Paleoclimate reconstruction using carbonate clumped isotope thermometry

Paleoclimate reconstruction using carbonate clumped isotope thermometry

Quaternary Science Reviews 30 (2011) 3575e3588 Contents lists available at SciVerse ScienceDirect Quaternary Science Reviews journal homepage: www.e...

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Quaternary Science Reviews 30 (2011) 3575e3588

Contents lists available at SciVerse ScienceDirect

Quaternary Science Reviews journal homepage:

Invited review

Paleoclimate reconstruction using carbonate clumped isotope thermometry John M. Eiler* Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 May 2011 Received in revised form 5 September 2011 Accepted 6 September 2011 Available online 24 October 2011

Carbonate clumped isotope thermometry is a relatively new paleotemperature proxy based on measurements of the degree of ordering of 13C and 18O into bonds with each other (making the 13C18O16O2 2 ion group) in lattices of carbonate minerals. This technique has several unusual properties that complement existing methods of paleoclimate reconstruction. Most importantly, it is based on a homogeneous isotope exchange equilibrium and thus constrains temperature independent of the isotopic composition of waters from which carbonates grew. This method also appears to be generally insensitive to ‘vital effects’ that compromise many other paleothermometers based on the chemical properties of biominerals or organic matter, at least for those organisms that have been subjected to systematic study to-date (corals and foraminifera); however, discrepancies among some calibrations, particularly at low temperatures, may point toward the existence of vital effects in mollusks and other organisms. This review discusses the principles and calibrations of the technique, its uses in combination with conventional stable isotope measurements to constrain the d18O of past waters, preservation of paleotemperatures in ancient materials, as well as current problems in our understanding of calibrations and interlaboratory data comparisons. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: Stable isotope Clumped isotope Paleothermometry Paleoclimate

1. Introduction Clumped isotope geochemistry is the study of the properties and distributions of naturally occurring isotopologues containing more than one rare isotope (Eiler, 2007). Carbonate clumped isotope thermometry (Ghosh et al., 2006a; Schauble et al., 2006) is the tool on which the largest applied part of this field is based, and involves measurement of growth temperatures of carbonate minerals by determining the extent to which 13C and 18O are chemically bound to one another within the same carbonate ion group. This method is based on a recent innovation in gas source mass spectrometry and generally unfamiliar principles of isotope geochemistry (Eiler and Schauble, 2004; Huntington et al., 2009), so is it less widely understood than many other methods of paleoclimate reconstruction. And, it is technically challenging and suffers from several intrinsic weaknesses (Huntington et al., 2009; Dennis et al., 2011), making it unattractive for the casual user. However, it also provides unique information d most importantly, temperatures that are independent of the compositions of coexisting waters d that is permitting progress with several longrefractory problems, including quantitative paleothermometry in

terrestrial materials (Passey et al., 2010; Csank et al., 2011), climate change deep in earth history (Came et al., 2007; Finnegan et al., 2011), shallow crustal diagenesis and low grade metamorphism (Huntington et al., 2011), paleoaltimetry (Ghosh et al., 2006b; Quade et al., 2007; Garzione et al., 2008), paleothermometry using organisms that exhibit prohibitively extreme ‘vital effects’ on conventional thermometers (Thiagarajan et al., 2011a,b), body temperatures of extinct vertebrates (Eagle et al., 2010, 2011), among others (Dennis and Schrag, 2010). For these reasons, carbonate clumped isotope thermometry has expanded rapidly in its breadth of applications and numbers of practitioners over the last several years. This review examines the application of carbonate clumped isotope thermometry to paleoclimate reconstructions, focusing on its ability to complement information obtained from other, better known techniques. Previous reviews of the general concepts behind clumped isotope geochemistry and the analytical methods for clumped isotope analyses of CO2 and carbonates can be found in Eiler (2007), and Huntington et al., 2009, respectively.

2. The context of paleoclimatological problems and tools

* Tel.: þ1 626 395 6942. E-mail address: [email protected] 0277-3791/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2011.09.001

Carbonate clumped isotope thermometry is a specialized tool that is most interesting for its ability to leverage information from more conventional paleoclimatological techniques. The study of


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past climates is concerned with many properties of the environment d temperature, atmospheric and oceanic composition and circulation, ecosystem structure, distributions of rainfall and nutrients, cloudiness, ice cover, sea level, solar irradiance, among others (e.g., Mayewski et al., 2004). However, temperature and the 18 O content of water have a special place in this list because both are connected in generally understood ways to other environmental properties. I.e., temperature depends on solar irradiance, abundances of atmospheric greenhouse gases and albedo (e.g., Hansen et al., 1997), the 18O content of seawater is related to global ice volume and sea level, the geographic distributions of ice, ocean circulation and rainfall (e.g., Dansgaard and Tauber, 1969; Adkins et al., 2002), and the 18O content of meteoric water varies complexly but predictably with patterns of atmospheric circulation (e.g., Jouzel et al., 1987). And, importantly, both temperature and d18O of water are reducible to scalar measurements and so are relatively straightforward to translate into quantitative stratigraphic and geographic records (e.g., Zachos et al., 2001). The oldest and, arguably, most widely used tool for quantitative paleoclimatology is Urey’s carbonate-water oxygen isotope exchange thermometer (Urey, 1947; McCrea, 1950; Epstein et al., 1951). Urey focused on the oxygen isotope fractionation between seawater and marine carbonate for several reasons: (1) both phases (water and carbonate) could be analyzed precisely for 18O/16O ratio using known techniques; (2) the size of the oceans suggests they could not have changed in isotope composition over geological timescales (incorrect but plausible in 1947); and (3) carbonate minerals are well preserved in the geological record. He could have added to this list a fourth important feature: (4) dissolved inorganic carbon undergoes rapid oxygen isotope exchange with water at near-neutral pH (e.g., Beck et al., 2005), meaning this system will reach thermodynamic equilibrium at earth-surface conditions more readily than heterogeneous equilibria involving most other easily imagined authigenic minerals. Urey’s thermometer remains one of the most widely used and enduring tools of geochemistry, and we can regard the first of these arguments (i.e., the precision of the analytical data) as an unmitigated success. However, for much the last 64 years we have struggled with failures of the remaining three premises of the technique. First, and most fundamentally, carbonate is a widespread component of the geological record, but one rarely has direct constraints on the isotopic composition of water from which it grew (one exception is the last glacial maximum, which can be reconstructed from marine pore water profiles; Adkins et al., 2002). In the absence of such constraints, any temperature based on oxygen isotope thermometry is only as accurate as one’s assumption about the isotopic compositions of waters in the past. Second, it has long been recognized that the oxygen isotope compositions of many biogenic carbonates exhibit ‘vital effects’ d species-specific differences between compositions of biominerals and expected compositions of their inorganic equivalents grown from their parental waters (i.e., seawater in the case of a marine animal). This effects have been carefully explored through empirical calibrations and controlled culturing experiments for a wide range of organisms (e.g., Erez and Luz, 1982; Grossman, 1984; McConnaughey, 1989a,b; Bemis et al., 1998; Adkins et al., 2003; Rollion-Bard et al., 2003), though they continue to present a major uncertainty for studies of ancient extinct organisms, and in some cases are so large and variable that they effectively preclude quantitative paleotemperature reconstructions. Finally, carbonate minerals commonly undergo postdepositional transformations in fabric, structure and composition during diagenesis and metamorphism. It is widely recognized that these processes can lead to profound (multiple per mil, to tens of per mil) changes in d18O of carbonate sediments and fossils, effectively erasing the oxygen isotope record of past climates. Some

of these preservational problems can be avoided by careful sample selection, textually resolved microsampling and/or trace metal analysis of carbonates (which show distinctive modifications during diagenesis). Nevertheless, it remains possible that poor preservation is responsible for some of the strongest signals in the geological record, such as the marked decrease in d18O of marine carbonates with increasing geologic age for sediments of greater than Mesozoic age (Jaffres et al., 2007). Many alternative methods have been proposed for reconstructing environmental temperature d in part as attempts to address the challenges to carbonate-water oxygen isotope paleothermometry. For example, thermometers based on organic biomarkers (e.g., alkenones; Muller et al., 1996; Sachs et al., 2000; Volkman, 2000) or partitioning of minor elements (e.g., Mg/Ca ratios) between seawater and biogenic carbonates (Delaney et al., 1995; Anand et al., 2003; Elderfield et al., 2006). The number and diversity of these other proxies provides a clue to the general difficulty of the problem. While these approaches can be rationalized by principles of physical and biochemistry, most ultimately reduce to empirical calibrations of complex non-equilibrium phenomena, and so are unsuitable for extrapolation to organisms or conditions beyond their calibrations. Furthermore, trace-metal thermometers, just as for Urey’s carbonate-water thermometer, are sensitive to the composition of waters from which analyzed minerals grew, and thus can be applied only to settings where water trace element chemistry can be confidently deduced. Trace metal thermometers also generally suffer from vital effects, and some appear to suffer from other, less obvious, species-specific or interlaboratory artifacts (e.g., intra-test variability, and sensitivity to cleaning protocols; Lea, 2003; Sadekov et al., 2008). Problems of preservation are less well explored for trace metal and organic thermometers as compared to oxygen isotopes, but they must suffer from analogous effects of diagenesis and metamorphism. The principle value of carbonate clumped isotope thermometry is its ability to give researchers yet another ‘bite at the apple’ of paleoclimate reconstruction, using principles of physical chemistry that differ in several important ways from these previous approaches.

