Additional information needed for carbon dating
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.
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 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.
Of more recent date was the overcompensating effect of man-made carbon-14 injected into the atmosphere during nuclear bomb testing.
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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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 this way, the deviations can be compensated for and the carbon-14 age of the sample converted to a much more precise date.