May 9, 2016 – A new process for analyzing ancient bubbles trapped in lava sheds light on the Earth’s atmosphere and its evolution over billions of years. Researchers at the Denver Museum of Nature & Science, along with colleagues at the University of Washington and the University of Western Australia, used a CT scanner to analyze the size and distribution of gas bubbles within lava flows that solidified an estimated 2.7 billion years ago.
“Since no one was around billions of years ago to collect and store atmospheric samples, we rely on proxies, such as the gas bubbles trapped in lava rocks, to determine what the atmosphere was like at the time and help inform how it has changed over time,” said James Hagadorn, curator of geology at the Denver Museum of Nature & Science. “And what these fossilized bubbles tell us is that our atmosphere has changed a lot.”
The scans, along with other proxies, such as raindrops fossilized in stone and rusting of minerals in ancient river deposits, provide evidence that Earth’s atmosphere was twice as thin 2.7 billion years ago as it is now. The results also point to an ancient atmosphere that was rich in greenhouse gases and that it greatly fluctuated over the course of time.
“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. “Our result is the opposite of what we were expecting.”
These findings contribute to a refined understanding of the degree to which temperatures varied and how resilient Earth is.
“The entire scientific community has a vested interest in these results,” said Hagadorn. “This CT scanning technique can be applied in a variety of ways to study other celestial bodies and their atmospheric evolution. Just imagine putting one of these on a Mars rover to study the lava flows that cover its surface!”
The idea of using bubbles trapped in cooling lava as a “paleobarometer” to determine the weight of air in our planet’s youth occurred decades ago to co-author Roger Buick, a UW professor of Earth and space sciences. Others had used the technique to measure the elevation of lavas a few million years old. To flip the idea and measure air pressure farther back in time, researchers needed a site where truly ancient lava had undisputedly formed at sea level.
Their field site in Western Australia was discovered by co-author Tim Blake of the University of Western Australia. There, the Beasley River has exposed 2.7 billion-year-old basalt lava. The lowest lava flow has “lava toes” that burrow into glassy shards, proving that molten lava plunged into seawater. The team drilled into the overlying lava flows to examine the size of the bubbles.
A stream of molten rock quickly cools from top and bottom, and bubbles trapped at the bottom are smaller than those at the top. The size difference records the air pressure pushing down on the lava as it cooled, 2.7 billion years ago. Rough measurements in the field suggested a surprisingly lightweight atmosphere and the scientists decided to take more rigorous x-ray scans.
Dozens of samples were taken and cored with a special drill. The researchers only took samples from lava solidified on ancient beaches and tidal flats to ensure elevation was known.
These samples were then scanned at the Denver Museum of Nature & Science on a microfocus X-ray CT scanner. Unlike scanners used on humans, this one emits higher-energy X-rays that can penetrate dense objects like rock, and can image microscopic structures really well, such as bubbles a tenth the size of a period. Each scan took half a day to complete. The data from the scans was then processed to make three-dimensional images of the bubbles located deep inside the rock samples.
Scientists already know that ancient Earth was home to only single-celled microbes, sunlight was about one-fifth weaker, and the atmosphere contained no oxygen, but the results point to conditions being even more otherworldly than previously thought. A lighter atmosphere could affect wind strength and other climate patterns, and would even alter the boiling point of liquids.
“We’re still coming to grips with the magnitude of this,” Buick said. “It’s going to take us a while to digest all the possible consequences.” Other geological evidence clearly shows liquid water on Earth at that time, so the early atmosphere must have contained more heat-trapping greenhouse gases, like methane and carbon dioxide, and less nitrogen.
“The levels of nitrogen gas have varied through Earth’s history, at least in Earth’s early history, in ways that people just haven’t even thought of before,” said co-author David Catling, a UW professor of Earth and space sciences. “People will need to rewrite the textbooks.”
The researchers will next look for other suitable rocks to confirm the findings and learn how atmospheric pressure might have varied through time.
While clues to the early Earth are scarce, it is still easier to study than planets outside our solar system, so this will help understand possible conditions and life on other planets where atmospheres might be thin and oxygen-free, like that of the early Earth.
The results were published online May 9 in Nature Geoscience.