3. Principles of carbonate clumped isotope thermometry Carbonate clumped isotope thermometry shares some principles of physical chemistry with Urey’s carbonate-water thermometer but differs in one key respect: the Urey thermometer examines a heterogenous isotope exchange reaction (i.e., an equilibrium between two or more separate phases) of the form:

CaC16 O3 þ H2 18 O ¼ CaC18 O16 O2 þ H2 16 O


where as the carbonate clumped isotope thermometer examines a homogeneous isotope exchange reaction (an equilibrium among components of a single phase) of the form:

Ca13 C16 O3 þ Ca12 C18 O16 O2 ¼ Ca13 C18 O16 O2 þ Ca12 C16 O3 13

(2) 18

I.e., a reaction involving ordering or ‘clumping’ of C and O ion group into bonds with each other making the 13C18O16O2 2 (Fig. 1). This difference has important implications for the application of stable isotope thermometry to paleoclimate problems, and in particular provides a new avenue for addressing the challenges to conventional carbonate-water thermometry outlined above. Reaction 2 has a temperature dependent equilibrium constant that approaches 1 at high temperatures and increases monotonically (though somewhat complexly) with decreasing temperature (Schauble et al., 2006). This temperature dependence reflects

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Fig. 1. Atomic structure of the calcite lattice, illustrating the homogeneous isotope exchange reaction between carbonate ion units on which carbonate clumped isotope thermometry is based. Analogous reactions exist among structural constituents of aragonite-structure solid carbonates and dissolved carbonate species (carbonic acid, carbonate and bicarbonate ion).

a balance between the relatively low vibrational energy (and therefore enhanced thermodynamic stability) of the 13Ce18O bearing isotopologue of the carbonate ion vs. the effect of configurational entropy, which favors random isotopic distributions and increases in importance with increasing temperature. Statistical thermodynamic models of reaction 2 and similar reactions involving other solid and aqueous carbonates (Schauble et al., 2006; Guo et al., 2009) indicate that the equilibrium constant controlling stable isotope ‘clumping’ in carbonates should equal w1.0005 at earth-surface temperatures and decrease by w0.000003e0.000004 per 1  C increase in temperature. To-date, all measurements of the extent of 13Ce18O ordering (or clumping) in carbonate solids have been made by acid digestion in anhydrous phosphoric acid, with a reaction temperature of 25e90  C, collection and purification of the product CO2, and analysis of that CO2 by gas source isotope ratio mass spectrometry (Ghosh et al., 2006a; Huntington et al., 2009). This analysis includes simultaneous collection of the ion beams cor13 16 þ 12 16 17 þ C O O ), responding to M/Z of 44 (12C16Oþ 2 ), 45 ( C O2 and 13 18 16 þ 46 (12C18O16O, 13C16O17O and 12C17Oþ 2 ) and 47 ( C O O , 12 18 17 þ C O O and 13C17Oþ 2 ). (Some laboratories also collect the mass 13 17 18 þ C O O d and 49 d 13C18Oþ 48 d 12C18Oþ 2 and 2 d ion beams, though little of this data has been presented in talks or papers and it is generally used only as an internal monitor of contamination by hydrocarbons, chlorocarbons and sulfur compounds; Eiler and Schauble, 2004; Guo and Eiler, 2007.) An involved set of ion correction calculations and standardizations to reference gases having a known state of isotopic ordering allows one to translate intensity ratios of these ion beams into measurements of d18O, d13C, and the variable ‘Δ47’, which measures the excess of mass-47 isotopologues relative to the abundances expected for a stochastic distribution of all isotopes among all possible isotopologues (Eiler and Schauble, 2004; Wang et al., 2004; Affek and Eiler, 2006; Affek et al., 2007; Huntington et al., 2009). Values of Δ47 are reported in units of per mil, generally vary between 0 and þ1&, and principally reflect the super-abundance of 13C18O16O. While the principles, methods and instruments involved in a clumped isotope analysis of a carbonate mineral resemble those

of conventional stable isotope analyses, there are several artifacts that must be carefully documented and corrected for in order to obtain accurate results. All data, published and unpublished, of which the author is aware indicate that the gas source mass spectrometers currently used for these measurements are subtly (w1%, relative, or less) non-linear in their relationship between measured current intensity ratios and actual isotope ratios (Huntington et al., 2009; Dennis et al., 2011). This non-linearity broadly resembles that long observed in gas source mass spectrometric measurements of the D/H ratio of H2, which has been addressed through instrument-specific calibrations based on measurements of two or more standards having known (or at least accepted) D/H ratios (Nief and Botter, 1959; De Wit et al., 1980). It is possible subtle artifacts of this sort exist for other isotope ratios but are not recognized because there are few examples of pairs of standards for which we independently and confidently know their difference in isotope ratio to with in 1%, relative, or better. In any event, clumped isotope measurements calibrate and correct for this effect by measuring gases that differ from one another in bulk isotopic composition but are known to be equal in Δ47 because they were equilibrated at a common temperature (generally through heating to w1000  C, but possibly also through water-CO2 equilibration at lower temperatures; Eiler and Schauble, 2004; Huntington et al., 2009; Dennis et al., 2011). A second common artifact is ‘scrambling’ or isotope exchange in the ion source through fragmentation/ recombination reactions. This must be corrected for by monitoring the measured difference in Δ47 between two materials that have accepted Δ47 values. Until recently, little was known about the methodological artifacts and interlaboratory consistency for measurements of Δ47 in CO2 derived from carbonate minerals. However, a recent study of the acid digestion fractionation (i.e., differences between the proportions of 13Ce18O bonds in reactant carbonate and product CO2 gas; Guo et al., 2009) and a four-lab intercalibration study (Dennis et al., 2011) have provided the foundation for integrating data measured in different laboratories and under different conditions. Guo et al. (2009) showed that the acid digestion reaction at 25  C enriches product CO2 in 13Ce18O bonds by 0.2& relative to their abundance in reactant carbonate, and this


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enrichment decreases with reaction temperature by w0.001& per degree C. This fractionation broadly resembles the long-recognized acid digestion fractionation of oxygen isotope ratios between reactant carbonate and product CO2 (Swart et al., 1991); Guo et al. (2009) present a quantum mechanical model of fractionations associated with irreversible dissociation of carbonic acid, which appears to explain at least the first-order behavior of both phenomena. Dennis et al. (2011) develop and ground-proof a scheme for standardizing and reporting Δ47 measurements in CO2 gases, including (but not limited to) those produced by acid digestion of carbonates. Briefly, one calibrates both instrument linearity and ‘scrambling’ through analysis of CO2 gases that vary in both bulk isotopic composition and temperature of equilibrium with respect to the reaction: 13 16

