Search

Low-Mg calcite, precipitating from meteoric fluids, is a common mineral that forms in a variety of near-surface diagenetic environments. However, recent studies, based on a combination of analyses of δ18O and Δ 47 values, have suggested that this mineral might form in disequilibrium and consequently yield kinetic bias in Δ 47 -derived temperatures and fluid δ18O values. Here, we use dual clumped isotope proxies (Δ 47 and Δ 48) to investigate the influence of kinetic isotope effects within different meteoric diagenetic zones of Holocene and Pleistocene carbonates from the southern Florida and the Dominican Republic. In the Miami Oolite, the primary aragonite ooids and secondary low-Mg calcite cements in the bulk sample were separated from each other and their isotopic compositions (δ13C, δ18O, Δ 47 and Δ 48 values) were measured. The Δ 47 and Δ 48 values of the separated aragonite are consistent with the modern ooid sediments and in approximate equilibrium with the surface seawater. In contrast, the low-Mg calcite cement shows the higher Δ 48 and lower Δ 47 values, than expected, with the disequilibrium arising as a result of CO 2 degassing in the vadose zone. Such deviations of Δ 47 and Δ 48 values are also observed in low-Mg calcite vadose cements in the Dominican Republic. While low-Mg calcites formed in the lower freshwater phreatic zone in the Dominican Republic have the Δ 47 - and Δ 48 -derived temperatures close to expected, the same mineral forming near the water-table and upper phreatic zone shows much higher Δ 48 -derived temperatures (up to ∼ 90 °C). The possible origin of such elevated temperatures can be attributed to non-equilibrium processes caused by changes in pH and pCO 2 , mediated by microbial sulfate reduction. Such differential kinetic behavior of Δ 48 values between vadose and phreatic zones could be used as a proxy marker for the presence and the location of a water-table. This study demonstrates the great potential of dual clumped isotopes in the investigation of meteoric diagenesis and will help understand the alteration of ancient sequences and the interpretation of stable C isotope trends that they contain. [ABSTRACT FROM AUTHOR]

In fewer than fifteen years, the study of Ni stable isotopes has advanced from early method development to application of a powerful tool for resolving a long-standing question: why does it appear that output fluxes of Ni from the global oceans far exceed input fluxes? The seawater concentration of Ni, a bioessential trace metal, is almost certainly at steady state on timescales comparable to its residence time of ∼20 kyr, so some of the current flux estimates must be inaccurate. Just as the input and output fluxes should balance, so should the flux-weighted isotopic compositions of the inputs and outputs. Thus, isotopic characterization of inputs and outputs provide an additional constraint on a balanced model of the marine Ni budget. Here, we report on experiments designed to elucidate fractionation mechanisms and magnitudes for sorption of Ni to Mn oxyhydroxide (birnessite), because Mn-rich sediments accumulating on abyssal plains represent the largest sink flux of Ni from seawater to marine sediments. Our results show remarkably large fractionations at low ionic strength (average Δ60/58Ni dissolved-sorbed = +1.38 ‰). Neither closed-system equilibrium trends nor Rayleigh curves fit the data well. Fractionations are even larger at high ionic strength (Δ60/58Ni dissolved-sorbed ranging from +2.0 to +4.0 ‰), and they decrease with experimental duration from 2 d (49 h) to 27 d. The high ionic strength data fit Rayleigh trends well. Here, we use X-ray absorption fine-structure spectroscopy (EXAFS) and results from previous studies to support interpretation of our data as combinations of kinetic and equilibrium isotope effects that vary in their proportional contributions to the total fractionation with time and with surface loading. One important consequence of this study is that none of the experimental results reported thus far, including ours, are directly applicable to building steady-state models of the Ni cycle. Even our longest duration experiments did not achieve equilibrium, which is likely to be manifest in the very slowly accumulating sediments on abyssal plains. Our work constrains further the mechanisms of Ni sorption to birnessite and clearly indicates that determination of equilibrium fractionation in this system, although challenging, will be a crucial step toward resolving the apparent marine Ni imbalance. [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]

Cave carbonate minerals are an important terrestrial paleoclimate archive. A few studies have explored the potential for applying carbonate clumped isotope thermometry to speleothems as a tool for constraining past temperatures. To date, most papers utilizing this method have focused on mass-47 clumped isotope values (Δ 47) at a single location and reported that cave carbonate minerals rarely achieve isotopic equilibrium, with kinetic isotope effects (KIEs) attributed to CO 2 degassing. More recently, studies have shown that mass-47 and mass-48 CO 2 from acid digested carbonate minerals (Δ 47 and Δ 48) can be used together to assess equilibrium and probe KIEs. Here, we examined 44 natural and synthetic modern cave carbonate mineral samples from 13 localities with varying environmental conditions (ventilation, water level, pCO 2 , temperature) for (dis)equilibrium using Δ 47 -Δ 48 values, in concert with traditional stable carbon (δ13C) and oxygen (δ18O) isotope ratios. Data showed that 19 of 44 samples exhibited Δ 47 -Δ 48 values indistinguishable from isotopic equilibrium, and 18 (95 %) of these samples yield Δ 47 -predicted temperatures within error of measured modern temperatures. Conversely, 25 samples exhibited isotopic disequilibria, 13 of which yield erroneous temperature estimates. Within some speleothem samples, we find Δ 47 -Δ 48 values consistent with CO 2 degassing effects, however, the majority of samples with KIEs are consistent with other processes being dominant. We hypothesize that these values reflect isotopic buffering effects on clumped isotopes that can be considerable and cannot be overlooked. Using a Raleigh Distillation Model, we examined carbon and oxygen isotope exchange trajectories and their relationships with dual clumped isotope disequilibria. Carbon isotope exchange is associated with depletion of both Δ 47 and Δ 48 relative to equilibrium, while oxygen isotope exchange is associated with enrichment of both Δ 47 and Δ 48 relative to equilibrium. Cave rafts collected from proximate locations in Mexico exhibit the largest average departures from equilibrium ( Δ Δ 47 ¯ = −0.032 ± 0.007, Δ Δ 48 ¯ = −0.104 ± 0.035, where ΔΔ i is the measured value – the equilibrium value). This study shows how the Δ 47 -Δ 48 dual carbonate clumped isotope framework can be applied to a variety of tcave carbonate mineral samples, enabling identification of isotopic equilibria and therefore quantitative application of clumped isotope thermometry for paleoclimate reconstruction, or alternatively, constraining the mechanisms of kinetic effects. [ABSTRACT FROM AUTHOR]

The Cedars is an area in Northern California with a chain of highly alkaline springs resulting from CO 2 -charged meteorological water interacting with a peridotite body. Serpentinization resulting from this interaction at depth leads to the sequestration of various carbonate minerals into veins accompanied by a release of Ca2+ and OH– enriched water to the surface, creating an environment which promotes rapid precipitation of CaCO 3 at surface springs. This environment enables us to apply the recently developed Δ 47 -Δ 48 dual clumped isotope analysis to probe kinetic isotope effects (KIEs) and timescales of CO 2 transformation in a region with the potential for geological CO 2 sequestration. We analyzed CaCO 3 recovered from various localities and identified significant kinetic fractionations associated with CO 2 absorption in a majority of samples, characterized by enrichment in Δ 47 values and depletion in Δ 48 values relative to equilibrium. Surface floes exhibited the largest KIEs (ΔΔ 47 : 0.163‰, ΔΔ 48 : −0.761‰). Surface floe samples begin to precipitate out of solution within the first hour of CO 2 absorption, and the dissolved inorganic carbon (DIC) pool requires a residence time of >100 h to achieve isotopic equilibria. The Δ 48 /Δ 47 slope of samples from the Cedars (−3.223 ± 0.519) is within the range of published theoretical values designed to constrain CO 2 hydrolysis-related kinetic fractionation (−1.724 to −8.330). The Δ 47 /δ18O slope (−0.009 ± 0.001) and Δ 47 /δ13C slope (−0.009 ± 0.001) are roughly consistent with literature values reported from a peridotite in Oman of −0.006 ± 0.002 and −0.005 ± 0.002, respectively. The consistency of slopes in the multi-isotope space suggests the Δ 47 -Δ 48 dual carbonate clumped isotope framework can be applied to study CO 2 -absorption processes in applied systems, including sites of interest for geological sequestration. [ABSTRACT FROM AUTHOR]

Several studies have exploited the stable calcium (Ca) and strontium (Sr) isotope compositions of marine carbonate rocks to investigate ancient carbon cycle dynamics, seawater geochemistry, and controls on carbonate production and preservation. However, the two proxies have rarely been applied together. Based on knowledge achieved to date, the δ44/40Ca-δ88/86Sr multi-proxy can distinguish signatures imparted by mass-dependent fractionation and reservoir mixing, including early diagenesis and shifts in the isotopic composition of seawater. In detail, the δ44/40Ca-δ88/86Sr multi-proxy represents a five-isotope system because the determination of δ88/86Sr values requires analysis of fractionation-corrected 87Sr/86Sr ratios, which provide additional constraints on mixing. Here, we apply the novel δ44/40Ca-δ88/86Sr multi-proxy to two Marinoan (ca. 635 Ma) "cap carbonate" sequences from Namibia. Cap carbonates were widely deposited after the Neoproterozoic Marinoan Snowball Earth Event, but controversy surrounds their origin. We find that the rocks archive primary environmental signals deriving from a combination of seawater-glacial meltwater mixing and kinetic isotope effects. In an outer platform section, dolostone δ44/40Ca and δ88/86Sr values define a line predicted for kinetic mass-dependent isotope fractionation. This dolostone mostly precipitated from meltwater. Moreover, stratigraphically higher samples exhibiting the fastest precipitation rates display elevated 87Sr/86Sr ratios, consistent with long-held expectations that a rapid deglacial weathering pulse forced cap carbonate formation. An inner-platform dolostone shows greater effects from water-mass mixing but still reveals that precipitation rates increased up-section. Overlying limestones show greater Ca and Sr contributions from seawater. Amplification of local coastal processes during global ice sheet collapse offers a simple but sufficient proposition to explain the Ca isotope heterogeneity of cap carbonates. Detection of kinetic isotope effects provides evidence for predominantly rock-buffered diagenesis and further offers a basis for developing the δ44/40Ca-δ88/86Sr multi-proxy as an indicator of saturation state and p CO 2. [ABSTRACT FROM AUTHOR]

The stable hydrogen isotope composition of persistent biomolecules is used as a palaeohydrological proxy. While much previous work has focused on plant leaf wax-derived n -alkanes, the potential of prokaryotic lipid biomarkers as carriers of H isotope signatures remains underexplored, particularly in the Archaea. Here we investigated H isotope distributions in the membrane lipids of the ammonia-oxidizing chemoautotroph Nitrosopumilus maritimus strain SCM1. Hydrogen isotope ratios were measured on the cleaved biphytane chains of tetraether membrane lipids extracted from steady-state continuous cultures cultivated at slow, medium, and fast growth rates. In contrast to recent work on bacterial fatty acids, where the direction and magnitude of isotopic fractionation varies widely (ca. 600‰ range) as a function of central C and energy metabolism, archaeal biphytane data in the present work are relatively invariant. The weighted average 2H/1H fractionation values relative to growth water (2 ε L/W) ranged from –272 to –260‰, despite a three-fold difference in doubling times (30.8–92.5 hr), yielding an average growth-rate effect <0.2‰ hr−1. These 2 ε L/W values are more negative than most heterotrophic microbial lipid H isotope measurements in the literature, and are on par with those from other autotrophic archaea, as well as with phytol from photoautotrophic algae. N. maritimus values of 2ε L/W also varied systematically with the number of internal rings (cyclopentyl + cyclohexyl), increasing for each additional ring by 6.4 ± 2.7‰. Using an isotope flux-balance model in tandem with a comprehensive analysis of the sources of H in archaeal lipid biosynthesis, we use this observation to estimate the kinetic isotope effects (KIEs) of H incorporation from water; from reducing cofactors such as flavins and NADPH, and for the transhydrogenation reaction(s) that convert the electron-donor derived NADH into these cofactors. Consistent with prior studies on bacteria and plants, our results indicate the KIEs of reducing cofactors in archaea are highly fractionating, while those involving exchange of water protons are less so. When combined with the observation of minimal growth-rate sensitivity, our results suggest biphytanes of autotrophic 3HP/4HB utilizing Nitrososphaerota (a.k.a. Thaumarchaeota) may be offset from their growth waters by a nearly constant 2ε L/W value. Together with the ring effect, this implies that all biphytanes originating from a common source should have a predictable ordering of their isotope ratios with respect to biphytane ring number, allowing precise reconstruction of the original δ2H value of the environmental water. Collectively, these patterns indicate archaeal biphytanes have potential as paleo-hydrological proxies, either as a complement or an alternative to leaf wax n -alkanes. [ABSTRACT FROM AUTHOR]

