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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, years. Unlike most isotopic dating methods, the conventional carbon dating technique is not based on counting daughter isotopes.
It relies instead on the progressive decay or disappearance of the radioactive parent with time. Newly created carbon atoms were pd to react with atmospheric oxygen to form carbon dioxide CO 2 molecules. Radioactive carbon thus was visualized as gaining entrance wherever atmospheric carbon dioxide enters—into land plants by photosynthesis, into animals that feed on the plants, into marine and fresh waters as a dissolved component, and from there into aquatic plants and animals.
In short, all parts of the carbon cycle were seen to be invaded by the isotope carbon 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 level that no one had detected natural carbon until Libby, guided by his own predictions, set out specifically to measure it. His success initiated a series of measurements deed to answer two questions: Is the concentration of carbon uniform throughout the plant and animal kingdoms?
After showing the essential uniformity of carbon 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, years from sources such as Egyptian tombs. With correction for radioactive decay during the intervening years, such old samples hopefully would show the same starting carbon level as exists today.
His conclusion was that over the past 5, years the carbon 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.
Expressed as a fraction of the contemporary level, they have been mathematically converted to ages through equation 5 above. Archaeology has been the chief beneficiary of radioactive-carbon dating, but late glacial and postglacial chronological studies in geology have also been aided greatly.
The occasional exceptions all involve nonatmospheric contributions of carbondepleted carbon dioxide to organic synthesis. Specifically, volcanic carbon dioxide is known to depress the carbon 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 addition to spatial variations of the carbon level, the question of temporal variation has received much study. Of more recent date was the overcompensating effect of man-made carbon injected into the atmosphere during nuclear bomb testing.
The result was a rise in the atmospheric carbon level by more than 50 percent. Fortunately, neither effect has been ificant in the case of older samples submitted for carbon dating. The ultimate cause of carbon variations with time is generally attributed to temporal fluctuations in the cosmic rays that bombard the upper atmosphere and create terrestrial carbon Whenever the of cosmic rays in the atmosphere is low, the rate of carbon production is correspondingly low, resulting in a decrease of the radioisotope in the carbon-exchange reservoir described above.
Studies have revealed that the atmospheric radiocarbon level prior to bce deviates measurably from the contemporary level. In the year bce it was about 8 percent above what it is today. In the context of carbon dating, this departure from the present-day level means that samples with a true age of 8, years would be dated by radiocarbon as 7, 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 content of the sample being dated is plotted against that of objects of known age.
In this way, the deviations can be compensated for and the carbon 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, years ; periglacial varve, or annual lake sediment, data see above ; and, in archaeological research, certain materials of historically established ages. It is clear that carbon dates lack the accuracy that traditional historians would like to have.
Until then, the inherent error from this uncertainty must be recognized. A final problem of importance in carbon dating is the matter of sample contamination. If a sample of buried wood is impregnated with modern rootlets or a piece of porous bone has recent calcium carbonate precipitated in its pores, failure to remove the contamination will result in a carbon age between that of the sample and that of its contaminant. Consequently, numerous techniques for contaminant removal have been developed. 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, years is relatively rare. Beyond that age, however, the fraction of contaminant needed to have measurable effect is quite small, and, therefore, undetected or unremoved contamination may occasionally be of ificance. A major breakthrough in carbon dating occurred with the introduction of the accelerator mass spectrometer. This instrument is highly sensitive and allows precise ages on as little as 1 milligram 0. The increased sensitivity from the fact that all of the carbon atoms of mass 14 can be counted in a mass spectrometer.
By contrast, if carbon is to be measured by its radioactivity, only those few atoms decaying during the measurement period are recorded. By using the accelerator mass spectrometer, possible interference from nitrogen is avoided, since it does not form negative ion beams, and interfering molecules are destroyed by stripping electrons away by operating at several million volts. The development of the accelerator mass spectrometer has provided new opportunities to explore other rare isotopes produced by the bombardment of Earth and meteorites by high-energy cosmic rays.
Many of these isotopes have short half-lives and hence can be used to date events that happened in the past few thousand to a few million years. In one case, the time of exposure, like the removal of rock by a landslide , can be dated by the presence of the rare beryllium 10 Be isotope formed in the newly exposed surface of a terrestrial object or meteoroidal fragment by cosmic-ray bombardment. Other applications include dating groundwater with chlorine 36 Cl , dating marine sediments with beryllium 11 Be and aluminum 26 Al , and dating glacial ice with krypton 81 Kr.
In general, the application of such techniques is limited by the enormous cost of the equipment required. Learning Guides. Videos Images. Additional Info. Load . Carbon dating and other cosmogenic methods The occurrence of natural radioactive carbon in the atmosphere provides a unique opportunity to date organic materials as old as roughly 60, years. Load Next .Other methods of carbon dating
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