When an organism dies it ceases to replenish carbon in its tissues and the decay of carbon 14 to nitrogen 14 changes the ratio of carbon 12 to carbon 14. At any particular time all living organisms have approximately the same ratio of carbon 12 to carbon 14 in their tissues. Carbon is naturally in all living organisms and is replenished in the tissues by eating other organisms or by breathing air that contains carbon.
The stable form of carbon is carbon 12 and the radioactive isotope carbon 14 decays over time into nitrogen 14 and other particles.
Archaeologists use the exponential, radioactive decay of carbon 14 to estimate the death dates of organic material.
