14 C is radioactive and decays back to 14 N with a half-life of 5730 years. The concentration or activity of 14 C is constant in the atmosphere because there is an equilibrium between the rate of its production and its decay. Living plants and animals also contain a constant level of 14 C, but when the organism dies, it becomes isolated from the atmosphere, and 14 C starts decaying. Because 14 C has a short half life, its use for dating is limited to about 8 or 9 half-lives or 50,000 years. Materials suitable for dating include wood, peat, shells, CaCO 3 accumulations, and bone.
Sample size can be a problem.
Usually, several grams of C are needed to get a measurement on the mass spectrometer. In many areas of Midwest, Wisconsin glaciers advanced quickly over forests and buried them. Wood is preserved in the till deposits. This wood is excellent for 14 C dating. 14 C dates for Wisconsin deposits are considered to be very accurate.