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At steady-state, sediment fluxes out of a drainage basin equal its average erosion rate. Quantifying relative sediment fluxes is therefore key in estimating spatial erosional variability among sub-basins and the consequential landscape evolution. Traditional approaches to quantify such fluxes in drainage basins include using the mineral and elemental compositions of sediments as markers for the relative contribution from sub-basins. Such an approach often fails to distinguish among bedrock sources, and have been shown to suffer from transport-related biases.Here, we aim to test and explore the combination of these traditional approaches together with oxygen, carbon and ‘clumped’ isotope analyses of detrital carbonate as a novel combined proxy for relative sediment fluxes in carbonate-dominated drainage basins. We test this approach at the Hatrurim Syncline in southern Israel, east of the Dead Sea margins. The area comprises of marine carbonate rocks of the Judea Gr., as well as Hatrurim Fm. rocks that have experienced different grades of combustion-metamorphism, and thereby registered a wide range of isotope values together with distinctive carbonate mineral assemblages – allowing for using both ‘traditional’ and isotope-informed approaches. We collected bedrock and sediment samples from the Morag Basin in the Hatrurim Syncline, and analyzed their mineral and isotope compositions in bulk and specific grain-size fractions.Our results show that: (a) Hatrurim Formation’s bedrock samples have a wide range of mineral and isotope values consistent with two main assemblages – high temperature metamorphic carbonates, and low temperature re-crystallized carbonates; and (b) Mineral and isotope compositions of fine grain sediment fractions (

The history of Earth’s carbon cycle reflects trends in atmospheric composition convolved with the evolution of photosynthesis. Fortunately, key parts of the carbon cycle have been recorded in the carbon isotope ratios of sedimentary rocks. The dominant model used to interpret this record as a proxy for ancient atmospheric CO 2 is based on carbon isotope fractionations of modern photoautotrophs, and longstanding questions remain about how their evolution might have impacted the record. Therefore, we measured both biomass (ε p ) and enzymatic (ε Rubisco ) carbon isotope fractionations of a cyanobacterial strain ( Synechococcus elongatus PCC 7942) solely expressing a putative ancestral Form 1B rubisco dating to ≫1 Ga. This strain, nicknamed ANC, grows in ambient pCO 2 and displays larger ε p values than WT, despite having a much smaller ε Rubisco (17.23 ± 0.61‰ vs. 25.18 ± 0.31‰, respectively). Surprisingly, ANC ε p exceeded ANC ε Rubisco in all conditions tested, contradicting prevailing models of cyanobacterial carbon isotope fractionation. Such models can be rectified by introducing additional isotopic fractionation associated with powered inorganic carbon uptake mechanisms present in Cyanobacteria, but this amendment hinders the ability to accurately estimate historical pCO 2 from geological data. Understanding the evolution of rubisco and the CO 2 concentrating mechanism is therefore critical for interpreting the carbon isotope record, and fluctuations in the record may reflect the evolving efficiency of carbon fixing metabolisms in addition to changes in atmospheric CO 2 .

Microbial coal-bed methane is an important economic resource and source of a potent greenhouse gas, but controls on its formation are poorly understood. To test whether the microbial degradability of coal limits microbial methane, we monitored methoxyl group demethylation—a reaction that feeds methanogenesis—in a global sample suite ranging in maturity from wood to bituminous coal. Carbon isotopic compositions of residual methoxyl groups were inconsistent with a thermal reaction, instead implying a substrate-limited biologic process. This suggests that deep biosphere communities participated in transforming plant matter to coal on geologic time scales and that methoxyl abundance influences coal-bed methane yield. Carbon isotopic enrichments resulting from microbial methylotrophy also explain an enigmatic offset in the carbon-13 content of microbial methane from coals and conventional hydrocarbon deposits.

