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We report quantitative carbon isotope evidence showing that porewater dissolved organic carbon (DOC) is produced from different sources in Arctic Ocean sediments, resulting in dramatic changes in both delta C-13 and Delta C-14 values of DOC. The very low delta C-13 (-42.7%) and Delta C-14 (-947%) values measured for porewater DOC indicate that bacteria consume methane hydrate-derived OC and convert a significant fraction of this delta C-13 depleted and old OC (44-97%) into DOC in the sediments of the Arctic Ocean. We suggest that the production of extremely old, methane-derived porewater DOC (5,430-23,600 years) in Arctic Ocean sediment is an important pathway for carbon cycling that provides not only a necessary energy source for microbial communities in this extreme environment but also an ancient DOC source that could flux and contribute to the old and biologically recalcitrant DOC found in the deep ocean.

Plain Language Summary Dissolved organic carbon (DOC) produced in sediment porewater is a crucial intermediary during the early diagenesis of organic matter in marine sediments, and DOC production is also an important pathway of carbon cycling in the ocean. In this study, we present radiocarbon and stable carbon isotope results measured for porewater DOC and sedimentary solid phase total organic carbon in sediment cores collected from the Arctic Ocean. We find that the carbon isotopic signatures of porewater DOC differed dramatically between samples produced from mostly marine-derived organic matter (delta C-13 = -22.9%, Delta C-14 = -80%) in the Chukchi Sea shelf and samples produced from methane hydrate-derived organic matter (delta C-13=-42.7%, Delta C-14=-947%) on the Arctic slope. We estimate that in the slope region, 44-97% of the extremely old DOC preserved in the sediment was produced from methane hydrate. The production of this ancient, methane-bearing porewater DOC could be an important but insufficiently studied pathway for carbon cycling that provides not only a necessary energy source for microbial communities in the Arctic sediment but also an ancient DOC source that could contribute to the old and biologically recalcitrant DOC found in the deep ocean.