Studies of marine shale and isotope data from the period of the Great Oxidation Event reveal dynamic oxygen fluctuations in Earth’s early atmosphere and oceans, highlighting the long-lasting and complex nature of this crucial evolutionary stage. Credit: SciTechDaily.com
According to recent discoveries, Earth’s “Great Oxidation Event” occurred over a period of 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 that lasted more than 200 million years, influenced by geological and biological factors crucial to the evolution of life.
The great oxidation event
About 2.5 billion years ago, free oxygen, or O2first began accumulating to meaningful levels in Earth’s atmosphere, paving the way for the emergence of complex life on our evolving planet.
Scientists call this phenomenon the Great Oxidation Event, or GOE for short. But the initial accumulation of O2 on Earth was not nearly as simple as that name suggests, according to new research led by a geochemist from the University of Utah.
This ‘event’ lasted at least 200 million years. And monitoring the accumulation of O2 in the oceans has been very difficult so far, says Chadlin Ostrander, assistant professor in the Department of Geology and Geophysics.
“Emerging data suggest that the initial emergence of O2 in Earth’s atmosphere was dynamic and unfolded in fits and starts until perhaps 2.2. billion years ago,” said Ostrander, lead author of the study published June 12 in the journal Nature. “Our data validates this hypothesis and goes even further by extending these dynamics to the ocean.”
Chadlin Ostrander. Credit: Chad Ostrander, University of Utah
Insights from marine shales
His international research team, which is supported by the NASA Exobiology programme, focusing on marine shales from the South African Transvaal Supergroup, providing insight into the dynamics of ocean oxygenation during this crucial period in Earth’s history. By analyzing stable thallium (Tl) isotope ratios and redox-sensitive elements, they found evidence for fluctuations in marine O2 levels that coincided with changes in atmospheric oxygen.
These findings help increase understanding of the complex processes that drive O2 levels during a critical period in the planet’s history that paved the way for the evolution of life as we know it.
Understanding early oceanic conditions
“We really don’t know what was going on in the oceans, where the earliest life forms on Earth likely originated and evolved,” said Ostrander, who joined the U faculty last year from the Woods Hole Oceanographic Institution in Massachusetts . “So knowing the O2 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 Britain and Andrey Bekker of the University of California, Riverside. In a 2021 study, their team of scientists discovered that O2 only became a permanent part of the atmosphere about 200 million years after the global oxygenation process began, much later than previously thought.
Atmospheric and oceanic oxygen fluctuations
The “smoking gun” evidence of an anoxic atmosphere is the presence of rare, mass-independent sulfur isotope signatures in sedimentary data before the Republican administration. Very few processes on Earth can generate these sulfur isotope signatures, and as far as we know, their preservation in rocks almost certainly requires the absence of atmospheric O.2.
For the first half of Earth’s existence, the atmosphere and oceans were largely devoid of O2. It appears that this gas was produced by cyanobacteria in the ocean before the government’s reign, but in these early days the O2 was quickly destroyed in reactions with exposed minerals and volcanic gases. Poulton, Bekker and colleagues found that the rare sulfur isotope signatures disappear but then reappear, indicating that multiple O2 rises and falls in the atmosphere during the GOE. This was not a single ‘event’.
Challenges in Earth’s oxygen supply
“The Earth was not ready to be supplied with oxygen when oxygen began to be produced. The Earth needed time to evolve biologically, geologically and chemically to promote oxygenation,” Ostrander said. “It’s like rocking a see-saw. You have oxygen production, but you have so much oxygen destruction that nothing happens. We are still trying to figure out when we have completely tipped the balance and Earth can no longer return to an anoxic atmosphere.”
Today O2 represents 21% of the atmosphere, by weight, second only to nitrogen. But after the GOE, oxygen remained a very small part of the atmosphere for hundreds of millions of years.
Advanced techniques for isotopic analysis
To avoid 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 an unequal number of neutrons, giving them slightly different weights. Ratios of the isotopes of a given element have made possible discoveries in archaeology, geochemistry and many other fields.
Thallium isotopes and oxygen indicators
Advances in mass spectrometry have allowed scientists to accurately analyze the isotope ratios for elements further down the periodic table, such as thallium. Fortunately for Ostrander and his team, thallium isotope ratios are sensitive to manganese oxide burial on the seafloor, a process that requires O.2 in seawater. The team examined thallium isotopes in the same marine shales recently shown to contain 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 the burial of manganese oxide on the seabed, and thus by the accumulation of O2 in seawater. These enrichments were found in the same samples without the rare sulfur isotope signatures, and thus when the atmosphere was no longer anoxic. The icing on the cake: the 203The enrichments disappear when the rare sulfur isotope signatures return. These findings were confirmed by redox-sensitive element enrichments, a more classical tool for monitoring changes in ancient O2.
“If sulfur isotopes say the atmosphere became oxygen-rich, thallium isotopes say the oceans became oxygen-rich. And when the sulfur isotopes say the atmosphere has become anoxic again, the thallium isotopes say the same for the ocean,” Ostrander said. “So the atmosphere and the ocean became oxygen-rich and oxygen-depleted together. This is new and cool information for those interested in ancient Earth.”
Reference: “Onset of Coupled Oxygen Supply between Atmosphere and Ocean 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, June 12, 2024 , Nature.
DOI: 10.1038/s41586-024-07551-5