Hydrogen Isotope Fractionation Factors Research Articles

Carbon and hydrogen isotope fractionation of phthalates during photocatalysis reactions in aqueous solution containing Fe(III) complexes or iron minerals

The hydrogen and carbon isotope fractionation factor (ε2H, ε13C) of dimethyl-, diethyl‑ and dibutyl phthalic acid ester during photosensitized degradation by artificial sunlight with Fe(III) ions and iron minerals (hematite, goethite and magnetite) in aqueous solution were examined by compound-specific isotope analysis (CSIA) in order to analyze the degradation mechanism. Hematite does not catalyze photosensitized degradation of phthalates. The correlation of 2H and 13C isotope fractionation (Λ = Δδ2H/Δδ13C) of phthalates with increasing chain length (dimethyl-; diethyl‑; and dibutyl-) were compared with values of the ∙OH radical model reaction with the aromatic ring as well as acidic and alkaline hydrolysis. The Λ values of die photosensitized reaction of diethyl phthalate with goethite (-5.1 ± 1.8) and magnetite (-18.9 ± 3.9) show a large difference compared to Fe(III) solutions (4.7 ± 0.9 to 4.8 ± 1.0) suggesting specific reaction mechanisms. The fractionation factors determined here have potential to characterize the degradation of phthalates catalyzed by photo-induced reaction of Fe(III), goethite and magnetite in natural system and in remediation approaches.

Carbon and hydrogen stable isotope fractionation due to monooxygenation of short-chain alkanes by butane monooxygenase of Thauera butanivorans Bu-B1211.

Multi element compound-specific stable isotope analysis (ME-CSIA) is a tool to assess (bio)chemical reactions of molecules in the environment based on their isotopic fingerprints. To that effect, ME-CSIA concepts are initially developed with laboratory model experiments to determine the isotope fractionation factors specific for distinct (bio)chemical reactions. Here, we determined for the first time the carbon and hydrogen isotope fractionation factors for the monooxygenation of the short-chain alkanes ethane, propane, and butane. As model organism we used Thauera butanivorans strain Bu-B1211 which employs a non-haem iron monooxygenase (butane monooxygenase) to activate alkanes. Monooxygenation of alkanes was associated with strong carbon and hydrogen isotope effects: εbulkC = -2.95 ± 0.5 ‰ for ethane, -2.68 ± 0.1 ‰ for propane, -1.19 ± 0.18 ‰ for butane; εbulkH = -56.3 ± 15 ‰ for ethane, -40.5 ± 2.3 ‰ for propane, -14.6 ± 3.6 ‰ for butane. This resulted in lambda (Λ ≈ εHbulk/εCbulk) values of 16.2 ± 3.7 for ethane, 13.2 ± 0.7 for propane, and 11.4 ± 2.8 for butane. The results show that ME-CSIA can be used to track the occurrence and impact of monooxygenase-dependent aerobic processes converting short-chain alkanes in natural settings like marine and terrestrial seeps, gas reservoirs, and other geological formations impacted by natural gas.

Hydrogen isotope biogeochemistry of plant waxes in paired lake catchments

Studies of plants in modern catchment systems can serve to improve the level of understanding of sedimentary plant wax hydrogen isotope (δ2H) data by directly investigating the link between local climate conditions, plant source water, leaf water, and plant lipids for individual plant species. Here we present such an application to compare two lake catchments with different basin morphologies in Estonia. We sampled leaf and xylem water, as well as leaf waxes from the seven common plant species in each catchment, and soil water. We then measured the δ2H values of all waters, and of n-alkanes (δ2Hn–alk) from the plant waxes, as well as surface lake sediments. We applied a Péclet modified Craig-Gordon leaf water model using local precipitation isotope and climate data to characterize the δ2H values of the biosynthetic source water pool throughout the entire growth season. The data and model results provide a detailed view of how the input hydro-climatic signals from the precipitation δ2H values were modified by environmental and plant physiological conditions and ultimately by the biosynthetic isotope fractionation associated with n–alkane production by each plant species. We report both average apparent (εapp = –92 ± 21 ‰) and biosynthetic (εbio = –132 ± 19 ‰) hydrogen isotope fractionation factors of all species from the studied catchments. This information serves as a foundation for sedimentary organic geochemistry and paleoclimate studies, which allows for more direct and quantitative links to be made between sedimentary plant wax δ2H values and the climate signal contained in plant source water.

