The coral Li/Mg temperature proxy is revisited through an in-depth trace element analysis of scleractinians collected live from tropical to polar environments. The dataset consists of Li/Ca, Mg/Ca, Sr/Ca and Li/Mg ratios of 64 coral specimens belonging to 8 different taxa, including both reef-building zooxanthellate and cold-water non-zooxanthellate species, from a wide range of water temperature (-1 to 29.5°C), salinity (34.71 to 38.61,) and depth (3 to 670m).
Our results showed that the reliability of the Li/Mg temperature proxy is strongly limited by the organic matter associated with the coral skeleton, which is most evident within the green bands observed in tropical corals. Organic-rich bands can double the Mg content otherwise present in the skeleton, which may ultimately lead to a temperature overestimation exceeding 15°C. We found that this bias can be overcome by the treatment of coral skeletons with a specific oxidizing cleaning protocol. We also detected the presence of calcite deposits within the aragonite skeleton of some Antarctic living coral specimens, which strongly affects the robustness of the Li/Mg proxy given its temperature sensitivity of ~1.5°C/1% calcite. Therefore, to obtain reliable reconstructions a correction needs to be applied when organic matter and/or calcite contamination is present.
The integrated results across a wide temperature range, from extreme cold to tropical shallow waters, yield to an overall precision for the Li/Mg-temperature proxy of ±1.0°C, with slight differences on the uncertainties depending on the environment: ±0.9°C, ±1.5°C and ±2.6°C for deep, intermediate, and tropical shallow water corals, respectively. However, the uncertainty for tropical corals can be reduced to ±0.6°C if a Li/Mg and Sr/Ca multi-regression approach is applied.  

Référence : Cuny-Guirriec K., Douville E., Reynaud S., Allemand D., Bordier L., Canesi M., Mazzoli C., Taviani M., Canese S., McCulloch M., Trotter J., Rico-Esenaro S.D., Sanchez-Cabeza J.A., Ruiz-Fernandez A.C., Carricart-Ganivet J.P., Scoot P.M., Sadekov A., Montagna P., 2019. Coral Li/Mg thermometry: caveats and constraints. Chemical Geology, 523, 167-178. Doi: 10.1016/j.chemgeo.2019.03.038