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

Abstract

The Li/Mg, Sr/Ca and oxygen isotopic (δ 18 O) compositions of many marine biogenic carbonates are sensitive to seawater temperature. Corals, as cosmopolitan marine taxa with carbonate skeletons that can be precisely dated, represent ideal hosts for these geochemical proxies. However, efforts to calibrate and refine temperature proxies in cold-water corals (<20 • C) remain limited. Here we present skeletal Li/Mg, Sr/Ca, δ 18 O and carbon isotope (δ 13 C) data from live-collected specimens of aragonitic scleractinian corals (Balanophyllia, Caryophyllia, Desmophyllum, Enallopsammia, Flabellum, Lophelia, and Vaughanella), both aragonitic and high-Mg calcitic stylasterid genera (Stylaster and Errina), and shallow-water high-Mg calcite crustose coralline algae (Lithophyllum, Hydrolithon, and Neogoniolithon). We interpret these data in conjunction with results from previously explored taxa including aragonitic zooxanthellate scleractinia and foraminifera, and high-Mg calcite octocorals. We show that Li/Mg ratios covary most strongly with seawater temperature, both for aragonitic and high-Mg calcitic taxa, making for reliable and universal seawater temperature proxies. Combining all of our biogenic aragonitic Li/Mg data with previous calibration efforts we report a refined relationship to temperature: Li/Mg All Aragonite = 5.42 exp(−0.050 × T (• C)) (R 2 = 0.97). This calibration now permits paleo-temperature reconstruction to better than ±3.4 • C (95% prediction intervals) across biogenic aragonites, regardless of taxon, from 0 to 30 • C. For taxa in this study, aragonitic stylasterid Li/Mg offers the most robust temperature proxy (Li/Mg Stylasterid (Arag) = 5.64 exp(−0.046 × T (• C)) (R 2 = 0.95)) with a reproducibility of ±2.3 • C. For the first time, we show that high-Mg calcites have a similar exponential relationship with temperature, but with a lower intercept value (Li/Mg = 0.63 exp(−0.050 × T (• C) (R 2 = 0.92)). This calibration opens the possibility of temperature reconstruction using high-Mg calcite corals and coralline algae. The commonality in the relationship between Li/Mg and temperature transcends phylogeny and suggests a similar abiogenic trace metal incorporation mechanism.

NIST RM 8301 Boron Isotopes in Marine Carbonate (Simulated Coral and Foraminifera Solutions): Inter‐laboratory δ11B and Trace Element Ratio Value Assignment

“…Equations () (Stewart et al . 2020) and () (Alpert et al . 2016) are based on multiple species of coral.…”

“…2019, Stewart et al . 2020). Laboratory mean Sr/Ca ratios for NIST RM 8301 (Coral) vary by only ± 1.7% (1 s ); however, using Equation (), such discrepancy results in more than 3 °C difference in calculated seawater temperatures.…”

Natural Variability of Skeletal Elemental Phosphorus (P/Ca), Lead (Pb/Ca), and Barium (Ba/Ca) in the Western Pacific Sclerosponges Acanthoceatetes wellsi and Astrosclera welleyana

“…The higher partition coefficient of A. wellsi compared to A. welleyana is consistent with the interpretation of a mineralogical effect on P/Ca in sclerosponges, as high-Mg calcite (polymorph of A. wellsi) has a higher partition coefficient than aragonite GROTTOLI ET AL. 10 of 14 10.1029/2020GC009245 (polymorph of A. wellyana) (Mucci, 1989;Stewart et al, 2020). In fact, high Mg can interfere with the uptake of Sr into calcite independent of temperature (Morse & Bender, 1990) and is assumed to be the root cause for the unpredictable Sr/Ca ratios in A. wellsi (Grottoli et al, 2010).…”

“…It has been used to reconstruct seawater changes related to ocean circulation patterns (Grottoli et al., 2010; Moore et al., 2000) and to reconstruct Caribbean seawater temperature (B. Rosenheim et al., 2009). However, δ 18 O of scleractinian corals are poor temperature proxies as they do not exhibit δ 18 O signature close to equilibrium with seawater (Samperiz et al., 2020) because they precipitate their calcium carbonate in semi‐enclosed system (Stewart et al., 2020). Sclerosponges appear to be more similar to stylasterid corals that have an open calcification system allowing them to precipitate δ 18 O close to seawater equilibrium (Samperiz et al., 2020), which also results in minimal vital effects on other proxies like Li/Mg (Stewart et al., 2020).…”

“…ies like Li/Mg (Stewart et al, 2020). Minor and trace elements have also been measured in sclerosponges, and their potential as paleo-proxies is still being explored (Table 1).…”