[jennyp]

I once read a report that had a scientist date a recently killed seal rib bone as being 5 thousand years old.

Dating organisms that feed on things at the bottom of the sea (or that feed on things that eat the things at the bottom of the sea) is problematic, because the carbon in those bottom-dwellers did not come from the atmosphere. C-14 dating is based on the fact that a certain amount of CO₂ in the atmosphere gets zapped by cosmic rays, thus turning its C into radioactive C-14 instead of the normal C-12. That's also where the popular creationist story of the live mollusk whose shell was dated as being millions of years old (or somesuch) came from.

[Blunderfromdownunder]

Its called the Marine Resevoir Effect, and the offset is about 400 years. Basically, the ocean (and therefore the organisms within it) exchange carbon with the atmosphere much more slowly than do terrestrial organisms - it takes about 400 years for carbon isotope ratios in the ocean to "catch up" with those on land. This is another problem with early radiocarbon dating that was recognised fairly early on and has since been largely solved by calibration , although local effects for different parts of different oceans are still in the process of being accounted for. This is done by dating known age samples (eg from museums) and using the differences in returned radiocarbon dates as a delta-R value for actual archaeological samples.

The whole story of getting a ludicrously ancient date from a modern bone/shell/piece of toast (which is the version I heard in church as a lad) is one of those creationist canards that tends to persist despite all arguments to the contrary.