Nitrogen dating archaeology
In the context of carbon-14 dating, this departure from the present-day level means that samples with a true age of 8,200 years would be dated by radiocarbon as 7,500 years old.
The problems stemming from temporal variations can be overcome to a large degree by the use of calibration curves in which the carbon-14 content of the sample being dated is plotted against that of objects of known age.
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?
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.
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.
Studies have revealed that the atmospheric radiocarbon level prior to 1000 it was about 8 percent above what it is today.
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.
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.