Hydrogen and carbon isotope fractionation factors of aerobic methane oxidation in deep-sea water

Abstract. Isotope fractionation factors associated with various biogeochemical processes are important in ensuring the reliable use of isotope tracers in biogeosciences at large. Methane is a key component of the subsurface biosphere and a notable greenhouse gas, making the accurate evaluation of methane cycles, including microbial methanotrophy, imperative. Although the isotope fractionation factors associated with methanotrophy have been examined under various conditions, the dual-isotope fractionation factors of aerobic methanotrophy in oxic seawater remain unclear. Here, we investigated hydrogen and carbon isotope ratios of methane as well as the relevant biogeochemical parameters and microbial community compositions in hydrothermal plumes in the Okinawa Trough. Methanotrophs were found to be abundant in plumes above the Hatoma Knoll vent site, and we succeeded in simultaneously determining hydrogen and carbon isotope fractionation factors associated with the aerobic oxidation of methane (εH=49.4±5.0 ‰, εC=5.2±0.4 ‰) – the former being the first of its kind ever reported. This εH value is comparable with values reported from terrestrial ecosystems but clearly lower than those from aerobic and anaerobic methanotroph enrichment cultures, as well as incubations of methanotrophic isolates. The covariation factor between δ13CCH4 and δDCH4, Λ (9.4 or 8.8 determined using two different methods), was consistent with those from methanotrophic isolate incubations. These values are valuable for understanding dynamics of methane cycling in the marine realm, and future applications of the approach to other habitats with methanotrophic activity will help reveal whether the small εH value observed is a ubiquitous feature across all marine systems.

Carbon and hydrogen isotope fractionation during aerobic oxidation of short-chain alkanes in experimental incubations of vent fluids

Aerobic oxidation of short-chain alkanes was observed in gas samples from the Lutao intertidal hydrothermal vents in Taiwan, during storage at 20 °C for up to 29 months without adding bacterial strains and replenishing substrates. The carbon isotope fractionation factors (εC) of methane (C1), ethane (C2), and propane (C3), were calculated using the Rayleigh fractionation equation to be −37.1 ± 7.5‰, −14.8 ± 4.8‰, and −4.7 ± 5.2‰, respectively. The hydrogen isotope fractionation factor (εH) of methane was determined to be −281 ± 187‰. DNA sequencing of the 16S rRNA gene in the vent fluids suggests that aerobic oxidation is dominated by methanotrophs of the genera Methylomicrobium and Methylophaga, which use the ribulose monophosphate pathway (RuMP). The degrees of isotope fractionation (εC and εH values) herein are larger than previously reported values, possibly due to the limited O2 supply and low abundance of aerobic methane-oxidizing bacteria in the experiments. Since the fractionation factor of methane is higher than those of ethane and propane, the aerobic oxidation of thermogenic or microbial alkanes could produce a carbon isotope reversal, which is frequently noted as a trait of abiotic hydrocarbons. This work demonstrates that in addition to anaerobic microbial oxidation, aerobic oxidation with a low cell density can also produce significant isotope fractionation of alkanes in geological closed/semi-closed environments and open flow reaction systems that are characterized by moderate temperatures and a limited supply of substrates and O2; these environments include cold seeps, mud volcanoes, and low-temperature hydrothermal plumes/aquifers/reservoirs.

