There is, in fact, no indication anywhere in the original reference that these samples were from the "Pennyslvanian"; nor is there any hint that they were expected to be "300 million years old"; these appear to be purely apocryphal embellishments to the original account.Surely, what the Russians intended to convey (and what nearly everybody would understand), is that these samples were charcoal from a not too ancient campfire.(Aardsma, 1989, page 2.) Other such stories have been circulated, but these examples make clear the nature of such stories.The truth is that radiocarbon dating is a very effective and valuable tool in the hands of competent chronologists.In the following article, some of the most common misunderstandings regarding radiocarbon dating are addressed, and corrective, up-to-date scientific creationist thought is provided where appropriate. Radiocarbon is used to date the age of rocks, which enables scientists to date the age of the earth.Radiocarbon is not used to date the age of rocks or to determine the age of the earth.Since no reliable historically dated artifacts exist which are older than 5,000 years, it has not been possible to determine the relationship of radiocarbon years to calendar years for objects which yield dates of tens of thousands of radiocarbon years.
This problem, known as the "reservoir effect", 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 the air, not the water.
This Radiocarbon reference must originally have been translated from Russian and it is not unreasonable to suppose that there was some loss of descriptive clarity as a result.
But it seems pretty clear that what is being described here is certainly not "Pennsylvanian coal".
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.