The age of the carbon in the rock is different from that of the carbon in the air and makes carbon dating data for those organisms inaccurate under the assumptions normally used for carbon dating.
This restriction extends to animals that consume seafood in their diet.
Carbon-14 cannot be used to date biological artifacts of organisms that did not get their carbon dioxide from the air.
This rules out carbon dating for most aquatic organisms, because they often obtain at least some of their carbon from dissolved carbonate rock.
Indeed, the initial amount of the daughter product can be determined using isochron dating.
It takes another 4,460,000,000 years for half of the remaining sample to decay into Pb206 and then another 4,460,000,000 years for half of what’s then left to decay, and so on.
The time it takes for half of a sample to decay is called a “half-life.” By measuring radioactive half-lives, by measuring how much parent and daughter are present in any given specimen, and by making certain key assumptions, scientists believe they are able to accurately determine the age of a specimen. The question is what are the underlying key assumptions and how reliable are they?
This technique can be applied if the daughter element has at least one stable isotope other than the daughter isotope into which the parent nuclide decays.
All forms of isochron dating assume that the source of the rock or rocks contained unknown amounts of both radiogenic and non-radiogenic isotopes of the daughter element, along with some amount of the parent nuclide.
Isochron dating is a common technique of radiometric dating and is applied to date certain events, such as crystallization, metamorphism, shock events, and differentiation of precursor melts, in the history of rocks.