Hydrogen Isotope Fractionation Research Articles

AgF desalination procedure for the routine determination of oxygen and hydrogen isotopic composition of saline waters and brines

ABSTRACTThe routine methods for stable oxygen and hydrogen isotope analysis of water involve water–CO2 gas equilibration and water reduction on hot metal (e.g. Zn, Cr, U) and subsequent mass spectrometric analysis of the evolved gases of CO2 and H2 for 18O/16O and 2H/1H ratios, respectively. Precise determination of the isotopic composition of water in brines with application of these standard methods is still problematic and technically often impossible due to detrimental influence of dissolved salts. The new method of brine desalination presented in this study overcomes the problem of the isotope salt effects encountered during the application of the routine techniques for the determination of the isotopic composition of high saline waters. The procedure combines two technical steps: (i) the chemical precipitation of Mg and Ca ions as insoluble non-hydroscopic fluorides, and (ii) the vacuum distillation of water from solution–precipitate mixture. The application of simple vacuum distillation allows full extraction of water and dehydration of remaining salts in a temperature range from 300 to 350 °C without hydrogen and oxygen isotope fractionation. The precision and accuracy of δ18O and δ2H determination of saline waters and brines with prior application of AgF desalination procedure is comparable with that usually obtained for fresh waters.

STXM-XANES analyses of Murchison meteorite samples captured by aerogel after hypervelocity impacts: A potential implication of organic matter degradation for micrometeoroid collection experiments

Author(s): Kebukawa, Y; Okudaira, K; Yabuta, H; Hasegawa, S; Tabata, M; Furukawa, Y; Ito, M; Nakato, A; David Kilcoyne, AL; Kobayashi, K; Yokobori, SI; Imai, E; Kawaguchi, Y; Yano, H; Yamagishi, A | Abstract: The Tanpopo mission is an astrobiology space experiment at the Japanese Experiment Module (JEM) 'Kibo' on the International Space Station (ISS). One of the sub-divided themes of the Tanpopo mission is for the intact capture of organic bearing micrometeoroids in low Earth orbit using ultralow density silica aerogel (0.01 g/cm 3 ). In order to evaluate damage to organic matter in micrometeoroids during hyper velocity impacts into the aerogel, Murchison meteorite powdered samples, analogs of organic bearing micrometeoroids, were fired into flight-grade silica aerogel (0.01 g/cm 3 ) using a two-stage light-gas gun with velocities of 4.4 and 5.9 km/s. The recovered Murchison grains were analyzed using scanning transmission X-ray microscopy/X-ray absorption near edge structure (STXM/XANES), transmission electron microscopy (TEM) and nanoscale secondary ion mass spectrometry (NanoSIMS). TEM observation did not show significant modifications of the recovered Murchison grains. Carbon-XANES spectra, however, showed a large depletion of the organic matter after the 5.9 km/s impact, but no such effects nor any significant hydrogen isotopic fractionation were observed after the 4.4 km/s impact.

2H/1H fractionation in microalgal lipids from the North Pacific Ocean: Growth rate and irradiance effects

