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 by
Willard Libby, who received the
Nobel Prize in Chemistry for his work in 1960. It is based on the fact that radiocarbon (14
C) is constantly being created in the atmosphere by the interaction of
cosmic rays with atmospheric
nitrogen. The resulting 14
C combines with atmospheric
oxygen to form radioactive
carbon dioxide, which is incorporated into plants by
photosynthesis; animals then acquire 14
C by eating the plants. When the animal or plant dies, it stops exchanging carbon with its environment, and from that point onwards the amount of 14
C it contains begins to decrease as the 14
Cundergoes
radioactive decay. Measuring the amount of 14
C 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 14
C there is to be detected, and because the
half-life of 14
C (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 around 50,000 years ago, although special preparation methods occasionally permit accurate analysis of older samples.
Research has been ongoing since the 1960s to determine what the proportion of 14
C 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 14
C in different types of organisms (fractionation), and the varying levels of 14
Cthroughout 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 14
C to decay below detectable levels, fossil fuels contain almost no 14
C, and as a result there was a noticeable drop in the proportion of 14
C in the atmosphere beginning in the late 19th century. Conversely, nuclear testing increased the amount of 14
C in the atmosphere, which attained a maximum in about 1965 of almost twice what it had been before the testing began.
Measurement of radiocarbon was originally done by beta-counting devices, which counted the amount of
beta radiation emitted by decaying 14
C atoms in a sample. More recently,
accelerator mass spectrometry has become the method of choice; it counts all the 14
C 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.
https://en.wikipedia.org/wiki/Radiocarbon_dating
