Plants and animals naturally incorporate both the abundant C-12 isotope and the much rarer radiocarbon isotope into their tissues in about the same proportions as the two occur in the atmosphere during their lifetimes.When a creature dies, it ceases to consume more radiocarbon while the C-14 already in its body continues to decay back into nitrogen.The amount of cosmic rays penetrating the earth's atmosphere is itself affected by things like the earth's magnetic field which deflects cosmic rays.Precise measurements taken over the last 140 years have shown a steady decay in the strength of the earth's magnetic field.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 decays slowly in a living organism, and the amount lost is continually replenished as long as the organism takes in air or food.Specimens which lived and died during a period of intense volcanism would appear older than they really are if they were dated using this technique.
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.
C-14 is produced in the upper atmosphere when nitrogen-14 (N-14) is altered through the effects of cosmic radiation bombardment (a proton is displaced by a neutron effectively changing the nitrogen atom into a carbon isotope).
The new isotope is called "radiocarbon" because it is radioactive, though it is not dangerous.
Once the organism dies, however, it ceases to absorb carbon-14, so that the amount of the radiocarbon in its tissues steadily decreases.
Carbon-14 has a half-life of 5,730 ± 40 years—, half the amount of the radioisotope present at any given time will undergo spontaneous disintegration during the succeeding 5,730 years.
After about 10 half-lives, the amount of radiocarbon left becomes too miniscule to measure and so this technique isn't useful for dating specimens which died more than 60,000 years ago.