The increased sensitivity also means that less material is required for analysis.
"We are able to select pure limestone crystals," said Ku, thus reducing the risk of a sample's being accidentally contaminated by older or younger non-carbonate material.
The mass spectrometric method also extends the time horizon for the method, allowing scientists to date material as far back as 600,000 years.
In addition to using a more sensitive dating method, Ku believes he has reduced error by analyzing undisturbed limestone from the strata in which the bones were found instead of studying the fossils themselves, as previous investigators had done.
According to Ku, there is evidence that radioisotopes can migrate in and out of bone during the fossilization process, affecting the accuracy of dating.
To double-check his data, Ku compared his mass spectrographic age determinations with results obtained from an alternate method - one using the isotopes Proactinium-231/Uranium 235. Though less precise than the U-238 isotopic ratio analysis, this method, too, confirmed the greater age of the finds.
The collaboration between Shen and Ku goes back more than a decade, to Shen's research on European caves for his dissertation at the University of Paris. Ku, then taking sabbatical leave from USC as a Fulbright Senior Scholar and Guggenheim Fellow visiting France, was brought in as an outside expert on geochronology. The two scientists have stayed in touch ever since.
In addition to his work at Zhoukoudian, Ku is currently engaged in numerous studies using subtle clues in ocean or lake sediments to study the past climate on Earth. One such study, for example, uses ratios of oxygen isotopes found in sediments from California's Mono Lake to reconstruct detailed climate fluctuations in the area since 10,000 years ago, shortly after the last Ice
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Contact: Eric Mankin
mankin@usc.edu
University of Southern California
13-May-1996