C O2 þ 12 C18 O16 O ¼

13 18

C O16 O þ 12 C16 O2


The temperature-dependent equilibrium constant for this reaction can be predicted with relative confidence because the vibrational dynamics, and thus partition function ratio, of gaseous CO2 is well known (Wang et al., 2004). Therefore, Dennis et al. argue, it is a relatively good reference frame for all laboratories to assume in order to establish some absolute Δ47 scale for interlaboratory standardization. And, there are methods for forcing CO2 to an internal isotopic equilibrium that appear to be reliable, reproducible and technically straightforward. Most importantly, Dennis et al. demonstrated that this procedure could be applied by 4 independent laboratories to obtain agreement on the Δ47 values of carbonate standards to within 0.01& . This corresponds to a roughly 2  C bias in reported temperature for materials grown at near-earth-surface temperatures d likely close to the best achievable interlaboratory precision given the experimental uncertainties in creating the equilibrated CO2 reference frame and intrinsic mass spectrometric errors. While the techniques of carbonate clumped isotope thermometry are rapidly becoming more widespread and standardized, there remain a number of refractory technical challenges: Most data generated to-date have used instruments and methods that require large (w8e10 mg) carbonate samples. Recent innovations (e.g., integration with a Kiel device or similar home-built devices; Schmidt and Bernasconi, 2010) indicate that this can be reduced significantly, but it remains to be seen whether these methodological changes result in changes in accuracy and precision. And, because clumped isotope measurements of CO2 demand relatively high precision (w0.01&) for a very low abundance species (13C18O16O; w40e50 ppm of natural CO2), long mass spectrometric integration is needed, making the method much slower than conventional stable isotope techniques (w3 h per sample, including both preparation and analysis, is typical; Ghosh et al., 2006a; Huntington et al., 2009). And, the low abundance of the target species means that special precautions, such as gas chromatography and sulfur ‘getters’, are required to assure the sample is free of contaminants that could produce isobaric interferences (Eiler and Schauble, 2004; Affek and Eiler, 2006; Guo and Eiler, 2007; Huntington et al., 2009; Yeung et al., 2009). This is a particular challenge for complex, organic-rich materials, such as soils, teeth or carbonaceous chondrites (e.g., Guo and Eiler, 2007; Eagle et al., 2010). We note that clumped isotope measurements, even when analytically successful, can be subject to peculiar artifacts that must be considered when making interpretations. The most counterintuitive of these is an effect of mixing, whereby a physical mixture of two CO2 gases that differ in bulk isotopic composition (i.e., d13C and/or d18O) but are equal in Δ47 value will produce a mixture that differs in Δ47 from the end members (Eiler and

Schauble, 2004; Eiler, 2007; equivalent effects occur for solid carbonate and other compounds; Thiagarajan et al., 2011a; Bristow et al., 2011). This phenomenon may be important to the study of texturally complex carbonate samples, where one might sample powders that are physical mixtures of compositionally distinct components. Finally, it has long been recognized that the oxygen isotope analysis of carbonate minerals depends on method (i.e., glass vacuum reaction vessel vs. common acid bath vs. Kiel device) and even details such as sample size or reagent preparation (Werner and Brand, 2001). It is possible (perhaps likely) that clumped isotope measurements of carbonates where unusual forms, types or sizes of samples are subjected to acid digestion, or the method or conditions of acid digestion differ from those previously tried, might involve systematic analytical artifacts. All such studies will have to be carefully standardized through analyses of relatively well-understood reference materials. 3.1. Calibrations and vital effects The calibration of the carbonate clumped isotope thermometer d i.e., determination of the temperature dependence of the equilibrium constant for reaction 2 d has been studied by theory (Schauble et al., 2006; Guo et al., 2009), inorganic experiments (Ghosh et al., 2006a; Dennis and Schrag, 2010), and empirical studies of biosynthetic carbonates and carbonate-apatites grown at known (or at least well constrained) temperatures (Ghosh et al., 2006a, 2007; Came et al., 2007; Eagle et al., 2010; Tripati et al., 2010a,b; Thiagarajan et al., 2011a), and empirical studies of speleothems and related synthetic analogues (Affek et al., 2008; Meckler et al., 2009; Daeron et al., 2011). A large amount of additional data for synthetic or biological carbonates (including dolomite, calcite and aragonite) exists in the ‘gray’ literature of non-peer reviewed abstracts and unpublished but privately circulated data sets. We do not discuss these data in detail here, but the issues raised in our discussion are influenced by knowledge of this work (much of which likely will appear in the peer reviewed literature relatively soon). Over the temperature range relevant to most paleoclimatological studies, approximately 40e0  C (Fig. 2), there is remarkable uniformity among most of the published calibrations, excepting only speleothems (which are omitted from Fig. 2 and discussed separately below). While we spend much of this section discussing the confusing picture that is emerging from alternate synthetic calibrations and some biosynthetic materials (particularly at temperatures below w10e15  C), and the difficulties of calibrating complex natural materials (soils, lake sediments, etc.), it is worth noting the simplicity of most of these data. The group of points that define the visually obvious trend of most of the data in Fig. 2 includes calcite, aragonite and carbonate-apatite, inorganic and biogenic minerals, salt water, fresh water and land animals, and a wide range of phyla and living environments. In this respect, carbonate clumped isotope thermometry contrasts with carbonate thermometers based on O, Ca and Mg isotopes or trace metal distributions, which commonly exhibit diverse species-specific and condition-specific calibrations (Lea, 2003). This uniformity comes as a pleasant surprise and surely is not due to anything clumped isotope analysts are doing unusually well; carbonate clumped isotope analysis is (and will likely remain) among the more difficult and imprecise (in C equivalent error) geochemical methods of paleothermometry. Rather, it suggests something distinctive about the physical chemistry of isotopic ordering or ‘clumping’ in carbonate d something that potentially provides clues about the causes of vital effects and inorganic kinetic isotope effects encountered by other carbonate paleotemperature proxies.



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Fig. 2. Summary of published calibrations of the carbonate clumped isotope thermometer between 0 and 40  C. Data sources: Ghosh et al., 2006a (synthetic calcite and corals); Came et al., 2007 (mollusks and brachiopods); Ghosh et al., 2007 (otoliths); Dennis and Schrag, 2010 (‘D&S synthetics’); Eagle et al., 2010 (bioapatite); Tripati et al., 2010a,b (forams, coccoliths and bulk marine carbonate); and Thiagarajan et al., 2011a (corals). Two outlier analyses from Thiagarajan et al. (2011a) were interpreted in that study to be the result analytical artifacts; these two points have been omitted from this figure.

3.2. Vital effects Vital effects d differences in composition between biogenic materials and their inorganic equivalents grown under the same conditions d present a significant challenge to many geochemical paleoclimate proxies (Lea, 2003 and references therein). A wide range of mechanisms have been suggested to explain these phenomena, including pH effects on the carbonate-water equilibrium (i.e., because many organisms force carbonate precipitation through control of pH in their body fluids; Zeebe, 1999; RollionBard et al., 2003), kinetic isotope effects on precipitation (McConnaughey, 1989a,b), and fractionations associated with preferential transfer of aqueous CO2 across membranes into the carbonate secreting organisms’s body fluids (Adkins et al., 2003). Regardless of the cause(s), many of the species that exhibit such vital effects still demonstrate temperature dependent carbonatewater fractionations broadly similar in slope (i.e., temperature dependence) to those for inorganic carbonates. In these cases, it is still possible to construct empirical, species-specific calibrations that appear to reliably return useful paleotemperature constraints. Much of the marine paleotemperature record for the Cenozoic is based on this approach. Despite these successes, vital effects remain problematic for several reasons: (1) There are organisms, such as deep sea corals, that are attractive as targets for paleoclimate studies, but exhibit such profound and variable vital effects that they are effectively useless for conventional oxygen isotope paleotemperature measurements (Adkins et al., 2003). (2) It is not obvious that empirical calibrations for modern species can be applied with confidence to ancient relatives. (3) There is no obvious way to directly calibrate thermometers for extinct organisms that might exhibit such vital effects. And (4) if one lacks a quantitative and predictive physical understanding of vital effects, it is difficult to judge whether an empirical calibration might be violated under some environmental conditions.

The most obvious distinguishing feature of carbonate clumped isotope thermometry is that it is based on a homogeneous isotope exchange equilibrium for a chemical species (carbonate ion) that undergoes rapid oxygen isotope exchange with aqueous solutions at room temperatures and near neutral pH. Therefore, it is imaginable that reservoir effects and fractionations that arise during transport make the growth medium of biogenic carbonates (i.e., organism body waters) differ in chemical and isotopic composition from surrounding bulk aqueous solutions (e.g., seawater), but that these differences are not observed in the state of isotopic ordering of dissolved carbonate and bicarbonate ions so long as they are able to undergo rapid, local oxygen isotope exchange with water and each other. The evidence regarding vital effects presented in the first paper to describe and calibrate the carbonate clumped isotope thermometer (Ghosh et al., 2006a) is ambiguous: deep-sea corals, an equatorial surface coral and summer growth bands from a Red Sea coral all conform, within analytical uncertainty, to the inorganic calcite calibration in that study. This suite includes natural materials that exhibit large vital effects in their bulk stable isotope compositions (particularly deep sea corals), suggesting that isotopic clumping might be free of such effects. On the other hand, the winter bands of a Red Sea coral examined in this study violate the Ghosh et al. inorganic calibration (though the growth temperature of this material was not confidently known; this is why the Red Sea data are not included in the calibration data sets in Fig. 2), implying that vital effects, or perhaps some other, previously unrecognized phenomenon, might increase 13Ce18O ‘clumping’ in excess of the equilibrium state in biogenic carbonate under some conditions. This remains possible, but has not been supported by subsequent studies. Since that time, two relatively large, detailed studies of vital effects in carbonate secreting organisms have been published: A study of foraminifera, noteworthy for the breadth of species and growth conditions that it considers (Tripati et al., 2010a,b), and