Microbial methane oxidation - or methanotrophy - is a key control of the global methane budget on Earth, and perhaps in other planetary systems. Here, we explore the potential role of mass-18 isotopologues of methane, expressed as Δ13CH 3 D and Δ12CH 2 D 2 values, in tracking both aerobic and anaerobic methanotrophy in nature. We examine two well documented methanotrophic environments: the Lake Pavin (France) water column, where methane degradation is dominated by aerobic methanotrophy (AeOM), and the Black Sea sediments (offshore Romania), dominated by anaerobic methanotrophy (AOM) coupled to sulfate-reduction. In both settings, lighter isotopologues are preferentially consumed, generating elevated 13CH 4 /12CH 4 , 12CH 3 D/12CH 4 , 13CH 3 D/12CH 4 and 12CH 2 D 2 /12CH 4 ratios. This results in increasing of δ13C and δD values in the residual methane for both settings, as observed commonly in systems dominated by methanotrophy. As a result, AeOM and AOM cannot be easily distinguished by the development of δ13C and δD. In contrast, the Δ13CH 3 D and Δ12CH 2 D 2 (departure from stochastic) values have opposite trajectories, with minimal decreases in the case of the AeOM-dominated system, but dramatic increases in the case of AOM, with Δ13CH 3 D and Δ12CH 2 D 2 reaching values as high as 15.7 ‰ and 76.6 ‰, respectively. This contrasting behavior of clumped isotopologues signatures illustrates fundamental distinction between the two processes and the way they segregate methane isotopologues. These data demonstrate that both AeOM and AOM have distinctive kinetic isotope effects in natural settings, consistent with preliminary laboratory work. In particular, we find that γ-values (which measure the deviation to the product of 'normal' bulk isotope fractionation factors) are close to unity in the case of AeOM (i.e. a negligible clumped isotope effect), but significantly below unity in the case of AOM (i.e. strong clumped isotope effect). In addition, our data also illustrate how AOM under low-sulfate conditions may promote methane isotopologue equilibration. Taken together, we suggest these data and apparent isotopologue fractionation factors extrapolated from these two environments may help refine the potential bio-signatures of methane affected by methanotrophy. [ABSTRACT FROM AUTHOR]

The diffusion of molecular water (H 2 O m) from the environment into volcanic glass can hydrate the glass up to several wt% at low temperature over long timescales. During this process, the water imprints its hydrogen isotope composition (δD H2O) to the glass (δD gl) offset by a glass-H 2 O fractionation factor (ΔD gl-H2O = δD gl – δD H2O) which is approximately −33‰ at Earth surface temperatures. Glasses hydrate much more rapidly at higher, sub-magmatic temperatures as they interact with H 2 O during eruption, transport, and emplacement. To aid in the interpretation of δD gl in natural samples, we present hydrogen isotope results from vapor hydration experiments conducted at 175–375 °C for durations of hours to months using natural volcanic glasses. The results can be divided into two thermal regimes: above 250 °C and below 250 °C. Lower temperature experiments yield raw ΔD gl-H2O values in the range of −33 ± 11‰. Experiments at 225 °C using both positive and negative initial ΔD gl-H2O values converge on this range of values, suggesting this range represents the approximate equilibrium fractionation for H isotopes between glass and H 2 O vapor (103lnα gl-H2O) below 250 °C. Variation in ΔD gl-H2O (−33 ± 11‰) between different experiments and glasses may arise from incomplete hydration, analytical uncertainty, differences in glass chemistry, and/or subordinate kinetic isotope effects. Experiments above 250 °C yield unexpectedly low δD gl values with ΔD gl-H2O values of ≤–85‰. While alteration alone is incapable of explaining the data, these run products have more extensive surface alteration and are not interpreted to reflect equilibrium fractionation between glass and H 2 O vapor. Fourier transform infrared spectroscopy (FTIR) shows that glass can hydrate with as much as 5.9 wt% H 2 O m and 1.0 wt% hydroxl (OH−) in the highest P-T experiment at 375 °C and 21.1 MPa. Therefore, we employ a 1D isotope diffusion–reaction model of glass hydration to evaluate the roles of equilibrium fractionation, isotope diffusion, water speciation reactions internal to the glass, and changing boundary conditions (e.g. alteration and dissolution). At lower temperatures, the best fitting model results to experimental data for low silica rhyolite (LSR) glasses require only an equilibrium fractionation factor and yield 103lnα gl-H2O values of −33‰ ± 5‰ and −25‰ ± 5‰ at 175 °C and 225 °C, respectively. At higher temperatures, ΔD gl-H2O is dominated by boundary layer effects during glass hydration and glass surface alteration. The modeled bulk δD gl value is highly responsive to changes in the δD gl boundary condition regardless of the magnitude of other kinetic effects. Observed glass dissolution and surficial secondary mineral formation are likely to impose a disequilibrium boundary layer that drives extreme δD gl fractionation with progressive glass hydration. These results indicate that the observed ΔD gl-H2O of ∼−33 ± 11‰ can be cautiously applied as an equilibrium 103lnα gl-H2O value to natural silicic glasses hydrated below 250 °C to identify hydration sources. This approximate ΔD gl-H2O may be applicable to even higher temperature glasses hydrated on short timescales (of seconds to minutes) in phreatomagmatic or submarine eruptions before H 2 O in the glass is primarily affected by boundary layer effects associated with alteration on the glass surface. [ABSTRACT FROM AUTHOR]

Stable isotope analysis has been widely used to aid the source identification of methane. However, the isotopic (13C/12C and D/H) and isotopologue (13CH 3 D and 12CH 2 D 2) signatures of microbial methane in natural environments are often different from those in laboratory cultures in which methanogens are typically grown under optimal conditions. Growth phase and hydrogen (H 2) concentration have been proposed as factors controlling the isotopic compositions of methane produced via hydrogenotrophic methanogenesis, but their effects on the relationship among carbon, hydrogen and doubly-substituted "clumped" isotopologue systems have not been assessed in a quantitative framework. Here we experimentally investigate the bulk (δ13C and δD) and clumped (Δ13CH 3 D) isotopologue compositions of methane produced by hyperthermophilic hydrogenotrophic (CO 2 -reducing) methanogens using batch and fed-batch systems at different growth phases and H 2 mixing ratios (Methanocaldococcus bathoardescens at 82 or 60 °C and on 80 or 25 % H 2 ; Methanothermobacter thermautotrophicus str. ΔH at 65 °C and on 20, 5 or 1.6 % H 2). We observed a large range (from 18 to 63 ‰) of carbon isotope fractionations, with larger values observed during later growth phase, consistent with previous observations. In contrast, hydrogen isotope fractionations remained relatively constant at –317 ± 25 ‰. Linear growth was observed for experiments with M. bathoardescens , suggesting that dissolution of gaseous H 2 into liquid media became the rate limit as cell density increased. Accordingly, the low (and undersaturated) dissolved H 2 concentrations can explain the increased carbon isotope fractionations during the later growth phase. The δD and Δ13CH 3 D values indicated departure from equilibrium throughout experiments. As the cell density increased and dissolved H 2 decreased, Δ13CH 3 D decreased (further departure from equilibrium), contrary to expectations from previous models. Our isotopologue flow network model reproduced the observed trends when the last H-addition step is less reversible relative to the first three H-addition steps (up to CH 3 -CoM). In this differential reversibility model, carbon, hydrogen and clumped isotopologue fractionations are largely controlled by the reversibility of the first three H-addition steps under high H 2 concentrations; the last H-addition step becomes important under low H 2. The magnitude of depletion and decreasing trend in Δ13CH 3 D values were reproduced when a large (≥6 ‰) secondary clumped kinetic isotope effect was considered in the model. This study highlights the advantage of combined bulk and clumped isotope analyses and the importance of physiological factors (growth phase) and energy availability (dissolved H 2 concentration) when using isotope analyses to better understand methanogenic metabolisms and methane cycling processes. [ABSTRACT FROM AUTHOR]

The phenomenon that laboratory pyrolysis experiments produce much wetter gases than those in natural reservoirs is a long-recognized and debated problem in the investigation of natural gases in sedimentary basins. In this study, we explore the discrepancy by pyrolyzing a type II kerogen from the Woodford Shale in Oklahoma, compared with the previous results on the produced natural gases from the Arkoma Basin generated from the same source rock (Liu et al., 2019) with the discussion of gas and isotopic compositions at bulk and position-specific (PS) levels. An improved GC-pyrolysis-GC IRMS method is applied for the determination of PS δ13C of propane produced in the pyrolysis of the Woodford Shale at Easy %R o from 0.76 to 3.27. Kinetic and thermodynamic considerations of the chemical and isotopic compositions of the natural and laboratory pyrolysis gases suggest that the generation of light hydrocarbons involves uni-directional cracking reactions, exchange reactions with water, and likely reversible reactions among light hydrocarbons and other H-containing volatiles. After the gas generation in the unconventional Woodford Shale reservoirs, the C 1 -C 4 gases might have approached close to chemical equilibrium of C 1 -C 3 and isotope equilibrium of C 2 -C 1 and C 3 -C 1 pairs at their peak temperatures. The capping H for the generation of C 1 -C 4 in the Woodford Shale gases appears to have experienced at least partial exchange with the water, while that in the pyrolysis gases is only originated from organic-bound compounds with large kinetic isotope effects (KIE). Our findings indicate that elevated compound-specific and PS δ13C values of propane in the wet-gas cracking stage are significantly influenced by the breakdown of the thermally stable compounds (e.g., remaining kerogen, residues). A first synthesis of PS δ13C and δ2H isotopic compositions of propane from this study and the literature data suggests relatively similar isotopic structures of propane precursors in kerogens. This study demonstrates that PS isotope analysis of propane can contribute to identifying various geological (e.g., maturation, wet-gas cracking, H exchange, diffusion) and biodegradation processes. [ABSTRACT FROM AUTHOR]

Gas hydrates stored in the continental margins of the world's oceans represent the largest global reservoirs of methane. Determining the source and history of methane from gas hydrate deposits informs the viability of sites as energy resources, and potential hazards from hydrate dissociation or intense methane degassing from ocean warming. Stable isotope ratios of methane (13C/12C, D/H) and the molecular ratio of methane over ethane plus propane (C 1 /C 2+3) have traditionally been applied to infer methane sources, but often yield ambiguous results when two or more sources are mixed, or when compositions were altered by physical (e.g., diffusion) or microbial (e.g., methanotrophy) processes. We measured the abundance of clumped methane isotopologue (13CH 3 D) alongside 13C/12C and D/H of methane, and C 1 /C 2+3 for 46 submarine gas hydrate specimens and associated vent gases from 11 regions of the world's oceans. These samples are associated with different seafloor seepage features (oil seeps, pockmarks, mud volcanoes, and other cold seeps). The average apparent equilibration temperatures of methane from the Δ13CH 3 D (the excess abundance of 13CH 3 D relative to the stochastic distribution) geothermometer increase from cold seeps (15–65 °C) and pockmarks (36–54 °C), to oil-associated gas hydrates (48–120 °C). These apparent temperatures are consistent with, or a few tens of degrees higher than, the temperature expected for putative microbial methane sources. Apparent methane generation depths were derived for cold seep, pockmark, and oil seep methane from isotopologue-based temperatures and the local geothermal gradients. Estimated methane generation depths ranged from 0.2 to 5.3 kmbsf, and are largely consistent with source rock information, and other chemical geothermometers based on clay mineralogy and fluid chemistry (e.g., Cl, B, and Li). Methane associated with mud volcanoes yielded a wide range of apparent temperatures (15–313 °C). Gas hydrates from mud volcanoes the Kumano Basin and Mediterranean Sea yielded δ13C-CH 4 values from −36.9 to −51.0‰, typical for thermogenic sources. Δ13CH 3 D values (3.8–6.0‰) from these sites, however, are consistent with prevailing microbial sources. These mud volcanoes are located at active convergent plate margins, where hydrogen may be supplied from basement rocks, and fuel methanogenesis to the point of substrate depletion. In contrast, gas hydrate from mud volcanoes located on km-thick sediments in tectonically less active or passive settings (Black Sea, North Atlantic) yielded microbial-like δ13C-CH 4 and C 1 /C 2+3 values, and low Δ13CH 3 D values (1.6–3.3‰), which may be due to kinetic isotope effects. Additionally, using samples from two sites, we found that Δ13CH 3 D values of hydrate-bound gas and vent gas agree within measurement error. This study is the first to document the link between methane isotopologue-based temperature estimates and key submarine gas hydrate seepage features, and validate previous models about their geologic driving forces. [ABSTRACT FROM AUTHOR]

