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Fortunately, neither effect has been significant in the case of older samples submitted for carbon-14 dating.The ultimate cause of carbon-14 variations with time is generally attributed to temporal fluctuations in the cosmic rays that bombard the upper atmosphere and create terrestrial carbon-14.
Invasion is probably not the proper word for a component that Libby calculated should be present only to the extent of about one atom in a trillion stable carbon atoms.
And, if so, has today’s uniform level prevailed throughout the recent past?
After showing the essential uniformity of carbon-14 in living material, Libby sought to answer the second question by measuring the radiocarbon level in organic samples dated historically—materials as old as 5,000 years from sources such as Egyptian tombs.
Until then, the inherent error from this uncertainty must be recognized.
A final problem of importance in carbon-14 dating is the matter of sample contamination.
Of more recent date was the overcompensating effect of man-made carbon-14 injected into the atmosphere during nuclear bomb testing.
The result was a rise in the atmospheric carbon-14 level by more than 50 percent.
Specifically, volcanic carbon dioxide is known to depress the carbon-14 level of nearby vegetation, and dissolved limestone carbonate occasionally has a similar effect on freshwater mollusks, as does upwelling of deep ocean water on marine mollusks.
In every case, the living material affected gives the appearance of built-in age.
If a sample of buried wood is impregnated with modern rootlets or a piece of porous bone has recent calcium carbonate precipitated in its pores, failure to remove the contamination will result in a carbon-14 age between that of the sample and that of its contaminant.
Consequently, numerous techniques for contaminant removal have been developed.
It is now clear that carbon-14 is not homogeneously distributed among today’s plants and animals.