Radiocarbon

Radiocarbon, commonly referred to as carbon-14 (14C), is a radioactive isotope of carbon containing six protons and eight neutrons. It is produced in the Earth's upper atmosphere through interactions of cosmic rays with nitrogen-14 and is oxidized to carbon dioxide. It mixes with atmospheric CO2 and is incorporated by plants during photosynthesis, entering the biosphere and carbon reservoirs. In living organisms, 14C is maintained in equilibrium with the atmosphere, making it possible to use its decay as a clock after death.

At death, exchange with the environment stops and the 14C decays to nitrogen-14 by beta decay with a half-life of about 5,730 years. The remaining 14C activity decreases predictably, allowing age estimates when compared to the initial level. The method is called radiocarbon dating.

Dating relies on measuring the current 14C concentration in a sample and comparing it to the atmospheric 14C concentration at the time the organism was alive. Techniques include measurement of beta emissions by decay counting and accelerator mass spectrometry (AMS), which counts 14C atoms directly and requires very small samples. The typical applicable time range is up to about 50,000 years.

Because atmospheric 14C levels have varied over time and by geographic region, calibration is required. Calibration curves built from tree-ring dating and other records correct radiocarbon ages to calendar ages. Additional factors such as the marine reservoir effect, sample contamination, and isotopic fractionation can affect results. Fossil fuels are devoid of 14C and can dilute measurements in mixed samples.

Radiocarbon dating has become a fundamental tool in archaeology, geology, paleoclimatology, and art authentication, providing chronological constraints and material histories for organic remains and carbon-containing artifacts.