Isotopes participate in the same chemical reactions but often at differing rates.
Radio carbon dating explained
Signals of this kind are often used by chemists studying natural environments.
A hydrocarbon found in beach sediments, for example, might derive from an oil spill or from waxes produced by plants.
They have the same ratio of carbon-14 to carbon-12 as the atmosphere, and this same ratio is then carried up the food chain all the way to apex predators, like sharks.
But when gas exchange is stopped, be it in a particular part of the body like in deposits in bones and teeth, or when the entire organism dies, the ratio of carbon-14 to carbon-12 begins to decrease.
The pathway from the plant to the molecule may have been indirect or lengthy, involving multiple physical, chemical, and biological processes.