Cycling of W and Mo species in natural sulfidic waters and their sorption mechanisms on MnO2 and implications for paired W and Mo records as a redox proxy

Abstract Redox-sensitive trace metal concentration patterns have been widely used to track redox conditions in aquatic environments. Among redox-sensitive trace metals, molybdenum (Mo) is of interest owing to its speciation behavior and associated changes in particle reactivity that cause significant changes in concentration patterns in oxic versus sulfidic conditions. However, current understanding of how to use Mo geochemistry of marine sediments as a paleoredox proxy is limited to determining presence or absence of, and in some cases the aerial extent of, sulfidic conditions in the past. Here, we present a combination work of field sampling analysis and experiments to link concentration patterns of another redox-sensitive trace metal, tungsten (W), to Mo in order to provide a framework for using these trace metals to track changes in sulfidic conditions in ancient aquatic environments. We analyzed W and Mo concentrations in the water column and sediment pore waters of the Chesapeake Bay. We found that dissolved Mo/W molar ratios varied under different redox conditions. Because concentrations of these trace metals are controlled by their speciation in solution, we defined three sulfidic zones based on W and Mo speciation and their impacts on particle reactivity: the weakly sulfidic zone (0.2 μM ∼600 μM). In all sulfidic zones, dissolved W concentrations were positively correlated with dissolved Mn concentrations. In contrast, the dissolved Mo concentrations were negatively correlated with dissolved Mn concentrations. As a result, Mo/W molar ratios were negatively correlated with dissolved sulfide concentrations in the weakly, moderately, and strongly sulfidic zones. Our sorption experiment results indicate that the negative correlation between Mo/W molar ratios and dissolved oxygen concentrations in the oxic zone is likely due to the stronger adsorption of WO42– onto Mn oxide surfaces compared with that of MoO42–. In comparison, the negative correlation between Mo/W ratios and sulfide concentrations in sulfidic waters is attributed to the differences in thiolation processes of W and Mo, with Mo exhibiting a higher degree of thiolation than that of W. Because W and Mo speciation controls their particle reactivities, these results together indicate that both speciation of W and Mo and their particle reactivity with respect to solid phases control the behaviors of Mo/W molar ratios in various reducing environments. Our results also suggest that in both oxic and weakly sulfidic conditions, both WO42– and MoO42– dominate. In contrast, different thiolated anions of W and Mo are likely to be present in the sedimentary records deposited in sulfidic systems. Thus, the results suggest that the dissolved Mo/W molar ratio combined with their speciation information is a strong indicator of redox changes (including oxic and sulfidic changes). Solid phase Mo/W molar ratios increase as sulfide levels increase and thus can be used to track variations in redox conditions (as well as the severity of sulfidic conditions) in ancient oceans.