10 December 2019
It may have been unexpectedly easy for the air to become rich in oxygen. A new simulation of the rise of oxygen suggests that it was driven by the planet itself, and needed little help from living organisms.
The finding implies that planets with oxygen-rich atmospheres could be more common than we thought. “It’s easier, not just for our planet, but possibly for others as well,” says Lewis Alcott at the University of Leeds, UK, one of the authors of the work.
For the first two billion years of Earth’s history, there was no oxygen in the air. That changed with the Great Oxidation Event around 2.4 billion years ago, when low levels of oxygen first appeared. This has often been attributed to the evolution of photosynthetic bacteria that release oxygen as a waste product.
Oxygen levels then rose twice more: once between 800 and 540 million years ago, and again 450-400 million years ago.
We have previously tried to explain the oxidation events by linking them to major evolutionary shifts or tectonic activity, says co-author Simon Poulton, also at the University of Leeds. For instance, the final rise has been linked to the spread of land plants.
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However, Alcott, Poulton and their colleague Benjamin Mills say there is no need to invoke any such dramatic events, other than the initial evolution of photosynthetic bacteria. They have shown that the behaviour of the planet is enough to explain the stepwise rises in oxygen levels.
The key is that Earth’s mantle has been gradually cooling since the planet formed, and as it cools, it releases fewer volcanic gases like sulphur dioxide, which react with oxygen and remove it from the air. When the team modelled how this shift affected the cycling of oxygen around the planet, they observed three sharp increases in oxygen that corresponded to the known oxidation events.
The initial Great Oxidation Event came about because the oxygen from bacteria overwhelmed the volcanic gases in the air. Levels then held steady for millions of years, because any extra oxygen reacted with minerals on land.
In the model, the second rise happened because the extra oxygen changed the nature of phosphorus-containing materials, making them more likely to be buried in sediments. Phosphorus is a vital nutrient, so this change meant fewer organisms that would otherwise have taken in oxygen could survive, allowing more oxygen to escape into the air and into surface layers of the sea. The same process led to a third sharp rise in oxygen, when it reached the deep ocean.
The same processes would play out on any planet that has oceans and continents, and where oxygen-releasing photosynthesis has evolved, says Poulton.
Journal reference: Science, DOI: 10.1126/science.aax6459
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