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Other processes may affect the local concentrations of carbon-14.These effects are corrected for by comparing samples from different locations.
If these three conditions are met, the initial concentration of C in the sample can be estimated.
Carbon-14 dating cannot be applied to materials that have no C dates are less than that figure.
It is sometimes thought possible to extend the dating range a few half-lives, so one occasionally sees dates as old as 70,000 years or more.
Violations in the atmosphere result from volcanic eruptions that occasionally add C.
Also, there is a lag time for atmospheric mixing in the northern and southern hemispheres which results in somewhat different ages for the two hemispheres.