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

Abstract

Coral skeletons are the most commonly used high‐resolution temperature proxy in the tropical oceans, providing paleoclimate reconstructions dating back centuries to millennia. However, physiological differences in skeletal formation modes together with artifacts arising from coral biomineralization (vital effects) can confound the temperature dependence of single element‐to‐calcium ratios. In efforts to reduce vital effects and isolate temperature, new approaches have been developed based on Sr‐U and Li/Mg, which combine Sr/Ca and U/Ca and Li/Ca and Mg/Ca, respectively. Here we examine the systematics of Sr/Ca, Sr‐U, and Li/Mg paleothermometry in 33 colonies of branching (Acropora, Pocillopora, and Stylophora) and foliose (Turbinaria) genera. To address the calibration of these morphologically complex calcifiers, we conducted repeat field trips every 1 to 3 months and collected the most recent (~1 month) uppermost growth of individual colonies over ~18‐ to 24‐month periods. This enables seasonally resolved calibration of genera that exhibit rapid extension and slower secondary calcification. Based on this experimental design, we show that all three proxies capture seasonal to annual temperature variations for their respective growth intervals, providing calibrations across an 11 °C range. Species effects on the temperature dependence were largest for Sr/Ca (22.7%) yet minor for Li/Mg (7.2%) and Sr‐U (6.3%). Residuals from proxy‐temperature regressions were correlated between Sr/Ca and Li/Mg, indicating similar biological processes may influence Sr/Ca and Li/Mg thermometry. The implications of this study are that Sr‐U and to a lesser extent Li/Mg are applicable to fossil branching coral skeletons identified to genus level without the need for modern‐day calibration. We further show that all three paleothermometers provide complementary temperature constraints, with Li/Mg and the more species‐dependent Sr/Ca showing greater effectiveness at resolving seasonal variability and Sr‐U showing greater reliability at capturing mean annual temperature.

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

“…Although correlation is weak, this relationship is similar to that reported for scleractinian corals by Ross et al (2019). Yet, we find no such correlation (R 2 = 0.01) for the residuals for aragonitic stylasterid coral temperature relationships: Sr/Ca residuals = 0.10 × Li/Mg residuals + 0.01 (R 2 = 0.01) (10) Li/Mg is thought to be unaffected by [Ca] of the ECF, however, biogenic carbonate precipitation experiments suggest that the partition coefficient of Li/Mg is sensitive to carbonate ion concentration (Holcomb et al, 2016;Ross et al, 2019). Sr/Ca may also be indirectly influenced by in the ECF if kinetic effects and/or Rayleigh fractionation become more dominant as [CO 2− 3 ] increases (Ross et al, 2019).…”

“…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.…”

“…Paired coral U/Ca ratios have been used to correct Sr/Ca values for growth rate effects, yielding a more robust relationship with seawater temperature (DeCarlo et al, 2016). However, this Sr-U approach is only applicable to large, growth-banded, shallowwater corals, from which seasonally resolved records can be obtained to characterise the Sr:U relationship for each individual (Ross et al, 2019). Alternative approaches are therefore required for developing robust temperature proxies in smaller cold-water coral taxa.…”

“…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).…”