So certain molecules or compounds can become preserved in sediments. When they are they act like a chemical fossil. This is an example of one of the molecules we study. This is a very large model of a single molecule.
This is a molecule called an alkenone, a fat-type molecule. Each of the black balls is a carbon atom and each of the white semi-spheres is a hydrogen atom. Then we have this little red oxygen atom.
This is a molecule of a fat produced by a particular type of algae called coccolithophores. So if we find this molecule in the sediments it tells us one thing to start with – at the time the sediments were deposited there were coccolithophores, this particular type of algae, living.
What they do that's really exciting is act like a kind of chemical or molecular thermometer.
So in any type of fat you can get unsaturations. So in this molecule we have these two little grey lines here. These are double bonds or unsaturations.
Now the algae vary the number of these unsaturations depending on the temperature of the water they live in. So they mainly produce an alkenone, this compound, with either two of these unsaturations or they produce a molecule like this which has three of these unsaturations. One, two, three.
The reason they do that, we think, is that they are trying to adjust the amount of unsaturation to keep those fat molecules at a particular level of fluidity. Think about the kind of fats you have in your kitchen at home. You might have margarine. At room temperature margarine is a saturated fat – there are none of these unsaturations – and that is solid at room temperature.
If you look at the fats in your olive oil or vegetable oil there'll be lots of these double bonds, these unsaturations. And those fats are fluid at room temperature.
So what the algae are trying to do is increase the amount of unsaturation when they have to live in colder water temperatures, to keep their fats at a particular level of fluidity.
They do that because they use the fats as a store of energy or in their cellular membranes. For them to function they have to be fluid, and if they solidify I guess they have the algal equivalent of a heart attack.
If that didn't happen the more solid, less fluid the fat would become and the organism would have trouble functioning. If it gets colder it increases the number of unsaturated bonds which makes it more fluid and that in turn because it remains in the sediment you can dig it up millions of years later and tell what they temperature was.
Millions of years later we can recover that sediment from an ocean drill ship or from sediments from a lake. We extract all the lipids, all the organic material we can, and we can analyse for compounds like alkenones. We measure the ratio of unsaturation we look at the individual molecules and find out how many of the molecules with the double or the triple bonds were being produced at that time.
That will tell us in quite a direct way the temperature of the sea or lake surface at that time. That's why it's a molecular thermometer or a palaeo-thermometer.