Complete oxidation of solid phase sulfides by manganese and bacteria in anoxic marine sediments

During the physical or biological reworking of surficial marine sediments, metal oxides are often brought into contact with both solid and dissolved sulfides. Experiments simulating these mixing processes demonstrate that in natural sediments Mn-oxides can completely oxidize solid phase sulfides to SO 4 − under anoxic conditions. The major source of sulfur is probably acid volatile sulfide. Minerals containing Mn +4 are apparently more effective than Mn +3 in driving the oxidation. There is slight or no evidence for complete sulfide oxidation by Fe-oxides under similar conditions. The reaction is inhibited by DNP (dinitrophenol) and azide, implying biological mediation by a group of chemolithotrophic bacteria such as the thiobacilli, having a well-organized cytochrome system, oxidative phosphorylation coupled with sulfide oxidation, and possibly aulolrophic CO 2 fixation. Lack of sensitivity to chlorate suggests that a No 3 − reductase complex is not involved. Because of metal reduction and the overall stoichiometry of reaction, this sulfide oxidation causes a rise in pH in contrast to oxidation by O 2. Alkalinity is also simultaneously depeleted by Mn, Ca carbonate precipitation. Both manganoan kutnahorite and manganoan calcite are observed to form rapidly (days) during Mn reduction. The oxidation of sulfides by Mn-oxides is likely to be important, but highly variable, in organic-rich shelf sediments and environments such as hydrothermal vents where sulfidic plumes contact oxidized metals. A substantial Proportion of sedimentary sulfide may be oxidized and Mn reduced by this pathway, particularly in bioturbated sediments. The relative roles of lithotrophic (S) and heterotrophic (C) Mn-reduction in marine sediments are presently unknown.