If you ever wondered why nuclear tests are now performed underground, this is why.
Other methods scientists use include counting rock layers and tree rings.
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.
Radiocarbon dating uses isotopes of the element carbon. Cosmic rays – high-energy particles from beyond the solar system – bombard Earth’s upper atmosphere continually, in the process creating the unstable carbon-14. Because it’s unstable, carbon-14 will eventually decay back to carbon-12 isotopes.
The age of the carbon in the rock is different from that of the carbon in the air and makes carbon dating data for those organisms inaccurate under the assumptions normally used for carbon dating.
This restriction extends to animals that consume seafood in their diet.