Li/Mg systematics in scleractinian corals: Calibration of the thermometer

Abstract

We show that the Li/Mg systematics of a large suite of aragonitic coral skeletons, representing a wide range of species inhabiting disparate environments, provides a robust proxy for ambient seawater temperature. The corals encompass both zooxanthellate and azooxanthellate species (Acropora sp., Porites sp., Cladocora caespitosa, Lophelia pertusa, Madrepora oculata and Flabellum impensum) collected from shallow, intermediate, and deep-water habitats, as well as specimens cultured in tanks under temperature-controlled conditions. The Li/Mg ratios observed in corals from these diverse tropical, temperate, and deep-water environments are shown to be highly correlated with temperature, giving an exponential temperature relationship of: Li/Mg (mmol/mol) = 5.41 exp (À0.049 * T) (r 2 = 0.975, n = 49). Based on the standard error of the Li/Mg versus temperature correlation, we obtain a typical precision of ±0.9°C for the wide range of species analysed, similar or better than that of other less robust coral temperature proxies such as Sr/Ca ratios.The robustness and species independent character of the Li/Mg temperature proxy is shown to be the result of the normalization of Li to Mg, effectively eliminating the precipitation efficiency component such that temperature remains as the ⇑ Corresponding author at: Istituto di Scienze Marine (ISMAR), Consiglio Nazionale delle Ricerche, Via Gobetti 101, 40129 Bologna, Italy. Tel.: +39 051 6398913.E-mail address: paolo.montagna@bo.ismar.cnr.it (P. Montagna). 1Published in which should be cited to refer to this work.http://doc.rero.ch main controller of coral Li/Mg compositions. This is inferred from analysis of corresponding Li/Ca and Mg/Ca ratios with both ratios showing strong microstructure-related co-variations between the fibrous aragonite and centres of calcification, a characteristic that we attribute to varying physiological controls on growth rate. Furthermore, Li/Ca ratios show an offset between more rapidly growing zooxanthellate and azooxanthellate corals, and hence only an approximately inverse relationship to seawater temperature. Mg/Ca ratios show very strong physiological controls on growth rate but no significant dependence with temperature, except possibly for Acropora sp. and Porites sp. A strong positive correlation is nevertheless found between Li/Ca and Mg/Ca ratios at similar temperatures, indicating that both Li and Mg are subject to control by similar growth mechanisms, specifically the mass fraction of aragonite precipitated during calcification, which is shown to be consistent with a Rayleigh-based elemental fractionation model. The highly coherent array defined by Li/Mg versus temperature is thus largely independent of coral calcification mechanisms, with the strong temperature dependence reflecting the greater sensitivity of the K dLi/Ca partition coefficient relative to K d Mg/Ca . Accordingly, Li/Mg ratios exhibit a highly coherent exponential correlation with temperature, thereby providing a more robust tool for reconstructing paleo-seawat...

Coral calcifying fluid aragonite saturation states derived from Raman spectroscopy

“…While there is some uncertainty in the mechanism of Mg 2+ incorporation into aragonite (Montagna et al, 2014), the K D nevertheless serves as an empirical measure of the distribution of Mg/Ca between aragonite and seawater under various conditions. DeCarlo et al (2015) reported K D for Sr/Ca from these same samples, and here we follow the same calculations for K Mg/Ca D using the Mg/Ca data from Holcomb et al (2016), except for the slight modifications to the calculations described below.…”

The Li isotope composition of marine biogenic carbonates: Patterns and mechanisms

“…Instead, culture experiments and core top studies have shown that the Li/Ca ratio of foraminifera is influenced by the solution Li/Ca ratio, DIC concentration, and possibly the growth rate (Delaney et al, 1985;Hall and Chan, 2004;Hathorne and James, 2006;Lear and Rosenthal, 2006;Vigier et al, 2015). In contrast, the Li/Mg ratio of corals and foraminifera is more strongly related to temperature than Li/Ca and has been recently proposed as being a reliable proxy for ocean temperature (Bryan and Marchitto, 2008;Case et al, 2010;Montagna et al, 2014;Rollion-Bard and Blamart, 2015;Fowell et al, 2016). The Li/Ca ratio of mollusks might be controlled by a combination of vital effects, growth rate, changes in ocean productivity, and/or dissolution of riverine fine sediments within the ocean (Füllenbach et al, 2015;Thébault et al, 2009;Thébault and Chauvaud, 2013).…”

Intra-skeletal calcite in a live-collected Porites sp.: Impact on environmental proxies and potential formation process

“…Variation of the Sr/Ca-SST dependency limits the proxy accuracy and therewith its utility, particularly in regions where seasonal temperature changes are limited. Recently, the Li/Mg ratio in aragoniticscleractinian coral skeletonshas been shown to be a species independent SST proxy that could complement previous temperature proxy (Case et al, 2010;Hathorne et al, 2013a;Montagna et al, 2014).To reconstruct the pH of the seawater, the U/Ca ratio in tropical corals was proposed (Inoue et al, 2011) but its strong sensitivity to temperature clearly limitspaleopH applications. Contrarily, boron isotopes in Porites sp.…”