About 2.5 billion years ago, free oxygen first started to accumulate to meaningful levels in Earth’s atmosphere, setting the stage for the rise of complex life. Scientists refer to this phenomenon as the Great Oxidation Event. But, according to new research led by University of Utah researchers, the initial accumulation of oxygen on Earth was not nearly as straightforward as that moniker suggests.
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The Great Oxidation Event refers to the transition from the mildly reducing Archean atmosphere-ocean system to the oxygenated atmosphere and shallow oceans of the early Paleoproterozoic. Image credit: Hadeano.
“Emerging data suggest that the initial rise of oxygen in Earth’s atmosphere was dynamic, unfolding in fits-and-starts until perhaps 2.2. billion years ago,” said Dr. Chadlin Ostrander, a researcher at the University of Utah.
“Our data validate this hypothesis, even going one step further by extending these dynamics to the ocean.”
By analyzing stable thallium isotope ratios and redox-sensitive elements, Dr. Ostrander and colleagues uncovered evidence of fluctuations in marine oxygen levels that coincided with changes in atmospheric oxygen.
The findings help advance the understanding of the complex processes that shaped Earth’s oxygen levels during a critical period in the planet’s history that paved the way for the evolution of life as we know it.
“We really don’t know what was going on in the oceans, where Earth’s earliest lifeforms likely originated and evolved,” Dr. Ostrander said.
“So knowing the oxygen content of the oceans and how that evolved with time is probably more important for early life than the atmosphere.”
In 2021, the researchers discovered that oxygen did not become a permanent part of the atmosphere until about 200 million years after the global oxygenation process began, much later than previously thought.
The smoking gun evidence of an anoxic atmosphere is the presence of rare, mass-independent sulfur isotope signatures in sedimentary records before the Great Oxidation Event.
Very few processes on Earth can generate these sulfur isotope signatures, and from what is known their preservation in the rock record almost certainly requires an absence of atmospheric oxygen.
For the first half of Earth’s existence, its atmosphere and oceans were largely devoid of oxygen. This gas was being produced by cyanobacteria in the ocean before the Great Oxidation Event, it seems, but in these early days the oxygen was rapidly destroyed in reactions with exposed minerals and volcanic gasses.
The scientists discovered that the rare sulfur isotope signatures disappear but then reappear, suggesting multiple oxygen rises and falls in the atmosphere during the Great Oxidation Event. This was no single ‘event.’
“Earth wasn’t ready to be oxygenated when oxygen starts to be produced. Earth needed time to evolve biologically, geologically and chemically to be conducive to oxygenation,” Dr. Ostrander said.
“It’s like a teeter totter. You have oxygen production, but you have so much oxygen destruction, nothing’s happening.”
“We’re still trying to figure out when we’ve completely tipped the scales and Earth could not go backwards to an anoxic atmosphere.”
To map oxygen levels in the ocean during the Great Oxidation Event, the authors relied on their expertise with stable thallium isotopes.
Thallium isotope ratios are sensitive to manganese oxide burial on the seafloor, a process that requires oxygen in seawater.
The team examined thallium isotopes in the same marine shales recently shown to track atmospheric oxygen fluctuations during the Great Oxidation Event with rare sulfur isotopes.
In the shales, the researchers found noticeable enrichments in the lighter-mass isotope thallium-203, a pattern best explained by seafloor manganese oxide burial, and hence accumulation of oxygen in seawater.
These enrichments were found in the same samples lacking the rare sulfur isotope signatures, and hence when the atmosphere was no longer anoxic. And they disappear when the rare sulfur isotope signatures return.
These findings were corroborated by redox-sensitive element enrichments, a more classical tool for tracking changes in ancient oxygen.
“When sulfur isotopes say the atmosphere became oxygenated, thallium isotopes say that the oceans became oxygenated,” Dr. Ostrander said.
“And when the sulfur isotopes say the atmosphere flipped back to anoxic again, the thallium isotopes say the same for the ocean.”
“So the atmosphere and ocean were becoming oxygenated and deoxygenated together. This is new and cool information for those interested in ancient Earth.”
The findings were published in the journal Nature.
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C.M. Ostrander et al. Onset of coupled atmosphere-ocean oxygenation 2.3 billion years ago. Nature, published online June 12, 2024; doi: 10.1038/s41586-024-07551-5
