Molecular hydrogen (H2) is a promising carbon-free energy carrier and a key metabolite in microbial ecosystems. Distinguishing microbially-produced H2 from abiotic H2 could improve our ability to recognize H2 sources. The molecular average deuterium content of H2 has been used previously for identifying its sources and sinks, and recent studies suggest the multiply-substituted (‘clumped’) isotopologue, DD, could provide additional constraints on molecular formation mechanisms and temperatures. However, the clumped isotope composition of biologically produced H2 is yet to be constrained. Here, we measured the molecular average and clumped isotope composition of H2 formed during batch fermentations of Clostridium pasteurianum and examined the effects of growth phase (exponential vs. stationary) and different H2 concentrations on the isotopic compositions of the produced H2. Our results show that microbially-produced H2 expresses a consistent deviation from the equilibrium fractionation during exponential growth, for both the clumped and molecular average (D/H) isotopic compositions, whereas during stationary phase the clumped isotope composition conforms to near-equilibrium isotopic compositions at the culturing temperature. Although lower H2 concentrations during stationary phase led to a consistent deviation from the molecular-average D/H equilibrium fractionation with respect to co-existing water, they did not affect the clumped isotope composition, which remained near equilibrium. We interpreted these results by way of a quantitative isotopic model that describes isotope effects associated with the enzymatic reaction steps of the [Fe-Fe]-hydrogenase, concluding that both the relative rates of the different reaction steps and the reversibility of the rate limiting step control the isotopic composition of the produced H2. Overall, the observed deviations from equilibrium were relatively small in comparison to the large natural variations of hydrogen D/H ratios and clumped anomalies, suggesting that despite the variations, the isotopic composition of microbially-produced H2 is generally expected to approach equilibrium at low formation temperatures. Recognition of non-equilibrium biosignatures in natural settings likely requires comparison of the H2-water D/H fractionation and H2 clumped isotope anomaly.

We apply a recently developed measurement technique for methane (CH4) isotopologues* (isotopic variants of CH4-13CH4, 12CH3D, 13CH3D, and 12CH2D2) to identify contributions to the atmospheric burden from fossil fuel and microbial sources. The aim of this study is to constrain factors that ultimately control the concentration of this potent greenhouse gas on global, regional, and local levels. While predictions of atmospheric methane isotopologues have been modeled, we present direct measurements that point to a different atmospheric methane composition and to a microbial flux with less clumping (greater deficits relative to stochastic) in both 13CH3D and 12CH2D2 than had been previously assigned. These differences make atmospheric isotopologue data sufficiently sensitive to variations in microbial to fossil fuel fluxes to distinguish between emissions scenarios such as those generated by different versions of EDGAR (the Emissions Database for Global Atmospheric Research), even when existing constraints on the atmospheric CH4 concentration profile as well as traditional isotopes are kept constant.

The Archean-Proterozoic transition marks a time of fundamental geologic, biologic, and atmospheric changes to the Earth system, including oxygenation of the atmosphere (termed the Great Oxygenation Event; GOE), and the emergence of continents above sea level. The impacts of the GOE on Earth's surface environment are imprinted on the geologic record, including the disappearance of mass-independent fractionation of sulfur isotopes (S-MIF). Temporally overlapping geologic and geo-chemical observations (e.g. a change in oxygen isotope ratio of sediments and an increase in subaerial volcanism) imply the widespread subaerial emergence of continents was coeval with atmospheric oxygenation. Here we present triple sulfur isotope ratios in pyrite and oxygen isotope ratios in garnet and zircon in a global suite of Archean and Proterozoic granitoids derived from the partial melting of sedimentary protoliths. These crustal melts record an increase in average garnet and zircon delta O-18 from 7.2 parts per thousand before 2.3 Ga to 10.0 parts per thousand post-2.3 Ga. Pre-2.3 Ga granitoids show small S-MIF signatures with Delta S-33 ranging from -0.29 parts per thousand to 0.13 parts per thousand, whereas post-2.3 Ga granitoids record S-MDF (i.e. Delta S-33 = 0 parts per thousand). The combination of sulfur and oxygen isotope signatures in the same sample with zircon U-Pb geochronology provides new insights on a potential causal link between the emergence of continents and Paleoproterozoic atmospheric oxygenation. (C) 2021 Elsevier Ltd. All rights reserved.

