For billions of years, Earth lacked the oxygen-rich atmosphere that sustains life today. A new study reveals a surprising truth about the Great Oxidation Event (GOE), the pivotal period when oxygen began to accumulate in our planet’s atmosphere. It wasn’t a single, swift event, but a prolonged process marked by fits and starts.

Rather than a single on-switch, the GOE lasted at least a staggering 200 million years. During this time, oxygen levels shot up only to be pulled back down by geochemical processes that absorbed the free oxygen from the atmosphere. This was an incredibly long chemical tug-of-war that lasted until about 2.2 billion years ago.

“Emerging data suggest that the initial rise of O₂ in Earth’s atmosphere was dynamic,” said lead author Chadlin Ostrander, an assistant professor at the University of Utah. “Our data validate this hypothesis, even going one step further by extending these dynamics to the ocean.”

The long road from deadly gas to fresh air

Ostrander and colleagues tracked fluctuations in oxygen during Earth’s ancient geological past by studying marine shale—sedimentary rocks formed from ancient ocean mud—from South Africa’s Transvaal Supergroup.

By analyzing stable thallium (Tl) isotope ratios and redox-sensitive elements, the team found evidence of fluctuating oxygen levels in the oceans that mirrored changes in the atmosphere. Redox-sensitive elements are elements that change their chemical form depending on the presence or absence of oxygen.

The study showed that the rise of oxygen in Earth’s atmosphere was chaotic until about 2.2 billion years ago. The data also shows that these fluctuations mirrored oxygen levels in the ocean. This is a particularly important observation because early life likely originated and evolved in the oceans.

“So the atmosphere and ocean were becoming oxygenated and deoxygenated together. This is new and cool information for those interested in ancient Earth,” said Ostrander.

“Knowing the O₂ content of the oceans and how that evolved with time is probably more important for early life than the atmosphere.”  

These findings build on previous work by Ostrander’s colleagues, Simon Poulton and Andrey Bekker. Their 2021 study revealed that oxygen did not become a permanent part of the atmosphere until about 200 million years after the GOE began. Scientists could tell when Earth’s atmosphere and oceans were largely devoid of oxygen by identifying sulfur isotope signatures in ancient sediments. This “smoking gun” signature cannot appear unless the environment lacks oxygen.

Ancient Atmosphere

Before the GOE, Earth’s atmosphere was predominantly methane, ammonia, water vapor, and nitrogen, with little to no free oxygen. Yet even under these hellish conditions during the Archean Eon, primitive life forms like anaerobic bacteria thrived.

Photosynthetic organisms, such as cyanobacteria (known as blue-green algae), later emerged but their activity was limited by the thick atmosphere that blocked a lot of the sun. Living in mats of sediment and cells on the ocean floor, they formed structures called stromatolites, which are common fossils from the Archean period. These fossils often show evidence of phototaxis, indicating that the cyanobacteria followed sunlight to photosynthesize.

When free oxygen was produced in more abundant quantities, it was absorbed by minerals and volcanic gases. The findings show that oxygen levels rose and fell repeatedly during the GOE. It was no “single” event but rather a series of ups and downs. Oxygen levels were like the stock market, and it apparently took 200 million years until Earth and life on it entered a radically new stage. Exactly what pushed the planet out of this repeating cycle is still an open question.

“Earth wasn’t ready to be oxygenated when oxygen starts 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 you have so much oxygen destruction, nothing’s happening. We’re still trying to figure out when we’ve completely tipped the scales and Earth could not go backwards to an anoxic atmosphere.”

The Great Oxidation Event represents a cornerstone in Earth’s history. It marks the advent of photosynthesis and biological oxygen production, setting the stage for the evolution of complex life and dramatically transforming our planet’s environment. We’re now stepping closer to unraveling this delicate but pivotal period. And we are finding it was much more complex and dynamic than previously thought.

The findings appeared in the journal Nature.