Background electrolytes affect calcite precipitation and dissolution rates and hence have the potential to impact kinetic isotope fractionation between calcite and the dissolved inorganic carbon (DIC) species. We grew inorganic calcite from NaCl-CaCl 2 solutions (T = 25 °C, pH = 8.3, and crystal growth rate = 10−6.3 ± 0.3 mol m−2 s−1) to test for potential ionic strength effects on the oxygen isotope fractionation between calcite and an isotopically equilibrated inorganic carbon pool (α c/EIC). Calcite was grown in the presence of the enzyme carbonic anhydrase from bovine erythrocytes (bCA) to promote isotopic equilibration of the DIC pool. No evidence of an ionic strength effect on α c/EIC was found for NaCl concentrations up to 0.35 M (1000lnα c/w = 28.0 ± 0.1; n = 7). For experiments conducted with [NaCl] > 0.35 M, the DIC pool could not be maintained in isotopic equilibrium with water due to the inhibitory effect of NaCl on bCA. The use of other types of CA may be required to maintain isotopic equilibration of DIC in solution with ionic strength close to or above that of seawater. The oxygen isotope results were modeled successfully with an isotopic box model for a CO 2 -fed solution that tracks the isotopic composition of each DIC species and CaCO 3. The experimental and modeling results suggest that the efficacy of bCA decreases exponentially with increasing [NaCl]. We quantify the salt effect on the quantity k cat / K M , which relates the catalyzed rate constant for CO 2 hydration to the concentration of bCA. The salt effect can be implemented in models of biomineralization with potential to extend our knowledge of oxygen isotope vital effects in marine organisms. [ABSTRACT FROM AUTHOR]

Strecker synthesis creates α-amino acids from prebiotically plausible substrates (cyanide, ammonia, and aldehydes) and is widely hypothesized to be a key mechanism in the chemistry that led to life on Earth and on other planets. To better constrain the synthetic environments and precursors of abiotic α-amino acids, and to determine unique signatures of abiogenic amino acids, we measured the molecular-averaged and site-specific carbon and nitrogen isotope effects for the Strecker synthesis of alanine. The reaction steps of the Strecker synthesis can be divided into two groups: an initial series of reversible amination and nitrile-addition reactions ('equilibration') and a second series of irreversible hydrolysis reactions ('hydrolysis'). The equilibration of cyanide, acetaldehyde, and ammonia with the intermediate, α-aminopropionitrile (α-APN), has a measured 55.1‰ equilibrium nitrogen isotope effect between the 15N-rich amine nitrogen in α-aminopropionitrile and the 15N-poor ammonia and a 20.0‰ equilibrium carbon isotope effect between the 13C-poor C-2 site in α-aminopropionitrile and the 13C-rich carbonyl carbon in acetaldehyde. The first irreversible hydrolysis step is inferred to have an up to 10‰ normal carbon fractionation (i.e., faster for 12C, slower for 13C) for the whole molecule, but it also has one or more side reactions that deplete the reactive α-APN reservoir by up to 15‰. The second hydrolysis step has a 15.4‰ normal kinetic isotope effect on the amide (C-1) site of alaninamide, which becomes the carboxyl site of alanine. Other α-amino acids will likely experience similar nitrogen isotope fractionations between ammonia and their amine sites, and similar carbon isotope fractionations between the carbonyl carbon in reactant aldehydes or ketones and the intermediate α-aminonitrile, and between cyanide and the carboxyl site. Therefore, these isotope effects allow us to predict the carbon and nitrogen isotopic contents and intramolecular structures of α-amino acids formed by Strecker synthesis based on their substrates' isotopic compositions, or to infer the isotopic compositions of substrates from which amino acids formed, for example in the case of the amino-acid-rich carbonaceous chondrites. The site-specific C and N isotopic compositions of amino acids formed by Strecker chemistry contrast with those typical of terrestrial biosynthetic amino acids, so these data also provide a means of discriminating between biogenic and abiogenic α-amino acids. [ABSTRACT FROM AUTHOR]

Here we present clumped isotope Δ 47 data of cave analogous experiments. We investigate the evolution of the Δ 47 values of both the dissolved inorganic carbon (DIC) and the CaCO 3 of thin-films flowing down in channels along inclined marble and glass plates precipitating CaCO 3 along the flow path. With increasing distance of flow and residence time on the plate, we observe large deviations from the initial Δ 47 values, along with increasingly over-estimated apparent Δ 47 - temperatures derived from the precipitated CaCO 3. With further increasing residence time on the plates (>250 s) and decreasing supersaturation with respect to calcite, the CaCO 3 Δ 47 values start to approach the initial equilibrium values again. This highlights the potentially strong effect of disequilibrium isotope effects on clumped isotope temperatures derived from speleothems, in particular because the disequilibrium effect on the Δ 47 values is most pronounced in the first few 100 seconds of CaCO 3 precipitation. The experimental Δ 47 values are consistent with a recent modeling study by Guo and Zhou (2019). The maximum degree of disequilibrium, however, seems to be more pronounced than predicted by the model for some of the experiments. A potential reason for this observation is that suitable kinetic fractionation factors are so far not available. While the Δ 47 values of the CaCO 3 decrease along the plate as the model suggests for the Δ 47 values of the HCO 3 −, those of the DIC increase and follow the modelled Δ 47 values of the dissolved CO 2. Based on our study, we can neither confirm nor exclude a temperature dependence of the Δ 47 -δ18O slope as well as a dependence on cave pCO 2 , which highlights the complexity of this relationship. In addition, the non-linear evolution of Δ 47 and δ18O values with time and along the flow path restricts a correction of the Δ 47 and δ18O values based on the Δ 47 -δ18O slope to the initial phase of the isotope evolution. So far, a correction of the kinetic isotope effects is not yet possible, but this might be important in future studies if the disequilibrium slope on a Δ 47 -δ18O diagram could be precisely determined for the initial phase of isotope evolution. [ABSTRACT FROM AUTHOR]

The stable carbon isotopic composition of Thaumarchaeota-produced crenarchaeol (δ13C cren) is thought to reflect the δ13C of dissolved inorganic carbon (DIC) and is valuable in carbon cycle research. Recently, an isotope flux-balance model was proposed that δ13C cren is dependent on passive CO 2 uptake and a slow rate of intracellular conversion to HCO 3 −, which has been validated in open ocean environments. However, further studies are required in environments with larger ranges of aqueous CO 2 and diverse archaeal communities to examine whether the δ13C cren value could still be predicted by the model. Here, we report δ13C values of biphytanes chemically released from the archaeal tetraether lipids (i.e., GDGTs) in sediments along a transect exhibiting shifts in the archaeal community and showing gradients of salinity, pH, and CO 2 in the Pearl River Estuary, South China. Methanogenic and methanotrophic Euryarchaeota are present in the upper estuary, although not significant, as revealed by the elevated relative abundances of GDGT-0 and higher values of methane index in suspended particulate matter (SPM) and surface sediments, as well as the more 13C-depleted C 40:0 , C 40:1 , and C 40:2 biphytanes than the C 40:3 biphytane released from sedimentary GDGTs. Toward the lower estuary and coastal sea, GDGT distributions in SPM and sediments tend to indicate archaeal communities under normal marine conditions. However, there are still differences in δ13C values between C 40:0 and C 40:3 biphytanes by up to 2.4‰ in sediments, although C 40:1 , C 40:2 , and C 40:3 were similar in δ13C, likely due to the existence of marine group II Euryarchaeota. Along such a physicochemical and biogeochemical gradient, the values of sedimentary δ13C 40:3 increase monotonically with the increase of mean annual pH and the decrease of mean annual CO 2. This phenomenon resembles the reported δ13C GDGT profiles, along with the variations in pH and CO 2 , in water column in the open ocean, and hence consistent in direction with the isotope flux-balance model for Thaumarchaeota lipids. However, our estimated apparent biosynthetic isotope effect (ε Ar) of crenarchaeol, ranging from 15.2‰ to 19.7‰, is mostly lower than those estimated in the open ocean. With the constraints of thaumarchaeal growth rates in such a dynamic estuarine environment as well as those in the open ocean, we suggest slight reductions in catalytic efficiency and intracellular kinetic isotope effect of intracellular CO 2 hydration relative to previously employed values. Such modifications would slightly change the sensitivities of ε Ar to archaeal growth rate and ambient CO 2 concentration but might be more suitable for variable hydrology and community environments. [ABSTRACT FROM AUTHOR]

In fire-prone biomes, the interpretation of paleoecological proxies is subjected to uncertainties since the plant biomarkers/molecules are thermally modified during vegetation fires, and the extent of alterations is yet to be well-constrained, particularly for oxic conditions. Towards this, we performed a series of controlled experiments where leaf samples from C 3 (tree and shrub) and C 4 (grass) plants were burned under ambient oxygen at temperatures between 200 °C and 500 °C. A topsoil sample was also heated to understand the effect of thermal degradation on organic matter (OM) already present in the soil. Our results show a reduction in the total organic carbon content and leaf wax concentrations which is consistent with the previous studies. We also observed a shift from predominantly long-chain homologues (with odd-over-even in n -alkanes or even-over-odd predominance in n -alkanoic acids; FAs) to balanced distributions with increased mid and short-chain homologues. We observed that the short, mid, and long-chain n -alkanes were mainly formed at the expense of FAs (with losses up to 100%), possibly due to oxygen-rich burning conditions. Burning of plant leaves also affected their stable isotopic compositions. In burned C 3 plant leaves (tree and shrub), the bulk carbon isotope values (δ13C OM) increased by 1.0–1.4‰, while in C 4 grasses, it was 0.4–1.6‰ lower than their unburned counterparts. The changes in the δ13C OM values are suggested to be a cumulative product of the kinetic isotope effect and source-driven isotopic fractionation. The carbon isotope values in long-chain n -alkanes (δ13C n -alk) mostly decreased (up to 4.6‰) in burned C 3 and C 4 plant leaves due to the isotopic modifications associated with the generation of secondary n -alkanes from FAs. In contrast, the hydrogen isotope values of n -alkanes (δ2H n -alk) in the burned samples were up to 86‰ higher than their initial values, mainly due to the kinetic isotope effect. Therefore, in biomes susceptible to frequent canopy and litter layer fires, unusually high δ2H n -alk values coupled with a disproportionate change between δ13C n -alk and δ2H n -alk values might indicate pyrogenic OM presence within the soil. We also observed that changes in the n -alkane characteristics due to heating of soil samples were substantially lower than those in plant leaves due to the protected lipid component within organomineral complexes. In summary, we recommend careful consideration and identification of vegetation fire history before using stable isotope-enabled organic proxies for generating paleoecological records. [ABSTRACT FROM AUTHOR]

