Ancient oceans cycled between oxygen-rich and oxygen-poor states, research shows

A research team led by Professor Chen Zhongqiang from the China University of Geosciences (Wuhan) in Hubei province has identified periodic oxygenation events in Earth's oceans dating back roughly 580 million years, offering new insight into environmental conditions preceding the rise of early complex life.

The study, published in Nature Geoscience, was conducted in collaboration with researchers from the University of Exeter, the United Kingdom, and Nanjing University. Using numerical modeling and geochemical evidence, the team examined environmental changes during the mid-Ediacaran period, a critical interval in Earth's evolutionary history.

By applying a "self-sustaining oscillation" model, the team reconstructed interactions among phosphorus, oxygen, and carbon in ancient oceans. Their results suggest that Earth's environment alternated between oxygen-rich and oxygen-poor conditions approximately every 5 million years. These oscillations appear to have occurred at least three times within about 20 million years and coincide with the Gaskiers Glaciation interval, dated to around 579 million years ago.

The researchers describe the process as a seesaw-like mechanism. During periods of higher oxygen levels, phosphorus was trapped in seafloor sediments, suppressing marine productivity. As oxygen levels later declined, phosphorus was released back into the ocean, fueling for the next biological boom and a surge in oxygen production.

To support their model, the team conducted carbon and uranium isotope analyses on carbonate samples extracted from the Egan Formation in northwestern Australia. The isotopic patterns revealed a close relationship between marine productivity and changes in ocean oxidation conditions.

According to the study, these periodic oxygenation pulses align with the emergence and flourishing of some of the earliest known complex multicellular life forms, including the Lantian and Weng'an biotas found in China. The findings suggest that repeated environmental oscillations, rather than a slow and steady rise in oxygen levels, may have played a key role in driving early biological complexity.

Professor Chen said the modeling approach provides a broader framework for understanding large-scale changes in Earth's systems and could help scientists investigate other major transitions in the planet's history.

Liu Xueru contributed to this story.