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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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