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The distribution of stable isotopes in organic compounds within geochemical systems reflects the reactions they undergo, which are often kinetically governed and can be elucidated through quantification based on first principles: employing quantum chemical simulations to derive energy barriers and kinetic isotope effects (KIEs), followed by kinetic calculations using these parameters. In this paper, the distribution of 13C in hydrocarbon gases during the thermal generation and decomposition of ethane and propane has been addressed. The work is based on abundant and systematic isotopic data, including the compound- and position-specific 13C compositions, recent improvements in kinetic networks in hydrocarbon generation, and state-of-the-art quantum chemical methods that can process organic geochemical systems involving solid phases. Density Functional Theory computations validate the carbonium path of hydrocarbon generation and the free-radical path of their decomposition under geological conditions. The first step of each path (alkyl sidechain protonation for generation and hydrogen abstraction for decomposition) is the rate-limiting step and determines the 13C fractionations of gaseous hydrocarbons, with the reaction rate constrained by the concentration or accessibility of the active species (hydrated protons and surface free radicals in the two paths, respectively). Alkyl protonation has normal 13C KIEs, but hydrogen abstraction on the surfaces of solid organic matter has inverse ones (substituting 12C by 13C accelerates the reaction). These inverse KIEs explain three isotopic phenomena in gaseous hydrocarbons in their late thermal evolution: 1) the depletion of 13C in ethane and propane accompanied by their decreased fraction in hydrocarbon gases (isotope "rollover"); 2) the reversed 13C sequence with carbon number (δ13C methane > δ13C ethane > δ13C propane); and 3) the depletion of 13C in the center carbon atoms of residual propane. Isotopic fractionation between gaseous hydrocarbons upon generation is evaluated using the Successive Random Chain-Scission model. The covariation of δ13C in alkanes shows a depletion in CH 4 compared with the values expected from the reciprocal of the carbon numbers, indicating that this deviation may be intrinsic instead of mixing with microbial methane. The comparison of intramolecular 13C fractionation of propane between computational and observed data suggests the contribution of isoprenoids at the beginning of the generation, in addition to isotopic homogenization between the center and end carbon atoms through the ring closure of intermediate cations. The 13C–13C clumping exhibits a weak KIE in both the generation and decomposition of ethane; the clumping is essentially stochastically governed. This work demonstrates how first-principle calculations can provide significant insights into the details of geochemical reactions governing the isotopic distributions. [ABSTRACT FROM AUTHOR]

Recently developed methane clumped isotope techniques provides fresh and novel insights into methane biogeochemistry, which have been unobtainable through other techniques such as conventional stable isotope determinations and molecular composition measurements of hydrocarbons. Nonetheless, the governing processes and mechanisms which control the clumped isotope signatures (Δ13CH 3 D and Δ12CH 2 D 2) of natural methane samples remain an active area of investigation. Here, we present paired clumped isotope measurements in methane hydrate, which is a major methane reservoir widely distributed along the continental margins and which plays an important role in the global carbon cycle and climate system. Our study aims to shed new light into the fundamental processes of methane clumped isotope effects and their potential to answer fundamental questions regarding the source and migration of methane found in naturally occurring gas hydrate accumulations. Gas hydrate samples were recovered from five shallow marine sediment sites on the eastern margin of the Japan Sea and most of them present Δ13CH 3 D and Δ12CH 2 D 2 temperatures ranging from ∼15 to ∼170 °C that apparently match expected methane formation temperatures. The distribution of these clumped isotope signatures along the equilibrium line is best explained by the mixing effect of equilibrated thermogenic methane formed at temperatures of 165 ± 15 °C and biogenic methane equilibrated at 1–2 °C, which may result from slow methanogenesis, anaerobic oxidation of methane (AOM), or their combination. The influences of gas migration/diffusion, hydrate formation and dissociation on Δ13CH 3 D and Δ12CH 2 D 2 values are insignificant. By combining clumped isotope results with other traditional approaches, a thermogenic and two microbial end-members as well as their isotopic compositions were identified and the relative contribution of each end-member was also quantified. The results not only demonstrate the applicability of methane clumped isotope data to identify potential end-members in natural methane samples, but also reveal that more conventional carbon isotope approaches may significantly underestimate the fraction of thermogenic methane present in global gas hydrate reservoirs. Improvements in the accuracy of source apportionment enable us to better understand the formation history and mechanisms of gas hydrate accumulation, as well as the role played by gas hydrate dissociation in past geological events. The estimated formation temperatures of thermogenic end-member can be further applied in reconstruction of the paleo geothermal gradient at the time when the thermogenic methane was formed at marine sedimentary environment. [ABSTRACT FROM AUTHOR]

