Shifting Volcanoes Made Early Complex Life Possible
for National Geographic News
|August 29, 2007|
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.)
So the switch to volcanoes that erupt above water might have allowed oxygen to build up.
A Planet Like Indonesia
At the time of the tectonic upheaval 2.5 billion years ago, much of the planet resembled Indonesia today.
"It was flooded, with fairly shallow seas and some islands above the water, but with lots of volcanoes erupting underwater," Kump said.
Then the Earth's crust thickened and became more buoyant, forming the first large continents. With more of the Earth above water, far more volcanoes erupted into the air.
In this week's issue of the journal Nature, Kump and Barley compiled information about volcanic rocks up to 3.5 billion years old—close to the age of the oldest rocks on Earth. Nearly all of the oldest rocks formed underwater.
"Sure enough, we found pretty good evidence for an increase in volcanoes erupting on land," Kump said.
"I think it is a provocative idea," said James Farquhar, a geologist at the University of Maryland. Farquhar's own work on the chemistry of ancient minerals helped determine when atmosphere began filling with oxygen.
But the change in atmosphere may have been subtle.
Tim Lyons, a geochemist at the University of California in Riverside, noted that the balance among gases would only have needed to shift slightly to match the changes geologists see in the minerals from the time.
The oxygen weathered rocks and reacted with minerals and metals to create micronutrients, he said. That may have been enough to fuel the development of more complex life.
"Even though it could have been a very small step, it really changed the nature of the world," Lyons said.
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