Uranium reduction in modern and ancient marine carbonate settings – insights from anaerobic U extractions and high-energy resolution X ray spectroscopy

Abstract

In the marine environment, hexavalent uranium, U6+, is incorporated into primary carbonate minerals with the same isotopic composition (δ238U) as the seawater in which they are formed. Yet, modern marine carbonate sediments carry heavier U isotope compositions. This enrichment of heavy U isotopes has been linked to biogenic U reduction in and below the Fe-reducing zone inside the sediment. Still, the oxidation state(s) of uranium in marine carbonate sediments undergoing syndepositional diagenesis has never been measured before. Here, we 1) present an anaerobic ion chromatographic technique based on the TEVA® resin to chemically separate and quantify abundances of tetravalent U4+ and hexavalent U6+ fractions in the carbonate, and 2) compare the results from ion chromatography to U L3 edge HERFD-XANES spectroscopic measurements of the total U in sediments to 3) estimate U oxidation states of fresh carbonate sediments from a modern seawater-fed lake and ancient limestones. We find that our anaerobic extraction technique can provide credible evaluations of reduced U4+ and oxidized U6+ contents, applicable to carbonate sediments and rocks. Our results show that U resides both in reduced and oxidized states in modern carbonate sediments and ancient carbonate rocks. By comparing air-exposed, oven-dried samples to samples always kept under strictly anaerobic condition, we find that the majority of authigenic U in modern carbonate sediments resides in oxidation-sensitive phases that accumulate with sediment depth, instead of being structurally bound in carbonate minerals (aragonite and calcite) We propose a model to account for the observed trends in U oxidation state, U phase associations, and U isotope fractionation, where a substantial fraction of U in the sediments is likely delivered via microbial reduction and precipitated as a non-crystalline, reduced form near the sediment–water interface. We suggest these oxidation-sensitive reduced U species participate in redox cycling where some U is re-oxidized and perhaps bio-reduced again later, for example in the presence of Fe(III) mineral surfaces that undergo reductive dissolution with depth. Simultaneously, a continued incorporation of recalcitrant and isotopically light (i.e. 238U-depleted) U from the pore fluids into diagenetic carbonate may occur. The determination of U oxidation states in modern carbonates in this study helps to bridge a gap in our knowledge of how U isotope signals are affected by syn-sedimentary diagenetic U transformations, opening new avenues for understanding sedimentary U cycling and improving the δ238U paleo redox proxy.