This is how carbon dating works: Carbon is a naturally abundant element found in the atmosphere, in the earth, in the oceans, and in every living creature.
C-12 is by far the most common isotope, while only about one in a trillion carbon atoms is C-14.
Beyond that timespan, the amount of the original C formed by irradiation of nitrogen by neutrons from the spontaneous fission of uranium, present in trace quantities almost everywhere.
For these samples, other dating methods must be used.
The new isotope is called "radiocarbon" because it is radioactive, though it is not dangerous.In contrast, from 1955 to 1963, atmospheric radiocarbon levels almost doubled.Since then they have been dropping back toward natural levels.And if it isn't constant, how do you calibrate your measurement so you can actually figure out how much carbon-14 there is relative to living plants and animals at that time?And the way that you can make that calibration, because it turns out it isn't perfectly constant, the way that you can make that calibration, there's two ways, and I have pictures here of both of them, one is to look at tree things. And I'm told this will work up to about 10,000 years. I don't know of any 10,000 year old trees, I don't think anyone does, but maybe there's some remains of old trees.