Manganese and iron reduction dominate organic carbon oxidation in deep continental margin sediments of the Ulleung Basin, East Sea
Rates and pathways of benthic organic carbon (C<sub>org</sub>) oxidation were investigated in surface sediments of the Ulleung Basin (UB) characterized by high organic carbon contents (> 2.5 %, dry wt.) and very high concentrations of Mn oxides (> 200 μmol cm<sup>−3</sup>) and Fe oxides (up to 100 μmol cm<sup>−3</sup>). The combination of geochemical analyses and independently executed metabolic rate measurements revealed that Mn and Fe reduction were the dominant C<sub>org</sub>
... C<sub>org</sub> oxidation pathways in the center of the UB, comprising 45 % and 20 % of total C<sub>org</sub> oxidation, respectively. By contrast, sulfate reduction was the dominant C<sub>org</sub> oxidation pathway accounting for 50 % of total C<sub>org</sub> mineralization in the continental slope. The relative significance of each C<sub>org</sub> oxidation pathway matched the depth distribution of the respective electron acceptors. The relative importance of Mn reduction for C<sub>org</sub> oxidation displays saturation kinetics with respect to Mn oxide content with a low half-saturation value of 8.6 µmol cm<sup>−3</sup>, which further implies that Mn reduction can be a dominant C<sub>org</sub> oxidation process even in sediments with lower MnO<sup>2</sup> content as known from several other locations. This is the first report of a high contribution of manganese reduction to C<sub>org</sub> oxidation in offshore sediments on the Asian margin. The high manganese oxide content in the surface sediment in the central UB was maintained by an extreme degree of recycling, with each Mn atom on average being reoxidized ~ 3800 times before permanent burial. This is the highest degree of recycling so far reported for Mn-rich sediments, and it appears linked to the high benthic mineralization rates resulting from the high organic carbon content that indicate the UB as a biogoechemical hotspot for turnover of organic matter and nutrient regeneration. Thus, it is important to monitor any changes in the rates and partitioning of C<sub>org</sub> oxidation to better understand the biogeochemical cycling of carbon, nutrients and metals associated with long-term climatic changes in the UB, where the fastest increase in sea water temperature has been reported for the past two decades.