Minor technical nit: Wood dates to the year when it *grew*, not
necessarily the year it was cut. Trees grow a new ring each year.
The previous years' rings no longer exchange carbon with their
environment, and therefore their carbon "clock" begins counting
when the next ring starts to grow. You can get wood that dates to
hundreds of years in age out of a live tree, if the tree is old
and the wood is taken from the inner rings.
> Chuck -- what answer is made when someone, having been told the
> above, objects that nobody knows how much C-14 the tree had 5730
> years ago, that C-14 levels may have been different then?
Since tree rings date to the year they grew, carbon dating can be
performed on series of tree rings whose age is known independently
(by counting rings backward from the present). The difference
between the age by "counting" and the (uncorrected) carbon-14 age
gives a "correction" which tells the difference in carbon-14 level
of the atmosphere back when the ring was grown.
The evidence suggests that over the past 10,000 years there have
been fairly rapid but small fluctuations in C-14 levels (on order
of 1% of present level, taking a small number of decades), as well
as larger longer-term fluctuations (on order of 10% of present
level, taking thousands of years). The latter correlate well with
changes to carbon-14 production rate that would be predicted from
fluctuations in the Earth's magnetic field.
For further information, see:
Dickin, Alan P., 1997. _Radiogenic Isotope Geology_. New York:
Cambridge University Press, ISBN 0-521-59891-5 (paper,
490 pp). Discussion of carbon dating is in Chapter 14;
Figures 14.7 and 14.9 indicate corrections for variations
in atmospheric carbon-14 levels.
-- Chris (firstname.lastname@example.org)