PATHWAYS OF ORGANIC-CARBON OXIDATION IN 3 CONTINENTAL-MARGIN SEDIMENTS

We have combined several different methodologies to quantify rates of organic carbon mineralization by the various electron acceptors in sed iments from the coast of Denmark and Norway. Rates of NH4+ and SIGMACO 2 liberation in sediment incubations were used with O2 penetration dep ths to conclude that O2 respiration accounted for only between 3.6-17. 4% of the total organic carbon oxidation. Dentrification was limited t o a narrow zone just below the depth of O2 penetration, and was not a major carbon oxidation pathway. The processes of Fe reduction, Mn redu ction and sulfate reduction dominated organic carbon mineralization, b ut their relative significance varied depending on the sediment. Where high concentrations of Mn-oxide were found (3-4 wt% Mn), only Mn redu ction occurred. With lower Mn oxide concentrations more typical of coa stal sediments, Fe reduction and sulfate reduction were most important and of a similar magnitude. Overall, most of the measured O2 flux int o the sediment was used to oxidized reduced inorganic species and not organic carbon. We suspect that the importance Of O2 respiration in ma ny coastal sediments has been overestimated, whereas metal oxide reduc tion (both Fe and Mn reduction) has probably been well underestimated.