Carbon dating, more precisely 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 produced naturally in the upper atmosphere and becomes incorporated into living organisms through the carbon cycle. While an organism is alive, its carbon exchanges with the environment and its proportion of carbon 14 broadly reflects the atmospheric level.
After death, carbon exchange largely stops and carbon 14 decays radioactively into nitrogen 14. By measuring the remaining carbon 14 and applying knowledge of its decay rate, scientists can estimate when the organism died. Radiocarbon dating is particularly important in archaeology, palaeoenvironmental research and Quaternary science because it provides numerical age estimates for material from the relatively recent geological past.
The method was developed in the twentieth century by Willard Libby and colleagues, with the first successful radiocarbon dating measurements produced in the late 1940s. Libby recognised that naturally occurring radioactive carbon could function as a chronological clock. Carbon 14 has a half life of approximately 5730 years, meaning that after one half life approximately half of the original carbon 14 remains, after two half lives approximately one quarter remains, and so forth.
Conventional radiocarbon dating is therefore most useful for material within roughly the last 50000 years, although practical limits vary according to sample quality, measurement technique and background contamination. Accelerator mass spectrometry allows very small quantities of carbon to be analysed and has substantially expanded the range of material that can be dated.
Radiocarbon dating can be applied to charcoal, wood, seeds, plant fibres, bone collagen, shells under appropriate conditions and other carbon bearing materials. Archaeologists commonly date charcoal from fireplaces, organic remains from occupation deposits and suitable bone from archaeological contexts. Palaeoenvironmental researchers can date peat, sediments containing appropriate organic material and other remains. The method dates the carbon in the sample rather than automatically dating the archaeological site, artefact or event associated with it. For example, a radiocarbon date from charcoal beneath a building may establish when the wood died, but it does not necessarily establish the exact date when the building was constructed. Interpretation therefore depends heavily on archaeological context.
Raw radiocarbon measurements cannot simply be converted into calendar years using the basic half life equation because atmospheric carbon 14 concentrations have varied through time. Radiocarbon results therefore require calibration against independently dated records. Tree rings provide an important calibration source because individual annual growth rings can sometimes be assigned precise calendar years. Longer calibration curves incorporate additional evidence and allow radiocarbon ages to be converted into calendar age ranges. Calibration can produce probability distributions rather than one exact date. A laboratory result might therefore indicate that a sample has a particular calibrated age range with a stated probability rather than producing a single definitive year. This distinction is essential when interpreting radiocarbon results.
Contamination is one of the principal challenges in radiocarbon dating. Modern carbon introduced into an ancient sample can make it appear younger, while certain forms of contamination can produce other distortions. Samples must therefore be carefully selected, cleaned and chemically processed. In bone dating, researchers often isolate collagen because it is the fraction most useful for radiocarbon analysis, while degraded collagen may produce unreliable results. Reservoir effects can also complicate interpretation. Aquatic organisms may incorporate carbon that is older than contemporary atmospheric carbon, causing apparent ages that are older than the actual time of death. Marine and freshwater reservoir effects therefore require appropriate corrections when relevant.
Radiocarbon dating is not suitable for every archaeological or geological question. It cannot directly date most stone tools because stone does not contain the relevant original biological carbon, although organic residues or associated materials may sometimes be dated. It is also unsuitable for material substantially older than its effective measurement range because very little original carbon 14 remains. Samples can also suffer from old wood effects, in which a long lived tree is cut and used centuries after the growth represented by the dated wood occurred. Multiple samples and independent dating methods can therefore be important when researchers need to establish the chronology of a complex site or event.
Radiocarbon dating has transformed archaeology because it allows prehistoric chronology to be anchored to numerical timescales. Before radiocarbon dating, archaeologists often relied heavily on typology, stratigraphy and comparisons with historically dated cultures. Radiocarbon measurements provided an independent chronological framework and contributed to major revisions of archaeological chronologies. The method has also been important in climate research, studies of past vegetation, sea level research and investigations of human migration. It is particularly valuable when combined with stratigraphy and other dating techniques because independent evidence can expose inconsistencies and strengthen chronological interpretations.
A common misconception is that carbon dating provides an exact age for anything found at an archaeological site. In reality, it dates suitable carbon containing material and produces an estimate with uncertainty. Another misconception is that the method directly measures the date an object was manufactured. It instead measures the age of the carbon containing material sampled. Modern radiocarbon dating can be highly precise when appropriate samples, laboratory procedures and calibration data are available, but precision is not the same as absolute certainty. Its scientific value comes from combining radioactive decay physics, careful sample preparation, calibration, statistical analysis and archaeological or geological context.
