Combined high-precision ∆48 and ∆47 analysis of carbonates

Geochimica Et Cosmochimica Acta, 2009

Phosphoric acid digestion has been used for oxygen-and carbon-isotope analysis of carbonate minerals since 1950, and was recently established as a method for carbonate 'clumped isotope' analysis. The CO 2 recovered from this reaction has an oxygen isotope composition substantially different from reactant carbonate, by an amount that varies with temperature of reaction and carbonate chemistry. Here, we present a theoretical model of the kinetic isotope effects associated with phosphoric acid digestion of carbonates, based on structural arguments that the key step in the reaction is disproportionation of H 2 CO 3 reaction intermediary. We test that model against previous experimental constraints on the magnitudes and temperature dependences of these oxygen isotope fractionations, and against new experimental determinations of the fractionation of 13 C-18 O-containing isotopologues ('clumped' isotopic species). Our model predicts that the isotope fractionations associated with phosphoric acid digestion of carbonates at 25°C are 10.72&, 0.220&, 0.137&, 0.593& for, respectively, 18 O/ 16 O ratios (1000 lna * ) and three indices that measure proportions of multiply-substituted isotopologues ðD Ã 47 ; D Ã 48 ; D Ã 49 Þ. We also predict that oxygen isotope fractionations follow the mass dependence exponent, k of 0.5281 (where a 17 O ¼ a k 18 O ). These predictions compare favorably to independent experimental constraints for phosphoric acid digestion of calcite, including our new data for fractionations of 13 C-18 O bonds (the measured change in D 47 = 0.23&) during phosphoric acid digestion of calcite at 25°C.

2020

2017

Carbonate minerals have been abundant throughout Earth's geological history and the carbon and oxygen isotope ratios of carbonates can be used for paleoclimate reconstruction based upon the recognized stable isotopic relationship with the environmental factors. However, their accuracy is obscured by "non-equilibrium isotope effects" caused by physicochemical factors, such as solution chemistry, pH and precipitation rate. This study aimed to better understand these factors to improve the robustness of isotope-based paleotemperature proxy and assist in providing a reference frame for future research. Carbonates were synthesized using passive CO2(g) degassing at two different pH levels (~8.2 and ~11.07) through the dissolution of 5, 15 and 25 mmolal sodium bicarbonate (NaHCO3) or sodium carbonate (NA2CO3) and calcium chloride dihydrate (CaCl2•2H2O) at 25 ± 0.1 o C. The 1000ln 18 α(CaCO 3-H 2 O) and 1000ln 13 α(CaCO 3-DIC) values were then compared with established isotopic equilibrium values for oxygen and carbon. Samples were also synthesized in the presence of various concentrations of carbonic anhydrase (CA) and it was found that this enzyme may not influence kinetic isotope effects at higher precipitation rates. A positive, concentration based trend was found for the mid-pH solutions between 1000ln 13 α(CaCO 3-DIC) and 1000ln 18 α(CaCO 3-H 2 O) values which began at the oxygen isotopic equilibrium value proposed by Kim and O'Neil (1997). This trend deviated upwards towards that of Coplen (2007) due to the kinetic influence of precipitation rate and degassing caused by the production of CO2(aq) as a by-product of the aforementioned reaction. The high pH solutions followed an opposite trend, with 1000ln 13 α(CaCO 3-DIC) continuing the enrichment trend but 1000ln 18 α(CaCO 3-H 2 O) values declining and may have been caused by CO2(aq) not being produced in the high pH reactions. The DIC equilibration time at this pH took 7 days as shown by Kim et al. (2006) and not 45 days as discussed in Beck et al. (2005).

