1. Institute of Geography and Geology and Nordic Center for Earth Evolution (NordCEE), University of Copenhagen, Øster Voldgade 10, 1350 Copenhagen, Denmark
2. Departamento de Geología, Facultad de Ciencias, Iguá 4225, 11400 Montevideo, Uruguay
3. School of Civil Engineering and Geosciences, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK
4. Nordic Center for Earth Evolution (NordCEE) and Institute of Biology, University of Southern Denmark, Campusvej 55, 5230 Odense, Denmark

Correspondence to: Robert Frei¹ Correspondence and requests for materials should be addressed to R.F. (Email: robertf@geo.ku.dk).

Abstract

Geochemical data^{1, 2, 3, 4} suggest that oxygenation of the Earth's atmosphere occurred in two broad steps. The first rise in atmospheric oxygen is thought to have occurred between 2.45 and 2.2 Gyr ago^{1, 5}, leading to a significant increase in atmospheric oxygen concentrations and concomitant oxygenation of the shallow surface ocean. The second increase in atmospheric oxygen appears to have taken place in distinct stages during the late Neoproterozoic era (800–542 Myr ago)^{3, 4}, ultimately leading to oxygenation of the deep ocean 580 Myr ago³, but details of the evolution of atmospheric oxygenation remain uncertain. Here we use chromium (Cr) stable isotopes from banded iron formations (BIFs) to track the presence of Cr(VI) in Precambrian oceans, providing a time-resolved picture of the oxygenation history of the Earth's atmosphere–hydrosphere system. The geochemical behaviour of Cr is highly sensitive to the redox state of the surface environment because oxidative weathering processes produce the oxidized hexavalent [Cr(VI)] form. Oxidation of reduced trivalent [Cr(III)] chromium on land is accompanied by an isotopic fractionation, leading to enrichment of the mobile hexavalent form in the heavier isotope. Our fractionated Cr isotope data indicate the accumulation of Cr(VI) in ocean surface waters 2.8 to 2.6 Gyr ago and a likely transient elevation in atmospheric and surface ocean oxygenation before the first great rise of oxygen 2.45–2.2 Gyr ago (the Great Oxidation Event)^{1, 5}. In 1.88-Gyr-old BIFs we find that Cr isotopes are not fractionated, indicating a decline in atmospheric oxygen. Our findings suggest that the Great Oxidation Event did not lead to a unidirectional stepwise increase in atmospheric oxygen. In the late Neoproterozoic, we observe strong positive fractionations in Cr isotopes (⁵³Cr up to +4.9), providing independent support for increased surface oxygenation at that time, which may have stimulated rapid evolution of macroscopic multicellular life^{3, 4, 6}.

1. Institute of Geography and Geology and Nordic Center for Earth Evolution (NordCEE), University of Copenhagen, Øster Voldgade 10, 1350 Copenhagen, Denmark
2. Departamento de Geología, Facultad de Ciencias, Iguá 4225, 11400 Montevideo, Uruguay
3. School of Civil Engineering and Geosciences, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK
4. Nordic Center for Earth Evolution (NordCEE) and Institute of Biology, University of Southern Denmark, Campusvej 55, 5230 Odense, Denmark

Correspondence to: Robert Frei¹ Correspondence and requests for materials should be addressed to R.F. (Email: robertf@geo.ku.dk).

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