Paleodynamics and organic carbon characteristics in a thermokarst affected landscape in West Alaska
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Global warming will continue to warm the Arctic resulting in the degradation of permafrost soils which leads to large-scale ground subsidence. Vast regions of the Arctic are covered with ice-rich silts, known as yedoma, containing large ice wedges. The formation of thermokarst lakes is one of the most important forms of permafrost degradation. Consequently, large amounts of previously freeze-locked organic carbon (OC) can be mobilized and released, which is of global significance for the carbon cycle. The aim of this research was to reconstruct the late Quaternary depositional environment and organic carbon characteristics of a thermokarst affected landscape, to better understand the processes involved in thermokarst development and identify the vulnerability of the organic carbon. Fieldwork was conducted on Baldwin Peninsula during the summer expedition in 2016 in West Alaska. Yedoma and drained thermokarst lake basin (DTLB) exposures were sampled and a thermokarst lake core was taken. Sedimentological and biochemical parameters, as well as lipid biomarkers were analyzed. A land cover classification map was made from the peninsula using Landsat imagery and a digital terrain model. By extrapolation of the field data, an estimation of the OC quantity on the peninsula and the loss since the Late Pleistocene was made. Yedoma deposition started > 50 cal ka BP whereas the DTLB deposits and thermokarst lake sediments originate from the Holocene. The grain size distributions show that the yedoma and DTLB deposits accumulated in a dominantly aeolian, stable regime. Yedoma was deposited in a drier and colder climate than the DTLB, as indicated by the lower BIT index (mean BIT: 0.94 for yedoma and 1.00 for DTLB) and MBT index. About 53 Mt of OC is stored in the frozen deposits on Baldwin Peninsula and it is estimated that the net loss since the Late Pleistocene is 3 Mt OC. The frozen DTLB deposits contain the largest share of OC (36 Mt, 70%). However, the yedoma deposits contain the most labile OC, as has been shown by the CPI (mean: 11.6 for yedoma and 8.8 for DTLB). This OC has been freeze-locked and not or barely altered by microbial degradation. The OC is terrestrially derived for the yedoma and DTLB deposits. The thermokarst lake shows a lacustrine input (mean δ13C -28.5‰). Because of the high ice content of the deposits on the Baldwin Peninsula, the deposits are highly susceptible and vulnerable to permafrost thaw. The high quality of the stored OC in the yedoma deposits makes these carbon pools an important source for microbial alteration. This poses an important input of carbon to the carbon cycle.