Sea surface temperature (SST) is one of the most important quantities used in reconstructions of past climate. The strontium-calcium ratios (Sr/Ca) of the coral mineral aragonite is a widely used (if imperfectly understood) proxy for SST, although calibrations of Sr/Ca performed on modern corals, for which accurate temperature records do exist, generally have confirmed the fidelity of the proxy.
However, a closer look at the mechanics of Sr/Ca variability in one type of massive, symbiotic coral reveals that kinetics, and not simply thermodynamics, play a role. Cohen et al. performed high-resolution ion microprobe analyses on samples of Porites lutea and found that the relation between Sr/Ca and temperature varied by a factor of nearly five between day and night. Only at night was the relation similar to that observed for inorganic (temperature-controlled) calcite precipitation. Because the bulk of the coral skeleton actually forms during the day, the possibility that the Sr/Ca of coral reflects the rate of photosynthetic symbiont-mediated calcification as well as chemical equilibrium means more sophisticated coral sampling strategies may be required. — HJS
Paleoceanography16, 20 (2001).