Research

Earth’s Oceans: Where Was the Oxygen?

As two rovers scour Mars for signs of water and the precursors of life, Rochester geochemists have uncovered evidence that Earth’s own ancient oceans were much different from today’s.

The research, published in the journal Science, cites new data that shows that Earth’s life-giving oceans had less oxygen dissolved in them than today’s and could have been nearly devoid of oxygen for a billion years longer than previously thought. The findings may help explain why complex life barely evolved for billions of years after it arose.

The Rochester team has pioneered a new method that indicates how ocean oxygen might have changed globally. Most geologists agree that there was virtually no oxygen dissolved in the oceans until about 2 billion years ago, and that they were oxygen-rich during most of the last half billion years—but there has always been a mystery about the period in between. Geochemists developed ways to detect signs of ancient oxygen in particular areas, but not in the Earth’s oceans as a whole. The team’s method, however, can be extrapolated to grasp the nature of all oceans around the world.

“This is the best direct evidence that the global oceans had less oxygen during that time,” says Gail Arnold, a doctoral student in earth and environmental sciences at the University and lead author of the research.

Arnold examined rocks from northern Australia that were at the floor of the ocean over a billion years ago, using the new method developed by her and her coauthors, Jane Barling of the University of British Columbia and Ariel Anbar, associate professor of earth and environmental sciences.

Previous researchers had drilled down into the rock and tested its chemical composition, confirming it had preserved original information about the oceans. The Rochester team brought the rocks back to their labs where they used newly developed technology—called a Multiple Collector Inductively Coupled Plasma Mass Spectrometer—to examine the chemistry of the rocks.