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The Thermo Scientific™ QExactive Orbitrap™ mass spectrometer combined with a Thermo Scientific™ Trace™ 1310 GC enables high-mass-resolution measurements of molecular isotopic structure (e.g., molecular-average, position-specific, and multiple substitution measurements), but thus far has employed non-traditional, slow sample introduction methods and long integrations lasting minutes or tens of minutes to optimize measurement precision. This study examines the performance of the Orbitrap for isotope ratio measurements of analytes eluting directly from the gas chromatograph (GC) as traditional GC peaks — i.e. eluting over a period of seconds and with rapidly varying signal intensities. Such a measurement holds potential for simultaneous compound identification and isotope ratio measurement of numerous analytes separated by GC within a single acquisition. We applied this “direct elution” measurement strategy to molecular and fragment ions of para-xylene, serine, and a mixture of polycyclic aromatic hydrocarbons (PAHs) at natural isotope abundances. We built a mathematical model and used a Monte Carlo simulation to evaluate how variations in data processing decisions and GC peak characteristics (e.g., peak shape and elution timing) affect the accuracy of the resulting absolute isotope ratios and sample-standard comparisons (δ values). These case studies inform our recommendations for applying direct elution measurements. The method is appropriate for systems with large position-specific, molecular, or multiply-substituted isotopic anomalies (e.g., isotopically labelled or extraterrestrial compounds), and for compounds that produce strong molecular ions. Precisions improve when experiments are designed to (1) target ions with relatively high mass spectral intensities, (2) optimize the number of ions and range of masses admitted into the Orbitrap, and (3) minimize the nominal resolution settings while still separating relevant isobars.

Over the last few decades, isotopic analysis of amino acids at the compound- and position-specific levels has been rapidly advancing across diverse fields. As these techniques progress, evaluation of isotopic fractionation associated with sample workup is essential. This critical review of analytical methods through the lens of isotope geochemistry provides a benchmark for researchers across disciplines seeking to make compound- and position-specific amino acid isotope measurements. We focus on preparation, acid hydrolysis, clean-up, derivatization, separation, and C, H, N, and S isotope measurement. Despite substantial customizability across these steps, the following general recommendations should maximize recovery while minimizing isotopic fractionation. Samples should be freeze-dried and stored anoxically at ≤ –20 °C prior to conventional acid hydrolysis (6N HCl, 110 °C, 20–24 h, anoxic), which suffices for many residues. Both gas and liquid chromatographic (GC and LC, respectively) techniques are well-established and separate about 15 amino acids; LC bypasses the need for derivatization, while GC provides higher sensitivity. When derivatization is needed, n-acetyl and alkoxycarbonyl esters provide the most reproducible C isotope ratios. For compound-specific analyses, online GC–IRMS and LC–IRMS systems offer the easiest workflow, but EA–IRMS enables potential multi-element isotope analysis. Emerging techniques like high-resolution mass spectrometry are also promising for multi-element analysis and recover position-specific isotopic information. Looking forward to the next decade of innovation, isotope geochemists and ecologists can improve amino acid isotope analysis by focusing on streamlining multi-element analysis and standardizing calibration practices across laboratories.

The abundance of 18O isotopes and 13C-18O isotopic “clumps” (measured as δ18O and Δ47, respectively) in carbonate minerals have been used to infer mineral formation temperatures. An inherent requirement or assumption for these paleothermometers is mineral formation in isotopic equilibrium. Yet, apparent disequilibrium is not uncommon in biogenic and abiogenic carbonates formed in nature and in synthetic carbonates prepared under laboratory settings, as the dissolved carbonate pool (DCP) from which minerals precipitate is often out of δ18O and Δ47 equilibrium. For this, a complete understanding of both equilibrium and kinetics of isotopic partitioning and 13C-18O clumping in DCP is crucial. To this end, we analyzed Δ47 of inorganic BaCO3 samples from Uchikawa and Zeebe (2012) (denoted as UZ12 ), which were quantitatively precipitated from NaHCO3 solutions at various times over the course of isotopic equilibration at 25 °C and pHNBS of 8.9. Our data show that, although the timescales for δ18O and Δ47 equilibrium in DCP are relatively similar, their equilibration trajectories are markedly different. As opposed to a simple unidirectional and asymptotic approach toward δ18O equilibrium (first-order kinetics), Δ47 equilibration initially moves away from equilibrium and then changes its course towards equilibrium. This excess Δ47 disequilibrium is manifested as a characteristic “dip” in the Δ47 equilibration trajectory, a feature consistent with an earlier study by Staudigel and Swart (2018) (denoted as SS18 ). From the numerical model of SS18 , the non-first-order kinetics for Δ47 equilibration can be understood as a result of the difference in the exchange rate for oxygen isotopes bound to 12C versus 13C, or an isotope effect of ~25‰. We also developed an independent model for the Exchange and Clumping of 13C and 18O in DCP (ExClump38 model) to trace the evolution of singly- and doubly-substituted isotopic species (i.e., δ13C, δ18O and Δ47). The model suggests that the dip in the Δ47 equilibration trajectory is due largely to kinetic carbon isotope fractionation for hydration and hydroxylation of CO2. We additionally examined the BaCO3 samples prepared from NaHCO3 solutions supplemented with carbonic anhydrase (CA), an enzyme known to facilitate δ18O equilibration in DCP by catalyzing CO2 hydration ( UZ12 ). These samples revealed that, while CA effectively shortens the time required for Δ47 equilibrium in DCP, the overall pattern and magnitude of the dip in the Δ47 equilibration trajectory remain unchanged. This suggests no additional isotope effects due to the CA enzyme within the tested CA concentrations. With the ExClump38 model, we test various physicochemical scenarios for the timescales and trajectories of isotopic equilibration in DCP and discuss their implications for the Δ47 paleothermometry.

