Isotopengeochemische Studien zur klimatischen Ausprägung des Jüngeren Dryas in terrestrischen Archiven Eurasiens

In this thesis, the late glacial climate and environment development with special focus on the Younger Dryas chronozone was studied for three different regions in Eurasia. For this purpose, lacustrine sedimentary archives of Lake Meerfelder Maar (Westeifel, Germany), Lake Sacrow (Brandenburg, Germany) and Lake Sihailongwan (NE-China) were investigated by means of isotopic and geochemical methods. Stable isotope ratios of organic carbon, nitrogen and oxygen as well as element contents of organic carbon and nitrogen were measured with high time-resolution. Oxygen isotope ratios of diatomaceous opal (Lake Meerfelder Maar, Lake Sihailongwan) and autochthonous inorganic carbonates (Lake Sacrow) were determined. Separation of diatom valves from bulk sediments was achieved by the method of SPLITT-fractionation for Lake Meerfelder Maar and a newly optimised heavy liquid density centrifugation for Lake Sihailongwan. Determination of oxygen isotope ratios followed the iHTR method and isotope ratio mass spectrometry (IRMS). The late glacial chronozones Bølling / Allerød, Younger Dryas and Preboreal are documented in the sediment profiles of all three investigation sites which basically confirms the hypothesis of a hemispheric character of these climate stages. The Younger Dryas is characterised as a time period of distinct cooling in all three archives. Moreover, an increased allochthonous influx into the sediments is observed for all archives during that time. This is due to a less dense vegetation cover and partly frozen soils during very cold winters. A shortening of the vegetation period associated with an intraannual concentration of lacustrine primary production is inferred from the alignment of $\delta^{18}$O$_{SiO_{2}}$-values of different diatom size fractions from Lake Meerfelder Maar. Oxygen isotope ratios were particularly useful for the reconstruction of climate since they depend directly on climatic driving factors. However, the direct dependence of $\delta^{18}$O-values on lake water temperature during formation of diatoms and carbonates was superimposed by larger variations in the $\delta^{18}$O$_{H_{2}O}$-source value of lake water. Temperature reconstructions based on oxygen isotope variations suggest a decrease of weighted mean annual temperatures in Central Europe in the range of approx. 4.5 – 6.5 °C during the Younger Dryas. Since precipitation most likely occurred mainly during spring and summer, the weighted signal is [...