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.
Even a hypothetical sample containing absolutely no radiocarbon will register counts in a radiocarbon counter because of background signals within the counter.
This gives the clam shell an artificially old radiocarbon age.
This problem, known as the "," is not of very great practical importance for radiocarbon dating since most of the artifacts which are useful for radiocarbon dating purposes and are of interest to archaeology derive from terrestrial organisms which ultimately obtain their carbon atoms from air, not the water. Samples of coal have been found with radiocarbon ages of only 20,000 radiocarbon years or less, thus proving the recent origin of fossil fuels, probably in the Flood.
If this water is in contact with significant quantities of limestone, it will contain many carbon atoms from dissolved limestone.
Since limestone contains very little, if any, radiocarbon, clam shells will contain less radiocarbon than would have been the case if they had gotten their carbon atoms from the air.
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.
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.
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.