Rate Of Isotopic Exchange Research Articles

2-Pyridone-Enhanced Mn-Catalysis for the Synthesis of ortho-Deuterated Aromatic Nitriles.

We herein present a method for the synthesis of ortho-deuterated aryl nitrile using Mn(CO)5Br as catalyst with CH3OD as deuteratium source, where the structure of aryl imidates is used for interconversion with a cyanide group. This method features a broad substrate range and excellent functional group tolerance, allowing late modification of various complex molecules with good yields and deuterium incorporation. Mechanistic studies suggest that 2-pyridone is crucial to the success of this chemistry, serving as an endogenous base that enhances the rate of hydrogen isotope exchange.

Kinetics of hydrogen isotope exchange in kaolinite and the prediction of δD signature retention over geological time

Retention of the pristine hydrogen isotope composition (expressed as δD) formed during mineral formation in equilibrium with water is the basis for any paleoenvironmental reconstructions and for tracing mineral reactions using hydrogen (or H and O) isotope composition in kaolinite. Not only do post-formation reactions cause partial equilibration with ambient water, altering the pristine signature, but also result in the decoupling of H and O diffusion.In this study, kaolinite samples of different structural order (expressed, for example, as the Hinckley Index) were tested for their susceptibility to H isotope exchange under D2O vapor in an open system. The H isotope exchange was detected by recording the kaolinite’s structural OH and OD stretching mode using infrared spectroscopy. A number of different in-situ and ex-situ testing protocols under diverse temperature ranges (90 °C, 125–275 °C, 300–700 °C), show consistent kinetics and mechanism. In each kaolinite sample, the H isotope exchange rate increases with temperature; however, the major control on the reaction rate is the kaolinite crystallite’s structural order, not the particle (agglomerated crystallites) size. The hydrogen isotope composition in kaolinite can be altered independently of oxygen atoms, likely via proton hopping mechanism through structural defects. The H isotope exchange reaction under vapour shows a two-mode kinetics: the time-to-the-quarter (TTTQ; t1/4) dependence dominates in the first part of the reaction or in low-temperature reactions, transitioning to the log10t (Elovich) relationship and with lower activation energy, in advanced stages and in higher temperatures. Extrapolating the TTTQ model to temperatures < 90 °C allows estimating the rate of H isotope exchange between water vapor and kaolinite. The paper presents the prediction of preservation of pristine δD signature and thus reliability of using isotope data in kaolinites of different origins and structural order when exposed to pore water / water vapor over geological timescales.

Exploring the Interfacial Hydrogen Transfer between Pt and the Siliceous Framework and Its Promotional Effect on the Isotope Catalytic Exchange.

Interfacial hydrogen transfer between metal particles and catalyst supports is a ubiquitous phenomenon in heterogeneous catalysis, and this occurrence on reducible supports has been established, yet controversies remain about how hydrogen transfer can take place on nonreducible supports, such as silica. Herein, highly dispersed Pt clusters supported on a series of porous silica materials with zeolitic or/and amorphous frameworks were prepared to interrogate the nature of hydrogen transfer and its promotional effect on H2-HDO isotope catalytic exchange. The formation of zeolitic frameworks upon these porous silica supports by hydrothermal crystallization greatly promotes the interfacial hydrogen bidirectional migration between metal clusters and supports. Benefiting from this transfer effect, the isotope exchange rate is enhanced by 10 times compared to that on the amorphous counterpart (e.g., Pt/SBA-15). In situ spectroscopic and theoretical studies suggest that the defective silanols formed within the zeolite framework serve as the reactive sites to bind HDO or H2O by hydrogen bonds. Under the electrostatic attraction interaction, the D of hydrogen-bonded HDO scrambles to the Pt site and the dissociated H on Pt simultaneously spills back to the electronegative oxygen atom of adsorbed water to attain H-D isotope exchange with an energy barrier of 0.43 eV. The reverse spillover D on Pt combines with the other H on Pt to form HD in the effluent. We anticipate that these findings are able to improve our understanding of hydrogen transfer between metal and silica supports and favor the catalyst design for the hydrogen-involving reaction.

