Biogeosciences, 14, 5253–5269, 2017 https://doi.org/10.5194/bg-14-5253-2017 © Author(s) 2017. This work is distributed under the Creative Commons Attribution 3.0 License.

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 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

Correspondence to: Thomas M. DeCarlo ([email protected])

Received: 17 May 2017 – Discussion started: 29 June 2017 Revised: 17 October 2017 – Accepted: 23 October 2017 – Published: 24 November 2017

Abstract. Quantifying the saturation state of aragonite (Ar) warming using samples from natural and laboratory settings. within the calcifying fluid of corals is critical for under- To demonstrate this, we also show a high-resolution time se- standing their biomineralization process and sensitivity to ries of Ar over multiple years of growth in a Porites skeleton environmental changes including ocean acidification. Re- from the Great Barrier Reef, and we evaluate the response of cent advances in microscopy, microprobes, and isotope Ar in juvenile Acropora cultured under elevated CO2 and geochemistry enable the determination of calcifying fluid temperature. 2− pH and [CO3 ], but direct quantification of Ar (where = 2− 2+] Ar [CO3 ][Ca /Ksp) has proved elusive. Here we test a new technique for deriving Ar based on Raman spec- troscopy. First, we analysed abiogenic aragonite crystals pre- 1 Introduction cipitated under a range of Ar from 10 to 34, and we found a The calcium carbonate (CaCO ) skeletons built by coral strong dependence of Raman peak width on Ar with no sig- 3 nificant effects of other factors including pH, Mg/Ca parti- polyps are the building blocks of massive coral reef struc- tioning, and temperature. Validation of our Raman technique tures that protect shorelines, bolster tourism, and host some for corals is difficult because there are presently no direct of the greatest concentrations of biodiversity on the planet measurements of calcifying fluid  available for compari- (Knowlton et al., 2010; Costanza et al., 2014). Critical to Ar 2+ son. However, Raman analysis of the international coral stan- the coral calcification process is the extraction of Ca and CO2− ions from seawater to grow aragonitic CaCO crys- dard JCp-1 produced Ar of 12.3 ± 0.3, which we demon- 3 3 strate is consistent with published skeletal Mg/Ca, Sr/Ca, tals. But corals today live in seawater that is less conducive to B/Ca, δ11B, and δ44Ca data. Raman measurements are rapid CaCO3 nucleation than it was just a century ago. In surface waters of the tropical oceans, carbonate ion concentrations (≤ 1 s), high-resolution (≤ 1 µm), precise (derived Ar ± 1 to 2− ∼ 2 per spectrum depending on instrument configuration), ac- ([CO3 ]) have declined by 15 % since 1900 due to inva- curate ( ±2 if  < 20), and require minimal sample prepa- sion of anthropogenic CO2, which dissociates into carbonic Ar 2− ration, making the technique well suited for testing the sensi- acid and decreases seawater pH and [CO3 ] through a pro- cess referred to as ocean acidification (Caldeira and Wick- tivity of coral calcifying fluid Ar to ocean acidification and ett, 2003; Doney et al., 2009). If anthropogenic CO2 emis-

Published by Copernicus Publications on behalf of the European Geosciences Union. 5254 T. M. DeCarlo et al.: Raman proxy for aragonite saturation state sions continue unabated, by the end of the 21st century the tool for mineral identification (Urmos et al., 1991; Dandeu 2− ∼ [CO3 ] of surface seawater is projected to decline to 50 % et al., 2006; Brahmi et al., 2010; Clode et al., 2011; Nehrke of pre-industrial levels (Hoegh-Guldberg et al., 2014). This et al., 2011; Stock et al., 2012; Foster and Clode, 2016; Sto- rapid change in ocean carbonate chemistry, likely unprece- larski et al., 2016; Roger et al., 2017). Importantly, Raman dented for hundreds of millions of years (Hönisch et al., peaks can also provide information regarding the chemical 2012; Zeebe et al., 2016), has sparked concerns for coral composition of crystals and the conditions of the fluid from growth. Indeed, laboratory experiments repeatedly demon- which they formed. For example, in abiogenic CaCO3, the −1 strate that coral calcification rates decrease in response to shapes and positions of the ν1 peak at ∼ 1085 cm (which 2− 2+ lower [CO3 ] or [Ca ](Gattuso et al., 1998; Chan and Con- represents symmetric stretching of the carbonate C–O bond) nolly, 2013; Comeau et al., 2017), leading to projections that have been correlated with Mg content and/or crystallinity as CO2 levels continue to rise calcification will decline to un- (Bischoff et al., 1985; Wang et al., 2012; Perrin et al., 2016). sustainable levels, such that there is net reef erosion (Hoegh- A highly crystalline CaCO3 with relatively few defects or im- Guldberg et al., 2007; Pandolfi et al., 2011). purities will have a narrow ν1 peak because the C–O bonds Yet signs of resilience do exist. Some coral species are throughout the crystals are of the same, or very similar, able to maintain normal calcification rates across large natu- length (Bischoff et al., 1985). Defects in the crystals and/or ral acidification gradients (Fabricius et al., 2011; Shamberger trace element impurities cause positional disorder of CO3 in et al., 2014; Barkley et al., 2015), indicative of adaptation the lattice. Positional disorder affects the length, and thus the or acclimation (Barkley et al., 2017). One potential mecha- strength and vibrational frequency, of C–O bonds (Bischoff nism to counteract acidification of surrounding seawater is et al., 1985). An increase in disorder leads to an increase in pH homeostasis at the site of calcification (Georgiou et al., the distribution of C–O bond lengths, causing an increase in 2015; Barkley et al., 2017). Corals calcify from an isolated ν1 peak width (Bischoff et al., 1985; Addadi et al., 2003; Lin fluid located between the living tissue and the existing skele- et al., 2007; Wang et al., 2012; McElderry et al., 2013; Perrin ton (Barnes, 1970; Cohen and McConnaughey, 2003; Venn et al., 2016). et al., 2011; Tambutté et al., 2012). Up-regulation of pH In biogenic calcium carbonates, correlations have been re- within this fluid, potentially achieved via proton pumping ported between ν1 peak widths and environmental condi- and/or symbiont photosynthesis, elevates the saturation state tions, including temperature and seawater pCO2 (Kamenos with respect to aragonite (Ar), contributing to rapid nucle- et al., 2013, 2016; Hennige et al., 2015; Pauly et al., 2015). ation and growth of aragonite crystals (Gattuso et al., 1999; These changes potentially reflect differences in calcifying Cohen and McConnaughey, 2003; Al-Horani et al., 2003; fluid carbonate chemistry. Crystals growing from more su- McCulloch et al., 2012; Kubota et al., 2015). persaturated solutions generally have more defects and incor- Characterizing the Ar of the calcifying fluid, and under- porate more impurities (Watson, 2004), resulting in relatively standing its sensitivities to variations in the reef environment, disordered crystal lattices and wide Raman peaks (Urmos is therefore essential for accurately forecasting coral calci- et al., 1991). This hypothesis is generally consistent with em- fication responses to 21st century ocean acidification. Ob- pirical observations of biogenic calcium carbonates, in which serving this fluid has proved difficult though, due to its small ν1 peak widths increase as ambient seawater pCO2 decreases size and isolation beneath the living polyp (Clode and Mar- and/or pH increases (i.e. higher Ar)(Kamenos et al., 2013; shall, 2002). Estimates of fluid carbonate chemistry have so Hennige et al., 2015; Pauly et al., 2015). However, corals ex- far been derived from micro-electrodes, pH-sensitive dyes, ert strong control on the carbonate chemistry of their calcify- boron isotopes, B/Ca, U/Ca, and bulk calcification rates (Al- ing fluid by elevating pH and/or Ar at the site of calcifica- Horani et al., 2003; Trotter et al., 2011; Venn et al., 2011; tion to facilitate more rapid crystal growth (Al-Horani et al., DeCarlo et al., 2015; Cai et al., 2016; Holcomb et al., 2016; 2003; Venn et al., 2011; McCulloch et al., 2012). Thus, the Raybaud et al., 2017). However, these approaches do not al- observed correlations between Raman peak widths and am- ways agree (Ries, 2011; Holcomb et al., 2014), and they have bient seawater conditions likely do not reflect the true sen- so far focused on calcifying fluid carbonate chemistry with- sitivity of aragonite Raman peaks to seawater chemistry, but 2+ out considering the effect of [Ca ] on Ar. rather the sensitivity of calcifying fluid chemistry to external A potential alternative approach to quantify calcifying pH and/or Ar. For this reason, information regarding the fluid Ar is based on the Raman scattering from a laser fo- relationship between carbonate chemistry and Raman peak cused onto the skeleton. When light interacts with a material, width in abiogenic experiments is required to quantitatively a small percentage (typically < 0.0001 %) of the photons are interpret Raman spectra of biogenic CaCO3 with respect to scattered inelastically (referred to as Raman or Stokes scat- the actual calcifying fluid conditions. tering), resulting in a change of energy and frequency (Smith Here, we first evaluate the controls on Raman ν1 peak and Dent, 2005). The frequency shifts associated with Raman width using abiogenically precipitated aragonites. These scattering are characteristic of both the internal vibrations samples allow us to test the sensitivities of ν1 peak width of a molecule and the lattice vibrations between molecules against geochemical composition (e.g. Mg/Ca) and fluid in a crystal, which makes Raman spectroscopy a valuable conditions (including Ar, pH, and temperature) directly,

