Libby carbon dating

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Carbon has different isotopes, which are usually not radioactive.Plants take up atmospheric carbon dioxide by photosynthesis, and are eaten by animals, so every living thing is constantly exchanging carbon-14 with its environment as long as it lives. In 1958 Hessel de Vries showed that the concentration of carbon-14 in the atmosphere varies with time and locality.In every case, the living material affected gives the appearance of built-in age.In addition to spatial variations of the carbon-14 level, the question of temporal variation has received much study.A dating method was thus available, subject only to confirmation by actual application to specific chronologic problems.Since Libby’s foundational studies, tens of thousands of carbon-14 measurements of natural materials have been made.The result was a rise in the atmospheric carbon-14 level by more than 50 percent.Fortunately, neither effect has been significant in the case of older samples submitted for carbon-14 dating.

Whenever the number of cosmic rays in the atmosphere is low, the rate of carbon-14 production is correspondingly low, resulting in a decrease of the radioisotope in the carbon-exchange reservoir described above.The occasional exceptions all involve nonatmospheric contributions of carbon-14-depleted carbon dioxide to organic synthesis.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.Studies have revealed that the atmospheric radiocarbon level prior to 1000 it was about 8 percent above what it is today.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.

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