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 iden tify the biogeochemical pathways that control the cycles of manganese and iron at the oxic-anoxic transition of the Orca Basin. The redox tr ansition coincides with an increase in salinity from 35 to 260 parts p er thousand; hence, mixing diagrams are used to constrain the salinity ranges over which consumption or production of solute species takes p lace. Analysis shows that the very high dissolved Mn(ll) levels (>400 mu M) at intermediate salinities (60-180 parts per thousand) result fr om dissimilatory (microbial) reduction of manganese oxides, coupled to organic matter oxidation. The manganese oxides are continuously regen erated in the oxygenated, low-salinity region (45-52 parts per thousan d) by microbial oxidation of dissolved Mn(ll). Precipitation of mangan ese carbonate in the high-salinity zone (>180 parts per thousand) is t he main removal mechanism of Mn to the sediments. Upward diffusing Fe( ll) ions are extracted from solution within the anoxic, high-salinity range (230-260 parts per thousand), through anaerobic oxidation by man ganese oxides or a nonoxidative sorption process. Ferric oxyhydroxides are reduced by reaction with dissolved sulfide and are, therefore, no t an important terminal electron acceptor for organic matter oxidation . Overall, the acid-base chemistry, redox transformations, and microbi al activity across the salinity transition are strongly coupled to the cycle of manganese.