More than 2 billion years ago, Earth underwent a dramatic transformation as oxygen began building up in its atmosphere. But that change left scientists with a major mystery: How did the planet maintain enough oxygen to remain habitable over millions of years?
Researchers have now uncovered evidence that the answer may lie in an ancient recycling system within Earth's oceans. By studying rocks from South Africa, they found that changes in ocean chemistry helped create a natural cycle in which biological activity supported the continued accumulation of atmospheric oxygen.
The research, published in Nature Communications , involved scientists including University of California, Riverside geologist Andrey Bekker. Their findings suggest that phosphorus, a nutrient essential for life, played a central role in sustaining the oxygen levels that would eventually help make more complex organisms possible.
How Ancient Oceans Helped Sustain Earth's Oxygen
About 2.3 billion years ago, Earth experienced the Great Oxidation Event, a major turning point when oxygen became a lasting component of the atmosphere. Scientists have understood this transition for decades, but explaining how oxygen remained sufficiently abundant afterward has been much more difficult.
The researchers identified a feedback loop that may help explain what happened. As oxygen spread into the oceans, it altered the chemistry of seawater and increased concentrations of sulfate, a compound containing sulfur and oxygen.
Certain microbes used this sulfate to break down organic material more efficiently. In the process, they released phosphorus back into the water, making this vital nutrient available to other organisms.
With more phosphorus available, biological productivity increased. That meant more organic carbon could eventually become buried in seafloor sediments rather than being broken down. Because the decomposition of organic matter can consume oxygen, burying more carbon helped favor the continued buildup of oxygen in the atmosphere.
This created a reinforcing cycle in which rising oxygen levels encouraged chemical changes that supported biological growth, which in turn helped maintain atmospheric oxygen.
"Living things cannot grow or function properly without phosphorus," Bekker said. "Once more of it became available in the oceans, it allowed more organic carbon to be buried. A side effect of that process is that more oxygen continued to be released into the atmosphere."
Ancient Rocks Reveal a Hidden Phosphorus Cycle
To investigate this process, the scientists examined ancient rock samples from South Africa using a technique that provides a more detailed picture of phosphorus availability in prehistoric oceans.
Earlier research could determine how much phosphorus was present in ancient rocks, but those measurements revealed little about whether organisms could actually use it. Some phosphorus becomes trapped in minerals, making it effectively inaccessible to living things.
The researchers addressed this limitation by separating phosphorus according to the minerals that contained it. They accomplished this by dissolving different mineral components one at a time, allowing them to distinguish phosphorus that had been available for biological activity from phosphorus locked away in mineral structures.
This distinction is crucial because the total amount of phosphorus in an ocean does not necessarily indicate how much is available to sustain life. The new approac…