Radiocarbon dating

Radiocarbon dating works because carbon-14 is radioactive and decays at a predictable rate. Cosmic rays create carbon-14 in the upper atmosphere, where it becomes part of carbon dioxide and enters plants, animals, and food webs. After an organism dies, carbon exchange largely stops; the remaining carbon-14 then declines while stable carbon isotopes remain. Measuring that decline gives an estimate of elapsed time, with a half-life of about 5,730 years. ¹

Willard Libby and his collaborators established the method in the late 1940s and early 1950s, transforming archaeology and the study of the recent geological past. Libby received the 1960 Nobel Prize in Chemistry for developing radiocarbon dating. ² The method applies to charcoal, wood, bone collagen, shell, seeds, textiles, peat, and other materials containing carbon of biological origin.

Radiocarbon ages are measured either by counting radioactive decay or by directly counting carbon-14 atoms with accelerator mass spectrometry. AMS requires very small samples and is especially valuable when material is rare, although careful chemical pretreatment remains essential because contamination can shift the result. Laboratories normally report a conventional radiocarbon age in years before present, with “present” conventionally fixed at 1950.

That laboratory age is not automatically a calendar date. Atmospheric carbon-14 has varied through solar activity, geomagnetic changes, carbon-cycle shifts, and fossil-fuel emissions, so results are calibrated against independently dated records such as tree rings, corals, lake sediments, and speleothems. The internationally maintained IntCal calibration curves provide the principal framework for terrestrial, marine, and Southern Hemisphere samples. ³ Calibration commonly produces probability ranges rather than one exact year.

Radiocarbon dating is most powerful for establishing chronologies from roughly a few hundred to several tens of thousands of years ago. Archaeologists use it to date settlements, burials, hearths, agricultural remains, and trade materials; Quaternary scientists apply it to climate records, glacier histories, coastal change, and ecosystem development. A date applies to the carbon-bearing event sampled, not necessarily to the whole object: charcoal may predate the fire in which it was found, while short-lived seeds can more closely represent the depositional moment.

Interpretation can be complicated by contamination, incomplete pretreatment, and reservoir effects. Marine organisms may appear artificially old because ocean carbon circulates slowly, and freshwater organisms can show even larger, locally variable offsets. Samples therefore require context-specific correction, and dates are strongest when combined with stratigraphy, typology, dendrochronology, or other independent evidence. ¹⁵

Radiocarbon dating also records recent environmental change, not only ancient history. Atmospheric testing of nuclear weapons in the 1950s and 1960s produced a rapid “bomb pulse” of carbon-14, which can help distinguish modern from pre-modern biological material and estimate the formation age of tissues such as tree rings, corals, and some human tissues. The signal is especially useful in forensic and ecological research.

Another subtle issue is that calibration curves contain plateaus, where several calendar years correspond to similar radiocarbon values; these can create broad or multimodal age ranges. Researchers sometimes date a sequence of samples and use “wiggle matching” against tree-ring structure to improve resolution. Samples can also be split among laboratories for comparison, while Bayesian chronological models combine radiocarbon likelihoods with archaeological order and stratigraphic information. International laboratory standards and intercomparison exercises help detect systematic differences between facilities. ³⁴

Carbon-14

A radioactive isotope of carbon produced naturally in the atmosphere and incorporated into living organisms.

AMS

Accelerator mass spectrometry, a technique that counts carbon-14 atoms directly and can analyze very small samples.

Calibration curve

A reference curve that converts a measured radiocarbon age into a calendar-age probability distribution.

Reservoir effect

An apparent age offset caused when organisms obtain carbon from a reservoir, such as deep ocean or old freshwater, with reduced carbon-14.

Bomb pulse

The sharp increase in atmospheric carbon-14 caused by mid-twentieth-century nuclear-weapons testing.