The hydrogen isotope ratio (2H/1H) of microalgal lipids has become a useful tool for reconstructing hydrologic conditions in the geologic past from marine and lacustrine sediments. Culture studies have shown that, in addition to the 2H/1H ratio of growth water, parameters such as salinity, growth rate, and irradiance, can have a large effect on lipid 2H/1H ratios. But aside from salinity, these effects have not been well studied in the field. In this work, hydrogen isotope fractionation relative to growth water was measured in four algal lipids - C37:3 and C37:2 methyl alkenones, (3β,4α,5α,22E)-4,23-Dimethylergost-22-en-3-ol (dinosterol), and 24-methylcholesta-5,22E-dien-3β-ol (brassicasterol) – in the upper water column of two North Pacific Ocean locations. By comparing lipid 2H/1H variations as a function of depth at tropical Station ALOHA (22°45′N, 158°00′W) and mid-latitude Gyre (41°30′N, 132°00′W in the Transition Zone) it was possible to evaluate the H isotope effects of nutrient limitation and irradiance on natural phytoplankton populations by comparing measured fractionation factors (αlipid) with those derived from laboratory-based empirical relationships. Significant distinctions were observed between the inferred causes of H isotope fractionation in different algal lipids at the two locations. At Station Aloha, 2H/1H fractionation in coccolithophorid-derived alkenones was primarily caused by nutrient-limited growth, while at Gyre it was caused by light limitation. Distinctions were less clear for the two sterols. 2H/1H fractionation in dinosterol was possibly influenced by nutrient-limited growth or light limitation at greater depths at both stations, while 2H/1H fractionation in brassicasterol matched values expected for nutrient-limited growth at both locations. Although systematic trends with depth in αlipid on the order of 10–30‰ were observed for each lipid, inter-day variability was some 3–7‰ depending on the lipid and site, and the magnitude of αlipid changes observed in the field were much less than those expected based on culture studies. This implies that the effects of nutrient and light limitation on natural phytoplankton populations in the ocean may not have a large influence on hydroclimate reconstructions based on 2H/1H ratios of microalgal lipids in marine sediments.

Degradation of 4-bromophenol by Ochrobactrum sp. HI1 isolated from desert soil: pathway and isotope effects.

Anthropogenic activities have introduced elevated levels of brominated phenols to the environment. These compounds are associated with toxic and endocrine effects, and their environmental fate is of interest. An aerobic strain Ochrobactrum sp. HI1 was isolated from soils in the vicinity of a bromophenol production plant and tested for its ability to degrade 4-bromophenol (4-BP). A ring hydroxylation pathway of degradation was proposed, using the evidence from degradation intermediates analysis and multi-element (C, Br, H) compound-specific isotope analysis. Benzenetriol and 4-bromocatechol were detected during degradation of 4-bromophenol. Degradation resulted in a normal carbon isotope effect (εC = -1.11 ± 0.09‰), and in insignificant bromine and hydrogen isotope fractionation. The dual C-Br isotope trend for ring hydroxylation obtained in the present study differs from the trends expected for reductive debromination or photolysis. Thus, the isotope data reported herein can be applied in future field studies to delineate aerobic biodegradation processes and differentiate them from other natural attenuation processes.

Origin of Earth's Water: Chondritic Inheritance Plus Nebular Ingassing and Storage of Hydrogen in the Core

AbstractRecent developments in planet formation theory and measurements of low D/H in deep mantle material support a solar nebula source for some of Earth's hydrogen. Here we present a new model for the origin of Earth's water that considers both chondritic water and nebular ingassing of hydrogen. The largest embryo that formed Earth likely had a magma ocean while the solar nebula persisted and could have ingassed nebular gases. The model considers iron hydrogenation reactions during Earth's core formation as a mechanism for both sequestering hydrogen in the core and simultaneously fractionating hydrogen isotopes. By parameterizing the isotopic fractionation factor and initial bulk D/H ratio of Earth's chondritic material, we explore the combined effects of elemental dissolution and isotopic fractionation of hydrogen in iron. By fitting to the two key constraints (three oceans' worth of water in Earth's mantle and on its surface; and D/H in the bulk silicate Earth close to 150 × 10−6), the model searches for best solutions among ~10,000 different combinations of chondritic and nebular contributions. We find that ingassing of a small amount, typically >0–0.5 oceans of nebular hydrogen, is generally demanded, supplementing seven to eight oceans from chondritic contributions. About 60% of the total hydrogen enters the core, and attendant isotopic fractionation plausibly lowers the core's D/H to ~130 × 10−6. Crystallized magma ocean material may have D/H ≈ 110 × 10−6. These modeling results readily explain the low D/H in core‐mantle boundary material and account for Earth's inventory of solar neon and helium.

