Coral calcifying fluid aragonite saturation states derived from Raman spectroscopy

Quantifying the saturation state of aragonite (ΩAr) within the calcifying fluid of corals is critical for understanding their biomineralisation process and sensitivity to environmental changes including ocean acidification. Recent advances in microscopy, microprobes, and isotope geochemistry allow determination of calcifying fluid pH and [CO32−], but direct quantification of ΩAr (where ΩAr =[CO32−][Ca2+]/Ksp) has proved elusive. Here we test a new technique for deriving ΩAr based on Raman spectroscopy. First, we analysed abiogenic aragonite crystals precipitated under a range of ΩAr from 10 to 34, and found a strong dependence of Raman peak width on ΩAr that was independent 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 ΩAr available for comparison. However, Raman analysis of the international coral standard JCp-1 produced ΩAr of 12.3 ± 0.3, which we demonstrate is consistent with published skeletal Sr/Ca, Mg/Ca, B/Ca, δ44Ca, and δ11B data. Raman measurements are rapid (≤ 1 s), high-resolution (< 1 μm), precise (derived ΩAr ±1 to 2), and require minimal sample preparation; making the technique well suited for testing the sensitivity of coral calcifying fluid ΩAr to ocean acidification and warming using samples from natural and laboratory settings. To demonstrate this, we also show a high-resolution time series of ΩAr over multiple years of growth in a Porites skeleton from the Great Barrier Reef, and we evaluate the response of ΩAr in juvenile Acropora cultured under elevated CO2 and temperature.

DeCarlo T. M., D’Olivo J. P., Foste, T., Holcomb M., Becker T. & McCulloch,M. T., 2017. Coral calcifying fluid aragonite saturation states derived from Raman spectroscopy. Biogeosciences Discussions 1-25. Article.

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