Die Sedimente des Steißlinger Sees (Hegau, Süddeutschland)
- Ein Archiv für zeitlich hochaufgelöste geochemische Untersuchungen zu Umweltveränderungen im Holozän.
The sediments of Lake Steisslingen (Southern Germany)
- A high resolution geochemical record of Holocene environmental change.
by Karin Eusterhues
Date of Examination:2000-04-20
Date of issue:2000-08-21
Advisor:Prof. Dr. Jürgen Schneider
Referee:Prof. Dr. Jürgen Schneider
Referee:Prof. Dr. Hans Ruppert
Persistent Address: http://hdl.handle.net/11858/00-1735-0000-0006-B353-3
Files in this item
EnglishLake Steisslingen is a small hardwater lake 20 kms west of Lake Constance, mostly fed by submerged springs. Sediment cores from the deepest part of the lake cover the complete Late-and Postglacial history and are dominated by annually laminated sediments. 480 samples from three core sections (12100- 9745, 8200-6200 and 2650-1440 cal a BP) have been analysed by high-resolution sampling (0.5 to 10 years per sample in the two younger intervals) for ICP-OES and ICP-MS major and trace elements, as well as total carbon, inorganic carbon and sulfur. Stratigraphy and absolute ages are derived from pollen analyses (Lechterbeck), varve counting (Wolf-Brozio) and (super14)C AMS dating using terrestrial organic detritus. Geochemical signals refer to specific lake sedimentation as well as to palaeo-redox conditions, bioproductivity and detrital input: In all the time slices investigated there is evidence for exceptionally high sulfate concentrations of the lake water, which can only be caused by S-rich water supplied by the submerged springs. The high S-concentrations in turn cause the fixation of all available Fe as FeS(sub2) instead of as FeS, Fe-carbonates or Fe-phosphates. Pyritization is limited by Fe-concentration. Pronounced anoxic conditions in the hypolimnion are not due to high bioproductivity (eutrophication); rather, they are accompanied by phases of low biological production. Thus, it must be external factors such as storm activity, wind protection, temperatures, and ice cover that control stable stratification of the water column. Since the springs probably constitute the most important nutrient carriers to the lake, nutrient transport into the biologically productive epilimnion depends on stratification or mixing of the waterbody. During Younger Dryas and Preboreal low lake levels and low precipitation rates are assumed. The water column was mixing regularly, bioproductivity was high, and high degradation of the organic matter led to redissolution of calcites. This resulted in the formation of a TOC-rich Mudde. The hydrological separation of the lake and the northern swamp (Seeried) probably took place in the course of this lake level lowering. Around 10450-10300 cal a BP the lake level rose again. Between 10000 and 7200 cal a BP, non-glacial varves are found even in shallower parts of the lake, and redox-sensitive elements like Mo, U, Fe, Mn and S point to a stable anoxic hypolimnion. 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 responsible factors. Since about 8000 cal a BP the calcites are characterized by lower concentrations of Ba and Sr. An abrupt change in delta(super13)C and delta(super18)O (Mayer & Schwark 1999), occurring roughly at the same time, suggests a change in water chemistry caused by a more pronounced spring activity. From 6300 to 1500 cal a BP reddish-brown sections alternate with light grey-green sections in periods of about 20-300 years. The distribution of Mo, U, Fe, Mn and S gives rise to the assumption that these changes reflect redox conditions. Cyclicity of the red-green changes and the lack of correlation with cultural phases or detrital input indicate a natural control. Variations in the North Atlantic Circulation could provide a possible explanation. Extensive forest clearings in the drainage area are marked by detrital input and associated eutrophication. Since 2000 cal a BP additional input of soil material can be traced, pointing to changing conditions of human impact and anthropogenic changes to the hydrological system (swamp drainage, clearing of shoreline vegetation). From then on high accumulation rates of detrital material, organic matter and carbonates demonstrate high pressure on natural ressources through clearings and agriculture.
Keywords: Europe; Southern-Germany; lake-sediments; geochemistry; trace-elements; major-elements; paleoenvironment; paleoclimate; Holocene; carbonates; lacustrine-environment; redox; productivity; varves; human-impact; Steisslingen; Hegau