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 < [H₂S] < ∼15 μM); the moderately sulfidic zone (∼15 μM < [H₂S] < ∼600 μM); and the strongly sulfidic zone ([H₂S] > ∼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 WO₄^2– onto Mn oxide surfaces compared with that of MoO₄^2–. 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 WO₄^2– and MoO₄^2– 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.

氧化还原敏感的痕量金属浓度模式已被广泛用于跟踪水生环境中的氧化还原条件。在氧化还原敏感的痕量金属中，由于钼的形态行为和相关的颗粒反应性变化而引起了关注，钼在氧化和硫化条件下会引起浓度模式的显着变化。但是，目前对如何利用海洋沉积物的钼地球化学作为古氧化还原的了解仅局限于确定过去是否存在硫化物，在某些情况下还可以确定其航空范围。在这里，我们将现场采样分析和实验结合起来，以链接另一种对氧化还原敏感的痕量金属钨（W）的浓度模式，为了为使用这些微量金属追踪古代水生环境中硫化物条件的变化提供一个框架。我们分析了切萨皮克湾水柱和沉积物孔隙水中的W和Mo浓度。我们发现，溶解的Mo / W摩尔比在不同的氧化还原条件下变化。由于这些痕量金属的浓度受溶液中的形态控制，我们基于W和Mo形态及其对颗粒反应性的影响定义了三个硫化区：弱硫化区（0.2μM<[H₂ S] <〜15μM）；中度硫化物带（〜15μM<[H ₂ S] <〜600μM）；以及强硫带（[H ₂ S]>〜600μM）。在所有硫化带中，溶解的钨浓度与溶解的锰浓度呈正相关。相反，溶解的Mo浓度与溶解的Mn浓度呈负相关。结果，Mo / W摩尔比与弱，中和强硫化带中溶解的硫化物浓度呈负相关。我们的吸附实验结果表明，Mo / W摩尔比与含氧区域中的溶解氧浓度之间呈负相关，这可能是由于与MoO相比，WO ₄ ^2–在Mn氧化物表面的吸附更强₄ ^2–。相比之下，硫化水中Mo / W比与硫化物浓度之间的负相关归因于W和Mo的硫醇化过程的差异，其中Mo的硫醇化程度高于W。因为W和Mo的形态控制着它们这些结果共同表明，W和Mo的形态以及它们相对于固相的反应性都控制着Mo / W摩尔比在各种还原环境中的行为。我们的结果还表明，在有氧和弱硫化条件下，WO ₄ ^2–和MoO ₄ ^2–支配。相反，沉积在硫化体系中的沉积记录中可能存在W和Mo的不同硫醇化阴离子。因此，结果表明，溶解的Mo / W摩尔比及其形态信息是氧化还原变化（包括氧化变化和硫化变化）的有力指标。固相Mo / W摩尔比随硫化物含量的增加而增加，因此可用于追踪古代海洋中氧化还原条件（以及硫化条件的严重性）的变化。