Ams dating method
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 atmospheric radiocarbon signal has, in effect, been diluted by about 2%.
De Vries (1958) was the first person to identify this 'Atom Bomb' effect.
The logical conclusion from this was that in order to obtain a modern radiocarbon reference standard, representing the radiocarbon activity of the 'present day', one could not very well use wood which grew in the 1900's since it was affected by this industrial effect.
Thus it was that 1890 wood was used as the modern radiocarbon standard, extrapolated for decay to 1950 AD.
The volcanic effect has a limited distance however. (1980) found that at 200 m away from the source, plants yielded an age in agreement with that expected.
They suggested that the influence of depleted CO2 declined rapidly with increasing distance from the source.
Since about 1955, thermonuclear tests have added considerably to the C14 atmospheric reservoir.