🚀 go-pugleaf

In article <9204080901.aa00587@Paris.ics.uci.edu> kibler@turing.ICS.UCI.EDU (Dennis Kibler) writes:
>How is radioactive dating done?

> I understand that by knowing the current % (say Z) of X in Y,
>and the half-life of X, that we can project backwards
>and figure out the % (say Z') of X in Y at any previous time.
>So to figure out the age of Y  we need to know the original
>percentage of X in Y.  But how is that known?

> Let me put it in a more concrete form. Suppose we use carbon
>dating to measure the age of a fossilized plant. I don't
>understand why we aren't measuring the age of the carbon,
>created I presume at the some time before the age of the earth,
>rather than the age of the plant.


Actually the carbon used for dating is being created all the time
and that is why it can be used for such dating.  C14 is created by
cosmic ray bombardment of the atmosphere.  Being radioactive it
decays of course.  However it is replaced by the constant creation
of new atoms, resulting in an approximately constant ratio of C12 to
C14 atoms in the biosphere.  However when an organism dies the
carbon atoms in its body cease to be replaced with new ones.  After
death the percent of C12 atoms stays constant while the percent of C14 
atoms decreases as they decay radioactively (I am talking about what
percent C12 or C14 are of the entire sample).  Therefore if we can 
get a relatively undisturbed sample of old biological material we can
determine its age by measuring the ratio of C12 to C14.  If the ratio 
is the same as the atmosphere today, the sample is very young.  If 
there is very little C14 the sample is old.  A few relatively simple
mathematical calculations will tell us how old.