The curvature and slope of speleothem surfaces have been shown to affect the reaction rates in the aqueous carbonate system by altering the thickness of the CaCO 3 -precipitating solution. However, the effects of speleothem geometry and drip rate on the speleothem's carbon and oxygen isotopic composition have yet to be investigated. Over more strongly sloping surfaces, solutions are thinner and flow faster. The effects of thinner and faster-flowing solutions on the isotopic composition of carbonate minerals precipitated from these solutions are of opposite sense. Thinner solutions enhance rates of CO 2 degassing and mineral formation, increasing the degree of isotopic distillation of the dissolved inorganic carbon (DIC) reservoir and leading to larger isotopic fractionation between the carbonate mineral and the initial DIC. Concurrently, faster flow over the steeper surfaces results in shorter residence times of the solutions on the growing speleothem, thereby limiting the degree of isotopic distillation and CaCO 3 -DIC fractionation. Consequently, predicting the CaCO 3 -DIC isotopic fractionation as a function of drip rate, surface slope and flow distance is not trivial. Using an advection-diffusion-reaction model, we tested the sensitivity of the isotopic composition of calcite precipitated along inclined surfaces to the solution discharge (drip) rate and the surface slope. Calcite δ13C and δ18O values correlate well with the degree of prior calcite precipitation (PCP), which is identified as a major determinant of isotopic compositions in speleothems. Our results show that at low PCP, speleothem δ13C and δ18O values may initially decrease relative to calcite-DIC and calcite-water equilibrium due to expression of kinetic isotope effects of mineral precipitation. Upon progressive PCP, δ13C and δ18O values gradually increase due to continuous CO 2 (aq) formation and degassing. This shift in the isotopic composition of the calcite to lower-than-equilibrium and then higher-than-equilibrium values expands the regime of near-equilibrium compositions during the isotopic evolution. In turn, this may allow quantitative environmental reconstructions, even when the isotopic system is in disequilibrium. Under the simulated conditions, our model predicts maximal potential enrichments of 7 and 3‰ in the calcite δ13C and δ18O values, respectively. In addition, we found a strong dependence of the calcite δ13C and δ18O values on the drip rate and distance of flow, and a weak dependence on the surface slope. In fact, changes in drip rate alone may drive isotopic offsets of several permil, when all other environmental parameters are kept constant. According to our model, higher drip rates and shorter stalactites promote closer-to-equilibrium isotopic compositions of stalagmites, providing a higher signal-to-noise ratio, and minimizing variability that is unrelated to climate. [ABSTRACT FROM AUTHOR]

The abundance of 18O isotopes and 13C-18O isotopic "clumps" (measured as δ18O and Δ 47 , respectively) in carbonate minerals have been used to infer mineral formation temperatures. An inherent requirement or assumption for these paleothermometers is mineral formation in isotopic equilibrium. Yet, apparent disequilibrium is not uncommon in biogenic and abiogenic carbonates formed in nature and in synthetic carbonates prepared under laboratory settings, as the dissolved carbonate pool (DCP) from which minerals precipitate is often out of δ18O and Δ 47 equilibrium. For this, a complete understanding of both equilibrium and kinetics of isotopic partitioning and 13C-18O clumping in DCP is crucial. To this end, we analyzed Δ 47 of inorganic BaCO 3 samples from Uchikawa and Zeebe (2012) (denoted as UZ12), which were quantitatively precipitated from NaHCO 3 solutions at various times over the course of isotopic equilibration at 25 °C and pH NBS of 8.9. Our data show that, although the timescales for δ18O and Δ 47 equilibrium in DCP are relatively similar, their equilibration trajectories are markedly different. As opposed to a simple unidirectional and asymptotic approach toward δ18O equilibrium (first-order kinetics), Δ 47 equilibration initially moves away from equilibrium and then changes its course towards equilibrium. This excess Δ 47 disequilibrium is manifested as a characteristic "dip" in the Δ 47 equilibration trajectory, a feature consistent with an earlier study by Staudigel and Swart (2018) (denoted as SS18). From the numerical model of SS18 , the non-first-order kinetics for Δ 47 equilibration can be understood as a result of the difference in the exchange rate for oxygen isotopes bound to 12C versus 13C, or an isotope effect of ~25‰. We also developed an independent model for the Ex change and Clump ing of 13 C and 18 O in DCP (ExClump38 model) to trace the evolution of singly- and doubly-substituted isotopic species (i.e., δ13C, δ18O and Δ 47). The model suggests that the dip in the Δ 47 equilibration trajectory is due largely to kinetic carbon isotope fractionation for hydration and hydroxylation of CO 2. We additionally examined the BaCO 3 samples prepared from NaHCO 3 solutions supplemented with carbonic anhydrase (CA), an enzyme known to facilitate δ18O equilibration in DCP by catalyzing CO 2 hydration (UZ12). These samples revealed that, while CA effectively shortens the time required for Δ 47 equilibrium in DCP, the overall pattern and magnitude of the dip in the Δ 47 equilibration trajectory remain unchanged. This suggests no additional isotope effects due to the CA enzyme within the tested CA concentrations. With the ExClump38 model, we test various physicochemical scenarios for the timescales and trajectories of isotopic equilibration in DCP and discuss their implications for the Δ 47 paleothermometry. [ABSTRACT FROM AUTHOR]

• Large D/H fractionation exists between Na-rich and -poor regions in silicate melt/glass. • Interstrutural D/H fractionation in silicate melt/glass responds to varying P-T-X H2O. • Large kinetic isotope effects can occur due to very different reactivity of Si-OD and Si-OH. • High D/H ratio of Na-rich melt regions carries over to silicate saturated fluid. Hydrogen isotope ratios (D/H) measured on geological samples are used to trace the pathways of water in magmatic and hydrothermal systems. Interpreting respective isotope data relies thereby on the theoretical and empirical constraints on hydrogen isotope fractionation. This study revisits a recently discovered hydrogen isotope fractionation effect between alkali-rich and alkali-poor regions within hydrous alkali silicate melts (Wang et al., 2015). In a series of experiments conducted at variable P-T- X H2O, D2O conditions, we studied this intramolecular isotope effect by 1H and 2H solid-state nuclear magnetic resonance (NMR) spectroscopy on quenched hydrous sodium tetrasilicate melts (Na 2 O⋅4SiO 2 with 1:1 H 2 O and D 2 O). The results indicate a marked difference in the reactivity of the protonated and deuterated hydrous species. It is observed that c. 70% of deuterium concentrates as deuterated silanols (Si-OD) in proximity to the sodium cations in the silicate melt structure, while the remaining 30% occupy the Na-poor regions in the silicate melt in the form of Si-OD and presumably molecular water (HDO m). This distribution remains nearly constant under all experimental conditions. In contrast, the concentration of protonated hydrous species (Si-OH, H 2 O m) in the Na-rich and -poor regions of the silicate melt varies as a response to changing experimental P-T- X H2O and cooling rate, which is linked to common water speciation reactions taking place within the melt (Si-O-Si + H 2 O m = 2Si-OH). Previously considered insignificant, 1H NMR indicates that the speciation equilibrium is sensitive to pressure and correlates positively with Si-OH concentration. The significantly different reactivity of the isotope substituted hydrous species results in large molecular variations in the D/H ratio inside the sodium silicate melt. Measured intramolecular hydrogen isotope fractionation yields a 2- to 9-fold (in average 3-fold) enrichment of deuterium in Na-rich depolymerized regions in the silicate network, which translates to intramolecular fractionations of 1000ln(α) = 400 to 2200. Generally, intramolecular fractionation tends to increase when the melt evolves towards fluid-saturated conditions. The pre-concentration of deuterium in the depolymerized Na-rich regions of the silicate melt results in a higher local D/H ratio that may carry over when these regions disintegrate into a silicate saturated aqueous fluid. In this way, intramolecular isotope fractionation poses a potent way of enriching silicate saturated aqueous fluids in deuterium, potentially explaining previous experimental in-situ observations of large hydrogen isotope fractionations in the hydrous-melt – silicate-saturated fluid system. The observed affinity of deuterium to combine with alkali-associated silanols could comprise an important molecular sieving mechanism for deuterium that enhances the D/H fractionation between silicate-rich fluids (and hydrous melts) and the surrounding host rock. Consequently, intramolecular hydrogen isotope fractionation in hydrous silicate melts and silicate-rich aqueous fluids may be a very effective deuterium filter acting in the deep geological water cycle. [ABSTRACT FROM AUTHOR]

Methane clumped isotope analysis is a tool used to constrain the formation or equilibration temperatures of methane, or to differentiate methane of thermogenic, microbial or 'abiotic' origins. Geothermometry applications are based on the temperature dependence of relative abundances of multiply-substituted isotopologues in thermodynamic equilibrium, whereas assignments of biogenicity or 'abiogenicity' rely on kinetic isotope effects associated with synthesis, which disturb clumped isotope abundances away from expected equilibrium proportions. However, kinetic processes in thermogenesis or during post-generation storage of thermogenic gas may cause isotopic disequilibrium, confounding thermometry applications or leading to 'false positive' identifications of microbial or abiogenic gases. Non-equilibrated clumped isotope compositions have been observed in thermogenic gases including unconventional oil-associated gases and from coal pyrolysis experiments. The isotopic disequilibria might be caused by kinetic isotope effects expressed during gas migration (including extraction), or by irreversible chemical processes, such as breaking carbon–carbon bonds in an alkyl precursor. In this study, we performed controlled pyrolysis experiments at 400 °C on n-octadecane (C 18 H 38). We characterized the gas chemistry, and compound-specific carbon and hydrogen isotope and methane clumped isotope compositions of the gas products. We found that Δ13CH 3 D values (anomalies relative to a stochastic distribution of isotopes) appear to be relatively close to equilibrium at the experimental temperature, whereas Δ12CH 2 D 2 values are 30–40‰ lower than expected for equilibrium. The large deficit in Δ12CH 2 D 2 can be explained by assembling hydrogen atoms affected by two distinct kinetic isotope effects into a methane molecule, previously referred to as a 'combinatorial effect'. We present a kinetic model that describes the full isotopic systematics, including anomalous Δ12CH 2 D 2 deficits, of pyrolysis product methane. Finally, we propose a model for the isotope signatures of natural thermogenic methane where the non-equilibrium Δ12CH 2 D 2 composition is a signature of the onset of catagenetic methane production. Our model also describes ways in which this signature disappears as further maturation drives Δ12CH 2 D 2 to equilibrium through hydrogen exchange. Our findings demonstrate that anomalous depletion in Δ12CH 2 D 2 is not a unique signature for microbial or putative abiotic methane, and specifically, it can be generated during pyrolytic chemistry. [ABSTRACT FROM AUTHOR]

The Cedars ultramafic block hosts alkaline springs (pH > 11) in which calcium carbonate forms upon uptake of atmospheric CO 2 and at times via mixing with surface water. These processes lead to distinct carbonate morphologies with "floes" forming at the atmosphere-water interface, "snow" of fine particles accumulating at the bottom of pools and terraced constructions of travertine. Floe material is mainly composed of aragonite needles despite CaCO 3 precipitation occurring in waters with low Mg/Ca (<0.01). Precipitation of aragonite is likely promoted by the high pH (11.5–12.0) of pool waters, in agreement with published experiments illustrating the effect of pH on calcium carbonate polymorph selection. The calcium carbonates exhibit an extreme range and approximately 1:1 covariation in δ13C (−9 to −28‰ VPDB) and δ18O (0 to −20‰ VPDB) that is characteristic of travertine formed in high pH waters. The large isotopic fractionations have previously been attributed to kinetic isotope effects accompanying CO 2 hydroxylation but the controls on the δ13C-δ18O endmembers and slope have not been fully resolved, limiting the use of travertine as a paleoenvironmental archive. The limited areal extent of the springs (∼0.5 km2) and the limited range of water sources and temperatures, combined with our sampling strategy, allow us to place tight constraints on the processes involved in generating the systematic C and O isotope variations. We develop an isotopic reaction–diffusion model and an isotopic box model for a CO 2 -fed solution that tracks the isotopic composition of each dissolved inorganic carbon (DIC) species and CaCO 3. The box model includes four sources or sinks of DIC (atmospheric CO 2 , high pH spring water, fresh creek water, and CaCO 3 precipitation). Model parameters are informed by new floe Δ44Ca data (−0.75 ± 0.07‰), direct mineral growth rate measurements (4.8 to 8 × 10−7 mol/m2/s) and by previously published elemental and isotopic data of local water and DIC sources. Model results suggest two processes control the extremes of the array: (1) the isotopically light end member is controlled by the isotopic composition of atmospheric CO 2 and the kinetic isotope fractionation factor (KFF (‰) = (α − 1) × 1000) accompanying CO 2 hydroxylation, estimated here to be −17.1 ± 0.8‰ (vs. CO 2(aq)) for carbon and −7.1 ± 1.1‰ (vs. 'CO 2(aq) + H 2 O') for oxygen at 17.4 ± 1.0 °C. Combining our results with revised CO 2 hydroxylation KFF values based on previous work suggests consistent KFF values of −17.0 ± 0.3‰ (vs. CO 2(aq)) for carbon and −6.8 ± 0.8‰ for oxygen (vs. 'CO 2(aq) + H 2 O') over the 17–28 °C temperature range. (2) The isotopically heavy endmember of calcium carbonates at The Cedars reflects the composition of isotopically equilibrated DIC from creek or surface water (mostly HC O 3 - , pH = 7.8–8.7) that occasionally mixes with the high-pH spring water. The bulk carbonate δ13C and δ18O values of modern and ancient travertines therefore reflect the proportion of calcium carbonate formed by processes (1) and (2), with process (2) dominating the carbonate precipitation budget at The Cedars. These results show that recent advances in understanding kinetic isotope effects allow us to model complicated but common natural processes, and suggest ancient travertine may be used to retrieve past meteoric water δ18O and atmospheric δ13C values. There is evidence that older travertine at The Cedars recorded atmospheric δ13C that predates large-scale combustion of fossil fuels. [ABSTRACT FROM AUTHOR]

