Radiocarbon dating




A radiometric method for measuring the decay of the radioactive isotope carbon- 14 in organic material up to 80 000 years old, developed in 1948-9 by Willard Libby. Living animals and plants take in carbon, which contains some radioactive carbon-14. When the organism dies, it stops taking in carbon, and as the carbon-14 decays, its proportion to the total amount of carbon decreases in away which is directly related to the time elapsed since death.

Using samples principally from wood and charcoal, the technique revolutionized archaeological dating across the world. Recent refinements allow reliable determinations of date from no more than a few fibres of cloth or a grain of wheat. Radiocarbon dating was the technique by which in 1988 the Turin Shroud was shown to date from the 14th-c.


Radiocarbon Dating and Archaeology
Radiocarbon dating has been the most significant development in twentieth-century archaeology, its revolutionary impact far more profound that of any discovery or ideological innovation.

In providing an absolute age for organic materials like wood, charcoal and bone often found sealed in archaeological deposits, it released archaeologists from having to spend so much time organizing and dating their material. New ideas could now be pursued and more important questions asked.

The radiocarbon technique was announced in 1949 by Willard Libby, a university of Chicago scientist interested in cosmic radiation and its effects on the earth's environment; archaeologists quickly realized its potential and Libby won a Nobel prize.

As a direct dating method, it can be used on any organic object and, theoretically at least, can provide dates up to 80,000 years old. carbon 14 (14c) is a radioactive isotope, produced in the atmosphere, that is absorbed by plants during photosynthesis and passed on to the animals that feed on those plants, or indeed on other animals.

All living things contain 14C. Because the isotope is unstable, it decays at a known rate, with a half-life of 5,730 years (in other words, half of the isotope is lost over that period). In living organisms, any 14C lost is replaced and the amount remains constant; but at death on more 14C is absorbed and the amount declines. As the rate of loss is known, Libby realized that he could measure how long ago an organism died by determining how much 14C was left.

Radiocarbon dating was an earth-shaking innovation for archaeology, but its impact from the outset has been much greater in the United States and western Europe than further east.
Since the technique was a product of atomic bomb research, archaeologists in eastern Europe have always been particularly poorly served: although the first radiocarbon laboratory behind the Iron curtain was inaugurated in Leningrad in 1955, it was not until 1961 that the East Berlin laboratory systematically began to date large numbers of samples. If access to western laboratories was wanted, hard currency or connections were required - both thin on the ground during the cold War.

More important still, eastern Europe was a region that had made a tremendous intellectual investment in artifact typology as a means of dating. For every major synthesis published in the west by a Schuchhardt or a Childe, there were hundreds of fine-tunings of regional pottery sequences and other diagnostic artifacts. Absolute dating therefore tended to be viewed as ancillary, even irrelevant, to the practice of archaeology.

Predictably therefore, the most sustained resistance to radiocarbon came from central and eastern Europe, most famously from the Heidelberg prehistorian Vladimir Milojcic. His monumental and meticulously documented book, The chronology of the Later Stone Age in Central and Southeastern Europe (1949), rested on the then widely-accepted premise that the great late neolithic tell at Vinca, near Belgrade, excavated in 1908-12, was an outpost of Aegean early Bronze Age civilization - a harbinger of the copper and bronze metallurgy soon to sweep Europe.

This belief lent support to Montelius' - short chronology' for later European prehistory; through Vinca, Milojcic argued, innumerable central European cultures could be tied to an Aegean sequence underpinned by sound, historically documented dates.

However, it soon became clear that something was adrift. Some of the first radiocarbon dates in the 1950s came from Dutch early neolithic sites, which if the - short chronology were correct should have been dated C. 3,000 BC.

In fact, they emerged over a thousand years older - a dating soon achieved so consistently that it became clear that either the radiocarbon technique itself was flawed
or the 'short chronology' was too short. Western Europe put its faith in radiocarbon and began to re-examine its archaeological data and chronologies.

Further east there was consternation: some, like Milojcic, rejected the radiocarbon method, pitching in with some well-aimed thrusts at its early shortcomings; others simply chose to ignore it.

It was not until the 1960s, especially when the East Berlin laboratory began to date numerous samples from the Balkans and eastern Europe, that support for the 'short chronology' faded away. Vinca was shown to predate the Aegean early Bronze Age - specifically early Troy - by at least a millennium, its early bronze metallurgy evidently developed locally rather than borrowed from the eastern Mediterranean.

Milojcic's scepticism long endured but most other archaeologists working in central and eastern Europe were quickly converted by the weight of the radiocarbon evidence to accept the new, - 'longer' chronology. By 1970, the validity of the method was established to the satisfaction of almost all.

Libby, Willard (Frank)
Life: 1908-80

US chemist: developed radiocarbon dating technique.

Libby taught at the University of California at Berkeley until 1941, when he joined the
Manhattan Project developing the atom bomb. After the war he moved to the Institute of Nuclear Studies at the University of Chicago, returning to California in 1959.
In 1939 Serge Korff (1906- ) discovered carbon- 14, a radioactive isotope of carbon with a half-life of 5730 years, and showed that it is produced in the upper Atmosphere by the action of cosmic rays on nitrogen atoms. In 1947 Libby and his colleagues used this discovery to develop their radiocarbon dating technique, which has proved to be invaluable in archaeology and Quaternary geology.
The technique is based on the fact that living biological material contains carbon- 14 and carbon- 12 in equilibrium with the Atmosphere (which contains a very small but approximately constant proportion of carbon- 14 to carbon- 12). However, when the organism dies it stops taking up carbon dioxide from the Atmosphere and so the proportion of carbon- 14 to carbon- 12 starts to diminish as the carbon- 14 undergoes radioactive decay.
By measuring the proportion of carbon- 14 to carbon- 12, therefore, the time since death may be determined. The technique is applicable with reasonable accuracy in dating organic objects up to about 40000 years old, but greater accuracy can be achieved by calibrating the technique with objects of known age, and this has been done back to about 5000 years ago. This calibration is desirable because the rate of production of carbon- 14 in the Atmosphere varies slightly with time. Libby was awarded the 1960 Nobel Prize for chemistry for his work.


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