RIVERSIDE, Calif. - Two teams of scientists, including three researchers from UC Riverside, report that traces of oxygen appeared in Earth's atmosphere roughly 100 million years before the "Great Oxidation Event" 2.4 billion years ago. The Great Oxidation Event is when most geoscientists think atmospheric oxygen rose sharply from very low levels and set the stage for animal life that followed almost two billion years later.

Analyzing layers of sedimentary rock in a kilometer-long core sample they retrieved in 2004 from the Hamersley Basin in Western Australia, the researchers found evidence for the presence of a small but significant amount of oxygen 2.5 billion years ago in the oceans and likely also in Earth's atmosphere.

Because the core was recovered from deep underground, it contains materials untouched by the atmosphere for billions of years. After retrieval, the scientists sliced the core longitudinally for analysis.

Study results appear in a pair of papers in tomorrow's issue of Science.

The UCR contribution:

Geochemists Timothy Lyons, Steven Bates, and Clinton Scott of the UCR Department of Earth Sciences -- working with teams from Arizona State University and the universities of Maryland, Washington, and Alberta -- generated elemental and isotopic data that provide indirect, or proxy, evidence for the evolving atmosphere and its relationship to the early evolution of life.

"This is the earliest convincing record for an ephemeral accumulation of oxygen in the biosphere before its irreversible rise beginning 2.4 billion years ago," said Lyons, a professor of biogeochemistry.

Scott, a graduate student working with Lyons, used metals in the ancient ocean--now trapped in sedimentary rocks--as proxies for the amount of oxygen in the early ocean and atmosphere. His doctoral research provided a baseline for the Australian samples, showing that the 2.5 billion-year old rocks look more like those from younger times when oxygen was higher in the atmosphere.

These results revealed to the UCR geochemists and their colleagues at Arizona State University that oxygen increased significantly but briefly 100 million years before its permanent place in Earth's atmosphere.

Working principally with colleagues at the University of Maryland, Bates, a research associate, and Lyons analyzed sulfur present in the Australian rocks as another fingerprint of oxygen concentrations at Earth's surface. Their analysis of the sulfur also confirmed that the world changed briefly but importantly 2.5 billion years ago, presaging the life-affirming oxygenation of the atmosphere 100 million years later.

"We were surprised to see evidence of increasing oxygen in rocks so old," Lyons said. "And the fact that two independent lines of evidence point in the same direction suggests that Earth's most dramatic shift in atmospheric composition and its relationship to the evolution of life began earlier and was more complex than most imagined."

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Funds from the Astrobiology Drilling Program of the NASA Astrobiology Institute and the U.S. National Science Foundation supported the work. The two research teams were led separately by Ariel Anbar of Arizona State University, Tempe, Ariz.; and Alan Jay Kaufman of the University of Maryland, College Park, Md.

About Tim Lyons:

Lyons joined UCR's Department of Earth Sciences almost three years ago. As a geochemist specializing in studies of how elements cycle in the ocean and atmosphere and their cause-and-effect relationships with the evolution of early life, he uses elemental and isotopic methods developed through studies of modern oxygen-poor settings, such as the Black Sea. These geochemical tools are then turned toward fundamental questions about Earth's early history recorded in the chemical properties of rocks formed many millions to billions of years ago.

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