ARTICLE in PRESS + MODEL EPSL-08806; No of Pages 16

ARTICLE IN PRESS + MODEL EPSL-08806; No of Pages 16

Earth and Planetary Science Letters xx (2007) xxx–xxx www.elsevier.com/locate/epsl

Sr/Ca and Mg/Ca vital effects correlated with skeletal architecture in a scleractinian deep-sea coral and the role of Rayleigh fractionation ⁎ Alexander C. Gagnon a, , Jess F. Adkins b, Diego P. Fernandez b,1, Laura F. Robinson c

a Division of Chemistry, California Institute of Technology, MC 114-96, Pasadena, CA 91125, USA b Division of Geological and Planetary Sciences, California Institute of Technology, MC 100-23, Pasadena, CA 91125, USA c Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA

Received 29 November 2006; received in revised form 15 May 2007; accepted 3 July 2007

Editor: H. Elderfield

Abstract

Deep-sea corals are a new tool in paleoceanography with the potential to provide century long records of deep ocean change at sub- decadal resolution. Complicating the reconstruction of past deep-sea temperatures, Mg/Ca and Sr/Ca paleothermometers in corals are also influenced by non-environmental factors, termed vital effects. To determine the magnitude, pattern and mechanism of vital effects we measure detailed collocated Sr/Ca and Mg/Ca ratios, using a combination of micromilling and isotope-dilution ICP-MS across skeletal features in recent samples of Desmophyllum dianthus, a scleractinian coral that grows in the near constant environment of the deep-sea. Sr/Ca variability across skeletal features is less than 5% (2σ relative standard deviation) and variability of Sr/Ca within the optically dense central band, composed of small and irregular aragonite crystals, is significantly less than the surrounding skeleton. The mean Sr/Ca of the central band, 10.6±0.1 mmol/mol (2σ standard error), and that of the surrounding skeleton, 10.58±0.09 mmol/ mol, are statistically similar, and agree well with the inorganic aragonite Sr/Ca-temperature relationship at the temperature of coral growth. In the central band, Mg/Ca is greater than 3 mmol/mol, more than twice that of the surrounding skeleton, a general result observed in the relative Mg/Ca ratios of D. dianthus collected from separate oceanographic locations. This large vital effect corresponds to a ∼10 °C signal, when calibrated via surface coral Mg/Ca-temperature relationships, and has the potential to complicate paleoreconstructions. Outside the central band, Mg/Ca ratios increase with decreasing Sr/Ca. We explain the correlated behavior of Mg/ Ca and Sr/Ca outside the central band by Rayleigh fractionation from a closed pool, an explanation that has been proposed elsewhere, but which is tested in this study by a simple and general relationship. We constrain the initial solution and effective partition coefficients for a Rayleigh process consistent with our accurate Metal/Ca measurements. A process other than Rayleigh fractionation influences Mg in the central band and our data constrain a number of possible mechanisms for the precipitation of this aragonite. Understanding the process affecting tracer behavior during coral biomineralization can help us better interpret paleoproxies in biogenic carbonates and lead to an improved deep-sea paleothermometer. © 2007 Elsevier B.V. All rights reserved.

Keywords: biomineralization; paleo-oceanography; deep-sea coral; Mg/Ca; Sr/Ca; thermometry

⁎ Corresponding author. Tel.: +1 626 395 2320; fax: +1 626 683 0621. E-mail addresses: [email protected] (A.C. Gagnon), [email protected] (J.F. Adkins), [email protected] (D.P. Fernandez), [email protected] (L.F. Robinson). 1 Present Address: Department of Geology and Geophysics, University of Utah, 135 S. 1460E, Room 719, Salt Lake City, UT 84112, USA.

Please cite this article as: Gagnon, A.C. et al. Sr/Ca and Mg/Ca vital effects correlated with skeletal architecture in a scleractinian deep-sea coral and the role of Rayleigh fractionation. Earth Planet. Sci. Lett. (2007), doi:10.1016/j.epsl.2007.07.013 ARTICLE IN PRESS

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1. Introduction attributable entirely to vital-effects. Thus, deep-sea corals are an ideal model system for the study of vital Deep-sea corals are a new tool in paleoceanography effects. As an additional advantage, deep-sea corals with several promising characteristics. Their optically grow far from sunlight and lack the photosymbionts banded, aragonitic, and unbioturbated skeletons provide typical of surface corals, enabling calcification to be century long records of deep ocean change with the investigated uncoupled from photosynthesis. Similar potential for sub-decadal resolution. High concentrations patterns of Me/Ca heterogeneity in both surface and of uranium allow for accurate independent calendar ages deep-sea corals imply some commonality in vital effect with ±1%, or better, precision (Cheng et al., 2000). mechanism and suggest results from deep-sea corals Together these features of deep-sea corals provide for a may be applicable to corals in general (Sinclair et al., new archive in paleoclimate with the potential for ice 2006). In this study we investigate the magnitude, core-like resolution in the deep ocean. In an attempt to pattern and mechanism of Sr/Ca and Mg/Ca vital effects utilize this new archive, both Sr/Ca and Mg/Ca in the scleractinian deep-sea coral Desmophyllum temperature proxies, long applied to surface corals dianthus (formerly D. cristagalli) a globally distributed (Smith et al., 1979; Beck et al., 1992; Mitsuguchi et al., species with a large available fossil coral collection. 1996), have been recently investigated in deep-sea corals Vital effects often correlate with skeletal structural (Shirai et al., 2005; Cohen et al., 2006). However, a direct features, relating spatially resolved geochemical measure- temperature relationship in corals from both surface and ments to the biologically controlled process of skeleton deep environments is complicated by the overprint of formation. The skeletons of both surface and deep-sea physiological processes during biomineralization, termed corals are composed of the same basic architectural units. vital effects (Weber and Woodhead, 1972). In transverse petrographic microsections of a deep-sea Most thoroughly investigated in surface corals, Me/ coral, two different crystal morphologies are clearly Ca vital effects are evidenced by: (1) large differences evident by visible light microscopy (Fig. 1). Acicular, or between temperature calibrations for different species needle-like, bundles of crystals radiate from regions of and even between individuals within the same species small-disorganized granular crystals or Centers of (Correge, 2006); (2) differences in temperature sensi- Calcification, COCs. In this study, the term COC refers tivity between corals and inorganically precipitated to interior regions of the coral skeleton typified by small aragonite (Kinsman and Holland, 1969; Mucci et al., irregular or fusiform crystals, following the terminology 1989; Zhong and Mucci, 1989; Dietzel et al., 2004; of Gladfelter (1982, 1983).InD. dianthus COCs combine Correge, 2006; Gaetani and Cohen, 2006); (3) the into a 30–100 μm wide optically dense central band. apparent growth dependence of Me/Ca temperature Crystal morphology is related to the process of skeletal calibrations (de Villiers et al., 1994) including similar growth with the initial precipitation of small fusiform sensitivity to both light and temperature induced crystals followed by the extension of needle-like crystals, changes to growth rate (Reynaud et al., 2006); (4) as hypothesized by Gladfelter (1982) and recently spatial variability of Me/Ca and stable isotope proxies documented in living surface corals (Raz-Bahat et al., across coral skeletal features (Cohen et al., 2001; 2006), although the relative growth rate of these two Allison et al., 2001; Adkins et al., 2003; Allison and crystal forms is still a topic of research. Finch, 2004; Shirai et al., 2005; Cohen et al., 2006; There is growing evidence that Mg/Ca variability is Robinson et al., 2006; Meibom et al., 2006a,b); and (5) strongly correlated with skeletal architecture. A study in differences in Me/Ca-temperature calibrations in the a surface coral shows Mg/Ca doubles in COCs when presence or absence of photosymbionts (Cohen et al., measured by SIMS with a 30 μm spot (Meibom et al., 2002). While Me/Ca temperature proxies continue to 2006a). Smaller scale sampling, at ∼0.4 μm using a provide important information on past climate, quanti- NanoSIMS instrument, reveals small regions where Mg fying and understanding the processes that cause non- increases by an order of magnitude (Meibom et al., temperature related variability in Me/Ca paleotherm- 2006b). Sr/Ca heterogeneity also correlates with skeletal ometers is a major goal of paleoceanography. structure in surface corals, with higher Sr/Ca ratios The growth environment of the deep-sea is a virtually reported in COCs (Cohen et al., 2001; Allison et al., constant “culture medium” during a coral's lifetime, 2001; Allison and Finch, 2004; Meibom et al., 2006a). outside specific regions that experience rapid ventila- In deep-sea corals, Cohen et al. (2006) used SIMS to tion. In properly chosen modern deep-sea coral samples, measure Sr/Ca and Mg/Ca across the thecal wall of the the observed variability of a proxy, isolated from a species Lophelia pertusa, observing increased Mg/Ca known and constant environmental background, is coincident with, and extending beyond, most opaque

