Degradation of Lena Delta Ice Complex by thermokarst and thermal erosion

During the past decade, we have studied thermokarst and thermo-erosional processes and landforms in Ice Complex deposits of the Lena Delta. Ice Complex deposits are very ice-rich permafrost up to tens of meters thick. They are widespread in the Arctic and have attracted raising attention due to their vulnerability to thaw under climatic warming. In the Lena Delta they occur on the third geomorphological main terrace, which is distributed as several islands in the southern delta. Degradation processes throughout the Holocene have affected these late Pleistocene deposits. Rapid permafrost thaw underneath ponding water (thermokarst) has created thermokarst lakes; when these lakes drain, thermokarst basins remain that can be several kilometers wide and up to twenty meters deep. Rapid permafrost thaw by running water (thermal erosion) has created gullies, valleys and valley networks that are also deeply incised into the terrain surface. All these landforms and associated processes play an important role for the landscape’s hydrology, energy budget and carbon cycle. Our investigations have aimed at 1) understanding the evolution of thermokarst and thermal erosion in the Lena Delta Ice Complex throughout the Holocene, 2) quantifying current terrain changes (permafrost degradation and aggradation) as well as the contribution of different landform types to organic matter export from the Ice Complex to the Lena River, and 3) deducing potential future thermokarst and thermo-erosional activity. Our methodological approach has been a combination of geomorphological analyses in the field and based on satellite imagery and digital elevation models (DEM), sediment and water sampling. As main results we would like to highlight the following: - Thermokarst development started during the transition from Pleistocene to Holocene and has evolved throughout the Holocene. - Only a minor part of the third terrace in the Lena Delta provides the conditions for the future formation of thermokarst and thermo-erosional landforms, because a large area has already been degraded by thermokarst and thermal erosion during the Holocene. - Newly developing thermokarst landforms will not be able to grow to such large sizes as the existing Holocene thermokarst landforms. - The existing landforms vary much in their activity: some thermokarst lakes and thermo-erosional valleys are expanding and actively erode the Ice Complex deposits, many have been stable over the last decades and some lakes have shrunk or drained thereby giving way for permafrost to reestablish. - This variation of erosion activity is also reflected in different hydrogeochemical compositions of the waters in thermokarst lakes and streams in thermo-erosional valleys: water samples from eroding sites have higher concentrations of dissolved organic carbon (DOC) than stable sites, while thermokarst lakes in the drainage pathway act as DOC-reducing landscape components along the transport route. Acknowledgments. We wish to thank all colleagues who supported our field work in the Lena Delta during the past ten years and helped with sample processing in the lab. A. Morgenstern was supported by the German Academic Scholarship Foundation and by the Helmholtz Association (grant PD-003).