Carbon Isotope Fractionation Factors Research Articles

Anaerobic methane oxidation by nitrate: kinetic isotope effect

The ratio of stable carbon isotopes (13C/12C) in different environments serves as a significant limitation in estimating the global balance of methane [Hornibrook et al., 2000]. In this case, the value of 13C/12C largely depends on the kinetic isotope effect associated with the metabolism of microorganisms that produce and consume CH4. The article suggests a dynamic model of the processes of methane formation and its anaerobic oxidation with nitrate by methanotrophic denitrifying microorganisms (DAOM), which allowed estimating the fractionation factor of stable carbon isotopes. In the experiment with peat from the minerotrophic bog [Smemo, Yavitt, 2007], the dynamics of the amount of methane and was measured. The dynamic model showed that the introduction of nitrate leads to a slow decrease in the partial pressure of methane. Since methane in the DAOM process is a substrate, methane is enriched with heavier carbon 13C in the system under study. This leads to an increase in the value . The carbon isotope fractionation factor during methane oxidation with nitrate was equal to 1.018 and comparable with the fraction of carbon isotope fractionation in the process of acetoclastic methanogenesis (1.01). Model calculations have shown that during incubation the apparent fractionation factor of carbon isotopes with the simultaneous formation of methane and DAOM slowly decreases. The ratio of 13C/12C isotopes in dissolved and gaseous methane practically does not differ. The model showed that an increase in the initial concentration of nitrate increases the rate of DAOM, which leads to a decrease in the concentration of dissolved methane. In this case, the value of 13C/12C increases. In field studies, Shi et al. (2017) showed that the presence of DAOM in peat bogs in which fertilizers penetrate can be controlled by the amount of nitrate used and the depth of penetration into the anoxic layer. Two MATLAB files describing DAOM are attached to the article.

Experimental determination of equilibrium CH4–CO2–CO carbon isotope fractionation factors (300–1200 °C)

Carbon isotope fractionation in the CO2–CO–CH4–C system was investigated at 300–1200°C at near-atmospheric pressures by thermally decomposing a variety of organic materials in sealed quartz tubes. Measured gas speciations correspond well to the expected range from thermodynamic calculations. We show that chemical and isotopic equilibrium among gas species is obtained when applying a nickel catalyst for CO2/CH4, CH4/CO, and CO2/CO at ≤600°C or without a catalyzing agent for CO2/CO at ≥800°C. The experiments define carbon isotope fractionation factors for the CO2/CH4, CO2/CO and CH4/CO pairs as(i)103ln⁡αCO2/CH4=8.9(±0.6)⋅105⋅(1T2)0.825(±0.005) (200–1200°C)(ii)103ln⁡αCO2/CO=1.07(±0.05)⋅106⋅(1T2)0.830(±0.003) (300–1200°C)(iii)103ln⁡αCH4/CO=1.1(±0.2)⋅103⋅(1T2)0.462(±0.001) (300–1200°C), which reproduce the experimental values within 0.2‰ for CO2/CH4 and CO2/CO and within 0.12‰ for CH4/CO (T in K, 1σ fit uncertainties in brackets, CO2/CH4 includes the ≤600°C experimental data of Horita, 2001). Carbon isotope fractionation factors at 1000°C are still large for CO2/CH4 and CO2/CO (6.6 and 7.5‰, respectively) but only 1.5‰ for CH4/CO.Elemental carbon precipitated through thermal decomposition of the organic starting materials yields δ13C values that depend on the X(O) = O/(O + H) of the organic starting material, i.e. the initial oxidation state of carbon in the organics. We further observe a catalytic effect of the quartz walls on chemical and isotopic exchange in the CO2/CO system, probably due to the activation of the silicate surface by H+ and OH− ions at >650°C. Our experimental results yield improved calibrations of the CO2/CH4 equilibria and the first experimental calibration of CO2/CO and CH4/CO carbon isotope fractionation. Applications are in the tracing of magmatic hydrothermal gas emissions, in carbon-precipitating COH-fluids, and in monitoring of coal-seam fires, but our results may also be applied for quality control during steel-making processes.