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Fig. 1. Coral skeletal architecture. (a) Sampling for petrographic thin sections begins with removal of “pie-shaped” wedges containing at least one exert septa (S1) from the calice in the transverse plane, and subsequent polishing to ∼100 μm thickness. The transverse plane is perpendicular to the body axis, roughly cutting the calice into an ellipse. Analysis in this plane gives the view of looking down the body axis toward the oral surface — as seen in figure inset. (b) Transmitted light photomicrograph, in negative, of thin section 47407A. White regions are optically dense with an obvious central band in the middle of septa S1. (c) Positive transmitted light view of a single septa, the dark, optically dense central band is surrounded by acicular (ac), or needle like, crystals in the outer septa. (d) Crystal morphology, granular (gr) irregular crystals making up the centers of calcification (COCs), and surrounding acicular crystals, is evident in this etched SEM image of a septum. (e) A 10 s of micrometer thick section viewed in positive cross polarized light. In addition to granular crystals, the central band appears to be composed of many sphere like bodies (sp) stacked together. Acicular crystals (ac) show c-axis alignment. bands, with Sr/Ca depleted by a smaller relative amount pattern of tracer heterogeneity suggests skeletal archi- in the same regions. For the deep-sea coral genus Car- tecture is a key factor in understanding vital effects. yophillia, maps of elemental distribution by electro- We examine Mg/Ca and Sr/Ca across skeletal nprobe X-Ray microanalysis suggest Mg and Sr features in D. dianthus using micromilling to sample heterogeneity may correlate with COCs (Shirai et al., with lateral spatial resolution of 10s of micrometers. 2005). In D. dianthus, the target of this study, previous Micromilled samples allow the use of a rapid isotope- work with other proxies show very depleted δ13C and dilution-ICP-MS method to determine precise and δ18O within the central band (Adkins et al., 2003) and a accurate metal ratios. We also use electronprobe X-ray region of low uranium content that follows but extends microanalysis to analyze Mg/Ca heterogeneity at fine beyond the central band (Robinson et al., 2006). The scale. We compare our measurements to inorganic

