MONTREAL— It was once such a clear story. For its first 2 billion years, Earth had almost no free oxygen in its air or oceans. Then, some 2.45 billion years ago, the ancestors of photosynthetic cyanobacteria began to flood the world with oxygen, creating the conditions that allowed complex life to evolve and prosper. The Great Oxidation Event (GOE), as it’s known, is “the most fascinating and most dramatic change in how the Earth’s surface works in its history,” says Mojtaba Fakhraee, a geochemist at the University of Connecticut.
But that story has clouded recently. Mineral analyses suggest oxygen-producing microbes evolved hundreds of millions of years before the GOE, leaving a huge, unexplained gap when oxygen remained low. And the GOE appears to have been not one event but several, with oxygen repeatedly rising and falling over a span of some 200 million years—a great oxidation followed by a great deoxidation, and then back again, several times.
Now, possible explanations are emerging. One talk at the Goldschmidt conference this month proposed that oxygen could not rise until extensive shallow areas formed in the world’s oceans. Another argued the oxygen pulses reflect ancient Earth’s periodic transformation into Snowball Earth, when ice engulfed the planet to low latitudes.
For several decades, scientists have recognized the GOE from a dramatic shift in the ratio of three sulfur isotopes in the rock record, reflecting changes in the breakdown of sulfur dioxide in the atmosphere by ultraviolet (UV) light. Those reactions would have slowed after the rise in oxygen spawned an ozone layer that blocked the UV. The isotope shift pegged the GOE to 2.45 billion years ago, a date still widely agreed on.
Yet a study last year in Nature reported carbonate deposits created by oxygen-producing cyanobacteria 2.85 billion years ago. Using the mutation rates of modern-day microbes to date the origin of photosynthesis pushed it back even further, to as much as 3.5 billion years ago. And for 2 decades, researchers have tracked what appear to be “whiffs” of oxygen prior to the GOE, recorded in minerals that require oxygen to form.
Put together, the studies paint a compelling picture that the GOE didn’t start until hundreds of million years after the arrival of photosynthetic microbes, Fakhraee says. “The big question is, why?”
Not everyone agrees photosynthetic oxygen production evolved so early, says Jena Johnson, a geochemist at the University of Michigan who has shown that microbes may have first developed a form of the process that did not generate oxygen. What’s more, some metal oxides that were once taken as signatures of biological oxygen have been shown to come from photochemical reactions driven by UV light. “There’s still work to do to understand these,” she says.
But assuming the gap between the world’s photosynthetic explosion and the GOE is real, some scientists explained it by assuming that reactions with reducing materials such as iron, floating in the ocean, consumed the early oxygen. The scavenged oxygen contributed to the deep-red banded iron formations, made of oxidized iron, found in Australia and elsewhere. Later, investigators proposed that a constant flux of reducing chemicals from deep in the Earth kept oxygen at bay. But it’s unclear how such a chemical sponge could keep working for hundreds of millions of years, Johnson says.
The new explanation for the lag, from Fakhraee and his co-authors and presented at Goldschmidt, centers on the continental shelves, the oceanic shallows that today are one of the planet’s hot spots for photosynthetic oxygen despite making up only 9% of its area. When marine cyanobacteria die on the shelves, their carbon is quickly buried in sediment, preventing it from oxidizing and drawing oxygen back out of the atmosphere. There are signs that the shelves began to expand around the time of the GOE, as the youthful continents began eroding. And a model showed that once the shelves expanded to more than 10% of the present-day area, oxygen growth would have taken off, Fakhraee says. “It does give feet to idea that tectonic activities could have driven large-scale change.”
That change was not steady, however. Early this decade, a team of geochemists reported that in marine sediments from South Africa, the sulfur signature of low oxygen returned multiple times during the 200 million years after the GOE’s start, including during what are believed to be three periods when the world plunged into a Snowball Earth state. “We realized that [the GOE] was a more episodic transition,” says geochemist Andrey Bekker of the University of California, Riverside and a co-author of the paper, published in 2021 in Nature.
At the Goldschmidt meeting, Colin Goldblatt, a climate scientist at the University of Victoria, argued that Snowball Earth could have given the GOE its shaky start. Geological records mostly agree that the first Snowball Earth occurred before the GOE, perhaps caused by a decline in carbon dioxide (CO2) levels, either from increased absorption by rocks or declining volcanic emissions.
Whatever the cause, this worldwide glaciation would have largely kept the oceans from absorbing any more CO2, allowing it to build up, warm the planet again, and melt off all the ice. This thaw would have only taken 10,000 years, whereas CO2 would have stayed in the atmosphere for 100,000 years. In the resulting hot climate, lots of phosphorous would have eroded from the continents into the ocean, causing a boom in marine microbes and boosting oxygen. An ozone layer would have formed, limiting the breakdown of oxygen by UV-driven chemistry.
Then, when the next Snowball Earth set in and decimated microbial production in the ocean, oxygen would have collapsed again. “Oxygen yo-yos with the climate of the Earth until it gets out of its cycle of low latitude glaciations,” Goldblatt says.
Early historians of Earth admit they are only beginning to re-create a clear story of the GOE—one that may include factors beyond continental shelves and global glaciation, Goldblatt said at the meeting. “There is probably an idea for every person in the room.”