Uranium isotope fractionation by abiotic reductive precipitation

Abstract

Significant uranium (U) isotope fractionation has been observed during abiotic reduction of aqueous U, counter to the expectation that uranium isotopes are only fractionated by bioassociated enzymatic reduction. In our experiments, aqueous U is removed from solution by reductive precipitation onto the surfaces of synthetic iron monosulfide. The magnitude of uranium isotopic fractionation increases with decreasing aqueous U removal rate and with increasing amounts of neutrally charged aqueous Ca-U-CO species. Our discovery means that abiotic U isotope fractionation likely occurs in any reducing environment with aqueous Ca ≥ 1 mM, and that the magnitude of isotopic fractionation changes in response to changes in aqueous major ion concentrations that affect U speciation. Our results have implications for the study of anoxia in the ancient oceans and other environments.

Uranium isotopes in marine carbonates as a global ocean paleoredox proxy: A critical review

“…Uranium (U) isotopes (238U/235U, reported as δ238U) in marine carbonates have emerged as an important tool for determining the oxygenation state of ancient oceans. The basic principles underlying this proxy are that (1) U is a redox-sensitive element whose solubility is strongly controlled by dissolved oxygen levels (e.g., Tribovillard et al, 2006), and (2) redox reactions result in large and detectable isotope fractionations that are unrelated to radioactive decay of U (e.g., Weyer et al, 2008;Andersen et al, 2016;Brown et al, 2018). As a consequence, seawater U isotopic compositions reflect changes in marine redox conditions.…”

“…Uranium reduction in natural environments today appears to be predominately biotic (e.g., enzymatic activity of sulphate or other dissimilatory metal-reducing bacteria; Barnes and Cochran, 1993;Rademacher et al, 2006;Basu et al, 2014;Stylo et al, 2015), and the fractionation associated with this process is caused by the nuclear volume effect (NVE) (Bigeleisen, 1996;Nomura et al, 1996;Schauble, 2007), which favors the heavy 238U isotope in the reduced state. However, abiotic reduction also results in observable isotope fractionation (e.g., Stylo et al, 2015), which can be due to the NVE as well as more standard mass-dependent effects (Brown et al, 2018). The direction of isotope fractionation depends on the rate of U removal (i.e., of insoluble U(IV) from solution): fast removal is primarily controlled by kinetic effects (mass-dependent fractionation, fractionation factor α < 1, favoring the light isotope; however, we should note that kinetic fractionation is very small with α ~ 1 due to the large mass number of U), whereas slow removal leads to equilibrium (NVE, fractionation factor α> 1, favoring the heavy isotope) (Brown et al, 2018).…”

“…However, abiotic reduction also results in observable isotope fractionation (e.g., Stylo et al, 2015), which can be due to the NVE as well as more standard mass-dependent effects (Brown et al, 2018). The direction of isotope fractionation depends on the rate of U removal (i.e., of insoluble U(IV) from solution): fast removal is primarily controlled by kinetic effects (mass-dependent fractionation, fractionation factor α < 1, favoring the light isotope; however, we should note that kinetic fractionation is very small with α ~ 1 due to the large mass number of U), whereas slow removal leads to equilibrium (NVE, fractionation factor α> 1, favoring the heavy isotope) (Brown et al, 2018).…”

“…However, in most natural systems with relatively slow rates of U reduction, the impact of this effect may be minor. U isotope fractionation can also be influenced by aqueous speciation-for instance, aqueous Ca concentrations [e.g., which determines the abundance of the neutrally charged U species, Ca2UO2(CO3)3(aq), in the solution] can affect the fractionation factor associated with reduction (Brown et al, 2018). In general, the magnitude of U isotope fractionation increases with greater proportions of neutrally charged aqueous Ca-U-CO3 species as a function of seawater Ca/Mg ratio, carbonate chemistry, and pH.…”

A Cenozoic record of seawater uranium in fossil corals

“…Bigeleisen, 1996;Schauble, 2007). This isotope effect has been observed in experiments where uranium has been reduced both by biological and abiotic means (Basu et al 2014, Stylo et al 2015, Stirling et al 2015Brown et al 2018).Thus, reconstructions of seawater δ 238/235 U can track the relative importance of uranium removal by reduction relative to other sinks (e.g., Weyer et al, 2008;Montoya-Pino et al, 2010;Brennecka et al, 2011;Kendall et al, 2013).…”