Measuring the elemental ratio of Mg to Ca in marine shells is one way to get at past ocean temperatures. This method is mostly used for Quaternary and Pliocene reconstructions, though it can be used most of the way back through the Eocene. This page is meant to provide a brief background on the technique and its uses in paleoceanography.
Why do we want paleoceanographic temperature records?
Almost anyone who looks at past climate has to (directly or indirectly) also understand what the oceans were doing at that time. One of the most common ways to not only tie oceanic changes to climate, but also tie climate changes back to physical changes within the oceans, is to study ocean temperatures. Ocean temperatures vary with the air and water masses that are present in an area. This means that if the configuration of ocean currents changes, so does the ocean temperature pattern. So, one benefit of studying ocean temperatures is increased understanding of ocean currents.
Another advantage to knowing past ocean temperatures is that we can tie those temperatures to changes in other aspects of the climate system. For instance, we’re currently facing warming oceans, and thus sea level rise from thermal expansion. How much will the oceans warm in the future? There are many considerations when answering that question, but we can help constrain the answer by looking at the response of oceans to past climate warming.
What is Mg/Ca paleothermometry?
Many marine organisms, like some foraminifera and coccolithophorids, build their shells out of calcium carbonate (calcite), CaCO3. A small amount of Mg gets incorporated into the shells during construction, forming impurities in the calcite. This can weaken the shell, so organisms often have mechanisms to exclude much of the Mg. However, this exclusion is temperature-dependent. Planktic foraminifera will incorporate more Mg into their shells (called tests) when they grow in warmer waters. Researchers have examined modern-day Mg to Ca ratios in planktic tests, and then compared the ratios to measured water temperatures, creating Mg/Ca-temperature calibrations.
How is Mg/Ca measured?
There are a variety of ways to determine the Mg/Ca of a calcite sample. Many researchers use inductively coupled plasma mass spectrometers (ICP-MS), where the sample is dissolved, ionized, and then separated into ions grouped by mass-to-charge ratios. Others use ICP-OES (OES = optical emission spectrometry), which ionizes the sample and then looks at the different wavelengths produced by the excited atoms.
A less common, but very useful, approach is to use laser-ablation ICP-MS, which removes very small, precise portions of a sample prior to ionization. This is beneficial if you want to examine how a sample varies spatially. Spatial variations in elemental ratios can also be detected by electron probe microanalysis (EPMA, or sometimes EMPA). EPMA involves hitting a sample with an electron beam, followed by analysis of x-rays produced by the different elements within the sample.