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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. 0012-821X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2007.07.013 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 2 A.C. Gagnon et al. / Earth and Planetary Science Letters xx (2007) xxx–xxx 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 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 A.C. Gagnon et al. / Earth and Planetary Science Letters xx (2007) xxx–xxx 3 Fig.
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