Testing the veracity of the Mg/Ca palaeotemperature proxy in cultured marine molluscan calcite

Climate change is an important and pressing issue for all of human society. Scientists need to understand how Earth's climate varies over different timescales so that they can inform politicians and the public about the likely scale and extent of the impacts of any future climate change. To understand climate it is necessary to gather information about how key oceanographic and climatic parameters, e.g. seawater temperature, changed in the recent and geological past. These data then can be used to model, and ultimately predict, how climate varies over different timescales of interest, i.e. years, decades and centuries. Unfortunately, however, instrumental records of these key parameters only extent back 100 years or so, but much critical information can be obtained from earlier times when the climate was in different states. Because direct measurements of the required parameters are not available for the earlier time intervals an indirect approach then must be taken, whereby measurements are made not of the actual climatic or oceanographic parameter but something else that responded in the same way and still is preserved in the recent or geological record. This approach is known as a proxy-based reconstruction, with the proxy being either a physical or chemical signal that can be measured and that also exhibits a relationship with the climatic or oceanographic parameter that needs to be determined. In addition to the physical or chemical proxy an archive material that preserves the information through recent and geological time also is required and these archives include tree rings, corals and marine molluscs. All of these organisms exhibit an incremental growth pattern, e.g. yearly growth bands, such that high temporal resolution records of the proxies and ultimately climatic and oceanographic parameters can be reconstructed. Corals and molluscs secrete calcium carbonate skeletons that include the mineral calcite and it has been shown that elements such as strontium and magnesium can replace, by a process termed substitution, some of the calcium in these minerals and that the amount of substitution can be related to the temperature of the seawater in which they lived. Therefore, the coral and mollusc skeletons are the archive materials and the measurement of the ratios of strontium and magnesium to calcium in the minerals are the proxy measurements, which can be used to reconstruct temperature in this case. Successful use of such a proxy-based approach is reliant, however, on there being sensitive and precise relationships between the proxy and the climatic or oceanographic parameter of interest. We have recently observed significant variation in magnesium to calcium ratios in mollusc shells cultured at constant temperature and therefore we intend to investigate the degree of such variability in more detail in this study. Detailed assessment of the level of variation in magnesium to calcium ratios is necessary so that we can improve the precision of temperature reconstructions made using measurement of this proxy in these organisms. To meet this objective we will grow two mollusc species, with contrasting shell structures, in laboratory aquaria under controlled temperature conditions. The degree of variability of the magnesium to calcium ratio within the new shell material then will be investigated using several analytical techniques, including one that can make the measurements of magnesium and calcium concentrations every one hundredth of a millimetre across and through the shell structure. By understanding how the magnesium to calcium ratio varies within the shells we should be able to adjust our sampling strategy to provide a more precise calibration and hence more precise seawater temperature reconstructions, thereby improving understanding of past and future climate change.