Nuclear tests almost doubled the radiocarbon in the air we breathe.
Radiocarbon dating works because every living thing absorbs a trace of radioactive carbon, then stops when it dies. The clock needs careful correcting, though. Burning coal diluted that carbon, and 1950s and 60s bomb tests nearly doubled it. Scientists now check the clock against thousands of years of tree rings.
Cosmic rays striking nitrogen high in the atmosphere constantly create carbon-14, a radioactive form of carbon. It ends up in carbon dioxide, plants take it in through photosynthesis, and animals eat the plants. When an organism dies it stops taking in new carbon, and its carbon-14 decays with a half-life of about 5,730 years. Measure how much is left in a bone or a piece of wood and you can estimate how long ago it died, back to about 50,000 years.
Willard Libby developed the method at the University of Chicago in the late 1940s. To test it, he dated samples from the tombs of the Egyptian kings Djoser and Sneferu, whose ages were known, and got a close match. Within about a decade, more than 20 radiocarbon labs had opened worldwide, and Libby received the 1960 Nobel Prize in Chemistry. Archaeologists speak of a 'radiocarbon revolution'.
But Libby assumed the amount of carbon-14 in the air had always been steady, and it has not. Tree rings exposed the problem and supplied the fix: each ring locks in the air's carbon balance for the year it grew, so a continuous sequence of rings, now stretching back almost 14,000 years, acts as a calibration ruler. Hans Suess published the first calibration curve in 1967. Radiocarbon results today are converted to calendar dates using curves such as IntCal20.
Humans have also disturbed the clock. Coal and oil are so old that their carbon-14 has gone, so burning them dilutes the atmosphere's share and makes recent samples look older, the Suess effect. Above-ground nuclear testing did the opposite, almost doubling atmospheric carbon-14 by the mid-1960s. That 'bomb pulse' is slowly fading. Handled carefully, the method still solves famous questions: in 1988 three labs dated the Shroud of Turin's linen to the 14th century.
Source: Wikipedia — Radiocarbon dating · Text summarised from Wikipedia (CC BY-SA 4.0)
