"In a broad brush view, the tectonic evolution of our planet has been a major contributor to oxygenating the atmosphere," Allen said. Seven times the continents have come together, she said, and each time there has been a notable jump in atmospheric oxygen. Allen and Campbell made their case using over 5,000 samples of sediment from 40 major river deltas around the world. They found that each time Earth's tectonic plates crashed together to form a supercontinent, the amount of sediment flowing from the rivers increased, which fits with their theory. But some scientists aren't convinced. "Did mountain building play an important role in oxygenation and specifically defining some of the [oxygen] steps? Sure. Are all the authors' steps significant? Probably not," Timothy Lyons of the University of California, Riverside, said. "The authors have synthesized a lot of data into what is likely too slick a story." Lyons said that the formation of the supercontinent Rodinia 1.1 billion years ago is associated with only a modest increase in oxygen, seemingly too small to have been caused by mountain ranges eroding simultaneously all over the planet. The assembly of Pangea 300 million years ago is another problem, Lyons said. Trees evolved around that time, contributing huge amounts of oxygen to the atmosphere, but they would be unaffected by sediments washed into the ocean. Related Links: Michael Reilly's blog: Strike Slip |
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