Brazilian Cave Paintings dates disputed at Pedra Furada
Topiltz...@webtv.net
HoustonChronicle.com -- http://www.HoustonChronicle.com | Section:
Local & State
July 2, 2003, 10:11AM
ROCK OF AGES
Method developed at A&M challenges claims over cave paintings
By ERIC BERGER
Copyright 2003 Houston Chronicle Science Writer
HOW PLASMA
EXTRACTION WORKS
• Scientists must delicately collect the limited quantities of carbon
embedded in inorganic paints on cave walls to date them with radiocarbon
methods.
• Texas A&M researchers have developed a method that injects oxygen
heated to 300 degrees into a pigment sample, knocking out carbon without
destroying it.
• The method allows such small samples of paint to be dated that
determining the age of an ancient cave painting is now possible.
They are pictures of people, deer, llamas, crocodiles and even pumas.
There are more than 350 stone walls filled with such paintings at Pedra
Furada, a prehistoric site in a remote area of Brazil. 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, a Texas scientist says 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, still very much
a young field.
Scientists have long used radiocarbon dating to determine the age of old
objects. Living organisms consume a rare form of carbon during their
lifetimes and by measuring the ratios of these carbons in dead material,
scientists can determine when the plant or animal died.
But most cave paintings were done with inorganic pigments -- reds,
browns and yellows -- that cannot be radiocarbon dated because they lack
carbon. Charcoal can be accurately dated, but it yields the oldest
possible date because the charred wood used for the painting could have
been dead or burnt long before being applied to a wall.
To tackle this problem, scientists can 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,
backing Guidon's claim of a very old site.
To add validity to the findings, however, the Guidon team 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.
Guidon did not accept the results, Rowe said.
Guidon was at the research site and unavailable for comment.
But the physics professor who performed the dating for Guidon, Shigueo
Watanabe of the University of São Paulo, said he stands by the older
dates. In an e-mail, Watanabe said he is flying this week to Pedra
Furada to obtain a new sample of the calcium found in another area to
date it.
He points out that there has been little independent verification of
Rowe's technique.
Human occupation at Pedra Furada 30,000 years ago, let alone 50,000,
would shatter scientific theories of the peopling of the Americas.
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, based
upon a notable archaeological site near Clovis, N.M., which dates to
11,500 years ago.
In 1985, an anthropologist at the University of Kentucky, Tom Dillehay,
reported finding human artifacts at the Monte Verde 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.
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Philip Deitiker
Topiltz...@webtv.net says in
news:8187-3F0...@storefull-2132.public.lawson.webtv.net:
> "We need to keep our minds open, but cautiously so,"
> Dillehay said.
Philip Deitiker <pde...@bcm.tmc.edu> wrote in message
news:<og69gvkg5va7vnuef...@4ax.com>...
> Article originally posted by Mikey Brass in PaleoAnthro.
>
> July 2, 2003, 10:11AM
> ROCK OF AGES
> Method developed at A&M challenges claims over cave
> paintings
> By ERIC BERGER
> Copyright 2003 Houston Chronicle Science Writer
>
>
> "
gary...@earthlink.net (Dr.GH) wrote:
There is no particular need to question Rowe's methods, they are
mostly well established C14 methods. His group's use of a low
temerature oxygen plasma to extract carbon from rock art
pigments has been in the literature (that I know of anyway)
since 1990 at the San Diego Museum of Man Rock Art '90 Symposia
(Published 1992 Rock Art Papers, San Diego Museum of Man Papers
#28:121-128 ).
There is nothing that is very difficult in the method. The most
likely problems are the same as for any C14 date: contamination.
For example, recent fungal, bacterial or even lichen growth
might alter the dated material. We are talking very small
samples. The method of oxygen plasma extraction is not an
significant issue, but sample collection and pre-extraction
procedure could be important.
On 5 Jul 2003 13:45:07 -0700, gary...@earthlink.net (Dr.GH)
wrote:
>There is no particular need to question Rowe's methods, they
are
>mostly well established C14 methods. His group's use of a low
>temerature oxygen plasma to extract carbon from rock art
pigments has
>been in the literature (that I know of anyway) since 1990 at
the San
>Diego Museum of Man Rock Art '90 Symposia (Published 1992 Rock
Art
>Papers, San Diego Museum of Man Papers #28:121-128 ).
PD posted:
Actually I question now any early C14 date, because there is
so little left after 20 ky (4 half lives leaves 1/16th the
original amount of C14) and because any contamination from
external sources can heavily affect dating. IMHO C14 is
probably the most overused and overinterpreted dating method
out their, other than comparative anatomy itself. early C14
dates are heavily affected by contamination. In this
particular case when you talk about paintings on cave walls
the continual leaching of -C03 from calcium carbonate can
replace carbonate in a sample if the sample has a high -CO3
composition.
>There is nothing that is very difficult in the method.
> The most likely problems are the same as for any
> C14 date: contamination.
PD posted:
Contamination is not much of a problem for later dates
because there is so much C14 and so little has decayed.
