Privileged Planet: more about oxidation event (Introduction)

by David Turell , Wednesday, June 12, 2024, 17:56 (96 days ago) @ David Turell

Occurred over 200 million years:

https://phys.org/news/2024-06-earth-great-oxidation-event-

"About 2.5 billion years ago, free oxygen, or O2, first started to accumulate to meaningful levels in Earth's atmosphere, setting the stage for the rise of complex life on our evolving planet.

"Scientists refer to this phenomenon as the Great Oxidation Event, or GOE for short. But the initial accumulation of O2 on Earth was not nearly as straightforward as that moniker suggests, according to new research led by a University of Utah geochemist.

"This "event" lasted at least 200 million years. And tracking the accumulation of O2 in the oceans has been very difficult until now, said Chadlin Ostrander, an assistant professor in the Department of Geology and Geophysics.

"Emerging data suggest that the initial rise of O2 in Earth's atmosphere was dynamic, unfolding in fits-and-starts until perhaps 2.2. billion years ago," said Ostrander, lead author on the study published June 12 in the journal Nature. "Our data validate this hypothesis, even going one step further by extending these dynamics to the ocean."

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"The "smoking gun" evidence of an anoxic atmosphere is the presence of rare, mass-independent sulfur isotope signatures in sedimentary records before the GOE. 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 O2.

"For the first half of Earth's existence, its atmosphere and oceans were largely devoid of O2. This gas was being produced by cyanobacteria in the ocean before the GOE, it seems, but in these early days the O2 was rapidly destroyed in reactions with exposed minerals and volcanic gases.

"Poulton, Bekker and colleagues discovered that the rare sulfur isotope signatures disappear but then reappear, suggesting multiple O2 rises and falls in the atmosphere during the GOE. 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," 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."

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"The team examined thallium isotopes in the same marine shales recently shown to track atmospheric O2 fluctuations during the GOE with rare sulfur isotopes.

"In the shales, Ostrander and his team found noticeable enrichments in the lighter-mass thallium isotope (203Tl), a pattern best explained by seafloor manganese oxide burial, and hence accumulation of O2 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. The icing on the cake: the 203Tl enrichments 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 O2.

"'When sulfur isotopes say the atmosphere became oxygenated, thallium isotopes say that the oceans became oxygenated. And when the sulfur isotopes say the atmosphere flipped back to anoxic again, the thallium isotopes say the same for the ocean," Ostrander said.

""So the atmosphere and ocean were becoming oxygenated and deoxygenated together. This is new and cool information for those interested in ancient Earth.'"

Comment: it is amazing how complex the oxygenation event was. It was not just cyanobacteria churning it out.