When an animal dies it is taken out of the carbon cycle.
After this, the proportion of 14-C to 12-C steadily declines. 141, page 636, August 16, 1963) showed the shell from a living mollusk to be dead for 3,000 years!
Radiocarbon dating: radioactive carbon decays to nitrogen with a half-life of 5730 years.
In dead material, the decayed 14C is not replaced and its concentration in the object decreases slowly.
The Carbon-14 within a living organism is continually decaying, but as the organism is continuously absorbing Carbon-14 throughout its life the ratio of Carbon-14 to Carbon-12 atoms in the organism is the same as the ratio in the atmosphere.
When this contamination occurs, it mixes the date for the item with the date of the contaminant.The unstable Carbon-14 within the organism begins to decay to form Nitrogen-14 by emitting a beta particle.Over time there is a gradual decrease in the amount of Carbon-14 and the ratio of Carbon-14 atoms to other Carbon atoms declines. Therefore half of the Carbon-14 has decayed after 5730 years.It is naturally unstable and so it will spontaneously decay back into N-14 after a period of time.It takes about 5,730 years for half of a sample of radiocarbon to decay back into nitrogen.Because of the somewhat short half-life of 14C, radiocarbon dating is not applicable to samples with ages greater than about 50,000 years, because the remaining concentration would be too small for accurate measurement.