A 200-Million-Year Delay in Permanent Atmospheric Oxygenation

Abstract

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 to above 10-5 of the present atmospheric level (PAL) constrained to ~2.43 billion years ago (Ga)4,5. Subsequent fluctuations in atmospheric oxygen levels have, however, been reported until ~2.32 Ga4, which currently denotes the perceived 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 Paleoproterozoic Era, as documented by marine sediments from the Transvaal Supergroup, South Africa. Based on multiple sulfur isotope and Fe-S-C systematics, we demonstrate continued oscillations in atmospheric oxygen levels after ~2.32 Ga, linked to major perturbations in ocean redox chemistry and climate. Oxygen levels thus fluctuated across the 10-5 PAL threshold for ~200 million years, with permanent atmospheric oxygenation finally arriving prior to the onset of the Lomagundi carbon isotope excursion at ~2.2 ± X Ga, some 100 million years later than currently estimated.