Radiocarbon dating

In 1949 the chemist Willard Libby and his colleagues at the University of Chicago announced radiocarbon dating, a way to learn how long ago a plant or animal died by measuring the radioactive carbon-14 left in its remains. They tested the idea on samples of known age, such as wood from Egyptian royal tombs, and the results matched. Libby won the Nobel Prize in Chemistry in 1960. The method works on wood, charcoal, bone, shell and cloth, and reaches back to about 50,000 years.¹²

The mechanism joins several discoveries. Cosmic rays strike the upper atmosphere and free neutrons, and a neutron hitting nitrogen-14 produces carbon-14, an isotope identified by Martin Kamen and Sam Ruben in 1940. It combines with oxygen to make carbon dioxide, which plants absorb and animals eat, so every living thing holds the same small share of it. At death, intake stops and the carbon-14 decays with a half-life of 5,730 years, so the remaining fraction gives the age. The idea rests on the discovery of radioactivity in 1896, and on knowledge of neutrons and cosmic rays.²

Before radiocarbon, archaeologists could order sites by pottery styles and layers of earth but rarely say how many years apart they were. Dates now tied together sites across continents. The method needed calibration, because the amount of carbon-14 in the air has varied, and tree rings and later cave and lake records produced the calibration curves in use today. Accelerator mass spectrometry, from the late 1970s, allows dating tiny samples. It has dated the Dead Sea Scrolls, Otzi the Iceman and Ice Age cave art, and showed in 1988 that the Shroud of Turin is medieval.¹²