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.

Biotransformation and inhibition effects of hexachlorocyclohexanes during biogas production from contaminated biomass characterized by isotope fractionation concepts

Hexachlorocyclohexane (HCH) production for pesticides was banned by Stockholm Convention (2009) due to its harmful and adverse effects on the environment. Despite this measure, many areas contaminated with former HCH production-waste products still require management. As a potential solution contributing to clean-up of these sites, anaerobic digestion (AD) of pesticide-contaminated biomass to produce biogas is a promising strategy. High pesticide concentrations, however, may inhibit biogas production. Therefore, laboratory-scale batch reactors were set up to investigate biogas reactor performance in presence of HCH. Inhibitory effects on biogas yield was observed with concentrations of HCH ≥ 150 mg/L. Carbon isotope composition of methane (δ13CCH4) showed significant fluctuation after an inhibition phase, indicating that HCH toxicity can affect the activity of acetoclastic methanogens. Furthermore, combined results of metabolites and carbon isotope fractionation factors (εc) demonstrated that α- and γ-HCH can be degraded to chlorobenzene and benzene via anaerobic reductive dechlorination.

Experimental, in-situ carbon solution mechanisms and isotope fractionation in and between (C–O–H)-saturated silicate melt and silicate-saturated (C–O–H) fluid to upper mantle temperatures and pressures

Our understanding of materials transport processes in the Earth relies on characterizing the behavior of fluid and melt in silicate–(C–O–H) systems at high temperature and pressure. Here, Raman spectroscopy was employed to determine structure of and carbon isotope partitioning between melts and fluids in alkali aluminosilicate–C–O–H systems. The experimental data were recorded in-situ while the samples were at equilibrium in a hydrothermal diamond anvil cell at temperatures and pressures to 825 °C and >1300 MPa, respectively.The carbon solution equilibrium in both (C–O–H)-saturated melt and coexisting, silicate-saturated (C–O–H) fluid is 2CO3 + H2O + 2Qn + 1 = 2HCO3 + 2Qn. In the Qn-notation, the superscript, n, is the number of bridging oxygen in silicate structural units. At least one oxygen in CO3 and HCO3 groups likely is shared with silicate tetrahedra. The structural behavior of volatile components described with this equilibrium governs carbon isotope fractionation factors between melt and fluid. For example, the ΔH equals 3.2 ± 0.7 kJ/mol for the bulk 13C/12C exchange equilibrium between fluid and melt. From these experimental data, it is suggested that at deep crustal and upper mantle temperatures and pressures, the δ13C-differences between coexisting silicate-saturated (C–O–H) fluid and (C–O–H)-saturated silicate melts may change by more than 100‰ as a function of temperature in the range of magmatic processes. Absent information on temperature and pressure, the use of carbon isotopes of mantle-derived magma to derive isotopic composition of magma source regions in the Earth's interior, therefore, should be exercised with care.

Dynamic changes of carbon isotope apparent fractionation factor to describe transition to syntrophic acetate oxidation during cellulose and acetate methanization

ABSTRACTTo identify predominant metabolic pathway for cellulose methanization new equations that take into account dynamics of 13C are added to the basic model of cellulose methanization. The correct stoichiometry of hydrolysis, acidogenesis, acetogenesis and methanogenesis steps including biomass is considered. Using experimental data by Laukenmann et al. [Identification of methanogenic pathway in anaerobic digesters using stable carbon isotopes. Eng. Life Sci. 2010;10:1–6], who reported about the importance of ace`tate oxidation during mesophilic cellulose methanization, the model confirmed that, at high biomass concentration of acetate oxidizers, the carbon isotope fractionation factor amounts to about 1.085. The same model, suggested firstly for cellulose degradation, was used to describe, secondly, changes in, and in methane and carbon dioxide during mesophylic acetate methanization measured by Grossin-Debattista [Fractionnements isotopiques (13C/12C) engendres par la methanogenese: apports pour la comprehension des processus de biodegradation lors de la digestion anaerobie [doctoral thesis]. 2011. Bordeaux: Universite Bordeaux-1;2011. Available from: http://ori-oai.u-bordeaux1.fr/pdf/2011/GROSSIN-DEBATTISTA_JULIEN_2011.pdf. French].The model showed that under various ammonium concentrations, at dominating acetoclastic methanogenesis, the value decreases over time to a low level (1.016), while at dominating syntrophic acetate oxidation, coupled with hydrogenotrophic methanogenesis, slightly increases, reaching 1.060 at the end of incubation.

