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Carbon-14 (14C) is a radioactive isotope of carbon with six protons and eight neutrons. It is produced in the Earth's upper atmosphere by interactions of cosmic rays with nitrogen-14 and has a half-life of about 5,730 years. In living organisms, 14C is incorporated into CO2 and enters the biosphere; after death, its radioactivity decays with no replenishment. The natural abundance of 14C in modern atmospheric carbon is about 1.2×10^-12, roughly one part per trillion, varying with solar activity and the Suess effect.

Radiocarbon dating uses the predictable decay of 14C to estimate the age of organic material. Living organisms maintain equilibrium with atmospheric 14C, but once dead, the 14C decays to 14N and the 14C/total carbon ratio decreases. Dates are obtained by measuring 14C activity or using accelerator mass spectrometry (AMS). Because atmospheric 14C has fluctuated over time, ages are calibrated against calendar ages using curves derived from tree rings and other records; typical reliable dating range extends to about 50,000 years.

Applications include dating archaeological samples, paleoclimatology materials, and art forensics. Limitations include contamination, preservation state, reservoir effects (marine or freshwater sources can yield apparent older ages), and mixing of carbon from different reservoirs. The method cannot date inorganic materials directly unless small carbon-containing inclusions are present or using indirect approaches.

History: radiocarbon dating was developed in the mid-20th century by Willard Libby, who received the Nobel Prize in Chemistry in 1960 for the method. Dates are commonly reported as “radiocarbon years before present” (BP), with “present” defined as 1950 CE.