It's long been known that ages based on radiocarbon dating tend to be too young. The radiocarbon dates can be calibrated against tree‐ring ages for the time period over which tree‐ring samples are available—that is, over the last 8000–9000 years—but the corrections needed for earlier times have been unknown. Now a new chronometer is available that may permit some calibration back tens of thousands of years before the present era. The new benchmark is based on measurements of the decay of uranium into thorium. Researchers have dated materials using the uraniumthorium decay for many years, but the method they used was not very precise. Nor was there any set of samples systematic enough to use for C14 calibration. Two developments in the past few years have changed that: A Caltech team in 1987 adapted a mass spectrometry technique to obtain high‐precision uranium‐thorium results with small samples and established the validity of the uraniumthorium ages. This year a group from Columbia University's Lamont‐Doherty Geological Observatory applied the new technique to samples of submerged corals collected off Barbados and was able to calibrate the C14 dates back at least 40 000 years.

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