Published online 25 July 2008 | Nature | doi:10.1038/news.2008.974

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Colliding continents may have oxygenated the Earth

Controversial theory proposes that tectonic activity fertilized cyanobacteria.

The clashing of supercontinents billions of years ago may have been responsible for the oxygen-rich atmosphere that sustains much of the life on Earth today.

PangeaComputer artwork showing the break-up of the supercontinent Pangea around 200 million years ago.CHRISTIAN DARKIN / SCIENCE PHOTO LIBRARY

That’s the controversial theory proposed in a paper published today in Nature Geosciences1. Geochemists at the Australian National University (ANU) in Canberra have suggested that as tectonic plates smashed into each other, reshaping supercontinents such as Pangaea, it set off a chain of events leading to increased oxygen in the atmosphere.

The Earth had almost negligible oxygen levels until the 'great oxidation event' roughly 2.5 billion years ago. Today, oxygen makes up about 21% of the atmosphere. Rising oxygen levels have been linked to the evolution of bilaterally symmetric animals, animals’ move onto land, and increased body size2.

Oxygen's origins

Cyanobacteria, the first organisms to emit oxygen, inhabited Earth at least 2.7 billion years ago, and geochemists have struggled to explain why oxygen levels remained low until 200 million years later. Most theories propose that the oxygen was being consumed, either by reacting with volcanic gases or by being trapped in the Earth through reactions with iron or sulphur. By around 2.5 billion years ago, such theories suggest, these oxygen sinks were full up, suddenly allowing the gas to accumulate in the atmosphere.

ANU’s Charlotte Allen and Ian Campbell propose instead that the trigger for oxygenation was the collision of tectonic plates. These collisions would push up land to form mountains — the same process that created the Himalayas, for example, when the Indian subcontinent met Eurasia. As those mountains eroded, it washed nutrient-rich sediments into the sea, which helped the oxygen-producing cyanobacteria to grow in vast numbers. At the same time, organic carbon fell to the ocean floor and was buried there. The extra oxygen, lacking the carbon it would normally react with to create carbon dioxide, remained in the atmosphere.

The scientists, whose main focus is not atmospheric sciences, had originally set out to understand continental growth by dating about 7,000 zircon crystals. Sifting through the scientific literature, they realised that more zircon crystals had been formed when several land masses were squashed together into a supercontinent; and that those time periods matched increases in atmospheric oxygen. “We had no inkling there would be a correlation with atmospheric oxygenation,” Allen says.

Successive rises

The researchers go on to link seven separate tectonic events to stepwise increases in atmospheric oxygen levels. “I think the possible connection between tectonic events and biogeochemistry and the rise of oxygen is intriguing, and will get people thinking,” says Ariel Anbar, a geochemist who studies the history of oxygen in Earth’s atmosphere at Arizona State University in Tempe.

However, others are not convinced. James Kasting, a geochemist at Pennsylvania State University in University Park who studies atmospheric evolution, says the theory is based on a faulty premise that organic carbon burial increases with time.

“I am in total disagreement with this paper with regards to the oxygen maximum on Pangaea,” adds Robert Berner, professor emeritus of geology at Yale University. “They have ignored the terrestrial carbon cycle, the rise of trees on the continents and the increased burial of resistant organic matter derived from wood.” 

  • References

    1. Campbell, I. H. & Allen, C. M. Nature Geosci. doi: 10.1038/ngeo259 (2008).
    2. Berner, R. A., VandenBrooks, J. M. & Ward, P. D. Science 316, 557–558 (2007).
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