Other radiometric dating methods such as potassium-argon or rubidium-strontium are used for such purposes by those who believe that the earth is billions of years old.
Radiocarbon is not suitable for this purpose because it is only applicable: a) on a time scale of thousands of years and b) to remains of once-living organisms (with minor exceptions, from which rocks are excluded).
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.
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.
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.
''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.