Land snails are widely distributed animals whose clumped isotope (∆47) in shell carbonates can serve as a proxy for temperature in terrestrial environments. However, it is unclear whether the ∆47 values of snail shells are affected by biological processes known as “vital effects.” Here, we report ∆47 values of whole Achatina fulica snail shells cultured at different temperatures at different growth ages (samples collected monthly). In general, the mean ∆47 values of the snail shells decrease with higher cultivation temperature, which indicates they have potential use in reconstructing temperature changes. However, the mean reconstructed ∆47 temperatures (T47) using the calibrations from Petersen et al. (2019), Bonifacie et al. (2017), and Kelson et al. (2017) are ~6 °C higher than the actual growth temperatures. Interestingly, this overestimation in T47 disappears when applying Zhai et al.'s (2019) calibration for the Bradybaena snail. This calibration also yields good agreement between the T47 data of all field-collected snails and their growing season temperatures. The above-mentioned phenomenon indicates the existence of the vital effect in land-snail shells. The variations in ∆47 with snail age and the correlations between ∆47, δ18O, and δ13C values also indicate the potential influence of vital effects. Finally, we suggest that the vital effect may be explained by kinetic fractionation through CO2 degassing associated with dehydration/dehydroxylation during the formation of snail shells.

Microbial methanogenesis is a significant component in the global carbon cycle, a driver of greenhouse warming from atmospheric methane, and contributes to natural gas resources. There are several metabolic pathways of methanogenesis, and it is challenging to discriminate between them and quantify their relative contributions in natural settings. Here we measure and compile four rare isotopologues of methane (13CH4, 12CH3D, 13CH3D and 12CH2D2) from the Qaidam Basin (China) and a large set of previous data across the world to identify distinctive fingerprints of two principal sources of subsurface microbial methane — hydrogenotrophic and methylotrophic methanogenesis — and use those fingerprints to deconvolve the budgets of microbial methane in the Qaidam Basin. Our data show that biogenic methanes in the Qaidam Basin have equilibrium Δ 18 / Δ 13 CH3D values respect to reservoir temperature without anaerobic methane oxidation. Our results suggest that methylotrophic methanogenesis produces methane with large deficits in 13CH3D and 12CH2D2 relative to that controlled by homogeneous equilibrium among methane isotopologues at ambient environmental temperatures, whereas hydrogenotrophic methanogenesis produces methane with 13CH3D abundance near equilibrium and relatively subtle 12CH2D2 deficits. We find a good linear correlation between the Δ 13 CH3D value of natural biogenic methane and independent estimates of the fraction of hydrogenotrophic (methylotrophic) methanogenesis, based on established interpretations of hydrogen isotope data. These findings are consistent with studies of laboratory cultures, which also show methylotrophic methane is more depleted in clumped isotopologues (13CH3D and 12CH2D2) than hydrogenotrophic methane, though both forms of cultured (hydrogenotrophic vs. methylotrophic) methane exhibit strong deficits in both isotopic species. The sensitivity of clumped isotopologues to methanogenesis pathways in natural settings provides a powerful tool for monitoring the activity of methanogenic microbial communities in the subsurface. Anomalies of both studied clumped isotopologues ( Δ 13 CH3D and Δ 12 CH2D2) decrease over time in wells that have been re-sampled repeatedly, suggesting that such measurements are capable of detecting and quantifying shifts in proportions of these two metabolic pathways over the timescales of gas production history.

Methane clumped isotope analysis is a tool used to constrain the formation or equilibration temperatures of methane, or to differentiate methane of thermogenic, microbial or ‘abiotic’ origins. Geothermometry applications are based on the temperature dependence of relative abundances of multiply-substituted isotopologues in thermodynamic equilibrium, whereas assignments of biogenicity or ‘abiogenicity’ rely on kinetic isotope effects associated with synthesis, which disturb clumped isotope abundances away from expected equilibrium proportions. However, kinetic processes in thermogenesis or during post-generation storage of thermogenic gas may cause isotopic disequilibrium, confounding thermometry applications or leading to ‘false positive’ identifications of microbial or abiogenic gases. Non-equilibrated clumped isotope compositions have been observed in thermogenic gases including unconventional oil-associated gases and from coal pyrolysis experiments. The isotopic disequilibria might be caused by kinetic isotope effects expressed during gas migration (including extraction), or by irreversible chemical processes, such as breaking carbon–carbon bonds in an alkyl precursor. In this study, we performed controlled pyrolysis experiments at 400 °C on n-octadecane (C18H38). We characterized the gas chemistry, and compound-specific carbon and hydrogen isotope and methane clumped isotope compositions of the gas products. We found that Δ13CH3D values (anomalies relative to a stochastic distribution of isotopes) appear to be relatively close to equilibrium at the experimental temperature, whereas Δ12CH2D2 values are 30–40‰ lower than expected for equilibrium. The large deficit in Δ12CH2D2 can be explained by assembling hydrogen atoms affected by two distinct kinetic isotope effects into a methane molecule, previously referred to as a ‘combinatorial effect’. We present a kinetic model that describes the full isotopic systematics, including anomalous Δ12CH2D2 deficits, of pyrolysis product methane. Finally, we propose a model for the isotope signatures of natural thermogenic methane where the non-equilibrium Δ12CH2D2 composition is a signature of the onset of catagenetic methane production. Our model also describes ways in which this signature disappears as further maturation drives Δ12CH2D2 to equilibrium through hydrogen exchange. Our findings demonstrate that anomalous depletion in Δ12CH2D2 is not a unique signature for microbial or putative abiotic methane, and specifically, it can be generated during pyrolytic chemistry.

