Past Lakes and Future Climate – Sediment Cores reveal Permafrost Dynamics in Beringia

Permafrost landscapes are highly dynamic. Effects of thawing and freezing processes are not only directly observed in inhabited Arctic regions by destructed building and infrastructure but also play a key role in the global climate system. For example, carbon stored in frozen deposits becomes by thawing available for microbiological decomposition which contributes to the positive feedback mechanism of carbon dioxide release and warming. Another important contributor of greenhouse gas emissions is methane produced in anaerobic conditions of shallow lakes. In this study we present two case studies of paleo-archives from thaw lakes on northern Seward Peninsula (western Alaska/Beringia). Beringia is of particular interest as it formed the land bridge between Eurasia and North-America allowing the first humans to migrate to America. By applying radiocarbon dating and various sedimentological, biogeochemical and micropaleontological methods on permafrost sediment cores from drained lake basins, a complex landscape history was revealed. The first study of a 400 cm permafrost core archived more than 45,000 years of sediment deposition including mostly terrestrial phases, but also a wetland phase and tephra deposition as well as a final lacustrine phase. The lake forming the today’s basins persisted only for the last 300 years before it drained in spring 2005. An earlier wetland formed at about 41,500 to 44,500 years before present did not result in a lake formation due to the fact that a 1 m thick layer of tephra (presumably from the South Killeak Maar eruption) deposited on top of the wetland and served as an thermal isolator of the underlying permafrost. The second study preserved 350 cm of lacustrine sediment except for the surficial terrestrial peat which formed after the lake drained around 1,060 years ago. The sediment core archives several generations of thaw lakes going back to 12,700 years before present. Here, a deep first generation lake drained partially about 9,500 years ago but persisted until 1,060 years ago as a shallow second generation lake. The refreezing of lake sediment after drainage is captured by the isotopic composition of intrasedimental ice. δ18O and δD clearly indicate the modern but also paleo-active layer after lake drainage as well as an open talik system due to the presence of a modern thaw lake next to the coring site. Both case studies demonstrate the interaction of global climate change, regional environmental dynamics and local disturbance processes. The complex nature of permafrost landscapes which are at the same time sensitive to rapid climate change is emphasized by paleoenvironmental investigations as presented here