Geochemistry, Geophysics, Geosystems, 2019

The clumped isotopic composition of carbonate-derived CO 2 (denoted Δ 47) is a function of carbonate formation temperature and in natural samples can act as a recorder of paleoclimate, burial, or diagenetic conditions. The absolute abundance of heavy isotopes in the universal standards VPDB and VSMOW (defined by four parameters: R 13 VPDB , R 17 VSMOW , R 18 VSMOW , and λ) impact calculated Δ 47 values. Here, we investigate whether use of updated and more accurate values for these parameters can remove observed interlaboratory differences in the measured T-Δ 47 relationship. Using the updated parameters, we reprocess 14 published calibration data sets measured in 11 different laboratories, representing many mineralogies, bulk compositions, sample types, reaction temperatures, and sample preparation and analysis methods. Exploiting this large composite data set (n = 1,253 sample replicates), we investigate the possibility for a "universal" clumped isotope calibration. We find that applying updated parameters improves the T-Δ 47 relationship (reduces residuals) within most labs and improves overall agreement but does not eliminate all interlaboratory differences. We reaffirm earlier findings that different mineralogies do not require different calibration equations and that cleaning procedures, method of pressure baseline correction, and mass spectrometer type do not affect interlaboratory agreement. We also present new estimates of the temperature dependence of the acid digestion fractionation for Δ 47 (Δ* 25-X), based on combining reprocessed data from four studies, and new theoretical equilibrium values to be used in calculation of the empirical transfer function. Overall, we have ruled out a number of possible causes of interlaboratory disagreement in the T-Δ 47 relationship, but many more remain to be investigated. Plain Language Summary Measured stable and clumped isotope values are fundamentally tied to established compositions of international standard materials. When these standard compositions are updated, it impacts previously published isotope measurements such as those used to define the clumped isotope calibration relationship (the foundation for use of this isotopic proxy as a paleothermometer, ©2019. American Geophysical Union. All Rights Reserved.

Geochimica et Cosmochimica Acta, 2011

We present a revised approach for standardizing and reporting analyses of multiply substituted isotopologues of CO 2 (i.e., 'clumped' isotopic species, especially the mass-47 isotopologues). Our approach standardizes such data to an absolute reference frame based on theoretical predictions of the abundances of multiply-substituted isotopologues in gaseous CO 2 at thermodynamic equilibrium. This reference frame is preferred over an inter-laboratory calibration of carbonates because it enables all laboratories measuring mass 47 CO 2 to use a common scale that is tied directly to theoretical predictions of clumping in CO 2 , regardless of the laboratory's primary research field (carbonate thermometry or CO 2 biogeochemistry); it explicitly accounts for mass spectrometric artifacts rather than convolving (and potentially confusing) them with chemical fractionations associated with sample preparation; and it is based on a thermodynamic equilibrium that can be experimentally established in any suitably equipped laboratory using commonly available materials. By analyzing CO 2 gases that have been subjected to established laboratory procedures known to promote isotopic equilibrium (i.e., heated gases and water-equilibrated CO 2), and by reference to thermodynamic predictions of equilibrium isotopic distributions, it is possible to construct an empirical transfer function that is applicable to data with unknown clumped isotope signatures. This transfer function empirically accounts for the fragmentation and recombination reactions that occur in electron impact ionization sources and other mass spectrometric artifacts. We describe the protocol necessary to construct such a reference frame, the method for converting gases with unknown clumped isotope compositions to this reference frame, and suggest a protocol for ensuring that all reported isotopic compositions (e.g., D 47 values; Eiler and Schauble, 2004; Eiler, 2007) can be compared among different laboratories and instruments, independent of laboratory-specific analytical or methodological differences. We then discuss the use of intra-laboratory secondary reference frames (e.g., based on carbonate standards) that can be more easily used to track the evolution of each laboratory's empirical transfer function. Finally, we show inter-laboratory reproducibility on the order of ±0.010 (1r) for four carbonate standards, and present revised paleotemperature scales that should be used to convert carbonate clumped isotope signatures to temperature when using the absolute reference frame described here. Even when using the reference frame, small discrepancies remain between two previously published synthetic carbonate calibrations. We discuss possible reasons for these discrepancies, and highlight the need for additional low temperature (<15°C) synthetic carbonate experiments.

