Most life needs oxygen to thrive, and science shows that O2 began to show up in Earth's atmosphere in serious amounts some 2.4 billion years ago. However, there was also a small injection of oxygen around 100 million years earlier than that – and the origin of that earlier breath has so far been a mystery.
A new study points to volcanoes as the likely cause of this shorter oxygenation event preceding the major one. Through an analysis of rock records, researchers have spotted a corresponding increase in mercury levels that indicates volcanic activity.
That activity would have led to nutrient-rich lava and volcanic ash fields, the researchers suggest, which then led to the release of those nutrients into rivers and coastal areas through weathering. That, in turn, would have enabled cyanobacteria and other single-celled organisms to flourish – and start pumping out oxygen.
"Our study suggests that for these transient whiffs of oxygen, the immediate trigger was an increase in oxygen production, rather than a decrease in oxygen consumption by rocks or other non-living processes," says geologist Roger Buick, from the University of Washington.
"It's important because the presence of oxygen in the atmosphere is fundamental – it's the biggest driver for the evolution of large, complex life."
Buick and his colleagues looked at drill-cores taken from the Mount McRae Shale formation in Western Australia, containing geological timelines stretching back 2.5 billion years to before the start of the Great Oxygenation Event.
Signs of both mercury enrichment and oxidation weathering convinced the researchers that volcanic eruptions and the subsequent introduction of phosphorus – a key nutrient for modulating biological activity across long timescales – had played a major part in the early oxygen spike.
And while it's not clear exactly where on Earth this volcanic activity might have taken place, geological records from locations in modern-day India and Canada, among other locations, back up the hypothesis of volcanism and lava flows around this time.
"During weathering under the Archaean atmosphere, the fresh basaltic rock would have slowly dissolved, releasing the essential macronutrient phosphorus into the rivers," says astrobiologist Jana Meixnerová from the University of Washington.
"That would have fed microbes that were living in the shallow coastal zones and triggered increased biological productivity that would have created, as a byproduct, an oxygen spike."
There may well have been other oxygen spikes before the Earth's atmosphere started transforming in earnest, but even if this study only explains one of them, it's still a useful bank of evidence for peering back into the earliest moments of life on our planet.
And as with any study of this type, there are implications for research into climate change (showing us how life adapts to less oxygen) and the search for life in space (showing us the sort of atmospheric conditions that microorganisms can exist in).
Questions remain about how life on Earth first got started in its most basic form, a billion years before the Great Oxygenation Event, and answering those questions will need a better understanding of the geology of the planet through time.
"What has started to become obvious in the past few decades is there actually are quite a number of connections between the solid, non-living Earth and the evolution of life," says Meixnerová.
The research has been published in PNAS.