for National Geographic News
Climate change on a grand scale occurred over much the Earth's early history, researchers say.
New geological data confirm the theory that complex life emerged only after a period of global cooling that lasted over three billion years.
Evidence that the world's oceans were once steamy cauldrons comes from the distribution of the element silicon in ancient rocks known as cherts.
In today's edition of the journal Nature, François Robert and Marc Chaussidon report that cherts formed from ocean sediments provide a kind of "paleo-thermometer" for seas during the Precambrian era.
The Precambrian is the vast stretch of time between Earth's formation more than 4.5 billion years ago and the rise of multicellular organisms about 600 million years ago.
The French researchers discovered that Precambrian ocean temperatures can be deduced from the ratios of different silicon variantsknown as isotopesfound in chert layers that formed on the ancient seafloor.
The scientists found that the heaviest of the three naturally occurring silicon isotopes is more common in rocks formed in cooler water.
"We found that these Precambrian cherts have very peculiar isotopic compositions," said Chaussidon, of France's Center for Petrographic and Geochemical Research in Vandoeuvre-lès-Nancy.
"We interpret these as reflecting a global cooling of the oceans from temperatures perhaps as high as 158 degrees Fahrenheit (70 degrees Celsius) 3.5 billion years ago to 68 degrees Fahrenheit (20 degrees Celsius) at the end of the Precambrian."
"Runaway Greenhouse" Effect
Previous studies of Precambrian cherts have also concluded that the seas in which early life evolved were far hotter than those today.
But critics have maintained that those studies, which focused on oxygen isotopes, might have been compromised by more recent geological processes that altered the geologic record.
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