Pathways of organic carbon oxidation in three continental margin sediments

We have combined several different methodologies to quantify rates of organic carbon mineralization by the various electron acceptors in sediments from the coast of Denmark and Norway. Rates of NH + 4 and ∑CO 2 liberation in sediment incubations were used with O 2 penetration depths to conclude that O 2 respiration accounted for only between 3.6–17.4% of the total organic carbon oxidation. Dentrification was limited to a narrow zone just below the depth of O 2 penetration, and was not a major carbon oxidation pathway. The processes of Fe reduction, Mn reduction and sulfate reduction dominated organic carbon mineralization, but their relative significance varied depending on the sediment. Where high concentrations of Mnoxide were found (3–4 wt% Mn), only Mn reduction occurred. With lower Mn oxide concentrations more typical of coastal sediments, Fe reduction and sulfate reduction were most important and of a similar magnitude. Overall, most of the measured O 2 flux into the sediment was used to oxidized reduced inorganic species and not organic carbon. We suspect that the importance of O 2 respiration in many coastal sediments has been overestimated, whereas metal oxide reduction (both Fe and Mn reduction) has probably been well underestimated.