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a study of deep-sea corals, which includes materials that exhibit exceptionally large vital effects in their d18O and d13C values (Thiagarajan et al., 2011a). No organism examined by either of these studies was found to exhibit a systematic, statistically significant vital effect in clumped isotope composition, and the few individual measurements that differed from the inorganic calibration were either irreproducible (i.e., possibly a result of a contaminant or other analytical artifact) or part of a relatively small sub-set of low temperature materials discussed below. Collectively, these studies suggest that the forms of vital effects that manifest in deep-sea corals and foraminifera have no influence on isotopic clumping of carbonate ion species. Thiagarajan et al. (2011a) compare their data for deep-sea corals to predictions of several models of the expected relationship between O isotope and clumped isotope compositions for various hypothesized mechanisms of vital effects. They found that the data are inconsistent with any model that calls on molecular diffusion as the mechanism for non-equilibrium fractionation, or with variable mixing of carbonate in seawater and isotopically distinct carbonate in body water, but that it could be consistent with models that call on pH dependence of the isotopic fractionations between dissolved inorganic carbon species and water. Similarly, Tripati et al.’s study of modern and recent foraminifera indicate that the inorganic calibration of the clumped isotope thermometer applies despite a range of vital effects in d18O and d13C, a trend they recognized as consistent with vital effects arising from pH variations of precipitating fluids. This study also noted that models in which vital effects reflect kinetic isotope effects on irreversible hydration or hydroxylation of aqueous CO2 are possible only if these reactions involve negligible departures from equilibrium from an equilibrium clumped isotope signature. The clumped isotope effects associated with these reactions are not yet known and present an attractive target for future study of this problem. A study of fish otoliths (Ghosh et al., 2007) yielded a calibration that is indistinguishable in slope and marginally different in intercept from the original calibration of Ghosh et al., 2006a; this small discrepancy in intercept might reflect a subtle vital effect, or a difference in standardization with respect to other studies (at the time the Ghosh et al. 2006a and 2007 studies were conducted, factors influencing data accuracy were relatively poorly understood; see Huntington et al., 2009 and Dennis et al., 2011), or some small but systematic error in estimating the temperatures at which fish grow (e.g., if growth is biased toward warmer seasons or water depths). On a practical level, the apparent simplicity of calibrations of carbonate clumped isotope thermometry make it an attractive approach to paleoclimate reconstructions involving organisms that exhibit large-amplitude, complex vital effects, or organisms with no direct modern equivalents. Nevertheless, no detailed studies of vital effects have yet been conducted for organisms that differ in their mechanisms of biomineralization from both corals and foramifera. Mollusks are a particularly attractive target for future studies of this problem. 3.3. Speleothems The first class of carbonates found to exhibit significant and systematic departures from the nominal equilibrium calibration of Ghosh et al., 2006a, are speleothems, which commonly have Δ47 values lower than predicted for their known growth temperatures (Affek et al., 2008; Meckler et al., 2009; Daeron et al., 2011). These offsets are observed to correlate inversely with d18O of carbonate (i.e., higher d18O corresponds to lower than expected Δ47), and are similar in amplitude and direction to isotope effects observed in artificial speleothems grown in the laboratory and in artificial

cryogenic carbonates grown by freezing carbonate saturated solutions. In a thesis by W. Guo, M. Daëron, P. Niles, W.A. Goddard and J.M. Eiler (Isotopic fractionations associated with degassing of CO2 from aqueous solutions: implications for carbonate clumped isotope thermometry) present a model that attempts to explain these phenomena as a consequence of irreversible dehydration or dehydroxylation of carbonic acid or bicarbonate (respectively), followed by outgassing of dissolved CO2. Models of the isotope effects associated with this process resemble the general trend of data for speleothems and related synthetic materials (though models and data differ from each other in detail in some cases). And, this explanation seems at least superficially reasonable because speleothems grow in environments where ground and soil waters having relative high pCO2 saturations drip or flow into caves that are ventilated with air, and thus become strongly supersaturated in CO2 and undergo rapid, extensive degassing. Perhaps the most peculiar finding in the studies of this subject to-date is that individual speleothems appear to record consistent values of Δ47 through time, or changes in Δ47 that are similar to those predicted from independent proxies for temperature change (Affek et al., 2008; Meckler et al., 2009; Daeron et al., 2011). It is difficult to understand how a process like disequilibrium degassing could lead to clumped isotope compositions that are offset from equilibrium by the same amount over thousands of years of growth of a single speleothem. Nevertheless, the data suggesting this are statistically robust and the phenomenon has been observed in several separate instances. A deeper exploration of this complex subject is warranted, but beyond the scope of this review. 3.4. Discrepant calibrations Perhaps the most puzzling and important issue facing the community of researchers working on carbonate clumped isotope thermometry concerns the calibration of the technique at low temperatures (below approximately 10e15  C). Dennis and Schrag (2010) present a set of synthetic calcite experiments that agree in average value with the Ghosh et al. 2006a calibration near room temperatures (w20e40  C), but fall to lower Δ47 values in the temperature range w0e15  C. Similarly, two arctic benthic foraminifera examined by Tripati et al., (2010a,b) are displaced to lower than predicted Δ47 values (these are the two blue diamonds at the extreme right side of the data array in Fig. 2). While these measurements represent a small fraction of all the materials examined by published calibration studies, the author is aware of several unpublished data sets on natural and synthetic materials that support the general result. There are several imaginable explanations of the discrepancies among calibration studies at low temperatures. Most simply, this discipline is sufficiently new that it isn’t yet clear whether differences in analytical methods (acid preparation procedures; acid digestion temperatures or durations; gas purification and transfer procedures) between or even within labs might be capable of generating systematic discrepancies among calibration data sets. The strongest argument against this as the sole explanation is that the two discrepant benthic foraminifera from Tripati et al. (2010a,b) were measured in the same laboratory using the same methods as other materials that conform to the Ghosh et al. (2006a) calibration at low temperatures. Nevertheless, caution is called for here and we suggest that continued exploration of this issue include detailed studies of the effects of laboratory technique on measured Δ47 values for various types of carbonates. If these discrepancies are not an analytical artifact, we consider two possible explanations that call on the physical and biochemistry if isotopic clumping: First, it is possible that the true equilibrium calibration is similar to that of Ghosh et al., 2006a, and the exceptions reflect kinetic

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isotope effects peculiar to the materials or experiments in question. The principal support for this argument is the large number and diversity of materials (formanifera, deep sea corals, at least some mollusks, brachiopods and fish otoliths) grown between 0 and 10  C that exhibit the high Δ47 values predicted by the Ghosh et al. calibration (Fig. 2). In this case, lower Δ47 values in other low temperature materials (i.e., Dennis et al.’s experiments in the 0e15  C range and the two discrepant arctic forams from Tripati et al., 2010a,b) might reflect a non-equilibrium isotopic distribution that occurs preferentially in cold aqueous solutions. The kinetics and thermodynamics of dissolved inorganic carbon provide some support for this possibility: pCO2 over an aqueous solution of constant total dissolved inorganic carbon content increases by a factor of three when temperature is decreased from 25 to 0  C (Dickson and Goyet, 1994). Thus, if all other factors are held constant, the driving force for degassing is substantially stronger in low-temperature aqueous synthesis experiments than in higher temperature ones. And, the activation energy for hydration of aqueous CO2 is large (82 kJ/mol), so the rate of this reaction, which is an essential step in exchange of oxygen between dissolved inorganic carbon and water (Zeebe, 2009), decreases by a factor of w20 between 25 and 0  C (Johnson, 1982). Both of these factors should make the kinetic isotope effects associated with degassing of aqueous solutions more difficult to avoid in low temperature experiments. If this explains the low temperature results of Dennis and Schrag (2010), it remains unclear why Ghosh et al., 2006a, avoided the artifact, nor why some cold-water benthic foraminifera exhibit a similar non-equilibrium state of isotope ordering when many other organisms conform to equilibrium. Alternatively, it is possible that the lower Δ47 values observed in some studies of low-temperature carbonates represent the true low temperature equilibrium, and the relevant synthesis experiment of Ghosh et al. (2006a), and the cold-water natural materials of Came et al. (2007), Ghosh et al. (2007), Tripati et al. (2010a,b) and Thiagarajan et al. (2011a) are influenced by some non-equilibrium isotope effect that elevates Δ47 values. The principle support for this scenario is that the theoretical model of Guo et al. (2009), which combines a thermodynamic model of clumping with a kinetic model of the acid digestion fractionation, more closely resembles the Dennis and Schrag (2010) result than it does the Ghosh et al. (2006a) calibration (however, this argument is weakened by the fact that translation of published data into the ‘absolute reference frame’ of Dennis et al., 2011, moves all of the calibration data away from the theoretical curve). In this case, the Ghosh et al. experiment near 0  C could reflect a kinetic isotope effect or analytical error, and the low-temperature biogenic calibrations of Came et al. (2007), Ghosh et al. (2007), Tripati et al. (2010a,b) and Thiagarajan et al. (2011a) reflect some vital effect that is shared by the relevant group of organisms, uncorrelated with vital effects in d18O and d13C, and noticeable only at low temperatures. This is a valid hypothesis, though it is challenged by another observation: the experimental data of Dennis and Schrag (2010) are actually uncorrelated with temperature between w15 and 0  C; i.e., they are scattered rather than exhibiting the gentler temperature dependence of the Guo et al. (2009) model (Fig. 2). This may simply reflect the small range in Δ47 at issue (i.e., the existing data do not resolve the true temperature dependence). But, it is peculiar given the relative success of several studies in resolving temperature dependencies at higher temperatures. Resolving this problem is a high priority for several reasons. On a practical level, any applications of carbonate clumped isotope thermometry at temperatures of w0e15  C will have to rely on empirical calibrations of closely similar materials until we understand the causes and controls of discrepancies among different studies at low temperatures. Perhaps more importantly, this


