δ13C Of CH4 Research Articles

Fluid geochemistry and geothermometry in the unexploited geothermal field of the Vicano–Cimino Volcanic District (Central Italy)

The Vicano–Cimino Volcanic District (VCVD) is related to the post-orogenic magmatic activity of the peri-Tyrrhenian sector of Central Italy. The chemical and isotopic compositions of 333 water discharges and 25 gas emissions indicate the occurrence of two main sources: 1) cold Ca-HCO3 to Ca(Na, K)-HCO3 type waters from relatively shallow aquifers hosted in volcanic and sedimentary formations; and 2) thermal Ca-SO4(HCO3) type waters located in a deep CO2-pressurized reservoir, hosted in carbonate–evaporite rocks and separated from the shallow aquifers by thick sequences of low-permeability formations. Carbon dioxide is mainly produced by thermal metamorphic decarbonation within the deepest and hottest parts of the carbonate–evaporite reservoir (δ13C–CO2 from −3.1 to +2.2‰ vs. VPDB), likely affected by a mantle-rooted CO2. Release of CO2-rich gases from the deep aquifer into the overlying shallow aquifers produces high-CO2 springs and bubbling pools. The spatial distribution of thermal waters and CO2-rich cold discharges is strongly controlled by fractures and faults located in correspondence with buried structural highs. Stable isotopes (δD and δ18O) suggest that meteoric water feeds both the shallow and deep reservoirs. The relatively low R/Ra values (0.27–1.19) indicate that He is mainly deriving from a crustal source, with minor component from the mantle affected by crustal contamination related to the subduction of the Adriatic plate. Consistently, relatively high N2/Ar and N2/3He ratios and positive δ15N–N2 values (from 0.91 to 5.7‰ vs. air) characterize the VCVD gas discharges, suggesting the occurrence of a significant “excess” nitrogen. Isotopic compositions of CH4 (δ13C–CH4 and δD–CH4 values from −28.9 to −22.1‰ vs. VPDB and from −176 to −138‰ vs. VSMOW, respectively), and composition of light alkanes are indicative of prevalent thermogenic CH4, although the occurrence of abiogenic CH4 production cannot be excluded. The δ34S–H2S values (from +9.3 to +11.4‰ vs. VCDT) are consistent with the hypothesis of H2S production from thermogenic reduction of Triassic anhydrites. Gas geothermometry in the H2O–H2–Ar–H2S system suggests that the VCVD gases equilibrated in a liquid phase at redox conditions controlled by interactions of fluids with the local mineral assemblage at temperatures lower (<200°C) than that and measured in deep (>2000m) geothermal wells. This confirms that secondary processes, i.e. steam condensation, gas dissolution in shallow aquifers, re-equilibration at lower temperature, and microbial activity, significantly affect the chemistry of the uprising fluids. Thermal water chemistry supports the occurrence in this area of an anomalous heat flow that, coupled with the recent demographic growth, makes this site suitable for direct and indirect exploitation of the geothermal resource, in agreement with the preliminary surveys carried out in the 1970's–1990's for geothermal exploration purposes.

Stable isotopic and molecular composition of desorbed coal seam gases from the Walloon Subgroup, eastern Surat Basin, Australia

This study used compositional and stable isotopic analysis to test hypotheses on the distribution and origins of Walloon Subgroup coal seam gas (CSG) in the eastern Surat Basin, Queensland, Australia. The Middle Jurassic Walloon Subgroup play differs from many other low-rank CSG plays—particularly in methane carbon isotopic signature, i.e., the CSG is not as ‘microbial’ as could be expected. The carbon isotope compositions of desorbed methane from three cored appraisal wells fall within the generally accepted range for thermogenic or mixed gas (δ13C −58.5‰ to −45.3‰). The δ13C–CH4 values from stratigraphically placed coal core samples increased (became more ‘thermogenic’) from the top of the upper (Juandah) coal measures to the base of the Tangalooma Sandstone. Below the Tangalooma Sandstone, in the lower (Taroom) coal measures, the δ13C–CH4 values decreased with increasing depth. These positively parabolic δ13C profiles tracked total measured gas content in two out of the three wells studied. The third well displayed lower variance of δ13C–CH4 and gas content increased uniformly with depth.A genetic classification based on methane stable carbon isotopes alone might interpret this pattern as a transition from microbially- to thermogenically-sourced methane in the central coal seams. However, a δ13C–CO2 profile for one well tracks total gas content and δ13C–CH4, and exhibits an inverse relationship with δD–CH4. These results, together with the mostly dry nature of the gas samples [(C1/(C2+C3)) ratios up to ~10,000] and relatively uniform δD–CH4 values (δD −238‰ to −202‰), suggest that microbial CO2 reduction is the primary source of Walloon Subgroup methane. As such, stratigraphic variations in gas content mainly reflect the extent of microbial methanogenesis. We suggest that peak microbial utilisation of H2–CO2 occurred at the Tangalooma Sandstone level, enriching the residual CO2 pool and derived methane in 13C. Carbon [Δ13C(CO2–CH4)] and deuterium isotopic differences [ΔD(H2O–CH4)], and cross-plots of δD–H2O and δ18O–H2O are also consistent with kinetic isotope fractionation during microbial-mediated carbonate reduction. The results are relevant for applying microbially enhanced coal bed methane (MECoM) in the Surat Basin.

