Oxygenation of the mid-Proterozoic atmosphere: clues from chromium isotopes in carbonates

Chromium (Cr) isotopes in marine sedimentary rocks can be used as a sensitive proxy for ancient atmospheric oxygen because Cr-isotope fractionation during terrestrial weathering only occurs when pO 2 exceeds a threshold value. This is a useful system when applied to rocks of mid-Proterozoic age, where fundamental questions persist about atmospheric pO 2 and its relationship to biological innovation. Whereas previous studies have focused on temporally limited iron-rich sedimentary rocks, we present new Cr-isotope data from a suite of mid-Proterozoic marine carbonate rocks. Application of the Cr-isotope proxy to carbonate rocks has the potential to greatly enhance the temporal resolution of Proterozoic palaeoredox data. Here we report positive 53 Cr values in four carbonate successions, extending the mid-Proterozoic record of Cr-isotope fractionation -and thus pO 2 above threshold values -back to ~1.1 Ga. These data suggest that pO 2 sufficient for the origin of animals was transiently in place well before their Neoproterozoic appearance, although uncertainty in the pO 2 threshold required for Cr-isotope fractionation precludes definitive biological interpretation. This study provides a proof of concept that the Cr-isotopic composition of carbonate rocks can provide important new constraints on the oxygen content of the ancient atmosphere.