This gives the clam shell an artificially old radiocarbon age.
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.
For this reason special precautions need to be exercised when sampling materials which contain only small amounts of radiocarbon.
Reports of young radiocarbon ages for coal probably all stem from a misunderstanding of one or both of these two factors.
I am not aware of any authentic research which supports this claim.
Also, it does not coincide with what creationist scientists would currently anticipate based upon our understanding of the impact of the Flood on radiocarbon.
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.