2005

Carbon and oxygen isotope measurements of 66 samples from the 60 m-thick variegated marble in the Upper Allochthon of the Norwegian Caledonides have a mean δ 13 C carb of −8.4 ± 0.9‰ (V-PDB), and a mean δ 18 O of 20.2 ± 2.2‰ (V-SMOW). The variegated marble is overlain by 150 m-thick pale grey marble characterised by mean δ 13 C carb of −6.5 ± 0.8‰ (n = 25) and underlain by dark grey marbles with a mean δ 13 C carb of +4.8 ± 1.1‰ (n = 61). This tripartite unit of an poorly constrained age-but between Neoproterozoic and Early Silurian-discontinuously developed over a distance of 500 km, is likely to represent one of the largest isotopically anomalous sedimentary carbonate formations yet reported. The marbles depleted in 13 C beyond the canonical mantle value of −6‰ show no obvious evidence of post-sedimentary repartitioning of carbon isotopes.

Geochimica et Cosmochimica Acta, 2010

Measurement of the ratio of 18 O to 16 O in CO 2 ðd CO 2 18 OÞ produced from rhombohedral carbonate minerals in the compositional range CaCO 3 -MgCO 3 by reaction with polyphosphoric acid (PPA), at temperatures of between 25 and 110 °C, shows that values of d CO 2 18 O are linearly correlated (r o > 0.99) with the reciprocal of absolute reaction temperature (K/T). This observation is consistent with earlier studies documenting the effect of temperature on the kinetic fractionation of oxygen isotopes between parent carbonate and product CO 2 and H 2 O during acid decomposition. However, analysis of the resultant data reveals: (1) a progressive increase in dd 18 O=dT À1 with increasing Mg content, and (2) a significant variation in dd 18 O=dT À1 between individual samples of carbonate of identical lattice symmetry and similar chemical composition. The overall increase in gradient with increasing Mg content is assumed to reflect cation radius dependent factors that control the bonding environment at the interface between the metal cation exposed at the surface of the reacting carbonate solid and a H 2 CO 3 transitional species during disproportionation of H 2 CO 3 to CO 2 and H 2 O ("cluster model" of . Phase-specific variations in dd 18 O=dT À1 might result from differences in lattice structure variables (e.g., degree of lattice distortion, extent of positional disorder, and non-ideal mixing of substituent cations where carbonates depart from end-member compositions). Lattice structure variables may be dependent on geochemical conditions pertaining at the time of carbonate precipitation (e.g., biosynthetic versus inorganic precipitates) and suggests that dd 18 O=dT À1 has the potential to vary, within limits, in response to both the chemical composition and structure of each carbonate sample. Because the oxygen isotope composition of carbonate minerals ðd MCO 3 18 OÞ measured on the VPDB scale is defined by the oxygen isotope composition of CO 2 prepared from NBS19 (calcite) by reaction with PPA at 25 °C , the variation in dd 18 O=dT À1 potentially introduces a non-systematic uncertainty to the calibration of the oxygen isotope composition of CO 2 produced at higher temperatures. Calibration of other carbonate phases, using a reference frame defined by d CO 2 18 O produced from calcite at 25 °C, is subject to additional uncertainty associated with poorly defined PPA fractionation factors ðaðT Þ CO 2 ; MCO 3 Þ required to compute values of d M CO 3 18 O from d CO 2 18 O. These additional sources of uncertainty suggest that levels of analytical precision routinely reported from the measurement of d CO 2 18 O at higher temperatures, and used as estimates of uncertainty for d MCO 3 18 O, do not adequately reflect real uncertainties in d MCO 3 18 O relative to VPDB.

