Because the cosmic ray bombardment is fairly constant, there’s a near-constant level of carbon-14 to carbon-12 ratio in Earth’s atmosphere.

A detailed description of radiocarbon dating is available at the Wikipedia radiocarbon dating web page.

Bottom line: Radiocarbon dating is a technique used by scientists to learn the ages of biological specimens from the distant past.

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.

Both organic or inorganic materials at the Earth's surface and in the oceans form in equilibrium with atmospheric carbon-14.

This makes it an important tool for the understanding of processes during the time-scale of modern humans, from the last glacial-interglacial transition, to recent archaeological studies of art works.Some specific examples including dating of famous artifacts of artistic, religious and scientific interest are discussed.AB - Radiocarbon dating is an important tool for the determination of the age of many samples and covers the time period of approximately the last 50,000 years.The unstable carbon-14 gradually decays to carbon-12 at a steady rate. Scientists measure the ratio of carbon isotopes to be able to estimate how far back in time a biological sample was active or alive.This plot shows the level of carbon-14 in the atmosphere as measured in New Zealand (red) and Austria (green), representing the Southern and Northern Hemispheres, respectively.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.