Meteorites contain prebiotic, bio-relevant organic compounds including amino acids. Their syntheses could result from diverse sources and mechanisms and provide a window on the conditions and materials present in the early solar system. Here we constrain alanine’s synthetic history in the Murchison meteorite using site-specific 13C/12C measurements, reported relative to the VPDB standard. The δ13CVPDB values of −29 ± 10‰, 142 ± 20‰, and −36 ± 20‰ for the carboxyl, amine-bound, and methyl carbons, respectively, are consistent with Strecker synthesis of interstellar-medium-derived aldehydes, ammonia, and low-δ13C nebular or interstellar-medium-derived CN. We report experimentally measured isotope effects associated with Strecker synthesis, and use them to constrain the δ13C values of the alanine precursors, which we then use to construct a model that predicts the molecular-average δ13C values of 19 other organic compounds of prebiotic significance found in Murchison if they were made by our proposed synthetic network. Most of these predictions agree with previous measurements, suggesting that interstellar-medium-derived aldehydes and nebular and/or pre-solar CN could have served as substrates for synthesis of a wide range of prebiotic compounds in the early solar system.

The crystal structure of goethite, FeO(OH), has two distinct oxygen sites, one with exclusively Fe-O bonds, the other with bonds to both iron and hydrogen. We developed a method to assess the oxygen isotope contrast between these sites by measuring both the bulk goethite and the oxygen released in the conversion of goethite to hematite. The method involves collecting the water released by dehydroxylation, fluorinating that population of extracted atoms, and measuring the resulting oxygen isotope composition (extracted δ O 18 ). Then, on a separate aliquot, all structural oxygen is fluorinated and measured (bulk δ O 18 ). Using synthetic goethite precipitates grown under controlled environmental conditions, we found significant temperature-dependent fractionation, e bulk-extracted = ( 5.51 ± 0.26 ) × ( 10 6 / T 2 ) − ( 44.5 ± 2.8 ) ; T in Kelvin). This intracrystalline fractionation forms the basis of a single-phase paleothermometer with an estimated uncertainty of ∼2-3°C. The temperature dependence appears to be independent of the isotopic composition of the parent fluid from which the goethite formed and the pH of that fluid. This intracrystalline thermometer can be used to simultaneously determine the formation temperature of a goethite and the isotopic composition of the water from which it formed. Natural goethites analyzed with this technique yield geologically reasonable formation temperatures of between 15 and 41°C.

The Gulf of Mexico (GOM) produces 5% of total U.S. dry gas production ( USEIA, 2016 ). Despite this, the proportion of microbial and thermogenic methane in discovered and producing fields from this area is still not well understood. Understanding the relative contributions of these sources in subsurface environments is important to understanding how and where economically substantial amounts of methane form. In addition, this information will help identify sources of environmental emissions of hydrocarbons to the atmosphere. We apply stable isotopes including methane clumped-isotope measurements to solution and associated gases from several producing fields in the U.S. Gulf of Mexico to estimate the proportions, properties and origins of microbial and thermogenic endmembers. Clumped isotopes of methane are unique indicators of whether methane is at thermodynamic isotopic equilibrium or affected by kinetic processes. The clumped methane thermometer can provide insights into formation temperatures and/or into kinetic processes such as microbial methanogenesis, early catagenetic processes, mixing, combinatorial processes, and diffusion. In this data set, we find that some fluids have clumped isotope methane apparent temperatures consistent with the methane component being produced solely by the thermogenic breakdown of larger organic molecules at substantially greater temperatures than those reached in shallow reservoirs. A portion of these reservoirs with hot clumped isotope methane temperatures are consistent with exhibiting a kinetic isotope effect. Other reservoirs have clumped isotope methane apparent temperatures, and other isotopic and molecular proportions, consistent with mixtures of microbial and thermogenic methane. We show that in certain cases the evidence is most consistent with formation of the microbial methane in the current reservoir. However, in other cases the methane is produced at significantly shallower depths and is then transported to greater depths as a result of post generation burial of methane bearing sedimentary sequences to the current reservoir conditions. For the first time, we show that methane of an unambiguously purely microbial origin (i.e. those that do not contain obvious contributions of thermogenic methane) is dominantly generated at temperatures less than 60 °C, despite burial to greater depths. This finding suggests that, while microorganisms are able to generate methane at temperatures up to 105 °C under laboratory conditions (Brock, 1985), in the Gulf of Mexico, microbial methane is dominantly produced in the 20–60 °C window.

We report stable (δ¹³C_(shell), δ¹⁸O_(shell)) and clumped isotope (Δ₄₇) compositions of modern and last glacial fossil snail shell carbonates from the Luochuan and Weinan sections on the central and southern Chinese Loess Plateau (CLP). Our study reveals that the average Δ₄₇ temperature (T₄₇) of modern snails is consistent with monitored temperatures during the snail growing season at the studied locations and is ~10 °C higher than that of fossil snails from glacial time. Moreover, the average δ¹³C_(shell) of modern snails is more depleted than that of fossils. We argue that the δ¹³C_(shell) cannot record changes in plant communities (i.e., the C3/C4 ratio) on the CLP and may mainly indicate arid conditions with depleted values reflecting reduced aridity. Additionally, the reconstructed snail body water δ¹⁸O (δ¹⁸O_(water)) of modern snails is more enriched than δ¹⁸O in modern growing season precipitation and δ¹⁸O_(water) of fossils. This contrast may be related to the high degree of evaporative enrichment of environmental water ¹⁸O in the body/ingested by modern snails under warm conditions. Therefore, we suggest that using δ¹⁸O_(shell) to directly reconstruct the oxygen isotopes of precipitation is difficult and that higher δ¹⁸O_(shell) and δ¹⁸O_(water) values probably indicate higher environmental temperature/stronger evaporative enrichment on glacial-interglacial timescales on the CLP.

