Isotopic dating is based on
Because the source of the industrial fuels has been predominantly material of infinite geological age ( e.g coal, petroleum), whose radiocarbon content is nil, the radiocarbon activity of the atmosphere has been lowered in the early part of the 20th century up until the 1950's.
The average difference between a radiocarbon date of a terrestrial sample such as a tree, and a shell from the marine environment is about 400 radiocarbon years (see Stuiver and Braziunas, 1993).
[A Conventional Radiocarbon Age or CRA, does not take into account specific differences between the activity of different carbon reservoirs.
A CRA is derived using an age calculation based upon the decay corrected activity of the absolute radiocarbon standard (1890 AD wood) which is in equilibrium with atmospheric radiocarbon levels (as mentioned previously, 1890 wood is no longer used as the primary radiocarbon standard, instead Oxalic Acid standards I and II were correlated with the activity of the original standard).
In order to ascertain the ages of samples which were formed in equilibrium with different reservoirs to these materials, it is necessary to provide an age correction.
Implicit in the Conventional Radiocarbon Age BP is the fact that it is not adjusted for this correction.
The effect has been suggested as providing dates in error for the eruption of Thera which has been linked to the demise of the Minoan civilisation in the Aegean.