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These dates will be seriously challenged by many.
Mike Ruggeri
******************************************** News in Science - Ancient hearth tests carbon dating - 17/11/2003
Ancient hearth tests carbon dating
Bob Beale
ABC Science Online
Monday, 17 November 2003
Rock art at Serra da Capivara National Park, home of the Pedra Furada
site in Brazil (Embassy of Brazil, London)
People were keeping warm by a fire in a rock shelter at least 56,000
years ago, according to new analysis of what may be the oldest known
human record in the Americas.
This is about 40,000 years earlier than generally agreed for when people
first arrived in the Americas.
The international team of researchers dated charcoal from a hearth at
the controversial Pedra Furada archaeological site in Brazil and
reported its findings in the latest issue of the journal Quaternary
Science Reviews. They used a new technique that pushes back the so-called radiocarbon
dating barrier, according to Dr Guaciara dos Santos and colleagues who
ran tests at the Australian National University.
Scientists have been polarised about the age of the Pedra Furada site
because estimates have been in "profound disagreement" with accepted
wisdom about who, when, where and how people first arrived in the
Americas. These were supposedly the Clovis people who walked from
Siberia into North America across an Ice Age land bridge only 12,000 to
14,000 years ago.
"These dates are good and reliable and there's no reason to doubt them,"
Dr Michael Bird, a member of the team who developed the new dating
technique, told ABC Science Online. "The question goes back to the
archaeology. If they are hearths, they are very old indeed."
The site at Pedra Furada, in the Serra da Capivara National Park, is a
rich archaeological area of sandstone rock shelters. It contains many
prehistoric sites, including hundreds of rock artworks, stone tools and
human remains.
Earlier tests on charcoal from the deepest layers of the excavations
suggested that it was at least 40,000 years old, the traditionally
accepted accurate "barrier" limit of radiocarbon dating. But scientists
were still puzzled about the authenticity of the hearths as human
artefacts and whether younger carbon sources could have contaminated the
samples and skewed the results.
The new study says that thermoluminscence testing of the hearthstones
showed that they "were heated independently from the stones found
outside the hearths in the same layer; thus, refuting the possibility
that the stones were heated by natural fires".
It revises the dates on those earlier charcoal tests using Bird's
technique to decontaminate it first. The procedure is known as ABOX
(acid-base-wet-oxidation) and involves chemically scouring a fine layer
off the charcoal surface.
"[This] reliably removes contamination from charcoal and wood enabling
credible radiocarbon dating to about 55,000 years before present," the
report said.
Bird said the method had been used in the past two years to secure
radiocarbon dates older than 40,000 years for archaeological sites in
South Africa and Australia, notably the famous Devil's Lair site in
Western Australia, which was redated at up to 50,000 years old.
Radiocarbon dates become progressively less reliable on older material
and until the ABOX technique was developed, few scientists would accept
their accuracy beyond the barrier limit, he said.
"At 50,000 years you have only about 0.1% of the original radiocarbon
present, so contamination with younger material is a major issue," Bird
said. "This is a much better way of pre-treating the samples to get rid
of any contamination. It's becoming the gold standard in archaeology for
getting good reliable dates that you can believe, particularly at these
old time scales."
Out of seven Pedra Furada charcoal samples scientists took from the
hearth structures in the deepest layers, five were beyond the limit of
the ABOX technique itself, returning ages greater than 56,000 years, the
report said. Analysis of the final two samples gave finite ages of
53,000 and 55,000 years.
© 2003 Australian Broadcasting Corporation
Tom McDonald -- remove 'nohormel' to reply
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