This idea of being able to relate the ratio of Oxygen-16 and Oxygen-18 directly to temperature is actually the method that's used most frequently to reconstruct paleotemperatures. Most of you will be familiar with the idea of glacial and inter-glacial cycles within the earth's recent history, so for about the last two and a half million years, the earth has been going through a series of glaciations and de-glaciations. And we actually have temperatures now for the most recent glacial cycles. These are, of course, important when we talk about things like global warming and so on. Well, those temperatures are almost, in every case, reconstructed by looking at the Del 0-18 value of marine fossils--deep sea marine fossils--and relating that directly to some sort of temperature measure. So, we can expect the composition of the carbonate within our clam shells to vary with temperature.

It will also vary based on the amount of O-16 and O-18 that you have in the water. Again, think of yourself as the shell building machine for the clams, so you're the skeleton building machine, and you're pulling particles out of the water to incorporate them into the skeleton. Well, simply because of the fact that you have so many more Oxygen-16 than Oxygen-18 molecules, there are times when you snag a molecule, your probability or your chance of snagging an Oxygen-16 is much higher than it is of snagging an Oxygen-18. Of course, by chance, that probability is going to vary according to exactly what that ratio is and that ratio is going to vary according to how much Oxygen-16 freshwater is entering the marine ecosystem. So that's essentially the chain, the logical chain that we're using to do our reconstruction.

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