discrepancy presents one of the first real mysteries in our understanding of the physical chemistry of isotopic clumping in carbonates. Therefore, it provides an opportunity for discoveries that could illuminate our understanding of vital effects and the chemical kinetics of the dissolved carbonate system. In any event, it is one or the more attractive targets for future studies of the fundamentals of this field. 3.5. Soil and lake carbonates Measurements of modern natural soil and lake carbonates present a challenge to independent calibrations of the carbonate clumped isotope thermometer because both sample environments that exhibit complex temporal and spatial temperature gradients, and generally one isn’t certain of the times or (in lakes) locations of carbonate growth. Nevertheless, their great importance as records of terrestrial climate change demand some effort to understand their suitability for carbonate clumped isotope thermometry. There have been 3 published studies that applied this technique to soil and lake carbonates and included measurements of modern or recent materials, for which one can make some sort of estimate of the air, ground and water temperatures at the growth site (Quade et al., 2007; Huntington et al., 2010; Passey et al., 2010), and a large body of related work is expected to be published within w one year of publication of this review. Collectively, these studies are consistent with the interpretation that both of these materials record the temperature and d18O value of water at their time and place of growth (i.e., they grow in isotopic equilibrium with their surroundings), but that seasonal, spatial and possibly diurnal temperature variations contribute importantly to their isotopic variability. Previous measurements of recent soil carbonates (Quade et al., 2007; Passey et al., 2010) reveal several telling trends: tropical savanah environments, which exhibit little seasonality in air temperature, provide carbonate clumped isotope temperatures for soil nodules that are similar to mean annual temperatures. Temperatures for higher latitude locations exhibit a bias toward warmer than mean annual temperatures, and this discrepancy increases with decreasing mean annual temperature. Individual soil horizons tend to preserve gradients of decreasing temperature with increasing depth, suggesting that carbonate growth occurs preferentially during warm-season months (most data supporting this result has yet to be published). This is plausible, as in the sites studied to-date late summer corresponds to the season when soils dry out following spring and early summer rains. Finally, the zerodepth intersection of these depth profiles ranges from similar to summer-season air temperature to several degrees C higher, plausibly due to radiative ground warming. Taken together, these observations suggest that soil carbonates present a valuable and more or less accurate record of ground temperature, but that ground temperature has a complex relationship to climate. The relevant factors (ground warming, seasonality, etc.) also influence the oxygen isotope composition of carbonates, and thus the lessons learned from clumped isotope studies may impact the interpretation of paleoclimate studies using conventional stable isotope approaches. Lake carbonates have been explored less extensively than soils (Huntington et al., 2010; there is relatively little unpublished data at present). Briefly, large samples (likely averaging a year or more of sediment accumulation) of modern lake carbonates from the western US exhibit a correlation of average apparent temperature with altitude that resembles the altitude gradient in surface air temperatures across the same region. This is loosely consistent with the notion that lake carbonate muds record the temperatures of their parent waters. However, individual analyses of these lake


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muds are highly variable in apparent temperature, and we have no information that illuminates how this variability relates to the large seasonal and depth variations in lake temperatures. It is tempting to assume the variations in measured apparent growth temperatures of carbonates reflect the known variations in water temperature, but no evidence clearly supports this conclusion at present. 3.6. The water problem The essential dilemma of conventional carbonate-water oxygen isotope paleothermometry (and other isotope thermometry based on isotopic compositions of authigenic minerals) is that one is generally attempting to solve one equation with two unknowns; i.e., the measured d18O of carbonate is a function of both temperature and the d18O of waters from which it grew, neither of which are independently known. The oxygen isotope composition of the oceans is sensitive to volume of water stored in continental ice, and it is imaginable that there are similar sensitivities to the size and properties of the Earth’s reservoir of pore waters (Dansgaard and Tauber, 1969; Adkins et al., 2002). And, the oxygen isotope composition of seawater varies significantly with geography, water depth and season (LeGrande and Schmidt, 2006). Variability of the 18 16 O/ O of seawater over longer timescales is among the more divisive and refractory issues in stable isotope geochemistry, with one side holding that the ocean is effectively buffered by interactions with the ocean crust (e.g., Land and Lynch, 1996; Muehlenbachs, 1998), and the other suggesting that the balance of high and low temperature reactions that ultimately control the d18O of the ocean might have differed dramatically in the past (e.g., Veizer et al., 1997; Jaffres et al., 2007). This debate has effectively precluded any consensus view of temperature reconstructions prior to the Mesozoic based on oxygen isotope paleotemperature techniques. Similarly, attempts to extend carbonate paleothermometry to terrestrial, lacustrine and sub-surface carbonates

struggle with the largely unconstrained variability of the oxygen isotope compositions of past meteoric and ground waters. A related problem occurs when the isotopic compositions of carbonates (and other authigenic minerals) are used as constraints on the 18O contents of past waters: A unique solution exists only when the growth temperature of the mineral is independently known (for example, teeth or bones of common mammals). Arguably the greatest strength of carbonate clumped isotope geochemistry is that it simultaneously solves both parts of this problem with no unresolved assumptions (other than the underlying premise of all thermodynamically based thermometers; i.e., that the system in question records equilibrium at some condition). Consider a calcite having a d18OSMOW of 25& (a common value for mid-Phanerozoic carbonate muds; Land and Lynch, 1996, and references therein). This composition is consistent with growth from modern seawater (d18OSMOW w 0&) at a temperature of 43  C (e.g., during early diagenesis in the presence of marine pore waters), or from mid-latitude meteoric water (d18OSMOW w 10&) at 0  C (e.g., precipitation from on the bottom of an ice-capped lake; or, similarly, a low-d18O ocean, if such a thing is possible), or from formation waters (d18OSMOW w þ10&) at 121  C (e.g., late diagenesis or anchimetamorphism) (Fig. 3). A carbonate clumped isotope thermometry measurement of this sample would constrain its growth temperature directly without reference to the d18O of water. One could then refer to the equilibrium carbonate-water fractionation at this temperature to calculate the d18O of water, turning an essentially uninterpretable measurement into a direct and quantitative constraint of both temperature and the origin of the fluid with which the carbonate last equilibrated (Huntington et al., 2011). This capacity to transform an ambiguous conventional oxygen isotope record into a rich source of relatively assumption-free paleoclimatological or diagenetic information is what motivates research in carbonate clumped isotope thermometry, despite its various challenges.

Fig. 3. Calculated stable isotope systematics of calcite and co-existing water equilibrated over a range of geological conditions. All calculated compositions assume the calcite-water oxygen isotope fractionation of O’Neil et al. (1969). Values of Δ47 corresponding to the plotted temperature assume the calibration of Ghosh et al., 2006a, from 0 to 50  C, and the model of Guo et al. (2009), at higher temperatures. Compositions of diagenetic and metamorphic systems evolving in a closed (i.e., rock-buffered) system are labeled ‘closed’; those in open (fluid-buffered) systems with marine or meteoric pore waters are labeled ‘open marine’ and ‘open meteoric’, respectively.

