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The occurrence of natural radioactive carbon in the atmosphere provides a unique opportunity to date organic materials as old as roughly 60,000 years.Unlike most isotopic dating methods, the conventional carbon-14 dating technique is not based on counting daughter isotopes.Among them are the removal of humic acids from charcoal and the isolation of cellulose from wood and collagen from bone.Today contamination as a source of error in samples younger than 25,000 years is relatively rare.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.
Consequently, numerous techniques for contaminant removal have been developed.
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.
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.
It is now clear that carbon-14 is not homogeneously distributed among today’s plants and animals.