Coral calcifying fluid aragonite saturation states derived from Raman spectroscopy

Quantifying the saturation state of aragonite (Omega(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 Omega(Ar) (where Omega(Ar) D [CO32-][Ca2+]/K-sp) has proved elusive. Here we test a new technique for deriving Omega(Ar) based on Raman spectroscopy. First, we analysed abiogenic aragonite crystals precipitated under a range of Omega(Ar) from 10 to 34, and we found a strong dependence of Raman peak width on Omega(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 Omega(Ar) available for comparison. However, Raman analysis of the international coral standard JCp-1 produced Omega(Ar) of 12.3 +/- 0.3, which we demonstrate is consistent with published skeletal Mg/Ca, Sr/Ca, B/Ca, delta B-11, and delta Ca-44 data. Raman measurements are rapid (<= 1 s), high-resolution (<= 1 mu m), precise (derived Omega(Ar) +/- 1 to 2 per spectrum depending on instrument configuration), accurate (+/- 2 if Omega(Ar) < 20), and require minimal sample preparation, making the technique well suited for testing the sensitivity of coral calcifying fluid Omega(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 Omega(Ar) over multiple years of growth in a Porites skeleton from the Great Barrier Reef, and we evaluate the response of Omega(Ar) in juvenile Acropora cultured under elevated CO2 and temperature.