MOLYBDENUM ACCUMULATION IN ANOXIC MARINE BASINS: MECHANISMS AND PALEOREDOX IMPLICATIONS

Arguments addressing Mo sequestration in low-oxygen settings have historically focused on the relative roles of Mo reduction, links to hydrogen sulfide availability in the pore waters and water column, and potential couplings with organic-matter deposition. Results from a range of modern anoxic marine settings of broadly contrasting depositional conditions--the Cariaco Basin, the Black Sea, the Orca Basin, and Effingham Inlet (an anoxic fjord in British Columbia)--suggest a strong mechanistic link between Mo enrichment and the presence of hydrogen sulfide in the water column. Ancient black shales from the Devonian (northern Appalachian Basin) and Pennsylvanian (midcontinent USA) corroborate interpretations from modern systems. Ultimately, magnitudes of Mo enrichment within the anoxic-sulfidic (euxinic) water columns, as expressed in Mo/Al ratios, are controlled by the relative roles of Mo scavenging versus dilution by continentally derived material. This continental influx has the potential to swamp the signal of euxinic Mo enrichment under conditions of high siliciclastic sedimentation along anoxic continental margins. Thus, Mo/Al ratios can also vary spatially and temporally by orders of magnitude as a function of sedimentation rather than strictly depositional redox.

The collective results also suggest that Mo enrichment can be confined to the sulfidic water column despite the potential for highly sulfidic pore waters beneath oxic to weakly oxic overlying waters. Further, data from both modern and ancient sediments suggest a coupling between water-column Mo scavenging and organic-matter deposition reflecting water-column sulfide availability and the quantity and spatially varying type of organic matter present. Finally, there is no clear direct link between pyrite and Mo burial in these oxygen-deficient settings. The ultimate goal of this ongoing study is to develop an unambiguous Mo proxy record of water-column euxinicity within the context of collaborative investigations of Mo isotope relationships and mass balance constraints during periods of ocean-scale anoxia.