Two-billion-year transitional oxygenation of the Earth's surface.

Earth's surface underwent stepwise oxygenation before persistently reaching modern levels late in its history, but the details of this transition remain unclear. Here we present a high-resolution 2.5-Gyr record of mass-independent oxygen isotopes in sedimentary sulfate (Δ'O), a proxy linked to the atmospheric partial pressure of O ( ). This record, together with existing sedimentary ΔS data, demonstrates a 2-Gyr transition characterized by generally low, fluctuating between an O-free state before 2.4 billion years ago (Ga) and a modern state after 0.41 Ga, with relatively elevated levels after 1.0 Ga. Our data also show coupled declines in Δ'O and sulfate-δS during major negative carbonate-δC excursions in the Neoproterozoic. Quantitative biogeochemical modelling indicates that these isotopic couplings reflect the increasing , which may have driven episodic ocean oxygenation through an increased atmospheric O influx. This process intensified the oxidation of marine organics and reduced-sulfur species, while triggering temporary drawdowns as negative feedback. These findings support a dynamic, lengthy co-oxygenation history for the atmosphere and oceans-marked by long-term positive coupling and short-term negative feedbacks-offering a coherent explanation for the anomalous Neoproterozoic carbon cycles and the protracted, episodic rise of complex life.