Biogeosciences, 14, 5253–5269, 2017 www.biogeosciences.net/14/5253/2017/ T. M. DeCarlo et al.: Raman proxy for aragonite saturation state 5255 without the confounding influence of a coral polyp. We de- ity of our results to measurements in other laboratories). All rive a calibration between ν1 peak width and Ar, and apply analyses of coral skeletons described below were conducted it to estimate Ar of the international coral skeleton standard with the WITec instrument using 1 s integration times. JCp-1, for which independent lines of geochemical evidence The Porites coral skeleton standard JCp-1 (Okai et al., allow us to test the accuracy of our approach. Finally, we 2002) was analysed to facilitate comparison between Raman demonstrate the applicability of Raman spectroscopy by (1) results and geochemical estimates of calcifying fluid Ar and 2+ reconstructing multiple years of Ar variability in a Porites [Ca ]. We collected 440 spectra from various grains spread coral collected from the Great Barrier Reef (GBR) (D’Olivo onto a glass slide. Although JCp-1 exists in ground form, we and McCulloch, 2017), and (2) comparing Ar in cultured ju- note that analysis of intact coral skeletons is also possible venile Acropora spicifera exposed to elevated CO2 and tem- (Wall and Nehrke, 2012; Hennige et al., 2014) and that peak perature treatments (Foster et al., 2015). widths apparently do not depend on whether the sample is powdered or intact (Zakaria et al., 2008), as long as the grain size exceeds the laser spot size. 2 Methods We also collected Raman spectra down-core in a Porites skeleton collected from the Great Barrier Reef (the sam- 2.1 Raman measurements ple is described in detail in D’Olivo and McCulloch(2017). Briefly, the core was collected near Havannah Island, an in- Raman spectroscopy was used to analyse the abiogenic arag- shore reef in the central GBR where corals bleached dur- onite precipitates described in DeCarlo et al.(2015) and Hol- ing 1998. The skeleton was cut in a slab and cleaned with comb et al.(2016) (Supplement Table S1). Briefly, aragonite bleach as described by D’Olivo and McCulloch(2017), but was precipitated by addition of Na2CO3 and NaHCO3 so- no additional preparation was required. D’Olivo and McCul- 11 lutions to seawater. Various Na2CO3 / NaHCO3 ratios and loch(2017) present δ B and trace element/Ca ratios from pumping rates produced a range of Ar from 10 to 34 while multiple tracks in the skeleton, and they show anomalies to achieving some independence between Ar and other car- the skeletal geochemistry corresponding to the 1998 thermal 2 bonate system variables (e.g. r between Ar and pH was stress event. We used the automated microscope stage and only 0.34). Most (22 of 28) experiments were conducted at area-mapping features of the WITec instrument to measure 25.5 ◦C, but two experiments were conducted each at 20, 33, Raman spectra in a down-core transect at 50 µm resolution, and 40 ◦C. Two experiments (f08 and g13) were conducted covering approximately 6 years of growth from 1996 to 2002. from seawater with [Ca2+] elevated by addition of dissolved The TrueSurface module was used to construct a topography CaCO3. map of the skeleton that was then followed by the instrument Raman spectra of the abiogenic aragonites were originally to ensure the sample was always in-focus for the Raman mea- reported in DeCarlo et al.(2015) and Holcomb et al.(2016). surements. A maximum depth of 1 mm below the cut surface These initial spectra were collected at Woods Hole Oceano- was set to limit the integration of signal from skeleton of graphic Institution (WHOI) with a Horiba LabRam HR800 different ages. Each time point represents the average of 10 Raman spectrometer using a 785 nm laser source, 40× ob- spectra spaced horizontally in 100 µm increments. jective, 600 mm−1 grating, and CCD detector maintained Finally, we analysed skeletons of the cultured juvenile at −70 ◦C. Spectral resolution was approximately 1.2 cm−1. Acropora described in Foster et al.(2015, 2016). Briefly, Three grains were analysed per experiment with an integra- larval planulae were maintained in “Control” (24 ◦C, 250 ◦ ◦ tion time of 5 s per spectrum. Subsequently, we repeated µatm CO2), “+T” (27 C, 250 µatm CO2), “+CO2” (24 C, ◦ analyses of a subset of these aragonites (samples f02, f03, 900 µatm CO2), and “+CO2 + T ” (27 C, 900 µatm CO2) f06, g07, g13, h09) at the Centre for Microscopy, Charac- treatments. Raman data were previously reported for these terisation, and Analysis (CMCA) at the University of West- corals in Foster and Clode(2016), but those earlier mea- ern Australia. These more refined measurements, which are surements were conducted with a 600 mm−1 grating, which the basis of this study, were performed with a WITec Al- made it difficult to resolve changes in peak width (see Sup- pha300 RA+ confocal Raman microscope using a 785 nm plement). We therefore collected 25 new Raman spectra from laser source, 20× objective with numerical aperture of 0.5, each of three broken skeletal pieces from each of three corals, 1200 mm−1 grating, and an Andor iDUS 401 CCD detector per treatment. The TrueSurface module was used to ensure maintained at −60 ◦C. The nominal spectral resolution was all measurements were taken with the optics well focused on 1.3 cm−1. A silicon chip was analysed to facilitate compar- the samples. ison of peak position with other laboratories (the strong Si peak was present at 522.9 cm−1). For analysis of the abio- 2.2 Calculations and statistics genic aragonites, at least five grains were analysed per ex- periment with 5–10 spectra per grain and integration times For all Raman spectra, we utilized the FWHM (full width at between 0.1 and 5 s (see Supplement for a comparison be- half maximum) intensity of the ν1 peak (Fig. 1). A Gaussian- tween instruments and discussion of the general applicabil- shaped curve was fitted to each spectrum between 1080 and www.biogeosciences.net/14/5253/2017/ Biogeosciences, 14, 5253–5269, 2017 5256 T. M. DeCarlo et al.: Raman proxy for aragonite saturation state

