The basis of radiocarbon dating is simple: all living things absorb carbon from the atmosphere and food sources around them, including a certain amount of natural, radioactive carbon-14. When the plant or animal dies, they stop absorbing, but the radioactive carbon that theyve accumulated continues to decay.

Why does radiocarbon dating involve carbon-14 and not carbon 12?

Carbon-14 is considered a radioactive isotope of carbon. Because its unstable, carbon-14 will eventually decay back to carbon-12 isotopes. Because the cosmic ray bombardment is fairly constant, theres a near-constant level of carbon-14 to carbon-12 ratio in Earths atmosphere.

Why is carbon-14 not used to date fossils?

These artifacts have gone through many carbon-14 half-lives and the amount of carbon-14 remaining in them is miniscule and very difficult to detect. Carbon dating cannot be used on most fossils, not only because they are almost always too old, but also because they rarely contain the original carbon of the organism.