An experimental simulation of oxygen isotope exchange reaction between amorphous silicate dust and carbon monoxide gas in the early Solar System

The reaction mechanism and kinetics of oxygen isotope exchange between tens of nanometer-sized amorphous silicate grains with forsterite composition (amorphous forsterite) and low-pressure carbon monoxide (CO) gas (PCO) of 0.05–1 Pa at 643–883 K were examined to investigate oxygen isotopic evolution in the protosolar disk that led to the mass-independent oxygen isotopic variation of planetary materials. Both CO gas supply- and diffusion-controlled isotope exchange reactions were observed. At 753–883 K and PCO of 0.05–1 Pa, the supply of CO gas controls the isotope exchange reaction, and its rate is 2–3 orders of magnitude smaller than that of the H2O supply-controlled isotope exchange reaction. The diffusion-controlled isotope exchange occurred at 643–703 K and PCO of 0.3 Pa, and the reaction rate of D (m2/s) = (3.1 ± 2.3) × 10−23 exp[−41.7 ± 9.6 (kJ mol−1) R−1 (1/T − 1/1200)] was obtained.We found that the oxygen isotope exchange rates of amorphous forsterite with CO and H2O gases are larger than those of gaseous isotope exchange between CO and H2O gases at a wide range of temperatures, wherein amorphous forsterite crystallization does not precede the isotope exchange reaction of amorphous forsterite with these gases. The most sluggish isotope exchange rate between H2O and CO in the gas phase suggests that amorphous forsterite would play a role in accelerating gaseous isotopic equilibrium through the isotope exchange of amorphous forsterite with both CO and H2O. We found that the oxygen isotopic equilibrium between 0.1 μm-sized amorphous forsterite, CO, and H2O would be accomplished through the isotope exchange of amorphous forsterite at temperatures as low as ∼600–700 K in the dynamically accreting protosolar disk, which is significantly lower than expected for the case of gaseous isotope exchange (>∼800 K).

Barium isotope (re-)equilibration in the barite-fluid system and its implications for marine barite archives

Variations in the Ba isotopic composition of seawater are largely driven by the extent of barite precipitation in the marine photic zone and replenishment of Ba by upwelling and/or continental inputs. Pelagic barites offer a robust tool for tracing sources and sinks of Ba in the (paleo)ocean as they record these isotopic variations. Knowledge of the Ba isotope fractionation between barite and ambient waters is therefore imperative. Here, the Ba isotope fractionation between barite and Ba2+ (aq) under equilibrium conditions has been estimated by the three-isotope method with a 135Ba-enriched reactive fluid. The estimated Ba isotope fractionation was Δ137/134BaBarite-Ba2+ = −0.07 ± 0.08‰. Textural observations of barite crystals recovered up to 756 days of reaction reveal smoothing of solid surfaces but also typical dissolution features such as development of pits and cracks. Thus, dissolution/re-precipitation is likely the mechanism controlling the observed isotope exchange that is facilitated by the further development of porosity in the crystals. Additionally, the isotope exchange in the experimental runs fits a second-order law yielding a surface normalized isotope exchange rate of ∼2.8 × 10−10 mol/m2/s. This exchange rate could theoretically result in complete isotope exchange between pelagic barite with a typical edge size of 1 μm and ambient seawater or pore fluid within years, altering the barite's Ba isotopic composition during settling towards the seafloor and/or after deposition in marine sediments. Although there is considerable uncertainty in extrapolating experimental results to natural conditions and longer time scales, the rapid rates of exchange observed experimentally over short timescales suggest that isotope exchange in pelagic barite should be considered during interpretation of the Ba isotope composition as a paleoarchive.

Theoretical and Experimental Assessments of Elementary Steps and Bound Intermediates in Catalytic H2–O2 Reactions on Dispersed Pt Nanoparticles

