Mistaken by Millenniums

Ever since its development in the 1940s, radiocarbon dating has been a vital tool for historians and paleontologists trying to pinpoint the ages of everything from ancient animal bones to prehistoric human settlements to Egyptian mummies. By measuring the decay of the natural radioactive isotope carbon 14, which almost all organisms ingest while they are alive, scientists can estimate how long it has been since an animal or plant died.

But those estimates, while valuable, are also known to be somewhat uncertain. Last week geologists at the Lamont-Doherty Geological Laboratory in - Palisades, N.Y., offered firm evidence of just how uncertain. Writing in Nature, they showed that some radiocarbon dates may be off by as much as 3,500 years -- possibly enough to force a change in current thinking on such important questions as exactly when humans first reached the Americas.

The technique the geologists used was based on another sort of radioactive decay. Organisms contain traces of uranium, which degrades into thorium. The rate of decay is known, and by measuring the relative amounts of the two substances in a sample, age can be accurately calculated.

In this case, samples came from a coral reef off Barbados. Carbon 14 and uranium-thorium dating largely agreed for pieces of coral up to about 9,000 years old. But for older pieces the findings diverged, with a maximum disparity of 3,500 years for coral about 20,000 years old.

Why did the scientists assume that the uranium-thorium tests were right and the carbon 14 tests wrong? For one thing, the carbon datings pointed to the strange conclusion that ice ages, thought to be related to changes in the earth's orbit around the sun, have mysteriously lagged behind those changes by a few thousand years. But uranium-thorium dating shows no such lag. Moreover, carbon 14 levels in the air -- and thus the amount ingested by organisms -- are known to vary over time, and that can affect the results of carbon dating.

Uranium-thorium has another advantage besides accuracy: it can be used to date objects up to 500,000 years old, while carbon 14 is good for only a few tens of thousands of years at best. The one drawback of the uranium-thorium technique is that it is useful mostly for marine animals and plants; uranium is more common in seawater than on the surface of the land. Scientists will no doubt continue to use all possible dating methods in the quest to construct an ever more accurate chronology of the earth's history.