For most of Earth's history there was nothing to breathe. Oxygen reached the air in three uneven steps, and the deep ocean lagged behind each one — in the last case by some 600 million years. That is the argument two geochemists at Chengdu University of Technology make in The Innovation , from a proxy that tracks the atmosphere rather than the sea.
Older narratives read ocean chemistry from the rock record as a stand-in for the air. But ocean redox also answers to nutrients, productivity, circulation and basin shape. Sulfate offers a cleaner line. Atmospheric oxygen carries a characteristic deficit in oxygen-17, manufactured by O2–O3–CO2 photochemistry, and that fingerprint passes into sulfate when sulfide minerals weather. Sedimentary sulfate therefore archives the atmosphere.
On that record the first rise, the Great Oxidation Event, still sits at 2.4–2.1 billion years ago, lifting oxygen from below one part in a million of the modern level to somewhere between a thousandth and a tenth. The second and third come earlier than the textbooks put them: 1.3–1.0 billion years ago, and 0.44–0.41 billion. The classic Neoproterozoic and Paleozoic oxygenation events, at 0.8–0.5 and 0.42–0.36 billion years, are then recast as the ocean's lagging replies — roughly 200 and 30 million years behind the air.
In between, oxygen stayed scarce. Modelling calibrated to the isotope data puts the mid-Proterozoic atmosphere at 0.1–1% of modern. Charcoal, which needs about three-quarters of the modern level to burn, appears only around 420 million years ago.
The two reservoirs are coupled, but not simply. Across a billion years they climb together; across a million they can part. A pulse of oxygen into the ocean oxidises stored organic carbon and sulfide, consuming the gas and pulling atmospheric levels down. Many spikes once read as rising oxygen may mark its retreat — arrows that should point down, not up.
Ocean oxygenation is patchy. Two regimes coexist: oxygen made on the spot by photosynthetic microbes, which opened oases from 3 billion years ago or earlier, and oxygen delivered from the air. The first dominates the Archean; the second takes over as the atmosphere thickens, and a fully oxygenated interior does not arrive until about 0.41 billion years ago.
Life tracked those conditions without being simply triggered by them. Under anoxic air, oxygen was a hard limit. Through the Proterozoic it became a permissive one, opening niches while animals evolved under fluctuating levels. Only after the third rise did oxygen actively amplify complexity — larger bodies, active predation, deeper habitats.
The authors are candid about the limits: isotope resolution is coarse, absolute levels after 1.0 billion years ago remain poorly constrained, and further steps in the long transition may be invisible. What the review changes is the order of events. The ocean did not lead the atmosphere into an oxygenated world; it followed, slowly, and the delay is where the biology happened.
The Innovation
Literature review
Not applicable
Stagewise oxygenation of the Earth's atmosphere and oceans
8-Sep-2026
The authors declare no competing interests.