New research led by U geochemist uses thallium isotopes to track the rise and fall of free oxygen on Earth 2.5 billion years ago, the process that enabled life as we know it.

About 2.5 billion years ago, free oxygen, or O₂, first started to accumulate to meaningful levels in Earth’s atmosphere, setting the stage for the rise of complex life on our evolving planet.

Scientists refers to this phenomenon as the Great Oxidation Event, or GOE for short. But the initial accumulation of O₂ on Earth was not nearly as straightforward as that moniker suggests, according to new research led by a University of Utah geochemist.

This “event” lasted at least 200 million years. And tracking the accumulation of O₂ in the oceans has been very difficult until now, said Chadlin Ostrander, an assistant professor in the Department of Geology & Geophysics.

“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.”

His international research team, which is supported by the NASA Exobiology program, focused on marine shales from South Africa’s Transvaal Supergroup, yielding insights 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 uncovered evidence of fluctuations in marine O₂ levels that coincided with changes in atmospheric oxygen.

These findings help advance the 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 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.”