Naturally occurring hydrocarbon fluids in sedimentary basins have economic, geological and environmental significance. Connecting sedimentary basin temperature-time evolution with petroleum generation and transformation is a long-studied problem. In this study, we investigate the use of a novel tool – multiply substituted isotopologues of methane, for distinguishing between different chemical mechanisms in catagenesis and for characterizing the extent of thermal maturation of thermogenic natural gases. We analyze the stable isotope compositions of a suite of thermogenic gas samples that are globally distributed and cover a wide range in composition and thermal maturation, from dominantly unconventional shale gas formations and a few conventional gas plays. Our data show that methane generated at early thermal maturity has a stable isotope composition governed by chemical kinetics, characterized by a pronounced deficit in Δ12CH2D2; this signature can be explained by its formation chemistry that combines a more D-rich methyl radical pool and more D-poor H radical pool. Methane from higher thermal maturity fluids increases in Δ12CH2D2, reaching equilibrium at vitrinite reflectance maturity (Ro) of approximately 1.5% (equivalent to 170–210 °C peak burial temperature) and higher, which is interpreted to be the result of isotope exchange erasing the disequilibrium signature of catagenetic chemistry, mediated by C H activation during either radical chain reactions or organic-inorganic interactions on mineral surfaces. We further examined hydrogen isotope fractionations among methane, ethane and propane for a compiled global dataset and found that the intermolecular fractionation exhibits a trend similar to that seen for the Δ12CH2D2 value of methane, departing from equilibrium at low thermal maturities and moving towards equilibrium as maturity increases. These findings indicate that the inter- and intra-molecular hydrogen isotope structures of components of thermogenic natural gas transition from chemical-kinetic control at low thermal maturities toward thermodynamic control at higher thermal maturities, mediated by hydrogen exchange reactions. We propose that these systematic relationships could be used to identify the exact thermal maturation stages for natural gases and their associated fluids, especially for oil-associated gas at early maturation.

Rationale: Position-specific ¹³C/¹²C ratios within amino acids remain largely unexplored in environmental samples due to methodological limitations. We hypothesized that natural-abundance isotope patterns in serine may serve as a proxy for plant metabolic fluxes including photorespiration. Here we describe an Orbitrap method optimized for the position-specific carbon isotope analysis of serine to test our hypothesis and discuss the generalizability of this method to other amino acids. Methods: Position-specific carbon isotope ratios of serine were measured using a Thermo Scientific™ Q Exactive™ GC Orbitrap™. Amino acids were hydrolyzed from Arabidopsis biomass, purified from potential matrix interferences, and derivatized alongside standards. Derivatized serine (N,O-bis(trifluoroacetyl)methyl ester) was isolated using gas chromatography, trapped in a reservoir, and purged into the electron ionization source over tens of minutes, producing fragment ions containing different combinations of atoms from the serine-derivative molecule. The ¹³C/¹²C ratios of fragments with monoisotopic masses of 110.0217, 138.0166, and 165.0037 Da were monitored in the mass analyzer and used to calculate position-specific δ¹³C values relative to a working standard. Results: This methodology constrains position-specific δ¹³C values for nanomole amounts of serine isolated from chemically complex mixtures. The δ¹³C values of fragment ions of serine were characterized with ≤1‰ precisions, leading to propagated standard errors of 0.7–5‰ for each carbon position. Position-specific δ¹³C values differed by up to ca 28 ± 5‰ between serine molecules hydrolyzed from plants grown under contrasting pCO₂, selected to promote different fluxes through photosynthesis and photorespiration. The method was validated using pure serine standards characterized offline. Conclusions: This study presents the first Orbitrap-based measurements of natural-abundance, position-specific carbon isotope variation in an amino acid isolated from a biological matrix. We present a method for the precise characterization of isotope ratios in serine and propose applications probing metabolism in plants. We discuss the potential for extending these approaches to other amino acids, paving the way for novel applications.

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.