Microbial production and consumption of methane are widespread in natural and artificial environments, with important economic and climatic implications. Attempts to use the isotopic composition of methane to identify its sources are complicated by incomplete understanding of the mechanisms of variation in methane's isotopic composition. Knowledge of the equilibrium isotope fractionations among the large organic intracellular intermediates in the microbial pathways of methane production and consumption must form the basis of any exploration of the mechanisms of isotopic variation, but estimates of these equilibrium isotope fractionations are currently unavailable. To address this gap, we calculated the equilibrium isotopic fractionation of carbon ( 13 C/ 12 C) and hydrogen (D/H) isotopes among compounds in the anaerobic methane metabolisms, as well as the abundance of double isotope substitutions ("clumping," i.e., a single 13 C–D bond or two 12 C–D bonds) in these compounds. The isotope fractionation factors were calculated using density functional theory at the M06-L/def2-TZVP level of theory with the SMD implicit solvation model, which we recently tested against measured equilibrium isotope fractionations. The computed 13 β and 2 β values decrease with decreasing oxidation state of the carbon atom in the molecules, resulting in a preference for enrichment in 13 C and D in the molecules with more oxidized carbon. Using the computed β values, we calculated the equilibrium isotope fractionation factors in the prominent methanogenesis pathways (hydrogenotrophic, methylotrophic and acetoclastic) and in the pathway for anaerobic oxidation of methane (AOM) over a temperature range of 0–700 °C. Our calculated equilibrium fractionation factors compare favorably with experimental constraints, where available, and were then used to investigate the relation between the apparent isotope fractionation during methanogenesis or AOM and the thermodynamic drive for these reactions. We show that a detailed map of the equilibrium fractionation factors along these metabolic pathways allows for an evaluation of the contribution of equilibrium and kinetic isotope effects to apparent isotope fractionations observed in laboratory, natural and artificial settings. The comprehensive set of equilibrium isotope fractionation factors calculated in this study provides a firm basis for future explorations of isotope effects in methane metabolism. [ABSTRACT FROM AUTHOR]

Redox transformations of nitrogen (N) play a critical role in determining its speciation and biological availability, thus defining the magnitude and extent of productivity in many ecosystems. A range of important nitrogen transformations often co-occur in regions hosting other redox-active elements, including sulfur, iron, and manganese (Mn), especially along sharp redox gradients within aquatic sediments. This proximity in "redox real estate" produces conditions under which multi-element interactions and coupled cycling are thermodynamically favored. While previous work has reported anoxic nitrification linked to the presence of manganese (Mn) oxides in sediments, a clear connection between the cycling of Mn and N has remained elusive. Soluble Mn(III), which is stabilized via ligand-complexation, has recently been shown to represent the dominant dissolved Mn species in many environments. Here, we examined the reactivity of ligand-stabilized Mn(III) with nitrite, using natural abundance stable nitrogen and oxygen isotopes to explore reaction dynamics under a range of conditions. Oxidation of nitrite to nitrate by Mn(III)-pyrophosphate proceeded abiotically under both oxygen replete and nitrogen-purged conditions. Kinetics and isotope systematics of this reaction were measured over a range of pH (5–8), with reaction rates decreasing with increasing pH. Under all treatments, an inverse kinetic isotope effect of −19.9 ± 0.7‰ was observed for N, remarkably similar to previously documented fractionation by nitrite-oxidizing organisms. Experiments using 18O-labeled water confirmed that the source of the additional oxygen atom was from water. These findings suggest that nitrite oxidation in environments hosting abundant ligand-bound Mn(III), including porewaters, estuaries, and coastal waters, may be facilitated in part by abiotic reactions with Mn, even under functionally anoxic conditions. [ABSTRACT FROM AUTHOR]

Kinetic isotope effect (KIE) reveals the transition state structure of an elementary reaction and can be predicted by quantum chemical calculation. Density Functional Theory calculation of an enzymatic reaction with large numbers of atoms is computationally prohibitive, especially if explicit solvent effect is considered. Cutoff method, which simplifies an entire molecule to a cluster around a target position, can simplify position-specific equilibrium isotope effect calculation for a large organic molecule. It should also be applicable to KIE calculation of an enzymatic reaction and allow us to introduce explicit solvent molecules to the system. If this treatment is feasible and trustable, it will provide an efficient method to estimate a number of KIEs produced by enzyme reactions. Obviously, its robustness must be tested. Here, using NO 3 − reduction by the active site of periplasmic dissimilatory nitrate reductase (Nap) in Rhodobacter sphaeroides as an example, we built 17 models to test the influence of cutoff size, implicit, and explicit solvent effects on KIE calculation. The results show that to estimate the KIE value of an enzymatic reaction accurately and efficiently, we can first simplify the reaction model to a cutoff model with 3 proximal bonds to the active position. Then, incorporating implicit-plus-explicit solvent models can simulate a reaction environment more realistically, which is necessary for accuracy. Our calculated ln KIE values for nitrate Nap reduction at 25 °C are −32.4 ± 1.8‰ for 15N and −20.9 ± 0.4 ‰ for 18O, respectively, with a ln18 KIE /ln15 KIE ratio of 0.65 ± 0.05. Although additional reservoir-transport processes need to be considered, our calculation results are consistent with calibrated isotope effects from laboratory experiments, suggesting that the transition state we calculated depicts the general reaction mechanism of NO 3 − reduction by Nap. [ABSTRACT FROM AUTHOR]

Chromium stable isotope ratios are used to trace the reduction of Cr(VI) to Cr(III) in both ancient and modern systems. However, quantitative interpretation of Cr isotopic signatures has been stymied by the large variability in isotopic fractionation factors for Cr(VI) reduction by different reductants. Here we determine Cr isotope fractionation factors during Cr(VI) reduction by Fe(II/III)-bearing clay minerals, which are abundant in subsurface environments. Several variables were tested: pH, total Fe content of the clay, and the fraction of reduced Fe within the clay (Fe(II)/Fe(total)). The latter controls the standard reduction potential of the clay. Our results demonstrate that neither pH nor total Fe content of the clay have major effects on isotopic fractionation. In contrast, as the effective standard reduction potential of the clay and thus the standard free energy of Cr(VI) reduction become more negative, Cr isotope fractionation factors decrease in magnitude from −4.9 to −1.3‰ according to a linear free energy relationship. This linear free energy relationship can be predicted from Marcus electron transfer theory and allows first-order predictions of Cr isotope fractionation factors to be made from the standard reduction potential or Fe(II)/Fe(total) of a clay, potentially improving our ability to model Cr isotope signatures in geochemical systems. Chromium is the first isotope system to show such a linear free energy relationship over a diverse range of reductants, including both aqueous and solid-phase reductants, and may provide a model for determining other redox-driven kinetic isotope effects in environmentally important isotope systems. [ABSTRACT FROM AUTHOR]

The formation of H 2 CO 3 and HCO 3 − from CO 2 is common in aqueous environmental systems and occurs via the CO 2 hydration and hydroxylation reactions. The reactions are especially important in the context of CaCO 3 mineral precipitation. The reactions carry with them equilibrium and kinetic isotope fractionations that impact paleoclimate proxy records. Herein, quantum mechanical calculations of dissolved inorganic carbon species embedded in H 2 O clusters are analyzed under a transition state theory framework to predict kinetic isotope fractionations associated with the reactions. Experimental measurements of reaction energetics and equilibrium isotope fractionations are well-reproduced by the models which suggests the computational methods are justified. The CO 2 hydration reaction discriminates against 13C by 19–23‰ and against 18O by 14–15‰ at 25 °C. The CO 2 hydroxylation reaction discriminates against 13C by 26–31‰ and against 18O by 27–30‰ at 25 °C. Our analyses of the model isotopic fractionations lead to the conclusion that hydrogen-bonds and C(CO 2)-O(H 2 O or OH−) distances influence the calculated values. We infer that when the model H-bond patterns more realistically represent the aqueous species, the accuracy and precision of equilibrium fractionation factor prediction is improved. Implications for coral carbonate paleothermometry are discussed. Results indicate that hydration and hydroxylation δ13C-δ18O slopes bound the values of slopes form corals and abiotic precipitation experiments, albeit with key uncertainties related to precipitation fractionations. The process of regressing molecular model fractionation results against H-bond patterns as presented here is a promising way to improve isotope fractionation calculations in aqueous systems. [ABSTRACT FROM AUTHOR]

Intramolecular isotope distributions, including isotope clumping and position specific fractionation, can provide proxies for the formation temperature and formation and destruction pathways of molecules. In this study, we explore the position-specific hydrogen isotope distribution in propane. We analyzed propane samples from 10 different petroleum systems with high-resolution molecular mass spectrometry. Our results show that the hydrogen isotope fractionation between central and terminal positions of natural propanes ranges from −102‰ to +205‰, a much larger range than that expected for thermodynamic equilibrium at their source and reservoir temperatures (36–63‰). Based on these findings, we propose that the hydrogen isotope structure of catagenic propane is largely controlled by irreversible processes, expressing kinetic isotope effects (KIEs). Kinetic control on hydrogen isotope composition of the products of thermal cracking is supported by a hydrous pyrolysis experiment using the Woodford Shale as substrate, in which we observed isotopic disequilibrium in the early stage of pyrolysis. We make a more general prediction of KIE signatures associated with kerogen cracking by simulating this chemistry in a kinetic Monte Carlo model for different types of kerogens. In contrast, unconventional shale fluids or hot conventional reservoirs contain propane with an isotopic structure close to equilibrium, presumably reflecting internal and/or heterogeneous exchange during high temperature storage (ca. 100–150 °C). In relatively cold (<100 °C) conventional gas accumulations, propane can discharge from its source to a colder reservoir, rapidly enough to preserve disequilibrium signatures even if the source rock thermal maturity is high. These findings imply that long times at elevated temperatures are required to equilibrate the hydrogen isotopic structure of propane in natural gas host rocks and reservoirs. We further defined the kinetics of propane equilibration through hydrogen isotope exchange experiments under hydrous conditions; these experiments show that hydrogen in propane is exchangeable over laboratory timescales when exposed to clay minerals such as kaolinite. This implies rather rapid transfer of propane from sources to cold reservoirs in some of the conventional petroleum systems. Propane is also susceptible to microbial degradation in both oxic and anoxic environments. Biodegradation of propane in the Hadrian and Diana Hoover oil fields (Gulf of Mexico) results in strong increases in central-terminal hydrogen isotope fractionation. This reflects preferential attack on the central position, consistent with previous studies. [ABSTRACT FROM AUTHOR]

