(Read related story: "Hell on Earth? Scientists Debate Planet's Early Years" [August 9, 2006].)
"This [new study] adds a major body of evidence and allows greater confidence in previous results," commented Paul Knauth, a geologist at Arizona State University in Flagstaff who conducted some of the earlier oxygen isotope studies.
"The large changes [in silicon isotopes] were quite surprising and were cleverly related to climatic temperatures," Knauth added. "It's a remarkable data set."
The French researchers say that the record based on ocean temperature is indicative of the changing climate of early Earth.
Early in the Precambrian, Earth received less heat from the sun, which is thought to have been 25 to 30 percent less luminous than it is today.
But high temperatures were maintained by an atmosphere extremely rich in greenhouse gases such as methane and carbon dioxide (CO2).
"If the temperature trend is correct, it appears that the early Earth almost went into a runaway greenhouse [effect], like Venus," Knauth said.
The Temperature of Life
Though inhospitable by today's standards, the hot Precambrian oceans were the setting for billions of years of biological evolution.
(See a National Geographic magazine feature about life that thrives near deep-sea hydrothermal vents.)
Single-celled organisms flourished in the ancient seas, and scientists believe some helped drive the process of global cooling.
Microbes capable of carrying out photosynthesis slowly reduced the amount of heat-trapping CO2 and helped create today's more temperate, oxygen-rich atmosphere.
Lower temperatures helped spur the emergence of complex plant and animal life at the end of the Precambrian.
Among the new life-forms at that time were abundant marine organisms that used silicon dissolved in seawater to build hard shells.
That development, Chaussidon says, spelled the end of the silicon-based temperature record, as marine life began to use more of the mineral, causing silicon levels in the sediments to drop.
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