Perhaps no concept in science is as misunderstood as "carbon dating." Almost everyone thinks carbon dating speaks of millions or billions of years.
Radiocarbon is the best and often the only way to quantify rates of exchange of carbon among reservoirs.
This is the key to achieving predictive understanding of the carbon cycle.
It is only useful for once-living things which still contain carbon, like flesh or bone or wood.
Rocks and fossils, consisting only of inorganic minerals, cannot be dated by this scheme.
Thus the ratio of stable C-12 to unstable C-14, which is known in today's open environment, changes over time in an isolated specimen. As long as the tree lives, it absorbs carbon from the atmosphere in the form of carbon dioxide, both C-12 and C-14.
Once the tree dies, it ceases to take in new carbon, and any C-14 present begins to decay.
Through the life of the organism, the proportion of C-14 to C-12 reaches the same proportion as in the rest of the environment.
When the organism dies, however, it ceases to incorporate carbon into its body.
More research is necessary to explain past changes in COC produced by atmospheric weapons testing (between 19), as it dissolves in surface oceans and is taken up and respired by land plants can be traced.