The history of Earth’s carbon cycle reflects trends in atmospheric composition convolved with the evolution of photosynthesis. Fortunately, key parts of the carbon cycle have been recorded in the carbon isotope ratios of sedimentary rocks. The dominant model used to interpret this record as a proxy for ancient atmospheric CO2is based on carbon isotope fractionations of modern photoautotrophs, and longstanding questions remain about how their evolution might have impacted the record. We tested the intersection of environment and evolution by measuring both biomass (εp) and enzymatic (εRubisco) carbon isotope fractionations of a cyanobacterial strain (Synechococcus elongatusPCC 7942) solely expressing a putative ancestral Form 1B rubisco dating to ≫1 Ga. This strain, nicknamed ANC, grows in ambient pCO2and displays larger εpvalues than WT, despite having a much smaller εRubisco(17.23 ± 0.61‰ vs. 25.18 ± 0.31‰ respectively). Measuring both enzymatic and biomass fractionation revealed a surprising result—ANC εpexceeded ANC εRubiscoin all conditions tested, contradicting prevailing models of cyanobacterial carbon isotope fractionation. However, these models were corrected by accounting for cyanobacterial physiology, notably the CO2concentrating mechanism (CCM). Our model suggested that additional fractionating processes like powered inorganic carbon uptake systems contribute to εp, and this effect is exacerbated in ANC. Understanding the evolution of rubisco and the CCM is therefore critical for interpreting the carbon isotope record. Large fluctuations in that record may reflect the evolving efficiency of carbon fixing metabolisms in addition to changes in atmospheric CO2.Significance StatementEarth scientists rely on chemical fossils like the carbon isotope record to derive ancient atmospheric CO2concentrations, but interpretation of this record is calibrated using modern organisms. We tested this assumption by measuring the carbon isotope fractionation of a reconstructed ancestral rubisco enzyme (>1 billion years old)in vivoandin vitro. Our results contradicted prevailing models of carbon flow in Cyanobacteria, but our data could be rationalized if light-driven uptake of CO2is taken into account. Our study showed that the carbon isotope record tracks both the evolution of photosynthesis physiology as well as changes in atmospheric CO2, highlighting the value of considering both evolution and physiology for comparative biological approaches to understanding Earth’s history.

Intramolecular isotope distributions, including isotope clumping and position specific fractionation, can provide proxies for the formation temperature and formation and destruction pathways of molecules. In this study, we explore the position-specific hydrogen isotope distribution in propane. We analyzed propane samples from 10 different petroleum systems with high-resolution molecular mass spectrometry. Our results show that the hydrogen isotope fractionation between central and terminal positions of natural propanes ranges from −102‰ to +205‰, a much larger range than that expected for thermodynamic equilibrium at their source and reservoir temperatures (36–63‰). Based on these findings, we propose that the hydrogen isotope structure of catagenic propane is largely controlled by irreversible processes, expressing kinetic isotope effects (KIEs). Kinetic control on hydrogen isotope composition of the products of thermal cracking is supported by a hydrous pyrolysis experiment using the Woodford Shale as substrate, in which we observed isotopic disequilibrium in the early stage of pyrolysis. We make a more general prediction of KIE signatures associated with kerogen cracking by simulating this chemistry in a kinetic Monte Carlo model for different types of kerogens. In contrast, unconventional shale fluids or hot conventional reservoirs contain propane with an isotopic structure close to equilibrium, presumably reflecting internal and/or heterogeneous exchange during high temperature storage (ca. 100–150 °C). In relatively cold (

Cold seep environments are characterized by methane-rich fluid migration and discharge at the seafloor. These environments are also intimately linked to microbial communities, which oxidize methane anaerobically, increase alkalinity and promote authigenic carbonate precipitation. We have analyzed a suite of methane-derived authigenic carbonate (MDAC) crusts from the North and Barents Sea using stable and clumped isotopes (δ¹³C, δ¹⁸O, δ⁴⁴Ca, and Δ₄₇) to characterize the sources of fluids as well as the environment of carbonate authigenesis. We additionally assess the potential of MDACs as a Δ₄₇-based paleotemperature archive. The MDACs occur as three main textural-mineralogic types: micritic Mg-calcite cements, micritic aragonite cements and cavity filling aragonite cements. We find that micritic Mg-calcite cements have low δ¹³C_(VPDB) values (−30 to −47‰), high δ⁴⁴Ca_(SW) values (−0.4 to −0.8‰), and Δ₄₇-temperatures (0–6 °C) consistent with shallow sub-seafloor precipitation in isotopic equilibrium. Micritic aragonite cements and cavity filling aragonite cements both have a wider range in δ¹³C_(VPDB) values (−18 to −58‰), lower δ⁴⁴Ca_(SW) values (−0.8 to −1.6‰) and a larger range in Δ₄₇-based apparent temperatures (–2 – 25 °C) with samples displaying equilibrium and disequilibrium clumped isotope values. The range in apparent temperatures as well as δ⁴⁴Ca_(SW) values seen in the aragonite MDACs suggest two kinetic processes: a kinetic isotope effect (KIE) due to the incomplete equilibration of carbon and oxygen isotopes among DIC species from the different sources of DIC (i.e., seawater, methane-sourced DIC and DIC residual to CO₂ degassing or diffusion) and a KIE due to a fast, irreversible precipitation affecting the cations, particularly Ca, bound to carbonate mineral. Our results improve the understanding of kinetic effects on clumped isotope temperatures in MDACs and demonstrate how the multi-isotopic approach combined with textural-mineralogic criteria can be used to identify MDACs for accurate paleotemperature reconstructions.

