Onset of coupled atmosphere–ocean oxygenation 2.3 billion years ago

The initial rise of molecular oxygen (O₂) shortly after the Archaean–Proterozoic transition 2.5 billion years ago was more complex than the single step-change once envisioned. Sulfur mass-independent fractionation records suggest that the rise of atmospheric O₂ was oscillatory, with multiple returns to an anoxic state until perhaps 2.2 billion years ago^1–3. Yet few constraints exist for contemporaneous marine oxygenation dynamics, precluding a holistic understanding of planetary oxygenation. Here we report thallium (Tl) isotope ratio and redox-sensitive element data for marine shales from the Transvaal Supergroup, South Africa. Synchronous with sulfur isotope evidence of atmospheric oxygenation in the same shales³, we found lower authigenic ²⁰⁵Tl/²⁰³Tl ratios indicative of widespread manganese oxide burial on an oxygenated seafloor and higher redox-sensitive element abundances consistent with expanded oxygenated waters. Both signatures disappear when the sulfur isotope data indicate a brief return to an anoxic atmospheric state. Our data connect recently identified atmospheric O₂ dynamics on early Earth with the marine realm, marking an important turning point in Earth’s redox history away from heterogeneous and highly localized ‘oasis’-style oxygenation.