Vanadium concentrations and speciation across the chemocline of a euxinic estuary. 1: geochemical cycling and fixation in euxinic sediments

Understanding V cycling in modern euxinic basins and its fixation in the underlying carbonaceous sediments may improve our ability to use - V as a paleo-redox proxy for the ancient ocean. To this end, V concentrations were determined in the water column and bottom sediments of the stratified Upper Basin of the Pettaquamscutt River estuary in Rhode Island as a model marine-influenced euxinic basin. Vanadium concentrations are correlated with dissolved silica in the Upper Basin, which suggests that V concentrations may reflect chemical weathering of silicate rocks in the watershed and that V cycling may be controlled, in part, by diatom bioassimilation and senescence. Vanadium also appears to be scavenged from the oxic surface waters by precipitating Fe oxide/oxyhydroxide minerals and subsequently released back into solution across the oxic-anoxic transition zone as these materials are redissolved by reductive dissolution. Equilibrium modeling indicates that vanadate oxyanions, (Formula presented) and (Formula presented), are the dominant forms of V in solution in the oxic surface waters and across the chemocline, whereas V(III) species [i.e., (Formula presented) and (Formula presented) ] predominate in the euxinic monimolimnion. Despite the equilibrium modeling, we cannot rule out the possibility that V(IV) species are also important in the Upper Basin water column. Chemical extractions of the bottom sediments using the Improved Lord method reveal that V is chiefly contained within the silicate (∼59%, on average) and reactive (∼39%, on average) fractions of these sediments. Very little of the V is associated with organic matter (< 2%) or pyrite (∼1.52%, on average) in these sediments. The silicate fraction is consistent with authigenic, 2:1 phyllosilicate minerals, which fix V(III) in their octahedral sites. The formation of the secondary clay minerals occurs during early diagenesis of the organic-rich, diatomaceous bottom sediments. The reactive fraction of the sediments includes V(IV) and/or V(III) sorbed onto or incorporated in mackinawite (FeS), V(III) associated with hydrous Al oxide minerals, and/or sparingly soluble V(III) oxides/oxyhydroxide like (Formula presented) and (Formula presented) .