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The carbon in seabed rocks may unlock Earth's early history

SCIENTISTS call it ``the faint young sun paradox.'' The sun was 30 percent cooler during our planet's youth than it is today. How, then, did earthly life have a warm enough environment to evolve? ``It's not really a paradox if you throw carbon dioxide [CO2] into it,'' says James Kasting of the NASA Ames Research Center in Mountain View, Calif. In the atmosphere, this heat-trapping gas absorbs outgoing infrared (heat) radiation and keeps Earth's surface warmer than it otherwise would be. Citing a new, more accurate analysis of Earth's CO2 cycle by his colleague David Des Marais, Kasting says there was probably plenty of CO2 in the air during Earth's first billion years to keep the surface warm.

It's an example of how an advance in laboratory technique can open a new window on our planet's history.

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CO2 from Earth's interior enters the atmosphere via volcanic vents. It leaves the air as the sea absorbs it and living plants incorporate its carbon in their tissues. Eventually the plants die. In the sea, their remains settle and gradually become part of sedimentary rocks. Old seabed plunges back into Earth's interior along deep ocean ridges and returns the carbon whence it came. It's a process that takes millions of years.

This long-term cycle with carbon (as CO2 gas) emerging from the interior and eventually returning to it establishes an equilibrium in which the atmosphere's CO2 content scarcely changes for extended periods of time. It helps stabilize climate.

Des Marais has developed a more accurate method of analyzing the carbon content of volcanic rocks. This enables him to make a much more accurate estimate of the rate of CO2 release.

His research also allows better estimates of CO2 outflow throughout Earth's 4.5 billion-ear geological history. Des Marais estimates that Earth may have had as much as 1,000 times as much CO2 in its atmosphere during its first billion years as is found there today.

This raises the question of why Earth didn't overheat, as did Venus. Venus's atmosphere has so much CO2 it keeps the planet's surface hot enough to melt lead (about 460 degrees C., 860 degrees F.).

Earth, explains Kasting, is farther from the sun than Venus is, and has more water. He estimates that, even if Earth had had as much CO2 as Venus now has, its temperature would not have gone beyond about 230 degrees C. (450 degrees F.). As it was, Kasting says, Earth appears to have had just enough CO2 to maintain a climate encouraging to life. Also, a relatively high level of carbon in the early sea would have made it easier for organisms to get hold of this nutrient.

We will probably never know for sure what happened on Earth several billion years ago. But as the Ames researchers have shown, new techniques to more accurately analyze what evidence remains can give more insight into the remote past.

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A Tuesday column. Robert C. Cowen is the Monitor's natural science editor.

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