Recycled phosphorus helped hold oxygen up after the Great Oxidation Event, South African rocks suggest

Until now, geochemists could measure only the total amount of phosphorus preserved in an ancient rock ^[8]. That total mixes phosphorus organisms could take up with phosphorus bound into minerals that life could not use ^[7]. The technique applied to the South African samples dissolves one type of mineral at a time, so each fraction can be assigned to one pool or the other ^[7]. "We can now separate the phosphorus that was available to organisms from phosphorus that was essentially locked away," Bekker said ^[9].

With the usable fraction measured, the team proposes a chain of cause and effect ^[6]. Oxygen entering the oceans raised sulfate concentrations. Microbes used the sulfate to break down organic matter more efficiently and released phosphorus back into seawater. That phosphorus fed new growth, more organic carbon was buried, and more oxygen accumulated in the atmosphere ^[6]. "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." ^[5]

As phys.org reports it, the cycle "helped keep oxygen levels high" after the first rise ^[4]. The claim concerns one contributing feedback, and that feedback addresses a question the Great Oxidation Event left open: how oxygen stayed high enough, over millions of years, to support more complex life ^[14].

The same findings suggest oxygen fluctuated more dramatically after the event than scientists had believed, with swings that likely reshaped ocean chemistry over tens of millions of years ^[10]. In my view the two results are compatible. A self-reinforcing loop amplifies a push in either direction, so it can hold oxygen high for a stretch and deepen a drop once one starts. The release itself runs the chain in reverse for the modern ocean: where climate change strips oxygen, phosphorus could become less available and marine productivity could fall ^[12].

The phosphorus measurements come from rocks in South Africa ^[7], so it is unclear how far one region's sediments speak for a whole ocean. The link from those sediments to atmospheric oxygen runs through the sulfate and carbon-burial steps of the proposed loop ^[6]. The phys.org account does not say how many samples were analysed, which formations they came from, or how large the oxygen swings were.

Bekker said that if oxygen remained abundant for extended periods, factors other than a severely limited oxygen supply may have slowed the emergence of complex organisms that depend on oxygen ^[11]. The larger swings reported in the same study make the length of those high-oxygen periods the open question ^[10].

"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." ^[13]