Around 2.5 billion years ago, free oxygen, also known as O2, began to accumulate in significant amounts in the Earth’s atmosphere. This was a crucial event that set the stage for the development of more complex forms of life on our evolving planet. Scientists call this phenomenon the Great Oxidation, or GOE (Great Oxidation Event). However, according to new research led by a geochemist from the University of Utah, the process of oxygen accumulation on Earth was not as simple as it seems.

This Great Oxidation event lasted at least 200 million years. Chadlin Ostrander, an assistant professor in the Department of Geology and Geophysics, explains that tracking how oxygen accumulated in the oceans has been very difficult until now. According to Ostrander, recent data suggests that the initial rise of oxygen in the Earth’s atmosphere was dynamic, with ups and downs that continued until about 2.2 billion years ago. New data from his team supports this idea, showing that these fluctuations were also reflected in the oceans.

The research team, which includes experts from different countries and is supported by NASA’s Exobiology program, focused on studying ancient marine rocks from South Africa. By analyzing these rocks, they found evidence of fluctuations in marine oxygen levels that coincided with changes in atmospheric oxygen. These findings help to understand the complex processes that shaped oxygen levels on Earth, a key factor for the evolution of life as we know it today.

Ostrander highlights the importance of knowing the oxygen content in the oceans, as it is likely that the first forms of life on Earth emerged and evolved in these marine environments. Through this research, it has been discovered that oxygen did not become a stable part of the atmosphere until about 200 million years after the global oxygenation process began, much later than previously thought.

Fossil of "Agaricocrinus americanus"
Fossil of “Agaricocrinus americanus”. Credit: Vassil / Wikimedia Commons

The evidence of an oxygen-free atmosphere is based on the presence of rare sulfur isotope signatures in sediment records before the GOE. These signatures indicate the absence of atmospheric oxygen. During the first half of the Earth’s existence, its atmosphere and oceans largely lacked oxygen. Although oxygen was produced by cyanobacteria in the oceans, it was rapidly destroyed in reactions with minerals and volcanic gases.

The researchers found that the rare sulfur isotope signatures disappear and reappear, suggesting multiple increases and decreases in oxygen levels during the GOE. This shows that oxygen accumulation was not a single event but a long and complex process. Earth needed time to evolve and be able to maintain stable oxygen levels.

Today, oxygen constitutes 21% of the atmosphere, being the second most abundant gas after nitrogen. But after the GOE, oxygen remained a minor component of the atmosphere for hundreds of millions of years. To track the presence of oxygen in the oceans during the GOE, the team used thallium isotopes, which are atoms of the same element with different numbers of neutrons.

The ratios of thallium isotopes are sensitive to the burial of manganese oxide on the seafloor, a process that requires oxygen in seawater. By studying these isotopes in marine rocks, they found patterns indicating oxygen accumulation in the ocean. These findings coincided with the disappearance of rare sulfur isotope signatures, suggesting that both the atmosphere and oceans were oxygenating and deoxygenating together.

When the sulfur isotopes say the atmosphere oxygenated, the thallium isotopes say the oceans oxygenated. And when the sulfur isotopes say the atmosphere became anoxic again, the thallium isotopes say the same for the ocean, Ostrander said. So, the atmosphere and ocean were oxygenating and deoxygenating together. This is new and cool information for those interested in ancient Earth.


Sources

University of Utah | Ostrander, C.M., Heard, A.W., Shu, Y. et al. Onset of coupled atmosphere–ocean oxygenation 2.3 billion years ago. Nature (2024). doi.org/10.1038/s41586-024-07551-5


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