4 A.C. Gagnon et al. / Earth and Planetary Science Letters xx (2007) xxx–xxx experiments, to previous measurements in deep-sea and understanding of vital-effect mechanisms during calcifi- surface corals, and interpret our results within the con- cation to help better interpret Me/Ca temperature proxies. text of models for vital effect mechanism. Geochemical modeling of biogenic carbonates 2. Materials and methods attempts to explain tracer abundance, tracer heterogeneity, and correlations between tracers using chemical and 2.1. Coral samples physical principles within a biological framework. McConnaughey (1989a,b) explained correlated δ13C Five individuals of recent D. dianthus were loaned and δ18O results in corals with an elegant model for from The Smithsonian Institution National Museum of isotope behavior paving the way for the systematic study Natural History (Fig. 2). Two individuals from the same of vital effects. More recently, an equilibrium model of South Pacific location, Smithsonian Collection number stable isotope vital effects was proposed to explain 47407, are the focus of most of this study. In situ tem- observed departures from a linear correlation between perature at the location and depth of 47407 is estimated δ13C and δ18O in corals (Adkins et al., 2003). Several to be 3.6±0.6 °C from the Levitus 1994 dataset (Levitus groups have expanded the geochemical approach of and Boyer, 1994). combined measurements and models to study Me/Ca vital effects in biogenic carbonates (Elderfield et al., 2.2. Micromilling 1996; Cohen et al., 2001, 2002; Russell et al., 2004; Sinclair, 2005; Bentov and Erez, 2006; Sinclair and Risk, Exert septa of two individuals from the same South 2006). Rather than develop a complete numerical model Pacific location were selected for obvious and wide of biomineralization to complement our measurements, central bands (samples 47407A and 47407B). Wedges we attempt to test for the presence of an important of coral are removed to allow analysis in the transverse parameter in most vital effect models, the “openness” or plane — equivalent to a top view down the body axis. “closedness” of the calcifying system to metal ions, and The corals are then mounted on glass slides with epoxy, the implied mechanism of Rayleigh fractionation. ground to a thickness of 200–400 μmona30μm Most models of biomineralization involve a privileged diamond wheel, sonicated without further polishing, and space closed to the external environment to some extent, imaged (background grey-scale images in Fig. 3). sometimes termed the calcifying fluid or extra-cellular A computer controlled micromill (Merchantek) is used fluid, where a coral isolates seawater and chemically to sub-sample parallel with banding to a depth of drives precipitation. Anytime there is a closed system and ∼200 μm, starting from one side of the septa and a tracer is discriminated against or preferentially incor- moving across. We believe growth in the transverse porated during coprecipitation, Rayleigh fractionation plane outward from the central band represents a growth will occur. Even without a complete understanding of the axis, with the oldest material deposited as COCs in the underlying processes that determine this partitioning, a center of the septa. However, the coral grew in near Rayleigh process is predicted and should be considered. constant conditions with largely time-invariant environ- Rayleigh fractionation has been applied previously to mental parameters, and the sampling method is designed explain single element Me/Ca behavior in foraminifera to clearly separate different skeletal regions rather than (Elderfield et al., 1996), and has been recently incorpo- to follow a particular growth axis. Dashed vertical lines rated as a key mechanism along with calcifying solution in Fig. 3 mark successively milled regions and corre- manipulation and temperature effects to explain Me/Ca spond to individual Me/Ca data points. The sup- variability in a surface (Gaetani and Cohen, 2006)and plemental information (S-1) contains a complete deep-sea coral (Cohen et al., 2006). description of sampling locations. For each data point Rayleigh fractionation predicts a quantitative relation- ∼100 μg of milled powder is dissolved in 5% trace ship between different Me/Ca ratios in an expression metal clean HNO3 (Seastar). previously developed to understand the evolution of magma melt-systems (Pearce, 1978; Albarède, 1995). 2.3. Isotope-dilution mass spectrometry Correlated Mg/Ca and Sr/Ca data from micromilled samples are used to test the role of Rayleigh fractionation The isotope dilution ICP-MS method used in this in Me/Ca vital effects across skeletal structural features study allows rapid and accurate determination of Mg/Ca and place this mechanism in the context of other tracer and Sr/Ca ratios. Unlike previous approaches, like vital effects. By examining a key component of many the work of Lea and Martin (1996) which uses a biomineralization models, our work aims to improve the combination of internal and external standardizations to

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Fig. 2. Collection locations of the recent deep-sea coral D. dianthus samples used in this study. Most of the analyses presented in this paper focus on two individuals from South Pacific location 47407. determine Me/Ca ratios, our method is a complete 11 measurements of the consistency standard resulted in isotope dilution method, as will be described in detail in Mg/Ca and Sr/Ca ratios that varied by1.3% and 2.1% a separate paper. Briefly, a combined 25Mg–43Ca–87Sr respectively (2σ standard deviation). As part of prelim- enriched spike is added to a subaliquot of each sample. inary work, three other individuals of coral were analyzed With a combined spike, Me/Ca ratios are determined using an identical method except with an uncalibrated accurately without an exact knowledge of the spike spike, yielding less precise relative Mg/Ca ratios. volume, simplifying analysis and limiting uncertainty. Isotope ratios are measured in analog mode on a 2.4. Electronprobe microanalysis ThermoFinnigan Element, a single collector magnetic sector ICP-MS. The 24Mg/25Mg and 48Ca/43Ca ratios Using a JEOL JXA-8200 electron probe X-ray are corrected offline for the interference of the doubly microanalyzer, Ca, Mg and Sr abundances were measured charged species 48Ca++ and 86Sr++. Measurement of a in a separate polished septal piece of a coral also collected combined Mg–Ca–Sr matrix matched standard of at Smithsonian sample location 47407. Multiple “lines” are known isotopic ratio before and after each sample is measured perpendicular to banding in wavelength disper- used to correct for instrumental mass fractionation. Mg/ sive mode (WDS) with a 15 keVelectron beam at a current Ca and Sr/Ca metal ratios are calculated using the of 30 nA focused to a 10 μm spot. Total measurement time isotope dilution relationship and assuming natural per spot is 90 s, including two backgrounds. Calcite, isotopic abundance of Mg, Ca and Sr in the coral. The strontianite and dolomite are used as standards and ZAF 16 sub-samples of 47407B were analyzed twice each matrix correction is applied. Due to its low abundance in over roughly a 30 h period. For a few hours near the deep-sea coral (∼2 mmol/mol), magnesium measurements middle of analysis, instrumental mass fractionation are very close to the detection limit of our method. On any drifted dramatically. Even though replicate samples given day, it is not always possible to measure the Mg peak. measured both during and outside the period of high Sr is more abundant than Mg in our coral (∼10 mmol/mol) drift compared well, all data collected during the high and therefore easier to quantify with the electron probe. drift period were disregarded (9 of 36 measurements). Mg/Ca and Sr/Ca measurements for each sample are 3. Results tabulated in the supplemental information (S-2). The reproducibility (external error) of our method is Mg/Ca is over twice as high in the optically dense assessed by the repeated measurement of a dissolved central band, as compared to the surrounding septa deep-sea coral consistency standard. Over three months, (Fig. 3a–b). The outer septal region is characterized by a

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Fig. 3. Isotope-dilution ICP-MS measurements of Mg/Ca and Sr/Ca across skeletal features in two D. dianthus septa. (a and b) Mg/Ca of microsamples from septa 47407B and 47407A, respectively, overlaid on negative transmitted light photomicrograph to show sampling locations in relation to skeletal structural features. Mg/Ca ratios double coincident with the optically dense central band. On this scale, ±2σ error bars are smaller than the line thickness. (c and d) Sr/Ca of microsamples across 47407B and 47407A, respectively, with smaller and less structured variability than Mg/Ca. Notice that the range of Sr/Ca is much smaller relative to variations in Mg/Ca. Error bars represent ±2σ external error. Septa are approximately 1.5 mm across. The optically dense central white band, corresponding to COCs, is ∼110 μm across. See supplemental information (Tables S-2a and S-2b) for metal ratio data.