Kinetic and isotopic data and H2 chemisorption uptakes measured under reaction conditions are combined here with theoretical assessments on model catalytic surfaces at relevant hydrogen (H*) coverages to establish the identity and kinetic relevance of elementary steps and bound species in H2–O2 reactions on Pt surfaces. Turnover rates are proportional to O2 pressure and decrease and then reach constant values as H2 pressures increase, leading to apparent first-order rate parameters (rO2/PO2) that reflect reactive collision probabilities of O2 with surfaces and which depend solely on the H2 pressure at all temperatures (540–680 K). H2–D2 isotopic exchange rates during reactions with O2 are much faster than water formation rates, consistent with quasi-equilibrated H2 dissociation steps and with prevalent H* coverages that can be determined independently from H2 uptakes; such measurements enable the decoupling of non-Langmuirian adsorption parameters from kinetic parameters in rate equations. These data indicate that H2–O2 reactions involve two kinetically-relevant O2 dissociation routes. One channel forms two bound O atoms on bare Pt atom ensembles available within H* adlayers, which react via fast subsequent reactions with H* to form H2O. A parallel O2 dissociation route involves reactions with a H*–H* pair to form weakly bound adsorbed hydrogen peroxide (*HOOH*), a highly reactive species that subsequently cleaves its O–O bond in steps that are not kinetically-relevant. The free-energy barriers are dominated by losses in translational entropy incurred upon the formation of the transition states from their gaseous O2 precursors. Kinetic isotope effects are near unity for both routes because H* is not involved in direct O2 dissociation steps, and the *HOOH* formation transition state occurs very early along the reaction coordinate (with nearly intact O–O and Pt–H bonds from reactant states), as confirmed by DFT-derived energies and isotope effects.

Equilibrium Ca isotope fractionation and the rates of isotope exchange in the calcite-fluid and aragonite-fluid systems at 25 °C

The Ca isotope composition of calcite and aragonite can provide insights into their formation conditions. The accurate interpretation of the Ca isotope composition of natural samples, however, requires precise knowledge of the equilibrium Ca isotope fractionation between the solid and fluid phases. In this study, the three-isotope method with 42Ca, 43Ca and 44Ca has been used to estimate the equilibrium isotope fractionation of Ca between the CaCO3 minerals calcite and aragonite and the Ca2+ aquo ion. Reactive fluids were enriched with 43Ca and equilibrated with synthetic calcite and aragonite of natural Ca isotope distribution up to 3745 h at 25°C. The isotopic composition of solids and fluids was measured using MC-ICP-MS and the estimated equilibrium fractionation for calcite-Ca2+(aq) and aragonite-Ca2+(aq) was Δ44/42Casolid-fluid=−0.02±0.13‰ and −0.80±0.10‰, respectively. Textural observations of the reacted solids suggest that isotope equilibration in aragonite experiments occurs via extensive Ostwald ripening, yielding large crystalline needles at the end of the experiments. In contrast, calcite did not exhibit an observable increase in size during the course of the experiments. Isotope exchange rates in the case of calcite are similar to those reported in previous studies and ∼4 orders of magnitude lower than the far-from-equilibrium calcite dissolution rate. Calcium isotope exchange rates for aragonite are more rapid than calcite driven by a greater extent of Ostwald ripening occurring via dissolution/precipitation reactions. The results of this study suggest that the Ca isotope compositions of calcite and aragonite crystals in chemical equilibrium, but isotope disequilibrium, with natural fluids could be significantly altered without overt evidence of diagenesis, especially in the case of calcite. The extent to which Ca isotope compositions are altered, however, would depend strongly on the environmental conditions, such as fluid:solid ratio and permeability of the solid facilitating fluid transport.

Incorporation of water-derived hydrogen into methane during artificial maturation of source rock under hydrothermal conditions

To investigate the origin of CH bonds in thermogenic methane (CH4), a solvent-extracted sample of organic-rich Eagle Ford shale was reacted with heavy water (D2O) under hydrothermal conditions (350 bar) in a flexible Au-TiO2 cell hydrothermal apparatus at a water-to-rock ratio of approximately 5:1. Temperature was increased from 200 to 350 °C over the course of one month and the concentrations of aqueous species and methane isotopologues were quantified as a function of time. In general, production of hydrogen, CO2, alkanes, and alkenes increased with time and temperature. Methane formed during the early stages of the experiment at 200 °C was primarily C1H4 with some CH3D. With progressively higher temperatures, increasing proportions of deuterated isotopologues were produced. Near the end of the experiment, the concentration of CD4 exceeded that of all other isotopologues combined. These results suggest that competition between rates of kerogen-water isotopic exchange and natural gas generation may govern the D/H ratio of thermogenic gases. Furthermore, hydrogenation of kerogen by water may be responsible for hydrocarbon yields in excess of those predicted by conventional models of source rock maturation in which hydrocarbon generation is limited by the amount of organically bonded hydrogen.

