14C Dating -     also called RADIOCARBON DATING; is a method of age determination that depends upon the decay of radiocarbon (carbon-14) to nitrogen . Carbon-14 is continually formed in nature by the interaction of neutrons with nitrogen-14 in the Earth's atmosphere; the neutrons required for this reaction are produced by cosmic rays interacting with the atmosphere.

Radiocarbon present in molecules of atmospheric carbon dioxide enters the biological carbon cycle: it is absorbed from the air by green plants and then passed on to animals through the food chain.

Radiocarbon spontaneously decays slowly in a living organism, and the amount lost is continually replenished as long as the organism takes in air or food.

Once the organism dies, however, it ceases to absorb carbon-14, so that the amount of the radiocarbon present in its tissues at death steadily decreases from that point on.

Carbon-14 has a half-life of 5,730 +/- 40 years--i.e., half the amount of the radioisotope present at any given time will undergo spontaneous disintegration during the succeeding 5,730 years.  A half-life is the time it takes for one-half of the parent isotope to decay to its daughter isotope (14C to 14N). Because carbon-14 decays at this constant rate, an estimate of the date at which an organism died can be made by measuring the amount of its residual radiocarbon.

The carbon-14 method was developed by the American physicist Willard F. Libby about 1946. It has proved to be a versatile technique of dating fossils and archaeological specimens from 500 to 50,000 years old. The method is widely used by Pleistocene geologists, anthropologists, archaeologists, and investigators in related fields. 
back