Available online at www.sciencedirect.com ScienceDirect Geochimica et Cosmochimica Acta 134 (2014) 120–136 www.elsevier.com/locate/gca Evaluation of kinetic effects on clumped isotope fractionation (D47) during inorganic calcite precipitation Jianwu Tang a,⇑, Martin Dietzel b, Alvaro Fernandez a, Aradhna K. Tripati c,d, Brad E. Rosenheim a,1 a Department of Earth & Environmental Sciences, Tulane University, New Orleans, LA 70118-5698, USA b Institute of Applied Geosciences, Graz University of Technology, Rechbauerstrasse 12, 8010 Graz, Austria c Department of Earth and Space Sciences, Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095, USA d Department of Atmospheric and Oceanic Sciences, Institute of the Environment and Sustainability, University of California, Los Angeles, CA 90095, USA Received 17 September 2013; accepted in revised form 4 March 2014; available online 24 March 2014 Abstract 13 18 Considerable efforts have been made to calibrate the D47 paleothermometer, which derives from the quantity of C– O bonds in carbon dioxide produced during acid digestion of carbonate minerals versus its expected stochastic abundance, in a range of materials. However the impacts of precipitation rate, ionic strength, and pH on carbonate D47 values are still unclear. Here we present a set of 75 measurements of D47 values from inorganic calcites grown under well-controlled experimental 2 conditions, where we evaluate the impact on D47 values of precipitation rate (logR = 1.8–4.4 lmol/m /h), pH (8.3–10.5; NBS pH scale), and ionic strength (I = 35–832 mM). With the data available and at the current instrumental resolution, our study does not resolve any clear effects of pH, ionic strength, growth rate effects on measured D47 when compared in mag- nitude to the effects on d18O over most of the ranges of parameters sampled by our analyses. If these relationships exist, they must be smaller than our current ability to resolve them within our dataset. Under our experimental conditions, a D47-tem- perature equation, which is apparently insensitive to variation in pH, precipitation rate, and ionic strength over the range of variables sampled, can be written as 6 2 2 D47 ¼ð0:0387 Æ 0:0072Þ10 =T þð0:2532 Æ 0:0829Þðr ¼ 0:9998; p ¼ 0:009Þ where D47 values were reported on the absolute D47 reference frame after normalizing to conventional 25 °C reaction temper- ature using an acid fractionation factor of À0.00141& °CÀ1. Ó 2014 Elsevier Ltd. All rights reserved. 1. INTRODUCTION traditional oxygen isotope thermometer is dependent on 18 knowledge of the oxygen isotope composition (d Owater) The stable oxygen isotopic fractionation between car- of the water from which the carbonate mineral precipitated. bonates and water can be used to estimate paleo-tempera- However, for most geological materials, we can only mea- 18 18 ture (Urey, 1947; Epstein et al., 1951, 1953). Use of this sure d O of the carbonate mineral (d Ocarbonate) without constraint on the two main variables that influence d18O (temperature and d18O ). Most work on ⇑ Corresponding author. carbonate water E-mail address: [email protected] (J. Tang). carbonates relies on independent paleothermometers such 1 Present address: College of Marine Science, University of South as carbonate Sr/Ca and Mg/Ca ratios to interpret changes 18 Florida, St. Petersburg, FL 33701, USA. in d Owater (Flower and Kennett, 1990; Spero and http://dx.doi.org/10.1016/j.gca.2014.03.005 0016-7037/Ó 2014 Elsevier Ltd. All rights reserved. J. Tang et al. / Geochimica et Cosmochimica Acta 134 (2014) 120–136 121 Williams, 1990; Hendy et al., 2002; Flower et al., 2004; calculations (Schauble et al., 2006; Guo et al., 2009; Hill Schmidt et al., 2004; Rosenheim et al., 2005; Lund and et al., 2013). All calibration datasets show evidence of cor- Curry, 2006; Moses et al., 2006; Schmidt et al., 2006; relation between D47 and temperature, although there are Schmidt and Lynch-Stieglitz, 2011) or assumptions about some discrepancies among the calibrations (Fig. 1, see also the isotopic composition of the waters of formation (e.g., Fernandez et al., 2014 for analysis of these calibrations). Rosenheim et al., 2009). Despite the high precision of Many biogenic carbonates (Ghosh et al., 2006, 2007; Came elemental ratio measurements, the propagation of uncer- et al., 2007; Eagle et al., 2010; Tripati et al., 2010; tainties from empirical calibration to temperature into Thiagarajan et al., 2011; Grauel et al., 2013) exhibit a D47 18 d Owater values can be significant (e.g., Schmidt, 1999; sensitivity to temperature that is similar to the original inor- Rosenheim et al., 2005). In addition, on long enough time ganic calibration of Ghosh et al. (2006), if the Ghosh et al. scales, changes in the cationic composition of seawater also (2006) data are transferred onto the absolute reference need to be considered when interpreting elemental proxies. frame (Dennis et al., 2011). The Ghosh et al. (2006) calibra- In contrast to stable oxygen isotope composition, the tion is generally used as the nominal equilibrium calibration carbonate clumped isotope thermometer is independent of due to the fact that it succeeds in explaining the D47 of most the isotopic composition of water from which the carbonate biogenic carbonates surveyed thus far. Recently, the Ghosh mineral precipitates (Ghosh et al., 2006). This paleother- et al. (2006) calibration was re-examined by Zaarur et al. mometer is based on the temperature dependence of (2013) who conducted additional carbonate precipitation 13C–18O bond abundance in the carbonate crystal lattice. experiments and ran their samples at higher analytical pre- The clumping of 13C and 18O into bonds with each other cision directly applying an absolute reference frame (Dennis in the crystal lattice involves a homogeneous isotope ex- change reaction within one single mineral phase (Schauble Ghosh et al., 2006 (a) et al., 2006). Therefore, the advantage of the carbonate Schauble et al., 2006 & Guo 0.75 et al., 2009 clumped isotope thermometer is the potential to more di- Dennis and Schrag, 2010 rectly estimate the precipitation temperature of carbonates Eagle et al., 2010 using the measurement of 13C–18O bond abundance in car- 18 Tripa et al., 2010 bonates. Subsequently, the d Owater can be calculated using 0.65 Thiagarajan et al., 2011 18 (‰) simultaneous measurements of d Ocarbonate. 47 Grauel et al., 2013 Early studies (Ghosh et al., 2006) linked the abundance Δ of 13C–18O bonds in CO liberated by phosphoric acid Henkes et al., 2013 2 0.55 digestion of a carbonate mineral to that in the reactant car- Eagle et al., 2013 bonate mineral. The isotopic species 13C18O16O accounts Hill et al., 2013 Zaarur et al., 2013 for most of the mass 47 CO2 measured by isotope ratio mass spectrometry. The abundance of this species is re- 0.45 8 101214 ported using the notation D , representing the mass 47 47 106/T2 (K) enrichment in CO2 relative to the amount of mass 47 ex- pected for a CO2 that has the same bulk isotopic composi- (b) 0.90 Ghosh et al., 2006 tion but a stochastic distribution of isotopologues (Wang Dennis and Schrag, et al., 2004). Specifically D47 is defined as: 2010 " 0.80 47 Schauble et al., 2006 R & Passey and Henkes, 2012 D47 ¼ 2 2R13 Á R18 þ 2R17 Á R18 þ R13 ðR17Þ Henkes et al., 2013 # 0.70 R46 R45 (‰) Eagle et al., 2013 47 À 2 À 13 17 þ 1 Á 1000 Δ 2R18 þ 2R13 Á R17 þðR17Þ R þ 2R Hill et al., 2013 0.60 where R refers to the ratio of the minor isotopologue to the Zaarur et al., 2013 major isotopologue of the molecule of interest. 0.50 In 2006, Ghosh et al. published the first D47-temperature 8101214 calibration that was based on inorganic calcite precipitation 106/T2 (K) at controlled temperatures using a classical active degassing method (i.e., CO2 was removed from a solution containing Fig. 1. Published D47-temperature calibration lines from biogenic 2+ À Ca and HCO3 and purged by N2 gas), as well as bio- carbonates and inorganic synthetic calcites: (a) D47 data are genic, aragonitic deep-sea and tropical corals. Following reported on the heated gas reference frame (Huntington et al., this pioneering work, further investigations of the relation- 2009); (b) D47 data are reported on the absolute reference frame (Dennis et al., 2011). Hill et al. (2013) theoretical calculations for ship between D47 and calcifying temperature were carried out using different biogenic carbonates (Came et al., 2007; calcite are shown with an acid digestion fractionation factor of 0.232& in the heated gas reference frame and an acid digestion Ghosh et al., 2007; Eagle et al., 2010, 2013; Tripati et al., fractionation factor of 0.268& (from Passey and Henkes, 2012)in 2010; Thiagarajan et al., 2011; Zaarur et al., 2011; Saenger the absolute reference frame. To facilitate comparison with other et al., 2012; Dennis et al., 2013; Grauel et al., 2013; Henkes studies, Henkes et al. (2013) data were corrected with an acid et al., 2013), inorganic synthetic carbonates (Dennis and digestion fractionation factor of 0.08& for the 90–25 °C offset from Schrag, 2010; Zaarur et al., 2013), and theoretical Passey et al. (2010). 122 J. Tang et al. / Geochimica et Cosmochimica Acta 134 (2014) 120–136 et al., 2011). The revised calibration by Zaarur et al. (2013) precipitation of BaCO3 by mixing the NaHCO3 solution has a similar slope as the original calibration, which further with BaCl2 and NaOH solution). confirms that the Ghosh et al. (2006) calibration can be Here, to evaluate the potential scope of kinetic isotope considered as the nominal equilibrium calibration.