For instance, even in the 1950s, when Willard Libby first developed the process, it was recognized that the scheme assumes that the level of carbon-14 in the atmosphere is constant.But researchers have known at least since 1969 that the carbon-14 level has not been constant, so that the radiocarbon clock needs to be "calibrated." As a result, various schemes are used to correct and calibrate radiocarbon dates, including: 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 relative width of the red calibration curve indicates the range of uncertainty: In October 2012, a team led by Christopher Ramsey of Oxford University published a new study, based on analyses of varves (alternating light/dark bands in sediments) from Lake Suigetsu, which is located about 350 kilometers west of Tokyo, near the coast of the Sea of Japan.These researchers collected core samples 70 meters deep, and then painstakingly counted the layers, year by year, to obtain a direct record stretching back 52,000 years.In 2009, several leading researchers in the field established a detailed calibration of radiocarbon dating, based on a careful analysis of pristine corals, ranging back to approximately 50,000 years before the present epoch [Reimer2009].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 2009 study).Comparing these counts with a series of 651 radiocarbon-dated samples spanning this record, they obtained a calibration curve that is very close to the 2009 calibration shown above [Callaway2012]. It should be emphasized that the actual calibrated dates are about 10%-20% older than the raw uncorrected radiocarbon dates that were once used.Compare, for example, the uncorrected line (blue dotted line) with the calibration curve (red curve).
However, in the scientific results mentioned by Brown, the dates come from different mammoth specimens.Because of this relatively short half-life, radiocarbon is useful for dating items of a relatively recent vintage, as far back as roughly 50,000 years before the present epoch.Radiocarbon dating cannot be used for older specimens, because so little carbon-14 remains in samples that it cannot be reliably measured.Since it is chemically indistinguishable from the stable isotopes of carbon (carbon-12 and carbon-13), radiocarbon is taken by plants during photosynthesis and then ingested by animals regularly throughout their lifetimes.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 5730 years.
Comparison of ancient, historically dated artifacts (from Egypt, for example) with their radiocarbon dates has revealed that radiocarbon years and calendar years are not the same even for the last 5,000 calendar years.