Theoretical calculation of position-specific carbon and hydrogen isotope equilibriums in butane isomers

Position-specific isotope analysis has shown its potential to reveal information regarding formation, migration, and conversion procesess of hydrocarbons. The intramolecular isotope compositions of butane are promising to serve as a new thermometer and tracer. Therefore, position-specific isotope signatures in butane at equilibrium are needed for calibrating experimental measurements, establishing new geothermometers, and recognizing kinetic isotope effects. Here we conduct quantum chemistry modeling with corrections beyond the harmonic approximation and the Born-Oppenheimer approximation to obtain accurate intramolecular and intermolecular carbon and hydrogen isotope fractionation factors for butane isomers at equilibrium. Temperature dependences of these equilibrium isotope effects are presented for the range from 0 °C to 726.85 °C. The contribution of higher-order energy terms to 1000lnα values beyond the Bigeleisen-Mayer equation is found to be comparable to the magnitude of current experimental precisions. In addition to the significance of anharmonicity, the contribution of hindered internal rotation and diagonal Born-Oppenheimer correction is found to be important for accurate predictions of position-specific hydrogen isotope equilibriums. The abundance ratio of n-butane to i-butane at equilibriums is also calculated at various temperatures. Our results provide fundamental understanding of equilibrium properties for studying position-specific isotope effects in butane.

Hydrogen isotope fractionation variations of n-alkanes and fatty acids in algae and submerged plants from Tibetan Plateau lakes: Implications for palaeoclimatic reconstruction

The hydrogen isotope compositions (δD) of n-alkanes and fatty acids (FAs) are widely applied in palaeoclimatic reconstructions, and the determinations of their hydrogen isotope fractionation factor values (ε) are vital for quantitatively reconstructing past precipitation variations. Currently, studies on n-alkane and FA ε values focus on terrestrial plants, which, however, show large uncertainties because of the influence of evapotranspiration. Therefore, in this study, we analysed the ε values of algae and submerged plants immersed in lakes, which are not affected by evapotranspiration, to understand the hydrogen isotope fractionation of plant lipid synthesis. By investigating the δD values of lipids (n-alkanes and FAs) in algae and submerged plants and the δD values of co-existing water (including lake bottom water, surface sediment water, and leaf water of algae and submerged plants) from five Tibetan Plateau lakes, we find that the n-alkane ε values of algae and submerged plants show narrow changes, ranging from −176 to −159‰ and −167 to −142‰, respectively. The FA ε values of algae and submerged plants also show small variations, ranging from −160 to −121‰ (except Chara) and −161 to −138‰, respectively. Therefore, the average biosynthetic hydrogen isotope fractionation of these plants is −162‰ for n-alkanes and −145‰ for FAs, and the small ε differences between FAs and n-alkanes can be related to the different magnitudes of FA utilization in n-alkane synthesis. Finally, we find that the biosynthetic hydrogen isotope fractionation factors of aquatic plants are close to those of terrestrial grasses but slightly more negative than those of terrestrial woody plants. Thus, our results are helpful for understanding the hydrogen isotope fractionation variations in terrestrial plant lipids, which is beneficial for palaeohydrological reconstructions.

Hydrogen isotope fractionation response to salinity and alkalinity in a calcifying strain of Emiliania huxleyi

Hydrogen isotope ratios of long-chain alkenones (δ2HC37) correlate with water isotope ratios and salinity, albeit with varying degrees of biological fractionation between alkenones and water. These differences in fractionation are the result of environmental and species related effects, which in some cases have consequences for the magnitude of the δ2HC37 response per unit increase in salinity. Earlier culture experiments have focused on constraining hydrogen isotope fractionation factor α in non-calcifying strains of Emiliania huxleyi. Here we studied isotopic fractionation in a calcifying strain of E. huxleyi and show that although absolute fractionation is different, the response to changes in salinity and alkalinity is similar to those of non-calcifying species. This suggests that calcification does not alter the δ2HC37 response to salinity significantly.

