About 2.5 billion years ago, free oxygen first began to accumulate in meaningful levels in Earth's atmosphere, setting the stage for the emergence of complex life. Scientists call this phenomenon Great Oxidation EventBut a new study led by researchers at the University of Utah suggests that Earth's early buildup of oxygen wasn't as simple as that moniker suggests.
The Great Oxidation Event refers to the transition from a slowly reducing Archean atmosphere-ocean system to an oxygen-rich atmosphere and shallow ocean during the early Paleoproterozoic. Image courtesy of Hadeano.
“The new data suggest that the early rise of oxygen in Earth's atmosphere was dynamic, possibly progressing intermittently up until 2.2 billion years ago,” said Dr Chadrin Ostrander, a researcher at the University of Utah.
“Our data validate this hypothesis and go a step further by extending this dynamics to the ocean.”
By analysing stable thallium isotope ratios and redox-sensitive elements, Dr Ostrander and his colleagues found evidence of fluctuations in ocean oxygen levels that are consistent with changes in atmospheric oxygen.
The discovery helps improve understanding of the complex processes that shaped oxygen levels on Earth at key times in its history and paved the way for the evolution of life as we know it.
“We have no idea what was going on in the oceans where Earth's earliest life forms are thought to have arisen and evolved,” Dr Ostrander said.
“So knowing the oxygen content of the ocean and how it evolved over time is probably more important for early life than the atmosphere.”
In 2021, researchers discovered that oxygen wasn't permanently present in the atmosphere until about 200 million years after the global oxygenation process began — much later than previously thought.
Definitive evidence for an anoxic atmosphere is the presence of rare mass-independent sulfur isotope signatures in the sediment record prior to the Great Oxidation Event.
There are very few known processes on Earth that could produce these sulfur isotope signatures, and atmospheric oxygen would almost certainly be absent for them to be preserved in the rock record.
For the first half of Earth's existence, its atmosphere and oceans were almost devoid of oxygen. This gas was likely produced by cyanobacteria in the oceans before the Great Oxidation Event, but during this early epoch the oxygen was rapidly destroyed in reactions with exposed minerals and volcanic gases.
Scientists found that traces of rare sulfur isotopes disappeared and reappeared, suggesting that atmospheric oxygen increased and decreased multiple times during the Great Oxidation Event – it wasn't a single “event.”
“When oxygen began to be produced, the Earth was not ready to be oxygenated. The Earth needed time to evolve biologically, geologically and chemically to encourage oxygenation,” Dr Ostrander said.
“It's like a seesaw. Oxygen is produced, but there's so much oxygen destruction that nothing happens.”
“We're still trying to figure out when the scales will tip completely and Earth will no longer be able to go back to an oxygen-free atmosphere.”
To map ocean oxygen levels during the Great Oxidation Event, the authors relied on expertise in stable thallium isotopes.
Thallium isotope ratios are sensitive to the burial of manganese oxides to the seafloor, a process that requires oxygen in seawater.
The team looked at thallium isotopes in the same ocean shales, which have recently been shown to be able to track fluctuations in atmospheric oxygen during the Great Oxidation Event, along with rare sulfur isotopes.
The researchers found a significant enrichment of the lighter isotope thallium-203 in the shale, a pattern best explained by the burial of manganese oxides on the ocean floor and the buildup of oxygen in the water.
These enrichments were found in the same samples that lacked the rare sulfur isotope signature, meaning the atmosphere was no longer anoxic, and they disappeared once the rare sulfur isotope signature reappeared.
These findings were supported by redox-sensitive element enrichments, a more classical means of tracing ancient oxygen changes.
“The sulfur isotopes indicate that the atmosphere was oxygenated, and the thallium isotopes indicate that the oceans were oxygenated,” Dr Ostrander said.
“Sulfur isotopes indicate that the atmosphere has become anoxic again, and thallium isotopes indicate the same for the oceans.”
“So the atmosphere and oceans were simultaneously oxygenating and deoxygenating. This is new and exciting information for people interested in the ancient Earth.”
of Investigation result Published in the journal Nature.
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Ostrander Commercial othersCoupled oxygenation of the atmosphere and oceans began 2.3 billion years ago. NaturePublished online June 12, 2024, doi: 10.1038/s41586-024-07551-5
Source: www.sci.news
