This standard content of C14 can then be used for wood not associated with a historically documented date.
Notice in the diagram that eight different isotopes of Carbon are illustrated.
Because of this, 14-C and it's breakdown product 12-C, typically stay in balance.
Carbon is passed between the ocean, atmosphere, and living things in the "Carbon Cycle".
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!