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Biogeosciences An interactive open-access journal of the European Geosciences Union
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Volume 14, issue 22 | Copyright
Biogeosciences, 14, 5253-5269, 2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 24 Nov 2017

Research article | 24 Nov 2017

Coral calcifying fluid aragonite saturation states derived from Raman spectroscopy

Thomas M. DeCarlo1,2, Juan P. D'Olivo1,2, Taryn Foster3, Michael Holcomb1,2, Thomas Becker4,5, and Malcolm T. McCulloch1,2 Thomas M. DeCarlo et al.
  • 1Oceans Institute and School of Earth Sciences, The University of Western Australia, 35 Stirling Hwy, Crawley 6009, Australia
  • 2ARC Centre of Excellence for Coral Reef Studies, The University of Western Australia, 35 Stirling Hwy, Crawley 6009, Australia
  • 3Australian Institute of Marine Science, Crawley 6009, Australia
  • 4Centre for Microscopy, Characterisation and Analysis, The University of Western Australia, Crawley 6009, Australia
  • 5Department of Chemistry, Curtin Institute of Functional Molecules and Interfaces, Curtin University, GPO Box U1987, Perth 6845, Australia

Abstract. Quantifying the saturation state of aragonite (ΩAr) 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 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 we found a strong dependence of Raman peak width on ΩAr 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 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 Mg∕Ca, Sr∕Ca, B∕Ca, δ11B, and δ44Ca data. Raman measurements are rapid ( ≤ 1s), high-resolution ( ≤ 1µm), precise (derived ΩAr±1 to 2 per spectrum depending on instrument configuration), accurate (±2 if ΩAr < 20), 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.

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We present a new technique to quantify the chemical conditions under which corals build their skeletons by analysing them with lasers at a very fine resolution, down to 1/100th the width of a human hair. Our first applications to laboratory-cultured and wild corals demonstrates the complex interplay among seawater conditions (temperature and acidity), calcifying fluid chemistry, and bulk skeleton accretion, which will define the sensitivity of coral calcification to 21st century climate change.
We present a new technique to quantify the chemical conditions under which corals build their...