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The Ordovician carbonate reservoirs in the Tarim Basin are hotspots for oil and gas exploration in recent years. However, due to the lack of effective paleo-geothermometers in carbonate formations, thermal history studies have been greatly restricted. In this study, we reconstructed thermal history of Ordovician carbonate intervals in Tarim Basin by applying clumped isotopes, equivalent vitrinite reflectance (R equ) and zircon (U–Th)/He ages as thermal indicators. The modeled results indicated that there are three phases of heat flow evolution in the Shuntuoguole low-uplift. (a) The gradual cooling stage during Ordovician-carboniferous. The heat flow is gradually decreasing during this period. (b) Rapidly raised stage in the early Permian period. The heat flow in Shunbei and Shuntuo area are raised rapidly during this period and the maximum is 51–65 mW/m2, but the SN3 well in Shunnan area increased slightly and the heat flow values of other single wells were still relatively stable. (c) The decline stage since Middle Permian. The tectonic activity is relatively stable and the geothermal heat flow is gradually reduced during this period, but the heat flow in Shunnan area is higher than the Shunbei and Shuntuo areas. Due to the control of thermal evolution, the dissolution of the Ordovician carbonate reservoir in Shuntuoguole area occurred earlier than the formation of large quantities of hydrocarbon materials, and the improvement of reservoir physical properties provided sufficient conditions for the late oil and gas filling, which was conducive to the preservation of large oil and gas reservoirs. [ABSTRACT FROM AUTHOR]

The oxygen isotope ratio 18O/16O (expressed as a δ18OVSMOW value) in marine sedimentary rocks has increased by ~8‰ from the early Paleozoic to modern times. Interpretation of this trend is hindered by ambiguities in the temperature of formation of the carbonate, the δ18Oseawater, and the effects of postdepositional diagenesis. Carbonate clumped isotope measurements, a temperature proxy, offer constraints on this problem. This thermometer is thermodynamically controlled in cases where carbonate achieves an equilibrium internal distribution of isotopes and is independent of the δ18O of the water from which the carbonate grew; therefore, it has a relatively rigorous chemical-physics foundation and can be applied to settings where the δ18O of the water is not known. We apply this technique to an exceptionally well-preserved Ordovician carbonate record from the Baltic Basin and present a framework for interpreting clumped isotope results and for reconstructing past δ18Oseawater. We find that the seawater in the Ordovician had lower δ18Oseawater values than previously estimated, highlighting the need to reassess climate records based on oxygen-isotopes, particularly where interpretations are based on assumptions regarding either the δ18Oseawater or the temperature of deposition or diagenesis. We argue that an increase in δ18Oseawater contributed to the long-term rise in the δ18O of marine sedimentary rocks since the early Paleozoic. This rise might have been driven by a change in the proportion of high- versus low-temperature water-rock interaction in the earth's hydrosphere as a whole. [ABSTRACT FROM AUTHOR]

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]

This study focuses on the origin of dolomite within the Middle Permian Qixia Formation in the northwestern Sichuan Basin by the approaches of petrographic examination and stable isotopic as well as clumped isotopic analysis. Two types of dolomite (matrix dolomite and dolomite cement) in the upper part of the Qixia Formation at the Chejiaba (CJB) section were classified. The gray matrix dolomite shows a clear inter-finger contact with limestone, while the milky white dolomite cement lines the edges of dissolved pores. Microscopic examination revealed three types of matrix dolomite including floating fine crystalline, planar-e(s) matrix dolomite (Md1); fine to medium crystalline, planar-e(s) matrix dolomite (Md2); and medium to coarse crystalline, planar-a matrix dolomite (Md3), along with saddle dolomite cement (Sd). The presence of Md1 dolomites along stylolites suggests they were formed due to pressure dissolution during shallow burial. The similar isotopic signatures of Md2 and Md3 with the host limestone indicate that the dolomitizing fluids likely originated from Permian seawater. However, elevated diagenetic temperatures suggest an influence from the Late Permian volcanic activity. The Sd dolomite and subsequent calcite cementation with negative δ18O values and radioactive 87Sr/86Sr ratios point to formation in high-temperature, externally-derived, radio-genic strontium-rich dolomitizing fluids, possibly from deep-seated thermal fluids within siliciclastic strata. The formation of dolomite in the Qixia Formation of the northwestern Sichuan Basin was influenced by both original sedimentary facies and faulting activities. The influx of hydrothermal fluids triggered widespread dolomitization, resulting in the creation of numerous dissolved and inter-crystalline pores, which played a beneficial role in reservoir development.

Determining the timing of fracturing is crucial for understanding reservoir evolution and hydrocarbon accumulation in foreland basins. Using fracturing data from cores, borehole images, and outcrops, combined with the clumped isotope (γ47) and fluid inclusion analyses of carbonate minerals filled in pores and fractures, this study ascertained the fracturing timing of the Jurassic reservoirs in the Dibei-Tuziluoke Gas Field, Kuqa Foreland Basin. Data from outcrops and borehole images show two dominant fracture sets in the study area: W-E and NE-SW striking fractures. Some W-E striking fractures are carbonate-filled, while NE-SW striking fractures lack mineral fillings. Bitumen veins, not easy to be identified in borehole images, are prevalent in cores. The petrographic analysis reveals that these bitumen veins formed before the calcite cementation in pores and display high viscosity and low maturity. Homogenization temperatures (Th) from primary fluid inclusion assemblages in two representative calcite vein samples were notably lower than TΔ47 values from corresponding samples. This suggests the γ47 signature underwent alteration due to partial reordering during burial. Thus, γ47-derived temperatures (apparent temperatures) may not faithfully represent the mineral precipitation temperatures. When plotting these apparent temperatures vs. the burial history, only the possible latest ages of fracturing emerged. These ages were further refined by considering petroleum charging, tectonic evolution, and stress orientation. Bitumen-filled fractures likely resulted from the Late Cretaceous uplift, marking the migration of low-maturity hydrocarbons in the study area. Carbonate-filled E-W striking fractures emerged during the late Miocene (~13e6.5 Ma) alongside fold development. NE-striking fractures that crosscut W-E ones possibly formed recently due to stress reorientation. [ABSTRACT FROM AUTHOR]

Clumped isotope measurements of carbonates and stable isotope analyses of water trapped in fluid inclusions are both promising techniques to determine carbonate formation temperatures. Cave-hosted carbonate deposits (speleothems) are excellent targets for such studies, but kinetic fractionations and diagenetic influences frequently deteriorate the temperature data obtained from these methods. However, subaqueous carbonate deposits may provide reliable data, as kinetic fractionations are less significant in underwater environments. In this study, subaqueous speleothems, whose formation temperatures were directly measured in the water, were investigated. Additionally, temperatures calculated from the oxygen isotope fractionations between calcite and fluid inclusion-hosted water were compared with clumped isotope temperatures obtained for subaqueous carbonate formations in cave-hosted lakes. The clumped isotope temperatures fit the measured and calculated fluid inclusion temperatures within the analytical precisions. Carbonate deposits formed at elevated temperatures (~50°C or above) may undergo post-formational calcite-water oxygen isotope exchange, altering the composition of the inclusion-hosted water. In contrast, subaqueous speleothems formed at about 20-25°C appear to preserve the primary isotopic compositions. Our study shows that subaqueous carbonate speleothems are useful targets for clumped isotope and inclusion water analyses, making them valuable paleotemperature archives. [ABSTRACT FROM AUTHOR]

