14500 yr History of Changes in Sediment Geochemistry - The Impact of Climate and Human Activities on Trace Elements and SR Isotopes in Lake Sediments

Erwin Schrösdinger Fellowship J 1963 Long term changes of geochemistry in lake sediments Karin Anne KOINIG 09.10.2000 In order to reconstruct the impact of changing climate and human activities on lakes, geochemistry will be studied in two sediment cores: 1) In a core of a low land lake (Soppensee, 14500 years old) that covers the climate oscillations at the end of the Late Glacial, the rapid warming after the Younger Dryas, and the changes over the Holocene. 2) In a core from an alpine lake (Sägistalsee, 8800 years old) that has been studied for trace metal changes during the first project year. In this lake, the gradual depletion of Ca and Sr indicated a change form less chemical to more physical weathering. I want to distinguish natural from anthropogenic changes in sediment geochemistry: Natural variability will be studied in the older core section: The steep climate gradients will largely affect bulk geochemistry and weathering rates. The gradual development of a closed vegetation cover and the consequent soil stabilisation will result in a depletion of erosion. In the younger core section, changes related to human activities will be studied: Human impact started as early as 6300 (low land lake) or 4500 (alpine lake) years ago when pollen profiles indicated deforestation and the onset of agriculture in the catchment. As a result, soil became less stable and erosion increased again which will affect lake geochemistry. In addition, I will analyse 87Sr/86Sr in the sediment sections in which geochemistry indicates a change in dust deposition (e.g. more heavy minerals) or a change in weathering (shift in the proportion of silicate to carbonate fraction). Deviations in the Sr isotope ratio will indicate, changes in the 1) the proportion of physical to chemical weathering, and 2) in the provenance and amount of dust deposited onto the lake.