Aerobic microbial oxidation of hydrocarbon gases: Implications for oil and gas exploration

Abstract The molecular and carbon and hydrogen isotope compositions associated with the aerobic consumption of methane (Cl), ethane (C2) and propane (C3) were determined from incubations of marine sediments collected from the seafloor surface of the gas- and oil-bearing Bohai Bay in eastern China. The fresh sediment samples were incubated aerobically at 28 °C and treated with three types of hydrocarbon gas mixtures as follows: mixture I (C1:C2:C3 = 100:0:0), II (C1:C2:C3 = 99.00:0.70:0.30) and III (C1:C2:C3 = 79.96:10.04:10.00), corresponding to incubations i, ii and iii, respectively. All the hydrocarbons are of thermogenic origin, except C1 (microbial) in incubation ii. The oxidation of C1-C3 was observed in all three treatments, with variations in carbon isotope fractionation factors (eC) of C1 and average values of −10.5 ± 0.1, −5.7 ± 1.6, and −10.9 ± 2.9, respectively. The hydrogen isotope fractionation factors (eH) of C1 in the three incubations also display variation, with average values of −118.4 ± 3.0, −68.3 ± 15.0 and −56.3 ± 8.4, respectively. Experiments with thermogenic C1 display greater carbon fractionation than those with microbial C1. The eC and eH values for C2 and C3 were obtained only in incubation iii. Compared to C1, C2 and C3 exhibit weaker carbon isotope fractionation but stronger hydrogen isotope fractionation. The fractionation factors presented in this study can be applied to estimate the extent of C1-C3 oxidation in natural environments, especially at seafloor and terrestrial seeps where aerobic microbes are commonly present. Our study has important implications in oil and gas exploration, with an alert for exercising caution when using the molecular and isotopic characteristics of hydrocarbon gases to evaluate the gas sources because these are susceptible to aerobic microbial oxidation.

Compound-specific δD and its hydrological and environmental implication in the lakes on the Tibetan Plateau

The hydrogen isotopic composition ( δ D) of n -alkanes in lacustrine sediments is widely used in palaeoenvironmental studies, but the heterogeneous origins and relative contributions of these lipids provide challenges for the interpretation of the increasing dataset as an environment and climatic proxy. We systematically investigated n -alkane δ D values from 51 submerged plants (39 Potamogeton , 1 Myriophyllum , and 11 Ruppia ), 13 algae (5 Chara , 3 Cladophora , and 5 Spirogyra ) and 20 terrestrial plants (10 grasses and 10 shrubs) in and around 15 lakes on the Tibetan Plateau. Our results demonstrate that δ D values of C29 n -alkane are correlated significantly with the lake water δ D values both for algae ( R 2=0.85, p n =9) and submerged plants( R 2=0.90, p n =25), indicating that δ D values of these algae and submerged plants reflect the δ D variation of lake water. We find that apparent hydrogen isotope fractionation factors between individual n -alkanes and water ( e a/w) are not constant among different algae and submerged plants, as well as in a single genus under different liminological conditions, indicating that the biosynthesis or environmental conditions (e.g. salinity) may affect their δ D values. The δ D values of submerged plant Ruppia in the Xiligou Lake (a closed lake) are significant enriched in D than those of terrestrial grasses around the lake (one-way ANOVA, p Chara in the Keluke Lake (an open lake) display similar δ D values with grasses around the lake (one-way ANOVA, p =0.826>0.05), suggesting that the n -alkane δ D values of the algae and submerged plants record the signal of D enrichment in lake water relative to precipitation only in closed lakes in arid and semi-arid area. For each algae and submerged plant sample, we find uniformed δ D values of different chain length n -alkanes, implying that, in combination with other proxies such as Paq and Average Chain Length, the offset between the δ D values of different chain length n -alkanes can help determine the source of sedimentary n -alkanes as well as inferring the hydrological characteristics of an ancient lake basin (open vs closed lake).

Biosynthetic hydrogen isotopic fractionation factors during lipid synthesis in submerged aquatic macrophytes: Effect of groundwater discharge and salinity