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Previous papers presenting the principles and applications of carbonate clumped isotope thermometry have noted this strength of the technique, but no previous paper presents a broader discussion of the combined clumped isotope and conventional stable isotope systematics of depositional, diagenetic and metamorphic environments. Fig. 3 and the following paragraphs present a general overview of this issue. Every clumped isotope analysis of a carbonate sample constrains three properties: The temperature of last equilibration of that carbonate (assuming the measured temperature corresponds to a specific growth event rather than a diffusional blocking temperature and/or physical mixing between two or more populations with different growth temperatures); the d18O of the carbonate mineral itself (i.e., because all clumped isotope measurements are also measurements of bulk isotopic composition); and the d18O of the water from which that carbonate grew or with which it last equilibrated (i.e., based on the known temperature-dependent oxygen isotope fractionation between carbonate and water). Fig. 3 presents the isotope systematics d including clumped isotope temperature, d18O of carbonate and d18O of water d for model scenarios spanning a range of depositional, diagenetic and metamorphic conditions commonly encountered at conditions of the earth’s surface and shallow crust. The horizontal axis of this figure plots the growth temperature of carbonate (the equivalent measured Δ47 value of carbonate, based on the calibration of Ghosh et al., 2006a, or, for temperatures greater than 50  C, the model of Guo et al., 2009, is shown on the top horizontal axis). The vertical axis plots the d18OSMOW of water in equilibrium with carbonate at its growth temperature. Dashed contours plot the d18OSMOW of carbonate consistent with each temperature and d18O of water (based on the calcite-water fractionation of O’Neil et al., 1969, chosen because it provides a single continuous function over the full temperature range explored; calculations of this type at near-earth-surface temperatures would be better made with more recent calibrations, such as Kim and O’Neil, 1997). The main panel of Fig. 3 shows model results at a scale most appropriate for discussing diagenetic and metamorphic processes, whereas the inset shows the range of compositions most relevant for carbonate deposition on or near the earth’s surface. Colored labels indicate broad categories of physical environment and fluid sources for carbonate formation. Several important features of the stable isotope systematics of carbonate-water systems can be deduced from Fig. 3. Most simply, a d18O value of a carbonate mineral (i.e., location along any one of the dashed contours), taken by itself, can be consistent with a wide range of physical environments and water sources. Two endmember behaviors can be expected of diagenetic and metamorphic processes: Closed system processes, in which the oxygen isotope budget of the rock and co-existing pore waters is buffered by solid carbonate, and heating results in evolution along a path of near constant d18O of carbonate and eventually creates pore waters with strongly elevated d18O values (þ10 to þ20&). Such fluid compositions have been observed in formation waters from deep (>1e2 km) wells in carbonate- or shale-dominated sequences. Open system diagenesis and metamorphism, in contrast, will follow paths through Fig. 3 of increasing temperature (decreasing Δ47 of carbonate) at constant d18O of water d near 0& if fluids are marine pore waters; lower if meteoric ground waters d and sharply decreasing d18O of carbonate. Natural systems undergoing burial diagenesis and metamorphism may follow either or (if heterogenous and/or time varying) both of these extremes, or intermediate cases in which d18O of water and carbonate co-vary complexly with varying temperature of watererock reaction. One of the so-far unrealized potentials of carbonate clumped isotope thermometry is the systematic study of such sub-surface water/


rock reaction processes (for initial efforts see Huntington et al., 2011; Bergmann et al., 2011 and Ferry et al., 2011). Combined clumped isotope and conventional stable isotope analysis of carbonates precipitated in near-surface environments (inset to Fig. 3) can permit one to simultaneously reconstruct climate and discriminate among marine, meteoric and evaporatively enriched parent waters. Recent studies of Paleozoic marine carbonates illustrate the usefulness of these approaches in de-convolving the effects of diagenesis from those of changing primary depositional conditions, and in separating the effects of temperature change from ice volume in the primary marine carbonate record. Came et al. (2007) present the first, relatively simple example of this approach. This study presents isotopic measurements of two sets of Paleozoic fossils d Silurian brachiopods and Pennsylvanian mollusks. Both suites exhibit ranges in bulk isotopic composition and apparent temperature, as recorded by clumped isotope composition, ranging from temperatures and implied d18O of water consistent with growth from marine waters at earth surface conditions, to higher temperatures and d18O of water that more plausibly reflect modification during burial, either through diagenetic replacement reactions (as suggested for Pennsylvanian samples by relationships of carbonate isotopic composition to trace metal content) or perhaps by partial resetting by oxygen self diffusion in the carbonate lattice (the atomistic mechanisms that modify clumped isotope compositions during burial are presently unclear and an active area of research; this issue is discussed further in a separate section below). The isotopic compositions of the apparently best preserved fossils indicate that these two suites grew under significantly different temperatures d a ‘hot house’ condition of w33e35  C for the Silurian vs. an ‘ice house’ condition of w25  C for the Pennsylvanian d from an ocean that differed little in d18O between these two times. These data are sparse, but open a new window on some of the oldest problems in paleoclimate research d the nature of Paleozoic climate and long-term variability in the oxygen isotope compositions of marine waters. More recently, Finnegan et al. (2011) conducted a more in-depth study of the end-Ordovician glaciation using these techniques. The larger, phylogenetically diverse suite of samples they examined also presented evidence of widespread diagenetic modification. In this case, trace metal indicators of diagenesis (e.g., Mn, Fe, Sr) were measured on the same splits of powders analyzed for clumped isotope composition, revealing a relationship between the two that implicates partial recrystallization in the presence of trace-metal rich pore fluids as the mechanism responsible for increasing the temperatures of some fossils during burial. After filtering the results in an effort to remove the effects of post-depositional modification, the record documents how tropical surface water temperatures and ocean d18O values co-varied through a major ancient glaciation. These data permit quantitative reconstruction of ice volume through a glaciation, and document the relationship between the sizes of high-latitude ice sheets and tropical temperatures. Most intriguingly, these data suggest that ‘hot house’ sea surface temperatures (w35  C) can persist during growth of significant volumes of ice at high latitude, but then plummet by w5e10  C in synch with further expansion of ice and the simultaneous end-Ordovician mass extinction. Data of this type permit one to ask relatively sophisticated questions about the dynamics of Paleozoic climate, and so provide an opportunity for significant conceptual advances to our understanding of climatic variations over the full span of earth history. Two studies have applied these techniques to more recent marine paleoclimate problems. Tripati et al. (2010b) reconstruct the temperature contrast between the last glacial maximum and present in the western pacific tropical warm pool surface ocean, based on measurements of foraminifera and coccolithophores. This


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problem has been approached with several other paleotemperature proxies, many of which are less labor intensive and more precise than carbonate clumped isotope thermometry. But, the data presented in Tripati et al. provide an independent ‘vote’ for resolving the large discrepancies that exist among past Mg/Ca, Sr/Ca and phylogenetic studies (generally speaking, clumped isotope results seem to support Mg/Ca reconstructions). One of the more interesting features of Tripati et al.’s study is the use of coccolithophores and diverse foraminiferal species as a collective archive of paleotemperature information for the surface ocean. These materials yield a coherent temperature record with no obvious materialspecific offsets, suggesting it should be possible to use carbonate clumped isotope thermometry to reconstruct marine temperatures without resorting to hand picking of individual taxa. Thiagarajan et al. (2011b) present a study of north Atlantic deep ocean temperatures in the Younger Dryas and Heinrich-I intervals, based on measurements of deep sea corals. This study takes advantage of the apparent lack of vital effects in the clumped isotope compositions of such corals (Thiagarajan et al., 2011a) to recover an accurate temperature record despite the samples’ pronounced vital effects in d13C and d18O. This study also presents perhaps the most extreme example in which the precision of carbonate clumped isotope temperatures has been refined through extensive replication and mutual standardization of the samples (i.e., to a common reference frame of accepted values for intralaboratory standards). It appears possible to reconstruct temperatures with precision of 1  C or better through such efforts. Terrestrial paleoclimatology presents a second set of problems where clumped isotope thermometry is useful principally because of its ability to provide quantitative paleoclimate information when water d18O is unknown. This is a subject where there is potential for many advances in understanding of even relatively recent (i.e., Pliocene or Pleistocene) climate because there are so few alternative approaches that can provide assumption-free measurements of air or ground temperatures on land. And, this benefit may be amplified by the fact that water d18O is the paleoclimate archive of greatest interest for some studies. Even if one is principally interested in reconstructing d18O of water rather than temperature, temperature must be known or assumed in order to convert d18O of conventional oxygen isotope proxies (e.g., carbonate or phosphatic fossils) into d18O of water. Common terrestrial materials that present useful clumped isotope records of climate include: carbonate shells (e.g., fresh water or land snails), soil concretions, and lacustrine carbonate muds. Most of the work done in this area to-date has been focused on paleoaltimetry as a component of tectonics studies (Ghosh et al., 2006b; Quade et al., 2007; Garzione et al., 2008; Huntington et al., 2010; Smith et al., in review). However, Passey et al. (2010) present a reconstruction of Pliocene climate in east Africa based on carbonate clumped isotope analysis of soil carbonates, and Csank et al. (2011) recently presented an application of these methods to quantifying Pliocene temperatures in the Arctic. Both studies provide examples of the ways in which carbonate clumped isotope thermometry can be used to gain insight about the connection of terrestrial climate to the relatively well known marine record. Finally, fossils of vertebrate land animals present another complex but attractive target for carbonate clumped isotope thermometry. Vertebrate land animals commonly feed off vegetation that contains water that is isotopically enriched through evaporation, and/or directly drink from ponds and streams that experience variable amounts of evaporative enrichment (fractionations associated with body water losses may further contribute to discrepancies with respect to meteoric water; Kohn and Cerling, 2002). As a result, the d18O of land vertebrate body water can be as much as a few per mil greater than local meteoric water; therefore, the d18O