We have experimentally determined the diffusivity of water in a representative lunar basaltic liquid composition (LG) and in an iron-free analog of a basaltic liquid (AD) at the low water concentrations and low oxygen fugacities (fO2) relevant to the eruption of lunar basalts. Experiments were conducted at 1 atm and 1350 °C over a range of pH2/pH2O from near zero to ∼10 and a range in fO2 spanning ∼9 orders of magnitude (from 2.2 log units below the iron-wustite buffer, IW−2.2, to IW+6.7). The water concentrations measured in our quenched experimental glasses by secondary ion mass spectrometry (SIMS) and Fourier transform infrared spectroscopy (FTIR) vary from a few ppm to ∼430 ppm. Water concentration gradients in the majority of our AD experiments are well described by models in which the diffusivity of water ( D w a t e r ∗ ) has a constant value of ∼2 × 10−10 m2/s, while our LG results indicate that D w a t e r ∗ in LG melt has a constant value of ∼6 × 10−10 m2/s under the conditions of our experiments. Water concentration gradients in hydration and dehydration experiments that were run simultaneously in H2/CO2 gas mixtures are well described by the same D w a t e r ∗ , and water concentrations measured near the melt-vapor interfaces of these experiment pairs are approximately the same. These observations strongly support an equilibrium boundary condition for our experiments containing >70 ppm H2O. However, dehydration experiments into nominally anhydrous CO2, N2, and CO/CO2 gas mixtures leave some scope for the importance of kinetics during dehydration of melts containing less than a few 10′s of ppm H2O. Comparison of our results with the modified speciation model (Ni et al., 2013) in which both molecular water and hydroxyl are allowed to diffuse suggests that we have resolved the diffusivity of hydroxyl ( D O H ) in AD and LG melts. Our results support a positive correlation between D O H and melt depolymerization. Best-fit values of D w a t e r ∗ for our LG experiments vary within a factor of ∼2 over a range of pH2/pH2O from 0.007 to 9.7 and a range of logfO2 from IW−2.2 to IW+4.9. The relative insensitivity of our best-fit values of D w a t e r ∗ to variations in pH2 suggests that H2 diffusion did not control the rate of degassing of H-bearing species from the lunar glasses of Saal et al. (2008); however, we cannot rule out a role for molecular H2 diffusion under lower-temperature and/or higher-pressure conditions than explored in our experiments. The value of D w a t e r ∗ chosen by Saal et al. (2008) for modeling the diffusive degassing of the lunar volcanic glasses is within a factor of ∼2 of our measured value in LG melt at 1350 °C. By coupling our LG results at 1350 °C with an activation energy of 220 kJ/mol (Zhang et al., 2017), we obtain the following Arrhenius relationship, which can be used to model syneruptive diffusive water loss from lunar melt beads: D w a t e r ∗ ( m 2 / s ) = 7.2 × 1 0 - 3 e x p - 2.6 × 1 0 4 T ( K ) .

The clumped isotope compositions of carbonate minerals are subject to alteration at elevated temperatures. Understanding the mechanism of solid-state reordering in carbonate minerals is important in our interpretations of past climates and the thermal history of rocks. The kinetics of solid-state isotope reordering has been previously studied through controlled heating experiments of calcite, dolomite and apatite. Here we further explore this issue through controlled heating experiments on aragonite. We find that Δ47 values generally decrease during heating of aragonite, but increase by 0.05–0.15‰ as aragonite starts to transform into calcite. We argue that this finding is consistent with the presence of an intermediate pool of immediately adjacent singly-substituted carbonate ion isotopologues (‘pairs’), which back-react to form clumped isotopologues during aragonite to calcite transformation, revealing the existence of kinetically preferred isotope exchange pathways. Our results reinforce the ‘reaction-diffusion’ model as the mechanism for solid-state clumped isotope reordering in carbonate minerals. Our experiments also reveal that the reordering kinetics in aragonite is faster than in calcite and dolomite, making its clumped isotope composition highly susceptible to alteration during early diagenesis, even before conversion to calcite. publishedVersion

Tektites contain inclusions of lechatelierite, nearly pure SiO_2 glass formed by quenching of quartz grains melted during hypervelocity impacts. We report the discovery in a tektite of chemically zoned boundary layers (ca 20 μm) between lechatelierite and host felsic glass. These boundary layers in tektites formed by chemical diffusion between molten silicainclusions (quenched to lechatelierite on cooling) and surrounding felsic melt. We reproduced the details of these boundary layers via experiments on mixtures of powdered natural tektite plus quartz grains heated to 1800–2400 °C for 1–120 s using an aerodynamic levitation laser heating furnace. The results of these experiments were used to provide quantitative constraints on possible thermal histories of the natural sample. The experiments successfully reproduced all major aspects of the concentration profiles from the natural sample including diffusion length scale, strong asymmetry of the concentration profiles with respect to the Matano plane (due to the strong concentration dependence of the diffusivities of all oxides on SiO_2 content), similarities in lengths of the diffusive profiles (due to control by the diffusion of SiO_2 on the diffusivity of the other oxides), and differences in the shapes of the profiles among the oxides (including a maximum in the diffusion profile of K_2O due to uphill diffusion). The characteristic lengths of all non-alkali oxide profiles are proportional to t from which diffusivities and activation energies can be derived; these results are consistent with measurements in melts with lower SiO_2 contents and at lower temperatures reported in the literature. We also fit the experimental profiles of SiO_2 and Al_2O_3 using simple formulations of the dependence of their diffusivities on SiO_2 content and temperature, yielding results similar to those obtained from the t dependence of the characteristic profile lengths. The quantitative characterization of diffusion in boundary layers based on our experiments allow us to set limits on the thermal history of the natural tektite in which the boundary layers were discovered. If the interdiffusion between the silica and felsic melts occurred at constant temperature, the duration of heating experienced by the natural tektite we studied depends on temperature; possible solutions include heating at ∼2000 °C for ∼70 s, −2400 °C for ∼3 s. We also explored non-isothermal, asymptotic cooling histories; for a maximum temperature of 2400 °C, a characteristic cooling time scale of ∼50 s is implied, whereas, for 2000 °C, the time scale is ∼1400 s. Further, a maximum temperature of ∼2360 °C yields an effective diffusive time scale of ∼5 s, a cooling time scale of ∼90 s, and a cooling rate at the glass transition temperature of ∼5 °C/s; results that are consistent with independent estimates of cooling time scales for ∼1 cm clasts (Xu and Zhang, 2002), as well as cooling rates at the glass transition temperature (Wilding et al., 1996) – thus satisfying all currently available relevant data. More complex T-t paths are possible and can also be modeled using our experimental results and compared with and used as tests of the accuracy of physical models of tektite-forming impact events.

