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18-Dec-2017 19:01

In the early days of radiocarbon analysis this limit was often around 20,000 radiocarbon years.Thus, all the researcher was able to say about samples with low levels of radiocarbon was that their age was greater than or equal to 20,000 radiocarbon years (or whatever the sensitivity limit of his apparatus was).It is not correct to state or imply from this evidence that the radiocarbon dating technique is thus shown to be generally invalid.The problem with freshwater clams arises because these organisms derive the carbon atoms which they use to build their shells from the water in their environment.By radiocarbon dating a piece of wood which has been dated by counting the annual growth rings of trees back to when that piece of wood grew, a calibration table can be constructed to convert radiocarbon years to true calendar years.Of course, the table, so constructed, will only give the correct calibration if the tree-ring chronology which was used to construct it had placed each ring in the true calendar year in which it grew.This gives the clam shell an artificially old radiocarbon age.

Some organic materials do give radiocarbon ages in excess of 50,000 "radiocarbon years." However, it is important to distinguish between "radiocarbon years" and calendar years.Long tree-ring chronologies are rare (there are only two that I am aware of which are of sufficient length to be of interest to radiocarbon) and difficult to construct.