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^1-5, but the details of this transition remain unclear^5-16. Here we present a high-resolution 2.5-Gyr record of mass-independent oxygen isotopes in sedimentary sulfate (Δ'¹⁷O_sulfate), a proxy linked to the atmospheric partial pressure of O₂ ( $p_{O_2}$ )^17-19. This record, together with existing sedimentary Δ³³S data^20-22, demonstrates a 2-Gyr transition characterized by generally low, fluctuating $p_{O_2}$ between an O₂-free state before 2.4 billion years ago (Ga) and a modern $p_{O_2}$ state after 0.41 Ga, with relatively elevated levels after 1.0 Ga. Our data also show coupled declines in Δ'¹⁷O_sulfate and sulfate-δ³⁴S during major negative carbonate-δ¹³C excursions in the Neoproterozoic. Quantitative biogeochemical modelling indicates that these isotopic couplings reflect the increasing $p_{O_2}$ , which may have driven episodic ocean oxygenation through an increased atmospheric O₂ influx. This process intensified the oxidation of marine organics and reduced-sulfur species, while triggering temporary $p_{O_2}$ drawdowns as negative feedback¹⁵. 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^23,24 and the protracted, episodic rise of complex life^25-27.