Isotopes, carbonate based skeletons and paleotemperatures...
PP12The ratio of Oxygen-16 to Oxygen-18 in the carbonate is a direct reflection of the ratio that was in the water within which the skeleton was growing. There are a couple of factors that influence the isotopic ratio, so of 16 to 18, that you'll find within a clam shell, water temperature is one of them. The water temperature of course is simply a measure of how much energy is in that system or is in the water and that energy is available to material, to particles within the water and that, as I'll explain in a moment, will influence the movement essentially, of the different isotopes.
The other factor is the relative amount of each isotope present in the water. This composition results in a shell composition that's probabilistic. I'll explain that in a moment but let's look a little bit more closely at the temperature. This little symbol down here which you'll see later on again, Delta 0-18 or Del 0-18 for short, is simply a measure of how much Oxygen-18 is present in your total sample of oxygen. It's actually equivalent to percentage. And what we'll notice, and this is just a straight physical process, is that as temperature increases the amount of Oxygen-18 in your sample decreases. This is just a function of the way that Oxygen-16 and -18 behave relative to each other. So, higher temperatures are actually going to serve to differentiate, or the lower temperatures will serve to differentiate those.