(a) ν1 (b)

Intensity FWHM = 4.24

ΩAr = 34

Measurements

Intensity Gaussian fit T 700 710

L ν4 Intensity FWHM = 3.22

ΩAr = 10

0 200 400 600 800 1000 1200 −1 1078 1080 1082 1084 1086 1088 1090 1092 Raman shift (cm ) Raman shift (cm−1 )

Figure 1. Example Raman spectra and peak width analyses. (a) Raman spectrum of precipitate from experiment f03, with the peaks labelled following White(1974) and Bischoff et al.(1985): T, translation (lattice mode); L, libration (lattice mode); ν4, in-plane bend (internal mode); ν1, symmetric stretch (internal mode). This spectrum is readily identifiable as CaCO3, based on the strong ν1 peak at approximately −1 1085 cm , and distinguished as aragonite rather than other CaCO3 polymorphs (calcite and vaterite) based on the positions of the T, L, −1 and ν4 peaks (Dandeu et al., 2006). The double peak between 700–710 cm is the feature that most clearly distinguishes aragonite from calcite or vaterite (Urmos et al., 1991). (b) Measurement of ν1 peak width. The grey points are the measured intensities at the corresponding Raman wavenumber shift. The black lines show the Gaussian curves fit to the data, and the grey boxes indicate the measured peak widths (ν1 FWHM). Note that measured ν1 FWHM is converted to true ν1 FWHM for analysis in subsequent figures. The y axis scale is in arbitrary intensity units.

1100 cm−1 with the following expression: In the analysis of the abiogenic aragonite, we used the lin- ear model function in R (R Core Team, 2016) to test for re- −(x−p)2 y = b + mx + ke 2s2 , lationships between ν1 FWHM and Ar, aragonite precip- (1) 2− itation rate, aragonite Mg/Ca, fluid [CO3 ], fluid pH, Ra- where y is the fit, x is the Raman shift between 1080 and man peak height (k), and Raman peak position (p). Since −1 1100 cm , b is the background intensity, m is the back- Ar was the single variable most strongly correlated with ν1 ground slope, k is the peak height, p is the peak position (i.e. FWHM, we also evaluated linear models with two explana- wavenumber), and s is the standard deviation. Peak intensity tory variables: Ar and the rest of the variables listed above. varies depending on the smoothness of the surface and the Analysis of variance (ANOVA) was used to test whether focus of the laser. We removed spectra with ν1 peak height multivariate models were significantly better than the model < 30 intensity units (signal : noise of approximately 5) due based on Ar alone. We tested for normality of residuals to the larger uncertainties associated with curves fit to these with Kolmogorov–Smirnov tests and homogeneity of vari- spectra. FWHM is calculated following Weisstein(2017): ances with Levene’s test. p For the cultured Acropora, we calculated the mean Ar of FWHM = 2s 2ln(2) ≈ 2.3548s, (2) each skeletal fragment using the abiogenic ν1 FWHM-Ar calibration, weighting the 25 measurements per fragment by where s is the standard deviation of the Gaussian curve from peak height. We then tested whether the data were signifi- Eq. (1). Measured Raman peak widths are known to be con- cantly different from normal distributions with Kolmogorov– volutions of the “true” peak widths and instrument noise Smirnov tests and homogeneity of variances was checked (Nasdala et al., 2001; Wang et al., 2012; Váczi, 2014). The with Levene’s test. Finding no significant differences from effect of instrument noise on Raman peak widths is directly normal distributions and no significant differences in vari- related to the spectral resolution, which must be accounted ance among treatments, we conducted a two-way ANOVA for when comparing measurements conducted on different and evaluated the effects with Tukey’s honest significant dif- instruments (see Supplement). We used the Nasdala et al. ference test. Statistical significance was defined as p < 0.05. (2001) formula to account for the effect of spectral resolu- tion and calculate true FWHM: 2.3 Mg/Ca partitioning between aragonite and r r seawater b = b 1 − 2( s )2, (3) t m b m We also report here the partitioning of Mg/Ca between these where bt is the true peak width, bm is the measured peak abiogenic aragonites and seawater because (1) previous stud- width, and rs is the spectral resolution. ies have suggested that Mg/Ca is important for interpret-

Biogeosciences, 14, 5253–5269, 2017 www.biogeosciences.net/14/5253/2017/ T. M. DeCarlo et al.: Raman proxy for aragonite saturation state 5257 ing Raman peak widths (Bischoff et al., 1985; Urmos et al., 1991), and (2) Mg/Ca is used as a constraint in our compar- ison of JCp-1 geochemistry and Raman spectroscopy. Parti- tioning of Mg2+ between aragonite and seawater can be de- scribed by an exchange for Ca2+ in the aragonite lattice, with a partition coefficient (KD) expressed as

