[Radioactive]

Exactly.

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

[Blunderfromdownunder]

Radiocarbondating cannot be used to date anything that was alive more recently than about 1750 AD (which includes your seal bone). The major reason for this cutoff is the vast increase and fluctuation in atmospheric carbon since the industrial revolution, along with more recent atomic/nucelar missile tests. The amount of carbon in the atmosphere has always fluctuated but can be accounted for (radiocarbon calibration) by dating known-age items (eg wood from long-lived tree species whose age can be detrmined accurately using dendrochronology or tree-ring dating) and noting the offset between the dates. Scientists have created several "calibration curves" that can be used in this manner, but sadly there are far two many wiggles in the curve beyond about 300 years ago for the technique to be used reliably. OTOH anything from 300-10,000 years ago can be calibrated fairly accurately and beyond 10,000 years, to within centuries (the 10,000 year point is where scientists run out of samples of old wood to create their calibration curves).

[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.