Comparative studies of pelagic microbial methane oxidation within the redox zones of the Gotland Deep and Landsort Deep (central Baltic Sea)

Abstract. Pelagic methane oxidation was investigated in dependence on differing hydrographic conditions within the redox zone of the Gotland Deep (GD) and Landsort Deep (LD), central Baltic Sea. The redox zone of both deeps, which indicates the transition between oxic and anoxic conditions, was characterized by a pronounced methane concentration gradient between the deep water (GD: 1233 nM, 223 m; LD: 2935 nM, 422 m) and the surface water (GD and LD &lt; 10 nM). This gradient together with a 13C CH4 enrichment (δ13C CH4 deep water: GD −84‰, LD −71‰; redox zone: GD −60‰, LD −20‰; surface water: GD −47‰, LD −50‰; δ13C CH4 vs. Vienna Pee Dee Belemnite standard), clearly indicating microbial methane consumption within the redox zone. Expression analysis of the methane monooxygenase identified one active type I methanotrophic bacterium in both redox zones. In contrast, the turnover of methane within the redox zones showed strong differences between the two basins (GD: max. 0.12 nM d−1, LD: max. 0.61 nM d−1), with a nearly four-times-lower turnover time of methane in the LD (GD: 455 d, LD: 127 d). Vertical mixing rates for both deeps were calculated on the base of the methane concentration profile and the consumption of methane in the redox zone (GD: 2.5 × 10–6 m2 s−1, LD: 1.6 × 10–5 m2 s−1). Our study identified vertical transport of methane from the deep-water body towards the redox zone as well as differing hydrographic conditions (lateral intrusions and vertical mixing) within the redox zone of these deeps as major factors that determine the pelagic methane oxidation.

On the interference of Kr during carbon isotope analysis of methane using continuous-flow combustion–isotope ratio mass spectrometry

Abstract. Stable carbon isotope analysis of methane (δ13C of CH4) on atmospheric samples is one key method to constrain the current and past atmospheric CH4 budget. A frequently applied measurement technique is gas chromatography (GC) isotope ratio mass spectrometry (IRMS) coupled to a combustion-preconcentration unit. This report shows that the atmospheric trace gas krypton (Kr) can severely interfere during the mass spectrometric measurement, leading to significant biases in δ13C of CH4, if krypton is not sufficiently separated during the analysis. According to our experiments, the krypton interference is likely composed of two individual effects, with the lateral tailing of the doubly charged 86Kr peak affecting the neighbouring m/z 44 and partially the m/z 45 Faraday cups. Additionally, a broad signal affecting m/z 45 and especially m/z 46 is assumed to result from scattered ions of singly charged krypton. The introduced bias in the measured isotope ratios is dependent on the chromatographic separation, the krypton-to-CH4 mixing ratio in the sample, the focusing of the mass spectrometer as well as the detector configuration and can amount to up to several per mil in δ13C. Apart from technical solutions to avoid this interference, we present correction routines to a posteriori remove the bias.