A few key methodological uncertainties remain for the carbonate clumped isotope community. One is how to compare data among published data sets that are not anchored to the InterCarb Carbon Dioxide Equilibrium Scale (I‐CDES). A second is how temperature calibrations of calcite compare to those of other carbonate minerals in the I‐CDES—particularly dolomite and apatite—which can elucidate several Earth system dynamics. Previous calibrations of the clumped isotope thermometer for dolomite are discrepant from one another and variably (dis)agree with calibrations developed for calcite; apatite calibrations have not yet been compared between laboratories using carbonate‐based standardization. Here we report I‐CDES standardized values for a suite of 11 carbonates that are commonly measured by the clumped isotope community to aid future comparisons of non‐I‐CDES data sets. In addition, 17 dolomite samples (25–1,200°C) and five apatite samples (1–38°C) of known precipitation temperature were measured using carbonate‐based standardization. Excellent agreement between calcites and dolomites heated to similar temperatures (1,100–1,200°C) suggests no mineral‐specific differences in absolute acid fractionation factor. We show that calcite and dolomite regressions largely agree but are sensitive to sample characteristics, regression method, and how equations are statistically compared. We suggest that there is no need for a dolomite‐specific clumped isotope calibration, although our results suggest that further work is necessary to determine the influence of sample characteristics on this relationship. The apatite calibration equation defined in this study is statistically indistinguishable from calcite‐based calibrations; we corroborate previous findings that an apatite‐specific calibration is unnecessary. Plain Language Summary: The clumped isotope composition of carbonate rocks can teach geoscientists about the temperature and isotopic composition of ancient water masses. To be confident in measurements of clumped isotopes between laboratories, measurements should be compared to a common set of carbonate standards. Calculations of temperature based on clumped isotopes require empirical calibrations of carbonates with known formation temperatures. It is still uncertain if all carbonate minerals can use the same calibration. Here, we produce clumped isotope thermometer calibrations for the carbonate minerals dolomite and apatite. We show that these calibrations are statistically similar to calibrations for calcite in most cases and that dolomite‐ and apatite‐specific clumped isotope calibrations are not necessary. We also extend the number and mineralogy of carbonate standards that have been assigned standardized values to facilitate easier data comparison between laboratories. Key Points: Non‐InterCarb carbonate standard values are presented and can be used to project data into the InterCarb Carbon Dioxide Equilibrium ScaleWe observe no dolomite‐specific absolute acid fractionation factorWe suggest that dolomite‐ and apatite‐specific clumped isotope calibrations are unnecessary [ABSTRACT FROM AUTHOR]

Carbonate clumped isotope, quantified by Δ47 value, has wide application potential in Earth science. A recently proposed carbonate standardisation scheme based on community‐available reference materials (i.e., ETH‐1–4) has proven to improve interlaboratory comparability in Δ47 measurement. However, the shortage of these reference materials may be problematic in future; therefore, developing traceable and supplementary in‐house reference materials may become necessary, especially when supplies run out. Here, we describe a method of producing synthetic witherite carbonates with variable bulk isotopic compositions and clumped isotope ordering states, for which Δ47 values were determined together with ETH carbonate reference materials. Results indicate that the witherite carbonates allow effective corrections for instrumental linearity and scale variations, improving long‐term reproducibility. Additionally, by normalising witherite Δ47 values to agreed ETH carbonate values on the Intercarb‐Carbon Dioxide Equilibrium Scale, the witherite reference materials can be used to determine Δ47 values without using equilibrated gases and reduce the consumption of ETH carbonates. [ABSTRACT FROM AUTHOR]

The study of thermal history of sedimentary basins is very important for oil-gas accumulation analysis and has decisive influence on oil and gas exploration and oil-gas resource evaluation. However, it is difficult to reconstruct the thermal history of carbonate strata in marine basins due to the lack of effective palaeo-thermometer. The carbonate cluster isotope is an index that can indicate the formation temperature of carbonate rock. It records temperature information in chemical bond through conjugated 13C-18O, and temperature information is revealed through changes in the abundance of conjugated 13C-18O, namely, the abundance of carbonate clumped isotope (Δ47) is closely related to the evolution of strata temperature. This paper reviews the application of carbonate clumped isotope palaeo-thermometer and discusses the existing problems and future development direction based on the basic principle and methods of carbonate clumped isotope thermometry and thermal history reconstruction, and combined with the case studies of thermal history reconstruction in marine basins in recent years. As a new palaeo-thermometer, carbonate clumped isotope shows great potential in the thermal history reconstruction of carbonate strata. Previous studies show that the maximum paleotemperature of the Bird Spring Formation in Arrow Canyon, Nevada, USA is about 175 ℃, and the maximum burial temperature of Permian Maokou Formation in the Sichuan Basin is 213-225 ℃. There is an abnormal high temperature of above 160 ℃ in the Ordovician strata of the Tarim Basin due to the Permian magmatic activity. We believe that the carbonate clumped isotope is still insufficient in the reconstruction of thermal history due to the immaturity of thermal evolution models, the recrystallization, and the change of burial temperature. Thus, we suggest that combining the carbonate clumped isotope, U-Pb dating, and other palaeo-thermometers to accurately reconstruct the thermal history of carbonate strata will be the future development direction.

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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 (13CH2D) and D–D (12CHD2) 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–CH3) groups as CH3F and CH3Cl, 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 CH3Cl, then placing our measurements onto this reference frame through experimental internal isotopic equilibration of CH3Cl 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.

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 (13CH2D) and D–D (12CHD2) 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–CH3) groups as CH3F and CH3Cl, 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 CH3Cl, then placing our measurements onto this reference frame through experimental internal isotopic equilibration of CH3Cl 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.

Acid extraction methods have been used in the last half century to selectively extract the CO2 produced from different carbonate minerals in mixed samples. However, these methods are often time-consuming and labor intensive. Their application to clumped isotope (Δ47) analysis has not been demonstrated. We propose here an acid extraction method with phosphoric acid for bulk stable and clumped isotope analysis that treats mixtures of calcite and dolomite the same regardless of the proportional composition. CO2 evolved from calcite is extracted by allowing a reaction with phosphoric acid to proceed for 10 min at 50 °C. We then extract CO2 evolved from dolomite by rapid ramping the acid temperature from 50 to 90 °C and allowing the reaction to complete. The experimental results show that our method yields accurate calcite and dolomite Δ47 values from mixed samples under different proportional compositions. Our method also displays equal or higher accuracy for calcite δ13C and dolomite δ13C and δ18O values from mixtures when compared to previous studies. Our approach exhibits higher sample throughput than previous methods, is adequate for clumped isotopic analysis and simplifies the reaction progression from over 24 h to less than 2 h, while maintaining relatively high isotopic obtaining accuracy. It yet poorly resolves calcite δ18O values, as found with previous methods. [ABSTRACT FROM AUTHOR]

In parallel with pronounced cooling in the oceans, vast areas of the continents experienced enhanced aridification and restructuring of vegetation and animal communities during the Late Miocene. Debate continues over whether pCO2-induced global cooling was the primary driver of this climate and ecosystem upheaval on land. Here we present an 8 to 5 Ma land surface temperatures (LST) record from East Asia derived from paleosol carbonate clumped isotopes and integrated with climate model simulations. The LST cooled by ~7 °C between 7.5 and 5.7 Ma, followed by rapid warming across the Miocene–Pliocene transition (5.5 to 5 Ma). These changes occurred synchronously with variations in alkenone and Mg/Ca-based sea surface temperatures and with hydroclimate and ecosystem shifts in East Asia, highlighting a global climate forcing mechanism. Our modeling experiments additionally demonstrate that pCO2-forced cooling would have altered moisture transfer and pathways and driven extensive aridification in East Asia. We, thus, conclude that the East Asian hydroclimate and ecosystem shift was primarily controlled by pCO2-forced global cooling between 8 and 5 Ma. [ABSTRACT FROM AUTHOR]

