Measurement of the time of formation of materials that contain carbon, such as bone, charcoal and plants, can be undertaken through the use of the natural change in the abundance of the radioactive isotope 14C (or Carbon-14; half-life ~5700 yrs) as it decays to 14N (or Nitrogen-14). Some of the Carbon-14 manufactured in the upper atmosphere by interaction of the carbon in CO2 with cosmic rays is ingested through food by all living tissue during its lifetime until that organism dies, whereby food uptake ceases and the ingested Carbon-14 continues to decay.
This form of dating is usually restricted to ages less than 100,000 years. For ages greater than this other radioactive dating systems must be used.
Atmospheric production of Carbon-14 is the result of an equilibrium between cosmogenic production and radioactive decay of Carbon-14 occurring in the atmosphere. During their lifetimes, living organisms exchange C with the atmosphere and remain in radioactive equilibrium with it. On death, this exchange ceases.
One complication of this technique is that the Carbon-14 production in the atmosphere by cosmic rays is assumed to have been constant but has probably varied through time, due to sunspot activity.