Here’s an example using the simplest atom, hydrogen. Carbon-14 is an unstable isotope of carbon that will eventually decay at a known rate to become carbon-12.
In The Cosmic Story of Carbon-14 Ethan Siegel writes: The only major fluctuation [in carbon-14] we know of occurred when we began detonating nuclear weapons in the open air, back in the mid-20th century.
Isotopic tracer, any radioactive atom detectable in a material in a chemical, biological, or physical system and used to mark that material for study, to observe its progress through the system, or to determine its distribution.
An isotopic tracer must behave as does the material being studied, but, in addition, it must have some distinguishing property by which it can be detected in the presence of the other material.
It is rapidly oxidized in air to form carbon dioxide and enters the global carbon cycle.
Plants and animals assimilate carbon 14 from carbon dioxide throughout their lifetimes.