How far has it come? Study seeks to settle debate over oxygen in Earth’s early atmosphere

Artist’s representation of what the Earth might have looked like in Archean times, 4 to 2.5 billion years ago. Credit: Peter Sawyer / Smithsonian Institution

Scientists have long debated the amount of molecular oxygen in Earth’s early atmosphere. About 2.4 billion years ago, there was an increase in oxygen that transformed the Earth’s atmosphere and biosphere, ultimately making life like ours possible. This transition is called the “Great Oxidation Event”. But how much oxygen was in the atmosphere before this time?

A team of scientists, led by former Arizona State University doctoral student Aleisha Johnson, set out to unravel the mystery of how the stage was set for the great oxidation event.

Using computer modeling, Johnson and his team determined how much oxygen might have been present on the Earth’s surface before the great oxidation event and the implications for life on early Earth.

“We all breathe oxygen and we all live on the only known planet where this is possible,” says Johnson. “With our study, we’re one step closer to understanding how this happened – how the Earth was able to pass through and maintain an oxygen-rich atmosphere.”

The results of their study were published in Scientists progress.

The long-standing puzzle

Geoscientists studying Earth’s rock records have found seemingly conflicting evidence about Earth’s early atmosphere. On the one hand, the oxygen “fingerprints” found after the great oxidation event are mostly missing before that date, leading some scientists to claim it was absent.

But recent findings suggest at least some degradation of common minerals that react vigorously in the presence of oxygen, and at least an input to the oceans of chemicals like molybdenum that accumulate in rivers and oceans when oxygen is present. The conflicting evidence creates a long-standing puzzle.

How far has it come?  Study seeks to settle debate over oxygen in Earth's early atmosphere

An emerging view of Archean terrestrial oxygen production. Before oxygen filled the Earth’s atmosphere, it may have been produced in oceans and shallow soils. Shallow soils near the microbial communities (in green in the figure) may have had oxygen, but the overlying atmosphere has not. As a result, weathering signatures such as molybdenum enrichments in shales predate the Great Oxidation Event. Credit: Johnson et al./ASU

“The evidence seemed conflicting, but we knew there had to be an explanation,” said Johnson, currently a National Science Foundation postdoctoral researcher at the University of Chicago.

To help solve this puzzle, Johnson and his team wrote a computer model that uses what is known about the environmental chemistry of molybdenum, the reactions of minerals with small amounts of oxygen, and the measurements that others have taken the abundance of molybdenum in ancient sedimentary rocks, to understand the range of oxygen levels that were possible in Earth’s atmosphere before 2.4 billion years ago.

“This computer model helps us quantify the amount of oxygen actually needed to produce the chemistry visible in the rock record,” Johnson said.

What the team found was that the amount of oxygen needed to explain the evidence for molybdenum was so low that it wouldn’t have left many more fingerprints.

“There is an old adage that ‘lack of evidence is not proof of absence,'” said study co-author Ariel Anbar, professor at the School of Earth Exploration and space and the ASU School of Molecular Sciences. “Until now, our ideas about the lack of oxygen before the great oxidation event were mostly shaped by a lack of evidence. Now we have reason to believe it was there, just at lower levels. to those who could be detected before. “

The results support other sources of evidence suggesting that oxygen was produced, perhaps by biology, long before the great oxidation event. This, in turn, helps scientists in their quest to understand what changes in Earth’s systems have caused one of the most significant transformations in Earth’s history.

“Our hope is that these stresses on ancient atmospheric oxygen will help us understand the cause and nature of the great oxidation event. But it’s not just about the history of Earth. exploring Earth-like worlds orbiting other stars, we want to know whether oxygen-rich atmospheres like ours are likely to be common or rare. So this research is also helping to inform the search for life on planets other than our own, ”Johnson said.

The other authors of this study are Chadlin Ostrander of the Woods Hole Oceanographic Institution, Stephen Romaniello of the University of Tennessee, Christopher Reinhard of the Georgia Institute of Technology, Allison Greaney of the Oak Ridge National Laboratory and Timothy Lyons of the University of California, Riverside.

Another 100 million years before Earth saw a permanent increase in oxygen

More information:
Aleisha C. Johnson et al, Reconciling the evidence for oxidative weathering and atmospheric anoxia on Archean Earth, Scientists progress (2021). DOI: 10.1126 / sciadv.abj0108

Provided by Arizona State University

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