that the suspended sediment in the lake is mainly distributed by underflows during the time of the largest sediment input from May to mid-August. This leads to the deposition of the basal silt layers of the varves. Settling of fine-grained suspension from interflows dominates sedimentation between August and October, forming the final layer of fine-grained material at the end of the annual deposition cycle. The median grain-size at the top of the varves is always below 5 μm and thus defines the varve boundaries. Varve thickness is a function of the annual discharge of suspended load from the catchment. This is supported by evidence from hydrological and meteorological data compiled for a glaciated catchment (Loetschental) adjacent to Oeschinensee. Suspended sediment discharge is related to runoff which in turn is correlated with summer air temperature (correlation coefficients 0.66 and 0.70 respectively). Between AD 1962 and 1982, annual thicknesses of the Oeschinensee varve record showed a positive correlation with the average summer air temperature of the region (correlation coefficient 0.44).