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

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 delta O-18, coinciding with the Florida landfall of hurricane Irma, as recorded at near-daily resolution in a continuous seawater sample collected with an osmotic pump.

Automated summary

A new ICP-AES method is introduced for rapid and routine analysis of Sr/Ca molar ratios in seawater, with a long-term precision of < 0.2%. However, there are larger Sr/Ca variations with distinct periodicity in some coastal waters, complicating the analysis. The method uses more accessible instrumentation and emphasizes the need for a certified seawater Sr/Ca reference standard.