Recent discoveries suggest that Earth’s “Great Oxidation Event” lasted for 200 million years.
The new study highlights the complexity of the Great Oxidation Event, revealing that the increase in oxygen in the atmosphere and oceans was a dynamic process lasting more than 200 million years, influenced by geological and biological factors that were crucial for the evolution of life.
Great Oxidation Event
About 2.5 billion years ago, free oxygen, or O2began to accumulate in meaningful levels in Earth’s atmosphere for the first time, setting the stage for the emergence of complex life on the evolving planet.
Scientists call this phenomenon the “Great Oxidation Event,” or GOE for short.2 According to a new study led by geochemists at the University of Utah, what happened on Earth wasn’t as simple as the name suggests.
This “event” lasted at least 200 million years, and tracking the accumulation of O2 Ocean exploration has historically been very difficult, said Chadrin Ostrander, assistant professor in the Department of Geology and Geophysics.
“New data shows that O2 “Evolution in Earth’s atmosphere was probably intermittent up until 2.2 billion years ago,” said Ostrander, lead author of a study published June 12 in the journal Nature. Nature“Our data validate this hypothesis and go a step further by extending this dynamic to the ocean.”
Insights from marine shales
His international research team: National Aeronautics and Space Administration (NASA) An exobiology program focused on marine shales from the Transvaal Superformation in South Africa has provided insights into the dynamics of ocean oxygenation during this critical period in Earth’s history. By analysing stable thallium (Tl) isotope ratios and redox-sensitive elements, we found evidence of variations in marine O.2 levels consistent with changes in atmospheric oxygen.
These discoveries help improve our understanding of the complex processes that shaped Earth’s O.2 This level occurred at a critical time in Earth’s history, and it paved the way for the evolution of life as we know it.
Understanding early ocean conditions
“We really don’t know what was going on in the oceans where Earth’s earliest life forms are thought to have arisen and evolved,” says Ostrander, who joined the university professorship last year from Woods Hole Oceanographic Institution in Massachusetts.2 The contents of the ocean and how it evolved over time is probably more important to early life than the atmosphere.”
The study builds on the work of Ostrander’s co-authors Simon Poulton of the University of Leeds in the UK and Andrei Becker of the University of California, Riverside. 2021 SurveyTheir team of scientists2 It didn’t become a permanent part of the atmosphere until about 200 million years after the global oxygenation process began, much later than previously thought.
Atmospheric and oceanic oxygen variations
Definitive evidence for an anoxic atmosphere is the presence of rare, mass-independent sulfur isotope signatures in the pre-GOE sediment record. Few processes on Earth could produce these sulfur isotope signatures, and their preservation in the rock record is a near-certain indication of the absence of atmospheric O.2.
For the first half of Earth’s existence, its atmosphere and oceans were almost devoid of O.2This gas appears to have been produced by marine cyanobacteria before the GOE, but early on it was O2 It was rapidly destroyed by reactions with exposed minerals and volcanic gases. Poulton, Becker and others found that traces of rare sulfur isotopes disappeared and then reappeared, and that multiple O2 During a GOE, you will rise and fall in the atmosphere – it is not a single “event”.
Challenges in oxygenating the planet
“When oxygen began to be produced, the Earth was not ready to be oxygenated. It needed time for the biological, geological and chemical evolution to encourage oxygenation,” Ostrander said. “It’s like a seesaw: oxygen is being 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 the Earth will no longer be able to go back to an oxygen-free atmosphere.”
Today, O2 By weight, oxygen makes up 21% of the atmosphere, second only to nitrogen, but since the GOE, oxygen has remained a very small component of the atmosphere for hundreds of millions of years.
Advanced isotope analysis techniques
Track the presence of O2 The team relied on Ostrander’s expertise to study stable isotopes of thallium that occurred in the ocean during the GOE.
Isotopes are atoms of the same element that have different numbers of neutrons and therefore slightly different weights. The ratios of isotopes of certain elements drive discoveries in archaeology, geochemistry, and many other fields.
Thallium isotopes and oxygen indicators
Advances in mass spectrometry have allowed scientists to precisely analyze the isotope ratios of elements further down the periodic table, such as thallium. Luckily for Ostrander and his team, thallium isotope ratios are sensitive to the burial of manganese oxides on the ocean floor, and that process involves O2 Thallium isotopes in seawater. The team looked at thallium isotopes in the same ocean shales that were recently shown to track O in the atmosphere.2 Variations in the GOE due to rare sulfur isotopes.
Ostrander and his team found that in the shale, lighter-mass thallium isotopes (203Tl), which is due to the burial of manganese oxides on the ocean floor and the resulting O2 These concentrations in seawater were found in the same samples that lacked the rare sulfur isotope signature, indicating that the atmosphere was no longer anoxic. 203The Tl enrichment disappears as the rare sulfur isotope signature returns. These findings were supported by redox-sensitive element enrichments, a more classical tool for tracing ancient O changes.2.
“When the sulfur isotopes tell us the atmosphere became oxygenated, the thallium isotopes tell us the oceans became oxygenated, and when the sulfur isotopes tell us the atmosphere became anoxic again, the thallium isotopes tell us the oceans became anoxic again,” Ostrander says. “So the atmosphere and oceans were simultaneously oxygenating and deoxygenating. This is new and exciting information for anyone interested in the ancient Earth.”
Reference: “The onset of coupled atmosphere-ocean oxygenation 2.3 billion years ago” by Chadrin M. Ostrander, Andy W. Hurd, Yunchao Xu, Andrej Becker, Simon W. Poulton, Kasper P. Olsen, and Sune G. Nielsen, 12 June 2024, Nature.
Publication date: 10.1038/s41586-024-07551-5