Earth's Oxygen: A Mystery Easy to Take for Granted

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To Donald E. Canfield, there's something astonishing in every breath we take. "People take oxygen for granted because it's just there and we breathe it all the time," said Dr. Canfield, a geochemist at the University of Southern Denmark. "But we have the only planet we know of anywhere that has oxygen on it."

What's even more astonishing is that Earth started out with an oxygen-free atmosphere. It took billions of years before there was enough of the element to keep animals like us alive.

Although scientists have been struggling for decades to reconstruct the rise of oxygen, they're still making fundamental discoveries. In just the past two weeks, for example, Dr. Canfield and his colleagues have published a pair of studies that provide significant clues about some of the most important chapters in oxygen's history. They're finding that our weirdly oxygen-rich atmosphere is the result of a complicated dance of geology and biology.

To study the ancient atmosphere, geochemists examine the chemical fingerprints left behind on rocks. Some rocks contain molecules that could have only formed in the presence of oxygen. The more of those molecules geochemists find in a rock, the more oxygen must have been in the atmosphere at the time.

When they look at the oldest rocks on Earth, they find no trace of oxygen in the atmosphere. Instead, their research indicates Earth's primordial air was made up mostly of carbon dioxide, methane and nitrogen. The Sun's rays created some free oxygen by splitting it off from carbon dioxide and other molecules. But the oxygen disappeared almost as soon as it was formed.

That's because oxygen is an enormously friendly element, forming bonds with a wide range of molecules. It attached to the iron in rocks, for example, creating rust. It joined with the hydrogen spewed out from volcanoes to form hydrogen peroxide and other compounds. Our planet, in other words, was a giant oxygen vacuum in its early years.

That changed by three billion years ago. In the Sept.26 issue of Nature, Dr. Canfield and his colleagues reported the fingerprints of oxygen in rocks from that time period. They estimate that the atmosphere three billion years ago had only 0.03 percent of today's oxygen levels. That may not sound like much, but it marked a huge shift in Earth's chemistry.

Sunlight alone couldn't have put that much oxygen in the atmosphere. Only life could.

By three billion years ago, some microbes had evolved the ability to carry out photosynthesis. Floating at the surface of the ocean, they used energy from sunlight to grow on carbon dioxide and water. They gave off oxygen as waste.

Much of the oxygen released by these photosynthetic microbes was sucked out of the atmosphere by Earth's vacuum. It reacted with hydrogen from volcanoes, for example. When microbes died, oxygen reacted with their carbon molecules.

But a tiny amount of oxygen remained behind because some of the organic matter from the dead microbes sank from the surface of the ocean to the sea floor, where oxygen couldn't react with it. The oxygen remained in the air.

Oxygen remained fairly scarce for the next few hundred million years. But during that time, Earth's vacuum was getting weak. The planet was cooling, and so its volcanoes spewed less hydrogen into the atmosphere to suck up oxygen.

In his forthcoming book, "Oxygen: A Four Billion Year History," Dr. Canfield suggests that this weak vacuum drove a sudden climb in oxygen that geochemists see in rocks from about 2.3 billion years ago. "Now we get to the point where the Earth has calmed down enough that the balance has tipped in the favor of oxygen," he said.

This oxygen boom may have added fuel to life's fire. The extra oxygen in the atmosphere attacked rocks exposed on land, freeing up phosphorus and iron to flow into the ocean to act as fertilizer. The microbes bloomed even more, sending up even more oxygen.

Reporting this week in The Proceedings of the National Academies of Sciences, Dr. Canfield and his colleagues report that there was so much oxygen in the atmosphere that it penetrated down a thousand feet into the ocean. Dr. Canfield speculates that oxygen may have become as abundant as it is today, at least for a while.

But this boom created its own bust. Microbes rained down onto the sea floor, creating carbon-rich rocks. Later, the rocks were lifted up to form dry land, where they could react with the oxygen, pulling it out of the atmosphere.

Life itself, in other words, turned Earth's vacuum back up again. By 2 billion years ago, oxygen levels were down to about 0.01 percent of current levels.

Life and Earth have continued to twiddle the oxygen knob over the past two billion years. When plants evolved, for example, they began storing away huge amounts of carbon in wood and other tough tissues, leaving less to react with oxygen and pull it out of the atmostphere. By 300 million years ago, oxygen levels had risen to levels as high as 50 percent higher than today.

But as continents moved across the globe, the planet's geography came to favor deserts. Forests shrank, bringing down the oxygen levels.

As Dr. Canfield gets better acquainted with the tumultuous history, he gets less certain about its future. Will Earth hold onto its remarkable supply of oxygen, or will it run low again? "I'm not sure we have a good prediction," said Dr. Canfield. "That depends a lot on the vagaries of geography."

science

This article originally appeared in The New York Times.


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