Nitrogen isotope dating
We can also compare radiocarbon dates with dates known on other grounds.
For example, we have discussed the use of varves for dating; now since varves incorporate organic material as they are formed, we can check that when we radiocarbon date a varve, we get the same date for it as we obtain by counting the varves.
This allows archaeologists to estimate the magnitude of this effect and correct for it.
Another source of carbon we have to take into account is the weathering of limestone.
Isotopes are different forms of an element that share the same chemical properties, but that differ in mass and the number of neutrons they contain.
Common elements that possess isotopes include carbon, oxygen, hydrogen, and nitrogen.
It is generally agreed that the dendrochronological dates should be considered the more accurate.
Since humans eat seafood, this can also affect the carbon dating of humans, and what is worse it does so in an inconsistent manner, since human consumption of seafood varies with location and culture.
However, the marine component of diet can be estimated by measuring the ratio of the stable isotopes C in the sample: this will be higher the more seafood the individual consumed.
One of the nice things about this method is that we don't have to worry about carbon being lost from the sample.
Because we are measuring the abundance of two isotopes of carbon, and because isotopes of the same element will be chemically identical, no ordinary process can preferentially remove C is going to be small enough to begin with, being only 0.0000000001% of atmospheric carbon, and then as the decay process progresses it's going to get smaller and smaller.