Methyl groups are found in numerous biogenic and synthetic materials including geologically preserved materials such as wood. The carbon and hydrogen isotope compositions of methyl groups are used as tracers in biogeochemical cycles, as paleothermometers, and to determine the hydrogen isotopic composition of ancient rain. Here we present analyses of resolved 13C–D (13CH 2 D) and D–D (12CHD 2) clumped isotope compositions of methyl groups as new variables for the study of methyl groups in the present and past. We first present chemical methods to extract, purify, and derivatize methyl groups from methoxyl (R –O–CH 3) groups as CH 3 F and CH 3 Cl, and high-resolution mass spectrometric techniques to determine the clumped isotope compositions of these species. We achieve precisions for 13C–D clumping of ±0.25‰ and D–D clumping of ±2.5‰. We anchor our clumped isotopic measurements to a thermodynamic reference frame by first calculating the theoretical temperature dependences of 13C–D and D–D clumping in CH 3 Cl, then placing our measurements onto this reference frame through experimental internal isotopic equilibration of CH 3 Cl at 200 °C. Finally, we provide and analyze an initial dataset of clumped 13C–D and D–D compositions of methyl groups from various commercial/synthetic monomers and environmental woods. We observe ranges in clumped isotope compositions of ∼11‰ in 13C–D and ∼48‰ in D–D, and systematic differences within these ranges between methyl groups from commercial monomers and wood. Specifically, commercial clumped 13C–D compositions are between 0 and 3‰, which correspond to apparent equilibrium temperatures between 170 °C and the infinite temperature limit. In contrast, the clumped 13C–D compositions of wood methoxyl groups are distinctively high (9.50–11.25‰) and 3–6‰ higher than would be expected if formed in internal isotopic equilibrium at Earth-surface temperatures. Commercial/synthetic methyl and wood methoxyl clumped D–D compositions are also distinct: −5 to +13‰ in commercial monomers vs. −35 to −8‰ in wood—such negative values cannot result from formation in isotopic equilibrium and require kinetic processes to have occurred. These results indicate that wood methoxyl groups are formed out of isotopic equilibrium and that clumped isotope compositions of methyl groups may be useful tracers of methyl group sources and sinks in the environment. For instance, isotopic clumping in methyl groups may be useful for understanding controls on isotopic clumping in methane produced by methylotrophic methanogens. [ABSTRACT FROM AUTHOR]

The carbonate clumped isotope thermometer is a powerful tool in paleoclimate research, because it constrains carbonate formation temperature based on the extent of 13C and 18O clumping within the carbonate lattice (Δ 47) and thus does not require knowledge of the isotopic composition of the water from which carbonates precipitate. However, there is growing evidence that kinetic processes in the precipitating solution, particularly the slow inter-conversion between CO 2 and HCO 3 −, can cause deviations of the clumped isotope composition of dissolved inorganic carbon (DIC) from their expected equilibrium values, leading to disequilibrium clumped isotope composition in the carbonate precipitates. Such disequilibrium effects impede the application of the carbonate clumped isotope thermometer to important climate archives (e.g., speleothems and corals), and hinder the realization of the full potential of this novel thermometer. Here, I systematically examine the patterns and controls of kinetic clumped isotope fractionations (i.e., Δ 47 , Δ 48 and Δ 49) in the DIC-H 2 O-CO 2 system through first-principles theoretical calculations and numerical modeling (IsoDIC), and explore their implications for the carbonate clumped isotope thermometry. I show that, in contrast to the large carbon and oxygen isotope effects, intrinsic clumped isotope fractionations associated with CO 2 hydration and hydroxylation and their reverse reactions (i.e., HCO 3 − dehydration and dehydroxylation) will lead to only relatively small disequilibrium Δ 47 effects in the HCO 3 − that are directly produced or consumed by these reactions. Instead, the disequilibrium Δ 47 effects observed in most natural and laboratory-synthesized carbonates arise in large part from the mixing or removal of DIC pools with similar Δ 47 compositions but distinct bulk carbon and oxygen isotope compositions. Further, my model simulations show characteristic evolutions of the clumped isotope composition