The higher abundance of dolomite in the Neoproterozoic compared with that of the Phanerozoic has been linked to elevated marine Mg/Ca ratios, and a hypothesized 'dolomite sea' that promoted widespread syn‐sedimentary fibrous dolomite cement precipitation. This article provides new uranium–lead (U–Pb) chronology, clumped isotope thermometry (TΔ47), combined with δ13C and δ18O, and trace element geochemistry measurements for different types of dolomite, including bulk dolomite matrix (BD), fibrous dolomite cements (DC1) and late sparry dolomite cements (DC2 and DC3), in three Neoproterozoic dolostone profiles from China. The Ediacaran U–Pb ages coupled with nearly identical, low TΔ47 temperatures (average at ca 55‐60°C) for BD and DC1 dolomites, imply that these dolomites were either syn‐depositional or early diagenetic. Some DC1 dolomites yield slightly younger (ca 20 to 50 Ma) U–Pb ages than the BD dolomites, suggesting that they were diagenetic products formed after matrix dolomitization. The estimates of δ18Owater values for dolomitizing fluids suggest that both DC1 and BD dolomites were precipitated from highly evaporated seawater. In contrast, DC2 and DC3 dolomites filling in vugs and fractures have much younger U–Pb ages (mostly <525 Ma), higher TΔ47 temperatures (>95°C), and lower Sr and U contents, which is consistent with the paragenetic sequence that they were deeper burial diagenetic products. A synthesized diagenetic model with evaporative dolomitizing water evolved from anoxic, ferruginous to sub‐oxic and/or oxic, non‐ferruginous conditions is responsible for the decrease of Fe and Mn concentrations in global DC1 and BD dolomites throughout Neoproterozoic time. This study suggests that the widely distributed Neoproterozoic fibrous dolomite cements may be a later diagenetic feature, and thus calls into question their use in palaeoclimate reconstruction and as a principal line of evidence for the Neoproterozoic 'dolomite sea' hypothesis. [ABSTRACT FROM AUTHOR]

• The Lunpola Basin remained at relatively low elevations between 40 and 20 Ma. • An abrupt surface uplift of 1.4 ± 0.8 km occurred at 20–19 Ma. • Additional elevation increases of 1.0 ± 0.7 km between 16 Ma and the present. • Subsurface geodynamic processes contributed to the Neogene surface uplift. • Including mainly convective removal and middle-lower crustal flow. Orogenic plateaus are known for their high and flat interiors, but the mechanism by which the high-elevation, low-relief topography is formed remains controversial. The Tibetan Plateau, which may have had a broad central valley along the Bangong suture zone during the Eocene, is a valuable research target to test potential driving mechanisms. In this study, we report both stable and clumped isotopes of lacustrine carbonates of the Lunpola Basin to better constrain the paleoelevation history of the Bangong suture zone in Central Tibet. Covariations of the stable oxygen and carbon isotopes indicate increasing evaporative enrichment of lake water reflecting enhanced aridity from the middle Eocene to early Miocene. After removing altered samples using multiple diagenesis screening methods and the potential influence of global cooling, our clumped isotope temperatures indicate consistent paleoelevations of 2.2 ± 1.1 km at 40–20 Ma, followed by an abrupt 1.4 ± 0.8 km surface uplift at 20–19 Ma and an additional 1.0 ± 0.7 km surface uplift between 16 Ma and the present to achieve the current high elevation of ∼4.6 km. These new paleoelevation results suggest that the Neogene was the primary period of topographic growth for the Lunpola Basin, which contradicts previous inferences emphasizing Paleogene growth. We argue that the Bangong suture zone in Central Tibet was probably a broad low-elevation valley throughout the Paleogene and uplifted mainly during the Neogene by multiple subsurface geodynamic processes, including convective removal of the lower lithosphere and middle–lower crustal flow. A comparison with the north-central Tibet Hoh Xil Basin and South American Altiplano indicates that these subsurface geodynamic processes may be common for uplifting and flattening orogenic plateaus. [ABSTRACT FROM AUTHOR]

The terrestrial Lower Cretaceous Cedar Mountain Formation, Utah, is a critical archive of paleoclimate, tectonics, and vertebrate ecology and evolution. Early Cretaceous carbon cycle perturbations associated with ocean anoxia have been interpreted from this succession, as expressed in stable carbon isotopes. However, refining the timing of the observed stable isotope excursions remains a key challenge in understanding how marine anoxia affects the Earth system, and is ultimately recorded in the terrestrial realm. The geochronology and geochemistry of a terrestrial carbonate near the base of this succession, which potentially records the Ap7 global carbon isotope excursion, is studied here. Petrographic and geochemical analyses are used to test plausible mechanisms for U incorporation into the calcite lattice in this sample. Using these methods, the hypothesis that the incorporation of U was at or close to the timing of carbonate precipitation is evaluated. U–Pb geochronology of calcite indicates a plausible Early Cretaceous age. However, comparison of the new U–Pb ages of calcite with detrital zircon maximum depositional ages immediately beneath the studied sample indicates a disparity in the apparent sedimentation rates if both types of geochronologic information are interpreted as reflecting the timing of sediment deposition. The totality of data supports an early, and high-temperature, diagenetic timing of U incorporation, with potential for minor leaching of U in subsequent fluid–rock interaction. The most likely mechanism for U transport and immobilization in these samples is hydrothermal fluid–rock interaction. Therefore, the radiometric ages, and corresponding stable isotope composition of U-bearing carbonate domains in this sample, indicate early subsurface fluid–rock interactions and not a record of atmosphere–soil geochemical reactions. [ABSTRACT FROM AUTHOR]

With more natural gas hydrate samples recovered and more research approaches applied to hydrate-associated gas studies, data concerning the geochemical characteristics of hydrate-associated gases have been increased significantly in the past decades. Although systematic reviews of hydrocarbons are available, fewer studies have focused on the systematic classification of gas hydrates, yet. In this study, the primary origins and secondary processes that affect the geochemical characteristics of the gases are discussed. The primary origins are affected mainly by the type and /or maturity of the organic matter, which determine the main signature of the gas is microbial gas or thermogenic gas in a broad scheme. Apart from primary origins, secondary processes after gas generation such as migration, mixing, biodegradation and oxidation occur during the migration and/or storage of gases can significantly alter their primary features. Traditional methods such as stable isotope and molecular ratios are basic proxies, which have been widely adopted to identify these primary origins and secondary processes. Isotopic compositions of C2+ gases have been employed to identify the precursor of the gases or source rocks in recent years. Data from novel techniques such as methane clumped isotope and noble gases bring additional insights into the gas origins and sources by providing information about the formation temperature of methane or proxies of mantle contribution. A combination of these multiple geochemical approaches can help to elucidate an accurate delineation of the generation and accumulation processes of gases in a gas hydrate reservoir.

Abstract Ice cores and other paleotemperature proxies, together with general circulation models, have provided information on past surface temperatures and the atmosphere's composition in different climates. Little is known, however, about past temperatures at high altitudes, which play a crucial role in Earth's radiative energy budget. Paleoclimate records at high‐altitude sites are sparse, and the few that are available show poor agreement with climate model predictions. These disagreements could be due to insufficient spatial coverage, spatiotemporal biases, or model physics; new records that can mitigate or avoid these uncertainties are needed. Here, we constrain the change in upper‐tropospheric temperature at the global scale during the Last Glacial Maximum (LGM) using the clumped‐isotope composition of molecular oxygen trapped in polar ice cores. Aided by global three‐dimensional chemical transport modeling, we exploit the intrinsic temperature sensitivity of the clumped‐isotope composition of atmospheric oxygen to infer that the upper troposphere (effective mean altitude 10–11 km) was 6–9°C cooler during the LGM than during the late preindustrial Holocene. A complementary energy balance approach supports a minor or negligible steepening of atmospheric lapse rates during the LGM, which is consistent with a range of climate model simulations. Proxy‐model disagreements with other high‐altitude records may stem from inaccuracies in regional hydroclimate simulation, possibly related to land‐atmosphere feedbacks.

