Cosmogenic dating methods
Newly created carbon-14 atoms were presumed to react with atmospheric oxygen to form ) molecules.
Radioactive carbon thus was visualized as gaining entrance wherever atmospheric carbon dioxide enters—into land plants by photosynthesis, into animals that feed on the plants, into marine and fresh waters as a dissolved component, and from there into aquatic plants and animals.
It is clear that carbon-14 dates lack the accuracy that traditional historians would like to have.
There may come a time when all radiocarbon ages rest on firmer knowledge of the sample’s original carbon-14 level than is now available.
In short, all parts of the carbon cycle were seen to be invaded by the isotope 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.
Studies have revealed that the atmospheric radiocarbon level prior to 1000 it was about 8 percent above what it is today.
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.
It is now clear that carbon-14 is not homogeneously distributed among today’s plants and animals.
The occasional exceptions all involve nonatmospheric contributions of carbon-14-depleted carbon dioxide to organic synthesis.
In this way, the deviations can be compensated for and the carbon-14 age of the sample converted to a much more precise date.