A 200-million-year delay in permanent atmospheric oxygenation.

The rise of atmospheric oxygen fundamentally changed the chemistry of surficial environments and the nature of Earth's habitability1. Early atmospheric oxygenation occurred over a protracted period of extreme climatic instability marked by multiple global glaciations2,3, with the initial rise of oxygen concentration to above 10-5 of the present atmospheric level constrained to about 2.43 billion years ago4,5. Subsequent fluctuations in atmospheric oxygen levels have, however, been reported to have occurred until about 2.32 billion years ago4, which represents the estimated timing of irreversible oxygenation of the atmosphere6,7. Here we report a high-resolution reconstruction of atmospheric and local oceanic redox conditions across the final two glaciations of the early Palaeoproterozoic era, as documented by marine sediments from the Transvaal Supergroup, South Africa. Using multiple sulfur isotope and iron-sulfur-carbon systematics, we demonstrate continued oscillations in atmospheric oxygen levels after about 2.32 billion years ago that are linked to major perturbations in ocean redox chemistry and climate. Oxygen levels thus fluctuated across the threshold of 10-5 of the present atmospheric level for about 200 million years, with permanent atmospheric oxygenation finally arriving with the Lomagundi carbon isotope excursion at about 2.22 billion years ago, some 100 million years later than currently estimated.