Sedimentary lipid biomarkers have become widely used tools for reconstructing past climatic and ecological changes due to their ubiquitous occurrence in lake sediments. In particular, the hydrogen isotopic composition (expressed as δD values) of leaf wax lipids derived from terrestrial plants has been a focus of research during the last two decades and the understanding of competing environmental and plant physiological factors influencing the δD values has greatly improved. Comparatively less attention has been paid to lipid biomarkers derived from aquatic plants, although these compounds are abundant in many lacustrine sediments. We therefore conducted a field and laboratory experiment to study the effect of salinity and groundwater discharge on the isotopic composition of aquatic plant biomarkers. We analyzed samples of the common submerged plant species, Potamogeton pectinatus (sago pondweed), which has a wide geographic distribution and can tolerate high salinity. We tested the effect of groundwater discharge (characterized by more negative δD values relative to lake water) and salinity on the δD values of n-alkanes from P. pectinatus by comparing plants (i) collected from the oligotrophic freshwater Lake Stechlin (Germany) at shallow littoral depth from locations with and without groundwater discharge, and (ii) plants grown from tubers collected from the eutrophic Lake Müggelsee in nutrient solution at four salinity levels. Isotopically depleted groundwater did not have a significant influence on the δD values of n-alkanes in Lake Stechlin P. pectinatus and calculated isotopic fractionation factors εl/w between lake water and n-alkanes averaged −137±9‰ (n-C23), −136±7‰ (n-C25) and −131±6‰ (n-C27), respectively. Similar ε values were calculated for plants from Lake Müggelsee grown in freshwater nutrient solution (−134±11‰ for n-C23), while greater fractionation was observed at increased salinity values of 10 (163±12‰) and 15 (−172±15‰). We therefore suggest an average ε value of −136±9‰ between source water and the major n-alkanes in P. pectinatus grown under freshwater conditions. Our results demonstrate that isotopic fractionation can increase by 30–40‰ at salinity values 10 and 15. These results could be explained either by inhibited plant growth at higher salinity, or by metabolic adaptation to salt stress that remain to be elucidated. A potential salinity effect on δD values of aquatic lipids requires further examination, since this would impact on the interpretation of downcore isotopic data in paleohydrologic studies.

Precise and accurate isotope fractionation factors (α17O, α18O and αD) for water and CaSO4·2H2O (gypsum)

Gypsum (CaSO4·2H2O) is a hydrated mineral containing crystallization water, also known as gypsum hydration water (GHW). We determined isotope fractionation factors (α17O, α18O and αD) between GHW and free water of the mother solution in the temperature range from 3°C to 55°C at different salinities and precipitation rates. The hydrogen isotope fractionation factor (αDgypsum-water) increases by 0.0001 units per °C between 3°C and 55°C and salinities <150g/L of NaCl. The αDgypsum-water is 0.9812±0.0007 at 20°C, which is in good agreement with previous estimates of 0.981±0.001 at the same temperature. The α18Ogypsum-water slightly decreases with temperature by 0.00001 per °C, which is not significant over much of the temperature range considered for paleoclimate applications. Between 3°C and 55°C, α18Ogypsum-water averages 1.0035±0.0002. This value is more precise than that reported previously (e.g. 1.0041±0.0004 at 25°C) and lower than the commonly accepted value of 1.004. We found that NaCl concentrations below 150g/L do not significantly affect α18Ogypsum-water, but αDgypsum-water increases linearly with NaCl concentrations even at relatively low salinities, suggesting a salt correction is necessary for gypsum formed from brines. Unlike oxygen isotopes, the αDgypsum-water is affected by kinetic effects that increase with gypsum precipitation rate. As expected, the relationship of the fractionation factors for 17O and 18O follows the theoretical mass-dependent fractionation on Earth (θ=0.529±0.001). We provide specific examples of the importance of using the revised fractionation factors when calculating the isotopic composition of the fluids.

In situ study of the fractionation of hydrogen isotopes between aluminosilicate melts and coexisting aqueous fluids at high pressure and high temperature – Implications for the δD in magmatic processes

