Dating a Fossil by Isotope Decay

Dating an old organic sample by isotope decay means comparing the surviving fraction of a radioactive isotope against a known half-life to calculate elapsed time.

A living organism keeps a ratio of carbon-14 to ordinary carbon-12 that matches the atmosphere around it, because it continually takes in carbon while alive. Once the organism dies, it stops taking in new carbon-14, and the carbon-14 already in its tissues decays toward nitrogen-14 at its fixed half-life of about 5,730 years, while the ordinary carbon-12 stays put. Comparing the sample's remaining carbon-14 to what a living organism would carry gives the fraction that has decayed. If a bone sample carries half the carbon-14 ratio of a living organism, one half-life has passed, and the organism died roughly 5,730 years ago; a quarter means two half-lives, near 11,460 years. This method works only up to about nine half-lives, since beyond that the remaining carbon-14 becomes too sparse to measure — dating is not used on samples believed far older than roughly 50,000 years, and isotopes with longer half-lives, such as potassium-40, take over for older material. The technique needs no known starting age, only the fixed decay rate and a careful measurement of what fraction of the isotope remains today.