of phosphatic and carbonate components of bio-apatite (bones and teeth) cannot be interpreted as simple measures of the temperature dependent fractionation with respect to meteoric waters. There have been several responses to this complexity of the land vertebrate stable isotope record: land mammals presumably have varied little in body temperature over geological time scales and thus provide an indirect record of variations in d18O of whatever materials they used as their water sources (assuming their diets and fractionations associated with body water losses remain constant or known; Luz and Kolodny, 1985). Alternatively, Amiot et al. (2004) have devised empirical paleotemperature proxies based on the latitudinal variations in isotopic compositions of teeth, bones and shells, averaged over a variety of mammals and non-mammals from similar environments. The recent demonstration that carbonate clumped isotope thermometry can be applied to the carbonate ion groups in bioapatite (Eagle et al., 2010, 2011) provides an opportunity to approach these materials differently d body temperatures can be measured directly and, therefore, the d18O of body waters can be calculated based on the relevant temperaturedependent mineral-water fractionations. It is not obvious that this approach adds anything to the study of ancient mammals (unless, of course, assumptions regarding the constancy of mammalian body temperatures over geological time scales are wrong). But it could be transformative for the study of ectotherms or organisms with uncertain metabolisms. Direct determination of body temperatures of such organisms will permit their use as quantitative paleoclimate archives (though the intrinsic complexity of the isotopic budgets of animal body waters will still make it challenging to translate such records into measures of the d18O of meteoric water), and will provide a new approach to studying the physiologies of extinct organisms (e.g., Eagle et al., 2011). 3.7. Diagenesis and metamorphism One of Urey’s primary motivations for focusing on the carbonate-water system as the first stable isotope thermometer was that dissolved inorganic carbon is soluble and undergoes rapid isotopic equilibration in aqueous solutions of near neutral pH (Urey, 1947); therefore, one generally expects carbonate minerals to record thermodynamic equilibrium with their growth environments even at the low temperatures of the earth’s surface. In this respect, carbonate is preferable to phosphate and sulfate, which are refractory to oxygen isotope exchange with water at earth surface conditions (Cole and Chakraborty, 2001). However, the solubility of solid carbonates and exchangeability of dissolved carbonate species also presents one of the discipline’s greatest challenges: preservation of primary (i.e., depositional) isotopic compositions through diagenesis, burial and metamorphism. Macroscopic carbonate fossils are a prominent component of many phanerozoic marine and lacustrine sediments, and limestones preserve evidence of carbonate precipitating environments throughout most of the geological record. However, a large proportion of this material has undergone post-depositional alteration through infilling of primary void spaces, dissolution and replacement, more cryptic recrystallization that preserves macroscopic fabrics but changes chemistry, and diffusional exchange with pore waters or other co-existing phases (Land, 1967; Schroeder, 1969; Brand and Veizer, 1980). These processes have the potential to modify the oxygen isotopes (and other chemical properties) of carbonates, confounding efforts to use them as paleoclimatological archives (e.g., Banner and Hanson, 1990). A variety of approaches have been taken to filtering or correcting for the effects of burial diagenesis and metamorphism on the stable isotope compositions of ancient carbonates: pre-selection of only certain textural and/or mineralogical types, with the presumption

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that the have escaped post depositional modification. For example, aragonitic mollusks that lack x-ray diffraction evidence for secondary calcite (Veizer et al., 2000); or, more qualitatively, foraminifera having a ‘glassy’ appearance (Wilson et al., 2002). Alternatively, many studies filter data sets based on trace element or other geochemical evidence for post depositional modification (e.g., Grossman, 1994; Mii et al., 1999; Jacobsen and Kaufman, 1999). Or, some studies have suggested that one can filter the stable isotope data based on the statistics of isotopic variation alone. For example, it has been suggested that the highest d18O observed in marine authigenic sediments at a given time, or the average of the highest 50% of all measured values in a given time window, represent the primary (depositional) composition (Knauth and Lowe, 2003; Jaffres et al., 2007). The validity of these approaches likely varies from case to case, and it seems inevitable that any chosen threshold in minor or trace element content, glassiness, etc., will fail to identify some altered materials or falsely reject some primary materials. Carbonate clumped isotope thermometry does not present a means of avoiding the post-depositional disturbance of stable isotope compositions. If anything, carbonate ‘clumping’ is more vulnerable to diagenesis and metamorphism than many bulk compositional indices because recrystallization of a carbonatedominated rock in a closed system may preserve bulk composition but re-order the isotopic distribution of CeO bonds to reflect equilibrium at the temperature of recrystallization. And, under some conditions solid state diffusion of oxygen through a crystalline carbonate could lead to a change in proportions of 13Ce18O bonds with little or no isotopic exchange between that crystal and its surrounding phases. Nevertheless, clumped isotope thermometry does provide a new set of constraints on the extents and mechanisms of post-depositional modifications of carbonate, and can be combined with conventional stable isotope, trace element, crystallographic and textural data to develop a more detailed understanding of the effects of these processes on paleoclimatological records. For this reason, it may be possible to use clumped isotope data to more reliably identify materials that have escaped the effects of burial diagenesis and metamorphism. Fig. 4 presents a simple example of the utility of clumped isotope data in detailing the effects of post-depositional processes on stable isotope compositions of carbonates. Panel A of this figure is a photomicrograph of a relatively homogeneous, micritic soil carbonate nodule recovered from a 54.1 Ma paleosol in the Bighorn Basin (data for this sample are from Snell et al., 2011; the images are previously unpublished and were provided by K. Snell). This material has a d18OSMOW value of 22.4& and a carbonate clumped


isotope temperature of 35.6  C; this composition is consistent with growth from meteoric or ground waters during the summer in this warm climatic period, and it resembles measurements of similar materials of the same age and location (Snell et al., 2011). Panel B is a photomicrograph of a portion of the same sample in which a vein of spar calcite cross cuts the (presumably) primary micritic fabric. This vein has a clumped isotope temperature of 56.6  C d higher than any plausible earth surface temperature d and a d18OSMOW of 14.4& d significantly lower than values commonly encountered in near-surface authigenic carbonate in mid latitudes and low or moderate altitudes. The combination of temperature and bulk stable isotope composition indicate this vein grew during burial diagenesis at a depth of w1 km from water having a d18OSMOW of 8.3& d plausibly a deep ground water or formation water derived from meteoric precipitation. While it is obvious from petrographic inspection that this sample has undergone some sort of post-depositional modification, a quantitative temperature determination on the secondary carbonate fabric lets one reach firm conclusions about the environment and fluid sources of diagenesis. A significant number of prior studies have examined the clumped isotope compositions of marbles, carbonatites and diagenetically or metamorphically modified limestones, dolostones and bioapatites (Ghosh et al., 2006a; Eiler et al., 2006; Came et al., 2007; Eiler, 2007; Dennis and Schrag, 2010; Eagle et al., 2010, 2011; Bergmann et al., 2011; Bristow et al., 2011; Ferry et al., 2011; Finnegan et al., 2011; Huntington et al., 2011). Fig. 5 and the following paragraphs summarize the findings of these studies and their broader implications. Slowly cooled regional metamorphic marbles generally record apparent temperatures of w175e200  C, up to w300  C for dolomite marbles (Ghosh et al., 2006a; Eiler, 2007; Ferry et al., 2011). This has been inferred to represent the blocking temperature with respect to diffusional resetting of the carbonate clumped isotope thermometer when cooling is gradual (a few, to perhaps tens of  C/ Ma). Similar temperatures are observed in carbonatites (Dennis and Schrag, 2010); these may be diffusional blocking temperatures, though many carbonatites undergo sub-solidus metasomatism and recrystallization, so in some cases these temperatures may reflect a metamorphic overprint event. The highest carbonate clumped isotope temperature measured to-date on a geological material is 476  C, obtained for a metasomatic calcite vein in a late-Precambrian dolostone (Bristow et al., 2011). In light of the lower temperatures observed for regional metamorphic marbles, this high temperature (if it is, in fact, a mineral that grew at equilibrium with respect to the relevant clumped isotope reaction) presumably reflects the rapid cooling this sample experienced following vein growth.

