Selenium isotope analysis of organic-rich shales: advances in sample preparation and isobaric interference correction

Abstract

Selenium has a complex biogeochemical cycle with many similarities to that of sulfur. Fractionations of selenium isotopes induced in bacterial respiration reactions have been recognized as potential recorders of environmental oxidation states, opening up the possibility of further constraining bioproductivity and the redox evolution of the Earth's atmosphere and ocean over time. However, analyses of selenium isotopes are complicated by interferences with organic compounds as well as a number of isobaric interferences. Application of existing sample preparation protocols to geological samples replete with organic carbon can result in incomplete extraction yields and incomplete removal of isobarically interfering germanium and arsenic. We present measurements of three different organic-rich shales of varying ages prepared with eight different sample preparation protocols and identify a method with which high selenium yields are obtained for all three samples while the concentration of germanium is greatly reduced. We further investigate the quantitative importance of isobaric interferences and present new post-analytical data correction protocols. If selenium concentrations in standards and samples are matched to within 5%, the ratios of five isotopes of selenium (74Se, 76Se, 77Se, 78Se and 82Se) can be measured with precisions better than 0.2‰ for δ76/78Se, δ77/78Se and δ82/78Se and 0.5‰ for δ74/78Se, allowing analytical accuracy to be monitored with three-isotope diagrams and thus enabling the detection of any mass-independent isotopic fractionation.