Mg/Ca of ∼1.5 mmol/mol while the central band is varies significantly less than the surrounding skeleton greater than 3 mmol/mol. The relative Mg/Ca ratios of (F-test, N98% loc, n=23 and 8) with a mean value in the three other D. dianthus individuals collected from central band of 10.6±0.1 (2σ standard error of the separate locations are also enriched up to two-fold mean). The standard deviation of Sr/Ca in the central within the central band (Fig. 4). Different amounts of band is similar to methodological external error and may smoothing, the incorporation of both central band and represent instrumental variability rather than the lower outer septal material in the same milling line, may limit of coral Sr/Ca variability. explain differences in the relative enrichment between Electron probe results, although qualitative, agree with each coral. Outside the central band, Mg/Ca variability milling, showing magnesium enrichment across four is ∼18% (2σ standard deviation). Also in this region, central bands (Fig. 5). Volcano like structures, up to Mg/Ca ratios increase with decreasing Sr/Ca ratios. 20 μm in size, grow during sampling with the electron Unlike Mg/Ca, Sr/Ca ratios show less structure and probe. Similar observations, attributed to beam damage, vary by a smaller relative amount across the coral septa, have been imaged and described for other biogenic with a standard deviation of less than 5% (2σ, Fig. 3c– carbonates (Gunn et al., 1992). Defocusing the beam and d). The average Sr/Ca of the two corals 47407A and reducing beam intensity failed to eliminate the volcano like 47407B, 10.56 and 10.62 mmol/mol, respectively, are structures. In the future, use of a more thermally conductive statistically identical, given our analytical uncertainty. coating such as gold or silver (Smith, 1986) may diminish Comparing all data by skeletal region rather than be- beam damage. In an experiment to determine the effect tween individual corals, Sr/Ca within the central band of beam damage, we monitored Ca and Sr counts every

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Fig. 4. Relative Mg/Ca ratios in several corals in relation to structural features. (a) Coral 48739, again showing a nearly 2-fold increase in Mg/Ca coincident with the central band. A region of optically dense material removed prior to milling is marked with a large “X”. (b and c) Relative Mg/Ca in coral 48738 and 85080 also increase in the central band. Although the relative increase in Mg/Ca is less than two-fold in these two corals, the consistent observation of increased Mg/Ca in the central band was observed in all the corals examined in this study. Relative Mg/Ca ratios were determined in reference to a particular microsample within the septa identified as a upside-down triangle in each plot. Error bars noted by black vertical lines.

5 seconds for the duration of a typical collection. Counts coral data are typically compared to inorganic experi- drift by less than 4% as beam damage occurs. While ments with kinetic or other effects which may or may limiting the quantitative power of the electron probe, this not be present during coral precipitation. Determining a magnitude of drift should not obscure the mapping of high valid inorganic reference frame to compare with coral is Mg regions assuming Mg and Sr drift are comparable. a topic of ongoing research. Unfortunately, the small relative variation of Sr/Ca in the Fortunately, there is general agreement, over a range central band, as measured by ICP-MS, is the roughly the of experimental conditions and methodologies, between same magnitude as beam damage effects, limiting the use most studies on the partition coefficient of Sr for in- of the electron probe method to determine Sr/Ca organic aragonite as a function of temperature (Kinsman heterogeneity, results whicharedominatedbynoiseand and Holland, 1969; Mucci et al., 1989; Zhong and not included here. Mucci, 1989; Dietzel et al., 2004; Gaetani and Cohen, 2006), where the partition coefficient from a solution of 4. Discussion constant composition is defined as: 4.1. Magnitude and pattern of Me/Ca vital effects and arag ¼ Sr Sr ð Þ DSr = 1 the inorganic reference frame Ca aragonite Ca seawater

To quantify the absolute magnitude of vital effects, As thoroughly discussed by Gaetani and Cohen the partitioning of elements between seawater and coral (2006), the low partition coefficient of Mucci et al. should ideally be compared to the thermodynamic (1989), by 16% in comparison to the consensus from partitioning of elements between seawater and abiotic other studies, may be the result of kinetic effects. A major aragonite, and then to kinetic effects. Unfortunately, control of precipitation rate in inorganic systems is the + 2− reaching thermodynamic equilibrium in precipitation saturation state of the solution: (Ω=[Ca2][CO3 ]/ksp). experiments has proven difficult; see, for example, the Mucci et al. precipitated aragonite from constant discussion in Gaetani and Cohen (2006). Therefore, composition solutions with saturation states ranging

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Fig. 5. Mg/Ca increases in optically dense bands as determined by electron probe X-ray microanalysis in a thin section of D. dianthus from Smithsonian location 47407. White horizontal lines mark path of analysis.

between 2.3 and 5. This is significantly lower than the Ω minimal vital effects for Sr/Ca in this region, while the of N20 in Kinsman and Holland (1969), Ω between 10 variability in the outer septa suggests the influence of and 31 for Dietzel et al. (2004), and Ω between 40 and 90 vital effects. Similar mean Sr/Ca values in both the in Gaetani and Cohen (2006). It is unlikely that coral central band and outer septa of D. dianthus are in contrast reach the high saturation states found in most of the to the results of Cohen et al. (2006) for another species of above mentioned inorganic experiments; possibly mak- deep-sea coral, L. pertusa, where opaque bands in the ing the data of Mucci et al. more applicable to theca are associated with decreased Sr/Ca relative to the understanding coral vital effects. However, it is also surrounding skeleton. In surface corals, Cohen et al. possible that effects other than kinetics may cause the (2001) explain both differences in Sr/Ca variability difference between Mucci et al. and the other studies. In between the COCs and the surrounding skeleton as well fact, Mucci et al. and Zhong and Mucci (1989) report the as different mean Sr/Ca ratios for these skeletal regions by partition coefficient for strontium is independent of the influence of photosynthesis. In an elegant test of this precipitation rate over the factor of 14 range in preci- hypothesis, Cohen et al. (2002) observed higher Sr/Ca pitation rates explored by their study. Gaetani and Cohen variability outside of COCs in a symbiont containing recognize the results of Mucci et al. and Zhong and coral as compared to an aposymbiont individual of the Mucci, but conclude that the range of low precipitation same species. We also observe higher Sr/Ca variability rates explored in that study is too small to see the outside COCs, but unlike the surface coral data we see differences obvious between Mucci et al. and the high Ω similar means. Our data suggests that some of the studies. Here, we choose to use the larger and more difference in Sr/Ca variability between the COCs and complete dataset of high Ω partition coefficients for Sr to the surrounding skeleton may be ubiquitous in corals both compare with deep-sea corals, although future work to with and without photosymbionts. arag determine the DSr at slow precipitation rates over a Unlike Sr/Ca, there is a two-fold difference in Mg/Ca range of temperatures is certainly necessary. Using the between the central band and outer septa of D. dianthus, combined empirical data of Kinsman and Holland following a similar pattern to skeletal features found in (1969), Gaetani and Cohen (2006), and Dietzel et al. surface corals (Meibom et al., 2006a,b), a commonality (2004) and assuming a Sr/Caseawter of 8.6 mmol/mol (de noted by Sinclair et al. (2006). As the deep-sea corals in Villiers, 1999), the Sr/Ca of inorganic aragonite can be this study grew in near constant environmental condi- predicted as a function of temperature (solid line and 2σ tions, the spatially structured 1.5 mmol/mol variation in error envelope in Fig. 6). Mg/Ca is a clear indication of vital effects and places a The range of Sr/Ca measured from coral microsam- lower limit on the magnitude of these effects. The ples both within and outside the central band agree with average Mg/Ca of D. dianthus compares well to the the expected Sr/Ca of inorganic aragonite precipitated at range of Mg/Ca found by Shirai et al. (2005) from bulk the temperature of coral growth, qualified by the error sampling of two different genus of deep-sea coral; but envelope of the inorganic relationship. Sr/Ca variability the results of Cohen et al. (2006) in Lopehelia pertusa in the central band is smaller and statistically different are significantly higher, ranging from 2.6 to 4.3 mmol/ from that of the outer septa. Inorganic behavior and low mol. Cohen et al. (2006), measure smaller spot sizes variability in the central band are consistent with than our method, and the larger range in Mg/Ca they

