Studies of sea shale and isotopic data from the Great Oxidation Event reveal dynamic oxygen fluctuations in Earth’s early atmosphere and oceans, highlighting the protracted and complex nature of this critical evolutionary phase. Credit: SciTechDaily.com
According to recent discoveries, the Earth’s “Great Oxidation Event” was spread over 200 million years.
New research highlights the complexity of the Great Oxidation Event, revealing that the rise of atmospheric and oceanic oxygen was a dynamic process lasting more than 200 million years, influenced by geological and biological factors critical to the evolution of life.
A major oxidation event
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.”
Chadlin Ostrander. Credit: Chad Ostrander, University of Utah
Insights from marine shale
Its international research team, which is supported NASA Focusing on marine shales from South Africa’s Transvaal Supergroup, the Exobiology Program provides insights 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.
Understanding early ocean conditions
“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 contents of the oceans and how they evolved over time are 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.
Fluctuations in atmospheric and oceanic oxygen
“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”.
Challenges in Earth Oxygenation
“The earth was not ready for oxygenation when oxygen starts to be produced.” Earth needed time to evolve biologically, geologically and chemically to help oxygenation,” Ostrander said. “It’s like a balancing act. You produce oxygen, but you destroy so much oxygen that 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.”
Today, O2 it makes up 21% of the mass, second only to nitrogen. But after the GOE, oxygen remained a very minor component of the atmosphere for hundreds of millions of years.
Advanced techniques of isotopic analysis
To monitor the presence of O2 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. Isotope ratios of a particular element have fueled discoveries in archaeology, geochemistry, and many other fields.
Thallium isotopes and oxygen tracers
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 the rare sulfur isotopic 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.
“Where sulfur isotopes say the atmosphere has become oxygenated, thallium isotopes say the oceans have become 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 the ocean were oxygenating and deoxygenating together.” This is new and great information for those interested in the ancient Earth.”
Reference: “Onset of Coupled Atmosphere-Ocean Oxygenation 2.3 Billion Years Ago” by Chadlin M. Ostrander, Andy W. Heard, Yunchao Shu, Andrey Bekker, Simon W. Poulton, Kasper P. Olesen, and Sune G. Nielsen, 12 June 2024, Nature.
DOI: 10.1038/s41586-024-07551-5