" Clumped-isotope " thermometry is an emerging tool to probe the temperature history of surface and subsurface environments based on measurements of the proportion of 13 C and 18 O isotopes bound to each other within carbonate minerals in 13 C 18 O 16 O 2 2À groups (heavy isotope " clumps "). Although most clumped isotope geothermometry implicitly presumes carbon-ate crystals have attained lattice equilibrium (i.e., thermodynamic equilibrium for a mineral, which is independent of solution chemistry), several factors other than temperature, including dissolved inorganic carbon (DIC) speciation may influence mineral isotopic signatures. Therefore we used a combination of approaches to understand the potential influence of different variables on the clumped isotope (and oxygen isotope) composition of minerals. We conducted witherite precipitation experiments at a single temperature and at varied pH to empirically determine 13 C– 18 O bond ordering (D 47) and d 18 O of CO 3 2À and HCO 3 À molecules at a 25 °C equilibrium. Ab initio cluster models based on density functional theory were used to predict equilibrium 13 C– 18 O bond abundances and d 18 O of different DIC species and minerals as a function of temperature. Experiments and theory indicate D 47 and d 18 O compositions of CO 3 2À and HCO 3 À ions are significantly different from each other. Experiments constrain the D 47 –d 18 O slope for a pH effect (0.011 ± 0.001; 12 P pH P 7). Rapidly-growing temperate corals exhibit disequilibrium mineral isotopic signatures with a D 47 –d 18 O slope of 0.011 ± 0.003, consistent with a pH effect. Our theoretical calculations for carbonate minerals indicate equilibrium lattice calcite values for D 47 and d 18 O are intermediate between HCO 3 À and CO 3 2À. We analyzed synthetic calcites grown at temperatures ranging from 0.5 to 50 °C with and without the enzyme carbonic anhydrase present. This enzyme catalyzes oxygen isotopic exchange between DIC species and is present in many natural systems. The two types of experiments yielded statistically indistinguishable results, and these measurements yield a calibration that overlaps with our theoretical predictions for calcite at equilibrium. The slow-growing Devils Hole calcite exhibits D 47 and d 18 O values consistent with lattice equilibrium. Factors influencing DIC speciation (pH, salinity) and the timescale for DIC equilibration, as well as reactions at the min-eral–solution interface, have the potential to influence clumped-isotope signatures and the d 18 O of carbonate minerals. In fast-growing carbonate minerals, solution chemistry may be an important factor, particularly over extremes of pH and salin-ity. If a crystal grows too rapidly to reach an internal equilibrium (i.e., achieve the value for the temperature-dependent mineral lattice equilibrium), it may record the clumped-isotope signature of a DIC species (e.g., the temperature-dependent equilibrium of HCO 3 À) or a mixture of DIC species, and hence record a disequilibrium mineral composition. For extremely slow-growing crystals, and for rapidly-grown samples grown at a pH where HCO 3 À dominates the DIC pool at equilibrium, effects of solution chemistry are likely to be relatively small or negligible. In summary, growth environment, solution chemistry , surface equilibria, and precipitation rate may all play a role in dictating whether a crystal achieves equilibrium or dis-equilibrium clumped-isotope signatures.

Sedimentology, 2015

Stable carbon and oxygen isotopes (δ18O and δ13C values) and trace elements have been applied to the study of diagenesis of carbonate rocks for over 50 years. As valuable as these insights have been, many problems regarding the interpretation of geochemical signals within mature rocks remain. For example, while the δ18O values of carbonate rocks are dependent both upon the temperature and the δ18O value of the fluid, and additional information including trace element composition aids in interpreting such signals, direct evidence of either the temperature or the composition of the fluids is required. Such information can be obtained by analysing the δ18O value of any fluid inclusions or by measuring the temperature using a method such as the ‘clumped’ isotope technique. Such data speak directly to a large number of problems in interpreting the oxygen isotope record including the well‐known tendency for δ18O values of carbonate rocks to decrease with increasing age. Unlike the δ18O, δ...

Palaeogeography, Palaeoclimatology, Palaeoecology, 2017

Although atmospheric CO 2 has been extensively described as a primary driver of Phanerozoic climate and carbon cycle disturbance, little is known about its carbon isotope composition (δ 13 C CO2) during pre-Cenozoic times. We reconstruct for the first time the evolution of δ 13 C CO2 during the whole Cretaceous period based on reference curves of δ 13 C values of Tethyan marine bulk carbonates (δ 13 C carb) and δ 18 O PO4 values of fish tooth enamel. We test this method against that based on the oxygen and carbon isotope ratios of highlatitude benthic foraminifera (δ 13 C foram) recently implemented for the Cenozoic and closely