Chromium isotope fractionation in ferruginous sediments

Abstract Ferrous Fe is a potent reductant of Cr(VI), and while a number of laboratory studies have characterized Cr isotope fractionation associated with Cr(VI) reduction by ferrous iron, the expression of this fractionation in real-world ferrous Fe-rich environments remains unconstrained. Here we determine the isotope fractionation associated with Cr(VI) reduction in modern ferrous Fe-rich sediments obtained from the previously well studied Lake Matano, Indonesia. Whole core incubations demonstrate that reduction of Cr(VI) within ferruginous sediments provides a sink for Cr(VI) leading to Cr(VI) concentration gradients and diffusive Cr(VI) fluxes across the sediment water interface. As reduction proceeded, Cr(VI) remaining in the overlying lake water became progressively enriched in the heavy isotope ( 53 Cr), increasing δ 53 Cr by 2.0 ± 0.1‰ at the end of the incubation. Rayleigh distillation modelling of the evolution of Cr isotope ratios and Cr(VI) concentrations in the overlying water yields an effective isotope fractionation of e eff = 1.1 ± 0.2‰ ( 53 Cr/ 52 Cr), whereas more detailed diagenetic modelling implies an intrinsic isotope fractionation of e int = 1.80 ± 0.04‰. Parallel slurry experiments performed using anoxic ferruginous sediment yield an intrinsic isotope fractionation of e int = 2.2 ± 0.1‰. These modelled isotope fractionations are corroborated by direct measurement of the δ 53 Cr composition on the upper 0.5 cm of Lake Matano sediment, revealing an isotopic offset from the lake water of Δ 53 Cr = 0.21–1.81‰. The data and models reveal that effective isotope fractionations depend on the depth at which Cr(VI) reduction takes place below the sediment water interface—the deeper the oxic non-reactive zone, the smaller the effective fractionation relative to the intrinsic fractionation. Based on the geochemistry of the sediment we suggest the electron donors responsible for reduction are a combination of dissolved Fe(II) and 0.5 M HCl extractable (solid phase) Fe(II). Our results are in line with the range of intrinsic fractionation factors observed for such phases in previous laboratory studies. We suggest that intrinsic isotope fractionations of around 1.8‰, may be broadly characteristic of ferruginous environments, but we note that the partitioning of ferrous Fe between dissolved and solid phases may modulate this value. These results indicate that seawater δ 53 Cr is only captured with high-fidelity by ferruginous sediments when oxygen penetration, and therefore the upper boundary of the zone of Cr(VI) reduction, extends to more than 10 cm below the sediment-water-interface, as can be the case in sediments deposited below oligotrophic waters. In more productive regions, with thinner oxic zones, ferruginous sediments would record δ 53 Cr as much as 1.8‰ lower than seawater δ 53 Cr. This implies that a range of sediment δ 53 Cr compositions, that include that of the igneous silicate earth (ISE), are possible even when seawater is isotopically heavier than the ISE.