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The variation in the / ratio in different parts of the carbon exchange reservoir means that a straightforward calculation of the age of a sample based on the amount of it contains will often give an incorrect result. There are several other possible sources of error that need to be considered. The errors are of four general types: * Variations in the / ratio in the atmosphere, both geographically and over time * Isotopic fractionation * Variations in the / ratio in different parts of the reservoir * Contamination == Atmospheric variation == In the early years of using the technique, it was understood that it depended on the atmospheric / ratio having remained the same over the preceding few thousand years. To verify the accuracy of the method, several artefacts that were datable by other techniques were tested; the results of the testing were in reasonable agreement with the true ages of the objects. However, in 1958, Hessel de Vries was able to demonstrate that the / ratio had changed over time by testing wood samples of known ages and showing there was a significant deviation from the expected ratio. This discrepancy, often called the de Vries effect, was resolved by the study of tree rings.〔Bowman (1995), pp. 16–20. 〕〔Suess (1970), p. 303. 〕 Comparison of overlapping series of tree rings allowed the construction of a continuous sequence of tree-ring data that spanned 8,000 years.〔 (Since that time the tree-ring data series has been extended to 13,900 years.)〔 〕 Carbon-dating the wood from the tree rings themselves provided the check needed on the atmospheric / ratio: with a sample of known date, and a measurement of the value of ''N'' (the number of atoms of remaining in the sample), the carbon-dating equation allows the calculation of ''N''0 – the number of atoms of in the sample at the time the tree ring was formed – and hence the / ratio in the atmosphere at that time.〔 Armed with the results of carbon-dating the tree rings, it became possible to construct calibration curves designed to correct the errors caused by the variation over time in the / ratio.〔Bowman (1995), pp. 43–49. 〕 These curves are described in more detail below. There are three main reasons for these variations in the historical / ratio: fluctuations in the rate at which is created, changes caused by glaciation, and changes caused by human activity.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Radiocarbon dating considerations」の詳細全文を読む スポンサード リンク
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