Hydrogen Isotope Fractionation in the Epidote–Hydrogen and Epidote–Water Systems: Theoretical Study and Implications

Experimental data for hydrogen isotope and, in general, stable isotope fractionation between mineral and water often tend to be widely divergent primarily because of solution and reprecipitation of...

Variations in hydrogen isotopic fractionation in higher plants and sediments across different latitudes: Implications for paleohydrological reconstruction

Sedimentary δDn-alkane value is widely utilized as a reliable proxy for paleo-hydrological reconstruction. Applications of this proxy must be based upon a globally clear understanding of the relationship between leaf wax δDn-alkane values and precipitation δD (δDp), defined as apparent fractionation (εapp). However, there is a critical concern about whether relatively constant εapp values exist across different latitudes. In this study, we systematically analyzed the variations of available εapp with latitudes based upon two compiled-new databases of higher plants and sediments over the world. We found that the total average εapp was relatively constant, i.e., −116 ± 5‰ (n = 941), in higher plants across different latitudes without consideration of plant types (e.g., dicots, monocots, gymnosperms), and was still constant but slightly lower average εapp, i.e., −125 ± 6‰ (n = 460), in sediments across the latitudes. The slightly lower average εapp in sediments relative to higher plants probably derived from the contribution of aquatic plants with isotopically D-depleted εapp in lake sediments. Interestingly, with consideration of plant types, average εapp increased in dicots but decreased in monocots slightly from low to high latitudes. The counteraction of these competing trends generates relatively constant average εapp values in higher plants, and resultantly constant average εapp values occur in sediments at the global scale. It is important to elaborate relatively constant εapp values from higher plants and sediments across different latitudes when sedimentary δDn-alkane is utilized as a proxy for paleohydrological reconstruction.

Variation in Hydrogen Isotope Composition Among Salt Marsh Plant Organic Compounds Highlights Biochemical Mechanisms Controlling Biosynthetic Fractionation

AbstractHydrogen isotopes of plant‐derived biomarkers can vary by >100‰ at a single location. Isotope fractionation associated with the movement of water in plant leaves cannot account for this variability alone. Biochemical processes therefore must play a fundamental role in controlling hydrogen isotope fractionation during secondary compound biosynthesis. Different biosynthetic pathways utilize discrete hydrogen pools and occur within distinct cell compartments. We analyzed hydrogen isotope compositions of C16 and C18 fatty acids and phytol from seven salt marsh plants and compared these data with (i) leaf water and n‐alkane δ2H, (ii) leaf carbon and nitrogen contents, and (iii) nitrogen isotopes of bulk tissue, to evaluate the relationship between biochemical processes, cellular compartmentalization, and hydrogen isotope fractionation. Interspecies variation in chloroplastic fatty acids and phytol δ2H exceeds leaf water δ2H, indicating that different commitments of metabolites among species at branching points in chloroplast metabolic processes may be important determinants of lipid δ2H values. Dominant osmoregulatory strategies, in particular, show strong correlation with leaf wax n‐alkane δ2H. Species that preferentially produce nitrogenous compounds (dicots/shrubs) as protective solutes have 2H‐enriched n‐alkanes relative to species that produce mainly carbohydrates (monocots). n‐Alkane δ2H values, in combination with δ15N data and elemental (C, N) composition, together provide information about biochemical environmental adaptations exhibited by different higher plant species in response to environmental stresses. Thus, while spatial and temporal integration of biomarkers may produce an isotopic record of ecosystem function, biomarkers from individual plant or microbial remains may hold additional details into biologic function and adaptation to ancient environments.