Rates of carbon and oxygen isotope exchange between calcite and fluid at chemical equilibrium

The isotopic composition of carbonate minerals provides a record of historical geochemical and environmental conditions, but the ability to interpret these compositions as paleo-proxies hinges on their preservation over thousand to million year timescales. At chemical equilibrium, alteration of initial isotopic compositions of calcite can occur in the presence of a fluid without visible changes in morphology at the submicron scale, complicating the interpretation of stable isotope compositions of carbonates. However, the rates and mechanisms of isotope exchange at chemical equilibrium are poorly understood. To evaluate the rates and processes by which C and O isotopes are exchanged between calcite and fluid, batch reactor experiments were conducted at chemical equilibrium between calcite and a fluid enriched in 13C and 18O relative to the solid at 25 °C. Both natural and synthetic calcite of different grain sizes were investigated to evaluate the impact of mineral surface area and size on C and O isotope exchange rates. Our experimental results indicate that rapid exchange of both C and O isotopes occurs within 72 h for all calcite grain sizes studied, likely indicative of exchange of surface species in combination with a backward reaction during dissolution and Ostwald ripening of high energy surface sites. After 72 h, C and O isotope exchange rates were slower but near constant for timescales of thousands of hours. Surface-area normalized C and O isotope exchange rates were similar for all calcite grain sizes studied, and O and C were exchanged in a ∼3:1 ratio consistent with exchange of CO32–. The rates of C and O exchange were ∼4 orders of magnitude lower than far-from-equilibrium calcite dissolution rates, suggesting exchange was controlled either by dissolution-precipitation of pre-existing reactive sites alone, or a combination of dissolution-precipitation and solid state/aqueous mediated diffusion. Overall, the results of this study suggest alteration of O and C isotope compositions of calcite at ambient temperatures can proceed readily over short time scales, though the extent to which this process continues to operate over geologic time scales is difficult to predict at present. The results of this study further highlight the importance of generating a mechanistic understanding of the process of isotope exchange at chemical equilibrium, and represent a step towards this understanding.

Experimental determination of hydrogen isotope exchange rates between methane and water under hydrothermal conditions

The hydrogen isotopic composition of methane (CH4) is used as a fingerprint of gas origins. Exchange of hydrogen isotopes between CH4 and liquid water has been proposed to occur in both low- and high-temperature settings. However, despite environmental evidence for hydrogen isotope exchange between CH4 and liquid water, there are few experimental constraints on the kinetics of this process. We present results from hydrothermal experiments conducted to constrain the kinetics of hydrogen isotope exchange between CH4 and supercritical water. Seven isothermal experiments were performed over a temperature range of 376–420 °C in which deuterium-enriched water and CH4 were reacted in flexible gold reaction cell systems. Rates of exchange were determined by measuring the change in the δD of CH4 over the time course of an experiment. Regression of derived second order rate constants (kr) vs. 1000/T (i.e., an Arrhenius plot) yields the following equation: ln(kr) = −17.32 (±4.08, 1 s.e.) × 1000/T + 3.19 (±6.01, 1 s.e.) (units of kr of sec−1 [mol/L]−1), equivalent to an activation energy of 144.0 ± 33.9 kJ/mol (1 s.e.). These results indicate that without catalysts, CH4 will not exchange hydrogen isotopes with liquid water on a timescale shorter than the age of the Earth (i.e., billions of years) at temperatures below 100–125 °C. Exchange at or below these temperatures is thought to occur due to the activity of life, and thus hydrogen isotopic equilibrium between methane and water may be a biosignature at low temperatures on Earth (in the present or the past) and on other planetary bodies. At temperatures ranging from 125 to 200 °C, hydrogen isotope exchange between CH4 and liquid water can occur on timescales of millions to hundreds of thousands of years, indicating that in thermogenic natural gas systems CH4 may isotopically equilibrate with water and achieve equilibrium isotopic compositions. Finally, the kinetics indicate that in deep-sea hydrothermal systems, the hydrogen (and thus clumped) isotopic composition of CH4 is likely set by formation and/or storage conditions isolated from the active flow regime. The determined kinetics indicate that once methane is entrained in circulating fluids, the expected time-temperature pathways are insufficient for measurable hydrogen isotope exchange between CH4 and water to occur.

