Land-derived organic matter drives benthic carbon and nutrient cycling on the Siberian Arctic Ocean shelves

The Arctic Ocean is currently changing at a high rate, and projections over the next decades include an increase in water temperature and retreat of sea ice, as well as increased input of freshwater and land-derived material released by permafrost thaw. These changes might substantially alter marine biogeochemical cycles and primary production, with repercussions for the Arctic Ocean greenhouse gas balance as well as ocean acidification. The large and shallow continental shelf seas north of Siberia are particularly affected by these changes as they receive land-derived material from strong coastal erosion and large rivers such as Ob, Yenisey and Lena. In recent studies, we have shown a transition from predominantly land- to marine-derived organic matter in sediments from the coast to the shelf break based on isotopes and biomarkers, an increased decomposition state of land-derived organic matter, as well as a decrease in sediment and water column nitrogen concentrations. We here combine this understanding with incubation experiments to assess the impact of these gradients on benthic CO₂ production and nutrient remineralization. We found that fresh, land-derived organic matter typical for near-shore environments showed highest decomposability to CO₂ in controlled, aerobic laboratory incubations, as indicated by correlations of CO₂ production with concentrations of terrigenous biomarkers (lignin, high molecular weight n-alkanes), and biomarker proxies indicating the decomposition state of these compounds. Fresh, land-derived organic matter was also associated with highest ammonium and nitrite release to the water column, measured during on-board incubation of intact sediment cores. The opposite pattern was observed for phosphate and silicate fluxes that were highest in more marine-influenced settings. Our data suggest that increased input of land-derived organic matter to the Siberian Arctic Ocean shelves could promote benthic decomposition processes near the coast, including CO₂ release that might contribute to the strong ocean acidification already observed in the region. Furthermore, the different controls on nutrient fluxes indicate a de-coupling of nitrogen, phosphorus and silicon remineralization, with implications for nutrient limitation and primary production in the Arctic Ocean.