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Single-grain rock samples are irradiated with neutrons to convert potassium-39 to argon-39, which is normally not present in nature.

The ratio of argon-40 to argon-39 then provides a measurement of the age of the sample."This should be the last big revision of argon-argon dating," Renne said.

Nevertheless, the technique had systematic errors that produced dates with uncertainties of about 2.5 percent, according to Paul Renne, director of the Berkeley Geochronology Center and an adjunct professor of earth and planetary science at UC Berkeley.

But consider three specimens that were excavated in Alaska, Montana, and Colorado.

These alternating climatic periods are caused by 100,000-year and 405,000-year cycles in Earth's orbital eccentricity.

Because Earth's orbit, and thus the relative ages of the sediment layers, can be precisely calculated, dating of the sediments by the argon-argon method provided a much-needed recalibration of the method and made it possible to pinpoint the Cretaceous/Tertiary boundary at 65.95 million years ago.

"A 1 percent difference at 4.5 billion years is almost 50 million years."One major implication of the revision involves the formation of meteorites, planetessimals and planets in the early solar system, he said.

Argon-argon dating was giving a lower date than other methods for the formation of meteorites, suggesting that they cooled slowly during the solar system's infancy."The new result implies that many of these meteorites cooled very, very quickly, which is consistent with what is known or suggested from other studies using other isotopic systems," he said.

"The importance of the argon-argon technique is that it is the only technique that has the dynamic range to cover nearly all of Earth's history," Renne said.

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