Manganese and iron reduction dominate organic carbon oxidation in surface sediments of the deep Ulleung Basin, East Sea

Abstract. Rates and pathways of benthic organic carbon (C_org) oxidation were investigated in surface sediments of the Ulleung Basin (UB) characterized by high C_org contents ( >  2.5 %, dry wt.) and very high contents of Mn oxides ( >  200 µmol cm⁻³) and Fe oxides (up to 100 µmol cm⁻³). The combination of geochemical analyses and independently executed metabolic rate measurements revealed that Mn and Fe reduction were the dominant C_org oxidation pathways in the center of the UB, comprising 45 and 20 % of total C_org oxidation, respectively. By contrast, sulfate reduction was the dominant C_org oxidation pathway, accounting for 50 % of total C_org mineralization in sediments of the continental slope. The relative significance of each C_org oxidation pathway matched the depth distribution of the respective electron acceptors. The relative importance of Mn reduction for C_org oxidation displays saturation kinetics with respect to Mn oxide content with a low half-saturation value of 8.6 µmol cm⁻³, which further implies that Mn reduction can be a dominant C_org oxidation process even in sediments with lower MnO₂ content as known from several other locations. This is the first report of a high contribution of manganese reduction to C_org 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 C_org content that indicate the UB as a biogeochemical hotspot for turnover of organic matter and nutrient regeneration.