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Isotopes and Archeology
Name: Zackery
Status: student
Grade: 9-12
Location: GA
Country: N/A
Date: 10/25/2005
Question:
How are isotopes used in archeology?
Replies:
Take carbon dating as an example: At any one moment, most of the carbon
atoms in the world exist in the form of the C-12 isotope, while a small
minority of the carbon atoms are in the form of C-14. Over time, C-14 atoms
undergo radioactive decay, at a rate specified by the half-life. At the same
time, new C-14 atoms are produced from atoms that are exposed to radiation
from the sun, so that the proportion of C-12 to C-14 stays about constant
over time.
Does it really stay constant? This is the unproven assumption on which
carbon dating is based. It is not certain that the rate is perfectly
constant; it probably undergoes some minor variations from time to time. And
it is certainly NOT proven whether the rate was constant over eons of
geological time. Because the half-life of carbon is 5730 years now, does
that mean that it was always 5730 years? As long as we understand this
assumption and its limitations, we can consider how measurement of carbon
and other isotopes is used to date archeological discoveries.
While an organism (whether plant or animal) is alive and metabolism is
ongoing, the proportion of its carbon atoms that are in the form of C-12 vs.
C-14 is the same as the proportion in the environment, since the organism is
in equilibrium with the environment, and exchange takes place between the
cells of the organism and the outside world. We are constantly replacing or
recycling the molecules in our bodies, faster for some tissues such as skin
or blood, and more slowly for other tissues such as bone. The raw materials
that replenish our cells come from the world around us.
When an organism dies, this exchange stops occurring. The cells of the dead
body will have the same atoms later as at the moment of death. The C-14
atoms in that dead body will continue to undergo radioactive decay, and will
not be replaced by new C-14 atoms. So over time, the percentage of C-14 will
decrease.
Note: Do not confuse the processes of radioactive decay and biological
decay. Of course, the actual tissues of the creature will decay and be
digested by whatever species are consuming dead bodies in that ecosystem.
But if biological remains are preserved and are discovered by an
archeologist, the proportion of C-12 to C-14 atoms can be measured, and used
to estimate how long the organism has been dead - or, in other words, how
long ago it lived. If it died a moment ago, the proportion of C-14 is about
the same as in the atmosphere today. If it died 5730 years ago, the
proportion of C-14 will have dropped to half of its original level. If the
organism died a long, long time ago, the proportion of C-14 will be much,
much less.
Just remember the assumption: Has the half-life of C-14 always been the same
as it is today? Food for thought ...
Sarina Kopinsky, MSc
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Update: June 2012
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