How does carbon dating work?

Cosmic rays striking the upper atmosphere continuously produce carbon-14, a radioactive isotope that mixes into carbon dioxide and enters plants through photosynthesis and animals through eating them. While an organism lives, its carbon-14 stays in equilibrium with the atmosphere. When it dies, intake stops and the carbon-14 decays into nitrogen at a fixed rate, with a half-life of about 5,730 years. Measuring the ratio of carbon-14 to stable carbon in a sample and comparing it to the atmospheric starting ratio gives the time since death.

The method has a range and several corrections. After about ten half-lives, so roughly 50,000 years, too little carbon-14 remains to measure, which is why it cannot date dinosaurs and why those ages come from other methods based on isotopes with far longer half-lives. The atmospheric ratio has not been constant, varying with solar activity and the carbon cycle, so raw results are calibrated against records from tree rings, cave formations, and lake sediments that extend back tens of thousands of years. Fossil fuel burning has diluted atmospheric carbon-14 and mid-century nuclear testing nearly doubled it, both of which require correction for recent samples.

Accelerator mass spectrometry, which counts carbon-14 atoms directly rather than waiting for decays, reduced the sample needed from grams to milligrams and made it possible to date a single seed or a thread. The technique earned Willard Libby the 1960 Nobel Prize in Chemistry and underpins the chronology of archaeology, dating the Dead Sea Scrolls, the Shroud of Turin, and the arrival of humans in the Americas. The same principle with different isotopes dates rocks and the Earth itself.