Radiocarbon dating is a scientific method used to estimate the age of once living organic material by measuring the amount of radioactive carbon 14 remaining in a sample. Carbon 14 is a radioactive isotope of carbon that is produced naturally in the atmosphere and incorporated into living organisms through the carbon cycle. While an organism is alive, its carbon isotope composition is continually influenced by exchanges with the environment.
After death, this exchange largely stops and carbon 14 begins to decrease through radioactive decay. Because carbon 14 decays at a known rate, the remaining amount can be used to estimate the time since the organism stopped exchanging carbon with its environment. The method is especially important in archaeology because it can date charcoal, wood, seeds, bone, shells and other appropriate organic materials over a substantial portion of human history.
Carbon 14 has a half life of approximately 5730 years, although the exact value used in radiocarbon calculations depends on the conventional radiocarbon framework. A half life means that approximately half of the radioactive carbon in a closed system will decay over one half life, approximately one quarter remains after two half lives, and so forth. Radiocarbon dating does not normally identify the exact year in which an organism died. Instead, laboratory measurement produces a radiocarbon age that must be interpreted statistically and usually calibrated against independent records. The method becomes increasingly uncertain as samples approach the practical limits of radiocarbon dating, and contamination or unusual carbon histories can significantly affect results.
Radiocarbon dating begins with careful sample selection and preparation. Archaeologists must establish exactly what material is being dated and how securely it relates to the archaeological event of interest. Laboratories remove contaminants and isolate suitable carbon fractions before measurement. Modern methods commonly use accelerator mass spectrometry, which directly counts carbon isotopes and allows very small samples to be analysed. The laboratory result is expressed as a radiocarbon age with an associated measurement uncertainty. The result is not automatically a calendar date. Calibration is required because atmospheric carbon 14 concentrations have varied over time due to changes in solar activity, ocean circulation, carbon reservoirs and other processes.
Calibration is performed using independently dated materials that preserve records of past atmospheric carbon 14 levels. Tree rings are especially important because individual annual rings can be dated precisely and analysed for their carbon isotope content. Long sequences of tree rings provide calibration information extending far beyond individual trees. Other archives, including corals and suitable sediments, also contribute to calibration curves. Because atmospheric carbon 14 concentration has fluctuated, a measured radiocarbon age can correspond to more than one possible calendar interval. Calibrated results are therefore commonly expressed as probability distributions or ranges rather than single dates.
The relationship between a radiocarbon date and the archaeological event being investigated must be considered carefully. If a short lived seed is found in a sealed hearth, its death may be closely related to the use of the hearth. A large old tree, however, may have been cut down long after the growth of the portion of wood that was sampled. Old wood can therefore produce an age older than the archaeological event.
Charcoal can also derive from wood that had already been dead or stored before burning. Bone can preserve useful carbon but requires careful assessment of preservation and contamination. Radiocarbon dating consequently measures the age of the carbon containing material, not automatically the age of the archaeological site or artefact associated with it.
Several sources of error and bias can affect radiocarbon measurements. Contamination with modern carbon can make a sample appear younger, while contamination with ancient carbon can make it appear older. Reservoir effects can occur when organisms obtain carbon from sources with different apparent radiocarbon ages. Aquatic organisms can sometimes appear older because dissolved carbon in water may have a different radiocarbon composition from the atmosphere. Marine and freshwater reservoir effects therefore require appropriate corrections. Researchers also consider laboratory uncertainty, sample pretreatment, calibration models and archaeological context. A reported radiocarbon date should consequently be understood as an estimate with uncertainty rather than an infallible timestamp.
Radiocarbon dating is most effective for materials that contain carbon derived from the atmosphere or appropriate carbon reservoirs and that fall within the method's usable chronological range. It cannot directly date most stone tools, metals or inorganic minerals because those materials do not normally contain the relevant biological carbon. Researchers sometimes date associated organic material rather than the artefact itself. Radiocarbon dating can also be integrated with stratigraphy and Bayesian modelling, allowing several measurements and known archaeological relationships to be analysed together. This can narrow chronological ranges when the underlying assumptions are justified and the samples are genuinely connected to the events being modelled.
Radiocarbon dating transformed archaeology because it provided an independent chronological method capable of dating many prehistoric and early historic contexts. Before its development, archaeologists often depended heavily on typology, stratigraphy and comparisons with historically dated cultures. Radiocarbon dating made it possible to test chronological assumptions and establish dates where written records were absent. Its results have influenced research into agriculture, migration, settlement, climate adaptation, monument construction and technological change.
Its power comes from the combination of a measurable physical decay process, laboratory analysis, calibration and archaeological context. Its limitations are equally important: it requires suitable material, depends on correct sampling and calibration, produces uncertainty, and dates the sampled carbon rather than automatically dating the human activity under investigation.