Experimentally-determined carbon isotope fractionation in and between methane-bearing melt and fluid to upper mantle temperatures and pressures

Abstract The behavior of melts and fluids is at the core of understanding formation and evolution of the Earth. To advance our understanding of their role, high-pressure/-temperature experiments were employed to determine melt and fluid structure together with carbon isotope partitioning within and between ( CH 4 + H 2 O + H 2 )-saturated aluminosilicate melts and ( CH 4 + H 2 O + H 2 )-fluids. The samples were characterized with vibrational spectroscopy while at temperatures and pressures from 475° to 850 °C and 92 to 1158 MPa, respectively. The solution equilibrium is 2 CH 4 + Q n = 2 CH 3 − + H 2 O + Q n + 1 where the superscript, n , in the Q n -notation describes silicate species where n denotes the number of bridging oxygen. The solution equilibrium affects the carbon isotope fractionation factor between melt and fluid, α melt/fluid . Moreover, it is significantly temperature-dependent. The α melt/fluid 1 with temperatures less than about 1050 °C, and is greater than 1 at higher temperature. Methane-bearing melts can exist in the upper mantle at f O 2 ≤ f O 2 ( MW ) ( Mysen et al., 2011 ). Reduced (C H)-species in present-day upper mantle magma, therefore, are likely. During melting and crystallization in this environment, the δ 13 C of melts increases with temperature at a rate of ∼ 0.6 ‰ / °C . From the simple-system data presented here, at T ≤ 1050 °C , melt in equilibrium with a peridotite-( CH 4 + H 2 O + H 2 )-bearing mantle source will be isotopically lighter than fluid. At higher temperatures, melts will be isotopically heavier. Degassing at T ≤ 1050 °C will shift δ 13 C of degassed magma to more positive values, whereas degassing at T ≥ 1050 °C , will reduce the δ 13 C of the degassed magma.

Estimating stable carbon isotope values of microphytobenthos in the Arctic for application to food web studies

Most studies on Arctic food webs have neglected microphytobenthos as a potential food source because we currently lack robust measurements of δ13C values for microphytobenthos from this environment. As a result, the role of microphytobenthos in high latitude marine food webs is not well understood. We combined field measurements of the concentration of aqueous carbon dioxide and the stable carbon isotopic composition of dissolved inorganic carbon (δ13CDIC) from bottom water in the Beaufort and Chukchi seas with a set of stable carbon isotopic fractionation factors reflecting differences in algal taxonomy and physiology to estimate the stable carbon isotope composition of microphytobenthos-derived total organic carbon (δ13Cp). The δ13Cp for Phaeodactylum tricornutum, a pennate diatom likely to be a dominant microphytobenthos taxon, was estimated to be −23.9 ± 0.4 ‰ as compared to a centric diatom (Porosira glacialis, δ13Cp = −20.0 ± 1.6 ‰) and a marine haptophyte (Emiliana huxleyi, δ13Cp = −22.7 ± 0.5 ‰) at a growth rate (µ) of 0.1 divisions per day (d−1). δ13Cp values increased by ~2.5 ‰ when µ increased from 0.1 to a maximum growth rate of 1.4 d−1. We compared our estimates of δ13Cp values for microphytobenthos with published measurements for other carbon sources in the Arctic and sub-Arctic. We found that microphytobenthos values overlapped with pelagic sources, yet differed from riverine and ice-derived carbon sources. These model results provide valuable insight into the range of possible isotopic values for microphytobenthos from this region, but we remain cautious in regard to the conclusiveness of these findings given the paucity of field measurements currently available for model validation.