Disequilibrium isotopic compositions in carbonate minerals often reflect the integration of several different kinetic isotope effects (KIEs), whose relative contribution to the overall composition depends on the specific mineral formation process and environment. Thus, potential environmental reconstructions from disequilibrium compositions in natural carbonates require (i) quantification of KIEs associated with reactions involved in mineral formation, and (ii) a theoretical framework linking physical properties of natural environments to the expression and preservation of these KIEs in carbonates. To constrain KIEs associated with carbonate mineral precipitation reactions, we performed a series of rapid witherite (BaCO 3) precipitation experiments over a range of pH (8.7–13.0), temperature (15–40 °C) and fractional yield of the dissolved inorganic carbon (DIC; a few percent to quantitative precipitation). Our experiments extend the range of pH and temperature explored in previous studies, and include measurements of both carbon and oxygen isotopes. We developed a dynamic model of the DIC system, with which we simulated the experiments. The model results identify isotopic distillation due to formation of aqueous CO 2 and mineral precipitation by either CO 3 2– or HCO 3 – as the main determinants of the carbon and oxygen isotopic evolution of the solid and of the solution. We estimate that the carbon kinetic fractionation factor (KFF) associated with unidirectional precipitation of witherite via CO 3 2–, 1000 l n 13 α C O 3 2 - → B a C O 3 k + 1 , is 0.5 ± 0.1‰ (15–40 °C), whereas the oxygen KFF, 1000 l n 18 α C O 3 2 - → B a C O 3 k + 1 , is –1.0 ± 0.3‰ (15 °C) and –0.5 ± 0.2‰ (25–40 °C). The carbon KFF associated with unidirectional HCO 3 – precipitation pathways, 1000 l n 13 α H C O 3 - → B a C O 3 k + 2 , + 3 , is –0.2 ± 1.0‰ (15–40 °C), whereas the oxygen KFF, 1000 l n 18 α H C O 3 - → B a C O 3 k + 2 , + 3 , is –6.7 ± 1.1‰ (15 °C), –4.5 ± 1.1‰ (25 °C) and –4.0 ± 1.1‰ (40 °C). Our oxygen KFF of witherite precipitation from CO 3 2– agrees well with the available literature estimates at 25 °C. In addition, our carbon and oxygen KFFs are comparable to literature KFFs associated with calcite precipitation, possibly suggesting a similarity of precipitation KFFs among carbonate minerals. Importantly, the relative magnitudes and uncertainties of the precipitation KFFs and the equilibrium fractionation between CO 3 2– and HCO 3 –, leads to the expectation that in many natural settings, the isotopic composition of carbonate minerals be an insensitive probe of the precipitation pathway. [ABSTRACT FROM AUTHOR]

Kinetic isotope effects (KIEs) during the hydration of carbon dioxide (CO 2) in aqueous solution are key to understanding various geochemical, physicochemical, and biological processes. Despite the growing interest in KIEs in natural systems (e.g., speleothem calcites, skeletal formation of corals, formation of cryogenic carbonates, and more), the currently available experimental data is limited to only a few studies. Here, we report results from laboratory experiments to constrain kinetic isotope fractionation (KIF) during the hydration of CO 2. The experimental setup consisted of a stock solution vessel containing a NaHCO 3 solution, a reaction chamber in which BaCO 3 was rapidly precipitated from a Tris-buffered BaCl 2 solution, and a pump to internally circulate gas throughout the system that was isolated from the ambient laboratory air. Resultant BaCO 3 samples, as well as the NaHCO 3 and deionized H 2 O used for our experiments were analyzed for stable carbon and oxygen isotopes by mass spectrometry. Our experimental results were grouped into four different batches, which were obtained during discrete time periods, while some adjustments to the experimental setup and protocols were implemented to improve the data quality. We discuss possible experimental errors among the four batches and conclude that Batch-4 provided the most reliable results. The average δ13C and δ18O values of the Batch-4 BaCO 3 samples produced at 25 °C and pH NBS = 8.0 are −29.7 ± 0.7‰ (VPDB) and 18.8 ± 0.6‰ (VSMOW), respectively, which is strongly depleted relative to gaseous CO 2 in the system (δ13C VPDB ≅ −10.7‰, δ18O VSMOW ≅ 37.9‰). From the isotope data, we calculate our experimental 13KIF and 18KIF, which refer to the 13C/12C and 18O/16O fractionation between CO 2(g) and BaCO 3 , where the δ13C and δ18O values of CO 2(g) were calculated using known equilibrium fractionation factors. From the mean of Batch-4 results, we conclude 13KIF = 19.6 ± 0.8‰ and 18KIF = 18.8 ± 0.6‰ at 25 °C and pH NBS = 8.0. If full carbon KIF of ∼ 2.0‰ is assumed between CO 2(g) and CO 2(aq) , then the mean 13KIF relative to CO 2(aq) is 17.6 ± 0.8‰. Our KIFs are the largest values compared to previously reported experimental KIFs (except for one study), suggesting that our values are closest to the full isotope disequilibrium during CO 2 hydration, as any partial re-equilibration tends to reduce observed KIFs. [ABSTRACT FROM AUTHOR]

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

Selenium (Se) isotope fractionation has been widely used for constraining redox conditions and microbial processes in both modern and ancient environments, but our knowledge of the controls on fractionation during microbial reduction of Se-oxyanions is based on a limited number of studies. Here we complement and expand the currently available pure culture data for Se isotope fractionation by investigating for the first time six phylogenetically diverse, mesophilic, and non-respiring bacterial strains that reduce Se-oxyanions to elemental Se [Se(0)]. Experiments were performed with either selenate [Se(VI)] or selenite [Se(IV)] at lower, more environmentally-relevant Se (9–47 μM) and carbon (500 μM) concentrations than previously investigated. Enterobacter cloacae SLD1a-1, Desulfitobacterium chlororespirans Co23 and Desulfitobacterium sp. Viet-1 were incubated with Se(VI) and Se(IV). Geobacter sulfurreducens PCA, Anaeromyxobacter dehalogenans FRC-W and Shewanella sp. (NR) were examined for their ability reducing Se(IV) to Se(0). Our data confirm that microbial reduction of both Se-oxyanions is accompanied by large kinetic isotopic fractionation (reported as 82/76 ε = 1000×(82/76α-1) in ‰). Under our experimental conditions, microbial reduction of Se(VI) shows consistently greater isotope fractionation (ε = −9.2‰ to −11.8‰) than reduction of Se(IV) (ε = −6.2 to −7.8‰) confirming the difference in metabolic pathways for the reduction of the two Se-oxyanions. For Se(VI), an inverse relationship between normalized cell specific reduction rate (cSRR) and Se isotope fractionation suggests that the kinetic isotope effect for Se(VI) reduction is governed by an enzymatically-specific pathway related to the bacterial strain-specific physiology. In contrast, the lack of correlation between normalized cSRR and isotope fractionation for Se(IV) reduction indicates a non-enzyme specific pathway which is dominantly extracellular. Our study highlights the importance to understand microbially-mediated Se isotope fractionation depending on Se species, and cell-specific reduction rates before Se isotope ratios can become a fully applicable tool to interpret Se isotopic changes in modern and ancient environments. [ABSTRACT FROM AUTHOR]

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

We present new nucleosynthetic, radiogenic and stable Sr isotopic data from fifteen previously studied CAIs from the Allende CV3 meteorite, including the highly altered Curious Marie inclusion. We use double-spike TIMS techniques to determine the degrees of isotopic mass fractionation, and also present internally normalised data for the same sample digestions to permit comparisons with previous studies and couple these isotopic data with Rb, Sr, Eu and Th abundance data to consider the origins and relationships of the isotopic variations documented here. Analysed CAIs display elevated μ84Sr anomalies of +58 ppm to +287 ppm, with variability far outside of analytical uncertainties (13 ppm 2 s.d.). We cannot tell at present whether these variations arise from heterogeneities in p -process 84Sr or in the other non-radiogenic isotopes of Sr (86Sr, 88Sr) that are produced by the main s -process, weak s -process, and r -process. All inclusions fall on an offset mass-dependent fractionation line in three-isotope space (δ88/86Sr vs δ84/86Sr) identical within error to that previously defined by bulk undifferentiated meteorites, and have a total range of δ88/86Sr of ∼5.3 ‰ (+1.67 ‰ to −3.67 ‰), reflecting kinetic isotope effects during partial condensation/evaporation and/or low-temperature alteration processes. CI-normalized Sr/Th ratios in our CAIs correlate with normalized Eu/Th ratios with a ∼ 1:1 relationship, regardless of texture or Sr-isotopic values. This indicates that Sr and Eu had similar condensation behaviors with Eu condensing as Eu2+ and having the same chemical behavior in minerals as Sr2+ under conditions relevant to CAI formation in the solar nebula. Rb/Th ratios are highly variable: fine-grained CAIs display elevated Rb/Th ratios, consistent with the introduction of Rb into the CAIs by alkali-rich secondary alteration fluids. The μ84Sr anomalies measured in our CAIs are similar (in magnitude) to those found in carbonaceous chondrites that formed in the outer part of the solar system. A way to reconcile this observation with the formation of CAIs near the Sun would be if the inventories of Sr and other refractory elements in carbonaceous chondrites are dominated by a cryptic refractory dust component (CRD) that was formed early and near the Sun, and was subsequently transported outwards to the carbonaceous chondrite-forming region. [ABSTRACT FROM AUTHOR]

The variation in the iron isotopic composition (δ56Fe) of sedimentary pyrite is often interpreted to reflect the degree of Fe redox cycling in modern and ancient environments. However, the degree to which precipitation pathways, isotopic exchange, and precipitation rates can affect the isotopic fractionation associated with pyrite precipitation from aqueous Fe(II) (Fe(II) aq) is poorly understood. In this study, pyrite is precipitated at 80 °C in batch reactors through the H 2 S and polysulfide pathways, in which the precipitation rates and the concurrent formation of a greigite (Fe 3 S 4) phase is modulated by the amount of initially added elemental sulfur and aqueous molybdenum. Our results indicate an average apparent isotopic fractionation (δ56Fe pyrite - δ56Fe FeSx , where FeS x includes FeS, Fe(II) aq , and greigite) of −0.51 ± 0.22‰ throughout the experiments irrespective of precipitation pathways and greigite formation. Early-stage precipitation is associated with ∼0.3‰ larger isotopic fractionation than late-stage precipitation, possibly indicating either a rate-dependent kinetic isotope effect (KIE) or a different isotopic fractionation factor for early-stage pyrite nucleation compared to later-stage growth. Overall, the magnitude of the apparent isotopic fractionation is significantly smaller than the <−2‰ isotopic fractionation determined in previous experiments (Guilbaud et al., 2011b). Numerical models indicate that isotopic exchange between pyrite and Fe(II) aq is necessary to explain the experimental data. The inferred rate of isotopic exchange decreases with time in our experiments, likely as a function of particle size, but shows no clear correlation with temperature across different studies. In the presence of isotopic exchange, modeling results indicate that pyrite precipitated from Fe(II) aq may theoretically have δ56Fe values ranging from −3 to + 4‰, which spans nearly the whole δ56Fe range observed in nature. Negative values reflect the expression of the KIE when isotopic exchange is slow (relative to net precipitation rate) while positive values reflect the expression of the equilibrium isotope effect (EIE) when isotopic exchange is relatively fast. We therefore propose that the variation in sedimentary pyrite δ56Fe can be explained in terms of varying expression of the KIE and the EIE, either during different stages of precipitation or as controlled by the availability of Fe(II), sulfide, and oxidants throughout Earth's history. The predominantly negative (but highly variable) pyrite δ56Fe values in modern marine sediments suggest a higher expression of the KIE in low temperature systems, but do not rule out the importance of isotopic exchange. The isotopic exchange rate is currently underconstrained in low temperature systems with an uncertainty range that spans 8 orders of magnitude. Our work suggests that isotopic exchange has the potential to affect sedimentary pyrite δ56Fe unless the current upper limit for isotopic exchange rate is overestimated by 5 orders of magnitude. [ABSTRACT FROM AUTHOR]

Abstract Paleoclimate reconstructions that use speleothem proxy data have increased our understanding of terrestrial climate change, but gaps remain in our understanding of in-cave processes that influence speleothem chemistry. The δ13C values of speleothem calcite are typically influenced by kinetic isotope effects that operate during CO 2 degassing and calcite precipitation. Therefore the identification and quantification of these isotopic effects is important in interpreting speleothem stable isotope records. Here we studied the change in water chemistry and δ13C values of dissolved inorganic carbon (DIC) along discreet flow paths at multiple drip sites in Inner Space Cavern, central Texas. We quantified the extent to which the water remains in C isotopic equilibrium during flow along speleothem surfaces as CO 2 degasses and calcite precipitates. Two locations in the study cave that have long in-cave flow paths were examined to determine the geochemical evolution and its driving processes along these paths. At each location cave water was sampled at two points 1–2 meters apart along each flow path. Among the key spatial changes observed is a <∼1‰ to ∼4‰ increase in δ13C values of DIC along the flow paths. The magnitude of the increase in δ13C values is controlled by the extent of DIC loss to CO 2 degassing. The extent of DIC loss and CO 2 degassing is controlled by the pCO 2 gradient between drip water and cave air. If the DIC loss is less than 15%, then the evolution of the δ13C value of the DIC reservoir can be accounted for by a Rayleigh distillation model with equilibrium C-isotope fractionation factors for (CO 2(g) -HCO 3 − (aq)) and (CaCO 3 -HCO 3 − (aq)). As the depletion of the DIC reservoir exceeds 15% the DIC δ13C values become progressively higher such that the (HCO 3 − (aq) -CO 2(g)) fractionation values needed to explain the observations change from equilibrium values of ∼8‰ to non-equilibrium values of up to ∼25‰. This variance in magnitude of C-isotope fractionation during CO 2 degassing cannot be attributed to changes in temperature, and thus we infer significant kinetic isotope effects at higher rates of DIC loss. Such kinetic effects have significant implications for speleothem C-isotope proxy interpretations, as these kinetic isotope effects are of a similar magnitude as those used to infer past changes in drought and vegetation. [ABSTRACT FROM AUTHOR]