The hydrogen isotopic composition of melt inclusions trapped in phenocrysts during their crystallization and growth in a magma may contribute to a better understanding of the water cycle between the atmosphere, the hydrosphere and the lithosphere. Such understanding relies on the knowledge of the hydrogen isotopic fractionation factors between aqueous fluids, silicate melts, and minerals at temperature and pressure conditions relevant to the Earth's interior. Significant D/H fractionation between silicate melts and aqueous fluids was reported at hundreds of MPa and °C by using in situ measurements in hydrothermal diamond anvil cell (HDAC) experiments (Mysen, 2013a, 2013b, Am. Mineral. 98, 376–386 and 1754–1764). However, the available dataset is focused on fluids and melts with D/H ratios close to unity. The relevance of such data for natural processes that involve per mil variations of δD-values may not always be clear. To address such concerns, the effect of the bulk D/H ratio on hydrogen isotope partitioning between water-saturated silicate melts and coexisting silicate-saturated aqueous fluids has been determined in the Na2O–Al2O3–SiO2–H2O–D2O system. To this end, in situ Raman spectroscopic measurements were performed on fluids and melts with bulk D/H ratios from 0.05 to 2.67 by using an externally-heated diamond anvil cell in the 300–800 °C and 200–1500 MPa temperature and pressure range, respectively.In these pressure/temperature ranges, the D/H ratios of fluids in equilibrium with melt barely change with temperature (in average ΔHfluid=0.47±1.15 kJ/mol). In contrast, the D/H ratios of coexisting melts display strong dependence on temperature (average ΔHmelt=7.18±1.27 kJ/mol). The temperature-dependence of the D/H fractionation factor between melt and fluid (αfluid-melt=D/Hfluid/D/Hmelt) is comparable in all the experiments and can be written: 1000⋅ln⁡(αfluid-melt)=263(±26)⋅T−2–126(±48). Therefore, the αfluid-melt is independent of the bulk D/H ratio of the melt + fluid system.Experimentally determined αfluid-melt using D-enriched fluids, therefore, can be applied to natural systems. It follows that for water-saturated magma strong isotopic fractionation of D and H between water dissolved in magmas and deep aqueous fluids may occur. The δD-values in melt inclusions in phenocrysts in such water-saturated magma will reflect such fluid/melt fractionation effects. A likely result is underestimation of the δD isotopic composition of slab fluids based on δD-values in melt inclusions. The temperature-dependent hydrogen isotope fractionation must be taken into account in the modeling of slab fluid–magma interaction in the mantle wedge.

Elemental and stable isotopic study of sweeteners and edible oils: Constraints on food authentication

Elemental concentrations and stable isotopic compositions of 41 sweeteners (syrups, honeys, and sugars) and 43 edible oils were determined to evaluate their potential as parameters for food authentication. The addition of as little as 10% of corn syrup to pure maple syrup can be detected using molar ratios of Na/(Na+K) and Na/(Na+Ca) for maple syrups. The detection of more than 15% adulterant in maple syrup and honey is also possible due to the 13‰ difference in carbon isotopic composition of C3 plants for authentic honeys and maple syrups (25.1±1.1‰) relative to the carbon isotopic composition of C4 plants for corn syrups and cane sugars (−11.4±0.8‰) as major adulterant agents. Nearly constant net oxygen (1.028±0.002) as well as hydrogen (0.974±0.005) isotope fractionation factors between honeys and average local meteoric waters from two areas, Kingston and Costa Rica, suggests that these isotope ratios of honeys are good candidates to authenticate their geographic locations. The net oxygen (1.03±0.001) as well as hydrogen (0.894±0.003) isotope fractionation factors between olive oils and local meteoric waters from different parts of Italy suggests that these isotope ratios are independent of the oil processing method and therefore can be used to trace geographic locations.

Fractionation of hydrogen and oxygen isotopes of gypsum hydration water and assessment of its geochemical indications

Fundamental knowledge of the isotopic fractionation between the hydration water and the mother solution and whether the primary information recorded in hydration water can be preserved or not in deposits or mines have long been unclear. In order to calculate the accurate hydrogen and oxygen isotopic fractionation factors between gypsum hydration water and its mother solution with new methods, to understand the mechanism of fractionation and synthetically assess the record-keeping abilities of the isotopic composition of hydration water during the process of diagenesis after deposition, experiments on the hydrogen and oxygen isotopic compositions of gypsum hydration water and its mother solution at different isothermal temperatures from 5 to 50°C were systematically conducted. In addition, samples from two typical gypsum deposits formed in different environmental conditions were also determined. Results show that during gypsum crystallisation, both hydrogen and oxygen isotopes show significant fractionation between the hydration water and the mother solution. The calculated hydrogen isotopic fractionation factors are <1, while the oxygen isotopic fractionation factors are >1 at temperatures from 5 to 50°C. The fractionation factors show no functional relationships with temperature. Isotopic compositions of gypsum hydration water in arid lake sediments can be used to trace the source of water and primary deposit environmental information. However, the isotopic composition of the gypsum hydration water can easily be altered by dissolution and secondary precipitation of gypsum during later diagenesis, particularly in areas with humid climate and abundant groundwater. A very careful assessment on record-keeping abilities of the primary isotopic composition of hydration water in gypsum during later diagenesis must be considered before application.

