Radiocarbon datingcarbon exchange reservoir

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.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.It relies instead on the progressive decay or disappearance of the radioactive parent with time.The discovery of natural 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 short, all parts of the carbon cycle were seen to be invaded by the isotope carbon-14.

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.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 clear that carbon-14 dates lack the accuracy that traditional historians would like to have.


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