Radiocarbon dating
Radiocarbon dating (also referred to as carbon dating or carbon-14 dating) is a method for determining the age of an object containing organic material by using the properties of radiocarbon, a radioactive isotope of carbon.
The method was developed
in the late 1940s at the University of Chicago by Willard Libby. It is based on the fact that
radiocarbon (14C) is constantly being
created in the Earth's atmosphere by the interaction of cosmic rays with atmospheric nitrogen. The resulting 14C combines with
atmospheric oxygen to form radioactive carbon dioxide, which is incorporated into
plants by photosynthesis;
animals then acquire 14C by eating the
plants. When the animal or plant dies, it stops exchanging carbon with its
environment, and thereafter the amount of 14C it contains begins
to decrease as the 14C undergoes radioactive decay. Measuring the amount
of 14C in a sample from a
dead plant or animal, such as a piece of wood or a fragment of bone, provides
information that can be used to calculate when the animal or plant died. The
older a sample is, the less 14C there is to be
detected, and because the half-life of 14C (the period of
time after which half of a given sample will have decayed) is about 5,730
years, the oldest dates that can be reliably measured by this process date to
approximately 50,000 years ago, although special preparation methods
occasionally make accurate analysis of older samples possible. Libby received
the Nobel Prize in
Chemistry for his work in 1960.
Research has been ongoing
since the 1960s to determine what the proportion of 14C in the atmosphere
has been over the past fifty thousand years. The resulting data, in the form of
a calibration curve, is now used to convert a given measurement of radiocarbon
in a sample into an estimate of the sample's calendar age. Other corrections
must be made to account for the proportion of 14C in different types
of organisms (fractionation), and the varying levels of 14C throughout
the biosphere (reservoir effects). Additional
complications come from the burning of fossil fuels such as coal and oil, and
from the above-ground nuclear tests done in the 1950s and 1960s. Because the
time it takes to convert biological materials to fossil fuels is substantially longer than
the time it takes for its 14C to decay below
detectable levels, fossil fuels contain almost no 14C. As a result, beginning
in the late 19th century, there was a noticeable drop in the proportion
of 14C as the carbon
dioxide generated from burning fossil fuels began to accumulate in the
atmosphere. Conversely, nuclear testing increased
the amount of 14C in the atmosphere, which reached a
maximum in about 1965 of almost double the amount present in the atmosphere
prior to nuclear testing.
Measurement of
radiocarbon was originally done by beta-counting devices, which counted the
amount of beta radiation emitted
by decaying 14C atoms in a sample. More
recently, accelerator
mass spectrometry has become the method of choice; it counts
all the 14C atoms in the
sample and not just the few that happen to decay during the measurements; it
can therefore be used with much smaller samples (as small as individual plant
seeds), and gives results much more quickly. The development of radiocarbon
dating has had a profound impact on archaeology. In addition to permitting more
accurate dating within archaeological sites than previous methods, it allows
comparison of dates of events across great distances. Histories of archaeology
often refer to its impact as the "radiocarbon revolution".
Radiocarbon dating has allowed key transitions in prehistory to be dated, such
as the end of the last ice age, and
the beginning of the Neolithic and Bronze Age in different regions.
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