Biogeochemical Cycles of Manganese and Iron at the Oxic-Anoxic Transition of a Stratified Marine Basin (Orca Basin, Gulf of Mexico)

Chemical distributions and microbial culture data are combined to identify the biogeochemical pathways that control the cycles of manganese and iron at the oxicanoxic transition of the Orca Basin. The redox transition coincides with an increase in salinity from 35 to 260‰; hence, mixing diagrams are used to constrain the salinity ranges over which consumption or production of solute species takes place. Analysis shows that the very high dissolved Mn(II) levels (>400 μM) at intermediate salinities (60-180‰) result from dissimilatory (microbial) reduction of manganese oxides, coupled to organic matter oxidation. The manganese oxides are continuously regenerated in the oxygenated, low-salinity region (45-52‰) by microbial oxidation of dissolved Mn(II). Precipitation of manganese carbonate in the high-salinity zone (>180‰) is the main removal mechanism of Mn to the sediments. Upward diffusing Fe(II) ions are extracted from solution within the anoxic, highsalinity range (230-260‰), through anaerobic oxidation by manganese oxides or a nonoxidative sorption process. Ferric oxyhydroxides are reduced by reaction with dissolved sulfide and are, therefore, not an important terminal electron acceptor for organic matter oxidation. Overall, the acid-base chemistry, redox transformations, and microbial activity across the salinity transition are strongly coupled to the cycle of manganese.