Mass independent isotopic fractionation: A key to plasma chemistry

We propose a numerical model allowing to calculate the relative variations of isotopic ratios involved in a mass-independent isotopic fractionation (MIF) effect. This model is derived from classical trajectory simulation performed to reproduce the reactions yielding the isotopomers of ozone. In the ozone simulation, we did not introduce quantum mechanical selection rules for trajectories or the potential surface, but we separated instead exchange and non-exchange collisions, in order to introduce the fundamental quantum mechanical requirement according to which, for indistinguishable isotopes, the two possible reaction channels (elastic scattering or particle exchange) have to be superposed. The MIF effect is related to the molecular symmetry of the complex by the result that a different fraction of isotopically asymmetric complexes (formed by distinguishable isotopes) is stabilized than for symmetric ones (formed by indistinguishable isotopes).The model is applied on the results obtained experimentally for Mg and Ti isotopes in plasma. In plasma, Mg and Ti radicals resulting from the molecular dissociation of chlorides react with their parent molecules. In presence of hydrocarbons, isotope exchange rates are greatly enhanced when the intermediate activated complexes are adsorbed at the surface of the carbonaceous grains growing in the plasma. If a chemical reaction with the grain stabilizes the complex faster than its dissociation, MIF effects are observed. In such a chemical situation, the isotopic fractionation greatly exceed the usual theoretical predictions. Several characteristics of the MIF isotopic patterns are reproduced by the model.

Fast and pervasive diagenetic isotope exchange in foraminifera tests is species-dependent

Oxygen isotope compositions of fossil foraminifera tests are commonly used proxies for ocean paleotemperatures, with reconstructions spanning the last 112 million years. However, the isotopic composition of these calcitic tests can be substantially altered during diagenesis without discernible textural changes. Here, we investigate fluid-mediated isotopic exchange in pristine tests of three modern benthic foraminifera species (Ammonia sp., Haynesina germanica, and Amphistegina lessonii) following immersion into an 18O-enriched artificial seawater at 90 °C for hours to days. Reacted tests remain texturally pristine but their bulk oxygen isotope compositions reveal rapid and species-dependent isotopic exchange with the water. NanoSIMS imaging reveals the 3-dimensional intra-test distributions of 18O-enrichment that correlates with test ultra-structure and associated organic matter. Image analysis is used to quantify species level differences in test ultrastructure, which explains the observed species-dependent rates of isotopic exchange. Consequently, even tests considered texturally pristine for paleo-climatic reconstruction purposes may have experienced substantial isotopic exchange; critical paleo-temperature record re-examination is warranted.

Clumped isotope acid fractionation factors for dolomite and calcite revisited: Should we care?

During the analysis of the Δ47 values of CO2 derived from carbonates, the acid fractionation factor not only allows Δ47 values produced from carbonates and reacted at different temperatures (usually 25 and 90 °C) to be compared to one another, but also provides an effective tool to better understand the mechanism and rate of isotopic exchange of oxygen with water during the phosphoric acid digestion of carbonate minerals. In this study, experiments have been conducted in which calcite (1 sample) and dolomite (4 samples), with varying Δ47 values, have been digested in phosphoric acid (H3PO4) at five different temperatures between 25 and 90 °C using both the common acid bath and sealed vessel methods. From these experiments, we show that calcite has an acid fractionation factor which is independent of the reaction method. In contrast, all four dolomites show significant differences between the two methods with the sealed vessel reactions showing much smaller acid fractionation factors than the common acid bath. The magnitude of the acid fractionation factor in the dolomites is also dependent upon the absolute Δ47 value of the sample, with samples possessing lower Δ47 values producing lower acid fractionation factors than those with higher Δ47 values. The explanation for the difference in the behavior of acid fractionation factors between calcite and dolomite, in regard to the technique used, is believed to be related to the rate of reaction of the two minerals and thus the time in which the CO2 is in contact with the acid and allowed to exchange with the H2O in the acid and produced during the reaction.

