Radiometric Dating

Imagine a plant absorbs carbon-12 and carbon-14 from the atmosphere during photosynthesis and is then eaten by another organism.

When it dies, it stops taking in carbon, and the existing carbon-14 gradually decays into nitrogen-14, while stable carbon-12 remains nearly constant.

Since the intake ratio of these isotopes is fixed, we can determine the initial amount of carbon-14 in the organism.

We also know that carbon-14 has a half-life of 5,730 years. By combining this with the current amount of carbon-14, we can calculate how many years have passed since the organism’s death.

For example, an organism with 400 g of carbon-14 at the time of death will have 200 g after 5,730 years and 100 g after another 5,730 years.

This method of determining the absolute age of fossils and rocks by measuring the decay rate of unstable isotopes is called radiometric dating.

While carbon-14 is used to date samples younger than 50,000 years, isotopes such as Potassium-40, Uranium-235, and Uranium-238 are used to date older specimens.