Abiotic hydrocarbon gas, typically generated in serpentinized ultramafic rocks and crystalline shields, has important implications for the deep biosphere, petroleum systems, the carbon cycle and astrobiology. Distinguishing abiotic gas (produced by chemical reactions like Sabatier synthesis) from biotic gas (produced from degradation of organic matter or microbial activity) is sometimes challenging because their isotopic and molecular composition may overlap. Abiotic gas has been recognized in numerous locations on the Earth, although there are no confirmed instances where it is the dominant source of commercially valuable quantities in reservoir rocks. The deep hydrocarbon reservoirs of the Xujiaweizi Depression in the Songliao Basin (China) have been considered to host significant amounts of abiotic methane. Here we report methane clumped-isotope values (Δ_(18)) and the isotopic composition of C_1–C_3 alkanes, CO_2 and helium of five gas samples collected from those Xujiaweizi deep reservoirs. Some geochemical features of these samples resemble previously suggested identifiers of abiotic gas (^(13)C-enriched CH_4; decrease in ^(13)C/^(12)C ratio with increasing carbon number for the C1–C4 alkanes; abundant, apparently non-biogenic CO_2; and mantle-derived helium). However, combining these constraints with new measurements of the clumped-isotope composition of methane and careful consideration of the geological context, suggests that the Xujiaweizi depression gas is dominantly, if not exclusively, thermogenic and derived from over-mature source rocks, i.e., from catagenesis of buried organic matter at high temperatures. Methane formation temperatures suggested by clumped-isotopes (167–213 °C) are lower than magmatic gas generation processes and consistent with the maturity of local source rocks. Also, there are no geological conditions (e.g., serpentinized ultramafic rocks) that may lead to high production of H_2 and thus abiotic production of CH_4 via CO_2 reduction. We propose that the Songliao gas is representative of an atypical type of thermogenic gas that can be mistaken for abiotic gas. Such gases may be encountered more frequently in future exploration of deep or over-mature petroleum systems.

Site-specific carbon isotope measurements of organic compounds potentially recover information that is lost in a conventional, ‘bulk' isotopic analysis. Such measurements are useful because isotopically fractionating processes may have distinct effects at different molecular sites, and thermodynamically equilibrated populations of molecules tend to concentrate heavy isotopes in one molecular site versus another. Most recent studies of site-specific ^(13)C in organics use specialized Nuclear Magnetic Resonance (NMR) techniques or complex chemical degradations prior to mass spectrometric measurements. Herein we present the first application of a new mass spectrometric technique that reconstructs the site-specific carbon isotope composition of propane based on measurements of the ^(13)C/^(12)C ratios of two or more fragment ions that sample different proportions of the terminal and central carbon sites. We apply this method to propane from laboratory experiments and natural gas samples to explore the relationships between site-specific carbon isotope composition, full-molecular δ^(13)C, thermal maturity, and variation in organic matter precursors. Our goal is to advance the understanding of the sources and histories of short-chain alkanes within geologic systems. Our findings suggest that propane varies in its site-specific carbon isotope structure, which is correlated with increasing thermal maturity, first increasing in terminal position δ^(13)C and then increasing in both center and terminal position δ^(13)C. This pattern is observed in both experimental and natural samples, and is plausibly explained by a combination of site-specific, temperature-dependent isotope effects associated with conversion of different precursor molecules (kerogen, bitumen, and/or oil) to propane, differences in site-specific isotopic contents of those precursors, and possibly distillation of reactive components of those precursors with increasing maturity. We hypothesize that the largest changes in site-specific isotopic content of propane occur when bitumen and/or oil replace kerogen as the dominant precursors. If correct, this phenomenon could have significant utility for understanding gas generation in thermogenic petroleum systems.

Site-specific isotope ratio measurements potentially provide valuable information about the formation and degradation of complex molecules—information that is lost in conventional bulk isotopic measurements. Here we discuss the background and possible applications of such measurements, and present a technique for studying the site-specific carbon isotope composition of propane at natural abundance based on mass spectrometric analysis of the intact propane molecule and its fragment ions. We demonstrate the feasibility of this approach through measurements of mixtures of natural propane and propane synthesized with site-specific^(13)C enrichment, and we document the limits of precision of our technique. We show that mass balance calculations of the bulk δ^(13)C of propane based on our site-specific measurements is generally consistent with independent constraints on bulk δ^(13)C. We further demonstrate the accuracy of the technique, and illustrate one of its simpler applications by documenting the site-specific carbon isotope signature associated with gas phase diffusion of propane, confirming that our measurements conform to the predictions of the kinetic theory of gases. This method can be applied to propane samples of moderate size (tens of micromoles) isolated from natural gases. Thus, it provides a means of studying the site-specific stable isotope systematics of propane at natural isotope abundances on sample sizes that are readily recovered from many natural environments. This method may also serve as a model for future techniques that apply high-resolution mass spectrometry to study the site-specific isotopic distributions of larger organic molecules, with potential applications to biosynthesis, forensics and other geochemical subjects.

