Molecular‐Multiproxy Assessment of Land‐Derived Organic Matter Degradation Over Extensive Scales of the East Siberian Arctic Shelf Seas

Abstract

AbstractGlobal warming triggers permafrost thaw, which increases the release of terrigenous organic matter (terr‐OM) to the Arctic Ocean by coastal erosion and rivers. Terrigenous OM degradation in the Arctic Ocean contributes to greenhouse gas emissions and severe ocean acidification, yet the vulnerability of different terr‐OM components is poorly resolved. Here, terr‐OM degradation dynamics are studied with unprecedented spatial coverage over the World's largest shelf sea system—the East Siberian Arctic Shelf (ESAS), using a multi‐proxy molecular biomarker approach. Mineral‐surface‐area‐normalized concentrations of terr‐OM compounds in surface sediments decreases offshore. Differences between terr‐OM compound classes (lignin phenols, high‐molecular weight [HMW] n‐alkanes, n‐alkanoic acids and n‐alkanols, sterols, 3,5‐dihydroxybenzoic acids, cutin acids) reflect contrasting influence of sources, propensity to microbial degradation and association with sedimenting particles, with lignin phenols disappearing 3‐times faster than total terr‐OM, and twice faster than other biomarkers. Molecular degradation proxies support substantial terr‐OM degradation across the ESAS, with clearest trends shown by: 3,5‐dihydroxybenzoic acid/vanillyl phenol ratios, acid‐to‐aldehyde ratios of syringyl and vanillyl phenols, Carbon Preference Indices of HMW n‐alkyl compounds and sitostanol/β‐sitosterol. The combination of terr‐OM biomarker data with δ13C/Δ14C‐based source apportionment indicates that the more degraded state of lignin is influenced by the relative contribution of river‐transported terr‐OM from surface soils, while HMW n‐alkanoic acids and stigmasterol are influenced by erosion‐derived terr‐OM from Ice Complex deposits. Our findings demonstrate differences in vulnerability to degradation between contrasting terr‐OM pools, and underscore the need to consider molecular properties for understanding and modeling of large‐scale biogeochemical processes of the permafrost carbon‐climate feedback.