Effects of environmental and biotic factors on carbon isotopic fractionation during decomposition of soil organic matter.

Decomposition of soil organic matter (SOM) plays an important role in the global carbon cycle because the CO2 emitted from soil respiration is an important source of atmospheric CO2. Carbon isotopic fractionation occurs during SOM decomposition, which leads to 12C to enrich in the released CO2 while 13C to enrich in the residual SOM. Understanding the isotope fractionation has been demonstrated to be helpful for studying the global carbon cycle. Soil and litter samples were collected from soil profiles at 27 different sites located along a vertical transect from 1200 to 4500 m above sea level (a.s.l.) in the south-eastern side of the Tibetan Plateau. Their carbon isotope ratios, C and N concentrations were measured. In addition, fiber and lignin in litter samples were also analyzed. Carbon isotope fractionation factor (α) during SOM decomposition was estimated indirectly as the slope of the relationship between carbon isotope ratios of SOM and soil C concentrations. This study shows that litter quality and soil water play a significant role in isotope fractionation during SOM decomposition, and the carbon isotope fractionation factor, α, increases with litter quality and soil water content. However, we found that temperature had no significant impact on the α variance.

Enrichment of nitrogen and 13C of methane in natural gases from the Khuff Formation, Saudi Arabia, caused by thermochemical sulfate reduction

Permian Khuff reservoirs along the east coast of Saudi Arabia and in the Arabian Gulf produce dry sour gas with highly variable nitrogen concentrations. Rough correlations between N2/CH4, CO2/CH4 and H2S/CH4 suggest that non-hydrocarbon gas abundances are controlled by thermochemical sulfate reduction (TSR). In Khuff gases judged to be unaltered by TSR, methane δ13C generally falls between −40‰ and −35‰ VPDB and carbon dioxide δ13C between −3‰ and 0‰ VPDB. As H2S/CH4 increases, methane δ13C increases to as much as −3‰ and carbon dioxide δ13C decreases to as little as −28‰. These changes are interpreted to reflect the oxidation of methane to carbon dioxide.Khuff reservoir temperatures, which locally exceed 150°C, appear high enough to drive the reduction of sulfate by methane. Anhydrite is abundant in the Khuff and fine grained nodules are commonly rimmed with secondary calcite cement. Some cores contain abundant pyrite, sphalerite and galena. Assuming that nitrogen is inert, loss of methane by TSR should increase N2/CH4 of the residual gas and leave δ15N unaltered. δ15N of Paleozoic gases in Saudi Arabia varies from −7‰ to 1‰ vs. air and supports the TSR hypothesis. N2/CH4 in gases from stacked Khuff reservoirs varies by a factor of 19 yet the variation in δ15N (0.3–0.5‰) is trivial.Because the relative abundance of hydrogen sulfide is not a fully reliable extent of reaction parameter, we have attempted to assess the extent of TSR using plots of methane δ13C versus log(N2/CH4). Observed variations in these parameters can be fitted using simple Rayleigh models with kinetic carbon isotope fractionation factors between 0.98 and 0.99. We calculate that TSR may have destroyed more than 90% of the original methane charge in the most extreme instance. The possibility that methane may be completely destroyed by TSR has important implications for deep gas exploration and the origin of gases rich in nitrogen as well as hydrogen sulfide.

