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 ~ 80 nM in surface nonsulfidic porewater, dominantly as Mo(VI). ΣMo increased up to 150 nM at a depth of 3.5 cm with low sulfide content (ΣH2S: 25–100 μM), and Mo(V) reached 20 nM. ΣMo decreased to ~ 70 nM at a depth of 7.5 cm in highly sulfidic deep sediment porewater (ΣH2S: > 100 μM) with Mo(V) accounting for ~ 10%. Mo(VI) dominated residual ΣMo, likely as MoS42−. 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 ~ 7 nM/h) and production of Mo(V) under low sulfide conditions (ΣH2S: 25–100 μM; Mo(V) as high as 160 nM). Mo(V) was subsequently lost rapidly from solution (dMo(V)/dt ~−5 nM/h) and residual ΣMo, presumably a mixture of Mo(VI) and a small percentage of Mo(IV), was gradually reestablished under highly sulfidic conditions (ΣH2S > 100 μM). 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.