In this article we shall look at how the composition of shells of marine organisms is affected by climate, and how we can therefore obtain clues about past climates by chemical and isotopic analysis of their shells. This collection of methods is known as scleroclimatology.
The ratio of the stable oxygen isotopes 16O and 18O in water (H2O) can be shown to vary with temperature, with a higher 16O/18O ratio associated with lower temperatures.
Oxygen from the seawater is used by shell-forming marine organisms when they form their calcium carbonate shells, and as the two isotopes of oxygen are chemically identical, no isotope is preferentially involved in this process, with the result that the oxygen ratios in the shells reflect the oxygen ratios in the seawater.
This means, of course, that if we look at shells in the geological record (the tests of foraminiferans being a favorite choice for this) and if the calcium carbonate in the shells hasn't been replaced by some other mineral, and if we can put a date on the shells, then we can find the temperature of the seawater at the time and place where the shells were deposited: this proxy is known as δ18O. What's more, since it is possible to distinguish between the fossils of plantonic and benthic species, we get two figures for each location: the surface temperature and the temperature at the sea floor.
Mg/Ca and Sr/Ca
The elements magnesium (Mg) and strontium (Sr) lie in the same column of the periodic table as calcium (Ca) and so possess similar chemical properties. This means that magnesium and strontium can substitute for the calcium in calcium carbonate (CaCO3). As these elements substitute more readily for calcium at higher temperatures, the proportions of magnesium and strontium substituting for calcium can be used as a temperature proxy.
Difficulties of the method
Obviously for this method to work the mineral composition of the shells must not be changed by fossilization. While this can present us with difficulties in obtaining appropriate material, it will not usually lead us to produce erroneous results; after all, if a shell has undergone mineral replacement so that it's no longer made of calcium carbonate then this is not something a geologist would easily be able to overlook.
One thing that interferes with the oxygen isotope method is that, paradoxically, actual glaciation has the opposite effect on global marine oxygen isotope ratios than mere low temperatures have on local isotope ratios. This is because when the glaciation of the Earth increases, water that evaporates from the seas is locked up in ice sheets; now 16O, being lighter than 18O, evaporates more readily, so that the result of increasing glaciation is a decrease in the 16O/18O ratio. To make sense of the data, it is necessary to disentangle these local and global effects by reference to other data.
When it comes to the Mg/Ca and Sr/Ca methods, the local mineral composition of the seawater can act as a confounding factor: for example, if the seawater in a particular location is particularly rich in magnesium for some reason, then this will also increase the Mg/Ca ratio.
How do we know?
We can take samples of shell-forming organisms from locations with a known temperate and measure their isotopic and chemical ratios, and see how they relate to temperature. It is also possible to grow shellfish in tanks kept artificially at a controlled temperature and see what happens.
It is unlikely that the biochemistry of shell formation has changed significantly since organisms first started forming calcium carbonate shells; and it is almost unthinkable that the physics of the evaporation of water has changed at all. It is reasonable to conclude that we can take chemical and isotopic ratios in the past as proxies for past temperatures.
Finally, we can note that there is a good if not exact correlation between the paleoclimatic data obtained from shells and other paleoclimatic proxies.