An experimental study on oxygen isotope exchange reaction between CAI melt and low-pressure water vapor under simulated Solar nebular conditions

Calcium-aluminum-rich inclusions (CAIs) are known as the oldest high-temperature mineral assemblages of the Solar System. The CAIs record thermal events that occurred during the earliest epochs of the Solar System formation in the form of heterogeneous oxygen isotopic distributions between and within their constituent minerals. Here, we explored the kinetics of oxygen isotope exchange during partial melting events of CAIs by conducting oxygen isotope exchange experiments between type B CAI-like silicate melt and 18O-enriched water vapor (PH2O = 5 × 10−2 Pa) at 1420 °C. We found that the oxygen isotope exchange between CAI melt and water vapor proceeds at competing rates with surface isotope exchange and self-diffusion of oxygen in the melt under the experimental conditions. The 18O concentration profiles were well fitted with the three-dimensional spherical diffusion model with a time-dependent surface concentration. We determined the self-diffusion coefficient of oxygen to be ∼1.62 × 10−11 m2 s−1, and the oxygen isotope exchange efficiency on the melt surface was found to be ∼0.28 in colliding water molecules. These kinetic parameters suggest that oxygen isotope exchange rate between cm-sized CAI melt droplets and water vapor is dominantly controlled by the supply of water molecules to the melt surface at PH2O < ∼10−2 Pa and by self-diffusion of oxygen in the melt at PH2O > ∼1 Pa at temperatures above the melilite liquidus (1420–1540 °C). To form type B CAIs containing 16O-poor melilite by oxygen isotope exchange between CAI melt and disk water vapor, the CAIs should have been heated for at least a few days at PH2O > 10−2 Pa above temperatures of the melilite liquidus in the protosolar disk. The larger timescale of oxygen isotopic equilibrium between CAI melt and H2O compared to that between H2O and CO in the gas phase suggests that the bulk oxygen isotopic compositions of ambient gas at ∼1400 °C in the type B CAI-forming region is preserved in the oxygen isotopic compositions of type B CAI melilite. Based on the observed oxygen isotopic composition, we suggest that a typical type B1 CAI (TS34) from Allende was cooled at a rate of ∼0.1–0.5 K h−1 during fassaite crystallization.

Extreme iron isotope variation of pyrite in the Muping gold deposit, Jiaodong: Implication for tracing metal origin

Pyrite has complicated iron isotope fractionation behavior in the hydrothermal condition. Here we present a detailed in-situ iron isotope analyses of different generation pyrite from the Muping gold deposit, eastern China. δ56Fe values of pyrite investigated range from −4.25‰ to +1.78‰, unlike previous bulk analytic data (−0.78‰ to +0.79‰). Iron isotopic homogeneity of single pyrite grain, which can be verified by in-situ analysis, was recommended as the preferred and reliable criterion to judge the isotopic fractionation condition in the hydrothermal gold deposit. Inter-grain variations in Fe isotopic composition suggest occurrence of both kinetic and equilibrium fractionation for pyrite at Muping. Electron backscattered diffraction analysis shows that the replacement likely occurred via dissolution-reprecipitation pathway. The replacement process resulted in obvious kinetic isotope effect, in which pyrites are possessed in extreme negative Fe isotopic composition (~−4‰). Maximum δ56Fe of pyrite up to +1.78‰ implied that kinetic isotopic effect also play a significant role in increasing the Fe isotopic composition of ore-forming fluids in the Muping gold deposit. Further model calculations suggested the kinetic isotopic effect was dominated by pyrite fast precipitation rather than pyrrhotite precipitation or pyrite precipitation via “FeS” pathway. This kinetic isotope effect ultimately will be partly or completely erased due to fast isotope exchange rate at medium–high temperature condition. Pyrite with homogeneous Fe isotopic composition can be used to trace iron source of hydrothermal gold deposit but be cautious.

