The effect of this has been to almost double the amount of C14 activity in terrestrial carbon bearing materials (Taylor, 1987).De Vries (1958) was the first person to identify this 'Atom Bomb' effect.In the northern hemisphere the amount of artificial carbon in the atmosphere reached a peak in 1963 (in the southern hemisphere around 1965) at about 100% above normal levels.
This C14 is 'artificial' or 'bomb' C14, produced because nuclear bombs produce a huge thermal neutron flux.
[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).
Thus, it dilutes the activity of the lake meaning that the radioactivity is depleted in comparison to 14C activity elsewhere.
The lake, in this case, has a different radiocarbon reservoir than that of the majority of the radiocarbon in the biosphere and therefore an accurate radiocarbon age requires that a correction be made to account for it.