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Radiocarbon dating measurements produce ages in "radiocarbon years", which must be converted to calendar ages by a process called calibration. Calibration is needed because the atmospheric / ratio, which is a key element in calculating radiocarbon ages, has not been constant historically.〔Taylor (1987), p. 133.〕 Although Willard Libby, the inventor of radiocarbon dating, had pointed out as early as 1955 the possibility that the / ratio might have varied over time, it was not until discrepancies began to accumulate between measured ages and known historical dates for artefacts that it became clear that a correction would need to be applied to radiocarbon ages to obtain calendar dates.〔 Radiocarbon years ago may be abbreviated ya (years ago). A general term used reflecting evidence from any method is Before Present (BP). ==Construction of a curve== To produce a curve that can be used to relate calendar years to radiocarbon years, a sequence of securely dated samples is needed which can be tested to determine their radiocarbon age. The study of tree rings led to the first such sequence: tree rings from individual pieces of wood show characteristic sequences of rings that vary in thickness because of environmental factors such as the amount of rainfall in a given year. These factors affect all trees in an area, so examining tree-ring sequences from old wood allows the identification of overlapping sequences. In this way, an uninterrupted sequence of tree rings can be extended far into the past. The first such published sequence, based on bristlecone pine tree rings, was created in the 1960s by Wesley Ferguson.〔Taylor (1987), pp. 19–21.〕 Hans Suess used this data to publish the first calibration curve for radiocarbon dating in 1967.〔Aitken (1990), p. 66–67.〕〔Bowman (1995), pp. 16–20.〕〔Suess (1970), p. 303.〕 The curve showed two types of variation from the straight line: a long term fluctuation with a period of about 9,000 years, and a shorter term variation, often referred to as "wiggles", with a period of decades. Suess said he drew the line showing the wiggles by "cosmic ''schwung''" – freehand, in other words. It was unclear for some time whether the wiggles were real or not, but they are now well-established.〔〔 The calibration method also assumes that the temporal variation in level is global, such that a small number of samples from a specific year are sufficient for calibration. This was experimentally verified in the 1980s.〔 Over the next thirty years many calibration curves were published using a variety of methods and statistical approaches.〔Bowman (1995), pp. 43–49.〕 These were superseded by the INTCAL series of curves, beginning with INTCAL98, published in 1998, and updated in 2004, 2009, and, most recently, 2013. The improvements to these curves are based on new data gathered from tree rings, varves, coral, and other studies. Significant additions to the datasets used for INTCAL13 include non-varved marine foraminifera data, and U-Th dated speleothems. The INTCAL13 data includes separate curves for the northern and southern hemispheres, as they differ systematically because of the hemisphere effect; there is also a separate marine calibration curve. Once testing has produced a sample age in radiocarbon years, with an associated error range of plus or minus one standard deviation (usually written as ±σ), the calibration curve can be used to derive a range of calendar ages for the sample. The calibration curve itself has an associated error term, which can be seen on the graph labelled "Calibration error and measurement error". This graph shows INTCAL13 data for the calendar years 3100 BP to 3500 BP. The solid line is the INTCAL13 calibration curve, and the dotted lines show the standard error range—as with the sample error, this is one standard deviation. Simply reading off the range of radiocarbon years against the dotted lines, as is shown for sample t2, in red, gives too large a range of calendar years. The error term should be the root of the sum of the squares of the two errors:〔Aitken (1990), p. 101.〕 : Example t1, in green on the graph, shows this procedure—the resulting error term, σtotal, is used for the range, and this range is used to read the result directly from the graph itself, without reference to the lines showing the calibration error.〔 Variations in the calibration curve can lead to very different resulting calendar year ranges for samples with different radiocarbon ages. The graph to the right shows the part of the INTCAL13 calibration curve from 1000 BP to 1400 BP, a range in which there are significant departures from a linear relationship between radiocarbon age and calendar age. In places where the calibration curve is steep, and does not change direction, as in example t1 in blue on the graph to the right, the resulting calendar year range is quite narrow. Where the curve varies significantly both up and down, a single radiocarbon date range may produce two or more separate calendar year ranges. Example t2, in red on the graph, shows this situation: a radiocarbon age range of about 1260 BP to 1280 BP converts to three separate ranges between about 1190 BP and 1260 BP. A third possibility is that the curve is flat for some range of calendar dates; in this case, illustrated by t3, in green on the graph, a range of about 30 radiocarbon years, from 1180 BP to 1210 BP, results in a calendar year range of about a century, from 1080 BP to 1180 BP.〔 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「Calibration of radiocarbon dates」の詳細全文を読む スポンサード リンク
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