Cosmogenic dating archaeology
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.
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?
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.
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.
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.In addition to spatial variations of the carbon-14 level, the question of temporal variation has received much study.A 2 to 3 percent depression of the atmospheric radioactive-carbon level since 1900 was noted soon after Libby’s pioneering work, almost certainly the result of the dumping of huge volumes of carbon-14-free carbon dioxide into the air through smokestacks.Since Libby’s foundational studies, tens of thousands of carbon-14 measurements of natural materials have been made.Expressed as a fraction of the contemporary level, they have been mathematically converted to ages through Improvements in measurement accuracy and the ever-mounting experience in applying carbon-14 dating have provided superior and more voluminous data with which to better answer Libby’s original questions.