Oxygen isotope fractionation in double carbonates

Oxygen isotope fractionations in double carbonates of different crystal structures were calculated by the increment method. Synthesis experiments were performed at 60 °C and 100 °C to determine oxygen and carbon isotope fractionations involving PbMg[CO3]2. The calculations suggest that the double carbonates of calcite structure are systematically enriched in 18O relative to those of aragonite and mixture structures. Internally consistent oxygen isotope fractionation factors are obtained for these minerals with respect to quartz, calcite and water at a temperature range of 0–1200 °C. The calculated fractionation factors for double carbonate–water systems are generally consistent with the data available from laboratory experiments. The experimentally determined fractionation factors for PbMg[CO3]2, BaMg[CO3]2 and CaMg[CO3]2 against H2O not only fall between fractionation factors involving pure carbonate end-members but are also close to the calculated fractionation factors. In contrast, experimentally determined carbon isotope fractionation factors between PbMg[CO3]2 and CO2 are much closer to theoretical predictions for the cerussite–CO2 system than for the magnesite–CO2 system, similar to the fractionation behavior for BaMg[CO3]2. Therefore, the combined theoretical and experimental results provide insights into the effects of crystal structure and exchange kinetics on oxygen isotope partitioning in double carbonates.

Isotopic constraints on the nature and circulation of deep mantle C–H–O–N fluids: Carbon and nitrogen systematics within ultra-deep diamonds from Kankan (Guinea)

Sublithospheric diamonds that sample the transition zone and uppermost lower mantle provide a unique view into the deep Earth. In order to investigate the origin of diamond-forming C–H–O–N fluids within the deep mantle, within the framework of the terrestrial deep volatile cycle, we conducted a δ13C-δ15N-[N] micro-analytical study, by secondary ion mass spectrometry, of five Kankan diamonds from the asthenosphere/transition zone and the lower mantle.Abrupt and large changes in δ13C within KK-99 (up to 10.2‰) and KK-200A (up to 6.9‰) illustrate distinct episodes of diamond growth, involving different fluids, possibly during transport of diamond to deeper mantle depths from the asthenosphere/transition zone into the lower mantle. Despite limited variability of δ13C within individual samples, diamonds KK-200B, KK-203, KK-204 and KK-207 display systematic δ13C–δ15N–[N] co-variations which can be modelled as a single diamond growth episode in a Rayleigh process from fluids/melts. These data constrain the carbon isotopic fractionation factors to be both negative (ΔC=−0.9‰ for KK-200B and −2.0‰ for both KK-203 and −207) and positive (ΔC=+1.0‰ for KK-204), consistent with equilibrium between diamond and oxidised (CO2 or carbonate) and reduced (CH4 or carbide) fluids respectively. The modelling of δ15N–[N] systematics suggests that the diamonds are depleted by ∼4‰ (KK-200B) and ∼0‰ (KK-204) relative to the oxidised and reduced sources respectively. Modelling the co-variation indicates a compatible behaviour of nitrogen in diamond relative to the growth medium (KN=4–16), independent of the redox state. The parental fluids to the ultra-deep diamonds exhibit geochemical characteristics (δ13C–δ15N–[N]–KN–ΔC–ΔN) comparable to fluids thought to form lithospheric diamonds, suggesting a common mechanism of diamond genesis.The metaperidotitic parageneses and the slightly negative δ13C signatures for both KK-204 and KK-207 are consistent with their formation in the lower mantle by fluids that originate either from mantle or subducted carbon sources. A carbon flux from subducted oceanic lithospheric mantle may be important in the latter case. The strictly positive δ15N signatures found both in KK-200B (δ13C>0‰) and KK-204 (δ13C slightly negative) illustrate that surficial carbon and nitrogen are potentially recycled as deep as the asthenosphere/transition zone and the lower mantle. Calculations of the diffusive relaxation of carbon isotope heterogeneity indicate that these ultra-deep diamonds may have a relatively young age and/or experienced rapid vertical movement to shallower mantle conditions, possibly by plume-related mantle.

Oxygen and carbon isotope fractionation in the system dolomite–water–CO2 to elevated temperatures

