How does radiocarbon dating work?

Living things take in a small, steady amount of radioactive carbon-14. When an organism dies, it stops taking in more, and the carbon-14 it already holds slowly decays at a predictable rate. By measuring how much is left, scientists can estimate how long ago the organism died. Raw results must then be calibrated to turn them into calendar dates. [1] [2]

Cosmic rays strike the upper atmosphere, and the neutrons they produce interact with nitrogen atoms to form carbon-14. The new atoms quickly oxidise into carbon dioxide, which mixes through the atmosphere and is taken up by plants through photosynthesis. Animals get the carbon by eating plants, or by eating animals that eat plants. As a result, all living organisms have about the same ratio of carbon-14 to stable carbon as the atmosphere does. [3] [4] [5]

After death the carbon is no longer replenished, and the carbon-14 decays into nitrogen-14. Its physical half-life is given as 5730 years, with an uncertainty of 40 years. The proportion of carbon-14 to stable carbon falls following an exponential decay law, so measuring the ratio in a sample and comparing it with a modern standard gives the time elapsed since death. [3] [4] [6]

The date refers to when the organism was alive, not when the material was later used. Anything that was once living, such as bone, seeds, wood or shell, can be dated. Stone, metal and pottery usually cannot be dated directly unless organic material is embedded in them or left as a residue. [4] [3]

Carbon-14 is hard to measure because it makes up only about one in every million million carbon atoms in living material. Samples are first pretreated to remove contamination, usually from the burial environment, packaging or conservation work. Early methods counted the radioactive decays of the atoms, using gas or liquid samples, and needed large samples and months of counting. [3] [1]

Accelerator mass spectrometry is now almost always used. It accelerates carbon atoms and separates the isotopes by mass with magnets, so it counts the atoms directly. This allows much smaller samples, for example about 1 gram of bone instead of about 200 grams. The practical limit is roughly 50,000 years. [3]

A radiocarbon age is not automatically a calendar age. The atmosphere's carbon-14 level has varied over time, and the half-life value used in calculations is the original one, kept for consistency. Both issues are handled by calibrating against samples of known calendar age. Reservoir effects also matter: the ocean surface is on average about 400 radiocarbon years older than the atmosphere, and freshwater offsets vary and must be measured locally. Stable isotope measurements can show whether such an effect is present. [4] [3]