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Fig. 6. Sr/Ca of D. dianthus compares well with inorganic aragonite. Black box marks total range of all D. dianthus samples where width represents estimated uncertainty in growth temperature. Gray box is smaller range of the optically dense central band. Solid line is an error weighted regression of the combined data from Kinsman and Holland (1969) (diamonds), Dietzel et al. (2004) (squares), and Gaetani and Cohen (2006) (circles) assuming a Sr/Ca of seawater of 8.6 mmol/mol. Dashed lines mark the 2σ error envelope of the regression. The slope of this inorganic relationship is 44±6 (μmol Sr) (mol Ca)− 1 °C− 1. The data of Mucci et al. (1989) is also plotted. For the experiments of Kinsman and Holland and Dietzel et al., the largest source of error is the variability between replicate precipitation experiments conducted at the same temperature, represented by 2σ error bars. In Gaetani and Cohen each temperature represents a single precipitation experiment. Since Gaetani and Cohen and Kinsman and Holland used similar methods, the average error of Kinsman and Holland was used to estimate the replicate error of Gaetani and Cohen for the purpose of the weighted linear regression. observe may represent skeletal variability with less organic results of Gaetani and Cohen (2006) suggest spatial averaging than our method; however, the higher either that there is a large vital effect of an absolute offset mean Mg/Ca measured by Cohen et al. (2006) as in Mg/Ca between corals and inorganic aragonite or that compared to Shirai et al. (2005) and this study cannot be the conditions of the inorganic experiment are not a valid explained by averaging and may represent differences reference frame for coral growth. The only other across species of deep-sea coral or differences in growth systematic study on magnesium partitioning in aragonite, environments between the studies. the inorganic experiments of Zhong and Mucci (1989), It is difficult to determine the correct inorganic find a Mg/Ca of 3.5 mmol/mol at the one temperature reference frame for magnesium in aragonite. Gaetani investigated, 25 °C, roughly half the result of Gaetani and and Cohen (2006) were able to establish a clear temper- Cohen (2006). Unlike the inorganic behavior of Sr/Ca, a ature relationship for Mg/Ca in aragonite, an important consensus of supporting studies over a range of experi- and significant achievement. However, assuming a mental conditions does not exist for Mg/Ca. The large seawater Mg/Ca of 5.1 mol/mol, the data of Gaetani and difference between Zhong and Mucci and Gaetani and Cohen predict aragonite should have a Mg/Ca ratio of Cohen suggest further research on the coprecipitation of ∼10 mmol/mol compared to the range of 1.5 to 3 mmol/ Mg in aragonite over a range of experimental conditions is mol found in D. dianthus and 4 to 5 mmol/mol Mg/Ca a high priority towards understanding inorganic behavior, ratios typical of surface corals. To explain the Mg/Ca and by extension, vital effects. ratios observed in the surface coral Diploria and in the deep-sea coral L. pertusa consistent with these high 4.2. Correlated Me/Ca ratios and Rayleigh inorganic Mg/Ca results, Gaetani and Cohen (2006) and fractionation Cohen et al. (2006) invoke a 3-fold to 8-fold reduction of Mg/Ca in the calcifying fluid with respect to seawater. The correlated variability of multiple tracers can be a Although there are no direct measurements of Mg/Ca in strong constraint on vital effect mechanism, as recog- the calcifying fluid, the energetic cost of manipulating nized and exploited by Sinclair (2005) in surface corals Mg/Ca by such a large amount seems prohibitive without and more recently by Sinclair et al. (2006) for Mg/Ca a clear biological purpose (Zeebe and Sanyal, 2002). If and U/Ca in deep-sea corals. A tracer–tracer plot of Sr/ Mg/Ca is not manipulated by a large amount, the in- Ca vs. Mg/Ca in D. dianthus using the combined data