An experimental study was conducted to determine oxygen and carbon isotope fractionation factors in the system dolomite–water–CO2 at 80–350 and 100–250°C, respectively, by means of direct precipitation (80°C) and dolomitization of CaCO3 (100–350°C). The products are protodolomite with slight Ca-excess (80–100°C) and well-ordered stoichiometric dolomite (150–350°C). Several experimental artifacts (inheritance, premature reactions, and kinetic effects) were tested, although attainment of isotope equilibrium cannot be proven. 18O/16O fractionation factors of (proto)dolomite–water at 80–350°C can be readily expressed with 1σ error: 103lnαdolomite–water=3.140(±0.022)·106T2-3.14(±0.11).Our experimental study, which is generally consistent with a majority of experimental and theoretical studies in the literature, provides for the first time an accurate equation over a wide range of temperature. In combination of the calcite–water equation (O’Neil et al., 1969; Friedman and O’Neil, 1977), 18O/16O fractionation factors of (proto)dolomite–calcite at 80–350°C can also be expressed with 1σ error: 103lnαdolomite–calcite=0.351(±0.028)·106T2-0.25(±0.13). Dolomite is slightly (0.7–2.6‰) enriched in 18O relative to calcite in this temperature range. Given the very good linearity with a 1/T2 term, the above two equations may be extrapolated beyond the temperature range. Our experimental results of 13C/12C fractionation between CO2 and dolomite at 100–250°C also show a linear function with a 1/T2 term with a cross-over temperature of 200°C, which differs from results of theoretical calculations.

Influence of climate change on carbon and oxygen isotope fractionation factors between glucose and α-cellulose of pine wood

Abstract We present the first analysis of the influence of climate change on carbon and oxygen isotope fractionation factors for two saccharides (glucose and α-cellulose) of pine wood. The conifers grew in the Niepołomice Forest in Poland and the annual rings covered a time span from 1935 to 2000 AD. Glucose samples from acid hydrolysis of α-cellulose were extracted from annual tree rings. The carbon and oxygen isotope fractionation factors between glucose and α-cellulose were not stable over time. The mean value for the carbon isotope fractionation factors between glucose and α-cellulose was greater than unity. The mean value for the oxygen isotope fractionation factors between glucose and α-cellulose was lower than unity. We established, with respect to climate change, the significance of the interannual and intraannual variation in the carbon and oxygen isotope fractionation factors between both saccharides. We used moving interval correlation results for May of the previous year through September of the current year using a base length of 48 years. The relationship with summer temperature is the main climate signal in the carbon isotope fractionation factor between glucose and α-cellulose. The relationship with autumn sunshine is the main climate signal in the oxygen isotope fractionation factor between glucose and α-cellulose for the tree ring chronology.

Carbon isotope fractionation of 11 acetogenic strains grown on H2 and CO2.

Acetogenic bacteria are able to grow autotrophically on hydrogen and carbon dioxide by using the acetyl coenzyme A (acetyl-CoA) pathway. Acetate is the end product of this reaction. In contrast to the fermentative route of acetate production, which shows almost no fractionation of carbon isotopes, the acetyl-CoA pathway has been reported to exhibit a preference for light carbon. In Acetobacterium woodii the isotope fractionation factor (ε) for (13)C and (12)C has previously been reported to be ε = -58.6‰. To investigate whether such a strong fractionation is a general feature of acetogenic bacteria, we measured the stable carbon isotope fractionation factor of 10 acetogenic strains grown on H(2) and CO(2). The average fractionation factor was ε(TIC) = -57.2‰ for utilization of total inorganic carbon and ε(acetate) = -54.6‰ for the production of acetate. The strongest fractionation was found for Sporomusa sphaeroides (ε(TIC) = -68.3‰), the lowest fractionation for Morella thermoacetica (ε(TIC) = -38.2‰). To investigate the reproducibility of our measurements, we determined the fractionation factor of 21 biological replicates of Thermoanaerobacter kivui. In general, our study confirmed the strong fractionation of stable carbon during chemolithotrophic acetate formation in acetogenic bacteria. However, the specific characteristics of the bacterial strain, as well as the cultural conditions, may have a moderate influence on the overall fractionation.

