for National Geographic News
Shifting volcanoes may have allowed Earth's atmosphere to fill with oxygen, spurring the development of complex life, a new study suggests.
A mysterious increase in oxygen levels occurred around 2.5 billion years ago. The new research says that a massive tectonic upheaval pushed submerged volcanoes above ground, where they stopped spewing oxygen-destroying chemicals. (Related news: "Volcanoes May Have Sparked Life on Earth, Study Says" [October 7, 2004].)
At that time, Earth's early atmosphere wasn't fit to breathe. Filled with nitrogen, carbon dioxide, and sulfurous fumes, the air would have left humans gasping.
These noxious fumes held a clamp on evolution: Complex life didn't really get going until the planet's skies began to fill with oxygen, allowing more efficient methods of extracting energy from nutrients.
Scientists have long speculated that early photosynthesizing organisms such as cyanobacteria shifted the chemical balance by using up some carbon dioxide and producing oxygen.
But cyanobacteria appeared in the fossil record at least 200 million years before the atmosphere's chemistry changed.
"How is it that cyanobacteria were probably producing copious amounts of oxygen and yet oxygen levels remained low?" asked lead study author Lee Kump, a geologist at Pennsylvania State University.
Kump and colleague Mark Barley of the University of Western Australia suggested that underwater volcanoes must have soaked up the oxygen as quickly as it formed.
Submerged volcanoes spew a different set of gases than those that erupt into the air. When magma emerges underwater, it chills in a snap and forms a glassy coating around pillow-shaped lava. (Related news: "Volcanic 'Fizz' That Triggers Explosive Eruptions Starts Deep" [July 12, 2007].)
That quick cooling, plus sea pressure, favor the formation of gases such as hydrogen sulfide, which grabs any available oxygen.
Magma from aerial volcanoes, on the other hand, stays hot and releases gases like carbon dioxide that don't react with oxygen. (Get the basics on volcanoes.)
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