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Where the Pecos and Devils rivers meet the Rio Grande, ancient peoples with a penchant for painting took refuge in shelters dug into limestone cliffs. Pictographs of panthers, spear-throwing shamans and other figures cover some of these places from stem to stern and from floor to ceiling.

Determining the dates of pictographs helps scientists reconstruct vanished cultures. To obtain their date data, archaeologists and anthropologists traditionally have relied on indirect evidence, including radiocarbon dates of deposits on and near the art, as well as imagery in the paintings.

Direct radiocarbon dating of cave paintings has been mostly out of the question because of scientists' inability to distinguish the inorganic carbon in the limestone "canvas" from the paint's organic carbon, says chemist Marvin W Rowe of Texas A&M University in College Station. The organic carbon in ancient paints derives from blood, plant resin, juice or other binders," which, like modern oil- or water-based binders, carried pigments and adhered to surfaces.

Several years ago, Texas A&M anthropologist Harry J. Shafer met Rowe on campus and posed the question: Is there any way to separate a sample's inorganic and organic carbon components to allow direct dating? In the Dec. 20/27, 1990 NATURE, Shafer and Rowe, along with chemists Marian Hyman and Jon Russ, describe a technique that seems up to the task.

The researchers tested their method on a thin, limestone-backed fragment of a pictograph that Shafer had found on the ground of a prehistoric shelter in southwest Texas. They scraped paint powder from the limestone and placed the powder in a partial vacuum for a week to let atmospheric carbon dioxide evaporate away. Next, they swamped the sample with an oxygen plasma. The reactive oxygen, although too cool to affect inorganic carbon in the limestone (calcium carbonate), converted the sample's organic carbon into carbon dioxide. The chemists collected this date-bearing gas, now free of contamination by inorganic carbon, by condensing it as dry ice onto a cold surface.

Others entered the picture at this point. A commercial lab purified the carbon dioxide and converted it into all-carbon graphite - a form well suited for sensitive dating techniques. Next, the graphite took a trip to a Swiss research facility where scientists analyzed it by accelerator mass spectrometry which precisely compared the relative amounts of radioactive and stable carbon isotopes.

The resulting ratio suggests the ancient artists prepared their paint 3,865 years ago, give or take a century That's far more precise than the "2000 to 6000 years ago" range indicated by indirect methods, Rowe says.

As organic carbon is a ubiquitous component of pictograph paints, this technique should be applicable to rock paintings throughout the world," the Texas researchers conclude.
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Publication:Science News
Date:Jan 19, 1991
Words:451
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