10 A.C. Gagnon et al. / Earth and Planetary Science Letters xx (2007) xxx–xxx sets of coral 47407A and 47407B (Fig. 7) shows two follow Eq. (4). Testing if correlated tracer behavior trends: (1) outside the central band, Sr/Ca decreases follows the functional form of Eq. (4) is a general test for with increasing Mg/Ca in a relationship that is roughly Rayleigh behavior that does not require knowledge of linear but may show some curvature, and (2) inside the initial solution conditions or partition coefficients. In the central band, corresponding to the centers calcification outer septal region of D. dianthus, the log–log tracer (COCs), variable and enriched Mg/Ca is associated with plot is linear, with an R2 of ∼0.6, (Solid line, inset constant Sr/Ca. We test for the presence of a Rayleigh Fig. 7), consistent with a Rayleigh mechanism. Equally process in these skeletal regions. important, the behavior of Mg/Ca and Sr/Ca in the Rayleigh fractionation as applied to individual trace central band cannot be explained by Rayleigh fraction- metal behavior has been discussed thoroughly elsewhere ation. In the deep-sea coral L. pertusa, Cohen et al. (Mcintire, 1963; Elderfield et al., 1996). Models of coral (2006) also observe decreasing Sr/Ca with Mg/Ca which trace element variability incorporating Rayleigh frac- they explain by a model emphasizing Rayleigh tionation as a key mechanism have also been recently fractionation. Indeed, while the plot of tracer–tracer proposed (Gaetani and Cohen, 2006; Cohen et al., 2006). behavior in Cohen et al. (2006) is roughly linear it may Here we develop a simple and general relationship for show some curvature toward low Sr/Ca and high Mg/ multiple Me/Ca ratio behavior during Rayleigh fraction- Ca, as predicted by Eq. (4). Recently, Sinclair et al. ation, similar to the approach of Albarède (1995), and (2006) showed that relative U/Ca and Mg/Ca ratios from compare this to our data set. both surface and deep-sea corals follow a power law Following Elderfield et al. (1996), the Sr/Ca of relationship and explained this behavior with end- aragonite precipitated at any point during a Rayleigh member mixing. As an alternative explanation to process from a closed solution can be calculated from endmember mixing, a Rayleigh process also predicts a the initial solution composition assuming a constant power law relationship between U/Ca and Mg/Ca, and effective partition coefficient: may explain some of the correlated observations. In Eq. (4) the slope of the log–log relationship is Sr Coral Sr DCoral1 ¼ D F Sr ð2Þ determined by the partition coefficients of each metal Ca Sr Ca coral SolO and the intercept is influenced by both the initial solution composition and the partition coefficients. The where the extent of precipitation is defined as: slope in Eq. (4) is predicted to be relatively less sensitive Coral Coral Coral Ca to variations in DMg than DSr , since DMg is ≪1, Fu ð3Þ Ca driving the denominator of the slope very close to 1 even o sol Coral over a large relative range of DMg . The low relative Coral Coral The effective partition coefficient, DSr relates the sensitivity of a Rayleigh process to variations in DMg Sr/Ca of the coral skeleton to the surrounding seawater has two implications, (1) the tracer–tracer behavior and is an empirical measure that integrates the thermo- predicted by a Rayleigh process is robust to small Coral dynamics of coprecipitation, kinetics, binding by variations in DMg and (2) it is difficult to determine a Coral organic molecules and all other process acting on Sr precise DMg by inverting the fitted slope and using an Coral under the specific growth conditions of the coral (Morse estimate of DSr . and Bender, 1990). Mg/Ca can be described similarly to Instead of inverting partition coefficients from a fit of Coral – Eq. (2), where Sr/Ca is replaced by Mg/Ca and DSr by the log log relationship, a predicted Rayleigh relationship Coral – DMg . Since Mg/Ca and Sr/Ca are linked by the extent for tracer tracer behavior can also be generated from of precipitation, F, the expressions can be combined, prescribed effective partition coefficients and initial eliminating F, yielding a linear log–log relationship: solution composition. For this forward modeling, we use an initial Mg/Ca ratio matching the seawater value of Sr ¼ DSr 1 Mg 5.1 mol/mol. We also make the functional assumption that ln ln Ca DMg 1 Ca the lowest Mg/Ca ratios measured in our corals correspond Sr D 1 Mg þ ln Sr ln to precipitation from the initial unfractionated solution Coral −4 Ca O DMg 1 Ca O (F=1), implying a DMg of 2.75×10 . We choose this ð4Þ partition coefficient because it describes our coral measurements well and does not imply a dramatic modification of Under the conditions of closed system precipitation the Mg/Ca of the calcifying fluid; however, it is sig- with constant partition coefficients and any initial fluid nificantly lower than the partition coefficient for Mg in composition, tracer–tracer behavior is predicted to inorganic aragonite as determined by Gaetani and Cohen

A.C. Gagnon et al. / Earth and Planetary Science Letters xx (2007) xxx–xxx 11

Fig. 7. Correlated Sr/Ca–Mg/Ca vital effects are consistent with Rayleigh fractionation in the outer septa, but not in the central band. Data points with 2σ error bars are measurements from both corals 47407A and 47407B, identified by skeletal location. Inset is a log–log plot of outer septa results with a linear model (bold black line) fit to the data (R2 =0.6). The fit parameters of the linear model in the inset were used to plot the Rayleigh Model in the larger plot (bold black line labeled “Rayleigh Fit”). As an alternative to fitting the data, Rayleigh behavior can be forward modeled with prescribed Coral parameters. In one scenario, initial Mg/Ca is set to a seawater value; initial Sr/Ca is enriched by ∼3% compared to seawater; DMg is set to − 4 Coral 2.75×10 , consistent with the lowest Mg/Ca measurements; and DSr is set to equal the partition coefficient of inorganic aragonite, 1.24±0.4. The results of this calculation are shown as the grey solid line in the main plot with the dashed error envelope representing a propagation of uncertainty in the inorganic strontium partition coefficient. While most of the discussion in this paper assumes that we sample instantaneous Rayleigh precipitant, the grey solid and dashed lines in the inset are the predicted composition of both instantaneous and accumulated solid, respectively, for the above forward Rayleigh model scenario, with relatively tight agreement.

Coral (2006). As described above, uncertainty in DMg has a whole plot vertically in log–log space, but otherwise small effect on the slope of the tracer–tracer relationship, leaving the slope and sign of the tracer–tracer correlation making our forward model conclusions relatively insensi- unchanged. In a second scenario, initial Sr/Ca is assumed tive to this functional assumption. to match seawater. In this case, our data are consistent Coral Given the above assumptions, two Rayleigh fraction- with a DSr in the outer septa of 1.28, within 2σ of the Coral ation scenarios can explain the observed Mg/Ca vs. Sr/ inorganic relationship but different from the DSr of the Ca relationship of the outer septa. If the abiotic aragonite central band. Our data support the presence of Rayleigh Arag partition coefficient, DSr , which matches the partition fractionation in the outer septa but are unable to coefficient of the central band, is also taken as an distinguish between these two scenarios. Coral estimate for DSr in the outer septa, then the correlated The above expressions ((1)–(4)) describe the com- data are consistent with a Rayleigh process and a non- position of instantaneous precipitate, or the most seawater initial Sr/Ca ratio of 8.8 mmol/mol, an recently formed precipitate at a given F. As it is unclear enrichment of less than 3% (solid grey line and dashed whether our analytical method samples instantaneous or error bounds Fig. 7). Evidence of active Sr pumping accumulated solid, both must be considered. The exists from cultured surface corals in a study with individual Me/Ca ratio of accumulated precipitate, chemical inhibitors, Ferrier-Pages et al. (2002), al- integrated from the start until a given extent of pre- though, as noted by Sinclair and Risk (2006), the results cipitation, (from 1 to F) can be derived (Doerner and of this study are also consistent with general inhibition of Hoskins, 1925; Elderfield et al., 1996). Even though the calcification and may simply evidence Sr coprecipitation combined tracer–tracer relationship for accumulated rather than active Sr pumping. If Sr pumping is present, solid is non-linear in a log–log plot, Me/Ca correlations this first Rayleigh scenario is consistent with an initial are nearly identical between accumulated and instanta- manipulation of the calcifying fluid followed by closed neous solids using the same partition coefficients and system behavior during precipitation. Changes to initial solution composition (solid and dashed lines in solution composition prior to precipitation only affect inset of Fig. 7). The most significant difference in the intercept of the log–log relationship, shifting the tracer–tracer behavior between instantaneous and

