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30 avril 2019
Thermométrie par le rapport Li/Mg sur corax : mise en garde et contraintes
Thermométrie par le rapport Li/Mg sur corax : mise en garde et contraintes

Paired Sr/Ca vs ln (Li/Mg) values for all the chemically-cleaned samples analysed in the present study (red squares), plotted together with data from Hathorne et al (2013), Raddatz et al (2013), Montagna et al (2014), Fowell et al (2016) and D’Olivo et al (2018).

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


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