Soil organic carbon and nitrogen pools in thermokarst-affected permafrost terrain

Permafrost soils are widespread in the Northern Hemisphere and contain high amounts of carbon (C) and nitrogen (N). Due to the changing climate, permafrost thaws and makes previously frozen C and N available for the active C and N cycles. While additional N might increase plant growth in the nutrient limited tundra, additional C will be decomposed by microbial activity and released through respiration as CO2 or CH4 into the atmosphere. This, in turn, increases the amount of C in the atmosphere, which triggers climate warming and is thus considered a positive feedback loop to climate warming termed the “permafrost-carbon-climate feedback”. \nThe focus of this dissertation is on deep, ice-rich permafrost deposits, which are prone to rapid degradation through thermokarst processes. Thermokarst involves the thawing of ice-rich permafrost and leads to surface subsidence, lake formation but also lake drainage. Therefore, once thaw is initiated, irreversible changes occur on the landscape and affect C and N that have been freeze-locked for millennia. However, thermokarst processes in permafrost environments are not yet implemented in global permafrost-carbon and climate models, even though they have the potential to degrade and change the surface of a landscape and to mobilize C and N within short time scales. \nIn this dissertation, I investigated different, rapidly changing permafrost environments in terms of their C and N contents in the top two meters of soil. This includes the yedoma-dominated study sites on Sobo-Sise Island and Bykovsky Peninsula in the north of east Siberia, the thermokarst lake landscape north of Teshekpuk Lake on the Arctic Coastal Plain of northern Alaska, and five different Arctic river deltas in the north and west of Alaska. For this analysis, a total of 77 permafrost soil cores were collected and more than 1,000 samples analyzed for their C and N contents. With a combination of field, laboratory, and remote sensing methods, landscape soil organic C and soil N was characterized. A coherent, continuous sampling collection and analyzing scheme was applied to compare the different study regions and to investigate the overarching research question “How much organic carbon and nitrogen is stored in the top two meters of dynamic thermokarst-affected permafrost environments?” \nIn particular, the analysis from the yedoma dominated sites in Siberia reveal that soils in drained thermokarst lake basins cannot be generalized. Especially on Sobo-Sise, soils in drained thermokarst lake basins are more depleted in organic C than the Holocene cover layer on top of yedoma deposits. The results of this study indicate a high variability in soil C and N contents within study regions, which is also reflected in a high variability in C accumulation rates. Nevertheless, there are still large amounts of frozen C (13 kg C m-2) potentially available for mobilization with future active layer deepening. \nA thermokarst lake landscape north of Teshekpuk Lake on the Arctic Coastal Plain was analyzed by comparing C, N and geochronological characteristics in sediment cores from a primary surface (upland), from a thermokarst lake and from several drained thermokarst lake basins of different lake level stages. This analysis along the thermokarst lake sequence (upland – lake – drained lake) revealed the degradation of organic C through the lake phase but also the accumulation of C and N rich organic matter after lake drainage. Nevertheless, both lacustrine and terrestrial sediments are C and N rich, leading to overall high C landscape contents (~60 kg C m-2 for 0-200 cm) for this thermokarst lake landscape, which is shaped by up to five different stages of lake levels indicating the dynamic nature of this thermokarst landscape over the last 7,000 years. \nAlso, soils in north Alaskan Arctic river deltas contain a large amount of frozen C and N. In relation to the entire two meter soil profile, there is a considerable amount of C (46%) and N (51%) stored in the 100-200 cm depth interval. This is particularly the case for sandy surface areas, where the first meter of soil is C and N poor. This finding shows the importance to include deeper Arctic river delta deposits in permafrost soil C and N estimations. In addition, radiocarbon dates from the delta sediments reveal coherent carbon and sediment accumulation rates among different sites in the investigated river deltas indicating stable depositional environments for the past 2,000 years. \nThis dissertation contributes to the circum-arctic C and N estimations by investigating rapidly changing, thermokarst-affected permafrost landscapes. The range of mean landscape C and N storages for these landscapes is between 31.3 and 60.2 kg C m-2 and 2.0 – 4.2 kg N m-2 for 0 200 cm. The consequent sampling down to two meter depth and the high sample resolution within cores allows estimates on the potential availability of C and N as a consequence of future permafrost thawing and active layer deepening which is expected to reach well beyond the standard soil survey depth (100 cm) in many Arctic permafrost regions by the end of the 21st Century. This assessment is complemented by extensive radiocarbon dating, which allows new insights into carbon and sediment characteristics for rapidly changing, thermokarst-affected permafrost landscapes. The high amounts of C and N stored in these deposits reveal the climate warming potential and urgent need to include thermokarst-affected permafrost terrain in future permafrost-climate models.