A knowledge of past climatic and environmental changes is of great interest because it allows us to view the recent, probably anomalous, climate change within a broader context, which should improve our ability to predict future climate. Existing climate reconstructions agree on major climatic episodes (e.g. the Little Ice Age and the Medieval Warm Period), but differ substantially with respect to the reconstructed absolute temperature amplitudes. New climate proxies and innovative reconstruction techniques are therefore in great demand.
Sediments from high-altitude lakes are known to respond especially sensitively to climatic and environmental changes. Lake Sils and Lake Silvaplana are located at an altitude of about 1800 m a.s.l. in the Upper Engadine region of south-eastern Switzerland. A detailed seismic survey of these lakes, based on a total of 32 km of seismic proiles, was used to determine optimum coring positions, and to study sediment volumes and distribution. In Lake Sils, 22 short cores were taken, and in Lake Silvaplana, a total of 16 short cores and two freeze cores.
Due to the absence of varves, the chronology of the 1300-year-long recovered sediment record of Lake Sils was determined using ¹³⁷Cs, ²¹⁰Pb, and ¹⁴C AMS dating. The average sedimentation rate in Lake Sils (40 mg cm^-2yr^-1) is only 20% of that in Lake Silvaplana (200 mg cm^-2yr^-1). This is because its surface area is 40% greater than that of Lake Silvaplana, while its catchment area is 60% smaller and its glaciated area 40% smaller. The impact of the late Little Ice Age (A.D. 1550 - 1850) on the sediments of Lake Sils is clearly apparent, with anomalously low concentrations of organic carbon and biogenic silica, but anomalously high values of median grain-size and mica content. In contrast, the period A.D. 800 - 1550 (partly coinciding with the Medieval Warm Period, A.D. 800 - 1300) is characterized by poorly sorted silici-clastics, high C/N ratios and high biogenic silica values, indicating major glacier recessions. The very recent sediments of both Lake Sils and Lake Silvaplana are signiicantly inluenced by waste water from the adjacent villages. Several steep, stepwise increases in organic carbon and biogenic silica concentrations have occurred since approximately A.D. 1900 in Lake Sils and A.D. 1960 in Lake Silvaplana. [...]