Mg Mg/Ca = Ca aragonite KD Mg , (4) Ca seawater where KD is dimensionless (Kinsman and Holland, 1969; Gaetani and Cohen, 2006). While there is some uncertainty in the mechanism of Mg2+ incorporation into aragonite (Montagna et al., 2014), the KD nevertheless serves as an em- pirical measure of the distribution of Mg/Ca between arag- onite and seawater under various conditions. DeCarlo et al. Figure 2. Sensitivity of true ν FWHM to  . The x axis rep- (2015) reported K for Sr/Ca from these same samples, and 1 Ar D resents the mean, and the horizontal error bars represent 1 stan- here we follow the same calculations for KMg/Ca using the D dard deviation, of Ar while aragonite precipitated in each exper- Mg/Ca data from Holcomb et al.(2016), except for the slight iment. The crosses indicate individual spectra, the filled points in- modifications to the calculations described below. Briefly, dicate means of each experiment, and the vertical error bars repre- Mg/Ca sent the standard error of replicate spectra. Colours indicate experi- KD was calculated from the measured Mg/Ca in the bulk precipitate (Holcomb et al., 2016), estimates of Mg/Ca ments conducted at various temperatures, and black circles indicate 2+ in the initial fluid (seawater), and modelling the evolution the two experiments conducted with elevated [Ca ]. Grey shad- of elemental concentrations in the fluid through the course ing represents the standard error of the curve fit to the individual spectra, and the dashed grey lines represent the standard error of of each experiment. However, since [Mg] in the initial sea- prediction. water was not reported, we instead calculated initial [Mg] and [Ca] from the established relationships between salin- ity and concentrations of these elements in seawater (Riley and Tongudai, 1967). Our approach also differs from De- nificant variable was Ar (Table 1). Together, this indicates Carlo et al.(2015) in that they combined element /Ca mea- that Ar is likely the variable controlling ν1 FWHM, and that surements of both the final solution and the bulk solid to the correlations between ν1 FWHM and other factors may arise as artefacts of the correlations between those variables calculate KD, whereas we used only the reported aragonite Mg/Ca because final fluid [Mg] was not measured. Two of and Ar. The relationship between Ar and ν1 FWHM is the experiments (f08 and g13) were conducted with initial el- shown in Table 2 for measured and true FWHM on both the emental concentrations modified from seawater by addition Horiba and WITec instruments (see Supplement for alternate calibration where  is fit to ν FWHM). Uncertainty of of dissolved CaCO , and they were excluded from KMg/Ca Ar 1 3 D the calibration propagates to an accuracy of derived  of calculations due to uncertainty of the fluid Mg/Ca ratio. Ar ±∼ 2 for Ar < 20, but increases to ±∼ 6 at Ar = 30 due to both the logarithmic shape of the relationship and the dis- 3 Results tance from the regression centre. The distributions of residu- als were not significantly different from normal distributions In the abiogenic aragonites analysed in this study, ν1 FWHM (Kolmogorov–Smirnov test, p > 0.8). FWHM data for each 2 was strongly correlated with seawater Ar (r = 0.70, p < experiment are listed in Supplement Table S2. 0.001; Fig. 2 and Tables 1–2). Aragonite precipitation rate Some uncertainty exists in defining the Ar associated (G) is a function of Ar and temperature (T ) (Burton and with each Raman spectrum, due to the variability of Ar over Walter, 1987), and thus the experiments conducted at dif- the course of each abiogenic experiment. We conducted a ferent T allow us to isolate the influence of Ar from G. Monte Carlo simulation using the data from the Horiba in- In contrast to the strong dependence of ν1 FWHM on Ar, strument to evaluate the variability in ν1 FWHM that is ex- 4 there was no significant correlation between ν1 FWHM and pected to arise from the uncertainty in Ar. In each of 10 G (Fig. 3; Table 1). The ν1 FWHM was significantly corre- Monte Carlo iterations, we added random error to the mean 2− lated with the aragonite Mg/Ca ratio, fluid [CO3 ], pH, T , Ar of each experiment, based on the standard deviation of and ν1 peak height (Fig. 3; Table 1). However, these correla- measured Ar and assuming a Gaussian distribution. Next, tions were all weaker than the correlation between ν1 FWHM we calculated ν1 FWHM in each iteration based on Ar (in- and Ar, and in multivariate models combining Ar with ei- cluding the random error added) using the equation in Ta- 2− ther Mg/Ca, [CO3 ], pH, T , or ν1 peak height, the only sig- ble 2. The average standard deviation of the Monte Carlo www.biogeosciences.net/14/5253/2017/ Biogeosciences, 14, 5253–5269, 2017 5258 T. M. DeCarlo et al.: Raman proxy for aragonite saturation state

Table 1. Linear model statistics for fits to ν1 FWHM.

Model F valuea p value(s) r2 Residual σ b

ln(Ar) 186.4 < 0.001 0.70 0.165 Mg/Ca 85.79 < 0.001 0.52 0.210 G 0.77 0.383 0.0 0.304 2− CO3 133.9 < 0.001 0.63 0.185 pH 35.46 < 0.001 0.31 0.253 T 6.75 0.011 0.07 0.293 ν1 height 4.50 0.037 0.04 0.297 ν1 position 0.06 0.807 0.02 0.289 ln(Ar) + Mg/Ca 92.97 < 0.001, 0.460 0.70 0.166 ln(Ar) + G 92.02 < 0.001, 0.944 0.70 0.166 + 2− ln(Ar) CO3 95.91 < 0.001, 0.135 0.71 0.164 ln(Ar) + pH 97.43 < 0.001, 0.079 0.71 0.163 ln(Ar) + T 92.18 < 0.001, 0.755 0.70 0.166 ln(Ar) + ν1 height 92.02 < 0.001, 0.954 0.70 0.166

a Notes: none of the multivariate models were significantly different from the Ar model. 1.77 degrees of freedom for univariate models, and 2.76 degrees of freedom for multivariate models; b except for ν1 position, which had 1.59 degrees of freedom. Standard deviation (1σ ) of the ν1 FWHM residuals from the model fit.

Table 2. Regression equations between mean ν1 FWHM and Ar. calibration (Table 2) to estimate calcifying fluid Ar, these seasonal changes in FWHM translate to approximately 3 Ar Calibration Intercept Slope r2 Residual σ units before the 1998 thermal stress event to approximately Horiba measured 1.62 (0.17) 0.70 (0.06) 0.85 0.09 1–2 Ar units afterwards. Horiba true 0.94 (0.19) 0.79 (0.06) 0.85 0.11 Measured ν1 FWHM of the cultured Acropora ranged −1 WITec measured 2.09 (0.14) 0.57 (0.05) 0.96 0.05 from mean of 3.45 (±0.017 standard error) cm for the WITec true 1.35 (0.16) 0.66 (0.06) 0.96 0.06 control treatment to 3.366 (±0.009 standard error) cm−1 for Notes: equations describe ν1 FWHM = slope×ln(Ar) + intercept, where the regressions the “+CO2 + T ” treatment. Derived Ar (from the abiogenic were performed on the mean values of each experiment. Parentheses indicate 1 standard error. calibration) was significantly different among treatments. Tukey’s test revealed significant decreases in Ar as both T (p < 0.001) and CO2 (p = 0.045) increased. There was no significant interactive effect of T and CO . ν1 FWHM residuals (the difference between the predicted 2 −1 Mg/Ca and calculated ν1 FWHM in each iteration) was 0.24 cm , KD determined from the abiogenic aragonites was which is 50 % greater than the observed standard deviation of significantly positively correlated with Ar (Fig. 4; Sup- the residuals (0.16 cm−1). Since the simulated variability ex- plement Table S1; r2 = 0.82 for experiments conducted at ◦ ceeds the observed variability, one or more of the following is 25 C): implied: (1) most (or all) of the scatter in ν1 FWHM around Mg/Ca (0.045 ± 0.004)Ar + 0.31 ± 0.11 the regression line is explained by uncertainty of Ar, (2) = KD . (5) the Raman spectra integrate signal from multiple crystals that 1000 together approximate the mean Ar during each experiment, The key finding of the KMg/Ca data for this study is the and/or (3) variability in Ar is overestimated. Regardless, the D data are consistent with a tight, logarithmic dependence of ν sensitivity to Ar at a single temperature. This dependence of 1 Mg/Ca FWHM on Ar, with scatter in Ar adding uncertainty to the KD on Ar or precipitation rate was predicted by Gae- observed relationship. tani and Cohen(2006) on the basis of the surface entrap- Analysis of 440 spectra collected from various grains of ment model proposed by Watson(2004), and was also ob- JCp-1 with the WITec instrument produced a mean measured served in a recent abiogenic aragonite study (AlKhatib and ± −1 Mg/Ca ν1 FWHM of 3.51 with 1σ of 0.09 cm (true ν1 FWHM Eisenhauer, 2017) (Fig. 4). Although our KD data show of 2.99 ± 0.11 cm−1). Applying the calibration equation in a similar trend to those of AlKhatib and Eisenhauer(2017), Mg/Ca Table 2 to calculate Ar for each of the 440 measurements of their KD are systematically lower. This offset is poten- ν1 FWHM gave a mean derived Ar of 12.3, with a standard tially explained by the different media used in the experi- deviation of 2.1, and a standard error of the mean of 0.3. ments: filtered seawater in DeCarlo et al.(2015) and Hol- In the Havannah Island coral, measured ν1 FWHM comb et al.(2016) as opposed to ammonium carbonate solu- changed seasonally by ∼ 0.15 cm−1. Using our abiogenic tions in AlKhatib and Eisenhauer(2017).

