The oxygenation of our atmosphere

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One of humanity&rsquo;s essential needs, oxygen, likely took 100 million years to accumulate into a permanent presence in the atmosphere 2.4 billion years ago, according to research carried out at UW as well as the University of Alberta, the University of California, and the Georgia Institute of Technology.</p>

“Based on previous work, we know that the Earth’s atmosphere became permanently oxygenated after about 2.4 billion years ago, widely known as the ‘Great Oxidation Event.’ We also knew that oxygenic photosynthesis evolved sometime before the Great Oxidation Event,” said Brian Kendall, associate professor of earth and environmental sciences at UW. “In an earlier 2007 study, we showed that small amounts of oxygen were present on Earth’s surface 2.5 billion years ago. However, we did not know if the oxygen concentration was stable or fluctuating.”

In a paper published Nov. 20 in Science Advances, Kendall, professor Robert Creaser of the University of Alberta, professor Timothy Lyons from the University of California Riverside, and assistant professor Chris Reinhard from the Georgia Institute of Technology, used osmium isotope geochemistry from ancient ocean sedimentary rocks to demonstrate that oxygen concentrations fluctuated and built up over time in what they call “whiffs” of oxygen. These whiffs were created by blue-green algae in shallow oceans around 2.5 billion years ago.

“Before the Great Oxidation Event began 2.4 billion years ago … the geochemical data from sedimentary rocks deposited back then tells us that the amount of oxygen in the atmosphere was typically less than 0.001 per cent of the amount in the atmosphere today. During the ‘whiff’ of oxygen described in our paper, the amount of oxygen may have temporarily risen to above 0.03 per cent of today’s atmosphere as that is most harmonious with our new data,” said Kendall. 

Kendall has been researching the evolution of our atmosphere for almost 10 years — he was part of the initial study in 2007 that found traces of oxygen 2.5 billion years ago. 

“It is an absolutely fascinating field of study. The evolution of oxygenic photosynthesis and initial accumulation of oxygen on Earth’s surface represents the beginning of the story on how complex life — animals and humans — evolved on Earth,” said Kendall. “Even after the Great Oxidation Event, there still wasn’t enough oxygen in the atmosphere for animals to live. The high-oxygen atmosphere required for animal life wasn’t achieved until a little over 0.5 billion years ago.”

There are still a lot of unanswered questions in terms of the development of our atmosphere. Some of them, according to Kendall, include: When did oxygenic photosynthesis evolve? Why did oxygen concentrations fluctuate before the Great Oxidation Event? Exactly what caused the Great Oxidation Event? 

“All are difficult questions to answer and are very active fields of research. There are few places in the world where pristine rocks more than 2.5 billion years old are preserved, which makes research on this time period challenging. The best places are Western Australia and South Africa. We continue to analyze ancient rocks from those areas to look for evidence for photosynthetic oxygen production more than 2.5 billion years ago,” said Kendall. “Some recent studies suggest that oxygenic photosynthesis may have evolved by three billion years ago, but we’re a long way from having a complete picture.”

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