#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.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.The method compares the abundance of a naturally occurring radioactive isotope within the material to the abundance of its decay products, which form at a known constant rate of decay.
''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.
According to carbon dating of fossil animals and plants, the spreading and receding of great ice sheets lagged behind orbital changes by several thousand years, a delay that scientists found hard to explain. The group theorizes that large errors in carbon dating result from fluctuations in the amount of carbon 14 in the air.
Accurate tree ring records of age are available for a period extending 9,000 years into the past.
But the tree ring record goes no further, so scientists have sought other indicators of age against which carbon dates can be compared.