As for the carbon dating of mollusk shells. I have posted on this before, at one of the radiometric dating topics. I pulled the info from my isotope geology text.
For radiocarbon dating to be accurate, the carbon in question must have been in equilibrium with the atmosphere at the time of the life forms death. Or in the above case, while the mollusk is living. But the carbon content of seawater is often not in equilibrium with the atmosphere of the time. Rather, the carbon ratios perhaps reflects an atmosphere contact of the past. Or perhaps the carbon ratios of the sea floor sediment. There are also other potential problems.
Reference: Principles of Isotope Geology, Gunter Faure, 1977, John Wiley & Sons, Inc.
The dating of shells is discussed on pages 315-317.
Professor, geology, Whatsamatta U Evolution - Changes in the environment, caused by the interactions of the components of the environment.
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