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.

Low-molecular-weight (LMW) aromatic compounds from petroleum fluids have not been widely studied for fluid-source correlations due to their volatility and their relatively low abundances in source rocks. However, LMW aromatics are important components in fluids, including condensates which lack biomarkers (molecular fossils typically used for correlation studies). Here, we have investigated the distribution of ethyltoluenes (o-ET, m-ET and p-ET; ortho, meta and para, respectively) in fluvial-deltaic condensates which contain relatively high abundances of the meta-isomer. The meta-selectivity found in these petroleum fluids is consistent with a mineral catalytic effect on the molecular distribution of these compounds, as it occurs during the clay-catalyzed synthesis of ethyltoluenes. Isomers differ up to 6‰ in δ13C values. Condensates from the Northern Carnarvon Basin, North West Shelf of Australia (NWS), have been analyzed by compound specific isotope analysis (CSIA) by GC-ir-MS, and site-specific isotope analysis (SSIA) using a Q-Exactive-GC Orbitrap™-based mass spectrometer. The SSIA revealed a 13C enrichment at the methyl end of the ethyl branch of m-ET, following a normal kinetic isotope effect during thermal maturation (cleavage). Continuous development of this first SSIA application will make possible high resolution analysis of light aromatics to outline the evolution of the organic matter to hydrocarbons in petroleum systems.

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.

International audience; High burial temperature and a lack of material for traditional thermal proxies significantly impede the understanding of the thermal history of carbonate stratigraphic sequences in deep time. Carbonate clumped isotope thermometry interpreted through models of solid-state isotopic reordering and in situ U-Pb dating provides a new methodology for reconstructing thermal histories of such successions. Calcite fabrics (micrite cements and veins) from deeply buried (5,000–8,000 m) Ordovician carbonate intervals in the Tarim Basin were analyzed for their carbonate clumped isotope compositions and U-Pb ages, and the results were used to estimate maximum peak burial temperatures. Reconstructed burial temperatures reveal a previously ignored thermal pulse in the range of 160°C–190°C during the Permian period. Maximum geothermal gradients at the time of peak burial temperature varied from 26.8°C/km to 69.3°C/km, with higher values in the northern Tarim Basin. We show that the spatial patterns of peak burial temperature and geothermal gradients closely correspond to the distribution of the Tarim Large Igneous Province, which emplaced during the Permian following the emplacement of a mantle plume beneath the Tarim Basin. We thus propose that the thermal pulse recorded in Tarim Ordovician limestones resulted from Permian mantle plume activity. The reconstructed thermal history has important implications for our understanding of source-rock hydrocarbon generating history, hydrocarbon accumulation, and fluid flow history in the Tarim Basin. This study also demonstrates the potential of the integration of Δ47, U-Pb dating of carbonates and solid-state reordering models as an integrated thermochronological method for reconstructing thermal histories of deeply buried carbonate intervals.

Increased use and improved methodology of carbonate clumped isotope thermometry has greatly enhanced our ability to interrogate a suite of Earth‐system processes. However, interlaboratory discrepancies in quantifying carbonate clumped isotope (Δ47) measurements persist, and their specific sources remain unclear. To address interlaboratory differences, we first provide consensus values from the clumped isotope community for four carbonate standards relative to heated and equilibrated gases with 1,819 individual analyses from 10 laboratories. Then we analyzed the four carbonate standards along with three additional standards, spanning a broad range of δ47 and Δ47 values, for a total of 5,329 analyses on 25 individual mass spectrometers from 22 different laboratories. Treating three of the materials as known standards and the other four as unknowns, we find that the use of carbonate reference materials is a robust method for standardization that yields interlaboratory discrepancies entirely consistent with intralaboratory analytical uncertainties. Carbonate reference materials, along with measurement and data processing practices described herein, provide the carbonate clumped isotope community with a robust approach to achieve interlaboratory agreement as we continue to use and improve this powerful geochemical tool. We propose that carbonate clumped isotope data normalized to the carbonate reference materials described in this publication should be reported as Δ47 (I‐CDES) values for Intercarb‐Carbon Dioxide Equilibrium Scale., Key Points The exclusive use of carbonate reference materials is a robust method for the standardization of clumped isotope measurementsMeasurements using different acid temperatures, designs of preparation lines, and mass spectrometers are statistically indistinguishableWe propose new consensus values for a set of seven carbonate reference materials and updated guidelines to report clumped isotope measurements

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.

