Quantifying cross-shelf degradation of terrigenous organic carbon in Eurasian Arctic Shelf Seas

The permafrost in the Northern Hemisphere holds approximately 50% of the global soil organic carbon, constituting a reservoir that is twice the size of atmospheric carbon storage. Permafrost carbon plays a vital role in governing the global carbon cycle through its reactivity and accessibility to microbial respiration to release greenhouse gases. Existing studies have revealed spatial variability of degradation of coastally-exported organic matter across regimes in the East Siberian Arctic Shelf Seas. While the degradation patterns and ambient rates are reasonably constrained for the Laptev Sea, these aspects are less well understood for the Kara Sea. Here, we quantified carbon isotopes (¹³C and ¹⁴C), TOC, specific surface area, lipid biomarkers, and lignin phenols along a Kara Sea cross-shelf transect to assess terrigenous organic matter degradation, and compare patterns with three East Siberian cross-shelf transects (Kara Sea, Laptev Sea, Western East Siberian Sea, and Eastern East Siberian Sea). The data demonstrate the highest degradation rate constant of 2.5 kyr^-1 in the Eastern East Siberian Sea, a moderate value of 2.0 kyr^-1 in the Laptev Sea, and the lowest value of 1.2 kyr^-1 in the Western East Siberian Sea. Intriguingly, no statistical trend in degradation was observed across the Kara Sea. The recalcitrant fractions of terrestrial organic carbon are determined to be largest (50%) in the Western East Siberian Sea, moderate (31%) in the Eastern East Siberian Sea, and smallest (11%) in the Laptev Sea. The spatial variabilities in degradation rate constants and recalcitrant fractions are likely attributed to different organic carbon speciation across regimes on land, e.g., from fibrous plant residues to mineral-associated organic carbon, and potential biological controls (e.g., priming effect) during transport.