Changes in the Earth's crust were responsible for the transformation of the planet's atmosphere from oxygen-free to oxygen-rich, a geological study has found.
In the Earth's early days, the atmosphere and oceans were devoid of oxygen. The land and oceans were bare and almost, but not entirely, lifeless. The only organisms capable of thriving in this almost oxygen-free world were cyanobacteria, which actually released oxygen as a waste product.
About 3 billion years ago, this changed. Free oxygen – in molecular O2 form – began to appear in the oceans. For a few hundred million years, this low-level oxygen environment persisted. Then about 2.4 billion years ago, something happened that changed the atmosphere much more dramatically. Known as the Great Oxidation Event, free oxygen levels shot up about 10,000 fold within just 200 million years.
Exactly what caused the Great Oxidation Event has been a long-standing discussion among geologists. Some have proposed it was iron dissolved in the oceans that soaked up the oxygen first released by cyanobacteria. Others have said that it was the advent of multicellularity that triggered this boom in free oxygen. More have said that it was a shift in volcanic activity that made the difference.
Now researchers at the University of British Colombia have a new explanation. At just about the same time as the Great Oxidation Event, there was a marked change in the make-up of continental rocks, they write in a study published in the journal Nature Geoscience.
"[The Great Oxidation Event] really appears to have been the starting point for life diversification as we know it," study author Matthijs Smit of the University of British Colombia said in a statement.
"After that change, the Earth became much more habitable and suitable for the evolution of complex life, but that needed some trigger mechanism, and that's what we may have found."
After analysing data from more than 48,000 rock samples, the researchers discovered that before the oxidation, the continental rocks contained a lot of the mineral olivine. This substance acts like a sponge for oxygen, locking up any produced by cyanobacteria. So it could have been the evolution of the crust from olivine-rich to olivine-poor rocks that allowed oxygen from these bacteria to accumulate in the oceans.
"Oxygenation was waiting to happen," said Smit. "All it may have needed was for the continents to mature."
The next question is to pin down exactly why the amount of olivine in continental rocks dropped off around the time of the Great Oxidation Event.