In a Surprise Discovery, Early Earth’s Atmosphere May Not Have Been Thicker Than Today’s

May 9, 2016 | Joanne Kennell

The layers on this 2.7 billion-year-old rock, a stromatolite from Western Australia, show evidence of single-celled, photosynthetic life on the shore of a large lake. The new result suggests that this microbial life thrived despite a thin atmosphere.
Photo credit: Roger Buick/University of Washington

"People will need to rewrite the textbooks."

It is currently accepted that 2.7 billion years ago, Earth not only had an oxygen-free atmosphere, but also a thicker one than today to compensate for weaker sunlight — which was about one-fifth the strength that it is now.  But it turns out that idea may be wrong.

According to new research from the University of Washington, bubbles trapped in 2.7-billion-year-old rocks show that air at the time was about half the pressure of today’s atmosphere.

"For the longest time, people have been thinking the atmospheric pressure might have been higher back then, because the sun was fainter," said lead author Sanjoy Som, who did the work as part of his UW doctorate in Earth and Space sciences, in a press release. "Our result is the opposite of what we were expecting."

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The results, published today (May 9) in Nature Geoscience, may also have implications for which types of gases were in the atmosphere, and how biology and climate worked on early Earth. For example, a lighter atmosphere could affect wind strength, climate patterns, and even alter the boiling point of liquids.

"We're still coming to grips with the magnitude of this," said co-author Roger Buick, a UW professor of Earth and Space sciences, in the release. "It's going to take us a while to digest all the possible consequences."

2.7 billion years ago, Earth was home only to single-celled microbes, and even though sunlight was about one-fifth weaker and the atmosphere contained no oxygen, the finding suggests that conditions were even more bizarre than previously thought. In fact, geological evidence has shown liquid water on Earth at that time, so the atmosphere must have contained more heat-trapping greenhouse gases, like methane and carbon dioxide, and less nitrogen.

Using bubbles trapped in cooling lava as a "paleobarometer" to determine the weight of air throughout Earth’s history is nothing new. Other scientists have used the technique to measure the elevation of lava millions of years old, but Som wanted to determine the air pressure much farther back in time. In order to do that, the researchers needed a site where ancient lava had once flowed at sea level.

So the team traveled to Beasley River located in Australia, and exposed 2.7 billion-year-old basalt lava. The lowest lava flow had "lava toes" that burrowed into glassy shards, proving that molten lava had plunged into seawater. They then drilled into the overlying lava flows to examine the size of the bubbles, which recorded the air pressure pushing down on the lava as it cooled 2.7 billion years ago.

Their measurements suggested a surprisingly lightweight atmosphere — an atmospheric pressure less than half of today's. However, more x-ray scans from several other rocks and lava flows will be needed to confirm the result.

Since it is easier to study planets outside our solar system, the results could help scientists understand the conditions and life on other planets where atmospheres might be thin and oxygen-free, just like early Earth.

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