Researchers using new techniques reported last week that the method most widely used to date prehistoric artifacts and fossils is inaccurate by thousands of years, meaning that many key dates for events such as ice ages and human migrations may have to be adjusted.

Columbia University scientists say in the current issue of the journal Nature that the popular method of dating materials using ratios of radioactive carbon-14 gives dates that are up to 3,500 years too young. In other words, the "carbon-14 clock" has been running fast.

The researchers drilled into an ancient coral reef off the coast of Barbados and then compared dates produced by the traditional carbon-14 technique and a more sophisticated technique based on the ratios of radioactive uranium and thorium. Carbon-14 dating consistently gave younger ages from about 8,000 to 30,000 before the present.

Based on the new dating system, the peak of the last ice age is now thought to be 21,500 years ago, not 18,000 as figured by carbon-14 dating. Many archaeologists may also need to fiddle with their dates, though the more important question of sequence -- of what happened first -- will not be altered.