Man Made Causes C-14 concentration can also be affected by human activities:i.
It was almost doubled for a short period due to atomic bomb tests in the 1950s and 1960
In order to circumvent this problem, Accelerator-based mass-spectrometric (AMS) techniques was introduced: All the 14C atoms can be counted directly, rather than only those decaying during the counting interval allotted for each analysis.
The AMS technique allows one to date samples containing only a few milligrams of carbon.2.
In order to resolve this discrepancy within a certain tolerance, calibration charts are often used to mediate the readings.
Various calibrations may include distortions caused by magnetic disturbances or abnormally high C-14 concentrations found in the vicinity of the fossil's environment.4.
In short, it takes 5,700 years to reduce C-14 by 50%, another 5700 years to reduce to 25%, and so on, at an exponential rate.