Organic matter characteristics of a rapidly eroding permafrost cliff in NE Siberia (Lena Delta, Laptev Sea region)

[摘要] Organic carbon (OC) stored in Arctic permafrostrepresents one of Earth's largest and most vulnerable terrestrial carbonpools. Amplified climate warming across the Arctic results in widespreadpermafrost thaw. Permafrost deposits exposed at river cliffs and coasts areparticularly susceptible to thawing processes. Accelerating erosion ofterrestrial permafrost along shorelines leads to increased transfer oforganic matter (OM) to nearshore waters. However, the amount of terrestrialpermafrost carbon and nitrogen as well as the OM quality in these depositsis still poorly quantified. We define the OM quality as the intrinsicpotential for further transformation, decomposition and mineralisation. Here,we characterise the sources and the quality of OM supplied to the Lena Riverat a rapidly eroding permafrost river shoreline cliff in the eastern part ofthe delta (Sobo-Sise Island). Our multi-proxy approach captures bulkelemental, molecular geochemical and carbon isotopic analyses of LatePleistocene Yedoma permafrost and Holocene cover deposits, discontinuouslyspanning the last ∼52  kyr. We showed that the ancientpermafrost exposed in the Sobo-Sise cliff has a high organic carbon content(mean of about 5 wt %). The oldest sediments stem from Marine IsotopeStage (MIS) 3 interstadial deposits (dated to 52 to 28 cal ka BP) and areoverlaid by last glacial MIS 2 (dated to 28 to 15 cal ka BP) and HoloceneMIS 1 (dated to 7–0 cal ka BP) deposits. The relatively high average chainlength (ACL) index of n -alkanes along the cliff profile indicates apredominant contribution of vascular plants to the OM composition. Theelevated ratio of iso- and anteiso-branched fatty acids (FAs) relative to mid- and long-chain (C  ≥  20) n -FAs in the interstadial MIS 3 and the interglacial MIS 1 depositssuggests stronger microbial activity and consequently higher input ofbacterial biomass during these climatically warmer periods. The overall highcarbon preference index (CPI) and higher plant fatty acid (HPFA) values aswell as high C / N ratios point to a good quality of the preserved OM and thusto a high potential of the OM for decomposition upon thaw. A decrease inHPFA values downwards along the profile probably indicates stronger OM decomposition in the oldest (MIS 3) deposits of the cliff. Thecharacterisation of OM from eroding permafrost leads to a better assessmentof the greenhouse gas potential of the OC released into river and nearshorewaters in the future.

[学科分类] 大气科学