12 A.C. Gagnon et al. / Earth and Planetary Science Letters xx (2007) xxx–xxx accumulated solids is the sensitivity to F, with the same 4.3. Me/Ca vital effect mechanism in the central band paired Sr/Ca and Mg/Ca ratios corresponding to larger extents of precipitation in the accumulated solid than for Large variations in Mg/Ca are uncoupled from Sr/Ca instantaneous solid. F values are sensitive to the choice in the central band suggesting that Rayleigh fraction- of partition coefficients and initial solution composition, ation is not the dominant process influencing Mg and Sr which, combined with the difference in F between correlation in this region. Understanding this vital effect accumulated and instantaneous precipitant, means that has implications beyond deep-sea corals, as increased F is underconstrained by our data set. Qualified by Mg/Ca in COCs appears to be a common feature in both this large uncertainty, the largest extent of precipitation surface and deep-sea corals (Meibom et al., 2006a,b; in our data, corresponding to the most depleted Sr/Ca Sinclair et al., 2006). Possible mechanisms for high Mg/ ratios and the most enriched Mg/Ca ratios in the outer Ca in the central band can be evaluated with our data set. septa, are consistent with an F value of roughly 0.8 for A surface entrapment model predicts precipitation instantaneous precipitation and an F value of 0.6 for rate dependent behavior of a tracer when growth rate and accumulated precipitation. near-surface solid diffusion are closely matched (Wat- In the above discussion of the Rayleigh mechanism we son, 2004). This model assumes chemical potentials at assume a system closed to all metal cations during pre- the surface and in the bulk-solid differ for a tracer, cipitation. However, active Ca2+ pumping in surface resulting in a difference of chemical composition as the corals has been demonstrated in culture experiments solid “traps” more or less of these endmembers. For (Tambutte et al., 1996) and is supported by the discovery surface entrapment to explain the enriched Mg of the of a plasma membrane Ca-ATPase localized to the central band, assuming rapid precipitation occurs in this calcifying region (Zoccola et al., 2004). The effect of region and traps more of a surface-like composition, the active calcium pumping on metal/calcium ratios during aragonite surface must be enriched in Mg compared to precipitation can be evaluated with a simple analytical the bulk crystal. Sr/Ca, however, is not observed to model that assumes constant rates of pumping and systematically vary with Mg/Ca in the central band. precipitation (see supplemental information S-3 for a Within the surface entrapment framework, this implies derivation and discussion). Provided that the rate of that either the bulk and surface composition of aragonite pumping is less than the rate of precipitation, this simple have identical strontium activities, or the balance of near- model predicts a linear relationship between Sr/Ca and surface diffusion and precipitation rate are different Mg/Ca in a log–log plot with a negative slope, similar to a enough between Sr and Mg such that Sr is insensitive to completely closed Rayleigh process. On the other hand, if growth rate over the range of rates that occur during coral pumping and precipitation occur at the same rate or if growth. While our data are not a direct test of the surface pumping outpaces precipitation, the model predicts a Mg/ entrapment model, they do highlight key model para- Ca to Sr/Ca relationship with a positive slope, unlike our meters and predictions if this process is at work in data. Calcium transport prior to precipitation has the po- D. dianthus. Furthermore, if surface entrapment explains tential to modify Me/Ca ratios in the initial calcifying the uncoupled variability of Mg/Ca and Sr/Ca in the fluid; however, Ca-ATPase driven calcium-proton ex- central band, the dominance of Rayleigh fractionation change is effectively alkalinity pumping and perturbs in the outer septa may suggest crystal growth rates and [Ca2+] relatively little while still increasing the saturation consequently surface entrapment effects do not vary state of aragonite dramatically through the speciation of significantly in the outer septa. the carbonate system. Together, small changes in initial There is evidence for organic material in coral skeletons [Ca+2] and a rate of calcium pumping less than the rate of that may help regulate biomineralization (Cuif et al., 2003; precipitation may result in initial Me/Ca ratios close to Puvereletal.,2005). It is possible that the “organic matrix” seawater that then progress along a Rayleigh curve, con- or another biomolecule is responsible for enriched Mg in sistent with both the presence of active calcium transport the central band. A biological component that selectively and our data. The constraint of a closed system with binds Mg but not Sr seems possible given the difference in regard to non-calcium metal cations does not apply to polarizability between these two cations. This putative dissolved inorganic carbon (DIC), which can diffuse biomolecule would have to be localized within the COCs, through bio-membranes as CO2(aq). A Me/Ca Rayleigh however, the existence and magnitude of Sr vs. Mg trace process is therefore consistent with the equilibrium stable element selectivity by organic components within the coral isotope vital effect model of Adkins et al. (2003) where skeleton has yet to be demonstrated. 13 18 δ C is set by net transmembrane CO2(aq) flux, and δ Ois While far from a comprehensive list of all processes set by the pH of the calcifying fluid. affecting magnesium, certain conclusions can be made.

