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'' 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.But the tree ring record goes no further, so scientists have sought other indicators of age against which carbon dates can be compared.

Accurate tree ring records of age are available for a period extending 9,000 years into the past.

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.

Uranium 234, a radioactive element present in the environment, slowly decays to form thorium 230.

Using a mass spectrometer, an instrument that accelerates streams of atoms and uses magnets to sort them out according to mass and electric charge, the group has learned to measure the ratio of uranium to thorium very precisely.

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