Trace Metal Geochemistry in Anoxic Waters: Molybdenum and Mercury

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Much current research is directed toward understanding the behavior of trace metals in anoxic aquatic environments. These studies create a need for accurate data on the stoichiometry and stability of metal complexes containing reduced sulfur ligands. Many existing stability constants are uncertain at the order of magnitude level and thus are far less accurate than the field data to which they are applied. In some cases, especially for polysulfide or thiol ligands, no data exist. When data are available, it is often uncertain whether the species described are mononuclear or polynuclear. This is an insidious problem that can introduce large, unsuspected errors into predictions of metal behavior. We have completed investigations of copper speciation in solutions equilibrated with reduced Cu-S assemblages, chalcocite+djurleite and anilite+covellite. This has included EXAFS studies which demonstrate that multinuclear complexes exist in this system. We have also completed studies of zinc and cadmium speciation under conditions appropriate for anoxic marine waters. We will continue this line of research, focusing in the next two years on mercury and molybdenum. Specific objectives are as follows: 1) Mercury. Measure the solubility of cinnabar under conditions approaching those in anoxic marine waters and compare with metacinnabar solubility to determine if polynuclear complexes exist. Determine if Hg-polysulfide complexes will be important in nature. Investigation the solubility of cinnabar over its solid solution range, and 2) Molybdenum. Characterize Fe-Mo-S precipitates formed under conditions representative of anoxic marine waters by elemental analysis, EXAFS and solubility. Investigate the possible importance of thiomolybdates as ligands for Cu, Zn, Cd and Fe. Extend preliminary EXAFS work on Mo in black shales and seeks links between the products produced in the laboratory and those preserved in shales from ancient sulfidic marine basins.