According to recent geochemical discoveries, the “Great Oxidation Event” on Earth has been extended to 200 million years

About 2.5 billion years ago, free oxygen, or O₂first began to accumulate at 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 According to new research led by a University of Utah geochemist, life on Earth wasn’t nearly as simple as that label suggests.

This “event” lasted at least 200 million years. And monitoring 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.

“New data suggest that the initial rise of O₂ in the earth’s atmosphere it was dynamic, unfolding in fits and starts until 2.2. billion years ago,” said Ostrander, lead author of the study published June 12 in the journal Nature. “Our data confirm this hypothesis, even going a step further by extending this dynamic to the ocean.”

His international research team focused on marine shales from South Africa’s Transvaal Supergroup, providing insight into the dynamics of ocean oxygenation during this pivotal period in Earth’s history. By analyzing the stable isotope ratios of thallium (Tl) and redox-sensitive elements, they revealed evidence of fluctuations in sea oxygen levels.₂ levels that coincided with changes in atmospheric oxygen.

These findings help advance understanding of the complex processes that shaped Earth’s O₂ levels during a critical period in the planet’s history that paved the way for the development of life as we know it.

“We really don’t know what happened in the oceans where Earth’s earliest life forms likely originated and evolved,” said Ostrander, who joined the U faculty last year from the Woods Hole Oceanographic Institution in Massachusetts. “So knowing O₂ The content of the oceans and how it evolved over time is probably more important to early life than the atmosphere.”

The research builds on the work of Ostrander’s co-authors Simon Poulton of the University of Leeds in the United Kingdom and Andrea Bekker of the University of California, Riverside. In a 2021 study, their team of scientists found that O₂ did not become a permanent part of the atmosphere until about 200 million years after the process of global oxygenation began, much later than previously thought.

“Smoking gun” evidence of an anoxic atmosphere is the presence of rare mass-independent sulfur isotope signatures in the pre-GOE sedimentary record. Very few processes on Earth can generate these sulfur isotopic signatures, and from what is known, their preservation in the rock record almost certainly requires the absence of atmospheric oxygen.₂.

During the first half of Earth’s existence, its atmosphere and oceans were largely free of O₂. This gas appears to have been produced by cyanobacteria in the pre-GOE ocean, but in these early days O₂ it was rapidly destroyed by reactions with exposed minerals and volcanic gases.

Poulton, Bekker and colleagues discovered that rare isotopic signatures of sulfur disappear but then reappear, suggesting more O₂ rises and falls in the atmosphere during the GOE. It wasn’t the only “event”.

“Earth was not ready for oxygenation when oxygen began to be produced. Earth needed time to evolve biologically, geologically and chemically to handle the oxygenation,” Ostrander said. “It’s like a balancing act. You have oxygen production, but you have so much oxygen destruction, nothing happens. We’re still trying to figure out when we tipped the scales completely and the Earth couldn’t go back into an anoxic atmosphere.” “

On the map O₂ levels in the ocean during the GOE, the research team relied on Ostrander’s expertise with stable thallium isotopes.

Isotopes are atoms of the same element that have unequal numbers of neutrons, giving them slightly different masses. The isotope ratios of a particular element have fueled discoveries in archaeology, geochemistry, and many other fields.

Advances in mass spectrometry have allowed scientists to accurately analyze isotope ratios for elements further and further up the periodic table, such as thallium. Fortunately for Ostrander and his team, thallium isotope ratios are sensitive to the burial of manganese oxide on the seafloor, a process that requires O₂ in sea water.

The team examined thallium isotopes in the same marine shales that were recently shown to trace atmospheric oxygen₂ fluctuations during the GOE with rare sulfur isotopes.

In the shales, Ostrander and his team found a noticeable enrichment of the lighter isotope thallium (²⁰³Tl), a pattern best explained by the burial of manganese oxide on the sea floor and thus the accumulation of O₂ in sea water.

These enrichments were found in the same samples lacking rare sulfur isotope signatures, and therefore the atmosphere was no longer anoxic. The icing on the cake: ²⁰³The Tl enrichment disappears when rare sulfur isotopic signatures return. These findings were confirmed by enrichment of redox-sensitive elements, a more classical tool for monitoring changes in ancient O₂.

“When sulfur isotopes say the atmosphere has become oxygenated, thallium isotopes say the oceans have become oxygenated. And when sulfur isotopes say the atmosphere has flipped back to anoxic again, thallium isotopes say the same for the ocean,” Ostrander said.

“So the atmosphere and ocean were oxygenating and deoxygenating together. This is new and great information for those interested in the ancient Earth.”