carbon dating for age determination

Bob xyzbbruner at uclink4.berkeley.edu
Sat Jun 28 20:54:46 EST 2003

On Sat, 28 Jun 2003 23:43:43 GMT, Tom McCloud
<mccloud-tom at worldnet.att.net> wrote:

>On 28 Jun 2003 23:07:09 GMT, mydogatemypw at aol.com (Mydogatemypw)
>>ok silly me, but substitute skeleton on corpse for ceramic bowl and explain
>>this to me then
>All living organisms, plants, animals, microbes, utilize carbon in
>some way.   Both isotopically stable C12 and radioactive C14  (as well
>as paramagnetic C13)  are naturally present in the environment in
>fixed amounts.  Since those enzymes carrying out metabolism in a
>living organism essentially cannot tell any difference among the
>carbon isotopes, all those isotopes get incorporated into sugars,
>amino acids, proteins, fats, nucleic acids, etc.
>When the organism dies, the carbon in that organism is 'fixed' as it
>were, presumably no flux or exchange is occurring with carbon in the
>environment any longer.   So since no C14 is being 'added to the
>closed system', the amount of C14 contained can only decrease with the
>age of the material.    Therefore,  if one carefully measures that
>admittedly very small amount of C14,  that quantity is related to the
>length of time since the organism died.   

Yes, that is the heart of the method.

A major uncertainty comes from variations in the isotopic composition
of C over time -- in part because biological systems do fractionate
the isotopes to some extent.

>Now there are assumptions
>being made.  The half life of C14 is an estimate, 

It is known to quite high precision; knowledge of the half life is not
a limitation of the method.

>and the rate of
>decay is assumed to be constant.   

It is _known_ to be constant.

Note that similar methods are available using  isotopes of other
elements. The use of C-14 only works over about 50,000 years or so
(the half life is about 5000 yr), but other isotopes allow access to a
wide range of ages (not all for biological materials).


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