Was methane responsible for early Earth’s oxygene boom?

The research part-supported by the EU-funded OXYGEN project, based at the University of St Andrews, UK, suggests that billions of years ago, Earth’s atmosphere was filled with a methane-rich haze over a period of about a million years. This haze drove a large amount of hydrogen out of the atmosphere, clearing the way for massive amounts of oxygen to fill the air, resulting in an atmosphere much like the one that sustains life today. Before this methane-driven transformation, Earth’s atmosphere was inhospitable, due to being filled with toxic gasses that drove wildly fluctuating surface temperatures.

The research was published in the journal ‘Proceedings of the National Academy of Sciences’, the international collaboration also included the University of Maryland, NASA’s Jet Propulsion Laboratory, the University of Leeds and the Blue Marble Space Institute of Science. In their study, the research team propose a new contributing cause for the Great Oxidation Event, which occurred 2.4 billion years ago, when oxygen concentrations in the Earth’s atmosphere increased more than 10 000 times.

‘The transformation of Earth’s air from a toxic mix to a more welcoming, oxygen-rich atmosphere happened in a geological instant,’ commented James Farquhar, a professor of geology at the University of Maryland and a co-author of the study. ‘With this study, we finally have the first complete picture of how methane haze made this happen.’

Specifically the researchers used detailed chemical records and sophisticated atmospheric models to reconstruct atmospheric chemistry during the time period immediately before the Great Oxidation Event. Their results suggest that ancient bacteria (the only life on Earth at the time) produced massive amounts of methane that reacted to fill the air with a thick haze. This current study is the first to show how rapidly these events began and how long they lasted. For a modern comparison, such an atmosphere reflects the current atmospheric conditions found on Titan, Saturn’s largest moon.

What made this research even more exciting was the discovery of anomalous patterns of sulphur isotopes in the geochemical records from this time. Sulphur isotopes are often used as a proxy to reconstruct ancient atmospheric conditions, but previous investigations into the time period has not revealed anything too unusual.

‘High methane levels meant that more hydrogen, the main gas preventing the build up of oxygen, could escape into outer space, paving the way for global oxygenation,’ said Aubrey Zerkle, a biogeochemist at the University of St Andrews and a co-author of the study. ‘Our new dataset constitutes the highest resolution record of Archean atmospheric chemistry ever produced, and paints a dramatic picture of Earth surface conditions before the oxygenation of our planet.’

Overall, the methane haze lasted for about one million years and after enough hydrogen left the atmosphere, the right chemical conditions took over and the oxygen boom got underway, allowing for the evolution of multicellular life.

‘Reconstructing the evolution of atmospheric chemistry has long been the focus of geochemical research,’ said Gareth Izon, lead author of the study, who contributed to the research while a postdoctoral researcher at St Andrews. ‘Our new data show that the chemical composition of the atmosphere was dynamic and, at least in the prelude to the Great Oxidation Event, hypersensitive to biological regulation.’

The OXYGEN (Quantifying the evolution of Earth''s atmosphere with novel isotope systems and modelling) project will continue at St Andrews until May 2021 and received nearly EUR 1.8 million of EU funding.

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