Chemical research

Rise and fall of oxygen

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The Jeerinah Formation in Western Australia, where researchers found a sudden shift in nitrogen isotopes. Photo: Roger Buick

The 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 (GOE) about 2.4 billion years ago, according to University of Washington (U.W.) researchers.

“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,” says Matt Koehler, the lead author of a new paper published in the recent issue of “Proceedings of the National Academy of Sciences” (PNAS).

In 2007, co-author Roger Buick was part of an international team of scientists that found evidence of an episode, a “whiff”, of oxygen some 50 million to 100 million years before the GOE. Analysis of samples obtained by deep-drilling into sedimentary rocks of Mount McRae Shale in Western Australia led to this conclusion. The team looked for the trace metals molybdenum and rhenium, accumulation of which depends on oxygen in the environment.

This new study found evidence of a second and much earlier “whiff” of oxygen—in the atmosphere and on the surface of a large stretch of ocean—showing that the oxygenation of the earth was a complex process of repeated trying and failing over a vast stretch of time.

According to the research group, led by Koehler, this second episode in the earth’s past happened roughly 150 million years earlier, or about 2.66 billion years ago, and lasted less than 50 million years. They have determined this by analysing two different proxies for oxygen—nitrogen isotopes and the element selenium.

“What we have… is another detection, at high resolution, of a transient whiff of oxygen,” says Koehler. “Nitrogen isotopes tell a story about oxygenation of the surface ocean, and this oxygenation spans hundreds of kilometres across a marine basin and lasts for somewhere less than 50 million years.” The team analysed drill samples taken by Roger Buick in 2012 at another site called the Jeerinah Formation in Western Australia.

The researchers drilled two cores about 300 kilometres 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’ years shows, Buick says, 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”.

The team found plentiful selenium only in the shallow hole, meaning that it came from the nearby land. Selenium is held in sulfur minerals on land; higher atmospheric oxygen would cause more selenium to be leached from the land through oxidative weathering—“the rusting of rocks”, Buick says—and transported to sea. “That selenium then accumulates in ocean sediments,” Koehler says. “So when we measure a spike in selenium abundances in ocean sediments, it could mean that there was a temporary increase in atmospheric oxygen.”

The finding, Buick and Koehler say, also has relevance for detecting life on exoplanets, or those beyond the solar system.

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