Understanding mechanisms, rates, and drivers of past carbonate formation provides insight into the chemical evolution of Earth’s oceans and atmosphere. We paired geological observations with elemental and isotope geochemistry to test potential proxies for calcium‐to‐alkalinity ratios (Ca:ALK). Across diverse carbonate facies from Pleistocene closed‐basin lakes in Owens Valley, CA, we observed less δ^(44/40)Ca variation than theoretically predicted (>0.75‰) for the very low Ca:ALK in these systems. Carbonate clumped isotope disequilibria implied rapid carbonate growth—kinetic isotope effects, combined with the diverse carbonate minerals present, complicated the interpretation of δ^(44/40)Ca as a paleo‐alkalinity proxy. In contrast, we observed that the high phosphate concentrations are recorded by shoreline and lake bottom carbonates formed in eleven Pleistocene lakes at orders of magnitude greater concentrations than in marine carbonates. Because the maximum phosphate content of water depends on Ca:ALK, we propose that carbonate P/Ca can inform phosphate levels and thereby Ca:ALK of aqueous environments in the carbonate record.

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.

The release of long‐stored carbon from thawed permafrost could fuel increased methanogenesis in northern lakes, but it remains unclear whether old carbon substrates released from permafrost are metabolized as rapidly by methanogenic microbial communities as recently produced organic carbon. Here, we apply methane (CH₄) clumped isotope (Δ₁₈) and ¹⁴C measurements to test whether rates of methanogenesis are related to carbon substrate age. Results from culture experiments indicate that Δ₁₈ values are negatively correlated with CH₄ production rate. Measurements of ebullition samples from thermokarst lakes in Alaska and glacial lakes in Sweden indicate strong negative correlations between CH₄ Δ₁₈ and the fraction modern carbon. These correlations imply that CH₄ derived from older carbon substrates is produced relatively slowly. Relative rates of methanogenesis, as inferred from Δ₁₈ values, are not positively correlated with CH₄ flux estimates, highlighting the likely importance of environmental variables other than CH₄ production rates in controlling ebullition fluxes.

Dolomite clumped isotope compositions are indispensable for determining the temperatures and fluid sources of dolomitizing environments, but can be misleading if they have modified since formation. Carbonate Δ_(47) values are susceptible to resetting by recrystallization during diagenesis, and, even in the absence of dissolution and reprecipitation reactions, alteration by solid-state reordering during prolonged residences at elevated temperatures. In order to understand the potential of dolomite Δ_(47) values to preserve the conditions of dolomitizationin ancient sections, the kinetic parameters of solid-state reordering in this phase must be determined. We heated mm-sized crystals of near-stoichiometric dolomite in a Rene-type cold seal apparatus at temperatures between 409 and 717 °C for 0.1–450 h. In order to prevent the decarbonation of dolomite to calcite, periclase, and CO_2 at these conditions, the system was pressurized with CO_2 to 0.45–0.8 kbar. Over the course of 31 temperature-time points and 128 individual Δ_(47) measurements of powdered dolomite crystals from these points, we observed the evolution of dolomite Δ_(47) values from the initial (unheated) composition of the crystals (0.452 ± 0.004‰, corresponding to a formation temperature of ∼145 °C) towards high-temperature equilibrium distributions. Complete re-equilibration occurred in the 563–717 °C experiments. As with previous heating experiments using calcite and apatite, dolomite Δ_(47) exhibited complex reordering behavior inadequately described by first-order Arrhenian-style models. Instead, we fit the data using two published models for clumped isotope reordering: the transient defect/equilibrium defect model of Henkes et al. (2014), and the exchange-diffusion model of Stolper and Eiler (2015). For both models, we found optimal reordering parameters by using global least-squares minimization algorithms and estimated uncertainties on these fits with a Monte Carlo scheme that resampled individual Δ_(47) measurements and re-fit the dataset of these new mean values. Because the exact Δ_(47)–T relationship between 250 and 800 °C is uncertain, we repeated these fitting exercises using three published high-temperature Δ_(47)–T calibrations. Regardless of calibration choice, dolomite Δ_(47) rate constants determined using both models are resolvably slower than those of calcite and apatite, and predict that high-grade dolomite crystals should preserve apparent equilibrium blocking temperatures of between ∼210 and 300 °C during cooling on geologic timescales. Best agreement between model predictions and natural dolomite marbles was found when using the exchange-diffusion model and the ab initio Δ_(63)–T calibration of Schauble et al. (2006), projected into the Δ_(47) reference frame by Bonifacie et al. (2017). Therefore, we recommend modeling dolomite Δ_(47) reordering using the exchange-diffusion model and this parameter set. In simple heating scenarios, the two models disagree. The transient defect/equilibrium defect model suggests that dolomite fabrics resist detectable reordering at ambient temperatures as high as 180 °C for tens of millions of years, while the exchange-diffusion model predicts incipient partial reordering perhaps as low as 150 °C. In either case, barring later recrystallization, dolomite Δ_(47) values should be faithful recorders of the conditions of dolomitization in sedimentary sections buried no hotter than ∼150 °C for tens of millions of years.

