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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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