Next time you pause to view a scenic mountain vista, consider that the oxygen your lungs are taking in resulted from the same process that raised those peaks. Researchers have connected the periodic formation of supercontinents in Earth's geological past to the nourishment of tiny, oxygen-producing sea creatures, and the process continues to this day.
At least seven times, the massive plates that make up Earth's continents have slammed together--sometimes two at a time, and sometimes all of them--forming what geologists call supercontinents. Those gradual collisions severely warped the intervening crust and pushed up high mountain ranges, such as the Himalayas. Each time, over millions of years, wind and rain wore down those mountains into dust that was flushed into the sea. There, minerals containing iron, phosphorus, and other elements became food for microscopic plant life that flourished and, through photosynthesis, boosted the amount of oxygen in the atmosphere. The result, a team reported on 27 July in Nature Geoscience, was that atmospheric oxygen content rose from what they call negligible levels about 2.65 billion years ago to about 21% today.
Earth scientists Ian Campbell and Charlotte Allen of the Australian National University in Canberra inferred atmospheric oxygen levels from the oxygen content of the minerals deposited in ocean sediments. When they compared those data with geological evidence for supercontinent formation, they found close correlations in six of the cases and some evidence for the seventh. "All of the recognized supercontinent-forming events correlate with documented increases in oxygen," Campbell says. During the past 40 million years, when Asia and the Indian subcontinent collided and produced the Himalayas, atmospheric oxygen has increased from about 15% to its present level.
There are other hypotheses for oxygenation of the atmosphere, Campbell says; some researchers pose that volcanoes released oxygen from Earth's mantle into the air, for instance. But if that were the mechanism, oxygen concentrations should be much higher than observed, Campbell says.
Without plate tectonics, our planet might have had a thin atmosphere with nearly no oxygen, says geophysicist Rainer Kind of GeoForschungsZentrum, a government research institution in Potsdam, Germany. "Earth would perhaps look like Mars," he says. And that would have made the emergence of life difficult or maybe even impossible.