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Scientists at the Lamont-Doherty Geological Laboratory of Columbia University at Palisades, N.Y., reported today in the British journal Nature that some estimates of age based on carbon analyses were wrong by as much as 3,500 years.The Lamont-Doherty scientists conducted their analyses on samples of coral drilled from a reef off the island of Barbados.

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But when a plant or animal dies, it can no longer accumulate fresh carbon 14, and the supply in the organism at the time of death is gradually depleted.

Since the rate of depletion has been accurately determined (half of any given amount of carbon 14 decays in 5,730 years), scientists can calculate the time elapsed since something died from its residual carbon 14.

Archaeologist Sturt Manning and colleagues have revealed variations in the radiocarbon cycle at certain periods of time, affecting frequently cited standards used in archaeological and historical research relevant to the southern Levant region, which includes Israel, southern Jordan and Egypt.

These variations, or offsets, of up to 20 years in the calibration of precise radiocarbon dating could be related to climatic conditions.

Carbon dating is unreliable for objects older than about 30,000 years, but uranium-thorium dating may be possible for objects up to half a million years old, Dr. The method is less suitable, however, for land animals and plants than for marine organisms, because uranium is plentiful in sea water but less so in most soils.

But even if the method is limited to marine organisms, it will be extremely useful for deciphering the history of Earth's climate, ice, oceans and rocks, Dr. Manning, professor of archaeology at Cornell University and director of the Cornell Tree-Ring Laboratory, is the lead author of "Fluctuating Radiocarbon Offsets Observed in the Southern Levant and Implications for Archaeological Chronology Debates," published in the .Pre-modern radiocarbon chronologies rely on standardized Northern and Southern Hemisphere calibration curves to obtain calendar dates from organic material.It is too soon to know whether the discovery will seriously upset the estimated dates of events like the arrival of human beings in the Western Hemisphere, scientists said.But it is already clear that the carbon method of dating will have to be recalibrated and corrected in some cases.These standard calibration curves assume that at any given time radiocarbon levels are similar and stable everywhere across each hemisphere. "We went looking to test the assumption behind the whole field of radiocarbon dating," Manning said.

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