Billion-year-old air reveals surprise about oxygen on ancient Earth
Oxygen levels much higher than expected, with implications for origin of complex life
Canadian scientists have found a way to analyze air from the ancient Earth's atmosphere that was trapped in salt crystals nearly a billion years ago.
What they found may have implications for the origin of complex life.
The air, which has been preserved, undisturbed, in tiny pockets in the crystals for about 815 million years, appears to contain 10.9 per cent oxygen — just half the amount in the atmosphere today.
But it's about five times more than scientists expected for that time period, which is about 200 million years before the first known multicellular fossils.
"I'm surprised and excited," said Nigel Blamey, a professor of earth sciences at Brock University in St. Catharines, Ont., who co-led the study with fellow Brock geochemist Uwe Brand.
Brand says the discovery answers a key question about the evolution of complex life — did animals arise before or after the oxygen needed to support larger, more complex organisms?
"And now with our research and our result, we know there was sufficient oxygen before they arose," he said, adding that the higher oxygen levels would have allowed animals to diversify and become more complex.
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That means it may be possible to find multicellular fossils older than the oldest that are currently known.
"Now paleobiologists will have reason to go looking for rocks with original traces of these first evolutionary steps," Brand said.
The team's results are published in the journal Geology.
When salty water called brine evaporates from a shallow pool, it forms crystals of salt called halite — very similar to the kind we sprinkle on our food. In the process, it often traps tiny bubbles of fluid or air from the atmosphere at the time it forms.
That happened in the Officer Basin in southwestern Australia about 815 million years ago, during a geological era called the Neoproterozoic. The halite was subsequently buried and preserved under about a kilometre of sediment.
For the study, researchers removed the deeply buried samples using a drill core. While it wasn't chemically possible to directly measure the age of the salt, the layers on either side of it contained radioactive isotopes of elements such as uranium that decay at a very specific rate. Those measurements showed that the layers on either side of the salt were 800 million and 830 million years old, suggesting the salt was in between those ages.
The samples were taken back to the lab where Blamey has built a geochemical analysis machine that is unique in the world. It can crush tiny samples of rock as small as a tenth of a gram, and measure, identify and compare minute quantities of the gas released from tiny pores in the rock using a device called a mass spectrometer. The device has previously been used to detect methane trapped in meteorites from Mars, among other things.
The team took multiple samples of ancient Australian halite, crushing each one four to 10 times and measuring the gas that was released.
"They all gave us very similar results."
The researchers also compared salt samples from other halite deposits of different ages — including the modern day — when oxygen concentrations were known and found their results matched up with known concentrations. That gave them confidence in their result, they said.
Scientists had previously estimated that the Earth's atmosphere contained just two per cent oxygen during the Neoproterozoic. Blamey said that was largely based on geochemical measurements of marine sediments — located deep underwater, far from the atmosphere itself — combined with computer modelling, leaving lots of room for error and incorrect assumptions.
"They're using an indirect method," he said. "We're using a direct measurement and we're measuring from a mineral that has actually trapped atmosphere. It's a huge step forward, really."
Blamey and Brand collaborated with co-authors from the U.S., Scotland, France and China who helped provide samples from different time periods from around the world, and provided input about their analysis. The study was funded by the Natural Sciences and Engineering Research Council of Canada and the National Natural Science Foundation of China.