We report stable (delta C-13(shell), delta O-18(shell)) and clumped isotope (Delta(47)) 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 Delta(47) temperature (T-47) of modern snails is consistent with monitored temperatures during the snail growing season at the studied locations and is similar to 10 degrees C higher than that of fossil snails from glacial time. Moreover, the average delta C-13(shell) of modern snails is more depleted than that of fossils. We argue that the delta C-13(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 delta O-18 (delta O-18(water)) of modern snails is more enriched than delta O-18 in modern growing season precipitation and delta O-18(water) of fossils. This contrast may be related to the high degree of evaporative enrichment of environmental water O-18 in the body/ingested by modern snails under warm conditions. Therefore, we suggest that using delta O-18(shell) to directly reconstruct the oxygen isotopes of precipitation is difficult and that higher delta O-18(shell) and delta O-18(water) values probably indicate higher environmental temperature/stronger evaporative enrichment on glacial-interglacial timescales on the CLP.

The oxygenation of the Earth’s atmosphere at ~2.35 Ga termed the Great Oxygenation Event (GOE) irreversibly changed major biogeochemical cycles on Earth and provided the base for highly efficient aerobic metabolism that allowed development of complex life. It remains a matter of debate whether a decrease in O 2 consumption or an increase in O 2 production led to the build-up of free oxygen. The drastic impacts of the GOE on the Earth’s surface environment are imprinted on the geologic record, such as the observation of predominantly mass-independent fractionation of sulfur isotopes (S-MIF) pre-GOE to predominantly mass-dependent fractionation post-GOE. Recent studies show that fluctuations in atmospheric O 2 level are also captured in the igneous rock record and the deeper crust, e.g., through a change in average oxygen fugacity of strongly peraluminous granites and recycled S-MIF in igneous rocks. Coevally with the rise of atmospheric O 2 , the triple oxygen isotope composition of shales (expressed as ∆ 17 O) and the 18 O/ 16 O composition of felsic magmas are subject to a rapid change at 2.35 Ga, which has been linked to the widespread emergence of continents above sea-level at that time. In this study we present triple sulfur isotopic signatures of pyrite and oxygen isotopic signatures of garnet and zircon in a global sample set of Archean and Proterozoic

Natural gas is a key energy resource, and understanding how it forms is important for predicting where it forms in economically important volumes. However, the origin of dry thermogenic natural gas is one of the most controversial topics in petroleum geochemistry, with several differing hypotheses proposed, including kinetic processes (such as thermal cleavage, phase partitioning during migration, and demethylation of aromatic rings) and equilibrium processes (such as transition metal catalysis). The dominant paradigm is that it is a product of kinetically controlled cracking of long-chain hydrocarbons. Here we show that C2+n-alkane gases (ethane, propane, butane, and pentane) are initially produced by irreversible cracking chemistry, but, as thermal maturity increases, the isotopic distribution of these species approaches thermodynamic equilibrium, either at the conditions of gas formation or during reservoir storage, becoming indistinguishable from equilibrium in the most thermally mature gases. We also find that the pair of CO2 and C1 (methane) exhibit a separate pattern of mutual isotopic equilibrium (generally at reservoir conditions), suggesting that they form a second, quasi-equilibrated population, separate from the C2 to C5 compounds. This conclusion implies that new approaches should be taken to predicting the compositions of natural gases as functions of time, temperature, and source substrate. Additionally, an isotopically equilibrated state can serve as a reference frame for recognizing many secondary processes that may modify natural gases after their formation, such as biodegradation.

The Archean-Proterozoic transition marks a time of fundamental geologic, biologic, and atmospheric changes to the Earth system, including oxygenation of the atmosphere (termed the Great Oxygenation Event; GOE), and the emergence of continents above sea-level. The impacts of the GOE on Earth’s surface environment are imprinted on the geologic record, including the attenuation of mass-independent fractionation of sulfur isotopes (S-MIF). Temporally overlapping geologic and geochemical observations (e.g. a change in oxygen isotope ratio of sediment melts) imply the widespread subaerial emergence of continents was coeval with atmospheric oxygenation. Here we present triple sulfur isotope ratios in pyrite and oxygen isotope ratios in garnet and zircon in a global suite of Archean and Proterozoic sediment-derived granitoids. These crustal melts record an increase in average 18O/16O isotope ratio and a disappearance of S-MIF in the Paleoproterozoic. The coupled behaviour of sulfur and oxygen isotope signatures imply a potential causal link between the emergence of continents and atmospheric oxygenation at ~2.3 Ga.

