In 1949 a team led by US scientist Willard Libby developed the technique of radiocarbon dating. Libby, who had worked on cosmic radiation during the Second World War, discovered that living things absorb carbon present in the atmosphere. However, he also observed that one form of carbon, the isotope 14C, is unstable – it is radioactive – and that it decays at a fixed rate.
He calculated that it took 5,568 years for half of the 14C in dead organic matter to decay, known as the ‘half-life’. By measuring the remaining 14C in a sample of excavated material, a calendar date could therefore be ascribed to it. Libby worked out a way to do this that involved measuring the emissions of beta particles using a Geiger counter.
Radiocarbon dating tweaked
Libby was wrong about two things. First, the half-life of 14C is actually 5,730 years, and second, he believed that the carbon content of the atmosphere was constant, whereas it is now known that atmospheric carbon levels have varied somewhat over time. That is why radiocarbon dates are now ‘calibrated’ using a tree-ring calibration curve. Tree-ring dates are precise to a single year, and dendro-samples can also be radiocarbon dated, allowing 14C measurements to be correlated with calendar dates. These results can then be applied to other 14C determinations where no tree-ring date is available.
Radiocarbon dating in use
Radiocarbon dating has revolutionised the study of prehistory. Before it became generally available, dating had depended on two techniques: the use of changes in the form of artefacts over time to establish relative dates; and occasional cross-referencing of artefacts with historically recorded dates. Archaeologists working on Mycenaean sites, for example, developed a detailed knowledge of the changing forms of pottery, and this relative sequence was then anchored by finds of diagnostic material in Egyptian contexts dated by inscriptions. Otherwise, absolute dating was little more than guesswork.
It was widely assumed that the great megalithic monuments of Western Europe were imitations of Egyptian and Near Eastern monuments – with ideas and influences ‘diffusing’ outwards from the ancient civilisations. But radiocarbon dates showed that many of the tombs, henges, and temples of prehistoric Europe were earlier than the pyramids and ziggurats. The diffusionist argument collapsed, and archaeologists started focusing on specific societies and internal processes of evolution. A science-driven dating revolution led directly to a revolution in how we think about social change.
This article is an extract from the full article published in World Archaeology Issue 33. Click here to subscribe