Biogeosciences, 14, 5253–5269, 2017 www.biogeosciences.net/14/5253/2017/ T. M. DeCarlo et al.: Raman proxy for aragonite saturation state 5259

Figure 3. Scatter plots of Raman true peak width (ν1 FWHM) and (a) aragonite Mg/Ca, (b) aragonite precipitation rate, (c) experimental 2− fluid [CO3 ], (d) experimental fluid pH, (e) Raman peak intensity, and (f) Raman peak position. Colours indicate experiments conducted at various temperatures, and black circles indicate the two experiments conducted with elevated [Ca2+]. While some apparent correlations exist, statistical models (Table 1) imply that Ar is the variable controlling ν1 FWHM, and that the patterns observed here may be artefacts of correlations between these factors and Ar.

4 Discussion

4.1 Controls on aragonite ν1 peak width

We conducted Raman spectroscopy analyses of abiogenic aragonites, precipitated under various carbonate chemistry and temperature treatments as previously described in De- Carlo et al.(2015) and Holcomb et al.(2016). Carbonate chemistry was manipulated in these experiments such that there is some independence between the carbonate system parameters, which allows us to isolate their effects on the Ra- man spectral peaks. For example, experiments conducted at the same Ar, but different temperatures, allow us to identify that ν1 FWHM is sensitive to Ar but not to aragonite pre- cipitation rate (G) (Figs. 2 and 3b). Similarly, since Sr/Ca is strongly controlled by temperature in these aragonites (De- Mg/Ca ◦ Figure 4. Sensitivity of KD to Ar at 25 C. Horizontal error Carlo et al., 2015), the lack of a temperature effect on ν1 bars represent 1 standard deviation of Ar while aragonite precipi- FWHM implies that Sr/Ca ratios exert minor or no effects on tated in each experiment. Grey shading represents the standard error ν1 FWHM. Other factors were more difficult to isolate – for of the curve, and the dashed grey lines represent the standard error instance Ar was correlated with both Mg/Casolid and fluid of prediction. The grey squares show the AlKhatib and Eisenhauer [CO2−]. Yet comparisons among statistical models (Table 1) (2017) data replotted against  (see their Fig. 4 for a plot against 3 Ar show that  is the only variable that consistently has a sig- precipitation rate). Ar nificant effect on ν1 FWHM, regardless of which additional variable is considered in multivariate models. This leads us to conclude that ν1 FWHM is primarily dependent upon Ar. We also found that the scattering of data around the regres- sion between ν1 FWHM and Ar can be explained entirely www.biogeosciences.net/14/5253/2017/ Biogeosciences, 14, 5253–5269, 2017 5260 T. M. DeCarlo et al.: Raman proxy for aragonite saturation state by the uncertainty of Ar in each experiment. Further, the variability in these aragonites arises from the sensitivity of absence of any significant correlations between ν1 FWHM Mg/Ca partitioning to Ar (Fig. 4), not manipulation of and either ν1 position or height indicates that the variation of Mg/Ca in the initial fluid. Recognizing this methodological ν1 FWHM in our study was not the result of instrumental or distinction is important for understanding why we did not curve-fitting artefacts. find a correlation between ν1 FWHM and position. Firstly, Our ability to isolate the effect of Ar from Mg/Casolid aragonite has several orders of magnitude less Mg than high- 2− and fluid [CO3 ] is best exemplified by the two experi- Mg calcites, potentially dampening the role of Mg in affect- ments in which [Ca2+] was elevated in the fluids by ad- ing the shape of the Raman peaks in aragonite. Secondly, if dition of dissolved CaCO3. Experiments f08 and g13 were the trend between Ar and ν1 FWHM were driven by Mg 2+ conducted with mean fluid Ar of 12 and 10, and [Ca ] content alone, shifts in Raman peaks to higher wavenumbers of 12.4 and 10.4 mmol kg−1, respectively, compared to the would be expected, because Mg is lighter than Ca (Bischoff mean [Ca2+] range of 6.9–8.7 mmol kg−1 in all other ex- et al., 1985). However, there was no significant correlation 2+ periments. The addition of Ca increased Ar, decreased between ν1 position and Ar (p > 0.7), indicating that the 2− Mg/Ca, and caused precipitation to occur at lower [CO3 ] mean C–O bond vibrational frequency did not change sys- than other experiments, thereby establishing some indepen- tematically with Ar. These observations imply that Mg con- dence among these factors. Over the relatively narrow Ar tent was not the primary source of lattice defects broadening range of 10–12, which includes f08 and g13 as well as two Raman peaks, an interpretation supported by the absence of a 2+ experiments with lower [Ca ], there is still a positive cor- correlation between Mg/Ca and ν1 FWHM (after accounting relation between ν1 FWHM and Ar (Fig. 5a). Conversely, for the effect of Ar; Table 1). The lack of correlation be- there are no clear patterns between ν1 FWHM and either tween ν1 FWHM and position also indicates that ν1 FWHM 2− Mg/Ca or [CO3 ] across these four experiments (Fig. 5b–c). was unaffected by crystallite size (Urmos et al., 1991; Za- While this further demonstrates that Ar is likely the factor karia et al., 2008). directly controlling ν1 FWHM, more experiments designed That fluid Ar, not Mg/Casolid, apparently controls ν1 2− to decouple Ar, Mg/Ca and [CO3 ] will be useful for con- FWHM in aragonite precipitated from seawater is critical straining ν1 FWHM sensitivities. In addition, whether the for interpreting the Raman spectra of coral skeletons. Corals 2+ 2− stoichiometry of [Ca ] and [CO3 ] affects ν1 FWHM will transport seawater to the micro-scale site of calcification be an important question to address in future experiments (Gagnon et al., 2012; Tambutté et al., 2012), and much like in (Nehrke et al., 2007). the abiogenic experiments of DeCarlo et al.(2015) and Hol- The influence of Ar on ν1 FWHM can be explained by comb et al.(2016), they precipitate aragonite crystals from carbonate ion disorder within the aragonite lattice, as origi- a seawater-like solution (McConnaughey, 1989). Changes in 2+ nally hypothesized by Bischoff et al.(1985). Increasing Ar calcifying fluid Mg/Ca can result from Ca transport and/or causes more defects and/or impurities to be incorporated into Rayleigh fractionation (Gaetani and Cohen, 2006; Gagnon the aragonite, increasing the positional disorder of carbon- et al., 2012). Yet Mg/Ca in coral skeletons is generally within ate in the lattice. The absence of a clear Mg/Ca effect on the limits of the abiogenic aragonites analysed here (2.5– −1 ν1 FWHM (discussed further below) suggests that our ob- 11 mmol mol ) and, critically, ν1 FWHM was insensitive served changes in ν1 FWHM are related more to disorder to Mg/Ca over this range. While it is still conceivable that 2− in the lattice (i.e. more amorphous-like aragonites formed at Mg/Ca or [CO3 ] could have some influence on coral ν1 higher Ar) than to trace element impurities. Disordered car- FWHM, these effects are likely subordinate to the Ar sen- bonate ions have a distribution of C–O bond strengths and sitivity based on our abiogenic results. This implies that we vibrational frequencies, which results in wider Raman peaks can interpret Raman spectra of coral skeletons using the abio- (Bischoff et al., 1985; Urmos et al., 1991; Perrin et al., 2016). genic sensitivity of ν1 FWHM to Ar. Raman spectroscopy studies of abiogenic CaCO3 have so far focused on calcite, vaterite, and amorphous CaCO3 4.2 Analysis of JCp-1 and relation to skeletal (ACC) precipitated with wide ranges of Mg/Ca (from 0 to geochemistry 45 mol %) (Bischoff et al., 1985; Wehrmeister et al., 2009; Wang et al., 2012; Perrin et al., 2016). These studies manip- Using Raman spectroscopy, we derived a mean calcifying ulated the Mg/Ca ratios of the solutions from which the pre- fluid Ar for the JCp-1 coral standard of 12.3 ± 0.3. JCp- cipitates formed, and they consistently reported strong corre- 1 is an internationally calibrated geochemical standard, and lations between ν1 FWHM and ν1 position, both of which in- has been analysed repeatedly for trace element and iso- creased with progressively higher Mg/Ca in the solid phase. topic composition. Because the geochemistry of the skele- The abiogenic precipitates used in our study are different in ton reflects the calcifying fluid conditions, we can evalu- that (1) they are aragonite, and (2) most of them (26 out ate the accuracy of our Ar estimates by comparing them of 28) were grown from the same parent solution (seawa- to the reported geochemical properties of the JCp-1 carbon- ter) but under different carbonate chemistry treatments (De- ate. Specifically, here we calculate calcifying fluid calcium 2+ Carlo et al., 2015; Holcomb et al., 2016). Most of the Mg/Ca concentration ([Ca ]cf) using two different sets of proxies