of DIC during three common isotope fractionation processes, i.e., DIC-H 2 O isotope exchange, CO 2 degassing, and CO 2 absorption, and predict correlated enrichments in δ13C, δ18O and Δ 48 but depletions in Δ 47 and Δ 49 of DIC during the early stage of CO 2 degassing and vice versa during CO 2 absorption, yielding typical disequilibrium Δ 47 /δ18O, Δ 48 /δ18O and Δ 47 /Δ 48 slopes of about −0.03, 0.03, and −1.0 for CO 2 degassing and about −0.02, 0.04, and −0.6 for CO 2 absorption. While both the physicochemical condition (e.g., T, pH, [DIC], pCO 2) and the isotopic composition (e.g., δ13C, δ18O, Δ 47, Δ 48 , Δ 49) of the aqueous solution and air CO 2 can affect the magnitudes of these disequilibrium isotope effects, the correlations among these disequilibrium effects are relatively insensitive to changes in most environmental parameters except the isotopic composition of the aqueous solution and air CO 2. The quantitative agreements between my model results and the existing observations from laboratory experiments and natural carbonates suggest that the model captures the key processes governing clumped isotope fractionations in the DIC-H 2 O-CO 2 -CaCO 3 system, and support the development of novel approaches, e.g., coupled Δ 47 -Δ 48 measurements, to discern the kinetic processes involved in carbonate formation and to correct for disequilibrium clumped isotope effects in carbonate archives and derive accurate estimates of their formation temperatures. [ABSTRACT FROM AUTHOR]

Stable isotope composition of speleothems reflects the physicochemical condition of their formation environment such as the local temperature and the isotopic composition of surface precipitation, making them one of the best archives of terrestrial climate. However, quantitative reconstruction of paleo-temperature from speleothem isotope records is challenging, because most speleothems do not form in isotope equilibrium with the drip water. These disequilibrium isotope effects, often thought to arise from the rapid degassing of CO 2 from a thin water film, vary among different speleothems and over time, and hinder the applications of many isotope thermometers in speleothems, including both the conventional carbonate-water oxygen isotope thermometer and the recently developed carbonate clumped isotope thermometer. Here we present an isotope-enabled reaction-diffusion model of speleothem formation (IsoCave), and use it to systematically examine the patterns and controls of the disequilibrium isotope effects in speleothems (δ13C, δ18O, Δ′17O, Δ 47 , Δ 48 and Δ 49) and explore their implications for isotope thermometry in speleothems. We show that prior calcite precipitation, cave air pCO 2 and δ 13 C CO 2 exert the strongest controls on the disequilibrium isotope effects in speleothems, followed by cave temperature, water film thickness and water drip rate. Together, changes in these environment parameters explain the variations of disequilibrium isotope effects in natural speleothems. Our model reproduces the apparent temperature dependence of oxygen isotope compositions of speleothems compiled from multiple caves over a large temperature range, but highlights the challenges in the application of speoleothem-specific calibration of isotope thermometers due to the effects of non-temperature factors. Further, we show the disequilibrium effects in different isotope systems are highly correlated. The slopes of these correlations reflect mainly the kinetic isotope fractionations associated with HCO 3 − dehydration and dehydroxylation reactions, and remain relatively constant despite changes in environmental conditions especially for speleothems formed during the early evolution period of drip water. Based on these correlations, specifically the correlation between disequilibrium Δ 47 and Δ 48 effects, we propose a novel approach to quantitatively correct for disequilibrium isotope effects in speleothems and derive more accurate estimates of speleothem formation temperatures. Application of this coupled Δ 47 -Δ 48 approach to synthetic speleothem isotope records reproduces the speleothem formation temperatures, suggesting new opportunities for quantitative paleo-temperature reconstruction based on speleothem isotope records. [ABSTRACT FROM AUTHOR]

• Land snail Δ 47 -derived temperature correlates well with growing season temperature. • Clumped temperatures of Cathaica are about 3 °C higher than those of Bradybaena. • A species-specific Δ 47 transfer function is needed to reconstruct paleotemperature. • Body water δ18O of Bradybaena shows a robust correlation with rainfall δ18O in northern China. Land snail fossils are abundantly distributed in geological deposits and their isotopic compositions provide a means to determine paleoclimatic changes. With the development of the clumped isotopes (Δ 47) geothermometer, many efforts have been made in recent years to study clumped isotopes in land snail shell carbonate. Although there have been several recent attempts, there is, as yet, no empirical calibration function to convert land snail Δ 47 to environmental temperature. Here, we systematically analyzed clumped