Carbonate clumped-isotope thermometry is a promising technique that has the potential to help decode the significance of the variability of both physical and geochemical compositions of ancient carbonate rocks. This study utilizes a 600-million-year record of marine carbonate rocks from the subsurface and surface of the Sultanate of Oman to explore how burial and exhumation affected the carbonate clumped-isotope thermometer. Samples span 6 km of burial depth, and include calcite and dolomite mineralogies and a range of carbonate rock textures. We find evidence for two broad patterns in the physical and geochemical behavior of carbonate rocks during burial. The first group of carbonates yield water δ^(18)O_(VSMOW) compositions slightly enriched or equal to an expected “ice-free” seawater composition of –1.2‰ and display good to fair textural preservation suggesting that cementation and lithification occurred within tens of meters of the sediment–water interface. Temperatures from the second group sit on the present-day geotherm, yield highly enriched water δ^(18)O_(VSMOW) compositions, and display fair to poor textural preservation. We find no evidence for solid-state reordering in paired analyses of calcites and dolomites. Our results contribute to a growing body of work that indicates that the seawater δ^(18)O_(VSMOW) composition has not changed significantly over 600 Myr and was not –6‰ in the Ediacaran.

We report volatile concentrations and hydrogen isotope compositions of impact melts and minerals in EETA 79001. We observed chemical changes in pyroxene, maskelynite (or feldspathic glass), and merrillite in contact with or inside impact melts. All pyroxene grains analyzed here are inside or close to impact melt pockets and contain 10–41 ppm H_2O and enriched in D (δD=+1729 to +3707‰), with the highest values found in a grain enclosed in an impact melt pocket. Maskelynite or feldspathic glass contains 6.3 to 98 ppm H_2O with δ D values of +1604 to +3938‰. The high H_2O and δD values were obtained in those enclosed inside or in contact with the impact melts, whereas low H_2O content (4 ppm) and terrestrial-like D/H value (δ D of −90±82‰) were found in one maskelynite grain away from impact melts contains. Rims of ∼5 μm thickness of merrillite grains next to impact melts display Na-depletion by ∼0.9 wt%, and the sides in contact with impact melts show Mg-enrichment by ∼0.5 wt%. However, the H_2O and δD values of merrillite interiors (39–242 ppm H_2O and δ D of +1682 to +3884‰) do not show correlation with their proximity to the impact melts. Rather, δD and 1/H_2O of merrillite form a negative trend different from that of impact melt pockets and maskelynite, suggesting post-crystallization or late-crystallization interactions with the crustal fluids. The impact melt pockets in EETA 79001 contain 121–646 ppm H_2O, 4.3–13 ppm F, 13–50 ppm Cl, 707–2702 ppm S, and the δ D values of +3368 to +4639‰. The correlations between H_2O, F, Cl, P_2O_5, and δD values of impact melts and feldspathic glass are consistent with mixing between a volatile-rich and high δ D (+3000 to +5000‰) endmember and a volatile-poor and low δD endmember. The volatile-poor and low δD endmember is consistent with magmatic volatiles stored in silicates. The volatile-rich and high δD endmember represents pre-impact alteration materials by subsurface water. Alteration from the subsurface water, equilibrated with the present-day-like Martian atmosphere, occurred after the crystallization of the rock (∼170 Ma) and before impact launch (∼0.7 Ma). Our conclusion is different from the previous suggestion of an isotopically distinct subsurface water reservoir with a δ D value of +1000 to +2000‰ in EETA 79001. Although heterogeneous subsurface water on Mars is possible, the previous study was likely biased by a limited number of analyses (n=2) and possible terrestrial contamination. The δ D value of the subsurface source in EETA 79001 is ∼+4200‰, similar to those in the Tissint meteorite (crystallization at ∼600 Ma, impact launch at ∼0.7 Ma) and LAR 06319 (crystallization at ∼200 Ma, impact launch at ∼3 Ma), suggesting stable water chemistry for the subsurface environment in the last 600 Myrs.

