Radioactive dating debate

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The Lamont-Doherty scientists conducted their analyses on samples of coral drilled from a reef off the island of Barbados.

The samples represented animals that lived at various times during the last 30,000 years. Alan Zindler, a professor of geology at Columbia University who is a member of the Lamont-Doherty research group, said age estimates using the carbon dating and uranium-thorium dating differed only slightly for the period from 9,000 years ago to the present.

Since 1947, scientists have reckoned the ages of many old objects by measuring the amounts of radioactive carbon they contain.

New research shows, however, that some estimates based on carbon may have erred by thousands of years.

#30,000-Year Limit The Lamont-Doherty group says uranium-thorium dating not only is more precise than carbon dating in some cases, but also can be used to date much older objects.

After all, textbooks, media, and museums glibly present ages of millions of years as fact.

Yet few people know how radiometric dating works or bother to ask what assumptions drive the conclusions.

''But at earlier times, the carbon dates were substantially younger than the dates we estimated by uranium-thorium analysis,'' he said.

''The largest deviation, 3,500 years, was obtained for samples that are about 20,000 years old.'' One reason the group believes the uranium-thorium estimates to be more accurate than carbon dating is that they produce better matches between known changes in the Earth's orbit and changes in global glaciation. Fairbanks, a member of the Lamont-Doherty group, said that if the dates of glaciation were determined using the uranium-thorium method, the delay - and the puzzle - disappeared.

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