Position-specific and clumped isotope compositions that reveal intramolecular isotope distributions can offer novel insights into the physical and chemical properties of substances. In particular, the intramolecular isotope effects observed in propane have demonstrated significant potential for constraining the formation and evolution of hydrocarbons. To calibrate measurements and interpret observations, a comprehensive understanding of equilibrium isotope fractionation within propane is required. However, previous studies yielded inconsistent calculations of position-specific isotope equilibria, and lacked any prediction of clumped isotope equilibria in the methyl group of propane. Here, we employ quantum chemical calculations beyond the harmonic approximation and the Born-Oppenheimer approximation to study the intramolecular isotope equilibria in propane. Benchmark coupled cluster calculations (CCSD(T)) were used to produce reliable frequencies of propane isotopologues. By integrating the CCSD(T) outcomes with effects beyond the harmonic and Born-Oppenheimer approximations, we present high-accuracy equilibrium carbon and hydrogen isotope fractionation between the methylene and methyl sites, and report for the first time the equilibrium Δ13CH 2 D and Δ12CHD 2 signatures of the methyl group in propane. Our results of position-specific isotope equilibria can serve as a reference frame for calibrating both theoretical calculations and experimental measurements. The equilibrium Δ13CH 2 D and Δ12CHD 2 values provide a theoretical framework for further studies of 13C D and D D clumping in methyl groups of alkanes, and are valuable for tracing the sources and sinks of methyl groups as well as processes related to methylation. • High-accuracy determination of molecular constants for propane isotopologues. • Prediction of intramolecular isotope equilibria using the most accurate theoretical treatments to date. • Evaluation of higher-order energy terms such as anharmonicity, internal rotation, and diagonal Born-Oppenheimer correction. [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]

The carbon, oxygen and clumped isotope compositions of carbonates are typically analyzed by carbonate-phosphoric acid digestion. Although this approach has been widely used since the 1950s, this procedure has problems in the operability. To address these issues, an improved apparatus with properly sized accessories, including an I-shaped vessel, a sample boat, a magnet, and a magnetic stir bar, has been developed for sample digestion. Similar to a long neck flask, this apparatus has a straight and axisymmetric structure, providing advantages in terms of cost and operability, and thus can be easily implemented in both open and sealed vessel digestion. To evaluate the reliability of the stable and clumped isotope compositions using this new vessel, international and interlaboratory carbonate standards were analyzed with this new apparatus and two commonly used Y-shaped and T-shaped vessels. The results using the new apparatus were consistent with those obtained by the others and in previous studies, suggesting suitability for the traditioinal phosphoric acid digestion of carbonates. [ABSTRACT FROM AUTHOR]

Carbonate clumped isotope thermometry is based on the ordering of 13C and 18O in the carbonate lattice and is based on the relative abundance of 13C18O16O in CO2 produced through acid digestion of carbonate minerals. The major advantage of this technique is its non‐dependency on the δ18O value of water from which the carbonate precipitated. Ghosh et al. (2007, https://doi.org/10.1016/j.gca.2007.03.015) previously published calibration data for fish otoliths referenced to heated gases and used the Gonfiantini 17O parameter set in their data evaluation. Herein, we present a new clumped isotope (Δ47) calibration for aragonitic fish otoliths in the absolute reference frame using the International Union of Pure and Applied Chemistry 17O correction. Our revised calibration equation for otolith is: Δ47CDES=0.0364±0.005×(106)T2+0.2619±0.0657(R2=0.9,p−value<0.001). To test the accuracy of this calibration, we apply it to otoliths of modern Lutjanus lutjanus from the Bay of Bengal. The estimated average temperature (22.3°C ± 4.2°C) for the Bay of Bengal and δ18OV‐SMOW composition of waters of −1.7‰ (±0.5) are consistent with the onsite observations. We also apply the new calibration to well‐preserved otoliths of "genus Ambassidarum" sp. and "genus Gobiidarum" sp. from lower Miocene (Burdigalian) sediments of the Quilon Formation, India to quantify coastal water conditions. Estimated average environmental water temperatures in their habitats were 12.9°C ± 1.7°C, and the average δ18OV‐SMOW of ambient waters calculated yielded a value between −3.5‰ and −2.6‰ (V‐SMOW) (mean: −2.9‰ ± 0.4) and −4.4‰, respectively. These results indicate δ18O values reflect the kinetic effects impacting the δ18O of fish otoliths independently of Δ47, although we cannot fully preclude diagenesis. Key Points: A revised clumped isotope calibration equation for otoliths was established based on the International Union of Pure and Applied Chemistry parameter setThe new calibration equation is validated by quantifying the environmental water δ18O of modern watersNew calibration equation allows for the reconstruction of lower Miocene coastal conditions in southwest India utilizing fossil otoliths [ABSTRACT FROM AUTHOR]

Abstract Carbonate clumped isotope thermometry is based on the ordering of 13C and 18O in the carbonate lattice and is based on the relative abundance of 13C18O16O in CO2 produced through acid digestion of carbonate minerals. The major advantage of this technique is its non‐dependency on the δ18O value of water from which the carbonate precipitated. Ghosh et al. (2007, https://doi.org/10.1016/j.gca.2007.03.015) previously published calibration data for fish otoliths referenced to heated gases and used the Gonfiantini 17O parameter set in their data evaluation. Herein, we present a new clumped isotope (Δ47) calibration for aragonitic fish otoliths in the absolute reference frame using the International Union of Pure and Applied Chemistry 17O correction. Our revised calibration equation for otolith is: Δ47CDES=0.0364±0.005×(106)T2+0.2619±0.0657(R2=0.9,p−value

The terrestrial Lower Cretaceous Cedar Mountain Formation, Utah, is a critical archive of paleoclimate, tectonics, and vertebrate ecology and evolution. Early Cretaceous carbon cycle perturbations associated with ocean anoxia have been interpreted from this succession, as expressed in stable carbon isotopes. However, refining the timing of the observed stable isotope excursions remains a key challenge in understanding how marine anoxia affects the Earth system, and is ultimately recorded in the terrestrial realm. The geochronology and geochemistry of a terrestrial carbonate near the base of this succession, which potentially records the Ap7 global carbon isotope excursion, is studied here. Petrographic and geochemical analyses are used to test plausible mechanisms for U incorporation into the calcite lattice in this sample. Using these methods, the hypothesis that the incorporation of U was at or close to the timing of carbonate precipitation is evaluated. U–Pb geochronology of calcite indicates a plausible Early Cretaceous age. However, comparison of the new U–Pb ages of calcite with detrital zircon maximum depositional ages immediately beneath the studied sample indicates a disparity in the apparent sedimentation rates if both types of geochronologic information are interpreted as reflecting the timing of sediment deposition. The totality of data supports an early, and high-temperature, diagenetic timing of U incorporation, with potential for minor leaching of U in subsequent fluid–rock interaction. The most likely mechanism for U transport and immobilization in these samples is hydrothermal fluid–rock interaction. Therefore, the radiometric ages, and corresponding stable isotope composition of U-bearing carbonate domains in this sample, indicate early subsurface fluid–rock interactions and not a record of atmosphere–soil geochemical reactions.