The stable isotope compositions of biogenic carbonates have been used for paleoceanographic and paleoclimatic reconstructions for decades, and produced some of the most iconic records in the field. However, we still lack a fully mechanistic understanding of the stable isotope proxies, especially the biological overprint on the environmental signals termed “vital effects”. A ubiquitous feature of stable isotope vital effects in marine calcifying organisms is a strong correlation between δ 18 O and δ 13 C in a range of values that are depleted from inorganic calcite/aragonite. Two mechanisms have been proposed to explain this correlation, one based on kinetic isotope effects during CO 2 (aq)-HCO 3 − inter-conversion, the other based on equilibrium isotope exchange during pH dependent speciation of the dissolved inorganic carbon (DIC) pool. Neither mechanism explains all the stable isotope features observed in biogenic carbonates. Here we present a fully kinetic model of biomineralization and its isotope effects using deep-sea corals as a test organism. A key component of our model is the consideration of the enzyme carbonic anhydrase in catalyzing the CO 2 (aq)-HCO 3 − inter-conversion reactions in the extracellular calcifying fluid (ECF). We find that the amount of carbonic anhydrase not only modulates the carbonate chemistry of the calcifying fluid, but also helps explain the slope of the δ 18 O-δ 13 C correlation. Differences in CA activity in the biomineralization process can possibly explain the observed range of δ 18 O-δ 13 C slopes in different calcifying organisms. A mechanistic understanding of stable isotope vital effects with numerical models can help us develop better paleoceanographic tracers. [ABSTRACT FROM AUTHOR]

The clumped and stable isotope (Δ 47 , δ 18 O, and δ 13 C) composition of pedogenic (soil) carbonates from cold, arid environments may be a valuable paleoclimate archive for climate change-sensitive areas at high latitudes or elevations. However, previous work suggests that the isotopic composition of cold-climate soil carbonates is susceptible to kinetic isotope effects (KIE). To evaluate the conditions under which KIE occur in cold-climate soil carbonates, we examine the Δ 47 , δ 18 O, and δ 13 C composition of soil carbonate pendants from Antarctica (Dry Valleys, 77°S), the High Arctic (Svalbard 79°N), the Chilean and Argentinian Andes, and the Tibetan plateau (3800–4800 m), and compare the results to local climate and water δ 18 O records. At each site we calculate the expected equilibrium soil carbonate Δ 47 and δ 18 O values and estimate carbonate Δ 47 and δ 18 O anomalies (observed Δ 47 or δ 18 O minus the expected equilibrium Δ 47 or δ 18 O). Additionally, we compare the measured carbonate δ 13 C to the expected range of equilibrium soil carbonate δ 13 C values. To provide context for interpreting the Δ 47 and δ 18 O anomalies, the soil carbonate results are compared to results for sub-glacial carbonates from two different sites, which exhibit large Δ 47 anomalies (up to −0.29‰). The Antarctic and 4700 masl Chilean Andes samples have negative Δ 47 anomalies and positive δ 18 O anomalies consistent with KIE due to rapid bicarbonate dehydration during cryogenic carbonate formation. In contrast, the lower elevation Chilean Andes, Argentinian Andes, Tibetan Plateau and High Arctic results are consistent with equilibrium, summer carbonate formation. We attribute the differences in Δ 47 and δ 18 O anomalies to variations in inter-cobble matrix grain size and its effects on the effective soil pore space, permeability (hydraulic conductivity), moisture, and bicarbonate dehydration rate. The Antarctic and 4700 masl Chilean Andean soils have coarse-grained matrices that facilitate rapid bicarbonate dehydration. In contrast, the lower elevation Chilean Andes, Argentinian Andes, High Arctic and Tibetan Plateau soils have finer-grained matrices that decrease the soil pore space, soil permeability and CO 2 gas flux, promoting equilibrium carbonate formation. The sub-glacial carbonate samples yield highly variable Δ 47 and δ 18 O anomalies, and we propose that the differences between the two glacier sites may be due to variations in local sub-glacial drainage conditions, pCO 2 , and pH. Our findings suggest that carbonates from soils with coarse-grained matrices may exhibit KIE in cold climates, making them poor paleoclimate proxies. Soils with fine-grained matrices are more likely to yield equilibrium carbonates suitable for paleoclimate reconstructions regardless of climate. Paleosol matrix grain size should therefore be taken into account in the evaluation of carbonate stable and clumped isotope values in paleoclimate studies. [ABSTRACT FROM AUTHOR]

Ammonia degassing is a common process in natural alkaline waters and in the atmosphere. To quantitatively assess the nitrogen cycle in these systems, the essential parameter of nitrogen isotope fractionation factors associated with ammonia degassing is required, but still not constrained yet. In this study, we carried out laboratory experiments to examine the nitrogen isotope behavior during ammonia degassing in alkaline conditions. The experiments started with ammonium sulfate solution with excess sodium hydroxide. The reaction can be described as: NH 4 +  + OH − (excess) → NH 3 ·nH 2 O → NH 3 (g)↑. Two sets of experiments, one with ammonia degassing under static conditions and the other with ammonia degassing by bubbling of N 2 gas, were carried out at 2, 21, 50, and 70 °C. The results indicate that kinetic isotopic effects are dominated during efficient degassing of ammonia in the bubbling experiments, which yielded kinetic nitrogen isotope fractionation factors α NH3(g)–NH3(aq) of 0.9898 at 2 °C, 0.9918 at 21 °C, 0.9935 at 50 °C and 0.9948 at 70 °C. These values show a good relationship with temperature as 10 3 ln α NH3(g)–NH3(aq)  = 14.6 − 6.8 × 1000/T. In contrast, isotopic effects during less efficient degassing of ammonia in the static experiments are more complicated. The results do not match either kinetic isotope fractionation or equilibrium isotope fractionation but sit between these two. The most likely cause is that back dissolution of the degassed ammonia occurred in these experiments and consequently shifted kinetic isotope fractionation toward equilibrium isotope fractionation. Our experimental results highlight complicated isotopic effects may occur in natural environments, and need to be fully considered in the interpretation of field data. [ABSTRACT FROM AUTHOR]

CO 2 (de)hydration (i.e., CO 2 hydration/HCO 3 − dehydration) and (de)hydroxylation (i.e., CO 2 hydroxylation/HCO 3 − dehydroxylation) are key reactions in the dissolved inorganic carbon (DIC) system. Kinetic isotope effects (KIEs) during these reactions are likely to be expressed in the DIC and recorded in carbonate minerals formed during CO 2 degassing or dissolution of gaseous CO 2 . Thus, a better understanding of KIEs during CO 2 (de)hydration and (de)hydroxylation would improve interpretations of disequilibrium compositions in carbonate minerals. To date, the literature lacks direct experimental constraints on most of the oxygen KIEs associated with these reactions. In addition, theoretical estimates describe oxygen KIEs during separate individual reactions. The KIEs of the related reverse reactions were neither derived directly nor calculated from a link to the equilibrium fractionation. Consequently, KIE estimates of experimental and theoretical studies have been difficult to compare. Here we revisit experimental and theoretical data to provide new constraints on oxygen KIEs during CO 2 (de)hydration and (de)hydroxylation. For this purpose, we provide a clearer definition of the KIEs and relate them both to isotopic rate constants and equilibrium fractionations. Such relations are well founded in studies of single isotope source/sink reactions, but they have not been established for reactions that involve dual isotopic sources/sinks, such as CO 2 (de)hydration and (de)hydroxylation. We apply the new quantitative constraints on the KIEs to investigate fractionations during simultaneous CaCO 3 precipitation and HCO 3 − dehydration far from equilibrium. [ABSTRACT FROM AUTHOR]

The oxygen isotope ratio (δ 18 O) of inorganic and biogenic carbonates is widely used to reconstruct past environments. However, the oxygen isotope exchange between CaCO 3 and H 2 O rarely reaches equilibrium and kinetic isotope effects (KIE) commonly complicate paleoclimate reconstructions. We present a comprehensive model of kinetic and equilibrium oxygen isotope fractionation between CaCO 3 and water ( α c / w ) that accounts for fractionation between both (a) CaCO 3 and the CO 3 2− pool ( α c / CO 3 2 - ) , and (b) CO 3 2− and water ( α CO 3 2 - / w ) , as a function of temperature, pH, salinity, calcite saturation state (Ω), the residence time of the dissolved inorganic carbon (DIC) in solution, and the activity of the enzyme carbonic anhydrase. The model results suggest that: (1) The equilibrium α c / w is only approached in solutions with low Ω (i.e. close to 1) and low ionic strength such as in the cave system of Devils Hole, Nevada. (2) The sensitivity of α c / w to the solution pH and/or the mineral growth rate depends on the level of isotopic equilibration between the CO 3 2− pool and water. When the CO 3 2− pool approaches isotopic equilibrium with water, small negative pH and/or growth rate effects on α c / w of about 1–2‰ occur where these parameters covary with Ω. In contrast, isotopic disequilibrium between CO 3 2− and water leads to strong (>2‰) positive or negative pH and growth rate effects on α CO 3 2 - / w (and α c / w ) due to the isotopic imprint of oxygen atoms derived from HCO 3 − , CO 2 , H 2 O and/or OH − . (3) The temperature sensitivity of α c / w originates from the negative effect of temperature on α CO 3 2 - / w and is expected to deviate from the commonly accepted value (−0.22 ± 0.02‰/°C between 0 and 30 °C; Kim and O’Neil, 1997) when the CO 3 2− pool is not in isotopic equilibrium with water. (4) The model suggests that the δ 18 O of planktic and benthic foraminifers reflects a quantitative precipitation of DIC in isotopic equilibrium with a high-pH calcifying fluid, leading to a relatively constant foraminifer calcite δ 18 O-temperature relationship (−0.21 ± 0.01‰/°C). The lower average coral δ 18 O data relative to foraminifers and other calcifiers is best explained by the precipitation of internal DIC derived from hydrated CO 2 in a high-pH calcifying fluid and minimal subsequent DIC-H 2 O isotopic equilibration. This leads to a reduced and variable coral aragonite δ 18 O-temperature relationship (−0.11 to −0.22‰/°C). Together, the model presented here reconciles observations of oxygen isotope fractionation over a range of CaCO 3 -DIC-H 2 O systems. [ABSTRACT FROM AUTHOR]

The equilibrium isotope fractionation factor α eq is a fundamental parameter in the study of stable isotope effects. Experimentally, it has been difficult to establish that a system has attained equilibrium. The three-isotope method, using the initial trajectory of changing isotope ratios (e.g. 16 O, 17 O, and 18 O) to deduce the final equilibrium point of isotope exchange, has long been hailed as the most rigorous experimental approach. However, over the years some researchers have cautioned on the limitations of this method, but the foundation of three-isotope method has not been properly examined and the method is still widely used in calibrating α eq for both traditional and increasingly non-traditional isotope systems today. Here, using water-water and dissolved CO 2 -water oxygen exchange as model systems, we conduct an isotopologues-specific kinetic analysis of the exchange processes and explore the underlying assumptions and validity of the three-isotope method. We demonstrate that without knowing the detailed exchange kinetics a priori the three-isotope method cannot lead to a reliable α eq . For a two-reservoir exchanging system, α determined by this method may be α eq , kinetic isotope effect, or apparent kinetic isotope effect, which can all bear different values. When multiple reservoirs exist during exchange, the evolving trajectory can be complex and hard to predict. Instead of being a tool for α eq determination, three-isotope method should be used as a tool for studying kinetic isotope effect, apparent kinetic isotope effect , and detailed exchange kinetics in diverse systems. [ABSTRACT FROM AUTHOR]

