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.
One modern plant growing near the emanations had an apparent age of 1390 yr.
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.
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).
A reservoir correction must therefore be made to any conventional shell dates to account for this difference.