Redox speciation and early diagenetic behavior of dissolved molybdenum in sulfidic muds

In order to further elucidate the early diagenetic behavior of Mo, we examined the redox speciation of dissolved Mo in organic rich sediment from a back-barrier salt marsh environment in eastern Long Island (Flax Pond, NY). Total dissolved Mo (ΣMo) was ~80nM in surface nonsulfidic porewater, dominantly as Mo(VI). ΣMo increased up to 150nM at a depth of 3.5cm with low sulfide content (ΣH₂S: 25-100μM), and Mo(V) reached 20nM. ΣMo decreased to ~70nM at a depth of 7.5cm in highly sulfidic deep sediment porewater (ΣH₂S: >100μM) with Mo(V) accounting for ~10%. Mo(VI) dominated residual ΣMo, likely as MoS₄^2-. Averaged in situ Mo speciation patterns are complicated by mixed redox conditions created by biogenic structures and reworking. Serial anoxic incubation of surface sediments revealed reductive redox reaction progression without complications from transport and biogenic microenvironments: Mo(VI) dominated initially, followed by increases in ΣMo (dMo/dt ~7nM/h) and production of Mo(V) under low sulfide conditions (ΣH₂S: 25-100μM; Mo(V) as high as 160nM). Mo(V) was subsequently lost rapidly from solution (dMo(V)/dt ~-5nM/h) and residual μMo, presumably a mixture of Mo(VI) and a small percentage of Mo(IV), was gradually reestablished under highly sulfidic conditions (ΣH₂S >100μM). Mo(V) is clearly produced as a transient dissolved intermediate during reductive redox reaction succession. Mo(V) may react with particles or disproportionate in the presence of polysulfides into Mo(IV), which likely rapidly adsorbs-precipitates as pyritic Mo-Fe-S, sulfidized organic complexes, or perhaps MoS₂. Mo(VI), which remains, at least temporarily in solution as thiomolybdate is removed more slowly. In contrast to reductive reactions, reoxidation of reduced sediment results in rapid release of Mo dominated by Mo(VI). Dynamic diagenetic cycling and the existence of Mo(V) as a dissolved reaction intermediate must be accounted for in models of Mo fixation and associated isotopic fractionation in sulfidic deposits.