How to Date a Rock Artist

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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. …