Life’s Struggle Against Low Oxygen Concentrations in the Early Ocean and Atmosphere

Tasmanian researchers, working with a UC Riverside scientist, have revealed ancient conditions in the ocean that may have limited the early development of life

Photo shows drill cores.

Ross et al. analyzed samples of pyrite from rocks collected around the world. Pictured here are the fresh drill cores collected in South Africa. These are representative of the kinds of materials used in the study.Photo credit: Lyons Lab, UC Riverside.

RIVERSIDE, Calif. — The relationship between oxygen and life in the ocean is at the heart of understanding the evolution of both.  Best estimates now place the first permanent accumulation of oxygen in the atmosphere at something like 2.3 billion years ago, and those concentrations rose to very high levels soon after.  But following that rise was an equally impressive fall, and the ocean and atmosphere were then poised at low levels that challenged life in the ocean for more than a billion years.

According to University of Tasmania geologist Professor Ross Large and his international team, the key was a lack of oxygen and nutrient elements, which placed evolution in a precarious position. “During that billion years, oxygen levels declined and the oceans were losing the ingredients needed for life to develop into more complex organisms,” he said.

By analyzing ancient seafloor rocks, Ross and his Australian, Russian, US, and Canadian colleagues were able to show that the slowdown in evolution was tightly linked to low levels of oxygen and biologically important elements in the oceans.

“We’ve looked at thousands of samples of the mineral pyrite in rocks that formed in the ancient oceans. And by measuring the levels of certain trace elements in the pyrite, using a technique developed in our labs, we’ve found that we can tell an accurate story about how much oxygen and nutrients were around billions of years ago.”

Their research will be published in the March issue of the journal Earth and Planetary Science Letters.

“We were initially looking at oxygen levels in the ancient oceans and atmosphere to understand how mineral deposits form, and where to look for them today. That’s a focus of the Centre for Ore Deposit and Exploration Science, which we established at the University of Tasmania in 1989,” Ross said. “But the technology we have developed to find minerals can also tell us much about the evolution of life.”

After an initial burst of oxygen, the study plots a long decline in oxygen levels during the ‘boring billion’ years before leaping up about 750-550 million years ago. “We think this recovery of oxygen levels led to a significant increase in trace metals in the ocean and triggered the ‘Cambrian explosion of life’.”

UC Riverside Professor of Biogeochemistry Timothy W. Lyons, and coauthor on the study, noted that the results of the study agree beautifully with chemical data measured on very different samples.

“We are delighted that the story seems to be converging on a common theme based on independent tests, suggesting that we might be capturing the primary chemical conditions in the very early ocean,” he said.

Lyons and his colleagues have published previously using organic-rich shales and iron formations.

“All these data point to an ocean during Earth’s middle chapters, roughly 2.0 to 0.8 billion years ago, that was poor in both oxygen and the life-sustaining nutrients needed to spawn large populations of diverse organisms, as well as higher oxygen concentrations, and these factors may have delayed the rise of animals,” Lyons said.  “Only after this interval did oxygen rise to the necessary levels through a complex combination of tectonic, climatic, and evolutionary events.  The data from the Ross et al. study are an important development in this emerging story of early life.”

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