Coral calcifying fluid aragonite saturation states derived from Raman spectroscopy

Abstract. Quantifying the saturation state of aragonite (Ω_Ar) within the calcifying fluid of corals is critical for understanding their biomineralization process and sensitivity to environmental changes including ocean acidification. Recent advances in microscopy, microprobes, and isotope geochemistry enable the determination of calcifying fluid pH and [CO₃²⁻], but direct quantification of Ω_Ar (where Ω_Ar =  [CO₃²⁻][Ca²⁺]∕K_sp) has proved elusive. Here we test a new technique for deriving Ω_Ar based on Raman spectroscopy. First, we analysed abiogenic aragonite crystals precipitated under a range of Ω_Ar from 10 to 34, and we found a strong dependence of Raman peak width on Ω_Ar with no significant effects of other factors including pH, Mg∕Ca partitioning, and temperature. Validation of our Raman technique for corals is difficult because there are presently no direct measurements of calcifying fluid Ω_Ar available for comparison. However, Raman analysis of the international coral standard JCp-1 produced Ω_Ar of 12.3 ± 0.3, which we demonstrate is consistent with published skeletal Mg∕Ca, Sr∕Ca, B∕Ca, δ¹¹B, and δ⁴⁴Ca data. Raman measurements are rapid ( ≤  1 s), high-resolution ( ≤  1 µm), precise (derived Ω_Ar ± 1 to 2 per spectrum depending on instrument configuration), accurate ( ±2 if Ω_Ar < 20), and require minimal sample preparation, making the technique well suited for testing the sensitivity of coral calcifying fluid Ω_Ar to ocean acidification and warming using samples from natural and laboratory settings. To demonstrate this, we also show a high-resolution time series of Ω_Ar over multiple years of growth in a Porites skeleton from the Great Barrier Reef, and we evaluate the response of Ω_Ar in juvenile Acropora cultured under elevated CO₂ and temperature.