Isotope of carbon dating
For example, a sample with a true radiocarbon age of 100,000 radiocarbon years will yield a measured radiocarbon age of about 20,000 radiocarbon years if the sample is contaminated with a weight of modern carbon of just 5% of the weight of the sample's carbon.It is not too difficult to supply contaminating radiocarbon since it is present in relatively high concentrations in the air and in the tissues of all living things including any individuals handling the sample.First, any instrument which is built to measure radiocarbon has a limit beyond which it cannot separate the signal due to radiocarbon in the sample from the signal due to background processes within the measuring apparatus.
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).
Other radiometric dating methods such as potassium-argon or rubidium-strontium are used for such purposes by those who believe that the earth is billions of years old.
Radiocarbon is not suitable for this purpose because it is only applicable: a) on a time scale of thousands of years and b) to remains of once-living organisms (with minor exceptions, from which rocks are excluded).
This gives the clam shell an artificially old radiocarbon age.