Stable carbon isotope fractionation of individual light hydrocarbons in the C6–C8 range in crude oil as induced by natural evaporation: Experimental results and geological implications

The natural evaporation of a condensate from the Tarim Basin in northwest China was conducted at room temperature to investigate the stable carbon isotopic fractionation of individual light hydrocarbons (LHs) in the C6–C8 range. Our results show that individual LHs in this range in liquid residuals are progressively enriched in 13C, indicating the preferential evaporation of 12C containing compounds from the crude oil. When evaporative losses of individual LHs in the C6–C8 range are <70%, there is no significant carbon isotope fractionation (usually with δ13Cfinal–initial<1‰). A δ13Cfinal–initial of individual LHs in the C6–C8 range >1‰ mainly occurs when the evaporative losses are >70%, but the magnitudes of stable carbon isotopic fractionation strongly depend on compound classes. Compared to the straight chain and branched alkanes and the monoaromatic hydrocarbons, there is almost no stable carbon isotopic fractionation observed for cycloalkanes in the C6–C8 range. Using the Rayleigh model, stable carbon isotopic fractionation of individual LHs in the C6–C8 range during natural evaporation is described and the stable carbon isotope fractionation factors (α) are 0.998976 for n-hexane, 0.998991 for n-heptane, 0.999426 for n-octane, 0.999138 for 3-methylhexane, 0.999401 for methylcyclohexane and 0.998518 for toluene. Correspondingly, the stable carbon isotope enrichment factors (ε) are −1.024 for n-hexane, −1.009 for n-heptane, −0.574 for n-octane, −0.862 for 3-methylhexane, −0.599 for methylcyclohexane and −1.482 for toluene.Both α and ε values clearly show a progressive enrichment in 13C in the residual liquids during natural evaporation in a crude oil system (what is referred to as “normal isotope fractionation”), rather than an enrichment in 13C in the vapor phase as reported by previous studies (what is referred to as “inverse isotope fractionation”). The intermolecular binding energies due to the van der Waals attraction forces in crude oil likely plays an important role on the diffusion velocity and the escape of 13C containing LHs from the system, thereafter exerting a significant influence on stable carbon isotope fractionation, which is controlled by a kinetic isotopic effect.Since we cannot be certain of the exact amount of LHs that evaporated for one given crude oil from the subsurface to the laboratory, the results presented here strongly suggest that caution must be taken in the application of stable carbon isotopic composition of individual LHs in petroleum geochemistry and in environmental studies associated with oil pollution. However, δ13C values of cycloalkanes in the C6–C8 range can still be used in confidence to decipher oil generation, migration, accumulation and secondary alteration in the subsurface of sedimentary basins and to address oil pollution issues.

Sphalerite from the Sterling Hill Mine, New Jersey: Origin and History

The Sterling Hill and Franklin zinc-iron-manganese deposits, Sussex County, New Jersey, are sedimentary-exhalative in origin but were highly deformed and metamorphosed to granulite grade during the Mesoproterozoic Ottawan orogeny. They are unique because the major zinc minerals, zincite, franklinite, and willemite, are oxides and silicates rather than sulfides. However, at Sterling Hill minor granular sphalerite occurs in the central black willemite zone associated with loelling and graphite and sparsely elsewhere in the deposit. Based on textural observations and stable isotope geochemistry, sphalerite was present during metamorphism and was most likely part of the protolith mineral assemblage. It likely formed from a fluid containing seawater sulfate that circulated through volcanic rocks and reacted with igneous sulfides; some contribution from biological reduction of sulfate seems likely. This interpretation agrees with the generally accepted model that the protolith formed on the floor of an ancient rift in an environment similar to the present-day Red Sea brine pool sediments. The presence of loellingite and absence of pyrite or arsenopyrite in the black willemite zone requires that fAs2−/fS2− >1 during metamorphism. The position of the black willemite zone, occupying most of the central cross member of the deposit, and the more reduced mineral assemblages of the zone relative to the rest of the deposit are compatible with it being the feeder zone for the stratiform ores that make up the rest of the Sterling Hill deposit, indicating that the deposit is inverted relative to its original position. Pressure-corrected temperatures from fluid inclusion geothermometry and temperatures determined from carbon isotope fractionation factors between coexisting graphite and calcite suggest a postmetamorphic event that occurred at ∼360°–280°C at between ∼820 and 750 Ma. The fluid inclusion and mineral assemblage data also suggest that at least one additional postmetamorphic event affected the sphalerite and loellingite and their coexisting minerals. This event involved the development of secondary fluid inclusions in sphalerite and the formation of a secondary manganese arsenosilicate mineral and may correlate with the pervasive serpentinization-like alteration of the deposit. The complex sequence of vents indicated by the textures, isotopic ratios, and fluid inclusions is compatible with the history of prolonged cooling and uplift experienced by the New Jersey Highlands in the Neoproterozoic.

