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

Abstract

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 δ 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.