Oxygen isotope exchange rates between phosphate and water catalyzed by inorganic pyrophosphatase: Implications for the biogeochemical cycle of phosphorus

The oxygen isotope composition of phosphate (δ18OPO4) has been widely used as (paleo)temperature and bio-signature proxies and a tracer of biogeochemical cycling of phosphorus (P). In natural aqueous systems, reactive inorganic phosphorus is dominantly present as dissolved inorganic phosphate (DIP), and reactions involving DIP are primarily carried out by microorganisms and catalyzed by enzymes. One enzyme suggested to dominate P-cycling in nature is inorganic pyrophosphatase (PPase), a ubiquitous intracellular enzyme that catalyzes oxygen (O) isotope exchange between DIP and water. Herein, we determined PPase-catalyzed O-isotope exchange rates between DIP and water over the range of typical Earth surface temperatures (3 - 37°C) using isotope ratio mass spectrometry (IRMS). Exchange rates determined at pH 7.4 and between 3 and 37°C, were described by first-order reaction kinetics (rate constant k = 1.51E-05 to 3.13E-04 sec−1; half-life of reaction t1/2 = 37 to 765 minutes), and strongly dependent on temperature. The temperature dependence of the exchange reaction was fitted by the Arrhenius equation, with an activation energy of 61.8 kJ/mole. The rate of PPase-catalyzed O-isotope exchange is ca. 7 - 8 orders of magnitude faster than the rate of abiotic reactions (pH 5) at 37°C calculated by the extrapolation of high-temperature rate data. The results of this study can be used to improve the interpretation of measured δ18O values of phosphate preserved in the rock record or during biogeochemical reactions (e.g., deconvolution of simultaneous abiotic and enzymatic/microbial processes).

Oxygen isotope exchange between water and carbon dioxide in soils is controlled by pH, nitrate and microbial biomass through links to carbonic anhydrase activity

Abstract. The oxygen isotope composition of atmospheric carbon dioxide (CO2) is intimately linked to large-scale variations in the cycling of CO2 and water across the Earth's surface. Understanding the role the biosphere plays in modifying the oxygen isotope composition of atmospheric CO2 is particularly important as this isotopic tracer has the potential to constrain estimates of important processes such as gross primary production at large scales. However, constraining the atmospheric mass budget for the oxygen isotope composition of CO2 also requires that we understand better the contribution of soil communities and how they influence the rate of oxygen isotope exchange between soil water and CO2 (kiso) across a wide range of soil types and climatic zones. As the carbonic anhydrases (CAs) group of enzymes enhances the rate of CO2 hydration within the water-filled pore spaces of soils, it is important to develop understanding of how environmental drivers can impact kiso through changes in their activity. Here we estimate kiso and measure associated soil properties in laboratory incubation experiments using 44 soils sampled from sites across western Eurasia and north-eastern Australia. Observed values for kiso always exceeded theoretically derived uncatalysed rates, indicating a significant influence of CAs on the variability of kiso across the soils studied. We identify soil pH as the principal source of variation, with greater kiso under alkaline conditions suggesting that shifts in microbial community composition or intra–extra-cellular dissolved inorganic carbon gradients induce the expression of more or higher activity forms of CAs. We also show for the first time in soils that the presence of nitrate under naturally acidic conditions reduces kiso, potentially reflecting a direct or indirect inhibition of CAs. This effect appears to be supported by a supplementary ammonium nitrate fertilisation experiment conducted on a subset of the soils. Greater microbial biomass also increased kiso under a given set of chemical conditions, highlighting a putative link between CA expression and the abundance of soil microbes. These data provide the most extensive analysis of spatial variations in soil kiso to date and indicate the key soil trait datasets required to predict variations in kiso at large spatial scales, a necessary next step to constrain the important role of soil communities in the atmospheric mass budget of the oxygen isotope composition of CO2.

