Re: isotopic analysis of fossils

A.J. Norman (
10 Jul 1996 14:14:03 +0100

In article <>,
Dan Evens <> wrote:
>I'm kind of confused about isotopic analysis of fossils. Could
>somebody give a brief description of what is going on?
>- what is measured?
>- how does an anomolous isotopic ratio get generated?
>- how does it survive fossilization?
>- what does it tell about the organism?
>Now, I know about isotope use in estimating ages of various
>rocks, things like Carbon-14 and Argon-40/39 are familiar to
>me. But when people talk about Oxygen isotopes in fossils,
>I sort of fuzz out. Any help appreciated.

Ten years since I graduated in Geochemistry, so this should be taken
with a pinch of salt, but here are my answers:

1. Ratio of O-18 to O-16 in the sample. Natural abundance is 0.2% O-18,
the remainder being O-16.

2. The two isotopes are absorbed into organisms with different ratios
(e.g. calcium carbonate formation takes more of one isotope than of
the other, though I can't for the life of me remember which one).
This also depends on the temperature of the environment - I dimly
remember being shown graphs of 16/18 ratios taken from fossil shells
which showed the rise and fall of the water temperatures as the
shells were formed. According to
(a study of ice cores) the heavy isotope is depleted in water vapour
(as you might expect). The phrase "Standard Mean Ocean Water (SMOW)"
has started bells ringing in my head.

3. Dunno. I think it's assumed that the carbonate in a fossil shell is
pretty much the same stuff as in the original (though perhaps

4. If you assume that 18/16 ratios in the ocean (or whatever environment
the creature lived in) have remained constant, you can tell what the
ambient temperature was. (And this goes for minerals, ice cores

Andrew Norman, Leicester, England 10/07/96,