The vast majority of carbon atoms, about 98.89%, are C12. And since carbon is an essential element in living organisms, C14 appears in all terrestrial (landbound) living organisms in the same proportions it appears in the atmosphere. Animals and fungi get C14 from the plant or animal tissue they eat for food. The C14 already in the organism doesn't stop decaying, so as time goes on there is less and less C14 left in the organism's remains.If we measure how much C14 there currently is, we can tell how much there was when the organism died, and therefore how much has decayed.The second assumption is that the organism in question got its carbon from the atmosphere.A third is that the thing has remained closed to C14 since the organism from which it was created died.So, if we know how much of the isotope was originally present, and how much there is now, we can easily calculate how long it would take for the missing amount to decay, and therefore how long it's been since that particular sample was formed.That's the essence of radiometric dating: measure the amount that's present, calculate how much is missing, and figure out how long it would take for that quantity of the isotope to break down.The fourth one is that we know what the concentration of atmospheric C14 was when the organism lived and died.
A mathematical formula can be used to calculate the half-life from the number of breakdowns per second in a sample of the isotope.So the dates derived from C14 decay had to be revised.One reference on radiometric dating lists an entire array of corrective factors for the change in atmospheric C14 over time. Radiometric dating methods are the strongest direct evidence that geologists have for the age of the Earth.When I first became interested in the creation-evolution debate, in late 1994, I looked around for sources that clearly and simply explained what radiometric dating is and why young-Earth creationists are driven to discredit it.However, after a few years a number of scientists got suspicious of this assumption, because dates obtained by the C14 method weren't tallying with dates obtained by other means.A long series of studies of C14 content produced an equally long series of corrective factors that must be taken into account when using C14 dating.I found several good sources, but none that seemed both complete enough to stand alone and simple enough for a nongeologist to understand them.Thus this essay, which is my attempt at producing such a source.Because it's a statistical measurement, there's always a margin of error in the age figure, but if the procedure is done properly, the margin is very small. We must know the original quantity of the parent isotope in order to date our sample radiometrically. In order to do so, we need an isotope that's part of a mineral compound. Because there's a basic law of chemistry that says "Chemical processes like those that form minerals can't distinguish between different isotopes of the same element." This is because an element's chemical behavior depends only on the number of electrons it has, which is the same as its number of protons.Obviously, the major question here is "how much of the isotope was originally present in our sample? So to a chemical process, U235 and U238 are identical.