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● ● ● ) ) ) 1 1 1 − − −

● ● ●

● ● ● FWHM (cm FWHM (cm FWHM (cm 1 1 1 ν ν ν ● ● ●

● Seawater [Ca] ● Elevated [Ca] 2.80 2.85 2.90 2.95 3.00 3.05 2.80 2.85 2.90 2.95 3.00 3.05 2.80 2.85 2.90 2.95 3.00 3.05 9 10 11 12 13 0 1 2 3 4 5 6 500 600 700 800 900 1000 −1 2− −1 ΩAr Mg/Ca (mmol mol ) [CO3 ] (µmol kg )

2− 2+ Figure 5. Isolation of Ar, Mg/Ca and [CO3 ] controls on true ν1 FWHM based on manipulation of fluid [Ca ]. The same data are plotted 2+ here as in Figs. 2 and 3, but only over the Ar range of 10–12 because it includes two experiments initiated with seawater [Ca ] (grey) and 2+ 2+ two experiments with elevated [Ca ] (black). With fluid [Ca ] manipulation, ν1 FWHM is still sensitive to Ar (a), but not to Mg/Ca (b) 2− or [CO3 ] (c).

44 – the first based on Mg/Ca, Sr/Ca, and δ Ca, and the sec- 4.3 High-resolution time series of calcifying fluid Ar ond based on Raman, δ11B (a pH proxy), and B/Ca (see Ap- pendix A for details of calculations). Our approach uses ex- perimentally determined partitioning of these trace elements Analysis of the Havannah Island coral demonstrates that Ra- and isotopes between aragonite and seawater, and a simple man spectroscopy can detect variability in coral calcifying 2+ fluid  . The pattern of variability in Raman-derived  model in which [Ca ]cf is initially elevated relative to sea- Ar Ar water (Al-Horani et al., 2003) before precipitation from the showed some similarities, but also some differences, com- isolated calcifying fluid (i.e. a closed system) drives Rayleigh pared to the boron-derived Ar presented in D’Olivo and fractionation (Gaetani and Cohen, 2006; Gaetani et al., 2011; McCulloch(2017). In a sampling track that avoided a 1998 Gagnon et al., 2012). We then compare these two indepen- partial-mortality scar (“path D”), boron-derived Ar showed 2+ annual oscillations of 2–3 units, but one seasonal oscillation dent [Ca ]cf estimates to test whether our Raman results are consistent with JCp-1 geochemistry. was completely missing during 1998/1999 (Fig. 7). Yet in a The published Mg/Ca, Sr/Ca, and δ44Ca of the JCp- track directly adjacent to the scar (“path B”), boron-derived 1 coral can be explained with initial Ca2+ enrichment of Ar showed overall lower values and less variability, com- 9–11 % and precipitation of 48–59 % of the total Ca2+. pared to path D. Raman-derived Ar closely tracked variabil- 2+ −1 ity of path D from 1996 to early 1998, then diverged from the This corresponds to mean [Ca ]cf of 8.2 ± 0.7 mmol kg 2+ boron-derived  between mid-1998 and mid-1999, before (Fig. 6). We then independently estimated [Ca ]cf by com- Ar bining our Raman spectroscopy results with boron systemat- returning to similar seasonal variability by 2000. ics. The δ11B and B/Ca of JCp-1 together imply a calcifying There are several possible reasons for the differences be- 2− ± −1 tween Raman- and boron-derived Ar. The first is that the fluid [CO3 ] of 987 78 µmol kg . To reconcile the boron- 2− ± Raman sampling path was located approximately halfway derived [CO3 ] with the Raman-derived Ar of 12.3 0.3 2+ −1 between boron paths B and D, and thus there could be dif- requires [Ca ]cf of 8.3 ± 0.7 mmol kg . This agrees al- + ferences in  across the skeleton. The boron data indi- most exactly with the mean [Ca2 ] based on the calcula- Ar cf cate lower  near the partial-mortality scar, while the Ra- tions described above for Mg/Ca, Sr/Ca, and δ44Ca system- Ar man data potentially reflect an intermediate level of bleach- atics (Fig. 6d), giving us confidence in the accuracy of our ing stress between the low values of path B and the higher Raman-based Ar results. One implication of these results + values of path D. There is also potential for the existence is that [Ca2 ] is less than seawater by 11–25 %, and while cf of secondary precipitates near the scar, although the Raman this finding should be replicated on additional corals, we note + spectra clearly indicated that only aragonite was present with that depleted [Ca2 ] is consistent with trace element vari- cf no sign of calcite. Second, the Raman measurements have ability in deep-sea and surface-dwelling corals modelled us- higher spatial resolution than the boron measurements, po- ing both “batch” and “flow-through” versions of the Rayleigh tentially allowing us to capture variability not observed in calculations (Gaetani et al., 2011; Gagnon et al., 2012). Fur- the boron sampling paths. Finally, boron-derived estimates ther, our Raman-derived  is generally consistent with es- Ar of  are based on the assumption that [Ca2+] remains timates made on the basis of coral skeleton crystal aspect Ar cf at, or close to, seawater concentrations, and therefore any ratios (Cohen and Holcomb, 2009). 2+ substantial changes in [Ca ]cf will not be recorded in the boron-derived time series. This means that the decoupling www.biogeosciences.net/14/5253/2017/ Biogeosciences, 14, 5253–5269, 2017 5262 T. M. DeCarlo et al.: Raman proxy for aragonite saturation state

Figure 6. Evaluation of Raman-derived Ar based on geochemistry of the JCp-1 coral. In each panel, the x axis f is fraction of total 2+ 2+ [Ca ]cf remaining, and the y axis is initial [Ca ]cf enrichment relative to seawater. Colours indicate calculated (a) skeletal Sr/Ca, (b) 44 2+ skeletal Mg/Ca, (c) skeletal δ Ca, and (d) [Ca ]cf. In (a–c) the grey lines bound the reported values of JCp-1 (±1σ ), and the black polygon 44 2+ shows the parameter space consistent with Sr/Ca, Mg/Ca, and δ Ca. Panel (d) shows [Ca ]cf and the grey lines show the constraints from 2+ panels (a–c). The dashed black line indicates the [Ca ]cf independently estimated based on combining our Raman data with published boron data (see text for details of the calculations). The two approaches are within error (i.e. the dashed black line intersects the solid black polygon) for Raman-derived Ar between 10.8 and 13.7, consistent with the observed Ar of 12.3 ± 0.3.

