Redox-independent chromium isotope fractionation induced by ligand-promoted dissolution

Abstract

The chromium (Cr) isotope system has emerged as a potential proxy for tracing the Earth’s atmospheric evolution based on a redox-dependent framework for Cr mobilization and isotope fractionation. Although studies have demonstrated that redox-independent pathways can also mobilize Cr, no quantitative constraints exist on the associated isotope fractionations. Here we survey the effects of common environmental ligands on the dissolution of Cr(III)-(oxy)hydroxide solids and associated Cr isotope fractionation. For a variety of organic acids and siderophores, δ53Cr values of dissolved Cr(III) are −0.27 to 1.23‰, within the range of previously observed Cr isotope signatures in rock records linked to Cr redox cycling. Thus, ligand-promoted dissolution of Cr-containing solids, a redox-independent process, must be taken into account when using sedimentary Cr isotope signatures to diagnose atmospheric oxygen levels. This work provides a step towards establishing a more robust framework for using Cr isotopes to track the evolution of the Earth’s atmosphere.

A systematic look at chromium isotopes in modern shells – implications for paleo-environmental reconstructions

“…A small extent of reduction would lead to low δ 53 Cr values, while quantitative reduction would lead to similar to seawater values (Wang et al, 2016). Direct incorporation of organic acid and/or siderophore-bound Cr(III), as recently proposed by Saad et al (2017) to have a significant impact on the Cr cycle via their release from the continents to the oceans, can also be considered to play a role in the marine biogenic calcification processes as these compounds have been shown to carry isotopically heavy Cr(III) compositions that are reached through redox-independent chromium isotope fractionation induced by ligand-promoted Cr(III) dissolution on land.…”

“…Such high D Cr values observed in biogenic carbonates produced by different marine organisms point to a strong biological control over the incorporation of Cr from seawater into CaCO 3 skeletons, where it could be incorporated either as Cr(III) and/or Cr(VI) depending on species-specific redox cycling of Cr (cf. Wang et al, 2016;Semeniuk et al, 2016) and/or, as recently suggested, directly assimilated as 4916 R. Frei et al: A systematic look at chromium isotopes in modern shells organic ligand-bound Cr during biological uptake (Saad et al, 2017). It is too premature to compare the biogenic distribution coefficients with abiogenic values, simply because there is a lack of suitable modern seawater-carbonate pairs from which such values could be calculated.…”

“…It is now known that oxidation of silicate-hosted and oxide-mineral-hosted Cr(III), potentially with the catalytic help of MnO 2 , to Cr(VI) is accompanied by an isotopic fractionation rendering the mobilized Cr(VI) isotopically heavier (Ellis et al, 2002;Zink et al, 2010;Døssing et al, 2011). Recently, an alternative, redox-independent pathway of Cr mobilization, through ligand-promoted dissolution of Cr-containing solids, was advocated by Saad et al (2017). This mobilization path is based on the ability of organic acids and siderophores to efficiently bind Cr(III) whereby respective ligand formation is accompanied by isotope fractionation effects, leading to Cr(III) being enriched in 53 Cr very much like in redox-dependent mobilization paths.…”

Cr isotopic insights into ca. 1.9 Ga oxidative weathering of the continents using the Beaverlodge Lake paleosol, Northwest Territories, Canada

“…Oxidation of Cr(III) by Mn oxides exhibits fractionations [Δ 53 Cr = δ 53 Cr(VI)–δ 53 Cr(III)] ranging between –0.5‰ and 1.0‰ (Bain & Bullen, ; Joshi, ; Joshi et al., ), but these results have not been evaluated in the peer‐reviewed literature. Nevertheless, the small fractionations reported for oxidation of Cr(III) to Cr(VI) are in the same range as redox‐independent Cr(III) fractionations (Babechuk et al., ; Saad et al., ), thus demonstrating the challenge of distinguishing redox‐dependent and redox‐independent controls on Cr isotope variability in the Earth's exogenic system.…”

“…Saad et al. () reported epsilon values for ligand‐promoted dissolution of amorphous Cr(OH) 3 ( ) ranging between ~ –0.3 and +1.3‰. Babechuk et al.…”