Stable Isotopes of Cr and Se as Tracers of Redox Processes in Earth Surface Environments

Abstract

Redox reactions play a central role in the environmental geochemistry of chromium (Cr) and selenium (Se). A small but growing body of research shows that the stable isotope abundances of both elements are altered by these redox reactions. As is observed with nitrate and sulfate, reduction of the higher valence oxoanions to lower valence forms induces isotopic fractionation, with the reaction product enriched in lighter isotopes. The magnitudes of isotope ratio shifts range up to 4.5‰ for 53Cr/52Cr during Cr(VI) reduction, and up to 18‰ for 82Se/76Se during Se(VI) or Se(IV) reduction, but they vary with reaction mechanism and can be much smaller for environmentally relevant conditions. Other chemical processes generally produce lesser fractionation. Accordingly, Cr and Se isotope ratios have potential as indicators of reduction. Application of Cr isotope data as a quantitative indicator of natural reduction of hexavalent Cr in groundwater appears to be successful. Application of Se isotope data to constrain selenate reduction in wetlands has not been successful because effective fractionations induced by redox reactions are small, but recent studies suggest that reduction in groundwater produces significant fractionation and may be detected using Se isotope measurements.