As soon as a living organism dies, it stops taking in new carbon.
The ratio of carbon-12 to carbon-14 at the moment of death is the same as every other living thing, but the carbon-14 decays and is not replaced.
Thus, as living things take in carbon, they inevitably will take up a small amount of radioactive carbon into their bodies.
When these lifeforms die, they stop taking in new carbon.
Isotopes of a particular element have the same number of protons in their nucleus, but different numbers of neutrons.
This means that although they are very similar chemically, they have different masses.
The total mass of the isotope is indicated by the numerical superscript.