VANADIUM CONCENTRATIONS AND SPECIATION MODELLING ACROSS THE CHEMOCLINE FOR PETTAQUAMSCUTT RIVER ESTUARY, RHODE ISLAND

Vanadium occurs in four oxidation states in nature, (+2, +3, +4, +5), V (+2) being thermodynamically unstable in water. The redox sensitive nature of V makes it a potentially viable proxy for determining paleo redox conditions. However, the speciation of V is poorly understood, especially in sulfidic waters. The processes through which V is fixed in water column are redox dependent and controlled either by Fe-Mn shuttling and/or reduction in H₂S(aq.) hyper enriched euxinic environments, or a combination of both. Moreover, studies have reported that X-Ray adsorption spectroscopic (XAS) analysis of some sediments have yielded an average oxidation state of 3.5 which signifies the presence of V (+4) in addition with V (+3) in sediments. The uptake of V by organisms from surface waters for productivity, which gets deposited associated with porphyrins may turn out to be a process which dictates V enrichment in bottom sediments. To determine the provenance and thermodynamic conditions of the sedimentary ecosystem, it is essential to look at V speciation in modern euxinic environments. For this study, we hypothesize that V speciation in the water column dictates enrichment of V in the sediments. The Pettaquamscutt River estuary in Rhode Island is a modern, marine influenced, euxinic basin with a H₂S(aq.) concentration of ~ 4.5 mmol/kg. Water column samples were collected the euxinic Upper Basin of the estuary to evaluate the speciation of V in this unique environment. Vanadium concentrations vary with the dissolved oxygen and H₂S(aq.) concentrations and are positively correlated (R² = 0.75) with dissolved silica. Thermodynamic modelling of V speciation in the water column shows a predominance of V (+5) in the oxic surface waters, while V (+3) is predicted to dominate in the euxinic bottom waters. The modelling can help understand V speciation and biogeochemical cycling in euxinic waters but will require speciation measurements to verify the actual forms of dissolved V in such systems.