Cosmogenic dating methods
So low is such a carbon-14 level that no one had detected natural carbon-14 until Libby, guided by his own predictions, set out specifically to measure it.
His success initiated a series of measurements designed to answer two questions: Is the concentration of carbon-14 uniform throughout the plant and animal kingdoms?
With correction for radioactive decay during the intervening years, such old samples hopefully would show the same starting carbon-14 level as exists today. His conclusion was that over the past 5,000 years the carbon-14 level in living materials has remained constant within the 5 percent precision of measurement.
A dating method was thus available, subject only to confirmation by actual application to specific chronologic problems.
It is now clear that carbon-14 is not homogeneously distributed among today’s plants and animals.
Willard Libby of the United States began with his recognition that a process that had produced radiocarbon in the laboratory was also going on in Earth’s upper atmosphere—namely, the bombardment of nitrogen by free neutrons.
In this way, the deviations can be compensated for and the carbon-14 age of the sample converted to a much more precise date.
Calibration curves have been constructed using dendrochronological data (tree-ring measurements of bristlecone pines as old as 8,200 years); periglacial varve, or annual lake sediment, data (); and, in archaeological research, certain materials of historically established ages.
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.