mj@husky.uucp (Mark A. Johnson) (07/30/85)
The reference to solar activity's role in the formation of
atmospheric C14 got me thinking about something I've been
wondering about for a couple of years now. Where do the
scientists who use radiocarbon dating get their correlation
between C14 concentrations in the organic material under study
and the amount of time that has passed since the creature (or
whatever) died? My understanding is that the C12/C14 ratio in
an organism approached zero asymptotically with passing time
(e^-at) following the creature's death, since C14 is no longer
being moved through the living organism. Where does the initial
atmospheric C14 concentratio data come from? Simply assuming
that C14 concentration is constant since the beginning of time
seems questionable to me, at best. How 'bout it, folks?
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Mark A. Johnson -- Eastman Kodak Company -- Information Products
UUCP:...allegra!rochester!ritcv!husky!mj W:(716) 726-9953 H:(716) 227-2356
(The Name Says It All)
"Lived here all your life?" "Not yet."ethan@utastro.UUCP (Ethan Vishniac) (07/30/85)
> > The reference to solar activity's role in the formation of > atmospheric C14 got me thinking about something I've been > wondering about for a couple of years now. Where do the > scientists who use radiocarbon dating get their correlation > between C14 concentrations in the organic material under study > and the amount of time that has passed since the creature (or > whatever) died? My understanding is that the C12/C14 ratio in > an organism approached zero asymptotically with passing time > (e^-at) following the creature's death, since C14 is no longer > being moved through the living organism. Where does the initial > atmospheric C14 concentratio data come from? Simply assuming > that C14 concentration is constant since the beginning of time > seems questionable to me, at best. How 'bout it, folks? > > Mark A. Johnson -- Eastman Kodak Company -- Information Products This is not my field, but I think I can handle this one. Initially, that's just what people did (assume constant C14/C12 in the atmosphere). However, for a while now people have used a calibration curve obtained by finding pieces of bristle-cone pine. The layers of the pine give a calibration curve provided that the specimen can be dated. Living pines take one back about 4000 years. (The surface is being made now. Count the rings down to the core and sample the core.) With that calibration curve one can date dead pines and extend the calibration sequence. I believe that sequence now goes back about 8000 years. In that time it shows a lot of small bumps and wiggles, and some large scale trends which correlate with the paleomagnetic data. It is therefore possible to crudely correct the C14 date using the paleomagnetic data at even earlier times. I am under the impression that the people who take ice cores think they will be able to get a calibrated sequence back several tens of thousands of years. There was a Sci Am article on this about 10 years back. -- "Don't argue with a fool. Ethan Vishniac Borrow his money." {charm,ut-sally,ut-ngp,noao}!utastro!ethan Department of Astronomy University of Texas
hull@hao.UUCP (Howard Hull) (07/31/85)
Says Mark A. Johnson: > My understanding is that the C12/C14 ratio in > an organism approached zero asymptotically with passing time > (e^-at) following the creature's death, since C14 is no longer > being moved through the living organism. Where does the initial > atmospheric C14 concentratio data come from? Simply assuming > that C14 concentration is constant since the beginning of time > seems questionable to me, at best. How 'bout it, folks? I'm no expert, but, I think the C14 comes from cosmic ray disintegrations (some of which may be due to energetic solar protons). Solar activity may not have been strictly constant over time, but researchers can compensate for this by referencing other isotope abundances, and by correlating this with ice strata thicknesses in Antarctica and Greenland. Another exciting thing is that modern Accelerated Mass Spectrometers can find as little as *10 Atoms* of, say, Beryllium 10 in some samples carefully prepared for spectroscopy! Such studies are being used to bolster the C14 data. Howard Hull {ucbvax!hplabs | allegra!nbires | harpo!seismo } !hao!hull