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 degrees C, despite burial to greater depths. This finding suggests that, while microorganisms are able to generate methane at temperatures up to 105 degrees C under laboratory conditions (Brock, 1985), in the Gulf of Mexico, microbial methane is dominantly produced in the 20-60 degrees C window. (C) 2020 Elsevier Ltd. All rights reserved.

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 (TM) Orbitrap (TM) mass analyzer, here the Thermo Scientific Q Exactive GC (TM) and Q Exactive HF (TM) instruments. Orbitrap mass analyzers achieve mass resolutions in the range similar to 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 QExactive 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 su-set of molecular sites. (C) 2017 Elsevier B.V. All rights reserved.

The isotopic composition of methane is of longstanding geochemical interest, with important implications for understanding petroleum systems, atmospheric greenhouse gas concentrations, the global carbon cycle, and life in extreme environments. Recent analytical developments focusing on multiply substituted isotopologues ('clumped isotopes') are opening a valuable new window into methane geochemistry. When methane forms in internal isotopic equilibrium, clumped isotopes can provide a direct record of formation temperature, making this property particularly valuable for identifying different methane origins. However, it has also become clear that in certain settings methane clumped isotope measurements record kinetic rather than equilibrium isotope effects. Here we present a substantially expanded dataset of methane clumped isotope analyses, and provide a synthesis of the current interpretive framework for this parameter. In general, clumped isotope measurements indicate plausible formation temperatures for abiotic, thermogenic, and microbial methane in many geological environments, which is encouraging for the further development of this measurement as a geothermometer, and as a tracer for the source of natural gas reservoirs and emissions. We also highlight, however, instances where clumped isotope derived temperatures are higher than expected, and discuss possible factors that could distort equilibrium formation temperature signals. In microbial methane from freshwater ecosystems, in particular, clumped isotope values appear to be controlled by kinetic effects, and may ultimately be useful to study methanogen metabolism. (C) 2017 Elsevier Ltd. All rights reserved.

We present new hydrogen isotope data for separated matrix, hydrated chondrules, and other hydrated coarse silicate fragments from nine carbonaceous chondrites. These data were generated using a micro-analytical method involving stepped combustion of tens to hundreds of micrograms of hydrous solids. We also re-evatuate hydrogen isotope data from previous conventional stepped combustion experiments on these and other carbonaceous chondrites.Hydrogen isotope compositions of matrix and whole-rock samples of CM chondrites are correlated with oxygen isotope indices, major and minor-element abundances, and abundance and isotope ratios of other highly volatile elements. These correlations include a monotonic decrease in deltaD with increasing extent of aqueous alteration and decreasing abundances of highly volatile elements (including C, N and Ar), between extremes of similar to0parts per thousand (least altered, most volatile rich) and -200parts per thousand (most altered, least volatile rich). In plots involving only abundances and/or isotope ratios of highly volatile elements, CI chondrites fall on the high-deltaD, volatile rich end of the trends defined by CM chondrites; i.e., CI chondrites resemble the least altered CM chondrites in these respects. These trends suggest the protoliths of the CM chondrites (i.e., before aqueous alteration) contained an assemblage of volatiles having many things in common with those in the CI chondrites. If so, then the volatile-element inventory of the CI chondrites was a more widespread component of early solar system objects than suggested by the scarcity of recognized CI meteorites. Differences in volatile-element chemistry between the CI and average CM chondrites can be attributed to aqueous alteration of the latter.Previous models of carbonaceous chondrite aqueous alteration have suggested: (1) the protoliths of the CM chondrites are volatile poor objects like the CO or CV chondrites; and (2) the CI chondrites are more altered products of the same process producing the CM chondrites. Both suggestions appear to be inconsistent with hydrogen isotope data and other aspects of the volatile-element geochemistry of these rocks. We present a model for aqueous alteration of the CM chondrites that reconciles these inconsistencies and suggests revised relationships among the major subtypes of carbonaccous chondrites. Our model requires, among other things, that the water infiltrating CM chondrites had a deltaD value of similar to - 158parts per thousand, consistent with initial accretion of CM parent bodies at similar to4 AU. Copyright (C) 2004 Elsevier Ltd.

Iceland contains abundant low δ^(18)O volcanic rocks, reflecting ^(18)O-depleted sources and/or crustal contamination by assimilation and/or mixing with crustal melts. Previous attempts to distinguish among these mechanisms have been based principally on oxygen isotope measurements of whole-rocks, glasses, or mixed populations of phenocrysts. Here we add further constraints from micro-analyses of individual phenocrysts and xenocrysts.