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from the issue of November 4, 2004
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Study looks at oxygen in our past
BY TOM SIMONS, UNIVERSITY COMMUNICATIONS
Oxygen: We can't live without it, and neither can most other organisms on Earth. For most of the planet's history, however, very little oxygen was available, and as a result, life was neither widespread nor diverse until the Cambrian explosion more than 500 million years ago, some 3 billion years after the earliest fossil records of life.
Now, a study indicates that low-oxygen conditions went on even longer than scientists had previously thought.
Writing in the Oct. 14 edition of Nature, the international weekly journal of science, a team of scientists that includes UNL geologist Tracy Frank reported that their studies indicate low-oxygen conditions persisting for more than 1 billion years after oxygen began to accumulate in the atmosphere 2.2 billion years ago. Further, they found no significant increase in atmospheric oxygen until at least 750 million years ago, just before the onset of a series of major glacial episodes during which ice is thought to have covered much of the Earth's surface.
For the study, the scientists examined Precambrian marine sedimentary rocks from arctic Canada, Australia and Glacier National Park in Montana. They examined molecules of sulfate, a mineral whose molecules consist of one sulfur atom and four oxygen atoms, that reside within limestones. Sulfate can only form under oxidizing conditions and thus can be used as a proxy for measuring the presence of oxygen in the atmosphere.
"Before there was much oxygen in the atmosphere, even though it was raining and rocks were weathering, you didn't have things like pyrite (a compound of iron and reduced sulfur) being oxidized into sulfate. It just wasn't possible without oxygen," Frank said.
"By measuring the rocks, we can use these chemical proxies to understand how the ocean evolved. And the oceans integrate all of the global signals of weathering, so we can also understand what was going on in the atmosphere by studying the marine record."
Earlier studies had shown that the sulfate concentrations in recent limestones are about 10 times higher than in Precambrian rocks, but Frank and her colleagues decided to look at sulfur isotopes, as well, which gave them a better picture of the rate of oxygenation.
Frank said that because the modern ocean contains so much sulfate, it's very difficult to shift the composition of sulfur isotopes in the sulfate (isotopes are atoms of the same element with different numbers of neutrons in their nucleus). Over the last 500 million years, she said, the concentration of sulfur isotopes has changed by about 10 parts per 1,000 over 300 million years. In the ancient ocean, however, the rate of change was much faster.
"When we started to look at what the sulfur isotope record was doing in more ancient rocks, we found that the rate of change was something like 10 times faster than what we see in younger periods of Earth history," Frank said. "That suggested to us that the sulfate signal in the oceans was much more easily perturbed than it would be now, suggesting a smaller reservoir of dissolved sulfate in the oceans.
"And we found that it remained relatively small much beyond the time when people thought that the Earth's atmosphere and oceans were more oxidizing. What we found was the size of the sulfate reservoir stayed at below about 30 percent of the size of the modern reservoir until about 750 million years ago, when it started to increase more rapidly."
Frank's colleagues on the Nature paper were lead author Linda Kah of the University of Tennessee-Knoxville and Timothy Lyons of the University of Missouri-Columbia. Their work was funded by the National Science Foundation, the National Geographic Society, the Polar Continental Shelf Project (Natural Resources, Canada) and the University of Missouri Research Board.
GO TO: ISSUE OF NOVEMBER 4
NEWS HEADLINES FOR NOVEMBER 4
Alumna Donates Flags That Flew in Afghanistan
Cather scholar Rosowski dies
Professor to speak on Muslim cultures
Study looks at oxygen in our past
Teaching project builds on successes
A Piece of University History
Lessons of Afghan conflict are topic of Forum
Professor helps track election results, projections
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