A review of mass-dependent fractionation of selenium isotopes and implications for other heavy stable isotopes

Measurements of selenium (Se) isotope fractionation began about 1960, with work by Roy Krouse and Harry Thode on Se reduction. Se isotope research to date has confirmed these measurements, developed new measurement techniques, and investigated Se isotope fractionation during several other critical processes. As the geochemistry of Se is complex, current understanding of Se isotope systematics involves studies of many reactions, and synthesis of these results. Fractionation during dissimilatory Se reduction by pure bacterial cultures varies according to experimental conditions, but bacterial reduction in sediment slurries from three wetlands induces consistent ⁸⁰Se/⁷⁶Se fractionations of about 2.8‰ and 5.6‰ for Se(VI) reduction and Se(IV) reduction, respectively. Assimilation by higher plants, Se(IV) oxidation, sorption, and biological volatilization induce little or no isotopic fractionation. Preliminary experiments suggest that fractionation of about 1.0‰ accompanies assimilation of Se by algae. Thus, significant fractionation occurs in only a few reactions and if fractionation is observed in nature, it provides evidence that those reactions occur. Understanding of the systematics of other "heavy stable isotopes", including those of calcium, iron, copper, molybdenum, chromium, and zinc, is currently being developed. Mass-dependent fractionation of these and other elements should provide a new set of tools for low-temperature biogeochemical applications, such as detecting redox reactions or distinguishing between potential contaminant sources. Some general conclusions from the Se isotopes work can be extended to the other heavy stable isotopes to aid in developing them and assessing their likely geochemical applicability.