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## Radiocarbon dating
Carbon has two stable isotopes: carbon-12 (
*n*+^{14}N →^{14}C +^{1}H
Plants take up atmospheric carbon dioxide by photosynthesis, and are eaten by animals, so every living thing is constantly exchanging
Measurements were originally made by counting the radioactive decay of individual carbon atoms, but this was relatively insensitive and subject to statistical errors: there is never much Radiocarbon labs generally report an uncertainty, e.g., 3000±30BP indicates a standard deviation of 30 radiocarbon years. Traditionally this includes only the statistical counting uncertainty and some labs supply an "error multiplier" that can be multiplied by the uncertainty to account for other sources of error in the measuring process. Additional error is likely to arise from the nature and collection of the sample itself, e.g., a tree may accumulate carbon over a significant period of time and the wood turned into an artifact some time after the death of the tree. It is sometimes stated that burnt material can be reliably dated to the time of burning.
The maximum range of radiocarbon dating appears to be about 50,000 years, after which the amount of
## Calibration
The raw BP date can not be used directly as a calendar date, because the assumption that the level of The BP dates are therefore calibrated to give calendar dates. Standard calibration curves are available, based on comparison of radiocarbon dates with other methods such as examination of tree growth rings (dendrochronology), ice and sediment cores and coral samples. The difference between the Julian calendar and the Gregorian calendar can be ignored, because it's insignificant compared to the measurement uncertainty. The calibration curves can vary significantly from a straight line, so comparison of uncalibrated radiocarbon dates (e.g., plotting them on a graph or subtracting dates to give elapsed time) is likely to give misleading results. There are also significant plateaus in the curves, such as the one at 10000 radiocarbon years BP, which is believed to be associated with changing ocean circulation at the end of the Younger Dryas period. The accuracy of radiocarbon dating is lower for samples originating from such plateau periods. ## Libby vs Cambridge half-lifeCarbon dating was developed by a team led by Willard Libby. Originally a Carbon-14 half-life of 5568±30 years was used, which is now known as the Libby half-life. Later a more accurate figure of 5730±40 years was measured, which is known as the Cambridge half-life. However laboratories continue to use the Libby figure to avoid confusion. An uncalibrated dating using the Libby figure could be improved by multiplying by the ratio of these numbers (approximately 1.03), but this is usually unnecessary since the adjustment is included in modern calibration curves. ## Examples of Carbon Dating and Historical Disputes## External links | |||||||

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