© 2016 Elsevier Ltd. Methane is a potent greenhouse gas, and there are concerns that its natural emissions from the Arctic could act as a substantial positive feedback to anthropogenic global warming. Determining the sources of methane emissions and the biogeochemical processes controlling them is important for understanding present and future Arctic contributions to atmospheric methane budgets. Here we apply measurements of multiply-substituted isotopologues, or clumped isotopes, of methane as a new tool to identify the origins of ebullitive fluxes in Alaska, Sweden and the Arctic Ocean. When methane forms in isotopic equilibrium, clumped isotope measurements indicate the formation temperature. In some microbial methane, however, non-equilibrium isotope effects, probably related to the kinetics of methanogenesis, lead to low clumped isotope values. We identify four categories of emissions in the studied samples: thermogenic methane, deep subsurface or marine microbial methane formed in isotopic equilibrium, freshwater microbial methane with non-equilibrium clumped isotope values, and mixtures of deep and shallow methane (i.e., combinations of the first three end members). Mixing between deep and shallow methane sources produces a non-linear variation in clumped isotope values with mixing proportion that provides new constraints for the formation environment of the mixing end-members. Analyses of microbial methane emitted from lakes, as well as a methanol-consuming methanogen pure culture, support the hypothesis that non-equilibrium clumped isotope values are controlled, in part, by kinetic isotope effects induced during enzymatic reactions involved in methanogenesis. Our results indicate that these kinetic isotope effects vary widely in microbial methane produced in Arctic lake sediments, with non-equilibrium δ18values spanning a range of more than 5‰.

The central Altiplano is inferred to have experienced ∼2.5±1km surface uplift between ∼10 and 6 Ma, while the southern Altiplano experienced a similar magnitude of surface uplift that began earlier, between ∼16 and 9 Ma. To properly constrain the along strike timing of the Altiplano plateau surface uplift, it is necessary to know how and when the northernmost part of the Altiplano plateau evolved. We reconstruct the paleoclimate and infer the corresponding paleoelevation from the Miocene–Pliocene deposits of the Descanso–Yauri basin (14–15°S) in the northernmost part of the Altiplano plateau using 4 different proxies, including carbonate clumped isotope composition (i.e., Δ_(47) values), carbonate δ^(18)O_c, leaf wax δD_(wax) and pollen assemblages from paleosol, lacustrine and palustrine carbonates and organic-rich sediments. The isotopic signatures reflect past climate conditions of mean annual air temperature (Δ_(47)) and meteoric water isotope values (δ^(18)O_c, δD_(wax)). Our results show that the northernmost plateau remained at low elevation (0.9±0.8 to 2.1±0.9km) until late Miocene time (∼9 Ma) characterized by ∼15 °C warmer than modern temperature (mean annual air temperature of 23±4°C, 2σ), low elevation vegetation and precipitation signature with reconstructed □ δ^(18)O_(mw) (VSMOW) of −8.3±2.0‰(2σ) from carbonate (δ^(18)O_c) and −8.6±1.8‰(2σ) from leaf wax (δD_(wax)). Modern elevations of 4 km were not reached until 5.4±1.0Ma, as indicated by a negative shift in δD_(wax) (VSMOW) from −143.4±12.8‰(2σ) to −209.2±21.1‰(2σ) between 9.1±0.7 and 5.4±1.0Ma. The timing of surface uplift of the northernmost Altiplano is consistent with the evidence for late Miocene surface uplift of the central Altiplano (16–19°S) between 10 and 6 Ma, and indicates that regional scale uplift in the northern–central plateau significantly postdates the onset of surface uplift in the southern Altiplano (19–22°S) between ∼16 and 9 Ma. These results are consistent with piecemeal removal of the lower dense lithosphere, combined with possible lower/middle crustal flow from south to north in the plateau acting as the main mechanisms for the formation of the Altiplano plateau.

The concentrations of volatile elements in the moon have important implications for the formation of the earth–moon system. There is currently a debate regarding the water content of the lunar mantle: Authors studying H_2O in lunar pyroclastic glass beads and in olivine-hosted melt inclusions in such pyroclastic samples and in plagioclase crystals in lunar highland anorthosites infer hundreds of ppm H_2O in the lunar mantle. In contrast, authors studying Zn/Fe ratios infer that the H_2O concentration in the lunar mantle is ≤1 ppm, and they argue that the glassy lunar basalts are a local anomaly. We contribute to a resolution of the debate by a broader examination of the concentrations of H_2O and other volatile components in olivine-hosted melt inclusions in a wider range of lunar mare basalts, including crystalline melt inclusions that are homogenized by melting in the laboratory. We find that F, Cl, and S concentrations in various lunar melt inclusions (including those in glassy lunar basalts) are similar to one another, and previously studied glassy lunar basalts are not a local anomaly in terms of these volatile concentrations. Furthermore, we estimate the pre-degassing H_2O/Ce, F/Nd, and S/Dy ratios of mare basaltic magmas to be at least 64, 4.0 and 100 respectively. These ratios are lower than those of primitive earth mantle by a factor of 3, 5, and 4 respectively. The depletion factors of these volatile elements relative to the earth's primitive mantle do not correlate strongly with volatility or bonding energy, and indeed they are roughly constant and similar to those of other volatile elements such as Li, Cs, Rb and K. This approximate constancy of volatile depletion in the moon relative to the earth can be explained by assuming that both the earth and the moon acquired volatiles from a similar source or by a similar mechanism but the earth was more efficient in acquiring the volatiles. We estimate the H_2O, F and S concentrations in the primitive lunar mantle source to be at least 110, 5.3, and 70 ppm, respectively – similar to or slightly lower than those in terrestrial MORB mantle.