Net fractionation of hydrogen isotopes in n-alkanoic acids from soils in the northern boreal forest

Plant-derivedn-alkyl lipids are well-preserved in sedimentary archives, and their stable hydrogen isotope ratio (δDwax) is a proxy for precipitation δD and climate. Net fractionation of H isotopes between plant source water andn-alkyl lipids (εwax/water) is the largest uncertainty for interpreting this proxy and depends on plant type and environment. Although popular proxies,n-alkanoic acids (n-acids) are less frequently calibrated in modern environments thann-alkanes. We constrain the net fractionation (εwax/MAP – Mean Annual Precipitation) for the northern boreal forest based onn-C24,26,28acids in topsoils from a 13-site network in Yukon, Alaska and Northwest Territories (60–68°N).n-Acid homologue distributions for common boreal plants (gymnosperm trees, shrubs, forbs, C3 grasses and mosses) show that soiln-acids are dominated by mosses, but with a reduced carbon preference index compared to fresh mosses, possibly owing to post-depositional degradation. Regionally averaged εwax/MAP values are −93 ± 10‰, −101 ± 11‰ and −95 ± 11‰ for n-C24,26,28 acids, respectively, which are similar to values reported from low and high latitude regions. We find that εwax/MAP for n-C26,28 acids (but not n-C24) is significantly (p ≤ 0.05) smaller with increasing latitude and decreasing mean annual temperature, factors that covary across our network. Since latitude is typically known for ancient deposits, we parsed the dataset to obtainlatitude-specific εwax/MAPvalues for 60–64°N (−107 ± 8‰ for n-C26; and −101 ± 8‰ for n-C28) and 65–68°N (−94 ± 11‰ for n-C26; and −89 ± 10‰ for n-C28).These soil-derived values are relevant for hydroclimate reconstructions using fossiln-acids in paleosols from a comparable paleoflora and latitude.

Carbon and hydrogen stable isotope analysis for characterizing the chemical degradation of tributyl phosphate

Tributyl phosphate (TBP) belongs to the group of trialkyl substituted organophosphate esters. Its chemical reactivity depends on the stability of various chemical bonds. TBP was used as a model compound for the development of a concept using stable isotope fractionation associated with bond cleavage reactions for better understanding the fate of TBP in the environment. Carbon isotope enrichment factors (εC) of TBP hydrolysis were found to be pH dependent (−3.8 ± 0.3‰ at pH 2, -4.6 ± 0.5‰ at pH 7, -2.8 ± 0.1‰ at pH 9, no isotope fractionation at pH 12), which is in accordance with the mode of a SN2 hydrolytic bond cleavage. Hydrogen isotope fractionation was negligible as no H bond cleavage is involved during hydrolysis. The apparent carbon kinetic isotope effect (AKIEC) ranged from 1.045 to 1.058. In contrast to hydrolysis, both carbon and hydrogen isotope fractionation were observed during radical oxidation of TBP by OH and SO4–, yielding εC from −0.9 ± 0.1‰ to −0.5 ± 0.1‰ and εH from −20 ± 2‰ to −11 ± 1‰. AKIEC and AKIEH varied from 1.007 to 1.011 and from 1.594 to 2.174, respectively. The correlation of 2H and 13C isotope fractionation revealed Λ values ranging from 17 ± 1 to 25 ± 6. Results demonstrated that the correlation of 2H and 13C isotope fractionation of TBP allowed to identify radical reactions and to distinguish them from hydrolysis. The presented dual isotope analysis approach has diagnostic value for characterizing the chemical transformation of TBP in the environment.

Assessing the metabolism of sedimentary microbial communities using the hydrogen isotopic composition of fatty acids

The hydrogen isotopic composition of fatty acids (FAs) has previously been shown to reflect the metabolism of microorganisms in pure culture, but has rarely been tested in the environment. Here, we report the abundance and hydrogen isotopic composition of polar lipid derived FAs extracted from surface sediments of the saline Lake Grevelingen (The Netherlands), at two different stations and during two seasons with oxic bottom water conditions during spring and hypoxic to anoxic conditions during late summer. These data are compared with the bacterial diversity revealed by 16S rRNA gene amplicon sequencing. All measured FAs were depleted in deuterium relative to the bottom water by 103‰ to 267‰. FAs associated with heterotrophic bacteria (i-15:0 and ai-15:0) showed the smallest fractionation (−103‰ to −185‰) while those derived from pelagic photoautotrophic phytoplankton (20:5) showed the largest fractionation (−230‰ to −267‰). Overall, the hydrogen isotope fractionation reflected in the majority of the more commonly occurring FAs (14:0, 16:0, 16:1ω7) is relatively large (−172‰ to −217‰). Together with the high relative abundance of the 20:5 FA, this suggests a substantial contribution from dead pelagic biomass settling from the water column to the sedimentary polar lipid derived FA pool and not from the in situ microbial communities. Therefore, the majority of the isotope signal in the fatty acids from surface sediments might not represent the general metabolism of the active sedimentary communities. Therefore, the input of pelagic biomass into sedimentary environments may bias the information contained in the hydrogen isotopic composition of FAs.