Insights into Interfacial Proton Transport in Protonic Ceramic Conductors

Understanding the mechanism of proton conduction at the interface of materials enables the development of a new generation of protonic ceramic conductors at low temperatures (< 300 °C) through water absorption and proton transport on the interface. In-situ Raman spectroscopy was performed on water-saturated, porous, and nanostructured TiO2 membranes directly observe the isotope exchange reactions suggested a Grotthuss-type proton transport and faster isotope exchange rate at 175 °C than that at 25 °C with a corresponding activation energy of 9 kJ mol-1. Comparing to traditional high-temperature protonic ceramic conductors through bulk transport, water absorption on the interface in simple oxides system has higher proton mobility and similar proton concentration, indicating the interface is a facile route for proton transport. In addition, the measured hydrogen isotope exchange rates in nanostructured BCZYYb by in-situ Raman spectroscopy in the temperature range of 100-300 °C were comparable or even higher than that in fully dense BCZYYb in the range of 400-600 °C, suggesting the possibility of using the nanostructured BCZYYb membrane as an electrolyte materials in protonic ceramic fuel cells at low temperatures (<300 °C).

Oxygen isotopic fingerprints on the phosphorus cycle within the deep subseafloor biosphere

Phosphorus (P) is essential for all known forms of life. The oxygen isotopic composition of phosphate can carry a strong imprint from the metabolic processes in Earth’s surface environments, including the deep subseafloor biosphere extending to > 1 km beneath the seafloor. Here, we report the 18O/16O ratios (δ18O) of dissolved inorganic phosphate (DIP) to identify different pathways of P cycling in deep-sea sediments sampled up to 200 meters below the seafloor during Ocean Drilling Program (ODP) Leg 201. Our results, along with a diagenetic model, indicate that the δ18O of DIP (δ18ODIP) is mainly controlled by three pathways of P cycling at Site 1230: (1) release of DIP by extracellular enzymatic degradation of organic matter (i.e., organophosphate), (2) precipitation of authigenic apatite, (3) enzyme-catalyzed oxygen isotopic exchange between phosphate and water. Our diagenetic model quantitatively deconvolves the rates of the three P cycling pathways. In particular, a shift of δ18ODIP towards equilibrium around 140 m below the seafloor corresponds well with a shift in microbial communities, suggesting that δ18ODIP has potential as a proxy for microbial activities in the deep subseafloor biosphere. We also find that the rate of oxygen isotopic exchange can be scaled with the rate of organic matter degradation, suggesting microbially controlled oxygen isotopic exchange. Further, the pattern of variation between authigenic phosphate in sediments and DIP in porewaters bolsters the case that authigenic phosphate (authigenic apatite and Fe-bound phosphate) should reflect the oxygen isotopic composition of contemporaneous porewater DIP and thus can be used to track biogeochemical cycling of P.

Stable isotope imprint of hypogene speleogenesis: Lessons from Austrian caves

Hypogene speleogenesis involves the formation of solution-enlarged permeability structures driven by ascending water acquiring its acidity from deep-seated sources. This process occurs in a wide range of geological settings and in different lithologies. Although hypogene speleogenesis has received increasing attention in the last two decades, the identification of former hypogene activity in caves remains challenging, unless unambiguous morphological, mineralogical and/or hydrogeological evidence is available. Here, we document that geochemical analyses (mainly stable oxygen and carbon isotopes) of cave wall rock samples may fill this methodological gap and provide a useful tool to identify hypogene karstification through geochemical fingerprinting of water-rock interaction.Stable isotope profiles of a comprehensive set of wall rock cores from eight hypogene caves in Austria in both limestone and dolomite settings were investigated. Alteration halos typically a few centimeters wide and characterized by up to 12‰ and 15‰ depletion in δ13C and δ18O, respectively, were identified in limestone and calcite marble caves. Besides isotopic alteration, bleaching and/or staining of the wall rock was observed due to oxidizing conditions of the paleowater. No isotopic change of the wall rock was identified in hypogene caves carved in dolomite, which may be attributed to low Ca/Mg ratios of the paleowater and/or aggressive fluids leading to fast wall rock retreat. On the other hand, dedolomitization was observed in some wall rocks implying the presence paleowaters with elevated Ca/Mg ratios. Similarly to dolomitic caves, no isotopic alteration front was observed in a cave formed by sulfuric acid speleogenesis. A likely explanation is the high rate of wall rock retreat outpacing the rate of isotope exchange in the wall rock. Our study also shows that water-rock interaction during epigene speleogenesis does not lead to isotopic halos.This geochemical fingerprinting approach offers a powerful tool to test the hypogene origin of caves and may provide additional insights into the temperature and provenance of the paleowater and the source(s) of its dissolved inorganic carbon.