Was Earth’s oxygenation a gradual process?

By Richard A Lovett

Earth’s oxygen comes from plants and other photosynthetic organisms. Scientists have long known that. 

But an enormous puzzle has been why, over the course of two billion years, the amount of oxygen in the atmosphere appears to have risen in a series of three large steps, separated by long plateaus. 

“Photosynthesis, which produces all of the oxygen on Earth, has existed for three billion years,” says Benjamin Mills, a biogeochemical modeller at the University of Leeds in the UK. “Why did oxygen rise in steps rather than slowly accumulating?”

Over the years, he says, geophysicists have come up with a number of hypotheses, all involving major events of one kind or another, ranging from evolutionary changes to large volcanic eruptions or the breakup of supercontinents.

The problem, he adds, is that such events have also happened without triggering giant changes in the Earth’s oxygen levels. “We don’t have consistency,” he says. 

In an effort to figure this out, he and colleagues “stripped back to the simplest things” and attempted to create a model of Earth’s atmospheric evolution based on one of the simplest possible changes: a slow decline in the amount of oxygen-consuming gases released by volcanoes. {%recommended 6201%}

One would expect such a gradual change would merely cause oxygen to slowly build up in the atmosphere, but it turns out that this gradual change is all that’s needed to replicate its stepwise increase.

The key, Mills says, is understanding the way photosynthetic bacteria and algae react to nutrients in ocean water, especially phosphorus, and how their reactions affect oxygen levels.

Phosphorus is a critical ingredient in fertilisers and is widely recognised as one of the major limiting factors to plant growth. 

However, if too much gets into the water it causes a massive overgrowth of algae and photosynthetic bacteria. When these organisms die, the water becomes choked with decaying organic matter in an oxygen-depleting process known as eutrophication.

In today’s ecosystems, this is bad. 

But on the ancient Earth, it would have produced a string of complex feedbacks in which phosphorous sometimes rose, and sometimes returned to baseline. The result was a process he describes as the reverse of eutrophication, in which during low-phosphorous interludes, oxygen builds up and escapes to the atmosphere.

When this model is run over billion-year time frames, Mills says, it does a remarkable job of replicating not only short-term oscillations in the Earth’s oxygen levels, which sometimes occurred over time cycles of five to 20 million years, but of producing tipping points that cause the type of stepwise increase known to have occurred. 

The findings aren’t important just for understanding the rise of oxygen in Earth’s atmosphere, says Joshua Krissansen-Totton, a planetary scientist and astrobiologist at the University of Washington, Seattle, who was not a member of the study team. They also make it more likely that astronomers will find other oxygen-atmosphere planets, elsewhere.

That’s because the new model means a planet doesn’t need an unlikely string of geological “events” in order for oxygen to build up to levels that not only might support complex life, but which might be detectable by today’s astronomical instruments.

All that’s necessary, it appears, is the combination of photosynthetic organisms, and time. 

Mills reported his team’s work to this week’s meeting of the American Geophysical Union, and it also is described in a paper in the journal Science.