Comparison of n-alkane molecular, carbon and hydrogen isotope compositions of different types of plants in the Dajiuhu peatland, central China

Long chain n-alkanes preserved in peat deposits have been widely used to reconstruct paleoenvironmental and paleoecological changes. However, our knowledge of how the leaf wax molecular and isotopic compositions of peat sequences might respond to changes in plant communities is limited. This study investigates the n-alkane molecular, carbon and hydrogen isotopic compositions of plant species collected from the Dajiuhu peatland, central China. The results show that forb and graminoid plants growing in peatlands have a relatively higher carbon preference index of long-chain n-alkanes than shrubs, ferns and aquatic plants. The δ13C values of individual n-alkanes generally decrease with increasing chain lengths, and different plant groups have similar δ13C values of C31 and C33n-alkanes. An exception is Sphagnum, which has more negative δ13C values of C23 and C25 than those of its longer chain length homologues. Relative to peat pore water δD values in different seasons and different peat depths, the Dajiuhu plants normally exhibit relatively large hydrogen isotope fractionations, and the fractionations gradually increase from graminoid to forb to shrub. Taken together, these results highlight the influence of different kinds of vegetation on the collective molecular and isotopic ratios of n-alkanes in peat deposits.

Large carbon isotope variability during methanogenesis under alkaline conditions

High carbon isotope values (δ13CCH4 > −40‰) have widely been used as evidence that methane in alkaline rock-hosted fluids was formed abiotically, particularly in serpentinizing systems. However, isotope fractionation during microbial methanogenesis is relatively understudied at high pH. We isolated a hydrogenotrophic Methanobacterium sp. from hyperalkaline subsurface fluids in the Samail ophiolite to assess how carbon and hydrogen isotope values of CH4 varied depending upon pH and carbonate mineral source (NaHCO3 or CaCO3). The hydrogen isotope fractionation αH20/CH4 (1.46–1.66) did not vary across pH. In contrast, the expressed carbon isotope fractionation, αCO2/CH4, ranged from 1.028 to 1.089. Carbon isotope fractionation increased with pH, reaching a maximum 13C depletion of −85‰. However, the 13C depletion significantly diminished at pH ≥ 9 for CaCO3-amended experiments, generating δ13CCH4 as high as −28‰. To evaluate the large variability in δ13CCH4, we developed a steady-state model to assess how the rates of carbonate dissolution, cellular uptake of CO2 and irreversible CH4 production can affect the net isotope fractionation during methanogenesis. Methanobacterium sp. can produce highly depleted δ13CCH4 in simulated alkaline serpentinizing fluids when dissolved inorganic carbon levels are high and methanogenesis rates are slow. However, small carbon isotope fractionation occurs when rates of carbonate dissolution are slower than cellular uptake, leading to relatively high δ13CCH4 values (>∼−35‰) that are traditionally interpreted to be purely “abiotic”. Thus, microbial CH4 can be produced in carbon-limited mafic and ultramafic rock-hosted environments on Earth and potentially other planetary bodies, but it may be difficult to isotopically identify biogenic methane when mineral carbonates are the dominant carbon source.

