Carbon Dating

• Accuracy of Fossils and Dating Methods [online article]
• BBC: Radiocarbon Dating [online article]
• The Usefulness of Carbon 14 [online article]

Background:

Most carbon atoms in our atmosphere have six protons and six neutrons in the nucleus, yielding a total mass number of 12. A small fraction of naturally-occuring carbon atoms, however, have extra neutrons, resulting in mass numbers of 13 or 14. C-14 is unstable and decomposes to smaller isotopes (C-13 and C-12) at a predictable rate. This rate is described by a "half-life" of 5700 years. In other words, half of the entire fraction of C-14 will decompose in approximately 5700 years. Thanks to this natural phenomenon, scientists are able to estimate the age of carbon-containing materials (bones, plant material, etc.) by measuring the fraction of C-14 remaining in the sample, a test referred to as Carbon Dating (see informative video below)

Radiocarbon dating, as it is also called, is not the "be-all, end-all" of dating. Consider these limitations:

1. First and most obvious, is the fact that samples must be carbonaceous. Don't waste your time attempting to carbon-date a rock.
2. Also, samples must be less than ~100,000 years old, after which all traceable amounts of C-14 have disappeared. (Some sources report a limit of 50,000 years, while some report 130,000 years, but the point still stands. Time is a limiting factor.)
3. Even for recent, carbonaceous samples, this method of dating may not be reliable, since it lies on the precarious assumption that atmospheric C-14 content has remained relatively constant throughout time.
4. Only terrestrial samples yield accurate estimates, since almost no C-14 is present in dissolved carbonate. As a result, scientists in the past have estimated the age of clams to be over 50,000 years old using carbon-dating, while the clam was still alive!
5. Still other complications have arisen, when historical artifacts from a known time period are assigned a date 20% older than expected.

Despite these limitations, carbon-dating has been calibrated by ice-core, tree-ring, and lake varve measurements supposedly as far back as 15,000 B.C., and confirmed by innumerable cases. Scientists accept varying degrees of reliability when it comes to carbon-dating, because of these contradicting forms of evidence. A few different perspectives, and further discussion, are shared in the articles above.