Ancient South African rocks reveal a self-sustaining loop of sulfate, phosphorus and life that kept the atmosphere oxygenated after the Great Oxidation Event.
About 2.3 billion years ago, oxygen became a lasting part of Earth’s atmosphere in what geologists call the Great Oxidation Event. Getting oxygen into the air was one challenge. Keeping it there was another. A new study led by researchers at the University of California, Riverside suggests the answer lay in the oceans, in a feedback loop built on an element life cannot do without.
“Living things cannot grow or function properly without phosphorus,” said Andrey Bekker, a UC Riverside geologist and member of the research team. The study, published in Nature Communications, argues that changing ocean chemistry made more of that phosphorus available to life, and that life in turn kept oxygen flowing into the atmosphere.
A self-sustaining cycle
The proposed loop runs like this. As oxygen entered the oceans, sulfate levels in seawater rose. Microbes used that sulfate to break down organic matter more efficiently, which released phosphorus that had been tied up in dead material back into the water. The freed phosphorus fueled more biological productivity, which buried more organic carbon in sediments, and burying organic carbon leaves oxygen behind in the atmosphere.
“Once more of it became available in the oceans, it allowed more organic carbon to be buried,” Bekker said. “A side effect of that process is that more oxygen continued to be released into the atmosphere.”
Reading phosphorus mineral by mineral
Testing the idea required a new way of reading ancient rocks. Until now, scientists could measure only the total amount of phosphorus preserved in a rock, which says little about how much of it living things could actually use. The team analyzed ancient rocks from South Africa with a technique that dissolves one type of mineral at a time, sorting phosphorus by the mineral it is bound to.
“We can now separate the phosphorus that was available to organisms from phosphorus that was essentially locked away,” Bekker said. “That gives us a much clearer picture of nutrient levels in ancient oceans than we had before.”
The results also suggest that oxygen levels after the Great Oxidation Event swung more sharply than previously thought, reshaping ocean chemistry over tens of millions of years. If oxygen was abundant for long stretches, the authors note, other environmental or biological factors may have been responsible for the long delay before complex life appeared.
Lessons for today and for other worlds
The findings carry a modern warning. Climate change is causing parts of today’s oceans to lose oxygen. If that loss continues, the researchers suggest, the same coupling could run in reverse, making phosphorus less available and reducing the productivity and resilience of marine ecosystems.
The work may also help astrobiologists recognize similar conditions on ocean-bearing worlds beyond Earth.
“Earth’s history shows that oxygen, nutrients, and life evolved together,” Bekker said. “Understanding those connections gives us a more nuanced perspective on our own planet’s future and what we might look for on other planets.”
Notes
- “A nutrient control on oxygenation dynamics during Earth’s Great Oxidation Episode.” Nature Communications, 27 August 2026. DOI: 10.1038/s41467-026-76597-y.
- University of California, Riverside. “How ocean chemistry helped life keep breathing.” EurekAlert!, 29 September 2026. https://www.eurekalert.org/news-releases/1145902.
Featured image: Cyanobacteria (Scientific Inquirer media library).