July 30, 2008 -- Ancient supercontinents with mountain ranges as high as the Himalayas may have played a critical role in producing the oxygen we breathe today, researchers report in a new study.
Early Earth was inhospitable to life as we know it, with almost no oxygen to speak of. That is, until 2.4 billion years ago, when concentrations of the gas suddenly started climbing.
Geologists call it the Great Oxygenation Event and believe newly evolved bacteria floating in the oceans were the cause: As the bacteria consumed carbon dioxide and sunlight to make food through photosynthesis, they emitted oxygen as a byproduct.
But Ian Campbell and Charlotte Allen of the Australian National University in Canberra point out that the planet's first great landmass, a giant ancestor of Pangea, formed at the same time.
What's more, each of the six other supercontinents in geologic history -- including Pangea, which existed 300 million years ago -- seem to coincide with steady upticks in atmospheric oxygen.
Coincidence? They think not. In a study published this week in the journal Nature Geoscience, the authors argue that each time a supercontinent assembled, it created huge mountain ranges. As the mountains eroded, the world's rivers washed them into the sea.
Among the detritus were vast quantities of phosphorous, iron and calcium -- nutrients the ocean-going bacteria could use to grow in tremendous numbers. As they did, they consumed carbon dioxide and flooded the atmosphere with oxygen.
"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.
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