This apparent age of oceanic water is caused both by the delay in exchange rates between atmospheric CO2 and ocean bicarbonate, and the dilution effect caused by the mixing of surface waters with upwelled deep waters which are very old (Mangerud 1972).
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.
In such a case, it is very difficult to ascertain the precise reservoir difference and hence apply a correction to the measured radiocarbon age.
Spurious radiocarbon dates caused by volcanic emanations of radiocarbon-depleted CO2 probably also come under the category of reservoir corrections.
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.
Radiocarbon discrepancies due to volcanic CO2 emissions are a popular source of ammunition for fundamentalist viewpoints keen to present evidence to show that the radiocarbon method is somehow fundamentally flawed.
Since about 1890, the use of industrial and fossil fuels has resulted in large amounts of CO2 being emitted into the atmosphere.
A shellfish alive today in a lake within a limestone catchment, for instance, will yield a radiocarbon date which is excessively old.
The reason for this anomaly is that the limestone, which is weathered and dissolved into bicarbonate, has no radioactive carbon.
The presence of bomb carbon in the earth's biosphere has enabled it to be used as a tracer to investigate the mechanics of carbon mixing and exchange processes.