between Raman- and boron-derived Ar from path D dur- above the maximum monthly mean water temperature) and 2+ ing 1998–1999 could reflect changes in [Ca ]cf (Fig. 7). to elevated CO2 (Fig. 8). In micro-computed tomography D’Olivo and McCulloch(2017) showed that trace element (micro-CT) analyses of corals from the same experiment, ratios (Mg/Ca and Sr/Ca) in path B indicated a strong re- Foster et al.(2016) showed that elevated CO 2 increased 2+ duction in [Ca ]cf during 1998, although a consistent pat- skeletal porosity and decreased total calcification, while el- tern was not observed in path D. Similarly, a recent study evated temperature had the effect of partially mitigating the showed that Mg/Ca and Sr/Ca responses to bleaching in response to elevated CO2. Thus, the effect of elevated CO2 Porites from Western Australia were indicative of reduced on Raman-derived Ar is consistent with the calcification re- 2+ [Ca ]cf (Clarke et al., 2017), consistent with our compari- sponse, but thermal stress decreased Ar without having a son between Raman and boron systematics in the Havannah negative impact on calcification. One possibility is that the 2+ Island coral. Mechanistically, reduced [Ca ]cf could result effect of decreasing Ar on aragonite accretion was balanced from decreases in the renewal rates of calcifying fluid (via by elevated temperature, because aragonite crystal growth 2+ increased Rayleigh fractionation) or decreases in Ca addi- rates increase with both Ar and temperature (Burton and tion to the calcifying fluid. While more studies are needed to Walter, 1987). Further, the physiological response to ele- 2+ test whether reduced [Ca ]cf is a consistent response to ther- vated temperature could be complex, potentially involving mal stress, the ability to detect such changes by using Raman decreases in Ar balanced by increases in calcifying time or and boron systematics in tandem highlights their combined surface area. Finally, Foster et al.(2016) found pitted skele- utility. tal surfaces indicative of dissolution, which may have com- pounded the measured CO2 effects on calcification and led to their dominance over any apparent effect of temperature in 4.4 Effects of temperature and CO2 on Ar of juvenile Acropora the “+CO2 +T ” treatment. A recent study employing a sim- ilar experimental design found that juvenile Acropora B/Ca and U/Ca increased with elevated CO2 (Wu et al., 2017), Our analyses of juvenile Acropora indicate that calcifying 2− ◦ both of which indicate decreases in calcifying fluid [CO3 ] fluid Ar decreased in response to elevated temperature (3 C

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(a) (b) 11.0 r A Ω 10.0

9.0 2 anomaly T T + O + Ar C + O 2 Ω Control C +

Boron path B Figure 8. Raman-derived Ar of Acropora cultured under different −2 −1 0 1 2 1 cm Boron path D temperature and CO treatments. The grey bars indicate treatment Raman 2 means and error bars represent ± standard error of the mean. Sig- 1996 1998 2000 2002 nificant effects were observed for both temperature and CO2.

Figure 7. Time series of boron- and Raman-derived Ar in a coral from Havannah Island. (a) X-ray image of central slab cut from tors such as trace element variability to have some influence, the core. The black line indicates the Raman sampling path. Boron our Raman results from abiogenic aragonites and the JCp-1 paths B and D were located approximately 1 cm into and out of the coral can be explained by Ar alone. Nevertheless, future in- page, respectively (see Fig. 1 in D’Olivo and McCulloch(2017) for vestigations of controls on Raman spectra of corals skeleton details). (b) The red and blue lines show boron-derived Ar assum- 2+ will be useful, for instance in checking for positive correla- ing a constant [Ca ]cf. Raman-derived Ar is plotted in black with grey error bars indicating 1σ . The means of the two data sets (mean tions between ν1 FWHM and wavenumber that would po- tentially indicate an effect of Mg/Ca (Bischoff et al., 1985; of Raman Ar was 9.5) have been removed to facilitate compari- 2+ son and because of uncertainty of absolute [Ca ]cf for the boron Perrin et al., 2016; Borromeo et al., 2017). We found no calculations. such relationships in either our Porites or Acropora analyses (p > 0.05, r2 < 0.01 in both cases). Our approach could potentially be applied to other marine based on abiogenic partitioning (DeCarlo et al., 2015; Hol- calcifiers that build aragonitic shells and skeletons, such as comb et al., 2016). However, while elevated temperature in- sclerosponges and some molluscs. However, the relationship creased δ11B (i.e. increased pH), it did not affect B/Ca or between Raman peak width and  is likely to be mineral- U/Ca (Wu et al., 2017), potentially reflecting the lower de- specific. For example, calcite often contains magnesium with gree of thermal stress (culture temperatures were within the concentrations several orders of magnitude greater than in seasonal range at the collection site) compared to Foster aragonite, which is likely the reason for the wider peaks in 2+ et al.(2015) and/or that [Ca ]cf played an important role in Raman spectra of calcite (Urmos et al., 1991). In contrast, our Raman data. Nevertheless, our results further highlight we have shown that Raman spectroscopy is well suited for that Raman spectroscopy can detect changes in coral calci- analysis of coral calcifying fluid Ar. Our initial applications fying fluid Ar, and that this information is complementary to corals revealed annual cycles of Raman-derived Ar in a to other analyses, including boron systematics (Sect. 4.3) as field-collected Porites and Ar responses to elevated temper- well as CT-derived porosity and calcification. ature and CO2 in cultured Acropora, highlighting that Raman spectroscopy is indeed capable of detecting changes in coral calcifying fluid Ar. Additionally, the high spatial resolution 5 Conclusions and outlook (< 1 µm) and rapid sample processing (≤ 1 s per spectrum) of Raman are currently unrivalled. Further, the same Raman We tested whether Raman spectroscopy records the satura- spectra used to estimate Ar can confirm that the sample is tion state of the fluid from which aragonite precipitates. In aragonite, effectively eliminating the possibility of contam- abiogenic aragonites with known fluid conditions, we found ination from other mineral phases. Finally, our analysis of a clear dependence of Raman peak width on Ar, an ob- JCp-1 suggests that the Raman ν1 FWHM may be an accu- servation that is not confounded by other factors, including rate proxy of fluid Ar, making it a complementary approach 2− 11 temperature, Mg/Ca, pH, and [CO3 ]. In the JCp-1 coral to B/Ca and δ B because combining information from the standard, for which calcifying fluid Ar is not known di- two approaches enables us to calculate the full carbonate sys- 2− 2+ rectly, Raman-derived Ar is consistent with multiple, in- tem via [CO3 ], pH, and possibly now [Ca ]cf. dependent lines of geochemical evidence based on Mg/Ca, The Ar of 12.3 that we derived for JCp-1 corresponds Sr/Ca, B/Ca, δ11B, and δ44Ca. These results support the use to an instantaneous inorganic (Burton and Walter, 1987) of Raman spectroscopy as a proxy for coral calcifying fluid aragonite precipitation rate of 0.2 g cm−2 yr−1. This result is Ar. While it remains theoretically possible for other fac- somewhat surprising because the average annual calcification www.biogeosciences.net/14/5253/2017/ Biogeosciences, 14, 5253–5269, 2017 5264 T. M. DeCarlo et al.: Raman proxy for aragonite saturation state rate of Porites living at 25 ◦C is typically 1.3 g cm−2 yr−1 Code availability. An R code for Raman curve fitting is in- (Lough, 2008), a factor of 6.5 faster than expected on the cluded in the Supplement. Codes for the full analysis is basis of the inorganic calcifying fluid Ar systematics. Since available at https://doi.org/10.24433/CO.ff54fd98-a010-43f5-ad6a- coral calyces have complex 3-D shape such that their skeletal c10c675387fc (DeCarlo, 2017a). surface area is actually much greater than the planar area of the colony surface (Barnes, 1970), this implies that calcifying surface area is an important factor in determining coral cal- Data availability. Raman data are summarized in the Supple- cification rates (D’Olivo and McCulloch, 2017) and it com- ment. Individual Raman spectra are available in the Code Ocean link above and in the Zenodo “Raman spectra of marine calci- plicates the use of bulk calcification rates for estimating  Ar fiers” community (https://zenodo.org/communities/calcifierraman/) (Raybaud et al., 2017). Multiple studies using micro-CT have at https://doi.org/10.5281/zenodo.1035493 (DeCarlo, 2017b). shown increases in porosity and surface-area : volume ratios in corals cultured under elevated CO2 levels (Tambutté et al., 2015; Foster et al., 2016). This may represent a strategy for controlling calcifying surface area (i.e. the area over which crystals are precipitating from the calcifying fluid at a given time) to maintain bulk calcification rates and partially offset the impacts of ocean acidification. In summary, recent studies and technological advances in boron isotope measurements (McCulloch et al., 2014), CT (DeCarlo and Cohen, 2016; Foster et al., 2016) and Raman spectroscopy (this study) are now making it increasingly fea- sible to quantify coral calcification responses with a multi- pronged approach. Future investigations combining Raman spectroscopy with B/Ca, δ11B, and porosity measurements on corals grown under elevated CO2 will be valuable for un- derstanding mechanisms of resilience to ocean acidification. These include pH homeostasis, maintenance of high calcify- 2+ 2− ing fluid Ar by [Ca ] or [CO3 ] regulation, and increases in calcifying surface area as potential strategies for coral cal- cification to persist in a high-CO2 world. Evaluating their relative importance in the face of both increasing reef-water 2− temperature and declining [CO3 ] will be critical to inform- ing our predictions of where and when resilience may be found.

