The source of the 2,300-year-old radiocarbon date (Keith and Anderson 1963, discussed by Strahler 1987, 156-157), has been abused and misused to discredit radiocarbon dating. The article discussed the potential errors that the presence of "dead carbon" would introduce into the dating of mollusks. For example, carbon dioxide in the water can partially come from Paleozoic limestone, which lacks carbon-14. As a result, the carbon dioxide in the water is deficient in carbon-14 relative to the atmosphere, and mollusks living in the water build shells that give apparent dates older than they really are. This is a type of "reservoir effect."
The 27,000 year old date comes from Riggs (1984, 224), who wrote:
Carbon-14 contents as low as 3.3 +/- 0.2 percent modern (apparent age, 27,000 years) measured from the shells of snails Melanoides tuberculatus living in artesian springs in southern Nevada are attributed to fixation of dissolved HCO3- with which the shells are in carbon isotope equilibrium.
In other words, the apparent age of 27,000 years for these snail shells is another example of the reservoir effect. The springs, from which the snails came, were fed by carbonate aquifers. As this water percolated through the enclosing carbonates, it dissolved limestone and dolomite hundreds of millions of years old. The dissolution of limestone and dolomite introduced considerable quantities of "dead carbon" into the groundwater. As a result, the groundwater which fed the spring and in which the snails lived was significantly deficient in carbon-14 relative to what is found in the atmosphere. When the snails made their shells, they incorporated an excess amount of "dead carbon," relative to modern atmosphere, into their shells, which resulted in the excessively old apparent date.
Contrary to the complaints of creationists, conventional scientists are well aware of this problem. They test for it and take it into account when interpreting radiocarbon data. In cases where corrections for presence of dead carbon cannot be made, such dates are readily recognized as erroneous and can be safely disregarded. This is not the fatal flaw to radiometric dating that some creationists claim it to be. It just shows that dates from mollusks from streams and lakes need to be carefully evaluated as to their reliability. Other materials, such as wood, charcoal, bone, and hide, would remain unaffected by this type of reservoir effect. If found with shells in the same layer, these materials could be dated to determine if shells are locally affected by the reservoir effect and, if so, how much their radiocarbon dates have been skewed by it.
(See also the C14-dating of a seal for another example of the reservoir effect.)
Tópico: Datação radiocarbono e resultados fictícios... (de 1963)! (Lida 1214 vezes)