Ancient air bubbles could revise history of Earth’s oxygen

If new findings are correct, rise in gas preceded earliest animals

air bubbles in a rock

NOT-SO-FRESH AIR  Ancient air embedded inside rock salt for 815 million years suggests that oxygen was already abundant when the first animals appeared. The microscopic air bubbles were trapped inside rectangular inclusions (one circled in red) in the rock.

Chris Lecuyer

Whiffs of ancient air trapped in rock salt for hundreds of millions of years are shaking up the history of oxygen and life on Earth.

By carefully crushing rock salt, researchers have measured the chemical makeup of air pockets embedded inside the rock. This new technique reveals that oxygen made up 10.9 percent of Earth’s atmosphere around 815 million years ago. Scientists have thought that oxygen levels would not be that high until 100 million to 200 million years later. The measurements place elevated oxygen levels well before the appearance of animals in the fossil record around 650 million years ago, the researchers report in the August issue of Geology.

“I think our results will take people by surprise,” says study coauthor Nigel Blamey, a geochemist at Brock University in St. Catharines, Canada. “We came out of left field, and I think some people are going to embrace it, and other people are going to be very skeptical. But the data is what the data is.”

Scientists have previously measured ancient Earth’s oxygen supply by looking for the fingerprints of chemical reactions that require oxygen to take place (SN: 11/29/14, p. 14). Such work has suggested that oxygen levels rose sharply around 600 million years ago, during the Neoproterozoic era.

That earlier work measured oxygen levels only indirectly, however, leading to uncertainty and mismatches between various studies of the Neoproterozoic oxygen rise. Blamey and colleagues instead went to the source: actual air left over from the time period.

Around 815 million years ago in what is now southwest Australia, rock salt formed on the surfaces of evaporating ponds. As the salt grew, microscopic air pockets formed. Hundreds of millions of years later, that air remains sequestered in the rock.

The researchers crushed match head–sized pieces of the salt in a vacuum, each piece releasing five to 12 puffs of gas. Oxygen levels in the newly liberated air were on average more than five times the 2 percent concentration predicted by previous studies, the researchers found. The team verified the technique by measuring oxygen in younger rock salts, including modern samples.

If the findings are correct, such an early appearance of abundant oxygen in Earth’s atmosphere adds a twist in a debate about whether limited oxygen stalled the evolution of animals (SN: 11/14/15, p. 18), says Nicholas Butterfield, a paleontologist at the University of Cambridge. Even with plenty of oxygen, animals still took a long time to evolve after the emergence of the first complex life, he argues. “The delayed evolutionary appearance of animals had nothing to do with limiting levels of atmospheric oxygen.”

Outside air may have tainted the new results, however, says Yale University geochemist Noah Planavsky. Over hundreds of millions of years, gases may have passed through the salt and boosted oxygen levels in the air pockets.

The possibility of outside contamination seems especially likely given the surprisingly high oxygen levels recorded, says Timothy Lyons, a geochemist at the University of California, Riverside. A concentration of 10.9 percent “is really high,” he says. “There is nothing about the shifts you see in life or climate that demands an oxygen jump that high. That could be a worry. We have had so much oxygen over the last half a billion years, and this is a number like that.”

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