A switch from predominantly undersea volcanoes to a mix of undersea and terrestrial ones shifted the Earth's atmosphere from devoid of oxygen to one with free oxygen, according to geologists. "The rise of oxygen allowed for the evolution of complex oxygen-breathing life forms," says Lee R. Kump, professor of geoscience, Penn State.

Before 2.5 billion years ago, the Earth's atmosphere lacked oxygen. However, biomarkers in rocks 200 million years older than that period, show oxygen-producing cyanobacteria released oxygen at the same levels as today. The oxygen produced then, had to be going somewhere.

"The absence of oxidized soil profiles and red beds indicates that oxidative weathering rates were negligible during the Archaean," the researchers report in today's (Aug. 30) issue of Nature.

The ancient Earth should have had an oxygen atmosphere but something was converting, reducing, the oxygen and removing it from the atmosphere. The researchers suggest that submarine volcanoes, producing a reducing mixture of gases and lavas, effectively scrubbed oxygen from the atmosphere, binding it into oxygen containing minerals.

"The Archaean more than 2.5 billion years ago seemed to be dominated by submarine volcanoes," says Kump. "Subaerial andesite volcanoes on thickened continental crust seem to be almost absent in the Archaean."

About 2.5 billion years ago at the Archaean/Proterozoic boundary, when stabilized continental land masses arose and terrestrial volcanoes appeared, markers show that oxygen began appearing in the atmosphere.

Kump and Mark E. Barley, professor of geology, University of Western Australia, looked at the geologic record from the Archaean and the Palaeoproterozoic in search of the remains of volcanoes.

They found that the Archaean was nearly devoid of terrestrial volcanoes, but heavily populated by submarine volcanoes. The Palaeoproterozoic, however, had ample terrestrial volcanic activity along with continuing submarine vulcanism. Subaerial volcanoes arose after 2.5 billion years ago and did not strip oxygen from the air. Having a mix of volcanoes dominated by terrestrial volcanoes allowed oxygen to exist in the atmosphere.

Terrestrial volcanoes could become much more common in the Palaeoproterozoic because land masses stabilized and the current tectonic regime came into play.

The researchers looked at the ratio of submarine to subaerial volcanoes through time. Because submarine volcanoes erupt at lower temperatures than terrestrial volcanoes, they are more reducing. As long as the reducing ability of the submarine volcanoes was larger than the amounts of oxygen created, the atmosphere had no oxygen. When terrestrial volcanoes began to dominate, oxygen levels increased.

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The National Science Foundation, NASA Astrobiology Institute and the Australian Research Council supported this work.