Different types of methane monooxygenases produce similar carbon and hydrogen isotope fractionation patterns during methane oxidation

We determined the stable carbon and hydrogen isotope fractionation factors for methane oxidation under oxic conditions using strains with known degradation pathways. The aerobic oxidation of methane can be initiated by two different forms of enzymes known as methane monooxygenases (MMO). The expression of these enzymes is type-specific and dependent upon the adjusted copper concentration in the medium (or environment). In this study, the expression of either the soluble MMO or the particulate MMO was supported by adjusting the copper concentrations in the growth medium. Taxonomically different aerobic methanotrophic strains, mainly belonging to the alpha- and gamma- classes of Proteobacteria, produced methane isotope enrichment factors ( ε bulk) ranging from −14.8 to −27.9‰ for carbon, and from −110.0 to −231.5‰ for hydrogen. The ratio of hydrogen versus carbon discrimination ( Λ = ( α H −1 − 1)/( α C −1 − 1) ≈ Δ( δ 2H)/Δ( δ 13C)) were similar for all tested cultures, and are also identical to values calculated from previously published enrichment factors for aerobic and anaerobic methane degradation. In contrast, Λ-values for the abiotic oxidation of methane with OH radicals (this process is considered as the main removal process for methane from the atmosphere) were significantly higher than the values derived from biotic oxidation. Due to the low variability of microbial methane isotope fractionation patterns, we propose that combined carbon and hydrogen isotope fractionation analyses can be used to monitor and assess the occurrence of microbial methane oxidation in marine or terrestrial environments. However, it is not possible to distinguish distinct aerobic or anaerobic methane-oxidation pathways by this approach.

Assessment of MTBE biodegradation pathways by two-dimensional isotope analysis in mixed bacterial consortia under different redox conditions

The fuel oxygenate, methyl tert-butyl ether (MTBE), although now widely banned or substituted, remains a persistent groundwater contaminant. Multidimensional compound-specific isotope analysis (CSIA) of carbon and hydrogen is being developed for determining the extent of MTBE loss due to biodegradation and can also potentially distinguish between different biodegradation pathways. Carbon and hydrogen isotopic fractionation factors were determined for MTBE degradation in aerobic and anaerobic laboratory cultures. The carbon isotopic enrichment factor (epsilonC) for aerobic MTBE degradation by a bacterial consortium containing the aerobic MTBE-degrading bacterium, Variovorax paradoxus, was -1.1 +/- 0.2 per thousand and the hydrogen isotope enrichment factor (epsilonH) was -15 +/- 2 per thousand. This corresponds to an approximated lambda value (Lambda = epsilonH/epsilonC) of 14. Carbon isotope enrichment factors for anaerobic MTBE-degrading enrichment cultures were -7.0 +/- 0.2 per thousand and did not vary based on the original inoculum source, redox condition of the enrichment, or supplementation with syringic acid as a co-substrate. The hydrogen enrichment factors of cultures without syringic acid were insignificant, however a strong hydrogen enrichment factor of -41 +/- 3 per thousand was observed for cultures which were fed syringic acid during MTBE degradation. The Lambda = 6 obtained for NYsyr cultures might be diagnostic for the stimulation of anaerobic MTBE degradation by methoxylated compounds by an as yet unknown pathway and mechanism. The stable-isotope enrichment factors determined in this study will enhance the use of CSIA for monitoring anaerobic and aerobic MTBE biodegradation in situ.

