Lectures on Physics has been derived from Benjamin Crowell's Light and Matter series of free introductory textbooks on physics. See the editorial for more information....

C Dating

Almost all the carbon on Earth is 12C, but not quite. The isotope 14C, with a half-life of 5600 years, is produced by cosmic rays in the atmosphere. It decays naturally, but is replenished at such a rate that the fraction of 14C in the atmosphere remains constant, at 1.3×10-12 . Living plants and animals take in both 12C and 14C from the atmosphere and incorporate both into their bodies. Once the living organism dies, it no longer takes in C atoms from the atmosphere, and the proportion of 14C gradually falls off as it undergoes radioactive decay. This effect can be used to find the age of dead organisms, or human artifacts made from plants or animals. Figure j shows the exponential decay curve of 14C in various objects. Similar methods, using longer-lived isotopes, prove the earth was billions of years old, not a few thousand as some had claimed on religious grounds.

j / Calibration of the 14C dating method using tree rings and artifacts whose ages were known from other methods. Redrawn from Emilio Segrč, Nuclei and Particles, 1965.

Last Update: 2009-06-21