Stable carbon isotopic evidence of methane consumption and production in three alpine ecosystems on the Qinghai–Tibetan Plateau

To understand the mechanisms of soil CH4 consumption and production in alpine ecosystems, we for the first time examined the stable carbon isotope ratio of CH4 (δ13C–CH4) at three major grassland vegetation types, alpine meadow, alpine shrub, and alpine wetland, on the Qinghai–Tibetan Plateau.The alpine meadow and shrub showed net CH4 absorption in their vertical profiles of CH4 concentration in summer, but a difference in their processes. Whereas the alpine shrub was dominated by CH4 consumption in its soil profile, CH4 production in the alpine meadow could slightly cancel consumed CH4 in shallow soil from −0.3 to −0.1 m. This potential CH4 production can be attributed to the relatively wet soil type of that ecosystem, which might allow methanogenesis to act in moist soil lumps in the shallow layer.The alpine wetland differed in methane production, consumption, and transport pathways between hummock and hollow plots. In summer, both plots were enriched in 13C–CH4 in dissolved CH4 in soil pore water, suggesting that CH4 production was conducted mainly by acetate fermentation. In autumn, CH4 production was shifted toward CO2/H2 reduction. Furthermore, in hummocks, plant-mediated transport of CH4 by vascular plants appeared to perform passive CH4 flow from deep soil to atmosphere, which allowed the produced CH4 to bypass the oxidation zone in shallow soil. In hollows, however, CH4 produced in shallow soil was subject to simultaneous oxidation. The fractional oxidation rate on gross CH4 production in hollows was estimated by simple mass balance model at 7–17% in summer and 35–36% in autumn.

Determining the origin of carbon dioxide and methane in the gaseous emissions of the San Vittorino plain (Central Italy) by means of stable isotopes and noble gas analysis

The chemistry and isotope ratios of He, C (δ13C) and H (δD) of free gases collected in the San Vittorino plain, an intramontane depression of tectonic origin, were determined to shed light on mantle degassing in central Italy. The C isotopic composition of CO2 (δ13C–CO2 −2.0‰ to −3.8‰) and He isotope ratios (R/RA 0.12–0.27) were used to calculate the fraction of CO2 originating from mantle degassing vs. sedimentary sources. The results show that CO2 predominantly (average of 75%) derives from the thermo-metamorphic reaction of limestone. Between 6% and 22% of the CO2 in the samples derives from organic-rich sedimentary sources. The mantle source accounts for 0–6% of the total CO2; however, in two samples, located in proximity to the most important faults of the plain, the mantle accounts for 24% and 42%. The presence of faults and fractures allows upward gas migration from a deep source to the Earth’s surface, not only in the peri-Tyrrhenian sector, as generally reported by studies on natural gas emissions in central Italy, but also in the pre-Apennine and Apennine belts. Isotope ratios of CH4 (δ13C–CH4 −6.1‰ to −22.7‰; δD–CH4 −9‰ to −129‰) show that CH4 does not appear to be related to mantle or magma degassing, but it is the product of thermal degradation of organic matter (i.e. thermogenic origin) and/or the reduction of CO2 (i.e. geothermal origin). Most of the samples appear to be affected by secondary microbial oxidation processes.

Assessment of diffusive isotopic fractionation in polar firn, and application to ice core trace gas records

During rapid variations of the atmospheric mixing ratio of a trace gas, diffusive transport in the porous firn layer atop ice sheets and glaciers alters the isotopic composition of that gas relative to the overlying atmosphere. Records of past atmospheric trace gas isotopic composition from ice cores and firn need to be corrected for this diffusive fractionation artifact.We present a novel, semi-empirical method to accurately estimate the magnitude of the diffusive fractionation in the ice core record. Our method (1) consists of a relatively simple analytical calculation; (2) requires only commonly available ice core data; (3) is not subject to the uncertainties inherent to estimating the accumulation rate, temperature, close-off depth and depth–diffusivity relationship back in time; (4) does not require knowledge of the true atmospheric variations, but uses the smoothed records obtained from ice cores; (5) arguably gives more accurate results than a combined firn densification–firn air transport modeling study would.We apply the method to records of CH4, CO2 and N2O mixing ratios, and we find that the correction is particularly important for δ13C–CH4. We apply the correction to δ13C–CH4 records over the last glacial termination and the 8.2ka event. In both cases the diffusive signal exceeds the analytical precision of the data, and has a significant impact on the observed isotopic trends. For the 8.2ka event the corrected data show an isotopic enrichment in δ13C–CH4 for the duration of the event, consistent with reduced wetland emissions.