A New Approach to Reconstruct Paleogene Atmospheric Hydrology at High Latitudes

Reconstruction of ancient atmospheric conditions through the analysis of precipitation patterns is a novel endeavor in the study of paleoclimate. A new approach is now available for a quantitative reconstruction of Paleogene atmospheric hydrological conditions in High Arctic. It is based on 1) the discovery of exceptionally-preserved Paleogene plant fossils from the Canadian Arctic which yielded in situ labile biomolecules; 2) the development of compound-specific hydrogen isotope analysis which can be applied to small amount of plant material; and 3) taxon-specific apparent hydrogen isotope fractionation factors obtained from empirical measurement. A new moisture recycling model is established to explain the reconstructed paleohydrologic pattern in the High Arctic during Paleogene.

Characterization of anaerobic xylene biodegradation by two‐dimensional isotope fractionation analysis

We determined stable carbon and hydrogen isotope fractionation factors for anaerobic degradation of xylene isomers by several pure and mixed cultures. All cultures initiated xylene degradation by the addition of fumarate to a methyl moiety, as is known from the literature or verified by the presence of methylbenzylsuccinates as metabolic intermediates. Additionally, the A subunit of benzylsuccinate synthase (bssA) was identified in the majority of the cultures by bssA-targeted primers. Xylene degradation was always coupled to a significant carbon and hydrogen isotope fractionation. The values of the apparent kinetic isotope effect (AKIE) for carbon and hydrogen indicate that the cleavage of a carbon-hydrogen bond is an isotope-sensitive step during fumarate addition to xylene isomers. The slopes of the linear regression for hydrogen (Δδ(2) H) versus carbon (Δδ(13) C) discrimination (Λ = Δδ(2) H/Δδ(13) C ≈ εHbulk /εCbulk ) ranged from 12 ± 4 to 29 ± 5 and were comparable to Λ values previously determined for anaerobic toluene degradation. The results suggest that combined carbon and hydrogen isotope fractionation analyses can be used to monitor anaerobic xylene degradation at contaminated sites.

Oxygen and hydrogen isotope fractionation in serpentine–water and talc–water systems from 250 to 450 °C, 50 MPa

Oxygen and hydrogen isotope fractionation factors in the talc–water and serpentine–water systems have been determined by laboratory experiment from 250 to 450 °C at 50 MPa using the partial exchange technique. Talc was synthesized from brucite + quartz, resulting in nearly 100% exchange during reaction at 350 and 450 °C. For serpentine, D–H exchange was much more rapid than 18O– 16O exchange when natural chrysotile fibers were employed in the initial charge. In experiments with lizardite as the starting charge, recrystallization to chrysotile enhanced the rate of 18O– 16O exchange with the coexisting aqueous phase. Oxygen isotope fractionation factors in both the talc–water and serpentine–water systems decrease with increasing temperature and can be described from 250 to 450 °C by the relationships: 1000 ln α talc – water 18 O – 16 O = 11.70 × 10 6/T 2 − 25.49 × 10 3/T + 12.48 and 1000 ln α serpentine – water 18 O – 16 O = 3.49 × 10 6/T 2 − 9.48 where T is temperature in Kelvin. Over the same temperature interval at 50 MPa, talc–water D–H fractionation is only weakly dependent on temperature, similar to brucite and chlorite, and can be described by the equation: 1000 ln α talc – water D / H = 10.88 × 10 6/T 2 − 41.52 × 10 3/T + 5.61 where T is temperature in Kelvin. Our D–H serpentine–water fractionation factors calibrated by experiment decrease with temperature and form a consistent trend with fractionation factors derived from lower temperature field calibrations. By regression of these data, we have refined and extended the D–H fractionation curve from 25 to 450 °C, 50 MPa as follows: 1000 ln α serpentine – water D – H = 3.436 × 10 6/T 2 − 34.736 × 10 3/T + 21.67 where T is temperature in Kelvin. These new data should improve the application of D–H and 18O– 16O isotopes to constrain the temperature and origin of hydrothermal fluids responsible for serpentine formation in a variety of geologic settings.