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Appendix A: Details of JCp-1 geochemistry calculations − The JCp-1 coral grew at an average temperature of 25 ◦C 2+ 2+ (1 γ )P [Ca ]cf = [Ca ]seawater − . (A2) (Okai et al., 2002), and the mean salinity in the area is 34.54 kzρ ◦ Sr/Ca (Levitus, 2010). At 25 C, the KD is 1.123 (DeCarlo et al., 2+ −1 Mg/Ca The [Ca ]cf calculated by this method is 8.1 mmol kg , ◦ ∼ ± × − 2015), and at 25 C and Ar of 12, KD is 6.5 ( 0.5) within error of the 8.2 ± 0.7 mmol kg 1 derived from our −4 10 (Fig. 4; in agreement with the mean value of AlKhatib “batch” model calculations above (see Fig. 6). and Eisenhauer(2017) at < 20 and within error of our 2+ Ar Our second approach to calculating [Ca ]cf combines regression). We calculated a range of possible skeletal Sr/Ca Raman spectroscopy with boron systematics. JCp-1 has 2+ and Mg/Ca ratios based on various combinations of Ca a B/Ca ratio of 459.6 ± 22.7 µmol mol−1 (1σ)(Hathorne 2+ pumping (enrichment of initial [Ca ]cf relative to seawa- et al., 2013) and δ11B of 24.3 ± 0.17 ‰ (1σ) (McCul- ter between 0 and 30 %) and Rayleigh fractionation (0– loch et al., 2014). We used the isotope fractionation fac- 2+ 100 % of total Ca precipitated), and we compared the re- tor of Klochko et al. (2006) to determine pH from δ11B −1 sults to the known Sr/Ca (8.838 ± 0.042 mmol mol ) and assuming seawater δ11B of 39.6 ‰ (Foster et al., 2010). Mg/Ca (4.199 ± 0.065 mmol mol−1) ratios of JCp-1 (Fig. 6) Next, we used the parameterization of pKB from Dick- (Hathorne et al., 2013). For δ44Ca, we used the abiogenic − son(1990) to calculate [B(OH) 4 ] from pH, temperature, aragonite fractionation factor (Gussone et al., 2003, 2005), salinity, and assuming [B] equal to seawater (411 µmol 44 seawater δ Ca = 1.88 ‰ (Hippler et al., 2003), Ca added via − B/Ca kg 1 at salinity 34.54) (Allison et al., 2014). The K Ca-ATPase (i.e. [Ca2+] enrichment) of 0.68 ‰ (Inoue et al., D cf of Holcomb et al.(2016) was used to determine [CO 2−] 2015), and an average coral skeleton δ44Ca of 0.73 ± 0.1 (In- 3 from measured B/Ca and calculated [B(OH)−] (com- oue et al., 2015; Chen et al., 2016; Gothmann et al., 2016). 4 puter code available at https://codeocean.com/2017/05/08/ Our calculation scheme utilized the Rayleigh equation boron-systematics-of-aragonite/interface, and see McCul- 2+ i i (1 − f D) loch et al., 2017). We then solve for [Ca ]cf with the fol- = , (A1) lowing formula: j coral j initial fluid 1 − f  44 40 2+ Ar where i/j is Mg/Ca, Sr/Ca, or Ca / Ca, f is the fraction [Ca ]cf = Ksp, (A3) [ 2−] Mg/Ca Sr/Ca CO3 of Ca remaining; D is KD , KD , or α44−40; and it is assumed that Mg and Sr concentrations in the initial fluid where Ar is derived from Raman spectroscopy and Ksp is prior to precipitation are equal to those of seawater. 2+ the solubility product of aragonite in seawater. [Ca ]cf cal- A similar result is achieved when considering the calci- culated in this way is 8.3 ± 0.7 mmol kg−1. fying fluid as semi-closed and using the steady-state “flow- In the first set of calculations with Mg/Ca, Sr/Ca, and through” model of Gagnon et al.(2012). We used the same δ44Ca, we assume only that Ca2+ is added to the fluid, Mg/Ca Sr/Ca KD and KD listed above and the equations of Gagnon whereas in the second set of calculations with Raman spec- et al.(2012) to solve for their “ γ ” term (defined as the ratio troscopy and boron systematics we assume carbonate chem- of calcium pumping to precipitation) that best fit the JCp- istry is modified from seawater. However, since we do not P 2+ 1 data (0.35). Next, we found the unique value of “ kzρ ” directly connect the initial Ca enrichment of the fluid with (3.1 mmol Ca kg−1; where P is precipitation flux, k is sea- the carbonate system calculations, we do not need to make water exchange rate, z is the ratio of calcifying volume to assumptions regarding the stoichiometry of any potential + surface area, and ρ is seawater density) that was consistent proton–Ca2 exchange. with this γ and the JCp-1 Mg/Ca and Sr/Ca. Finally, using these terms matched to JCp-1 geochemistry, we follow the equation given in Gagnon et al.(2012):

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