Much of what we know about the history of Earth’s climate derives from the chemistry of carbonate minerals in the sedimentary record. The oxygen isotopic compositions (δ^(18)O) of calcitic marine fossils and cements have been widely used as a proxy for past seawater temperatures, but application of this proxy to deep geologic time is complicated by diagenetic alteration and uncertainties in the δ^(18)O of seawater in the past. Carbonate clumped isotope thermometry provides an independent estimate of the temperature of the water from which a calcite phase precipitated, and allows direct calculation of the δ^(18)O of the water. The clumped isotope composition of calcites is also highly sensitive to recrystallization and can help diagnose different modes of diagenetic alteration, enabling evaluation of preservation states and identification of the most pristine materials from within a sample set—critical information for assessing the quality of paleoproxy data generated from carbonates. We measured the clumped isotope composition of a large suite of calcitic fossils (primarily brachiopods and corals), sedimentary grains, and cements from Silurian (ca. 433 Ma) stratigraphic sections on the island of Gotland, Sweden. Substantial variability in clumped isotope temperatures suggests differential preservation with alteration largely tied to rock-buffered diagenesis, complicating the generation of a stratigraphically resolved climate history through these sections. Despite the generally high preservation quality of samples from these sections, micro-scale observations of calcite fabric and trace metal composition using electron backscatter diffraction and electron microprobe analysis suggest that only a subset of relatively pristine samples retain primary clumped isotope signatures. These samples indicate that Silurian tropical oceans were likely warm (33 ± 7 °C) and similar in oxygen isotopic composition to that estimated for a “modern” ice-free world (δ^(18)O_(VSMOW) of −1.1 ± 1.3‰). This result joins the growing body of evidence that suggests the δ^(18)O of Earth’s ocean waters has remained broadly constant through time.

As archives of past climate variability, the micron and sub-micron scales of element:calcium (Me/Ca) variability in both biogenic and inorganic carbonates contain important geochemical information. Ideally working at smaller and smaller scales leads to higher temporal resolution of past changes, but more often it has revealed the strong overprint of other processes on the climate signal. Therefore, the role of SIMS and NanoSIMS techniques in studying paleoenvironmental proxies continues to increase. We evaluate the accuracy and precision of the CAMECA ims 7F-GEO and NanoSIMS-50L ion probes for measurements of Sr/Ca, Mg/Ca, and Ba/Ca ratios in carbonate minerals. Nine carbonate reference materials were examined for their ^(88)Sr/^(42)Ca, ^(24)Mg/^(42)Ca, and ^(138)Ba/^(42)Ca ratios using a primary O^− beam with spot sizes of 20–40 μm (SIMS) and 0.8–2 μm (NanoSIMS). To assess accuracy, seven of these standards were analyzed for Sr/Ca and Mg/Ca with ID-ICP-MS. Variability in the elemental ratios arising from drift and changes in the tuning of the ims 7F-GEO over a nine month period is smaller than the chemical heterogeneity of the most frequently analyzed standards (OKA and Blue-CC). Across a whole crystal, Blue-CC is more homogeneous (1σ of 2.39% and 1.60% for Sr/Ca and Mg/Ca respectively) than OKA, but in the bulk matrix of OKA there is even less variability (1σ of 0.85% and 0.83% respectively). We find that carbonate samples can be accurately normalized to carbonate standards with significantly different absolute Me/Ca ratios. NanoSIMS intensity ratios follow counting statistics better than ± 1% (2σ) at any one spot, but conversion to Me/Ca ratios increases the uncertainty. Two factors give rise to this limitation. First, the spatial heterogeneity of nominally homogeneous standards can lead to accuracy offsets that are as large as the ranges quoted above. Second, the NanoSIMS generates higher instrumental mass fractionation relative to SIMS. The combination of three different analytical techniques demonstrates that Blue-CC and homogeneous calcite zones in OKA are promising reference materials for precise analyses. Accuracy is crucially dependent on making independent measurements on exactly the same crystal of standard used in the SIMS and NanoSIMS machines.

Using a new and rapid NanoSIMS-based method, we quantified the sensitivity of skeletal Sr/Ca in coral to the aragonite saturation state of seawater (Ω_(SW)). Skeletal Sr/Ca is a common proxy for temperature while Ω_(SW) is a parameter that varied in the past ocean and is predicted to change with continued ocean acidification. Five adult branches of the surface coral Stylophora pistillata were grown at different Ω_(SW) from 2.7 to 4.9 (pH of 7.9–8.5) but at a constant temperature of 25 °C. Despite a large range of growth parameters and a twofold range in calcification rates, the average skeletal Sr/Ca of coral exposed to each condition are within 1.2% of each other (2σ std. dev. of the 5 means). Furthermore, the average skeletal Sr/Ca measured in this study agrees with the results of two previous coral culture experiments conducted at the same temperature but where Ω_(SW) was not controlled. These results suggest that aragonite saturation has little or no influence on Sr/Ca paleothermometry over the range of Ω_(SW) investigated. Combined with existing data for low Ω_(SW) conditions, our results were used to elucidate the mechanisms controlling calcifying fluid acid–base chemistry during coral biomineralization. Assuming that coral drive precipitation through alkalinity pumping, our data suggest that this pumping occurs until the calcifying fluid reaches a target pH. Below a threshold Ω_(SW) bounded by 1 < Ω_(SW) < 2.4, however, coral do not pump enough alkalinity to reach the target pH and instead pump a maximal but finite amount of alkalinity. In this low Ω_(SW) regime, calcifying fluid pH is expected to decrease with Ω_(SW). The interplay between these two alkalinity pumping regimes and external seawater composition explain the full range of observed Sr/Ca sensitivity to Ω_(SW) and suggest that surface coral may become increasingly sensitive to ocean acidification below a threshold Ω_(SW) bounded by 1 < Ω_(SW) < 2.4.

