Chromium isotope fractionation during adsorption of chromium(III) by soils and river sediments

Chromium (Cr) isotope compositions of sedimentary rocks have been widely used to unravel fluctuations in atmospheric oxygen levels during geologic history. A fundamental framework of this application is that any Cr isotope fractionation in natural environments should be related to the redox transformation of Cr species Cr(VI) and Cr(III). However, the behavior of Cr isotopes during non-redox Cr cycling is not yet well understood. Here, we present laboratory experimental results which show that redox-independent adsorption of Cr(III) by natural river sediments and soils can be accompanied by obvious Cr isotope fractionation. The observed Cr isotope fractionation factors (−0.06‰ – −0.95‰, expressed as 10³lnα) are much smaller than those caused by redox processes. Combined with previous studies on redox-independent Cr isotope fractionation induced by ligand-promoted dissolution, we suggest that the systematic shift to highly fractionated Cr isotope compositions of sedimentary rocks is likely to represent atmospheric oxygenation, but muted signals observed in some geologic periods may be attributed to non-redox Cr cycling and should be interpreted with caution.