Speciation, oxidation state, and reactivity of particulate manganese in marine sediments

Particulate manganese oxides take a significant part in benthic biogeochemical processes of modern marine sediments. Thermodynamics calculations tell us that the reactivity of the different Mn oxides is variable. We describe here a method adapted for marine sediments, which allowed us to extract Mn oxides and determine the mean oxidation state of particulate Mn. Mn oxides are known to be enriched in the oxic layer of marine sediments. We have observed on a continental slope sediment that the oxidation state of particulate Mn changed with depth, within the 8 cm thick oxic layer. Mn(III) oxyhydroxides dominated at the sediment-water interface, and at the bottom of the oxic layer. In between, Mn(IV) oxides were more abundant. Mn(III) oxides are metastable phases which results from the primary oxidation of Mn2+. Mn oxides of the bottom of the oxic layer are enriched in Mn(III) phases because they result from the current in situ oxidation/precipitation of dissolved Mn2+ at the oxic-anoxic interface. Mn(III) oxyhydroxides of the surface sediment originate from the precipitation of Mn shortly before settling, presumably in estuaries where redox oscillations are intense. By contrast, Mn(IV) oxides result from the aging of Mn(III) oxyhydroxides during sediment burial. We have tested the reactivity of different types of Mn oxides with ammonia in anoxic environment. The formation of nitrate was observed after 13 days of incubation in experimental units that contained Mn(III) oxyhydroxides, but not in the units that contained Mn(IV) oxides. Therefore, Mn(III) oxyhydroxides may play a major role in the oxidation of ammonia to nitrate in marine sediments.