However as C14 decays the 'signal to noise' ratio drops and
small amounts of contamination can affect the datings. A
sample bury in a dry cave where little leaching or
intrustion has occurred may be a good choice for early
sample, but something that is continually exposed to the
elements is not.
> For
>example, recent fungal, bacterial or even lichen
>growth might alter the dated material. We are
>talking very small samples. The method of
>oxygen plasma extraction is not an significant issue,
>but sample collection and pre-extraction procedure could be
>important.
Fungal growth would act to decrease the C14 date.
pde...@worldnet.att.net (Philip Deitiker) wrote in message
news:<3f0757ff...@netnews.worldnet.att.net>...
> On 5 Jul 2003 13:45:07 -0700, gary...@earthlink.net (Dr.GH)
> wrote:
>
> Actually I question now any early C14 date, because there is
> so little left after 20 ky (4 half lives leaves 1/16th the
> original amount of C14) and because any contamination from
> external sources can heavily affect dating. IMHO C14 is
> probably the most overused and overinterpreted dating method
> out their, other than comparative anatomy itself. early C14
> dates are heavily affected by contamination. In this
> particular case when you talk about paintings on cave walls
> the continual leaching of -C03 from calcium carbonate can
> replace carbonate in a sample if the sample has a high -CO3
> composition.
gary...@earthlink.net (Dr.GH) wrote:
I more or less agree. When I taught medical students, I noticed
that the residents with the least grasp of labwork ordered the
most tests. It will always depend on the technical experience of
the fieldworker, and the pre-extraction labwork. I have samples
of rock art pigments that were not from limestone substrates
(granite and sandstone). Again, it is a question of the
appropriatness of the situation.
> Contamination is not much of a problem for later dates
> because there is so much C14 and so little has decayed.
> However as C14 decays the 'signal to noise' ratio drops and
> small amounts of contamination can affect the datings. A
> sample bury in a dry cave where little leaching or
> intrustion has occurred may be a good choice for early
> sample, but something that is continually exposed to the
> elements is not.
>
> Fungal growth would act to decrease the C14 date.
gary...@earthlink.net (Dr.GH) wrote:
Again depending on substrate, which is the better reason to
question the "late" as opposed to an "early" date as due to
contamination. As a matter of fact, I doubt that the early
pigment dates are reliable. But, if I did want to challenge the
late date, I would not do so based on some unfounded objection
the Lowe's methods.
What we really need to see is multiple methods employed when
dating sites and features. Too few hearths are sampled for
thermo-remnant magnetism AND C14. Even better, find a hearth
with a sherd in it with a matching piece outside, a partially
burnt jaw, and an obsidian flake in it with a matching piece
outside the hearth: C14, hydration rind measurment, fission
tract, TRM, TLD, electron spin resonance, U/Th, amino acid
racimization all on one tight context.
LOL. That would be so cool.
On 6 Jul 2003 16:18:50 -0700, gary...@earthlink.net (Dr.GH)
wrote:
>amino acid racimization all on one tight context.
PD posted:
Agreed but being able to triangulate 3 different methods is, I
think, a better ruler to use.
Philip
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http://groups.yahoo.com/group/DNAanthro/
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Other good sites
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Jeff Gonor
the Piedra Furada cave painting dates. It is not clear to me where this
discussion was originally posted: Paleo Anthro?
The Houston Chronicle article also started a discussion of this dating problem
on the Rock Art List last week. The PD-GH exchange would fit right into that
discussion. Interested readers of this newsgroup might want to go there (use
Google Groups to join) to get more details from rock art researchers more
familiar with this particular case. The author of the paper from Rowe's lab
also posted the entire text of the paper, which has not yet appeared in print,
to the Rock Art List.
The RA List discussion emphasized the difference between the siginificance of
the dates for the rock art and that of the dates obtained for materials
excavated from the cave floor. They are independent and the validity of the
dates from the materials the Guidon team recovered from excavation of deposits
on the cave floor are not in question here. The lead sentence of the Houston
Chronicle article is a bit erroneous on this point. An early date for the rock
art would be in concordance with the deposit dates, but a late date for the
rock art would not indicate that the deposit dates are wrong. If valid, it
would mean only that the rock art is not of the same age as the material at
depth in the deposits. The oxygen plasma method was designed to extract organic
carbon from the sample in the presence of mineral forms, which is the case
here. The Watanabe team dated the overlying mineral crust, which is susceptable
to contamination from wind blown limestone dust, the carbon of which of course
is exceedingly old, possibly skewing the date. There are other more complex
issues discussed on the Rock Art List.
Jeff Gonor
Jeff Gonor
the Rock Art List. An interview with Guidon and others is available at Athena
Review online. Several of the issues with the rock art in the region of Brazil
in question is discussed from the perspective of those who have been working on
it since the early 1970s (Guidon) and 1980s (the others).
"Pedra Furada, Brazil: Paleoindians, Paintings, and Paradoxes"
An interview with Drs. Niède Guidon, Anne-Marie Pessis, Fabio Parenti, Claude
Guérin, Evelyne Peyre, and Guaciara M. dos Santos
Athena Review, Vol.3, no.2: Peopling of the Americas
see: http://www.athenapub.com/10pfurad.htm
Jeff Gonor