Because past oxygen levels cannot be quantified directly, obtaining information about Earth’s paleoredox history requires interpretation of the sedimentary record. Vanadium offers a means to acquiring such knowledge because it experiences preferential enrichment in sulfidic sediments. The sequestration mechanism is largely believed to involve precipitation of insoluble V(III or IV) phases. However, various researchers have noted instances of elevated V solubilities in sulfidic waters. Unfortunately, information about the sulfidic inorganic geochemistry of V is severely limited and results in speciation models devoid of thiolated participants; HS- is solely regarded as a reducing agent. To hasten full exploitation of the V proxy, this work seeks to characterize experimentally the chemistry governing V speciation in sulfidic waters. Deoxygenated mother solutions initially containing 5-300 mM V(V)O43-, 3 mM SS2-, and 20 mM TRIS at pH = 8.50 give visually colorless and transparent solutions and yield UV-vis spectra consistent with those of polysulfides (Sx2- x = 2-6), indicating S2- oxidation. Extinction coefficients for the polysulfides were used to calculate S0 produced in the test solutions. Plots of S0 vs. initial VO43- yield a bimodal relationship. Above ~25 mM VO43-, the slope is ~1/2 (m = 0.51), implying reduction of V(V) to V(IV). Below ~25 mM VO43-, the slope approaches 1 (m = 0.83), inferring primarily reduction of V(V) to V(III). Tyndall tests indicate the presence of colloids in either redox regime. After flocculating (0.1 M MgCl2) and filtering (0.45 mm) the colloids, AAS data show that &gt;80% of the initial SV remains dissolved. These data hint that presently unverified V ions such as thiovanadyl, V(IV)S2+, and thiovanadites, V(III)O3-xSx3-, may be important contributors to aquatic V speciation. These aqueous phases may assist in explaining instances of elevated V solubility in sulfidic waters. Experiments are ongoing and aim ultimately to identify reactions, quantify thermodynamic constants, and develop V speciation models.

Advancing the vanadium paleoredox proxy: Defining the chemistry controlling vanadium speciation in sulfidic natural waters

255th National Meeting (2018)

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