Abstract
Organic carbon (OC) stored in Arctic permafrost represents one of Earth’s largest and most vulnerable terrestrial carbon pools. Amplified climate warming across the Arctic results in widespread permafrost thaw. Permafrost deposits exposed at river cliffs and coasts are particularly susceptible to thawing processes. Accelerating erosion of terrestrial permafrost along shorelines leads to increased transfer of organic matter (OM) to nearshore waters. However, the amount of terrestrial permafrost carbon and nitrogen as well as the OM quality in these deposits are still poorly quantified. Here, we characterise the sources and the quality of OM supplied to the Lena River at a rapidly eroding permafrost river shoreline cliff in the eastern part of the delta (Sobo-Sise Island). Our multi-proxy approach captures bulk elemental, molecular geochemical and carbon isotopic analyses of late Pleistocene Yedoma permafrost and Holocene cover deposits, discontinuously spanning the last ~52 ka. We show that the ancient permafrost exposed in the Sobo-Sise cliff has a high organic carbon content (mean of about 5 wt%).We found that the OM quality, which we define as the intrinsic potential to further transformation, decomposition, and mineralization, is also high as inferred by the lipid biomarker inventory. The oldest sediments stem from Marine Isotope Stage (MIS) 3 interstadial deposits (dated to 52 to 28 cal kyr BP) and is overlaid by Last Glacial MIS 2 (dated to 28 to 15 cal ka BP) and Holocene MIS 1 (dated to 7–0 cal ka BP) deposits. The relatively high average chain length (ACL) index of n-alkanes along the cliff profile indicates a predominant contribution of vascular plants to the OM composition. The elevated ratio of iso and anteiso-branched FAs relative to long chain (C ≥ 20) n-FAs in the interstadial MIS 3 and the interglacial MIS 1 deposits, suggests stronger microbial activity and consequently higher input of bacterial biomass during these climatically warmer periods. The overall high carbon preference index (CPI) and higher plant fatty acid (HPFA) values as well as high C / N ratios point to a good quality of the preserved OM and thus to a high potential of the OM for decomposition upon thaw. A decrease of HPFA values downwards along the profile probably indicates a relatively stronger OM decomposition in the oldest (MIS 3) deposits of the cliff. The characterisation of OM from eroding permafrost leads to a better assessment of the greenhouse gas potential of the OC released into river and nearshore waters in future, which is important to understand the consequences of a warming climate in Arctic environments on the global carbon cycle.
Highlights
The terrestrial Arctic is highly vulnerable to climate warming as large areas are underlain by ice-rich permafrost (e.g. Strauss et al, 2021b)
We focused on n-alkanes in the aliphatic organic matter (OM) fraction and n-fatty acids in the polar hetero-compound fraction
4.1 Sedimentological organic matter parameters 230 total organic carbon content (TOC) content was highest in the topmost sample and ranged from
Summary
The terrestrial Arctic is highly vulnerable to climate warming as large areas are underlain by ice-rich permafrost (e.g. Strauss et al, 2021b). The Arctic river discharge increased significantly in recent decades, transporting organic-rich waters to the nearshore area (Holmes et al, 2012; Holmes et al, 2015). Very ice-rich permafrost deposits (i.e. 50-90 vol% ice) such as the late Pleistocene Yedoma Ice Complex (Schirrmeister et al, 2013; Strauss et al, 2017) are vulnerable to rapid thermo-denudation and thermo-erosion processes along river shores (Costard et al, 2014; Kanevskiy et al, 2016; Stettner et al, 2018; Fuchs et al, 2020). Vonk et al (2013b) showed that Yedoma icewedge meltwater can increase the decomposition of OM due to co-metabolizing effects Another potential impact of the 65 decomposition of terrestrial OM and discharge with Arctic river water is the change in biochemical properties that may increase ocean acidification and anthropogenic carbon dioxide uptake from the atmosphere (Semiletov et al, 2016). Semiletov et al (2016) estimated that 57% of the terrestrial organic carbon in the East Siberian Shelf originates from ancient
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