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By measuring the ratio of the radio isotope to non-radioactive carbon, the amount of carbon-14 decay can be worked out, thereby giving an age for the specimen in question.
But that assumes that the amount of carbon-14 in the atmosphere was constant — any variation would speed up or slow down the clock.
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).
For instance, creationist Walt Brown has pointed out inconsistencies in some radiocarbon dates of mammoths -- one part was dated to 40,000 years, another to 26,000 years (and wood surrounding it to 10,000 years), and yet another to between 15,000 and 21,000 years before the present epoch [Brown2001].
These and numerous other claimed anomalies in radiocarbon dating are explained in detail in Mark Isaak's book [Isaak2007, pg. 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.As a rule, carbon dates are younger than calendar dates: a bone carbon-dated to 10,000 years is around 11,000 years old, and 20,000 carbon years roughly equates to 24,000 calendar years.The problem, says Bronk Ramsey, is that tree rings provide a direct record that only goes as far back as about 14,000 years.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.