Fig. 4. Textures and stable isotope (including clumped isotope) compositions of micritic (panel A) and sparry vein (panel B) calcite from a 54.1 Ma soil carbonate nodule from the Bighorn Basin. Both images are transmitted light photomicrographs taken with plane polarized light and a field of view 1.4 mm wide. Stable isotope data are reproduced from Snell et al., 2011. Images are previously unpublished and were taken by K. Snell.


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Fig. 5. Synopsis of common behaviors of the carbonate clumped isotope thermometer for calcite (blue) and dolomite (red), including both micritic muds (solid lines) and coarse materials (fossils, early diagenetic cements, etc.) that retain a primary fabric through compaction (dashed lines), during burial diagenesis and early metamorphism along a geothermal gradient shown by the black line. Shallow (2e3 km) burial may be accompanied by early diagenetic modification of some components, but does not drive comprehensive recrystallization. Micritic matrices of limestones and dolostones subjected to depths greater than 2e3 km (temperatures greater than w50e80  C) are commonly pervasively rerystallized, perhaps in response to compaction. Coarse primary and early diagenetic fabrics commonly survive this process. At burial depths in excess of 6e7 km (temperatures greater than w160e180  C), calcite approaches the blocking temperature observed in slowly cooled calcite marbles and resetting should affect all coarse and fine materials (presumably by solid state diffusion). Dolomite appears to have a higher blocking temperature in slowly cooled marbles (Ferry et al., 2011) and may preserve depositional and early diagenetic temperatures to depths approaching 12 þ km (temperatures of w300  C). Higher temperatures may be observed in materials that undergo geologically brief heating events (e.g., Bristow et al., 2011).

Diagenetic modification of macroscopic fossils in the temperature range w30e80  C generally involves coupled increases in apparent temperature and trace metal content and/ or changes in bulk stable isotope composition (Came et al., 2007; Finnegan et al., 2011), suggesting dissolution/reprecipitation reactions are the common mechanisms of exchange in this temperature range. This is also a common interpretation of oxygen isotope variations of carbonate fossils, though there are discrepancies between the two approaches which call for further study: some samples that pass established thresholds in trace element chemistry for identification of ‘pristine’ d18O values are recognized as altered by combined trace-element/clumped isotope systematics (Finnegan et al., 2011). And, aragonitic and low-Mg calcite fossils that are identified as pristine based on XRD or textural observations may also be identified as diagenetically modified by combined trace element/clumped isotope criteria (Came et al., 2007; Finnegan et al., 2011). The micritic matrices of limestones and dolostones commonly record temperatures of w50e80  C; some samples recovered by drill core yield temperatures lower than the borehole temperatures; and related samples exhumed by erosion and exposed in outcrop appear unmodified relative to their drill-core equivalents; these observations suggest temperatures in this range reflect recrystallization during compaction and lithification rather than retrograde re-equilibration (Bergmann et al., 2011). Finally, carbonate clumped isotope analyses of bioapatites, combined with conventional oxygen isotope analyses of phosphate and carbonate groups, reveal complex patterns of diagenetic alteration (Eagle et al., 2010, 2011). Generally, bone and dentin are pervasively altered in even texturally well-preserved materials. Enamel better resists diagenetic modification of its clumped isotope and conventional stable isotope signatures, but is only dependably well preserved in the most pristine and shallowly buried materials. On the basis of these initial studies, it seems likely that common approaches to the stable isotope study of phosphatic

materials substantially under estimate the extent to which their d18O values have been modified after deposition. 4. Summary and prospectus Carbonate clumped isotope thermometry has expanded greatly in the last 2e3 years: there are currently more than 30 relevant papers published (most in 2009e2011), the PI is aware of more than a dozen independent laboratories that are productive and more-orless mutually standardized, and the second annual clumped isotope workshop, recently held at Imperial College, London, was attended by nearly 100 people. It seems likely that the foundation summarized here will soon be outgrown, and in many respects supplanted, by a larger body of work from this community. The following issues are likely to be central to this next phase of development of this tool:  Work to-date on calibrations is significant and at least superficially coherent (Fig. 2), but there is an emerging problem in our understanding of the temperature dependence of 13Ce18O ordering in carbonates at temperatures near the freezing point of water (Fig. 2 inset). It is key that we understand the origin of these discrepancies, the nature of the true equilibrium calibration, and the physical processes responsible for deviations from that calibration.  Studies of biogenic carbonates to-date have failed to reveal systematic, well resolved vital effects on the carbonate clumped isotope thermometer. But, our understanding of the chemical kinetics of clumping during some modes of carbonate growth (e.g., speleothems and cryogenic carbonates) suggest such effects should be possible. There is need for detailed, systematic studies of several classes of biogenic carbonate that have not yet been examined carefully. Mollusks are perhaps the most important immediate target for such work.  Field calibrations of complex natural materials (soils, lake sediments, caliches, speleothems) are in their infancy, and the little

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that is known suggests a complex world of kinetic isotope effects and seasonal or other biases stands between our measurements of clumped isotope composition and a record of secular changes in mean annual temperatures. These issues must be explored through systematic experiments and analyses of materials recovered from instrumented field sites, where spatial and temporal gradients in T and d18O of water are well understood.  The carbonate clumped isotope compositions of pre-Cenozoic materials are commonly modified by burial diagenesis and low grade metamorphism. These processes have consequences for virtually every geochemical paleo-environmental proxy, but it is particularly important that new proxies explore their effects, both to develop informed judgment regarding the identification of well preserved materials and because the process of diagenesis is itself worthy of study and likely to advance with application of a new approach. Laboratory study of the kinetics of clumped isotope reactions will be a key component of this work.  Finally, the ultimate arbiter of success for paleoclimate proxies is their ability to contribute to scientific discoveries. Carbonate clumped isotope thermometry has enough discoveries to its credit that it is no longer just an idea with promise. But, its long-term significance cannot be judged until it has been taken up by several groups, multiple studies are conducted of related materials and problems, and the successes and failures of clumped isotope work can be fit within the broader dialectic of debate in paleoclimatology. Subjects that seem primed for this sort of focused, multi-group debate include: the d18O history of seawater in the Paleozoic and Precambrian; altimetry of orogenic plateaus; and the Cenozoic climate history of terrestrial environments. Acknowledgments This manuscript is founded on the work of a large number of present and past members of the Caltech stable isotope research group; the author thanks them for their creativity, persistence and collegiality. Katherine Snell provided images used to make Fig. 4. Kristin Bergmann consulted with the author to help develop Fig. 3. Aradhna Tripati and Nithya Thiagarajan helped compile data used to construct Fig. 2. This manuscript was improved by thoughtful reviews from Ben Passey and the QSR editorial staff. References Adkins, J., McIntyre, K., Schrag, D.P., 2002. The salinity, temperature, and delta O-18 of the glacial deep ocean. Science 298, 1769e1773. Adkins, J., Boyle, E., Curry, W., Lutringer, A., 2003. Stable isotopes in deep-sea corals and a new mechanism for "vital effects. Geochimica et Cosmochimica Acta 67, 1129e1143. Affek, H.P., Eiler, J.M., 2006. Abundance of mass-47 CO2 in urban air, car exhaust and human breath. Geochimica et Cosmochimica Acta 70, 1e12. Affek, H.P., Xu, X.M., Eiler, J.M., 2007. Seasonal and diurnal variations of 13 Ce18Oe16O in air: initial observations from Pasadena, CA. Geochimica et Cosmochimica Acta 71, 5033e5043. Affek, H.P., Bar-Matthews, M., Ayalon, A., Eiler, J.M., et al., 2008. Glacial/interglacial temperature variations in Soreq cave speleothems as recorded by ‘clumped isotope’ thermometry. Geochimica et Cosmochimica Acta 72, 5351e5360. Amiot, R., et al., 2004. Latitudinal temperature gradient during the Cretaceous Upper Campanian-Middle Maastrichtian: d18O record of continental vertebrates. Earth and Planetary Science Letters 226, 255e272. Anand, P., Elderfield, H., Conte, M.H., 2003. Calibration of Mg/Ca thermometry in planktonic foraminifera from a sediment trap time series. Paleoceanography 18, 28e31. Banner, J.L., Hanson, G.N., 1990. Calculation of simultaneous isotopic and traceelement variations during water-rock interaction with applications to carbonate diagenesis. Geochimica et Cosmochimica Acta 54, 3123e3137. Beck, W.C., Grossman, E.L., Morse, J.W., 2005. Experimental studies of oxygen isotope fractionation in the carbonic acid system at 15, 25, and 40  C. Geochimica et Cosmochimica Acta 69, 3493e3503.


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