Carbon and oxygen isotopic exchange between external and internal fluids of carbonate minerals can reset the clumped isotope composition (Δ 47) of paleotemperature archives. Understanding the nature of this exchange during carbonate diagenetic alterations is essential for correctly interpreting the paleoenvironmental significance of Δ 47 -derived temperatures. Through batch hydrothermal experiments on aragonitic coral carbonates, we examine the kinetic signatures of clumped isotope fractionation from the initial disequilibrium to final equilibrium states during the dolomitization of the aragonite. By analyzing oxygen isotope fractionation and Sr partitioning between carbonate and fluid, we simulate the effect of exchange reactions on the Δ 47 values. As the resetting feature of Δ 47 values in our aragonite samples disagrees with the fractionation pattern observed in previous aragonite solid-state reordering experiment, we rule out the effect of the solid-state reordering. Our findings demonstrate that isotope exchanges with internal or external fluids coexist, particularly during the early stage of aragonite self-recrystallization. The results of this study indicate that the Δ 47 value of coral aragonite is susceptible to internal exchange reactions in Mg-bearing solutions at 160–220 °C. Additionally, by modeling the bulk recrystallization and internal fluid-mineral exchange and evaluating their relative importance, we determine that the internal exchange reaction results in a nonlinear correlation between δ18O and Δ 47 values. Importantly, we highlight the previously unrecognized diagenetic scenario in which the internal fluid-mineral exchange effect on the Δ 47 value can outweigh the bulk recrystallization effect with external fluid coexisting. Consequently, our findings emphasize the need for a comprehensive assessment on the reaction kinetics for different types of recrystallization effects, to support the application of clumped isotope proxies for altered carbonate samples in diagenetic environments. [ABSTRACT FROM AUTHOR]

Global sea level fell progressively during the Pliocene in response to the growth of ice sheets at high northern latitudes. However, not until ∼2.7 Ma did the climate system actually generate and maintain major ice sheets in the Northern Hemisphere. A decline in atmospheric CO 2 levels (p CO 2) is thought to be responsible for the Pleistocene ice ages. However, the link between p CO 2 and Pliocene glaciations, or their delay, is not well understood, mainly because the magnitude of p CO 2 remains uncertain beyond the maximum age of ice cores (∼0.8 Ma). Here, we develop a carbonate clumped isotope (∆ 47) geothermometer specifically for pedogenic carbonate in Chinese loess, which we use to reconstruct temperatures in northern China, and p CO 2 , for the past 7.24 Ma. The palaeotemperature results show an oscillatory cooling trend over time in the East Asian monsoon region, in good agreement with coeval tropical sea surface temperature records. The p CO 2 reconstruction using the pedogenic carbonate palaeobarometer reveals low values (150–325 ppmv) during the intervals of 7.24–4.3 Ma and 2.6–0 Ma. In contrast, there is a three-step rise in p CO 2 up to 535 ppmv from 4.3 to 2.6 Ma, coincident with an oscillatory decrease in sea level and in benthic δ13C. We suggest that the Pliocene CO 2 rise was causally related to the sea-level fall and associated organic carbon oxidation on exposed continental shelves, thereby delaying the onset of the ice age. This implies a negative feedback mechanism induced by long-term cooling since the Late Miocene, which may have delayed the onset of the ice age but did not prevent it. • A carbonate clumped isotope (∆ 47) geothermometer was developed for Chinese loess. • Palaeotemperature reconstruction shows an oscillatory cooling trend since 7.24 Ma. • Low p CO 2 during 7.24–4.3 and 2.6–0 Ma estimated from calcic paleosols. • Sea-level falls during 4.3–2.6 Ma led to CO 2 release from continental shelves. • Cooling-induced p CO 2 rise as a negative feedback mechanism for the delayed ice age. [ABSTRACT FROM AUTHOR]

The lack of effective paleogeothermometers restricts the study of the thermal history of carbonate strata. Carbonate clumped isotope thermometry and in-situ U–Pb dating are appropriate for reconstructing the thermal history of carbonate strata. Herein, this study analyzed clumped isotope compositions and U–Pb ages of calcite fabrics, including micrite matrix and veins, from Permian carbonate strata in the eastern Sichuan Basin. U–Pb dating of the micrite matrix was performed to rule out the influence of recrystallization and determine the diagenetic age of the samples. Clumped isotope temperatures of the four micrite matrix samples ranged from 102 °C to 137 °C. The maximum paleotemperature derived from a solid-state reordering model was between 210 °C and 220 °C. From all four samples, three calcite veins yielded crystallization temperatures corresponding to 160 ± 10 °C, 60 ± 10 °C, and 29 ± 4 °C, as determined by clumped isotope and solid-state reordering model analyses. In-situ U–Pb dating provided corresponding crystallization ages of 159 ± 17 Ma, 256.6 ± 5.5 Ma, and 17.86 ± 0.8 Ma. The crystallization ages and temperatures of the three calcite veins align well with the thermal history of the Permian strata, indicating a high degree of accuracy. This study introduces a new strategy for reconstructing the thermal history of carbonate strata. The thermal history can be quantified using the constraints of the crystallization ages and temperatures from different stages of calcite veins or cements. This approach is suitable for determining the maturation progress of the source rock and conducting oil and gas exploration in petroliferous basins. • Permian strata's maximum paleotemperature simulated by clumped isotope is 210–220 °C. • Solid-state reordering model can get the calcite vein's crystallization temperature. • Calcite veins are essential to reconstructing the thermal history of carbonate strata. [ABSTRACT FROM AUTHOR]

• Intramolecular isotopes of 40 propane and 5 methane samples were analyzed. • Different intramolecular C isotopes of propane from coal and coal mudstone. • Extreme 13C depletion in central C of over mature propane at cooling setting. • Different effect on intramolecular isotopes by various post-generation processes. Evaluating the origin and fate of hydrocarbons holds significance in many fields, such as energy, geology, astrobiology, biosphere, and environment. However, challenges arise in many cases owing to the limitations of conventional methods. Intramolecular isotope analysis of propane is a new technique that offers the potential to provide insights into gas formation mechanisms; however, a comprehensive understanding of the method remains limited. Specifically, there is little knowledge about its ability to distinguish gases generated by source rocks deposited in similar sedimentary environments such as coal and coaly mudstone. Therefore, this study was undertaken to investigate position-specific carbon isotopes of 40 propane samples from 15 coal-type gas accumulations across four basins in China (Sichuan, Ordos, Qaidam, and Songliao); additionally, clumped isotopologues of methane samples were also analyzed. These coal systems cover a wide range of thermal maturity (from marginally mature to over-mature) at various age strata from the Permian, Triassic, and Jurassic periods, to the Cretaceous period. Our results revealed that propane generated from coal and coaly mudstone differed greatly in intramolecular isotope compositions despite having similar bulk δ13C 3 values. Propane generated from coaly mudstone at a wide range of maturity had relatively more stable δ13C values in both the central (δ13C cental) and terminal carbons (δ13C terminal), near the theory generation line of chain-alkane cracking in the plot of δ13C terminal vs. δ13C cental. However, propane generated from coal had higher stable δ13C terminal values and lower δ13C central values that progressively increased with maturity. Under cooling-down conditions, propane from over-mature natural gases became extremely 13C-depleted in both the terminal and central positions, with Δ13C central (δ13C central minus δ13C terminal) values being as low as –10‰, suggesting a partial origin of methane polymerization. Two propane samples from Cretaceous brown sandstone in the Songliao Basin exhibited an increase in both central and terminal carbons, suggesting that they may have surpassed chemical oxidation owing to high-valence Fe(Mn) oxides. These results indicate that intramolecular isotopes of propane can effectively differentiate dominant gas sources in a coal system with coal or coaly mudstone, as well as easily identify isotopic fractionation caused by post-generation processes, such as methane polymerization, mixing and chemical oxidation. [ABSTRACT FROM AUTHOR]

