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7 July 2003
Looking for the real age of Pedra Furada petroglyphs

At Pedra Furada, a prehistoric site in a remote area of Brazil, there are more than 350 stone walls filled with paintings of people, deer, llamas, crocodiles and even pumas. They also may be the oldest cave paintings ever found in the Americas, and their discovery could radically change scientists' understanding of how and when the first people came to this hemisphere.
     The problem is the paintings, drawn with charcoal and other pigments, are not 30,000 years old, as a team of archaeologists led by Brazilian Niede Guidon there claim, but just a few thousand years old. At issue is the method used to date the cave paintings: scientists can now measure trapped electrons in the thin layer of calcium that builds up on cave paintings over the centuries. It's a proven method for dating stalactites and stalagmites in caves. But it's problematic for dating human art because limestone in cave walls, at millions of years old, can contaminate the calcium deposits and give a much older age.
     Brazilian scientists recently used this method for setting the age of several Pedra Furada paintings at between 27,000 and 44,000 years old. To add validity to the findings, they sent material to Marvin Rowe, a professor of archaeological chemistry at Texas A&M University, who has developed a novel method of dating cave art called plasma extraction. Rowe's method is unique because he can extract traces of carbon within the non-living pigments, giving scientists a potentially powerful new tool to date cave painting.
     In his lab, Rowe tested 12 mostly reddish paint samples and found ages ranging from 1,230 to 3,730 years ago, hardly eyebrow-raising dates for human habitation in South America. "I was so disappointed myself because it would really have been a fantastic result," Rowe said.
     Until the last few decades the long-held belief was that mammoth-hunting humans crossed a Bering Sea land bridge about 12,000 years ago. In 1985, an anthropologist at the University of Kentucky, Tom Dillehay, reported finding human artifacts at a site in central Chile that were 12,500 years old. The finding has led scientists to believe that humans were here earlier, and possibly came by boat rather than land. Dillehay says his own excavations at Pedra Furada support people living there 11,500 years ago, but not earlier. "We need to keep our minds open, but cautiously so," Dillehay said.

Source: Houston Chronicle (2 July 2003)

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