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C14 was originally calibrated using Egyptian artifacts of "known" age on the "standard" chronology. (1991) Radiocarbon Dating: Recent Applications and Future Potential, Quaternary Proceedings, Number 1, 1991, Wiley Even though this is not my field of study, I happen to have several of these in my files already.
However, the "plateau" certainly does not equate to the Flood, for that would put the Flood in the middle of Egyptian history, the archaeological evidence of which is sitting on top of kilometers of Flood-deposited sediments. Third, some creationists like the members of the RATE group theorize there was a pulse of accelerated radioactive decay around the time of the Flood, but this would not apply to the post-Flood era.Fourth, while it is true that we cannot know the past (this is the great limitation of experimental science), it is sometimes convenient to use the opposition's numbers against them.A variable rate would only make the case worse for secular archaeology.Second, while we have discovered in recent years that certain radiometric decay rates do vary, the measured effect is slight, so far.