Earth's oxygen levels rose and fell more than once hundreds of millions of years before the planet-wide success of the Great Oxidation Event about 2.4 billion years ago, a study has found.
The finding, published in the journal Proceedings of the National Academy of Sciences, may have implications in the search for life beyond Earth.
Coming years will bring powerful new ground- and space-based telescopes able to analyse the atmospheres of distant planets, the researchers said.
The work could help keep astronomers from unduly ruling out "false negatives," or inhabited planets that may not at first appear to be so due to undetectable oxygen levels, they said.
"The production and destruction of oxygen in the ocean and atmosphere over time was a war with no evidence of a clear winner, until the Great Oxidation Event," said Matt Koehler, a doctoral student at the University of Washington in the US.
The Great Oxidation Event was the biologically induced appearance of oxygen in the Earth's atmosphere around 2.45 billion years ago.
"These transient oxygenation events were battles in the war, when the balance tipped more in favour of oxygenation," said Koehler.
The researchers confirmed the appearance of oxygen in Earth's past, roughly 150 million years earlier - or about 2.66 billion years ago - and lasting for less than 50 million years.
They used two different proxies for oxygen - nitrogen isotopes and the element selenium - substances that, each in its way, also tell of the presence of oxygen.
"Nitrogen isotopes tell a story about oxygenation of the surface ocean, and this oxygenation spans hundreds of kilometers across a marine basin and lasts for somewhere less than 50 million years," said Koehler.
The researchers drilled two cores about 300 kilometers apart but through the same sedimentary rocks - one core samples sediments deposited in shallower waters, and the other samples sediments from deeper waters.
Analysing successive layers in the rocks shows, Buick said, a "stepwise" change in nitrogen isotopes "and then back again to zero.
"This can only be interpreted as meaning that there is oxygen in the environment. It is really cool - and it is sudden," Buick said.
The nitrogen isotopes reveal the activity of certain marine microorganisms that use oxygen to form nitrate, and other microorganisms that use this nitrate for energy.
The data collected from nitrogen isotopes sample the surface of the ocean, while selenium suggests oxygen in the air of ancient Earth. Koehler said the deep ocean was likely anoxic, or without oxygen, at the time.
The team found plentiful selenium in the shallow hole only, meaning that it came from the nearby land, not making it to deeper water.
Selenium is held in sulphur minerals on land; higher atmospheric oxygen would cause more selenium to be leached from the land through oxidative weathering - "the rusting of rocks," and transported to sea, Buick said.
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