An absolute radiometric dating technique for determining the age of carbon-bearing minerals, including wood and plant remains, charcoal, bone, peat, and calcium carbonate shell back to about 50,000 bp. The technique is based on measuring the loss of radiocarbon (carbon-14) that begins disintegration at death at a known rate. It is one of the best-known chronometric dating techniques and the most important in archaeology presently. It can be used for the dating organic material up to 75,000 years old. It is based on the theory of Willard F. Libby (1947); his radioactive-carbon dating provided an extremely valuable tool for archaeologists, anthropologists, and earth scientists. When organic matter dies it ceases to exchange its carbon, as carbon dioxide, with the atmosphere, so its C14 dwindles by decay and is not replenished. Determination of the radioactivity of carbon from a sample will reveal the proportion of C14 to C12, and this will in turn, through the known rate of decay of C14, give the age of, or more accurately the time elapsed since the death of, the sample. Two things in the method have to be allowed for: first, the 'date' given is never exact. The +/- figure, which should always be quoted, is a statistical one, meaning that there is a 2 to 1 chance that the correct date lies within that bracket. Secondly, the rate of decay of C14 is based in all published examples on a half-life of 5730 +/- 40 years (after 5730 years, one half of the C14 will have disintegrated, after another 5730 years one half of the remainder, and so on). Correction tables are used to correct 'raw' radiocarbon dates (quoted as years ad or BC) into true dates (AD or BC). The method yields reliable dates back to about 50,000 bp and under some conditions to about 75,000 bp. One of the basic assumptions of the technique is that the amount of radiocarbon in the atmosphere has remained constant through time. It has now been established, with the dendrochronological sequence for the bristlecone pine, that the C14 concentration has fluctuated. The reasons for the fluctuation are not yet fully understood. The calibration of radiocarbon dates is therefore necessary in order to achieve an approximate date in calendar years. Dates quoted in radiocarbon years, before calibration, are written BC or bp (before present), as opposed to calibrated dates, written BC or BP. The original half-life for radiocarbon of 5,568 ? 30 years has been revised to 5,730 ? 40 years, though dates are normally published according to the old half-life in order to avoid confusion (the date can be adjusted for the new half-life by multiplying the old date by 1.029). All radiocarbon dates are quoted with a standard deviation. Ideally, a series of dates should be obtained for any deposit as a series may cluster around a central point. New refinements continue to improve the technique's accuracy as well as extend the range of dates which can be achieved. A previous limit of 50,000 years on the age of material which could be dated, set by the limits on the ability of the proportional counter used to record beta particle emissions, has been extended to 70,000 years by the use of isotopic enrichment, the artificial enrichment of the C14 to C12 ratio.