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.
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-14 age between that of the sample and that of its contaminant.
It is clear that carbon-14 dates lack the accuracy that traditional historians would like to have.
There may come a time when all radiocarbon ages rest on firmer knowledge of the sample’s original carbon-14 level than is now available.
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?
Of more recent date was the overcompensating effect of man-made carbon-14 injected into the atmosphere during nuclear bomb testing.
The result was a rise in the atmospheric carbon-14 level by more than 50 percent.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.Since Libby’s foundational studies, tens of thousands of carbon-14 measurements of natural materials have been made.Expressed as a fraction of the contemporary level, they have been mathematically converted to ages through Improvements in measurement accuracy and the ever-mounting experience in applying carbon-14 dating have provided superior and more voluminous data with which to better answer Libby’s original questions.And, if so, has today’s uniform level prevailed throughout the recent past?