Coral Li/Mg thermometry: Caveats and constraints

Abstract

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 from 64 coral specimens belonging to 8 different taxa, including both reefbuilding zooxanthellate and coldwater 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 670 m). 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. Organicrich 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, which requires the scrupulous assessment of the integrity of the aragonite prior to geochemical analyses. Given that some species entrap more organic matter than others, and that some are more prone to calcite contamination, a taxon-related effect is apparent. Here we show that the tropical species Porites spp., Pseudodiploria strigosa and Orbicella annularis, and the cold-water species Madrepora oculata, Caryophyllia antarctica and Flabellum impensum, are all suitable candidates for reconstructing seawater temperatures. The integrated results across a wide temperature range, from extreme cold to tropical shallow waters, yield an overall precision for the Li/Mg-temperature proxy of ± 1.0 °C, as quantified by the standard error of estimates. If calculated from the 95% prediction intervals, the uncertainty of the temperature estimates is ± 0.9 °C at 1 °C, ± 1.5 °C at 12 °C and ± 2.6 °C at 25 °C. However, the uncertainty for the tropical corals (e.g. Porites) can be reduced to ± 0.6 °C if a Li/Mg and Sr/Ca multi-regression approach is applied.

Refining trace metal temperature proxies in cold-water scleractinian and stylasterid corals

“…Thus the ratio of coral Li/Mg has great promise; preserving the temperature dependency of these elements while minimising growth-rate effects (Case et al, 2010). A universal Li/Mg relationship to temperature appears robust across published data for both zooxanthellate and azooxanthellate scleractinia (Andersson et al, 2020;Case et al, 2010;Cuny-Guirriec et al, 2019;Fowell et al, 2016;Hathorne et al, 2013a;Montagna et al, 2014;Raddatz et al, 2013;Ross et al, 2019) and also aragonitic foraminifera (Hoeglundina elegans; Bryan and Marchitto, 2008;Hall and Chan, 2004;Marchitto et al, 2018) (Fig. 1A).…”

“…1A; Chaabane et al, 2019). Without high-Mg calcite Compiled published Li/Mg temperature calibration data for shallow-water zooxanthellate (black) and azooxanthellate (grey) scleractinian corals (Andersson et al, 2020;Case et al, 2010;Cuny-Guirriec et al, 2019;Fowell et al, 2016;Hathorne et al, 2013a;Montagna et al, 2014;Raddatz et al, 2013;Ross et al, 2019), aragonitic foraminifera (green; H. elegans; Bryan and Marchitto, 2008;Hall and Chan, 2004;Marchitto et al, 2018), crustose coralline algae (pink squares; Clathromorphum compactum; Anagnostou et al, 2019), and high-Mg calcite "red coral" (pink diamonds; Corallium rubrum; averaged solution ICP-MS data Chaabane et al, 2019). Two azooxanthellate points identified as anomalous by Case et al (2010) due to preservation have been removed.…”

“…and is based on Li/Mg data compiled from the literature that we now adjust for interlaboratory analytical offsets (see Methods and Supplementary Information). More than three quarters of the data characterising this Li/Mg relationship to seawater temperature however come from shallow-water coral specimens harbouring zooxanthellae (Fowell et al, 2016;Hathorne et al, 2013a;Montagna et al, 2014;Ross et al, 2019), with few observations obtained from azooxanthellate deep-water taxa (Case et al, 2010;Cuny-Guirriec et al, 2019;Montagna et al, 2014;Raddatz et al, 2013). Fewer still of these samples are from locations with wellcharacterised temperature (Case et al, 2010).…”

Calibration of Sr/Ca, Li/Mg and Sr‐U Paleothermometry in Branching and Foliose Corals

“…Similarly, Rayleigh fractionation within the calcifying fluid reservoir influences trace element‐calcium ratios in the skeleton because ratios with K D > 1 (e.g., Sr/Ca) become depleted in the fluid, whereas ratios with K D < 1 (e.g., Mg/Ca) become enriched in the fluid during precipitation (Gaetani & Cohen, ). Yet, since Li and Mg are both largely incompatible in aragonite ( K D << 1), Li/Mg is relatively insensitive to Rayleigh fractionation, and this characteristic formed the primary basis for the development of Li/Mg thermometry (Cuny‐Guirriec et al, ; Montagna et al, ).…”

Morphological, elemental, and boron isotopic insights into pathophysiology of diseased coral growth anomalies

“…This is of particular concern given the differential tissue characteristics in unaffected and GA samples which potentially react differently to cleaning protocols. Therefore, we implement a thorough, two-step cleaning protocol (compared to single-step protocols such as in 66 ) that is demonstrated to effectively remove organics 67 . Furthermore, we show that Li/Mg ratios (a temperature proxy prone to organic matter contamination) in our unaffected corals fall within the Li/Mg range of published tropical corals living at similar sea surface temperature (Supplemental Fig.…”