Absence of hexavalent chromium in marine carbonates: implications for chromium isotopes as paleoenvironment proxy

The oxygenation of Earth's atmosphere is widely regarded to have played an important role in early-life evolution. Chromium (Cr) isotopes recorded in sedimentary rocks have been used to constrain the atmospheric oxygen level (AOL) over geological times based on the fact that a positive Cr isotopic signature is linked to the presence of Cr(VI) as a result of oxidative continental weathering. However, there is no direct evidence of the presence of Cr(VI) in sedimentary rocks yet. Carbonates are most widely distributed over geological times and were thought to have incorporated Cr(VI) directly from seawater. Here, we present results of Cr valence states in carbonates which show Cr(III) is the dominant species in all samples spanning a wide range of geological times. These findings indicate that Cr(VI) in seawater was reduced either before or after carbonate precipitation, which might have caused Cr isotopic fractionation between seawater and carbonates, or marine carbonates preferentially uptake Cr(III) from seawater. As Cr(III) can come from non-redox Cr cycling, which also can cause isotopic fractionation, we suggest that positively fractionated Cr isotopic values do not necessarily correspond to the rise in AOL.