Molybdenum geochemistry in salt marsh pond sediments

The concentration and isotopic composition of sedimentary molybdenum (Mo) has been used to distinguish different redox environments in modern marine settings and in the geological record. We report Mo concentrations and delta Mo-98 from porewaters and sediments in three anoxic East Anglian salt marsh pond environments: (1) 'iron-rich' sediments containing high concentrations of dissolved ferrous iron (up to 2 mM), (2) 'sulfide-rich' sediments containing very high concentrations of aqueous sulfide (up to 10 mM) and, (3) sediments that we consider to be intermediate between 'iron-rich' and 'sulfide-rich' conditions. In iron-rich sediments, we suggest that iron speciation and mineralogy controls the concentration and isotopic composition of Mo. Despite similar aqueous sulfide profiles, the intermediate and sulfide-rich pond sediment have different porewater Mo concentrations and delta Mo-98. In the sulfide-rich pond sediment, we suggest that the concentration and isotopic composition of Mo is controlled by solubility equilibrium with an Fe-Mo-S mineral species (e.g. FeMoS4) due to similarities in sediment and porewater delta Mo-98 throughout the sediment column. In the intermediate pond sediment, we conclude that active breakdown of iron oxides redistributes porewater Mo, observable as a peak of dissolved Mo (>100 ppb), which diffuses within the sedimentary porewaters. The sedimentary delta(98) Mo is higher in sulfide-rich and intermediate pond sediment (mean = 1.66 parts per thousand, range = 0.98-1.92 parts per thousand) than in iron-rich pond sediment (mean = 1.10 parts per thousand, range = 0.28-1.65 parts per thousand) with all ponds having sedimentary delta Mo-98 that is lower than seawater. The maximum sedimentary delta Mo-98 observed in these anoxic sediments, which is 0.5-0.7 parts per thousand lower than seawater, appears to be set by Fe-Mo-S equilibration with ambient thiomolybdate species. We suggest diagenetic overprinting can cause more efficient capture of pond water Mo and causes sediment delta Mo-98 of originally iron-rich pond sediment to evolve to higher values at progressively higher aqueous sulfide concentrations. (C) 2020 Elsevier Ltd. All rights reserved.