On Jul 17, 2007, at 2:48 PM, Brian Riordan wrote:
Calibration curves that have been developed to take into account
KNOWN radiation fluxuations are based on radiation fluxuations
observed during the past century. If you've done any
extrapolation, you'd know that 60,000 years is fr outside the
reliable extrapolation range. I don't know what type of
extrapolation you'd use for this information (linear, conic etc.)
but at any rate it's far beyond the limits of safe extrapolation.
(fyi, this is like making a graph of your income every month for
the last 5 years, using a 'curve fit' in Excell and stating with
confidence what you'll make October 3008). Calibration curves have
changed even in the last 50 years due to noticeable changes in the
suns solar activity.
But Brian, scientists who use radiocarbon dating don't pretend to be
able to date things in the recent past (less than 60,000 years ago--
as opposed to 4+ billion years ago) with extreme accuracy. They
always report their radiocarbon dates with an error estimate. And
they are always willing to update their estimations (and calibration
curves) when new evidence comes to light. What independent sources of
evidence do they use to adjust their calibration curves?
tree growth rings
Antarctic ice cores
deep ocean sediment cores
lake sediments
coral samples
cave speleothems (see, I knew I could get the discussion back to caves!)
True, the relative amount of 14-C available in the atmosphere
fluctuates a lot dependent upon factors such as
solar activity
volcanic eruptions
climate change
burning of fossil fuels
atomic bomb testing
hence the need to acquire independent observations from sources other
than 14-C dating.
The scientific method demands that one gather evidence from as many
different sources as possible, and to develop theories (and testable
hypotheses!) that are consistent with the data at hand. The reason
that science is an exciting, dynamic field is that the amount of data
keeps increasing, and the reliability of the data acquired is often
higher due to modern-day instruments.
Diana
P.S. I've always wanted to date a speleothem, but they never seemed
interested in me. :)
* * * * * * * * * * * * * * * * * * * * * * * * * * * *
Diana R. Tomchick
Associate Professor
University of Texas Southwestern Medical Center
Department of Biochemistry
5323 Harry Hines Blvd.
Rm. ND10.214B
Dallas, TX 75390-8816, U.S.A.
Email: diana.tomch...@utsouthwestern.edu
214-645-6383 (phone)
214-645-6353 (fax)
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