Radiocarbon dating is based on the fact that the interaction of cosmic rays from outer space with nitrogen atoms in the atmosphere produces an unstable isotope of carbon, namely radiocarbon. In each case, radiocarbon dates, determined by well-established procedures and calculations, are compared directly with dates determined by the above methods, thus permitting the radiocarbon dates to be accurately calibrated with distinct and independent dating techniques. When a plant or animal organism dies, however, the exchange of radiocarbon from the atmosphere and the biosphere stops, and the amount of radiocarbon gradually decreases, with a half-life of approximately years. But researchers have known at least since that the carbon level has not been constant, so that the radiocarbon clock needs to be "calibrated. Comparing these counts with a series of radiocarbon-dated samples spanning this record, they obtained a calibration curve that is very close to the calibration shown above [ Callaway ]. This scheme can be used to date items between about years to over , years, and thus can be used to double-check and calibrate radiocarbon dates [ Optical ]. The relative width of the red calibration curve indicates the range of uncertainty: Measurement of N, the number of 14 C atoms currently in the sample, allows the calculation of t, the age of the sample, using the equation above. For instance, creationist Walt Brown has pointed out inconsistencies in some radiocarbon dates of mammoths -- one part was dated to 40, years, another to 26, years and wood surrounding it to 10, years , and yet another to between 15, and 21, years before the present epoch [ Brown ]. Also, at least one of these dates comes from a hide that had been soaked in glycerin, rendering the date invalid. Creationists often criticize radiocarbon dating in the context of discussions of the age of the earth. In other words, those hoping that uncertainties in radiocarbon dating, say in the assumption of constancy of atmospheric carbon levels, will mean that specimens are really much younger than the measured dates, are in for a big disappointment -- it is now clear that specimens are actually somewhat older than the raw, uncalibrated reckonings.
How does radiocarbon dating work? Conclusion In short, while like any other method of scientific investigation, radiocarbon dating is subject to anomalies and misuse, when used correctly in accordance with well-established procedures and calibration schemes, the method is a very reliable means of dating relatively "recent" artifacts. Creationists often criticize radiocarbon dating in the context of discussions of the age of the earth. This is based on the fact that stimulating mineral samples with blue, green or infared light causes a luminescent signal to be emitted, stemming from electron energy that is proportional to the amount of background radiation the specimen has undergone since burial. Measurement of N, the number of 14 C atoms currently in the sample, allows the calculation of t, the age of the sample, using the equation above. Optically stimulated luminescence dating. In each case, radiocarbon dates, determined by well-established procedures and calculations, are compared directly with dates determined by the above methods, thus permitting the radiocarbon dates to be accurately calibrated with distinct and independent dating techniques. The calculations involve several steps and include an intermediate value called the "radiocarbon age", which is the age in "radiocarbon years" of the sample: Additional background is available in a well-written Wikipedia article on the topic [ Radiocarbon ], and in Richard Wiens' article [ Wiens ]. Since it is chemically indistinguishable from the stable isotopes of carbon carbon and carbon , radiocarbon is taken by plants during photosynthesis and then ingested by animals regularly throughout their lifetimes. When a date is quoted, the reader should be aware that if it is an uncalibrated date a term used for dates given in radiocarbon years it may differ substantially from the best estimate of the actual calendar date, both because it uses the wrong value for the half-life of 14 C, and because no correction calibration has been applied for the historical variation of 14 C in the atmosphere over time. These and numerous other claimed anomalies in radiocarbon dating are explained in detail in Mark Isaak's book [ Isaak , pg. But, as is clear even from the very brief discussion in the previous paragraph, radiocarbon dating can say nothing one way or the other about whether the earth is many millions of years old, since such dates are far beyond this method's range of resolution. Measurements of coral or other carbonate structures such as stalagmites, corroborated using uranium-thorium radiometric dating. When a plant or animal organism dies, however, the exchange of radiocarbon from the atmosphere and the biosphere stops, and the amount of radiocarbon gradually decreases, with a half-life of approximately years. However, in the scientific results mentioned by Brown, the dates come from different mammoth specimens. The above calculations make several assumptions, such as that the level of 14 C in the atmosphere has remained constant over time. Compare, for example, the uncorrected line blue dotted line with the calibration curve red curve. Radiocarbon dating cannot be used for older specimens, because so little carbon remains in samples that it cannot be reliably measured. The relative width of the red calibration curve indicates the range of uncertainty: Also, at least one of these dates comes from a hide that had been soaked in glycerin, rendering the date invalid. Creationist criticisms of radiocarbon dating As mentioned above, young-earth creationist writers have cited various anomalies and potential difficulties with radiocarbon dating, and have used these examples to justify their conclusion that the entire scheme is flawed and unreliable. This affects the ratio of 14 C to 12 C in the different reservoirs, and hence the radiocarbon ages of samples that originated in each reservoir. In any event, it must be emphasized once again that radiocarbon dating has no relevance one way or the other for the overall question of whether the earth is many millions of years old, since the scheme can only be used to reliably date specimens less than approximately 50, years old. Here is a graph showing radiocarbon dates on the vertical axis and the calibrated age on the horizontal axis shown here with permission from Johannes van der Plicht, one of the authors of the study. Reliability of radiocarbon dating Radiocarbon dating has been studied at great length over the past few decades, and its strengths and weaknesses are very well understood at this point in time.
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