A series of experiments were carried out to determine the clumped ( 13 CH 3 D) methane kinetic isotope effects during oxidation of methane by OH and Cl radicals, the major sink reactions for atmospheric methane. Experiments were performed in a 100 L quartz photochemical reactor, in which OH was produced from the reaction of O( 1 D) (from O 3 photolysis) with H 2 O, and Cl was from photolysis of Cl 2 . Samples were taken from the reaction cell and analyzed for methane ( 12 CH 4 , 12 CH 3 D, 13 CH 4 , 13 CH 3 D) isotopologue ratios using tunable infrared laser direct absorption spectroscopy. Measured kinetic isotope effects for singly substituted species were consistent with previous experimental studies. For doubly substituted methane, 13 CH 3 D, the observed kinetic isotope effects closely follow the product of the kinetic isotope effects for the 13 C and deuterium substituted species (i.e., 13,2 KIE = 13 KIE × 2 KIE). The deviation from this relationship is 0.3‰ ± 1.2‰ and 3.5‰ ± 0.7‰ for OH and Cl oxidation, respectively. This is consistent with model calculations performed using quantum chemistry and transition state theory. The OH and Cl reactions enrich the residual methane in the clumped isotopologue in open system reactions. In a closed system, however, this effect is overtaken by the large D/H isotope effect, which causes the residual methane to become anti-clumped relative to the initial methane. Based on these results, we demonstrate that oxidation of methane by OH, the predominant oxidant for tropospheric methane, will only have a minor (∼0.3‰) impact on the clumped isotope signature (Δ 13 CH 3 D, measured as a deviation from a stochastic distribution of isotopes) of tropospheric methane. This paper shows that Δ 13 CH 3 D will provide constraints on methane source strengths, and predicts that Δ 12 CH 2 D 2 can provide information on methane sink strengths. [ABSTRACT FROM AUTHOR]

Various iron and stony-iron meteorites have been characterized for their Ni and Fe isotopic compositions using multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS) after sample digestion and chromatographic separation of the target elements in an attempt to further constrain the planetary differentiation processes that shifted these isotope ratios and to shed light on the formational history and evolution of selected achondrite parent body asteroids. Emphasis was placed on spatially resolved isotopic analysis of iron meteorites, known to be inhomogeneous at the μm to mm scale, and on the isotopic characterization of adjacent metal and silicate phases in main group pallasites (PMG), mesosiderites, and the IIE and IAB complex silicate-bearing iron meteorites. In a 3-isotope plot of 60/58 Ni versus 62/58 Ni, the slope of the best-fitting straight line through the laterally resolved Ni isotope ratio data for iron meteorites reveals kinetically controlled isotope fractionation ( β exper = 1.981 ± 0.039, 1 SD), predominantly resulting from sub-solidus diffusion (with the fractionation exponent β connecting the isotope fractionation factors, as α 62 / 58 = α 60 / 58 β ). The observed relation between δ 56/54 Fe and Ir concentration in the metal fractions of PMGs and in IIIAB iron meteorites indicates a dependence of the bulk Fe isotopic composition on the fractional crystallization of an asteroidal metal core. No such fractional crystallization trends were found for the corresponding Ni isotope ratios or for other iron meteorite groups, such as the IIABs. In the case of the IIE and IAB silicate-bearing iron meteorites, the Fe and Ni isotopic signatures potentially reflect the influence of impact processes, as the degree of diffusion-controlled Ni isotope fractionation is closer to that of Fe compared to what is observed for magmatic iron meteorite types. Between the metal and olivine counterparts of pallasites, the Fe and Ni isotopic compositions show clearly resolvable differences, similar in magnitude but opposite in sign (Δ 56/54 Fe met-oliv of +0.178 ± 0.092‰ and Δ 60/58 Ni met-oliv of −0.212 ± 0.082‰, 2SD). As such, the heavier Fe isotope ratios for the metal (δ 56/54 Fe = +0.023‰ to +0.247‰) and lighter values for the corresponding olivines (δ 56/54 Fe = −0.155‰ to −0.075‰) are interpreted to reflect later-stage Fe isotopic re-equilibration between these phases, rather than a pristine record of mantle-core differentiation. In the case of mesosiderites, the similarly lighter Ni and Fe isotopic signatures found for the silicate phase (−0.149‰ to +0.023‰ for δ 60/58 Ni, −0.214‰ to −0.149‰ for δ 56/54 Fe) compared to the metal phase (+0.168‰ to +0.191‰ for δ 60/58 Ni, +0.018‰ to +0.120‰ for δ 56/54 Fe) likely result from Fe and Ni diffusion. Overall, the Fe and Ni isotopic compositions of iron-rich meteorites reflect multiple, often superimposed, processes of equilibrium or kinetic nature, illustrating convoluted parent body histories and late-stage interaction between early-formed planetesimal reservoirs. [ABSTRACT FROM AUTHOR]

Small differences in triple isotope relationships, or Δ 17 O in the case of oxygen, have been increasingly used to study a range of problems including hydrological cycles, stratosphere-troposphere exchange, biogeochemical pathways and fluxes, and the Moon’s origin in the geochemical and cosmochemical communities. A Δ 17 O value depends on the triple isotope exponent θ of involved reaction steps. However, the probabilistic distribution of the intrinsic and apparent θ values has not been examined for elemental processes and for processes that are out of equilibrium or bearing reservoir-transport complexities. A lack of knowledge on the confines of θ may hamper our understanding of the subtle differences among mass-dependent processes and may result in mischaracterization of a set of mass-dependent processes as being in violation of mass-dependent rules. Here we advocate a reductionist approach and explore θ confines starting from kinetic isotope effects ( KIE s) within the framework of transition state theory (TST). The advantage of our KIE approach is that any elemental or composite, equilibrium or non-equilibrium process can be reduced to a set of KIE s with corresponding θ KIE . We establish that the KIE between a reactant and a transition state (TS) is intrinsic. Given a range of KIE s known for Earth processes involving oxygen, we use a Monte Carlo calculation method and a range of oxygen-bonded molecular masses to obtain a distribution of θ KIE values and subsequently that of θ eq . Next, complexities are examined by looking into expected effects due to reaction progress, unbalanced fluxes, and reference frame. Finally, compounded reservoir-transport effects are examined using two simple processes – Rayleigh Distillation (RD) and Fractional Distillation (FD). Our results show that the apparent θ values between two species or two states of the same evolving species have much broader confines than the commonly used “canonical” confines of 0.51–0.53, particularly when the overall fractionation factors are close to 1.000. Equilibrium processes exhibit the narrowest α-θ value distribution. More complex processes or non-equilibrium further broaden the confines of the apparent θ values for reaction systems. The compounded reservoir-transport effects of RD and FD demonstrate that non-canonical apparent θ and large non-zero Δ 17 O values are achievable even when all involved elemental steps are strictly mass-dependent. This study calls for a research effort to determine KIE and θ KIE for important natural processes, and for cautions in interpreting a slope value drawn in a δ′ 17 O–δ′ 18 O space, as the slope is only a superficial manifestation of a set of complex reaction pathways and dynamics. [ABSTRACT FROM AUTHOR]

There is growing evidence that speleothem calcite grows out of isotopic equilibrium with cave drip water, with clumped isotope analysis providing a sensitive indicator for disequilibrium. This disequilibrium is primarily the result of CO 2 degassing from a thin film of water, leading to irreversible 13 C enrichment and reversible 18 O enrichment and Δ 47 depletion. Here we examine isotopic disequilibrium in Soreq Cave (Israel) using multiple modern-day and late Holocene speleothems. The variability observed in Δ 47 is small, within the analytical uncertainty, but the Δ 47 -derived temperature is offset from the modern cave temperature by ∼4 °C, reflecting degassing related disequilibrium that is fairly constant spatially. δ 18 O is more heterogeneous, reflecting short-term variability in drip water δ 18 O w combined with variability in the speleothem growth rates and related fractionation between dissolved carbonate species and the growing calcite mineral. This complexity, however, is markedly reduced by spatial or temporal averaging, enabling an interpretation of the cave paleoclimate record. We examine the Soreq Cave speleothems through a comparison with 2 types of thermometers: one is based on CaCO 3 precipitation from a bulk solution and is typically used for calibration of the Δ 47 and δ 18 O thermometers; a second that is based on CaCO 3 precipitating at the surface of the solution thus focusing and amplifying the thin film characteristics of speleothem formation. Soreq Cave speleothems are intermediate between these thermometers, providing a cave-specific thermometer calibration. [ABSTRACT FROM AUTHOR]

Laboratory precipitation experiments provide the basis for the common calibration of both the oxygen isotope and the clumped isotope thermometers. These focus on CaCO 3 crystals that form deep in the bulk of the solution, often by the bubbling of N 2(g) through a saturated Ca(HCO 3 ) 2 solution, following the classic experiments on McCrea (1950). Here we examine oxygen and clumped isotopes in CaCO 3 that was precipitated at the surface of a solution that undergoes passive CO 2 degassing. This CaCO 3 is affected by enhanced disequilibrium that is associated with degassing, and has therefore higher δ 18 O and lower Δ 47 values than those of the accepted thermometer calibrations. These offsets from the common thermometers calibrations increase with decreasing temperature for Δ 47 but decrease with decreasing temperature for δ 18 O. The Δ 47 -δ 18 O co-variance is therefore strongly temperature dependent, in contrast to theoretical predictions. This suggests an influence of additional fractionation in fast growing minerals, between DIC and calcite, that affects δ 18 O and counteracts some of the degassing related enrichment. This effect seems not to influence Δ 47 . The physical setup of CaCO 3 minerals growing at the solution surface is analogous to the degassing and precipitation processes that occur in stalagmites, such that these laboratory experiments may help shed light on the isotopic disequilibrium that is observed in speleothems. [ABSTRACT FROM AUTHOR]

Abstract: We modeled the Δ17O of atmospheric O2 using 27 ordinary differential equations comprising a box model composed of the stratosphere, troposphere, geosphere, hydrosphere and biosphere. Results show that 57% of the deficit in 17O in O2 relative to a reference water fractionation line is the result of kinetic isotope fractionation attending the Dole effect, 33% balances the positive Δ17O of O(1D) in the stratosphere, and 10% is from evapotranspiration. The predicted Δ′17O O2 relative to waters is −0.410‰ as measured at the δ18O of air. The value for Δ′17O O2 varies at fixed δ18O with the concentration of atmospheric CO2, gross primary production, and net primary production as well as with reaction rates in the stratosphere. Our model prediction is consistent with our measurements of the oxygen isotopic composition of air O2 compared with rocks if rocks define a fractionation line with an intercept in δ′17O=103ln(δ17O/103 +1) vs. δ′18O=103ln(δ18O/103 +1) space less than SMOW but more positive than some recent measurements imply. The predicted Δ17O is less negative than that obtained from recent measurements of O2 directly against SMOW. Underestimation of Δ′17O O2 can only be ameliorated if the integrated (bulk) Δ′17O for stratospheric CO2 is significantly greater than measurements currently allow. Our results underscore the need for high-precision comparisons of the 17O/16O and 18O/16O ratios of atmospheric O2, VSMOW, and rocks. [Copyright &y& Elsevier]

Abstract: In isotope geochemistry, the Rayleigh equation describes the evolution of isotope ratios in a parent compound as a function of reaction progress, and associated equations describe isotope ratios in an instantaneous product and an accumulated product. The Rayleigh equation is commonly used for fitting fractionation factors of processes undergoing kinetic isotope fractionation such as biochemical reactions. This work extends the equations associated with the Rayleigh equation for describing the isotope ratios in intermediate products in a chain of reacting species degrading with first-order kinetics. A general solution is presented for decay chains of any length, and explicit examples are presented for the biodegradation of a substrate or a mixture of substrates through 3 intermediate products to a final product. Applications of these analytical solutions for the fitting of enrichment factors for intermediate compounds in laboratory experiments are demonstrated with a spreadsheet. This avoids separate experiments to measure each intermediate product. The utility of the equations for the assessment of slopes in dual isotope plots is furthermore illustrated, and limitations of its use are critically discussed. [Copyright &y& Elsevier]