Radiocarbon dating

Radiocarbon dating, also known as carbon dating or carbon-14 dating, is a widely used scientific method for determining the age of objects that contain organic material. Developed in the late 1940s at the University of Chicago by Willard Libby, the technique transformed archaeology, geology, and palaeoenvironmental research by providing a reliable means of placing organic remains within a chronological framework. The method is based on the properties of carbon-14, a radioactive isotope that forms naturally in the atmosphere through cosmic-ray interactions and decays predictably over time.
By measuring the amount of carbon-14 remaining in organic samples such as wood, bone, seeds, or textiles, researchers can estimate the time that has elapsed since the organism died. This capability has allowed major prehistoric transitions to be dated with unprecedented accuracy, marking what scholars often call the “radiocarbon revolution”.

Background and Origins of the Technique

The scientific foundations of radiocarbon dating emerged from early twentieth-century research into radioisotopes. In 1939, experiments by Martin Kamen and Sam Ruben demonstrated that carbon-14 possessed a much longer half-life than previously believed, establishing its potential for scientific applications. Around the same time, Serge A. Korff predicted that interactions between cosmic rays and atmospheric nitrogen would continually generate carbon-14 in the upper atmosphere.
During the Second World War, Willard Libby became aware of this work and proposed that carbon in living organisms must contain small but detectable amounts of carbon-14. After moving to the University of Chicago in 1945, Libby and his collaborators designed experiments to test this hypothesis. Using methane drawn from sewage works, they demonstrated in 1947 that recent organic material contained measurable carbon-14, whereas methane derived from petroleum showed no radioactivity owing to its great age.
Libby further tested the method by dating samples of known historical age, including materials from Egyptian royal tombs. The radiocarbon dates closely approximated the known dates, providing strong evidence of the technique’s reliability. By 1949, the first scientific results were published, and within little more than a decade radiocarbon laboratories had been established around the world. In recognition of his pioneering work, Libby received the Nobel Prize in Chemistry in 1960.

Physical and Chemical Basis

Carbon exists naturally in three isotopic forms: the stable isotopes carbon-12 and carbon-13, and the unstable isotope carbon-14. Carbon-14 is continually formed in the lower stratosphere and upper troposphere when cosmic rays produce neutrons that collide with nitrogen-14, transforming it into carbon-14. Once produced, the radioactive isotope combines with oxygen to form carbon dioxide, which circulates throughout the atmosphere.
Plants absorb this carbon dioxide through photosynthesis, and animals in turn obtain carbon-14 by consuming plants or other animals. Consequently, all living organisms maintain an approximate equilibrium with atmospheric carbon-14 levels. The ratio of carbon-14 to carbon-12 in atmospheric carbon is roughly one atom of carbon-14 per trillion atoms of carbon-12, with about one per cent of overall carbon being carbon-13.
When an organism dies, it ceases to exchange carbon with its surroundings. The carbon-14 present at the time of death then begins to decay at a fixed rate. The half-life of carbon-14 is about 5,730 years, meaning that after this period half of the original carbon-14 atoms have decayed into nitrogen-14 through beta emission. The decay process follows an exponential law, which allows researchers to calculate the time elapsed since death based on the remaining proportion of carbon-14 compared to stable carbon isotopes.

Principles of Measurement and Age Determination

Radiocarbon dating relies on measuring the amount of carbon-14 in a sample relative to carbon-12 or carbon-13. Because the decay rate is known, the age of the sample can be determined using the standard radioactive decay equation. The technique is most effective for samples up to about 50,000 years old; beyond this range, the remaining carbon-14 becomes too small to detect reliably.
Originally, radiocarbon measurements were obtained through beta-counting devices, which measured the beta particles emitted during carbon-14 decay. Although effective, these methods required relatively large sample sizes and long counting times. Since the late twentieth century, accelerator mass spectrometry (AMS) has become the preferred method. AMS counts the actual number of carbon-14 atoms present in the sample rather than waiting for decay events, allowing for far greater sensitivity. This enables dating of extremely small samples, including pollen grains and individual seeds, and produces results more rapidly.

Atmospheric Variation and Calibration

One of the complexities of radiocarbon dating is that atmospheric carbon-14 levels have not remained constant over time. Since the 1960s, extensive research has sought to reconstruct fluctuations in atmospheric carbon-14 across the past fifty millennia. These studies draw upon tree rings, corals, and other independently dated materials to produce a detailed calibration curve. The curve is essential for converting radiocarbon years (often called “radiocarbon ages”) into true calendar ages.
Several factors affect atmospheric carbon-14 concentration:

• Natural fluctuations due to changes in cosmic-ray intensity and Earth’s magnetic field.
• Fractionation, whereby biological processes preferentially absorb lighter or heavier carbon isotopes, requiring correction.
• Reservoir effects, particularly in marine environments, where dissolved carbon may differ significantly from atmospheric values.

Human activities have also altered atmospheric carbon-14 levels. The burning of fossil fuels since the Industrial Revolution introduced large quantities of carbon dioxide devoid of carbon-14, lowering its relative concentration, an effect known as the Suess effect. Conversely, nuclear weapons testing in the 1950s and 1960s sharply increased atmospheric carbon-14, peaking in the mid-1960s at nearly twice pre-industrial levels.

Scientific and Archaeological Impact

Radiocarbon dating has revolutionised archaeology by providing a means of directly dating organic remains, enabling chronological reconstruction with much greater precision than earlier methods. Its introduction allowed archaeologists to synchronise dates across regions and to build global timelines for climatic and cultural developments.
Key prehistoric transitions have been dated using radiocarbon techniques, including the end of the Younger Dryas cold phase, the emergence of Neolithic farming, and the spread of Bronze Age technologies. These advances have reshaped interpretations of human prehistory, leading scholars to describe the advent of radiocarbon analysis as a major turning point in archaeological methodology.
Beyond archaeology, radiocarbon dating plays an important role in climatology, geology, oceanography, and environmental science. It helps trace carbon pathways through ecosystems, date geological events, reconstruct palaeoclimates, and analyse environmental changes over time.