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. Key Points: Δ47, U‐Pb dating and solid‐state reordering model as an integrated thermochronological tool suitable for deeply buried carbonate intervalsIdentified a previously ignored thermal pulse induced by mantle plume activity in the Tarim BasinUsing natural samples to constrain the sensitivity of solid‐state reordering models to kinetic parameters [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 mid Miocene represents an important target for paleoclimatic study because the atmospheric CO2 concentration ranged from near modern values to ∼800 ppm, while a large, dynamic Antarctic ice sheet was likely to have been present throughout much of this interval. In this special issue, Modestou et al. (2020) (doi.org/10.1029/2020PA003927) reconstruct deep ocean warmth based on the clumped isotopic composition of benthic foraminifera, a technique that allows the ice volume and thermal components of the benthic oxygen isotope stack to be separated. These data reveal a very warm deep ocean while simultaneously suggesting that continental ice volume may, at times, have been greater than today. Here, I review these results in the context of recent developments in geochemical proxies and ice sheet modeling, and explore how the presence of a large Miocene ice sheet could be reconciled with CO2 at least as high as present. More broadly, I argue that many of the 'paradoxes' that pepper the paleoclimate literature result as much from our imperfect understanding of the proxies, as from our understanding of the climate system. Robust proxies with a well‐understood mechanistic basis, as employed by Modestou et al. (2020), as well as advances in model‐data comparability usher in a new era of palaeoclimate research; an exciting future of untangling Earth's myriad past climate states awaits. Plain Language Summary: Reconstructing climate variation in Earth's geologic past informs us of the broad features of warm climates, which is relevant to preparing for climate change over the coming centuries. Moreover, these data can be compared to state‐of‐the‐art climate models, which provides a test of the degree to which our models can reproduce warm climate states. A paper recently published in this journal applies a new method in order to reconstruct the temperature of the deep ocean in the middle Miocene (between 17 and 12 million years ago), when the atmospheric CO2 concentration was naturally similar to or higher than it is today. Coupled with decades of previous study, these exciting results depict an unfamiliar world characterized by a warm deep ocean, and yet a large ice sheet was present on Antarctica. Both models and data agree that the Antarctic ice sheet in the Miocene was highly responsive to changes in the atmospheric CO2 concentration, a clear cause of concern in the context of ongoing anthropogenic climate change. Key Points: The importance of a paper by Modestou et al. (2020) is explained, which reports middle Miocene benthic foraminifera clumped isotope dataWays in which a large Antarctic ice sheet can be reconciled with a warm deep ocean and moderately high CO2 are exploredMore broadly, I argue that recent advances in proxy methodology are resulting in ever increasing confidence in paleoclimate reconstructions [ABSTRACT FROM AUTHOR]

Carbonate clumped isotope (Δ47) is a new and reliable geothermometer. Endogenic carbonates in lake sediments are good archives to reconstruct lake water temperature using clumped isotope thermometry. However, applications of carbonate clumped isotope thermometry to lacustrine carbonates are still scarce because the existed organic and/or sulfur contaminants in such samples interfere with clumped isotope analysis and cause notable temperature biases. Therefore, exploring an effective way to remove contamination is a prerequisite to widely applying carbonate clumped isotope thermometry to lake sediments. By pretreating fine-grained endogenic carbonates from Dali lake sediments with hydrogen peroxide of different concentrations for different lengths of time, we conducted a series of conditional experiments to seek the optimal pretreatment condition for clumped isotope analysis. Δ47, Δ48 offset and 49 parameter were obtained from clumped isotope measurements to assess the effect of contamination removal. Results showed that untreated samples ("0%–0") had a very high 49 parameter. After the treatment with hydrogen peroxide, the 49 parameter was significantly reduced while the Δ48 offset evidently increased. The Δ47, Δ48 offset and 49 parameter of samples pretreated under different conditions showed significant variability, even though the measured Δ47 temperatures changed within a relatively narrow range (i.e., from 17.0±1.3 to 22.8±1.6°C). Among various treated samples, the sample "3%–8" yielded a smaller Δ48 offset and 49 parameter, and lower Δ47 temperature of 17.0±1.3°C. Therefore, "reacting with 3% hydrogen peroxide for 8 h" was suggested to be the optimal condition for removing contaminants from lacustrine endogenic carbonates prior to clumped isotope analysis. At the time when the pretreatment condition was too intense (i.e., H2O2 concentration >3% and/or reaction time >8 h), secondary contaminants might have been generated in the closed reaction system, whose ultimately transformed state may interfere with masses 47, 48. Using the optimal pretreatment procedure, we obtained reasonable Δ47 temperature changes during the last deglaciation and found a temperature decrease of about 6°C in Northern China during the Younger Dryas period. Our study demonstrates a great potential of applying carbonate clumped isotope thermometry to lacustrine carbonates for paleo-temperature and paleo-elevation reconstructions in the future. [ABSTRACT FROM AUTHOR]

Abstract Preglacial and synglacial low‐latitude carbonate sediments of the Elbobreen Formation, NE Svalbard, preserve facies changes associated with low‐latitude glacial advance in Cryogenian “Snowball Earth” episodes (717–635 Ma). We present the first application of carbonate clumped (Δ47) isotope thermometry on synglacial Snowball Earth carbonates and combine results with sedimentologic and petrographic observations and stable isotope (δ13C and δ18O) geochemistry to assess Neoproterozoic environmental change. We find elevated calcite Δ47 temperatures, which likely reflect solid‐state reordering during burial. Dolomites, however, record lower temperatures that vary with facies and stratigraphy. Preglacial dolomite Δ47 temperatures range from 48–77°C, with a reconstructed fluid δ18OVSMOW value of +0.6‰ in the coldest sample. Glacial diamictites and dolomicrites comprise (1) reworked detrital clasts similar to preglacial strata in stable isotope composition and petrographic textures and (2) autochthonous dolomicrite with more positive δ18O values than those of preglacial dolomites or cooccurring detrital clasts. Mean glacial autochthonous dolomicrite Δ47 temperatures are 26 ± 10°C (95% CL) cooler than preglacial strata, with four samples

The occurrence of the quartz‐carbonate alteration assemblage (listvenite) in ophiolites indicates that ultramafic rock represents an effective sink for dissolved CO2. However, the majority of earlier studies of ultramafic rock carbonation had to rely on the surface exposure of reaction textures and field relationships. Here we present the first observations on ultramafic rock alteration obtained from the 300 m deep BT1B drill hole, ICDP Oman Drilling Project, allowing for a continuous and high‐resolution investigation. Hole BT1B recovered continuous drill core intersecting surface alluvium, 200 m of altered ultramafic rock comprising mainly listvenite and minor serpentinite bands at 90 and 180 m depth, and 100 m of the underlying metamorphic sole. Textural evidence suggests that the carbonation of fully serpentinized harzburgite commenced by non‐equilibrium growth of spheroidal carbonate characterized by sectorial zoning resulting from radially oriented low‐angle boundaries. In the serpentinite, carbonate spheroids are composed of alternating magnesite cores and dolomite rims, whereas texturally similar carbonate in the listvenite is composed of Fe‐rich magnesite cores and Ca‐Fe‐rich magnesite rims. The distinct compositions and mineral inclusions indicate that the carbonation extent was controlled by fluid accessibility resulting in the simultaneous formation of limited carbonate in the serpentinite bands and complete carbonation in the listvenite parts of BT1B. The presence of euhedral magnesite overgrowing spheroidal carbonate in the listvenite suggests near‐equilibrium conditions during the final stage of carbonation. The carbonate clumped isotope thermometry constrains carbonate crystallization between 50 °C and 250 °C, implying repeated infiltration of reactive fluids during ophiolite uplift and cooling. Key Points: Large‐scale carbonation resulted in pervasive replacement of serpentinite by the magnesite‐quartz‐fuchsite (listvenite) assemblageClumped isotope thermometry indicates carbonation temperatures between 50 °C and 250 °CCarbonation textures indicate a transition from non‐equilibrium spheroidal growth to near‐equilibrium euhedral growth [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]

