Landscape development and organic matter characteristics of thermokarst lake deposits in Yakutia - Siberia


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julia.wiedmann [ at ] awi.de

Abstract

Withongoing climate warming, Arctic permafrost undergoes fast degradation, resulting inthe deepening of the seasonally unfrozen surface and deep permafrost thaw. As permafrost landscapes store huge amounts of carbon, they are becoming a source of greenhouse gases in the course of remobilization of former freeze-locked organic carbon. Large regions of the Arctic are coveredby ice-rich silt deposits with huge ice wedges, known as Yedoma. Due to their high ice content,these deposits are explicitly vulnerable to changing environmental conditions. One form of permafrost degradation is the formation of thermokarst lakes. These waterbodies play an important role regarding the thermal energy balance in the ground.Understanding the development of permafrost landscapes and the processes which cause their degradationis essential for the estimation of future changes of the permafrost-carbon feedback. The aim of this study was the reconstruction of the development of athermokarst affected late Quaternary landscape, by analyzing the deposits’ sedimentology to understand past depositional processes. Furthermore, I analysed the organic matter characteristics to identify the vulnerability of the organic carbon. Two sediment cores below two thermokarst lakes in the Lena Aldan interfluve region in Central Yakutia were investigated. One core originates from a Yedoma site (YUK15-YU-L15), where sediments accumulated during the late Pleistocene. The other core was retrievedfromthe bottom of an Alas lake (YUK15-YU-L7), consisting of diagenetically and thermal altered Yedoma deposits. Underneath both lakes, a talik (unfrozen ground or thaw bulb) was present. The talik was about 12 m deep for YUK15-YU-L15 and exceeded the core depth of YUK15-YU-L7. The fieldwork was conducted in March 2015 during a joined German-Russian expedition. The two cores were analysed for hydrochemical, biogeochemical and sedimentological parameters. The grain size distribution shows that the lake deposits of both cores were mainly accumulated by fluvial sedimentation processes. The very low organic carbon content in the deposits stand in contrast to other investigated Yedoma study sites. Reasons for the lack of carbon can be deep-thawing processes and related organic matter decomposition in the existing talik or the input of organic-poor sediments in the past. Water isotopes from pore water show a permanently frozen state for the lowerpartof the Yedoma lake core, thereby ruling out strong organic matter decomposition. For the Alas lake core,stable water isotopes reflect more recent precipitation values, resulting from rain and lake water infiltration in the unfrozen ground. An assumption is that the Alas lake deposits had characteristics similar tothe Yedoma lake deposits before permafrost degradation. This can help to estimate future developing stages of the study site. These findings indicate that Yedoma deposits are very heterogeneous on a global scale. The high water and ice content make these deposits vulnerableto fast permafrost degradationand ground subsidence. With the current warming scenarios, thermokarst activity will probably stay on a high level and an increase of thermokarst lake formation is likely in Central Yakutia.



Item Type
Thesis (Master)
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Not peer-reviewed
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Published
Eprint ID
50903
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Wiedmann, J. (2019): Landscape development and organic matter characteristics of thermokarst lake deposits in Yakutia - Siberia Master thesis,


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