Setting the Second Hand on the Rock Clock

The Argon-Argon Test Pinpoints Pompeii's
Demise to Within Seven Years

by Robert Sanders

A powerful geologic dating technique called argon-argon dating has pegged the 79 A.D. eruption of Vesuvius so precisely that it establishes one of the most solid and reliable anchors for any dating method.

With such validation, the radioactive argon dating technique now can reliably establish the age of rocks as old as the solar system or as young as 2,000 years, say researchers at Berkeley and the Berkeley Geochronology Center.

"Argon-argon dating is by far the most important technique in documenting the history of human evolution, and this new result is an important validation of the technique," says Paul Renne, adjunct associate professor of geology and geophysics here and director of the privately funded Berkeley Geochronology Center.

The center has now used the argon-argon method to date many recent important fossil finds, from the highly touted human ancestor dubbed "Lucy" and the major Ethiopian discoveries of Berkeley anthropologist Tim White to Homo erectus remains from Java.

Argon-argon dating also has been used to establish the age of meteorites several billion years old, mass extinctions, climate changes and other geologic events in the last several hundred million years.

The new results were published in the Aug. 29 issue of Science magazine. Renne's co-authors are Warren D. Sharp and Alan L. Deino of the Berkeley Geochronology Center, and Giovanni Orsi and Lucia Civetta of the Department of Geophysics and Vulcanology at the University of Naples. Civetta also is head of the Vesuvian Vulcanological Observatory.

Orsi and Civetta are working with the center to obtain argon-argon dates for numerous past volcanic eruptions in the Campi Flegrei or Phlegraean Fields surrounding Naples, in search of clues to the periodicity of activity that might allow prediction of future eruptions. Naples and vicinity, with more than two million inhabitants, is one of the world's most vulnerable populations to volcanic hazard.

According to the Roman historian Pliny the Younger, Vesuvius erupted in the afternoon of Aug. 24, 1,918 years ago, destroying Pompeii, Herculaneum and other Roman cities.

The certainty of the date tempted the team to test the ability of the argon-argon dating technique to establish the age of recent historic events. If it gave an accurate age for the pumice thrown out by the volcano, it would be the youngest rock ever dated by the technique.

The most common method for obtaining the age of objects as young as this is carbon-14 dating, a technique limited to organic material such as wood or bone.

To everyone's surprise the date given by the argon-argon dating technique was 1,925 years ago-off by only seven years. The scientific error on the estimate was plus or minus 94 years.

"We nailed the date to five percent on our first attempt, so we could probably get the error down to one percent or less," Renne says.

The result is so amazing because every dating technique invokes assumptions or involves uncertainties that limit its ability to pinpoint dates with extreme precision.

With the new dating system it should be possible to date even younger samples, perhaps a mere 1,000 years old, with 10 percent accuracy.

"Dating things that are really young has always been the Holy Grail of potassium-argon and argon-argon dating," Renne says.

The work was supported by grants from the National Science Foundation, the Ann and Gordon Getty Foundation, and the Osservatorio Vesuviano.



Copyright 1997, The Regents of the University of California.
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