I was told by a professor (I am assuming he was right) that organisms prefer lighter isotopes. So if a tree is in an atmosphere containing 99% O-16 and 1% O-18, the concentration of O-16 in the organism itself (in water, carbon dioxide, etc) will be MORE than the amount in the atmosphere (so >99%). These numbers are completely made up, by the way. So what I am getting at.... if you want to date a CaCO3 shell with radiocarbon dating, what is the reference point? Is the reference point the estimated concentration of C-14 in the water column ~5000 years ago? I started to think "well if organisms prefer lighter isotopes, then the C-14 concentration in the organism would not reflect the concentration in its environment. It would be slightly less in the organism". So, if the reference point of C-14 concentration is the amount in the water, and you are using that to measure the age of the organism, and the organism has a lower concentration of C-14 in its body than its surroundings had during its lifetime, then you would observe that the organism is much younger than it actually is (depending on the variance in heavy isotope concentration in the organism versus its surroundings). EDIT: I guess I forgot to ask a question. Is this something that is taken into account when carbon dating? Or is it a non-issue? Anybody care to comment on this?