Rapid climatic events as recorded in Middle Weichselian thermokarst lake sediments

From a Middle Weichselian sediment sequence in the opencast brown coal mine of Reichwalde (eastern Germany), a similar to 40cm thick thermokarst gyttja deposit has been sampled. The AMS C-14 dates, although at the limit of detection, indicate an early Middle Weichselian age of the gyttja. Pollen, botanical, and zoological (e.g. chironomids) macroremains have been analyzed. Botanical and chironomid taxa indicate warm climatic conditions in the bottom part of the sequence. For this lower part the botanical data suggest a minimum mean July temperature of 12-14 degrees C. Following this, a cooling is indicated, coinciding with an increased clastic deposition in the lake. A return to permafrost conditions is reconstructed for the upper part of the sequence. The combined evidence strongly suggests a degradation of permafrost due to increased warming in response to a D/O event as a forcing factor for the thermokarst lake formation.