Carbon and hydrogen isotope analysis of parathion for characterizing its natural attenuation by hydrolysis at a contaminated site

The applicability of compound-specific isotope analysis (CSIA) for assessing in situ hydrolysis of parathion was investigated in a contaminated aquifer at a former pesticide wastes landfill site. Stable isotope analysis of parathion extracted from groundwater taken from different monitoring wells revealed a maximum enrichment in carbon isotope ratio of +4.9‰ compared to the source of parathion, providing evidence that in situ hydrolysis took place. Calculations based on the Rayleigh-equation approach indicated that the natural attenuation of parathion was up to 8.6% by hydrolysis under neutral and acidic conditions. In degradation experiments with aerobic and anaerobic parathion-degrading microbes, no carbon and hydrogen isotope fractionation of parathion were observed. For the first time, CSIA has been applied for the exclusive assessment of the hydrolysis of phosphorothioate-containing organophosphorus pesticides at a contaminated field site.

Hydrogen isotope fractionation in the system brucite-water ± NaCl to elevated temperatures and pressures: Implications for the isotopic property of NaCl fluids under geologic conditions

A series of hydrothermal experiments have been conducted to determine equilibrium D/H fractionations between brucite and water as a function of temperature (200–600 °C), pressure (2.1–800 MPa), and dissolved NaCl (0–5 molal or 0–22.6 wt%). Along with our previous study, a total of 39 data points show that pressure and dissolved NaCl both increase the D/H fractionation factor at a given temperature. Theoretical treatment allowed a first direct comparison of experimental and theoretical results of brucite-water D/H fractionation at a zero pressure limit. After correcting the pressure effects on brucite D/H partition function ratios (β-factor), the isotope effects of pressure and dissolved NaCl on the D/H β-factor of water were for the first time rigorously evaluated. There is a good linear relationship between the D/H β-factor of the aqueous NaCl solutions and their densities of at a given temperature. A good, pseudo-linear relationship observed between the density of aqueous NaCl solutions and the maximum intensity of the asymmetric OH vibrational frequency of water in turn suggests that this frequency is a fundamental property that determines the β-factor of water at elevated temperatures and pressures. Our study suggests that the density of aqueous solutions (ρaq-soln) could be used as a primary parameter to predict the effects of pressure and NaCl compositions on the β-factor of water under geologic conditions encountered in the crust and upper mantle: Δ103lnβwat/Δρaq-soln = −0.0111 (±0.0012), where Δ is a difference in the value of 103lnβwat orρaq-soln, and ρaq-soln in kg m−3.

A unified Craig-Gordon isotope model of stable hydrogen and oxygen isotope fractionation during fresh or saltwater evaporation

Evaporation of water from the oceans and terrestrial environment governs the global water cycle and climate. Heavy isotope (2H,18O,17O) enrichment during evaporation led to the development of the well-known Craig-Gordon (C-G) model for quantifying evaporation. Several variables control the H and O isotopic composition of evaporating water including; the isotopic composition of the water, temperature, relative humidity, ambient vapor isotopic composition, diffusion and/or mixing at the water-air interface, and the thermodynamic activity (salinity) of water. Previous C-G modeling efforts considered these controlling variables separately; here we propose a newly unified C-G analytical model that allows for simultaneous quantification of all controlling variables in the evaporation of fresh and saline waters. Our unified model accurately predicted the results of laboratory water evaporation experiments conducted under a variety of molecular diffusion and turbulent diffusion conditions. We demonstrate the general applicability of the model by successfully predicting historical O and H isotope data for fresh and saltwater evaporation obtained from the scientific literature. The unified C-G model allows for improved estimates environmental parameters controlling the H and O isotope fractionation during water evaporation under natural conditions and can be used to better inform modeling efforts in regional and large-scale water balance studies.