isotopes (Δ 47) of two common land snail species (Bradybaena and Cathaica) from China. Results showed that temperatures calculated using the Δ 47 (T 47) of both species did not correlate with the mean annual temperatures (MAT) at the study sites. However, the T 47 -MAT offset is negatively correlated to MAT, suggesting that land snails tend to add shell during the warmer months at colder sites or modulate their body temperature differently in colder regions. Meanwhile, clumped temperatures of Cathaica are 3.4 ± 1.5 °C higher than those of Bradybaena at 18 sites, indicating that a species-specific transfer function is needed to reconstruct paleotemperature using land snail clumped isotopes. After determining the proper duration of the growing season for land snails at different locations, we developed a Δ 47 -growth season temperature (GST) transfer function for the two species. The calibration function for Bradybaena land snails is expressed by a linear regression between 1/T2 and absolute Δ 47 (R2 = 0.94): Δ 47 = (0.0513 ± 0.0036) × 106/T2 + (0.0930 ± 0.0413), where Δ 47 is expressed in ‰ and T in K. The calibration function for Cathaica is as follows (R2 = 0.80): Δ 47 = (0.055 ± 0.011) × 106/T2 + (0.035 ± 0.129). The function for Cathaica was successfully applied to reconstruct mean summer (June-July-August) temperatures during the Last Glacial Maximum and modern times on the central Chinese Loess Plateau, based on Δ 47 data of Cathaica sp. provided by Eagle et al (2013a). This testifies to the validity of the aforementioned constructed transfer function. In addition, the calculated δ18O of body water (δ18O BW) for Bradybaena showed a robust correlation with the δ18O of rainfall (δ18O p), particularly in northern China, which points to the potential to trace hydrological changes in the region. In contrast, Cathaica δ18O BW did not show a straightforward relation to δ18O p. This inter-species complexity warrants further study. [ABSTRACT FROM AUTHOR]

Abstract This work investigated the kinetic fractionation of clumped isotopes (13CH 3 D and 12CH 2 D 2) during thethermal generation and hydrogen exchange of methane. Kinetic reaction networks involving the five most abundant isotopologues of methane are applied. Hydrogen isotope exchange is modeled with a chain reaction network constrained by thermodynamic parameters of methane isotopologues. Analytical solutions under isothermal conditions are obtained for isotope fractionations during methane generation. Both methane generation and isotope exchange are possible to result in "anticlumping" at low conversions and clumped isotope reversal at high conversions. Sensitivity of anticlumping to 13C and deuterium kinetic isotope effects (KIEs) are tested for methane generation. D-D clumping is found to be sensitive to the deuterium KIE of the capping hydrogen, especially when the quantum tunneling effect is present. Clumped isotopic compositions of methane in natural gas accumulations are affected by multiple geochemical and geological factors. Precursor methyl groups are expected to be enriched in clumped isotopes, and the enrichment results in clumped isotopic compositions more positive than the equilibrated ones at the beginning of methane generation. In the oil window to early gas window, clumped isotopes deplete with thermal maturity due to combinatorial anticlumping and gas expulsion. These two factors may bring about clumped isotopic compositions more negative than the equilibrated values. In late wet gas to dry gas window (the same range of ethane and propane decomposition), C-H bonds of methane are activated, and isotope exchange occurs through free radical reactions; as a result, clumped isotopes eventually approach equilibrium. Clumped isotopic composition of methane records the chemical, thermal and flow histories of its source rock, rather than merely the temperature state of a gas accumulation. [ABSTRACT FROM AUTHOR]

Abstract The Δ 47 clumped isotope thermometer has been applied to multiple studies aimed at reconstructing the paleoelevation and paleoclimate of sedimentary basins and paleosol sequences. Ideally, this technique directly preserves the temperature of carbonate formation, avoiding any speculation on the composition of surface water from which the carbonate precipitated. Recently, however, concerns about post-depositional alteration of the Δ 47 isotope signature from the effects of burial and/or diagenetic alteration have arisen, potentially complicating the application of the Δ 47 clumped isotope thermometer for determining paleo-surface temperatures. Here we investigate the effect of burial history on mass-47 clumped isotope. To this purpose we collected samples, from the surface and from drill cores, in two different areas of the Greater Green River basin: the Washakie Basin near Rock Springs, Wyoming and the Green River Basin near Pinedale, Wyoming. Both basins are filled with a thick sequence of Eocene lacustrine strata and the thermal histories of both basins are well documented from petroleum prospecting studies. Clumped isotope Δ 47 compositions were measured from lacustrine micritic limestones with a range