The secular increase in δ^(18)O values of both calcitic and phosphatic marine fossils through early Phanerozoic time suggests either that (1) early Paleozoic surface temperatures were high, in excess of 40 °C (tropical MAT), (2) the δ^(18)O value of seawater has increased by 7–8‰ VSMOW through Paleozoic time, or (3) diagenesis has altered secular trends in early Paleozoic samples. Carbonate clumped isotope analysis, in combination with petrographic and elemental analysis, can deconvolve fluid composition from temperature effects and therefore determine which of these hypotheses best explain the secular δ^(18)O increase. Clumped isotope measurements of a suite of calcitic and phosphatic marine fossils from late Cambrian- to Middle-late Ordovician-aged strata–the first paired fossil study of its kind–document tropical sea surface temperatures near modern temperatures (26–38 °C) and seawater oxygen isotope ratios similar to today’s ratios.

As the most abundant transition metal in the Earth’s crust, iron is a key player in the planetary redox budget. Observations of iron minerals in the sedimentary record have been used to describe atmospheric and aqueous redox environments over the evolution of our planet; the most common method applied is iron speciation, a geochemical sequential extraction method in which proportions of different iron minerals are compared to calibrations from modern sediments to determine water-column redox state. Less is known about how this proxy records information through post-depositional processes, including diagenesis and metamorphism. To get insight into this, we examined how the iron mineral groups/pools (silicates, oxides, sulfides, etc.) and paleoredox proxy interpretations can be affected by known metamorphic processes. Well-known metamorphic reactions occurring in sub-chlorite to kyanite rocks are able to move iron between different iron pools along a range of proxy vectors, potentially affecting paleoredox results. To quantify the effect strength of these reactions, we examined mineralogical and geochemical data from two classic localities where Silurian-Devonian shales, sandstones, and carbonates deposited in a marine sedimentary basin with oxygenated seawater (based on global and local biological constraints) have been regionally metamorphosed from lower-greenschist facies to granulite facies: Waits River and Gile Mountain Formations, Vermont, USA and the Waterville and Sangerville-Vassalboro Formations, Maine, USA. Plotting iron speciation ratios determined for samples from these localities revealed apparent paleoredox conditions of the depositional water column spanning the entire range from oxic to ferruginous (anoxic) to euxinic (anoxic and sulfidic). Pyrrhotite formation in samples highlighted problems within the proxy as iron pool assignment required assumptions about metamorphic reactions and pyrrhotite’s identification depended on the extraction techniques utilized. The presence of diagenetic iron carbonates in many samples severely affected the proxy even at low grade, engendering an interpretation of ferruginous conditions in all lithologies, but particularly in carbonate-bearing rocks. Increasing metamorphic grades transformed iron in carbonates into iron in silicate minerals, which when combined with a slight increase in the amount of pyrrhotite, drove the proxy toward more oxic and more euxinic conditions. Broad-classes of metamorphic reactions (e.g. decarbonation, silicate formation) occurred at distinct temperatures-pressures in carbonates versus siliciclastics, and could be either abrupt between metamorphic facies or more gradual in nature. Notably, these analyses highlighted the importance of trace iron in phases like calcite, which otherwise might not be included in iron-focused research i.e. ore-system petrogenesis, metamorphic evolution, or normative calculations of mineral abundance. The observations show that iron is mobile and reactive during diagenesis and metamorphism, and these post-depositional processes can readily overprint primary redox information held by iron speciation. However, in principle, additional mineralogical and petrographic approaches can be combined with iron speciation data to help untangle many of these post-depositional processes and arrive at more accurate estimates of paleoenvironmental redox conditions and processes, even for metamorphosed samples.