Assessing the climatic perturbations during the Cretaceous when atmospheric CO 2 levels were high is critical for understanding the nature of environmental responses to greenhouse forcing in the past and future. Paleoclimate reconstructions utilizing various geochemical proxies have revealed a disturbed interval during the Late Jurassic–Cretaceous period. This interval comprised multiple instances of cooling and warming, including notable events such as the Kimmeridgian Warm Interval, Faraoni Thermal Excursion, and Tithonian-early Barremian Cool Interval. Among these perturbations, the late Valanginian stands out as the coolest identified period. Reconstructions of the duration and magnitude of these warming and cooling events mostly rely on biostratigraphy and the δ 18O values of carbonate minerals, but they remain controversial. In this study, we applied in situ U Pb geochronology, 87Sr/86Sr ratios, and clumped isotope thermometry to belemnite calcite from the Mahajanga Basin, Madagascar. We used these data to reconstruct seawater temperatures that are linked to absolute ages. The U Pb dates represent the timing of when diagenetic cements replaced the original organic matter in the apical line of the belemnite rostrum and filled the space after the death of the organisms. The lower ∆ 47 -derived temperatures obtained in the apical zone compared to other well-preserved regions of the rostrum could be explained by the possibility that diagenetic calcite formed on the seafloor and reflects bottom water temperature. Our results identified an abrupt Early Cretaceous cooling event of up to 6 °C in the Southern Hemisphere that followed the main phase of carbon isotope excursion, which is explained as a combination of global cooling and the change in belemnite habitat depth. These results demonstrate the necessity for better constraints on the absolute timing and magnitude of climatic fluctuation events and their potential consequences in order to achieve a more complete understanding of the relationships among volcanism, carbon cycle, atmospheric p CO 2 , ocean temperature, and continental ice volume. • The first in situ absolute U Pb ages (138.1 and 135.9 Ma) were obtained from belemnite calcite. • Clumped isotope data indicate that the coolest seawater temperature during the Valanginian in the Tethys (35° S) was ∼17 °C. • Reconstructed oxygen isotope results of seawater are more positive than typically assumed in an ice-free world. • Global cooling may have been triggered by the burial of organic matter and feedback from the Paraná–Etendeka LIP. [ABSTRACT FROM AUTHOR]

Southeastern (SE) Tibet forms the transition zone between the high interior Tibetan Plateau and the lowlands of southwest China. So understanding the elevation history of SE Tibet, a biodiversity hotspot, enlightens our understanding of the interactions between tectonics, monsoon dynamics and biodiversity. Here we reconstruct the uplift history of the Markam Basin, SE Tibet, during the middle−late Eocene based on U − Pb dating, plant fossil assemblages, and stable and clumped isotope analyses. Our results suggest that the floor of the Markam Basin was at an elevation of 2.6 ± 0.9 km between 42 Ma and 39 Ma, where the mean annual air temperature (MAAT) was 13.2 ± 2.4 °C. The basin then rose rapidly to 3.8 (+0.6/−0.8) km before 36 Ma. Integrated with existing paleoelevation data, we propose that the high plateau boundary (∼3.0 km) of SE Tibet formed during the late Eocene. Numerical climate modeling with realistic paleo-landscapes shows that with the rise of SE Tibet, a Mediterranean-like climate developed in the region characterized by bi-modal precipitation with two wet seasons in boreal spring and autumn. The high topographic relief of SE Tibet, coupled with this distinctive Mediterranean-like climate system, helped develop the high biodiversity of the Hengduan Mountains. • An absolute U Pb age framework of the Markam Basin gave a range of ∼42–36 Ma. • The Markam Basin stayed at ∼2.6 km during 42–39 Ma. • The Markam Basin uplifted to its modern elevation (3.8 km) before 36 Ma. • The high plateau boundary of SE Tibet formed during the late Eocene. • A late Eocene Mediterranean-like climate formed across SE Tibet and adjacent areas. [ABSTRACT FROM AUTHOR]

Outgassing of CO2 from thin water layers of a solution of CaCO3 in an H2O -CO2 system plays a crucial role in the precipitation of calcite. Understanding the process of outgassing of CO2 during precipitation of calcite to the surface of stalagmites is important for the interpretation of isotope signals in the calcite deposited to the speleothem. There is, however, some confusion in the literature about the physics and chemistry of this process. Indistinct terms like forced, enhanced, rapid, intense, slow, increased, equilibrium and progressive outgassing are used widely in the literature to explain the impact on isotope composition of the calcite deposited. It is shown that in all the variety of conditions occurring in nature only two distinct processes of outgassing exist. 1. Diffusion controlled outgassing: In the first step, whenever a thin water layer of calcareous solution is present, either on the cave wall or on the surface of a stalagmite, molecular CO2 escapes within several seconds by physical diffusion and after about 40 seconds pH and DIC in the solution achieve chemical equilibrium with respect to the CO2 in the cave atmosphere. 2.) Controlled by precipitation: In the second step this supersaturated solution precipitates calcite, whereby for each unit CaCO3 deposited one molecule of CO2 is generated and escapes from the solution by molecular diffusion. This precipitation controlled outgassing is active during precipitation only. All variations of outgassing mentioned in the literature can be explained by one of these two types of outgassing. Furthermore it is shown that the first step of outgassing driven by diffusion has no influence on the isotope composition of the HCO3 - reservoir in the solution and consequently on that of calcite precipitated from it. The isotope composition of HCO3 for 13C as well as for 18O solely is determined by the second step of precipitation controlled outgassing. An experiment is presented proving that the amount of CO2 escaping from the solution during precipitation of calcite at any time is equal to the amount of calcite precipitated. The results are used for a critical application to the Δ47 clumped isotope thermometer that explains why in most stalagmites the calcite is not a good candidate to obtain correct temperatures at the time of its deposition. Key words: isotope, clumped isotope, speleothem, calcite, paleo- thermometer. Fizika in kemija razplinjanja CO2 pri odlaganju sige, s posebnim ozirom na signale izotopa 18O in izotopskega skupka 13C18O v raztopljenem organskem ogljiku in kalcitu Razplinjanje CO2 iz tanke plasti raztopine sistema CaCO3 in H2O–CO2 je pomembno za izločanje kalcita in interpretacijo izotopskih signalov v odloženem kalcitu. V literaturi je precejšnja zmeda pri obravnavanju fizike in kemije procesa razplinjanja, saj raziskovalci uporabljajo različne izraze, kot so prisiljeno, poudarjeno, počasno, povečano in progresivno razplinjanje. V članku pokažem, da sta pri vseh različnih razmerah v naravi bistvena le dva procesa razplinjanja. 1) Difuzijsko razplinjanje: v prvem koraku molekularni CO2 v nekaj sekundah z difuzijo preide iz tanke plati vode, ki polzi ali po jamski steni ali po površini sige. Po približno 40 sekundah pH in raztopljeni organski ogljik v raztopini dosežeta ravnovesje z atmosferskim CO2. 2) Razplinjanje pri izločanju: v drugem koraku prenasičena raztopina izloča kalcit, pri čemer se za vsako odloženo molekulo CaCO3 iz raztopine sprosti molekula CO2, ki potem z difuzijo uide v jamsko atmosfero. S tema procesoma lahko pojasnimo vse druge načine razplinjanja, ki jih omenja literatura. Nato pokažem, da CO2, ki se razplini v prvem koraku, ne vpliva na izotopsko sestavo zaloge HCO3- v raztopini in zato tudi v izločenem kalcitu. Izotopska sestava HCO3- je tako za 13C in za 18O povsem določena z razplinjanjem med izločanjem kalcita. Ujemanje količine razplinjenega CO2 in izločenega kalcita pokažem tudi s poskusom. Rezultati omogočajo kritično obravnavo uporabe termometra na osnovi izotopskega skupka Δ47 in pojasnjujejo, zakaj kalcit ni primeren za določanje temperature v času izločanja. Ključne besede: izotop, izotopski skupek, siga, kalcit, paleo temperatura.