of peak burial depths from 1 to 6.5 km. For samples from the Washakie Basin that did not experience burial depths exceeding ∼2000 meters, the measured Δ 47 values vary from 0.623‰ to 0.684‰, yielding carbonate formation temperatures consistent with previously hypothesized Paleocene-Eocene surface temperatures. In contrast, samples from the Green River Basin, collected from greater burial depths, demonstrate significantly lower Δ 47 values (higher temperatures). The consistency of δ18O values and lack of CL evidence for recrystallization suggest closed-system resetting of the clumped isotope thermometer by either partial solid-state re-ordering or carbonate reprecipitation during burial. The Δ 47 values of the deeply buried samples have been compared to values derived from recrystallization and temperature history reordering models (THRMs). Both recrystallization and reordering models are successful at predicting Δ 47 reordering at burial temperatures >100 °C. These observations, from samples collected from a basin with a remarkably well-constrained thermal history and geothermal gradient, highlight the challenges in elucidating burial histories or diagenetic processes solely from clumped isotopic compositions. [ABSTRACT FROM AUTHOR]

Abstract Carbonate clumped isotope thermometry is based on the thermodynamically dependent relative abundance of 13C 18O bonds (quantified as Δ 47) within the carbonate crystal lattice. The clumping of 13C 18O in carbonates is based on a self-reaction of isotope exchange that occurs rapidly at near neutral pH. Similar Δ 47 -temperature relationships between biogenic and inorganically precipitated carbonate in initial studies led to the promise of a proxy free of biologically driven disequilibrium effects, commonly referred to as "vital effects". This has been largely the case for most organisms investigated. Biologically mediated disequilibrium precipitation has however been reported in corals and cephalopods and brachiopod molluscs. Echinoderms, despite their complex inter-cellular bio-mineralization strategy, large inter-skeletal fractionation of δ18O, δ13C and rapid calcite precipitation have however not been previously investigated with regards to their clumped isotope composition. We present clumped isotopic composition (Δ 47) of 25 inter-skeletal elements of 5 echinoid species with varying growth temperatures. We found no statistically significant inter-skeletal variation in Δ 47 in all echinoid species measured, a surprising find given the important inter-skeletal variability reported for δ13C and δ18O. Our echinoid Δ 47- temperature calibration however shows a statistically significant positive offset from Δ 47- temperature calibration for inorganic calcite of 0.014‰. The pattern of isotopic fractionation in δ18O and Δ 47 of echinoderms is not consistent with CO 2 hydration or hydroxylation, diffusion or the high-Mg calcite composition of echinoid calcite. Positive offsets in the Δ 47 of echinoid calcite may however relate to deviations in the pH of the calcifying fluid from the pH at which equilibrium calcite is precipitated. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The use of stable oxygen isotopes to understand the mechanisms of dolomite formation has been hampered by the inability to precipitate well-ordered dolomite under normal Earth surface conditions. Several studies have attempted to address this problem, either by precipitating high-temperature ordered dolomites and extrapolating the data to low temperatures or by using more disordered very-high Mg-calcites as a proxy for low temperature dolomites. The result is eight equations that disagree significantly from each other (by as much as ∼3.6‰ in the δ 18 O value of the precipitating fluid at 25 °C), and produce differences which can greatly affect the interpretation of the formation mechanisms for low temperature dolomites. However, by combining the recently developed clumped-isotope paleothermometer, an independent isotopic measurement (∆ 47 ) that directly relates to the temperature of formation, to Late Miocene to Pleistocene aged dolomites from the Bahamas with a well-constrained thermal and fluid history, we have attempted to narrow down the viable equations used to interpret the δ 18 O values of dolomites. The clumped-isotope temperatures measured on the Bahamian dolomites (16–37 °C) agrees with the range of temperatures expected in the Bahamas. Pairing these temperatures with geological and mineralogical arguments, we favor the equation suggested by Matthews and Katz (1977), as it is the only one that produces realistic δ 18 O fluid values across the range of clumped-isotope temperatures. Both the clumped-isotope temperatures and δ 18 O values of the precipitating fluid show a strong positive covariance that we have interpreted as reflecting the mixing of surface brines that have undergone varying amounts of evaporation and normal seawater. The different mechanisms driving these fluids included formation by normal marine seawater driven by the compensatory flow of the mixing zone, bank wide Kohout convection, and evaporative brine reflux. [ABSTRACT FROM AUTHOR]