Variable apparent hydrogen isotopic fractionation between sedimentary n-alkanes and precipitation on the Tibetan Plateau

The hydrogen isotope ratio values of sedimentary leaf wax n-alkanes (δDwax) have been considered a reliable proxy for precipitation isotope ratios (δDP) across various transects on a global basis. Records of δDwax have been used to infer past changes in hydrology, climate and altimetry, assuming nearly constant apparent hydrogen isotopic fractionation (εwax-P) between sedimentary leaf wax components and precipitation. Here, we present the results of an analysis of δDwax values from the surface sediments of 22 lakes across a precipitation and vegetation gradient on the Tibetan Plateau. The δDwax values correlated significantly with mean annual precipitation δD (r 0.90) and summer precipitation δD (r 0.91), suggesting that δDwax can record δDP in very dry regions. However, εwax-P varies with annual precipitation as well as with relative humidity. Calculation using Craig-Gordon isotope models showed that the variable εwax-P values likely resulted from intensive soil evaporation and plant evapotranspiration in this relatively arid region. This implies that variable isotopic fractionation should be taken into consideration when reconstructing paleohydrology, paleoclimate and paleoaltimetry.

Mass spectrometric measurement of hydrogen isotope fractionation for the reactions of chloromethane with OH and Cl

Abstract. Chloromethane (CH3Cl) is an important provider of chlorine to the stratosphere but detailed knowledge of its budget is missing. Stable isotope analysis is a potentially powerful tool to constrain CH3Cl flux estimates. The largest degree of isotope fractionation is expected to occur for deuterium in CH3Cl in the hydrogen abstraction reactions with its main sink reactant tropospheric OH and its minor sink reactant Cl atoms. We determined the isotope fractionation by stable hydrogen isotope analysis of the fraction of CH3Cl remaining after reaction with hydroxyl and chlorine radicals in a 3.5 m3 Teflon smog chamber at 293 ± 1 K. We measured the stable hydrogen isotope values of the unreacted CH3Cl using compound-specific thermal conversion isotope ratio mass spectrometry. The isotope fractionations of CH3Cl for the reactions with hydroxyl and chlorine radicals were found to be -264±45 and -280±11 ‰, respectively. For comparison, we performed similar experiments using methane (CH4) as the target compound with OH and obtained a fractionation constant of -205±6 ‰ which is in good agreement with values previously reported. The observed large kinetic isotope effects are helpful when employing isotopic analyses of CH3Cl in the atmosphere to improve our knowledge of its atmospheric budget.

Carbon and hydrogen isotope fractionation of phthalate esters during degradation by sulfate and hydroxyl radicals

This study investigated 13C and 2H isotope fractionation associated with oxidation of three phthalate esters (PAEs) by radical species, including sulfate radical (SO4−) induced by heat-activated persulfate (PS) and hydroxyl radical (HO) induced by UV/H2O2. For persulfate oxidation at pH = 2 and pH = 7, similar carbon isotope fractionation (εC) but distinct hydrogen isotope enrichment factors (εH) were observed. The UV/H2O2 reaction of three PAEs showed smaller εH values in comparison with persulfate oxidation. The correlation of 2H and 13C fractionation (Λ) allows to distinguish the persulfate oxidation (25.7 ± 2.6) and UV/H2O2 oxidation (2.4 ± 0.2) of diethyl phthalate (DEP) highlighting the potential of compound-specific stable isotope analysis (CSIA) to characterize chemical oxidation mechanism of PAEs. Moreover, study of radical quenching and CSIA were combined to explore the dominant radical species during persulfate oxidation of DEP. SO4− was found to be the predominant radical at pH = 2. Both SO4− and HO contributed to DEP degradation at pH = 7 and HO was estimated to have a contribution of 21–63% according to dual CH isotope fractionation values. Carbon and hydrogen apparent kinetic isotope effects (AKIEs) (13C-AKIE = 1.017, 2H-AKIE = 2.41) obtained from dominating sulfate radical reaction of DEP both supported the hypothesis of CH bond cleavage. Thus, carbon and hydrogen isotope enrichment factors clearly distinguish the different reaction mechanisms and hence, are a promising approach to improve understanding of radical species reaction pathways for chemical oxidation of PAEs.

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).