Clumped isotopes provide a theory‐based proxy to reconstruct formation temperatures of carbonates. With the introduction of the empirical transfer function (ETF) and several carbonate standards, interlaboratory comparison has become achievable. Due to the rare occurrence of the multiply substituted isotopologues, the analytical precision of these measurements is very low. In order to improve precision, we investigate the optimal strategy for the conversion into the absolute reference frame using a simulation approach. We find that changing the relative proportions of the standards to include more standards that are closer to the unknown target Δ47 value can greatly improve attainable precision. The inclusion of a hypothetical 4 °C standard results in only modest improvements in final temperature estimates for Earth surface temperature (0 and 40 °C) samples, indicating that the set of ETH‐1–3 standards is suitable for most applications. Full interpolation between two subsequent hypothetical standards, with Δ47 values of CO2 equilibrated at 1000 and 25 °C, results in modest improvements for samples with extreme Δ47 values. With a more optimal distribution of standards it is possible to measure more sample replicates before the uncertainty derived from the ETF becomes limiting. We provide suggestions for the optimal distribution of standards for all target sample Δ47 values and the R code to perform these simulations based on different laboratory settings. These optimizations can also be applied for ETFs using heated and equilibrated gases. We demonstrate numerically how optimizing the distribution and relative abundance of standards can increase measurement precision. Plain Language Summary: Carbonate clumped isotope measurements are a tool to reconstruct the temperature at which a carbonate has formed. Recent studies suggest that repeated measurements of carbonate standards allow for greater interlaboratory consistency and higher accuracy of measurements. In this study we demonstrate that the relative proportion of these standards should be changed to include more standards that are similar to the target clumped isotope composition of the unknown sample. We quantify how the composition of the standards used in a measurement session influences the measurement precision, based on simulations. Key Points: Measuring relatively more standards that are similar in Δ47 to the target sample improves precisionWith an optimal distribution of standards, the number of sample versus standard replicates should be approximately equal per session for smallest uncertaintiesAdding standards with extreme Δ47 values to the ETH‐1–3 set would lead to only minor error reduction at extreme Δ47 values [ABSTRACT FROM AUTHOR]

• Putative stromatolites of the Mesoproterozoic Stoer Group show no evidence for biogenicity. • Some are laminated clastic sediment draped over an ancient land surface. • Clastic carbonate is calcite, probably derived from weathering of older rocks. • Other possible stromatolites result from soft-sediment deformation. • Crystalline calcite in these is a result of syn -depositional to burial diagenesis. Putative stromatolites and associated carbonate minerals in 1.1 Ga Stoer Group lacustrine sedimentary rocks were analysed to deduce their likely origins. Potential stromatolite examples included finely laminated and sometimes wrinkled carbonate-siliciclastic rocks of the Clachtoll Formation at Clachtoll and Bay of Stoer, and laminated limestone domes of the Poll a'Mhuilt Member (Bay of Stoer Formation) from Enard Bay. Petrography shows that the lamination and wrinkling of Clachtoll Formation specimens can most logically be explained by abiotic siliclastic sedimentary processes, namely rippling and soft-sediment deformation probably related to de-watering. Electron backscatter diffraction shows that the carbonate in these laminated Clachtoll Formation specimens was calcite, and petrography combined with clumped isotope palaeothermometry indicates it was likely to be part syn -depositional and part burial diagenetic in origin. The laminated domes of the Poll a'Mhuilt Member are shown to comprise clasts of limestone interlayered with clay, quartz, Na-rich feldspars and micas. Cathodoluminescence revealed the limestone clasts to be composite and built of sub-grains that must have been derived from an earlier, potentially Palaeoproterozoic, carbonate unit. Support for this hypothesis comes from clumped isotope palaeotemperature measurements that indicate the limestone clasts were precipitated or recrystallized under higher temperature conditions than the burial diagenetic calcite found in the Clachtoll Formation. Raman spectra of an organic carbon particle within a laminated dome of the Poll a'Mhuilt Member at Enard Bay are consistent with the organic carbon having been re-worked from the ∼2 Ga Loch Maree Group, and we speculate that this might also be true of the calcite. Microbial fossils are well known from elsewhere in the Stoer Group, but no conclusive examples were found within the thin-sections examined herein. No conclusive evidence was found to suggest that any of the examined putative stromatolites were biogenic, leading to the conclusion that they are best considered stromatolite-like sedimentary rocks (pseudostromatolites). [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]

The Ordos Basin is the foremost tight-oil reservoir in China with respect to location and production, where tight-oil reservoirs are characterized by low porosity, low permeability, low pressure, and high heterogeneity. Until now, the cause of the oil-bearing heterogeneity of such reservoirs has not been reported. In this study, the cause of the oil-bearing heterogeneity and threshold of tight sandstone reservoirs in the seventh member of the Triassic Yanchang Formation (Chang7 Member), Ordos Basin, were investigated by core and thin-section observations using a fluorescence microscope, cathodoluminescence microscope, isotope ratio mass spectrometer, and thermal stage, among other devices. The results revealed that calcitic cementation is the major genetic mechanism for the oil-bearing heterogeneity of tight sandstone reservoirs in the study area, and that it is mainly in Phase 1. The clumped isotope revealed that the calcitic cements in Phase I were formed at 18–42 °C during the Middle–Late Triassic to Middle Jurassic periods; thus, they are the diagenetic products, and may be related to the early small-scale tectonic movements in the basin. The synchronous brine inclusions associated with the hydrocarbon inclusions correspond to a homogenization temperature of 90–120 °C. In conjunction with the basin simulation, this suggests that the hydrocarbons were charged in one phase in the Early Cretaceous period (100–130 Ma). Moreover, the modified cements in the Chang7 oil-free tight sandstone were formed earlier than the period in which the oil was charged in the oil-bearing sandstone. The tight sandstone reservoirs mainly contain nanoscale throats (diameter < 1 μm). With respect to the residual oil saturation, average size of mesopores, and the correlation between the oil saturation and porosity, it was concluded that tight oil may be present above the threshold characterized by a 15 mm mesopore average size and 2% porosity. It is first specified that the formation of oil-free calcitic-cemented sandstone is the diagenetic response of the early tectonic movement, which provides a basis for the determination of the influence of calcitic-cemented sandstone on hydrocarbon accumulation. Moreover, the results of the study on the oil-bearing threshold can be helpful in selection of the exploration zone and the resource estimation of unconventional tight-oil reservoirs. • Calcite cement is the factor for the oil-bearing heterogeneity in the sandstone. • Clumped isotope proofs that calcite cement was the product in early diagenesis. • The cement formation time is earlier than the oil charging time in sandstone. • The lowest oil bearing aperture is 15 nm in this tight oil play. [ABSTRACT FROM AUTHOR]

Abstract Shallow marine to peritidal carbonates of the Triassic Csukma Formation in the Mecsek Mts. of SW Hungary are made up of dolomites, limestones and dolomitic limestones that show evidence of a complex diagenetic history. Integration of petrographic, conventional stable oxygen and carbon isotope, clumped isotope, and strontium isotope data with the paleogeography, paleoclimate, and burial history of the region revealed four major diagenetic stages. Stage 1: The peritidal carbonates were dolomitized penecontemporaneously during the Middle Triassic by refluxing evaporatively concentrated brines. Stage 2: Increasing burial during the Late Triassic–Jurassic resulted in recrystallization of the Kán Dolomite Member in an intermediate burial setting. Stage 3: During the Early Cretaceous seawater was drawn down and circulated through rift-related faults, causing renewed recrystallization of the Kán Dolomite Member as well as dolomitization of the Kozár Limestone Member and the underlying limestones in a deep burial setting, but only in the vicinity of the faults. Stage 4: During the Late Cretaceous and Cenozoic thrusting resulted in tectonic expulsion of basinal fluids and precipitation of multiple saddle dolomite cement phases near the faults. The results of this study imply that the clumped isotope method integrated with other geochemical data can successfully be applied to identify the nature and potential sources of extra-formational diagenetic fluids responsible for dolomitization and recrystallization. This study provides conclusive evidence for multi-phase dolomitization and dolomite recrystallization over several millions of years (Middle Triassic through Early Cretaceous) and several thousands of meters of burial in the Csukma Formation in SW Hungary. Furthermore, this study is the first to identify fault-controlled dolomitization by circulating Cretaceous seawater within Triassic carbonates of central Europe, further supporting the viability of the interpretation of dolomitization by seawater initially drawn down and then geothermally circulated through faults in extensional basins. Highlights • Multi-phase dolomitization and dolomite recrystallization in SW Hungary. • Reflux dolomitization of Middle Triassic peritidal carbonates. • Intermediate burial recrystallization of reflux dolomites in connate waters. • Fault-controlled dolomitization/recrystallization by convecting Cretaceous seawater. • Synergistic application of carbon, oxygen, strontium and clumped isotope methods. [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]