The oxidation of Earth's atmosphere was one of the most important events in our planet's history. It's also one we don't understand very well. New evidence has dated the event to 2.33 billion years ago and indicates it happened over a timespan of less than 10 million years.
Oxygen is so reactive that it will not stay in the atmosphere for a long period of time. This is why scientist James Lovelock once suggested that oxygen in the atmosphere of another planet would be evidence for life, although the conclusiveness of this is now debated.
It's also why there was excitement about the discovery of oxygen on Mars this week, although the Martain concentration is low enough to allow other possible sources.
Life alone is not enough, however. For almost the first 2 billion years of life on Earth, oxygen levels are thought to have been minimal, although even this was challenged by another paper this week. Only after what is known as the Great Oxidation Event (GOE), did the planet become capable of sustaining multi-cellular organisms.
Few rocks remain from the time of the GOE, and what we have found has proven sufficiently contradictory that geologists have struggled to make sense of when, and how quickly, oxygen appeared. However, new evidence from South Africa may resolve these questions.
How long has Earth had oxygen? NASA
In Science Advances, a team led by Dr. Genming Luo of the Massachusetts Institute of Technology (MIT) reports on the results from three drill cores in the Transvaal, South Africa. All three provide a date of 2.33 billion years ago for the GOE, and indicate oxidation took between 1 million and 10 million years. Unimaginably long as a million years is to us, by geological standards, these are quite rapid.
Luo and co-authors also concluded that the concentrations of sulphate in the oceans, an indication of the presence of oxygen to bond with the sulfur, lagged the GOE by approximately 6 million years, and was a slower process.
Another of the big questions about the GOE has been whether it was caused by, or caused, radical climate change.
On this point the paper reports that “a series of glaciations took place between 2.45 and 2.2 billion years ago,” and that at least one of these “Snowball Earth” events occurred shortly before the GOE and may have precipitated it. The timing of other massive glaciations remains sufficiently uncertain that the authors cannot make any conclusions about their relationship to the GOE.
The GOE did not stand alone. Even after it was over, oxygen concentrations continued to rise for hundreds of millions of years, albeit at a much slower pace, contributing to the dating uncertainty.
On the other hand, Luo detected “significant compositional changes” in the atmosphere prior to the GOE based on the sulfur compounds deposited in the lead up. These may indicate that pockets of oxygen existed before the GOE, but were not consistent throughout the atmosphere, although the paper raises alternative explanations that might be tested in future.