Improving analytical method of Sr/Ca ratios in coral skeletons for paleo‐SST reconstructions using ICP‐OES

Abstract

Strontium/calcium ratios (Sr/Ca) in reef coral skeletons have been used for sea surface temperature (SST) proxy. Because skeletons of reef corals are accumulated throughout more than 100 years, SST records from coral skeletons were important archives to reveal climate mechanisms. Here, we improved the coral Sr/Ca analytical method using an inductively coupled plasma optical emission spectrometer which method has advantages in terms of initial/running cost and analysis time. We checked variations of Sr and Ca sensitivities as increases of Ca concentrations using various wavelengths of Sr and Ca. Our results suggested that sensitivities of Sr/Ca in a range of coral skeletons were modulated by selections of wavelengths and sensitivity changes of Sr and Ca. A combination of Sr(421.5 nm) and Ca(373.6 nm) is stable for the Sr/Ca analysis at a low Ca concentration. A SD of Sr/Ca(421.5/373.6 nm) was 0.006 mmol mol−1 (1σ, n = 40) based on the replicate measurement of the reference material (JCp‐1) using our new method. By applying a Porites colony from Kikai island, Japan, a precision of reconstructed SST was over ± 0.3°C at weekly resolution.

An ICP‐AES method for routine high‐precision measurement of seawater Sr/Ca ratios to validate coral paleothermometry calibrations

“…One issue of primary importance for an accurate and sensitive intensity ratio method is the selection of appropriate wavelengths for each element. Watanabe et al (2020), who recently presented an improved intensity ratio ICP‐AES method for coral aragonite Sr/Ca on a Thermo Fisher iCAP6200, considered 4 Sr lines (216.596, 346.446, 407.771, and 421.552 nm) and 4 Ca lines (315.887, 317.933, 370.603, 373.690 nm), all from ionic emission. In terms of sensitivity, suppression of Sr emission as a function of Ca concentration, and fidelity of the coral Sr/Ca–SST relation, best results were obtained with an intensity ratio calculated from Sr‐421 and Ca‐373.…”

“…On the PE Optima 8300 ICP‐AES, emission wavelengths above and below about 400 nm are recorded with separate detectors. Since measuring the Sr and Ca signal on different detectors is not conducive to a stable Sr/Ca intensity ratio, this precludes the combination of wavelengths used by Watanabe et al (2020). Moreover, both lines should be recorded in the same viewing mode (radial vs. axial view) and, in this case, radial view is dictated by the high Ca concentration of seawater, which exceeds the axial view linear range.…”

“…We recorded the intensity of 2 Sr and 3 Ca lines as a function of p , for different combinations of h and z , whereby the latter varied between −5.0 and −0.5 mm, in steps of 0.5 mm. The ionic Sr‐407 and Sr‐421 lines and the two ionic Ca‐315 and Ca‐317 lines were also considered by Watanabe et al (2020). We added the atomic Ca line at 422.673 nm.…”

“…We added the atomic Ca line at 422.673 nm. These are among the most sensitive lines for Sr (Watanabe et al 2020) and the least sensitive for Ca (Ramsey et al 1987), to compensate for the difference in concentration. Results are presented in Fig.…”

Corals Reveal an Unprecedented Decrease of Arabian Sea Upwelling During the Current Warming Era

“…Sr/Ca was determined using an inductively coupled plasma optical emission spectrometer, Thermo scientific iCAP6200, installed at Hokkaido University. 90–110 μg of coral powder were dissolved in 25% nitric acid and diluted to a Ca concentration of 7 ppm with Milli‐Q water (Watanabe et al., 2020). The precisions of Sr/Ca and δ 18 O coral were ±0.07%RSD and ±0.05‰ VPDB (1σ), respectively.…”