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High Mg/Ca in the central band is not dominated by Benninger, 1993; Russell et al., 2004), although the Rayleigh fractionation; may be the result of surface relative contributions of these factors and the mechanism entrapment, given key constraints; and, biomolecules of influence are a topic of current research. may be at work, although it is impossible to quantita- tively assess their role without further characterization. 4.5. Prospect for a Me/Ca paleothermometer in D. dianthus 4.4. Uranium/calcium and a continuous biomineralization model Agreement between the mean Sr/Ca of D. dianthus and the abiotic Sr/Ca temperature relationship at a single Differences in Me/Ca behavior between the outer temperature point is promising, if preliminary, for the septa and central band raise the possibility of fundamen- development a Sr/Ca paleothermometer. Individual tally different biomineralization mechanisms in these deep-sea corals collected from a range of ocean regions. However, previous measurements of U/Ca temperatures, following the approach of Shirai et al. argue against this. In fission track experiments on (2005), and analyzed with improved precision is D. dianthus coral sections similar to ones used in this necessary to determine if a Sr/Ca paleothermometer in study, there is a greater than three-fold range in [U] D. dianthus can be developed. Improved precision is across the coral skeleton (Robinson et al., 2006). The necessary because the external error of our current Sr/Ca pattern of this variation is crucial: the region of low method corresponds to roughly 5 °C using the slope of uranium follows but extends well beyond the central the relationship between Sr/Ca and temperature for band, with [U] largely unchanged across the interface inorganic aragonite. Since Sr/Ca in the central band of between the central band and the outer septa. If the D. dianthus exhibits low variance, with our results central band were under the control of a fundamentally possibly limited by analytical error rather than coral different mechanism than the outer septa, it seems variability, localized sampling in this region should unlikely that [U] behavior would remain unchanged yield the most precise test of a Sr/Ca paleothermometer. across the interface of these two mechanisms. The pat- Outside the central band, vital effects may still present a tern of [U] suggests a continuous general mechanism challenge to the development of a Sr/Ca deep-sea describing vital effects and biomineralization. Conclu- paleothermometer in D. dianthus, as the slope of the sions about uranium can be generalized to other tracers to inorganic Sr/Ca temperature relationship is shallow and the extent that they are influenced by the same processes. even small vital effects translate into large differences in Unfortunately the mechanism of U/Ca vital effects is still temperature. arag an open question. Experiments suggest DU varies by The impact of vital effects on the Mg/Ca paleotherm- ∼3 fold due to pH and precipitation rate (Meece and ometer can be estimated, in the absence of clear

Fig. 8. Large Mg/Ca vital effects in D. dianthus compared to extrapolated surface coral data. Surface coral calibrations: (line a) Mitsuguchi et al. (1996), which compares well with recent coral culture experiments of Reynaud et al. (2006), circles. (line b) Fallon et al. (1999), (line c) Sinclair et al. (1998). Calibrations are marked as solid lines within the temperature range of the respective experiments, and dashed lines over the extrapolated temperature range.

14 A.C. Gagnon et al. / Earth and Planetary Science Letters xx (2007) xxx–xxx inorganic data, by comparing D. dianthus with surface linear tracer–tracer relationship in a logarithmic plot. To be corals (Fig. 8)). From Mg/Ca-temperature relationships consistent with our accurate measurements of Mg/Ca and calibrated in surface corals (Mitsuguchi et al., 1996; Sr/Ca, Rayleigh fractionation can occur from an initial Sinclair et al., 1998; Fallon et al., 1999; Reynaud et al., solution where Me/Ca ratios match seawater, provided that Coral 2006) the total ∼2 mmol/mol range of Mg/Ca in the effective strontium partition coefficient (DSr ) differs D. dianthus corresponds to ∼10 °C. Surface coral tem- between the outer septa and central band. Alternatively, if Coral perature calibrations correspond best with the outer DSr is the same throughout D. dianthus and matches the septa of our deep-sea corals. While the slope of the Mg/ abiotic partition coefficient, our data is consistent with an Ca temperature relationship, as determined in surface initial solution enriched in Sr/Ca by ∼3% compared to corals, is steeper than that of Sr/Ca, the presence of large seawater. Rayleigh fractionation implies a closed system vital effects can introduce a false and difficult to separate during precipitation, at least with respect to trace or minor temperature-like signal if different proportions of skel- metal cations. In the central band, Me/Ca ratios are etal regions are sampled during analysis. dominated by a non-Rayleigh process with our data constraining a number of possible explanations for vital 5. Conclusion effects in this region. That Rayleigh fractionation cannot explain the large variability in Mg/Ca within the central We determined accurate and detailed co-located Sr/ band is equally important as our evidence for the presence Ca and Mg/Ca ratios across different skeletal regions in of Rayleigh fractionation in the outer septa. Given the two individuals of the deep-sea coral D. dianthus widespread feature of enriched Mg/Ca in COCs, the through a combination of micromilling and isotope mechanism of this process is an important target of future dilution—ICP-MS. Overall, Sr/Ca variability was research. relatively small, with Sr/Ca in the optically dense central band varying significantly less than in the Acknowledgements surrounding skeleton. The mean Sr/Ca of all skeletal regions generally agree with the predicted Sr/Ca of Special thanks to Dr. Stephen Cairns and The inorganic aragonite. This finding combined with the Smithsonian Institution National Museum of Natural agreement in mean Sr/Ca ratios between two individuals History for allowing the analysis of the deep-sea coral from the same location is a promising preliminary result specimens in this study. A.C.G. would like to thank D. towards the development of a deep-sea coral Sr/Ca Rees of the California Institute of Technology for paleothermometer. Since Sr/Ca in the central band continued scientific advice and guidance. Comments by exhibits small variance, sampling localized to this Glen Gaetani and Anne Cohen as well as two anonymous region should yield the most precise test of a Sr/Ca reviewers contributed to improving this manuscript. paleothermometer. Unlike Sr/Ca, mean Mg/Ca varies dramatically Appendix A. Supplementary data between different skeletal regions. Coincident with the optically dense central band, Mg/Ca was at least Supplementary data associated with this article can 3 mmol/mol, more than twice that of the surrounding be found, in the online version, at doi:10.1016/j.epsl. skeleton. This result appears to be general, as relative 2007.07.013. Mg/Ca ratios of three other D. dianthus individuals collected from separate oceanographic locations also References nearly double within the central band. The difference in the mean Mg/Ca of the central band and surrounding Adkins, J.F., Boyle, E.A., Curry, W.B., Lutringer, A., 2003. Stable skeleton implies an ∼10 °C temperature signal, when isotopes in deep-sea corals and a new mechanism for “vital ” – calibrated via Mg/Ca-temperature relationships devel- effects . Geochim. Cosmochim. Acta 67, 1129 1143. Albarède, F., 1995. Introduction to Geochemical Modeling. Cam- oped in surface coral. A large non-environmental effect, bridge University Press, Cambridge, pp. 36–38. or vital effect, can obscure and complicate application of Allison, N., Finch, A.A., 2004. High-resolution Sr/Ca records in modern a Mg/Ca paleothermometer. Porites lobata corals: effects of skeletal extension rate and While complicating interpretation of a paleotherm- architecture. Geochem. Geophys. Geosyst. 5, Q05001. doi:10.1029/ ometer, vital effects are useful to help understand the 2004GC000696. Allison, N., Finch, A.A., Sutton, S.R., Newville, M., 2001. Strontium mechanism of biomineralization. Outside the central band, heterogeneity and speciation in coral aragonite: Implications for Mg/Ca increases with decreasing Sr/Ca. This relationship the strontium paleothermometer. Geochim. Cosmochim. Acta 65, can be explained by Rayleigh fractionation as shown by a 2669–2676.

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