Atom used in radiocarbon dating
Since the carbon in these fuels was ancient, it contained no radiocarbon.
Therefore, prior to atmospheric bomb testing, the proportion of radiocarbon to C was relatively low, giving relatively old ages.
The surplus "bomb" radiocarbon is just one of the effects human have had on the ratio of C.
During the industrial revolution (1850 - present) increasing amounts of fossil fuels were combusted.
This can be overcome by calibration curves calculated by dating materials of precisely known age.
Radiocarbon dating is an radioactive isotope dating technique used in dating materials which contain the unstable carbon-14 isotope.Ocean corals, dated by another radiometric method - Uranium-Thorium dating - have also helped to extend the calibration curve beyond the age of the most ancient treering chronologies.