By Corina Fiore Updated Mar 24, 2022
Many rocks and organisms contain unstable radioactive isotopes, such as uranium‑235 (U‑235) and carbon‑14 (C‑14). These isotopes decay at a predictable, logarithmic rate, emitting particles from their nuclei and transforming into stable daughter isotopes. The original unstable isotope is the parent, while the decay product is the daughter. The half‑life is the time required for half of the parent isotopes to decay. For example, C‑14 has a half‑life of 5,730 years, meaning that every 5,730 years an organism loses half of its remaining C‑14 atoms.
When fossils are recovered, they are often found embedded in the same rock layers (strata) as their surrounding matrix. Scientists carefully catalog these samples and analyze them with a mass spectrometer, which determines the precise types and amounts of isotopes present. By measuring the ratio of parent to daughter isotopes and comparing this ratio to the known half‑life of the parent isotope, researchers calculate the age of the fossil or the rock in which it is encased.
U‑235 is the most widely used isotope for dating older rocks and fossils. It decays to lead‑207 (Pb‑207) with a half‑life of 704 million years, making it ideal for determining ages far beyond the range of C‑14. C‑14, which decays to stable carbon‑12 (C‑12), is present in all living organisms. After an organism dies, its C‑14 content begins to decay. Because C‑14 has a relatively short half‑life, its measurable levels become negligible after roughly 50,000 years, limiting its usefulness to younger artifacts and fossils, especially those related to human activity.
