Crowe, Sean2; Døssing, Lasse Nørbye5; Beukes, Nicolas J.6; Bau, Michael4; Kruger, Stephanaus J6; Frei, Robert5; Canfield, Donald Eugene2
1 Geology, Department of Geosciences and Natural Resource Management, Faculty of Science, Københavns Universitet2 Nordisk Center for Jordens Udvikling (NordCEE)3 University of Johannesburg4 Jacobs University Bremen5 Geology, Department of Geosciences and Natural Resource Management, Faculty of Science, Københavns Universitet6 University of Johannesburg
It is widely assumedthat atmospheric oxygen concentrations remained persistently low (less than 1025 timespresent levels) for about the first 2 billion years of Earth’s history1. The first long-term oxygenation of the atmosphere is thought tohave taken place around2.3 billion years ago, during the GreatOxidation Event2,3.Geochemical indications of transient atmospheric oxygenation, however, date back to 2.6–2.7 billion years ago4–6. Here we examine the distribution of chromium isotopes and redox-sensitive metals in the approximately 3-billionyear- old Nsuze palaeosol and in the near-contemporaneous Ijzermyn iron formation fromthe Pongola Supergroup, South Africa.We find extensivemobilization of redox-sensitive elements through oxidative weathering. Furthermore, using our data we compute a best minimumestimate for atmospheric oxygen concentrations at that time of 331024 times present levels.Overall, our findings suggest that there were appreciable levels of atmospheric oxygen about 3 billion years ago, more than 600 million years before the Great Oxidation Event and some 300–400million years earlier than previous indications for Earth surface oxygenation.