Gilleaudeau, G.J.Frei, R.Kaufman, A.J.Kah, L.C.Azmy, K.Bartley, J.K.Chernyavskiy, P.Knoll, Andrew2017-09-292016Gilleaudeau, G.J., R. Frei, A.J. Kaufman, L.C. Kah, K. Azmy, J.K. Bartley, P. Chernyavskiy, and A.H. Knoll. 2016. “Oxygenation of the Mid-Proterozoic Atmosphere: Clues from Chromium Isotopes in Carbonates.” Geochemical Perspectives Letters: 178–187. doi:10.7185/geochemlet.1618.2410-339Xhttp://nrs.harvard.edu/urn-3:HUL.InstRepos:33973839Chromium (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 pO2 exceeds a threshold value. This is a useful system when applied to rocks of mid-Proterozoic age, where fundamental questions persist about atmospheric pO2 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 palaeo-redox data. Here we report positive δ53Cr values in four carbonate successions, extending the mid-Proterozoic record of Cr-isotope fractionation – and thus pO2 above threshold values – back to ~1.1 Ga. These data suggest that pO2 sufficient for the origin of animals was transiently in place well before their Neoproterozoic appearance, although uncertainty in the pO2 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.en-USJournal Article2017-09-2910.7185/geochemlet.1618