Ethane is the second most abundant alkane in most natural gas reservoirs. Its bulk isotopic compositions (δ^(13)C and δD) are used to understand conditions and progress of cracking reactions that lead to the accumulation of hydrocarbons. Bulk isotopic compositions are dominated by the concentrations of singly-substituted isotopologues (^(13(CH_3-^(12)CH_3 for δ^(13)C and ^(12)CDH_2-^(12)CH_3 for δD). However, multiply-substituted isotopologues can bring additional independent constraints on the origins of natural ethane. The ^(13)C_2H_6 isotopologue is particularly interesting as it can potentially inform the distribution of ^(13)C atoms in the parent biomolecules whose thermal cracking lead to the production of natural gas. This work presents methods to purify ethane from natural gas samples and quantify the abundance of the rare isotopologue ^(13)C_2H_6 in ethane at natural abundances to a precision of ±0.12‰ using a high-resolution gas source mass spectrometer. To investigate the natural variability in carbon-carbon clumping, we measured twenty-five samples of thermogenic ethane from a range of geological settings, supported by two hydrous pyrolysis of shales experiments and a dry pyrolysis of ethane experiment. The natural gas samples exhibit a range of ’clumped isotope’ signatures (Δ^(13)C_2H_6) at least 30 times larger than our analytical precision, and significantly larger than expected for thermodynamic equilibration of the carbon-carbon bonds during or after formation of ethane, inheritance from the distribution of isotopes in organic molecules or different extents of cracking of the source. However we show a relationship between the Δ^(13)C_2H_6 and the proportion of alkanes in natural gas samples, which we believe can be associated to the extent of secondary ethane cracking. This scenario is consistent with the results of laboratory experiments, where breaking down ethane leaves the residue with a low Δ^(13)C_2H_6 compared to the initial gas. Carbon-carbon clumping is therefore a new potential tracer suitable for the study of kinetic processes associated with natural gas.

Organic compounds are ubiquitous in the Earth's surface, sediments and many rocks, and preserve records of geological, geochemical and biological history; they are also critical natural resources and major environmental pollutants. The naturally occurring stable isotopes of volatile elements (D, ^(13)C, ^(15)N, ^(17,18)O, ^(33,34,36)S) provide one way of studying the origin, evolution and migration of geological organic compounds. The study of bulk stable isotope compositions (i.e. averaged across all possible molecular isotopic forms) is well established and widely practised, but frequently results in non-unique interpretations. Increasingly, researchers are reading the organic isotopic record with greater depth and specificity by characterizing stable isotope ‘structures’ – the proportions of site-specific and multiply substituted isotopologues that contribute to the total rare-isotope inventory of each compound. Most of the technologies for measuring stable isotope structures of organic molecules have been only recently developed and to date have been applied only in an exploratory way. Nevertheless, recent advances have demonstrated that molecular isotopic structures provide distinctive records of biosynthetic origins, conditions and mechanisms of chemical transformation during burial, and forensic fingerprints of exceptional specificity. This paper provides a review of this young field, which is organized to follow the evolution of molecular isotopic structure from biosynthesis, through diagenesis, catagenesis and metamorphism.

Several technologies are being developed to examine the intramolecular isotopic structures of molecules (i.e., site-specific and multiple substitution), but limitations in sample size and type or (for mass spectrometry) resolution have so far prevented the creation of a general technique. We have now demonstrated the capacity for precise and accurate study of molecular isotopic contents and structures by Fourier transform mass spectrometry, using instruments containing a Thermo Scientific™ Orbitrap™ mass analyzer, here the Thermo Scientific Q Exactive GC™ and Q Exactive HF™ instruments. Orbitrap mass analyzers achieve mass resolutions in the range ∼250,000–1 M (FWHM) in the mass range of greatest interest to studies of molecular isotopic structure, 50–200 amu. This allows for resolution of many nearly isobaric interferences for compounds containing H, C, N, O and/or S. In this paper we show that internal and external experimental reproducibilities of isotope ratio analyses using the Orbitrap analysis can conform to shot-noise limits down to levels of tenths of per mil (1SE), with similar accuracy when standardized to reference materials. Precision reaches ±0.015‰ for exceptionally long integrations. Such measurements do not call for modifications to the ion optics of the Q Exactive instruments, but do require specially designed sample introduction devices to permit sample/standard comparison and long integration times. The sensitivity of the Q Exactive instruments permit analysis of sub-nanomolar samples and quantification of multiply-substituted species. Site-specific capabilities arise from the fact that mass spectra of molecular analytes commonly contain diverse fragment ion species, each of which samples a specific sub-set of molecular sites.