Geologic record reveals how the oceans were oxygenated

Roughly 2.5 billion years in the past, the accumulation of free oxygen (O2) in Earth’s atmosphere marked the beginning of a pivotal era for the development of complex life on our planet. This significant event is commonly known as the Great Oxidation Event (GOE).

However, recent research spearheaded by a University of Utah geochemist suggests that the process of O2 accumulation was far more complex and protracted than previously understood, spanning at least 200 million years. Chadlin Ostrander, an assistant professor in the Department of Geology & Geophysics, highlights the longstanding challenge of tracking O2 accumulation in the oceans, a challenge that is now being addressed.

“Emerging data suggest that the initial rise of O2 in Earth’s atmosphere was dynamic, unfolding in fits-and-starts until perhaps 2.2. billion years ago,” said Ostrander, lead author on the study published June 12 in the journal Nature. “Our data validate this hypothesis, even going one step further by extending these dynamics to the ocean.”

The international research team, supported by the NASA Exobiology program, investigated marine shales from South Africa’s Transvaal Supergroup to gain insights into the dynamics of ocean oxygenation during a crucial period in Earth’s history.

Through the analysis of stable thallium (Tl) isotope ratios and redox-sensitive elements, they uncovered compelling evidence of fluctuations in marine O2 levels that correlated with changes in atmospheric oxygen.

These significant findings contribute to our understanding of the intricate processes that influenced Earth’s O2 levels during a critical period, ultimately paving the way for the evolution of life as we know it.

“We really don’t know what was going on in the oceans, where Earth’s earliest lifeforms 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 content of the oceans and how that evolved with time is probably more important for early life than the atmosphere.”

The research is built upon the work of Ostrander’s co-authors Simon Poulton of the University of Leeds in the UK and Andrey Bekker of the University of California, Riverside. In a 2021 study, their team of scientists made a significant discovery that challenges previous beliefs about the timeline of oxygen becoming a permanent part of the atmosphere. They found that O2 did not become a permanent part of the atmosphere until about 200 million years after the global oxygenation process began.

The evidence of an anoxic atmosphere before the Great Oxidation Event (GOE) is the presence of rare, mass-independent sulfur isotope signatures in sedimentary records. These unique signatures are indicative of very few processes on Earth and their preservation in the rock record almost certainly requires an absence of atmospheric O2.

The study also revealed that for the first half of Earth’s existence, its atmosphere and oceans were largely devoid of O2. While cyanobacteria were producing this gas in the ocean before the GOE, it was rapidly destroyed in reactions with exposed minerals and volcanic gasses.

Additionally, the observation of the rare sulfur isotope signatures disappearing and reappearing suggests multiple rises and falls of O2 in the atmosphere during the GOE, indicating that the process was not a single ‘event.’

“Earth wasn’t ready to be oxygenated when oxygen started to be produced. Earth needed time to evolve biologically, geologically, and chemically to be conducive to oxygenation,” Ostrander said. “It’s like a teeter-totter. You have oxygen production but so much oxygen destruction that nothing’s happening. We’re still trying to figure out when we’ve completely tipped the scales, and Earth could not go backward to an anoxic atmosphere.”

Today, oxygen makes up 21% of the Earth’s atmosphere by weight, second only to nitrogen. However, after the Great Oxidation Event (GOE), oxygen remained a very small part of the atmosphere for hundreds of millions of years.

The research team relied on Ostrander’s expertise in stable thallium isotopes to study the presence of oxygen in the oceans during the GOE.

Isotopes are atoms of the same element with different numbers of neutrons, resulting in slightly different weights. Ratios of an element’s isotopes have led to discoveries in archaeology, geochemistry, and other fields.

Advancements in mass spectrometry have allowed scientists to precisely analyze isotope ratios for elements further down the Periodic Table, such as thallium. Fortunately, thallium isotope ratios are sensitive to manganese oxide burial on the seafloor, a process dependent on oxygen in seawater. The team examined thallium isotopes in the same marine shales that recently revealed atmospheric oxygen fluctuations during the GOE using rare sulfur isotopes.

In these shales, Ostrander and his team detected significant enrichments in the lighter-mass thallium isotope (203Tl), a trend best explained by seafloor manganese oxide burial and the accumulation of oxygen in seawater. These enrichments were found in the same samples lacking the rare sulfur isotope signatures, indicating the absence of anoxic conditions in the atmosphere.

Furthermore, the 203Tl enrichments vanished when the rare sulfur isotope signatures reappeared. These findings were supported by enrichments of redox-sensitive elements, a conventional method for tracing ancient oxygen variations.

“When sulfur isotopes say the atmosphere became oxygenated, thallium isotopes say that the oceans became 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 ocean were becoming oxygenated and deoxygenated together. This is new and cool information for those interested in ancient Earth.”

Journal reference:

Chadlin M. Ostrander, Andy W. Heard, Yunchao Shu, Andrey Bekker, Simon W. Poulton, Kasper P. Olesen & Sune G. Nielsen. Onset of coupled atmosphere–ocean oxygenation 2.3 billion years ago. Nature, 2024; DOI: 10.1038/s41586-024-07551-5

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