Ecosystem

Physics

Geology

Meteorology

Organic chemistry

Mathematical physics

Atmosphere (unit)

Biology

Atmospheric sciences

Anoxic waters

Oceanography

Paleontology

Geography

Chemistry

Ecology

Early Earth

Earth science

Earth (classical element)

Atmospheric oxygen

Tectonics

Environmental science

Oxygen

Astrobiology

Proterozoic

Proceedings of the National Academy of Sciences

Significance Earth’s transition from anoxic oceans and atmosphere to a well-oxygenated state led to major changes in nearly every surficial system. However, estimates of surface oxygen levels in the billion years preceding this shift span two orders of magnitude, suggesting a poor understanding of the evolution of the oxygen cycle. We use the isotopic record of iron oxides deposited in ancient shallow marine environments to show that oxygen remained at extremely low levels in the ocean–atmosphere system for most of Earth’s history, and that a rise in oxygen occurred in step with the expansion of complex, eukaryotic ecosystems. These results indicate that Earth is capable of stabilizing at low atmospheric oxygen levels, with important implications for exploration of exoplanet biosignatures.

Strong evidence for a weakly oxygenated ocean–atmosphere system during the Proterozoic