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

Abstract

A new inductively coupled plasma atomic emission spectrometry (ICP‐AES) method is presented for rapid and routine analysis of Sr/Ca molar ratios in seawater, with a long‐term precision of < 0.2%. It is an adaptation of a method widely employed for the analysis of coral aragonite Sr/Ca ratios in marine paleothermometry studies, which are based on the assumption that the seawater Sr/Ca ratio is constant in space and time. While prior studies have shown variations of up to 1% with depth, smaller variations at the ocean surface are generally accounted for via empirical, species‐specific calibrations of coral Sr/Ca vs. temperature. We found Sr/Ca variations in some coastal waters to be even larger, with distinct periodicity, complicating this approach. Although the high precision necessary for measurements of seawater Sr/Ca has previously relied on advanced mass spectrometry, long analysis times, and expensive isotopic spikes, our method uses more accessible instrumentation and is both time‐ and cost‐saving. The intricate composition of seawater, relative to coral aragonite solutions, requires an intensity ratio calibration technique combined with rigorous normalization to a suitable seawater standard. Key aspects of our method are discussed, including the choice of wavelengths, instrument parameters, accuracy, precision, and matrix effects. Special attention is given to the need for a certified seawater Sr/Ca reference standard, which does not presently exist. Analytical validation is provided by concurrent sharp gradients in Sr/Ca and δ18O, coinciding with the Florida landfall of hurricane Irma, as recorded at near‐daily resolution in a continuous seawater sample collected with an osmotic pump.

Apparently Seasonal Variations of the Seawater Sr/Ca Ratio Across the Florida Keys Reef Tract

“…The renormalized values of CASS‐5 and CASS‐6 (Table 1), now invariant across all runs, are 0.045 mmol/mol lower than those reported by Khare et al. (2021), reflecting the offset between the HPH and AK batches of GoM. Nonetheless, the difference between CASS‐5 and CASS‐6 is 0.014 mmol/mol, exactly as before, confirming that the renormalization does not affect relative values.…”

“…In terms of the full data set of individual grid points, the maximum difference was 0.29 mmol/mol (station 1B in 19S vs. station 3A in 18W). Khare et al (2021) reported a sudden decrease of the seawater Sr/Ca ratio by 0.16 mmol/mol at station OS1 (Figure 1) over an interval of only 2 days around the landfall of hurricane Irma in September 2017. This was matched by a concurrent decrease in δ 18 O values, both apparently caused by a sudden influx of low-Sr/Ca (in summer) and less evaporated Atlantic water into Florida Bay, associated with the storm surge.…”

Identifying Hydro‐Sensitive Coral δ18O Records for Improved High‐Resolution Temperature and Salinity Reconstructions

“…Monitoring efforts should include long time series from key reef sites to assess the space‐for‐time assumption often made in paleoclimate studies (i.e., that the spatial relationship across sites is representative of the relationship through time at individual sites). Such data could be obtained by installing networks of OsmoSamplers, which do not require power and can be deployed for up to a year collecting water samples on daily to monthly time intervals (Jannasch et al., 2004; Khare et al., 2021), or by making continuous shipboard measurements with spectroscopic water isotope analyzers fed seawater from the intake line (Munksgaard et al., 2012). In each case, precise SSS measurements must also be obtained.…”

Environmental records from coral skeletons: A decade of novel insights and innovation

“…Other proxy records explicitly leverage spatio‐temporal variations in TE concentration (e.g., runoff and upwelling proxies); however, local, long‐term monitoring studies are rarely performed to validate these proxies at individual sites. In recognition of this gap and recent advancements in methods for cost‐effective, long‐term TE monitoring (Khare et al, 2021), there are a growing number of seawater TE monitoring studies to improve the calibration and interpretation of TE proxies (e.g., nutrient availability and upwelling strength from Ba/Ca, LaVigne et al, 2011, 2016; Lewis et al, 2018; Tanzil et al, 2019; P/Ca, LaVigne et al, 2010; Cd/Ca, Grottoli et al, 2013; and multi‐TE/Ca proxies, Lewis et al, 2018). While the relationships among these proxies and climate have been reasonably well replicated among sites (e.g., Grove et al, 2012; LaVigne et al, 2010; Sayani et al, 2021), complex circulation and mixing patterns may complicate the interpretation among sites and cores (Lewis et al, 2018).…”

“…While such variability is presumed to be small (relative to the climate signal) due to the conservative behavior of Sr and other key TE proxies, whether this assumption may break down (de Villiers et al, 1994; Shen et al, 1996) under circumstances is still debated. For example, Sr/Ca departures may occur with riverine and coastal runoff following cyclones and other heavy‐rainfall events (e.g., Khare et al, 2021). Other proxy records explicitly leverage spatio‐temporal variations in TE concentration (e.g., runoff and upwelling proxies); however, local, long‐term monitoring studies are rarely performed to validate these proxies at individual sites.…”