Inferences on gas transport based on molecular and isotopic signatures of gases at acoustic chimneys and background sites in the Ulleung Basin

We present results of a comparative study of hydrocarbons collected at acoustic chimney sites and nearby non-chimney (background) sites in the Ulleung Basin (offshore Korea). At all sites, molecular (C 1/C 2+ ratios ⩾ 600) and isotopic (δ 13C CH4, δD CH4 and δ 13C C2H6) indicators point to a microbial source of methane and ethane. Similarly, at all sites the δ 13C CH4 and δ 13C CO2 have minimum values at the sulfate–methane transition zone (SMTZ) throughout the basin. These data document intense carbon cycling at this horizon, whereby anaerobic oxidation of methane (AOM) at the SMTZ generates a dissolved inorganic carbon pool depleted in 13C that is subsequently reduced to generate the observed minima in δ 13C CH4. At the background sites, the carbon isotope fractionation factor ( ε c = δ 13C CO2 − δ 13C CH4) for all gases ranges from 56–66, consistent with values predicted for methanogenesis driven predominantly by microbial CO 2 reduction. However, ε c values measured in the two acoustic chimney sites that contain gas hydrate are significantly lower (30–46). We conclude that these abnormally small ε c values reflect two phase fluid transport at gas hydrate bearing sites. The composition of the hydrocarbons sampled below the SMTZ at these sites (higher C 2+ abundance and δ 13C enriched methane) indicates a deep source origin for the microbial gas, which we postulate migrates upwardly and sustains the gas hydrate formation near the seafloor. If so, abnormally small ε c values can be used as a potential geochemical indicator for the migration of methane in the gas phase and the consequent development of gas hydrate accumulations in shallow marine systems.

Effect of diamond growth rate on carbon isotope fractionation in Fe–Ni–C system

The carbon isotope fractionation factor between diamond and carbon dissolved in the Fe–Ni metal melt varies from 4.1‰ down to 2.4‰ in HTHP crystallization experiments with the average diamond growth rates ranging from 1 to 12.5 mg/h. The observed fractionation of heavy carbon isotope is in agreement with the Burton–Prim–Slichter model for the compatible impurity distribution associated with crystallization. The variations in the carbon isotope composition within a single diamond crystal can result from appropriate variations of the crystal growth rate. The scale of the observed fractionation effect is comparable with the internal isotope inhomogeneity of natural stones.

First application of mass spectrometry and gas chromatography in investigation of α‐cellulose hydrolysates: the influence of climate changes on glucose molecules in pine tree‐rings

We present the first results of the quantitative and qualitative gas chromatographic and isotope ratio mass spectrometric analysis of monosaccharides derived from acid hydrolysis of α-cellulose extracted from annual pine tree-rings. The conifers investigated in this study grew in the Niepolomice Forest in Poland, and the annual rings covered the time span from 1940 to 2000 AD. The main components of the α-cellulose samples were two saccharides: glucose and mannose. The amount of glucose in the annual rings varied between 17 and 44%. The δ(13)C of glucose was found to be less negative than that of α-cellulose and the δ(18)O values in glucose were less positive than those in α-cellulose. The content of monosaccharides in the α-cellulose samples has an influence on the isotope fractionation factors. The values of the carbon isotope fractionation factor increase with an increase in the monosaccharides concentration in α-cellulose, while the values of the oxygen isotope fractionation factor decrease with an increase in monosaccharides concentration in α-cellulose. The challenge is to establish, with respect to climate changes and environmental conditions, the significance of the interannual variations in the observed monosaccharide concentration.