Wang, H., Li, C.#, Peng, Y#., Zhang, J., Cheng, M., Cao, X., Qie, W., Zhang, Z., Dodd, M.S., Hou, M., Wallace, M., v. S. Hood, A., Lyons, T.W., Bao, H., 2025. Two-billion-year transitional oxygenation of the Earth’s surface. Nature 645, 665-671.

ABSTRACT

Earth's surface underwent stepwise oxygenation before persistently reaching modern levels late in its history1, 2, 3, 4–5, but the details of this transition remain unclear5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15–16. Here we present a high-resolution 2.5-Gyr record of mass-independent oxygen isotopes in sedimentary sulfate (Δ'17Osulfate), a proxy linked to the atmospheric partial pressure of O2 (pO2)17, 18–19. This record, together with existing sedimentary Δ33S data20, 21–22, demonstrates a 2-Gyr transition characterized by generally low, fluctuating pO2 between an O2-free state before 2.4 billion years ago (Ga) and a modern pO2 state after 0.41 Ga, with relatively elevated levels after 1.0 Ga. Our data also show coupled declines in Δ'17Osulfate and sulfate-δ34S during major negative carbonate-δ13C excursions in the Neoproterozoic. Quantitative biogeochemical modelling indicates that these isotopic couplings reflect the increasing pO2, 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 pO2 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 cycles23,24 and the protracted, episodic rise of complex life25, 26–27.