Late- and Post-Glacial evolution of Lake Steisslingen (I).: Sedimentary history, palynological record and inorganic geochemical indicators

Piston cores from Lake Steisslingen, a small hard water lake situated 20 km west of Lake Constance and mainly supplied by submerged springs, reveal a record of the environmental history going back to 15 600 cal yr BP. From the Bølling to ca. 9800 cal yr BP the sediments are partly seasonally laminated. From then on up to the Middle Ages the lamination is mostly undisturbed providing a high resolution time scale by varve counting, a detailed pollen profile and 13 AMS 14C-analyses. All investigated sediment sections show evidence of exceptionally high sulphate concentrations in the lake water, which lead to the fixation of all reactive iron as pyrite. The high sulphur content can only be attributed to S-rich water supplied by the submerged springs, thus indicating the Miocene ‘Obere Meeres Molasse’ as the dominant aquifer. Since soil erosion is strongly reduced due to increased forest density in the Preboreal/Boreal, the submerged springs are the main nutrient source for the lake. Mixing or stable stratification of the waterbody mostly depends on nutrient availability and bioproductivity in the epilimnion. The controlling external factors are wind, and changes in water level and/or temperature. At the end of the Younger Dryas and during the Preboreal low water levels and precipitation rates are assumed. The water column mixing was regular, bioproductivity high, and high degradation activity of organic matter led to a redissolution of calcite. The hydrological separation of a northern shallow lake (Seeried) probably fell into the period of this low lake level. Around 10 450–10 300 cal yr BP the lake level rose, but never again reached the former level. Between 10 000 and 7200 cal yr BP, non-glacial varves are even found in shallower parts of the lake, indicating a stable anoxic monimolimnion with a rising chemocline. Bioproductivity was low. Wind protection provided by the now dense forestation and favourable climatic conditions (e.g. high summer temperatures and low winter temperatures) seem to be the responsible factors. Fluctuating human impact is identified from about 7400 cal yr BP until permanent settlement by the Alemanni about 1500 cal yr BP. Strong human influence is observed from 2000 cal yr BP onwards, when agricultural activity and clearing of the shoreline vegetation by man caused high soil erosion rates and thus eutrophication. At about 800 cal yr BP the recent hydrological conditions were established by implementing an artificial drain, thus lowering the lake level.