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Abstract
Permafrost thaw leads to thermokarst lake formation and talik growth tens of meters deep, enabling microbial decomposition of formerly frozen organic matter (OM). We analyzed two 17-m-long thermokarst lake sediment cores taken in Central Yakutia, Russia. One core was from an Alas lake in a Holocene thermokarst basin that underwent multiple lake generations, and the second core from a young Yedoma upland lake (formed ~70years ago) whose sediments have thawed for the first time since deposition. This comparison provides a glance into OM fate in thawing Yedoma deposits. We analyzed total organic carbon (TOC) and dissolved organic carbon (DOC) content, n-alkane concentrations, and bacterial and archaeal membrane markers. Furthermore, we conducted 1-year-long incubations (4°C, dark) and measured anaerobic carbon dioxide (CO2 ) and methane (CH4 ) production. The sediments from both cores contained little TOC (0.7±0.4 wt%), but DOC values were relatively high, with the highest values in the frozen Yedoma lake sediments (1620mgL-1 ). Cumulative greenhouse gas (GHG) production after 1year was highest in the Yedoma lake sediments (226±212µgCO2 -Cg-1 dw, 28±36µg CH4 -Cg-1 dw) and 3 and 1.5 times lower in the Alas lake sediments, respectively (75±76µgCO2 -Cg-1 dw, 19±29µg CH4 -Cg-1 dw). The highest CO2 production in the frozen Yedoma lake sediments likely results from decomposition of readily bioavailable OM, while highest CH4 production in the non-frozen top sediments of this core suggests that methanogenic communities established upon thaw. The lower GHG production in the non-frozen Alas lake sediments resulted from advanced OM decomposition during Holocene talik development. Furthermore, we found that drivers of CO2 and CH4 production differ following thaw. Our results suggest that GHG production from TOC-poor mineral deposits, which are widespread throughout the Arctic, can be substantial. Therefore, our novel data are relevant for vast ice-rich permafrost deposits vulnerable to thermokarst formation.
Highlights
Rapid warming of the Arctic results in permafrost warming (Biskaborn et al, 2019) and thaw, enabling microbial decomposition of previously frozen organic matter (OM; Schuur et al, 2008; Walter Anthony et al, 2016)
Our results suggest that greenhouse gas (GHG) production from total organic carbon (TOC)-p oor mineral deposits, which are widespread throughout the Arctic, can be substantial
CO2 production could mainly be explained by the average chain length (ACL) and dissolved organic carbon (OC) (DOC) content, suggesting that OM source and quality is the main driver for CO2 production
Summary
Rapid warming of the Arctic results in permafrost warming (Biskaborn et al, 2019) and thaw, enabling microbial decomposition of previously frozen organic matter (OM; Schuur et al, 2008; Walter Anthony et al, 2016). Thermokarst and talik formation, especially in ice-rich Late Pleistocene Yedoma deposits, result in deep thaw and subsequent ground subsidence. The study of talik sediments is highly relevant for climate studies as it allows important insights into the pathways of previously frozen OM upon rapid thaw and the potential for GHG production below thermokarst lakes. We studied GHG production, long-chain n-alkanes, branched GDGTs, and archaeal microbial markers in talik sediments to determine the OM characteristics of Yedoma sediments thawed under subaquatic conditions underneath two different thermokarst lakes. The combination of biomarker degradation proxies and incubation data from >10 m deep permafrost sediments that are thawed underneath a thermokarst lake is unique so far
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