Guesswork? Atmospheric carbon levels are irrelevant to this. C14 date calibration is about the *ratio* of C14 to C12 and C13, not the absolute quantity. Biol

Guesswork? Atmospheric carbon levels are irrelevant to this. C14 date calibration is about the *ratio* of C14 to C12 and C13, not the absolute quantity. Biological processes to not meaningfully select between the isotopes of carbon, anything that grows while exposed to the atmosphere will contain carbon in the same ratio as the atmosphere--but over time the C14 decays, the ratio shifts. That tells you how long since it grew. (Note the "exposed to the atmosphere" provision--one of the arguments the creationists love is to cite some examples of living organisms that date old. They didn't run the test for giggles, but rather to examine the environment in which the organism was found. Pull something out of a deep cave, date it and you now know how fast the air is exchanged between that location and surface atmosphere. This is because virtually all C14 is created by high energy particles hitting the upper atmosphere.)