Multi-proxy evaluation of terrestrial organic matter degradation across the East Siberian Arctic Seas

    Ongoing global warming is expected to accelerate the thaw of permafrost on land and to increase the input of terrigenous organic matter (terrOM) into the Arctic Ocean through coastal erosion and river discharge. Large remobilization of terrOM into the East Siberian Arctic Shelf (ESAS) dominates the organic matter in surface sediments over large parts of the shelf and its degradation contributes to ocean acidification. Previous studies have focused on the source apportionment of terrOM and the releases of CO₂ and CH₄ to the atmosphere from terrOM degradation; this study focuses on its diagenetic state during cross-shelf transport, since degradation is the link between permafrost thawing and greenhouse gases emissions. This study probes the degradation status of different terrOM components across the ESAS using various molecular and isotopic proxies and hence evaluates their differences to infer degradation.

    High-molecular weight (HMW) lipid compounds and lignin phenols are exclusively produced by terrestrial plants, providing protection, strength and rigidity to the plant structure. Owing to diagenesis, microbial degradation leads to 1) loss of functional groups, thus the ratios of HMW n-alkanoic acids, HMW n-alkanols and sterols relative to HMW n-alkanes decrease; 2) reduction of unsaturated to saturated carbons, so ratios of stanols relative to stenols increase; 3) a higher formation of carboxylic acids in the lignin polymer and hence ratios of acids to aldehydes of vanillyl (Vd and Vl) and syringyl (Sd and Sl) increase.

    The concentrations of lipid- and lignin-derived products per sediment specific surface area decreased with offshore distance of the samples. During cross-shelf transport, the biomarker degradation proxies showed an increasing degradation for Sd/Sl, Vd/Vl, the “tannin-like” compound 3,5-dihydrobenzoic acid to vanillyl (3,5-Bd/V), β-sitostanol/ β-sitostenol and Carbon Preference Index (CPI) of HMW n-alkanes. Some other proxies showed no clear trend from inner to outer shelf and such inconsistent patterns are currently being investigated to better understand both the usefulness/response of different proxies and of the lability of terrOM in the ESAS. While β-sitostanol/β-sitostenol and CPI HMW n-alkane did not show strong differences between the East Siberian Sea and the Laptev Sea, Vd/Vl and Sd/Sl ratios indicated stronger degradation on the outer Laptev Sea and 3,5-Bd/V ratios indicated stronger degradation in the outer eastern East Siberian Sea. Such differences could reflect source properties of terrOM entering the ESAS, such as differences in source vegetation or transport pathway, i.e. coastal erosion or river discharge.