An international team of researchers has determined there was a "whiff" of oxygen in Earth's atmosphere about 2.5 billion years ago, the earliest time any significant amount of oxygen has been detected on Earth. Up to now, scholars believed oxygen levels on Earth were negligible before the "Great Oxidation Event" (GOE) about 2.3 to 2.4 billion years ago.
This latest discovery indicates there was at least a little oxygen in Earth's atmosphere 50 to 100 million years before the GOE. It also provides scholars with more information to help them solve the mystery of the origins of oxygen on Earth.
The research has been published this week in the journal Science.
"Fifty to 100 million years may not seem like a lot in the overall scheme of things—the Earth is about 4.5 billion years old—but this finding provides us with a record of something we didn't know before, and that's significant in itself," said Dr. Robert Creaser, a geologist at the University of Alberta and a co-author of the study.
Some researchers suggest the sudden evolution of organisms that produce oxygen by photosynthesis triggered the GOE. However, the latest research indicates that the evolution of oxygen producing organisms occurred prior to the GOE, and that oxygen buildup in Earth’s atmosphere was more gradual than sudden.
"Our research indicates there were oxygen producing organisms for many millions of years before the Great Oxidation Event, and then perhaps a 'tipping point' was reached, which launched the Great Oxidation Event," Creaser said.
Creaser and Ph.D student Brian Kendall made a key contribution to the research by using rhenium, a trace element present in a sample of shale taken from the Hamersley Basin in Western Australia, to precisely date the deposition of the shale. Rhenium accumulation in shale occurs only when oxygen is present in the overlying seawater. Rhenium is a radioactive element and can be used for absolute dating. Creaser and Kendall were able to prove that the rhenium enrichment was the same age as the shale it was found in—2.5 billion years old – proving that the rhenium enrichment and other indicators of oxygen in the shale predate the GOE.
Kendall was at Arizona State University as a Visiting Student in 2006, and, upon seeing the shale, brought core samples back to Edmonton for analysis.
"We've been working on a method to precisely date rocks like shale using the radioactivity of rhenium, and we've finally been able to nail it down—it's a complex process that only a few laboratories in the world are able to do," Creaser said.
"It's really exciting to conduct research in the forefront of your field," Creaser added. "I think that's why most of us go into research in the first place, to make fundamental discoveries like this one."
The findings are reported in “A Whiff of Oxygen Before the Great Oxidation Event?”. Authors on the paper include AD Anbar, Y Duan, GL Arnold and GW Gordon from Arizona State University, C Scott and TW Lyons from the University of California Riverside, B Kendall and RA Creaser from the University of Alberta, AJ Kaufman from the University of Maryland, and J Garvin and R Buick from the University of Washington.
Dr. Robert Creaser may be reached at 780-492-2942 or robert.creaser@ualberta.ca.
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Science