The association between diamonds and high-Cr, sub-calcic garnets from kimberlites suggests that garnets have experienced high pressure (and presumably, high temperature) conditions in the subcontinental lithosphere mantle (SCLM). The oxygen isotope compositions of these garnets are generally reported to be lower than average values of mantle olivines (~5.2‰)—opposite the sense of oxygen isotope fractionation expected at high-temperature equilibrium, and to correlate negatively with their Cr-contents. These observations were interpreted to indicate the sub-calcic garnets either are derived from melting Cr-enriched but ^(18)O-depleted source in the SCLM, or have experienced cryptic metasomatism by high-Cr, high-Mg#, but ^(18)O-depleted fluids/melts in the SCLM or during their ascent to the surface. We investigate the possibility that oxygen isotope characteristics of these garnets instead reflect an analytical artifact that reduces the measured δ^(18)O of garnets in proportion to its Cr content—a so-called ‘Cr-effect’. Mixtures of garnet standards (UWG-2) plus various amounts of pure Cr-metal were measured for δ^(18)O-values by infrared-laser fluorination technique (ILFT). Our results show that oxygen isotope compositions of sample mixtures vary systematically as a function of Cr-content and O_(2)-yield. Mass-balance analyses indicate that the correlation between the measured δ^(18)O-value and Cr-content is an analytical artifact mostly due to isotopic fractionation between the extracted-O_(2) and chromium oxyfluorides left as residues in the sample chamber during the ILFT analyses, which amounts to a decrease in the measured δ^(18)O values of ~0.064‰ for the increase of every wt.% Cr_(2)O_(3) present in garnets. After applying our experimentally-calibrated correction factor to garnet samples in garnet peridotite xenoliths from South Africa, our study reveals an equilibrium oxygen isotope fractionation between the Cr-poor garnet and coexisting olivine (i.e., garnet≥olivine) with a fractionation factor of ~0.28‰. More importantly, high-Cr garnets (Cr_(2)O_(3)>5%), after correction for the ‘Cr-effect’, are actually enriched, rather than depleted, in 18O compared with low-Cr garnets, suggesting the involvement of ^(18)O-enriched crustal materials during their formation. We hypothesize a fluid-flow model using depthdependent Cr partitioning. This model simultaneously explains relatively high-δ^(18)O-values and Cr and Mg enrichment in garnets, implying that the reduced C\H\O fluids that are responsible for the formation of diamonds in both eclogite and peridotite xenoliths in kimberlites, are also responsible for transporting Cr to oxidized locations where the Cr partition coefficient between garnet and fluids is high.

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_2CO_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 lnα^*) and three indices that measure proportions of multiply-substituted isotopologues (Δ^*_(47), Δ^*_(48), Δ^*_(49). We also predict that oxygen isotope fractionations follow the mass dependence exponent, λ of 0.5281 (where α_(17)_O = α^λ_(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 Δ_(47) = 0.23‰) during phosphoric acid digestion of calcite at 25 °C. We have also attempted to evaluate the effect of carbonate cation compositions on phosphoric acid digestion fractionations using cluster models in which disproportionating H_2CO_3 interacts with adjacent cations. These models underestimate the magnitude of isotope fractionations and so must be regarded as unsucsessful, but do reproduce the general trend of variations and temperature dependences of oxygen isotope acid digestion fractionations among different carbonate minerals. We suggest these results present a useful starting point for future, more sophisticated models of the reacting carbonate/acid interface. Examinations of these theoretical predictions and available experimental data suggest cation radius is the most important factor governing the variations of isotope fractionation among different carbonate minerals. We predict a negative correlation between acid digestion fractionation of oxygen isotopes and of ^(13)C–^(18)O doubly-substituted isotopologues, and use this relationship to estimate the acid digestion fractionation of Δ^*_(47) for different carbonate minerals. Combined with previous theoretical evaluations of ^(13)C–^(18)O clumping effects in carbonate minerals, this enables us to predict the temperature calibration relationship for different carbonate clumped isotope thermometers (witherite, calcite, aragonite, dolomite and magnesite), and to compare these predictions with available experimental determinations. The success of our models in capturing several of the features of isotope fractionation during acid digestion supports our hypothesis that phosphoric acid digestion of carbonate minerals involves disproportionation of transition state structures containing H_2CO_3.

We present analyses of stable isotopic ratios ^(17)O/^(16)O, ^(18)O/^(16)O, ^(34)S/^(32)S, and ^(33)S/^(32)S, ^(36)S/^(32)S in sulfate leached from volcanic ash of a series of well known, large and small volcanic eruptions. We consider eruptions of Mt. St. Helens (Washington, 1980, ∼1 km^3), Mt. Spurr (Alaska, 1953, 0.2‰) and sulfur isotopes exhibiting mass-dependent behavior. Products of large eruptions account for most of‘ these isotopic ranges. Sulfate with Δ^(17)O > 0.2‰ is present as 1–10 μm gypsum crystals on distal ash particles and records the isotopic signature of stratospheric photochemical reactions. Sediments that embed ash layers do not contain sulfate or contain little sulfate with Δ^(17)O near 0‰, suggesting that the observed sulfate in ash is of volcanic origin. Mass-dependent fractionation of sulfur isotopic ratios suggests that sulfate-forming reactions did not involve photolysis of SO2, like that inferred for pre-2.3 Ga sulfates from Archean sediments or Antarctic ice-core sulfate associated with few dated eruptions. Even though the sulfate sulfur isotopic compositions reflect mass-dependent processes, the products of caldera-forming eruptions display a large δ^(34)S range and exhibit fractionation relationships that do not follow the expected equilibrium slopes of 0.515 and 1.90 for ^(33)S/^(32)S vs. ^(34)S/^(32)S and ^(36)S/^(32)S vs. ^(34)S/^(32)S, respectively. The data presented here are consistent with modification of a chemical mass-dependent fractionation of sulfur isotopes in the volcanic plume by either a kinetic gas phase reaction of volcanic SO_2 with OH and/or a Rayleigh processes involving a residual Rayleigh reactant—volcanic SO_2 gas, rather than a Rayleigh product. These results may also imply at least two removal pathways for SO_2 in volcanic plumes. Above-zero Δ^(17)O values and their positive correlation with δ^(18)O in sulfate can be explained by oxidation by high-δ^(18)O and high-Δ^(17)O compounds such as ozone and radicals such as OH that result from ozone break down. Large caldera-forming eruptions have the highest Δ^(17)O values, and the largest range of δ^(18)O, which can be explained by stratospheric reaction with ozone-derived OH radicals. These results suggest that massive eruptions are capable of causing a temporary depletion of the ozone layer. Such depletion may be many times that of the measured 3–8% depletion following 1991 Pinatubo eruption, if the amount of sulfur dioxide released scales with the amount of ozone depletion.