Aerobic methanotrophy within the pelagic redox-zone of the Gotland Deep (central Baltic Sea)

Abstract. Water column samples taken in summer 2008 from the stratified Gotland Deep (central Baltic Sea) showed a strong gradient in dissolved methane concentrations from high values in the saline deep water (max. 504 nM) to low concentrations in the less dense, brackish surface water (about 4 nM). The steep methane-gradient (between 115 and 135 m water depth) within the redox-zone, which separates the anoxic deep part from the oxygenated surface water (oxygen concentration 0–0.8 mL L−1), implies a methane consumption rate of 0.28 nM d−1. The process of microbial methane oxidation within this zone was evident by a shift of the stable carbon isotope ratio of methane between the bottom water (δ13C CH4 = −82.4‰ and the redox-zone (δ13C CH4 = −38.7‰. Water column samples between 80 and 119 m were studied to identify the microorganisms responsible for the methane turnover in that depth interval. Notably, methane monooxygenase gene expression analyses for water depths covering the whole redox-zone demonstrated that accordant methanotrophic activity was probably due to only one phylotype of the aerobic type I methanotrophic bacteria. An imprint of these organisms on the particular organic matter was revealed by distinctive lipid biomarkers showing bacteriohopanepolyols and lipid fatty acids characteristic for aerobic type I methanotrophs (e.g., 35-aminobacteriohopane-30,31,32,33,34-pentol), corroborating their role in aerobic methane oxidation in the redox-zone of the central Baltic Sea.

Assessing methane oxidation under landfill covers and its contribution to the above atmospheric CO2 levels: The added value of the isotope (δ13C and δ18O CO2; δ13C and δD CH4) approach

We are presenting here a multi-isotope approach (δ13C and δ18O of CO2; δ13C and δD of CH4) to assess (i) the level(s) of methane oxidation during waste biodegradation and its migration through a landfill cover in Sonzay (France), and (ii) its contribution to the atmospheric CO2 levels above the surface. The isotope approach is compared to the more conventional mass balance approach. Results from the two techniques are comparable and show that the CH4 oxidation under the landfill cover is heterogenous, with low oxidation percentages in samples showing high biogas fluxes, which was expected in clay covers presenting fissures, through which CH4 is rapidly transported. At shallow depth, more immobile biogas pockets show a higher level of CH4 oxidation by the methanotrophic bacteria. δ13C of CO2 samples taken at different heights (from below the cover up to 8m above the ground level) were also used to identify and assess the relative contributions of its main sources both under the landfill cover and in the surrounding atmosphere.

Stable carbon isotope fractionation, carbon flux partitioning and priming effects in anoxic soils during methanogenic degradation of straw and soil organic matter

Straw amendment is a common practice for improving the fertility of rice field soils, but it also enhances production of the greenhouse gas methane. To quantify carbon flux partitioning and priming effects due to straw amendment, we measured δ13C in CH4 and CH4 precursors produced in anoxic slurries of soil from Italy, China and Thailand after addition of straw from either rice (C3 plant) or maize plants (C4 plant), which have different δ13C signatures. The δ13C values of the CH4, acetate and CO2 produced were similar when expressed as the difference to the δ13C value of the straw applied. These results indicated that the 13C-isotopic fractionation involved in methanogenic decomposition was similar for rice straw and maize straw. However, measurement of CH4 produced in soil without or with straw showed that isotopic fractionation during methanogenic degradation of straw was smaller than during degradation of soil organic matter. Isotopic fractionation during hydrogenotrophic methanogenesis, measured in the presence of methyl fluoride, with straw was also smaller than with soil organic matter. The results show that 13C-isotopic analysis after application of rice straw and maize straw is a convenient approach for quantifying carbon flux partitioning during methanogenic degradation of straw and soil organic matter. In our experiments, straw degradation accounted for most of the CH4 production and caused a negative priming effect on the methanogenic degradation of soil organic matter.

Origin of light hydrocarbons in gases from mud volcanoes and CH4-rich emissions

This paper presents new chemical data of hydrocarbon-rich gases discharged from mud volcanoes and CH4-rich emissions located in different areas of Italy (Northern Apennines and Sicily). The determination of C5–C10 alkanes, cyclics and aromatics was carried out by GC–MS, while the main gas species and C1–C4 hydrocarbons were analyzed by GC-TDC and GC-FID, respectively. Methane is by far the most abundant component of all the investigated gas emissions, with the exception of part of those discharging close to Mt. Etna volcano, which are CO2-rich. The gas samples collected from the Emilia Apennine and Sicily show δD–CH4 and δ13C–CH4 values typical of thermogenic gases. Most gases from the Romagna Apennine discharge methane with a biogenic isotopic signature. Non-methane hydrocarbons of biogenic gases are almost exclusively C2–C4 alkanes, with minor amounts (fraction of μmol/mol) of C4+ compounds, including few aromatics and cyclics, likely deriving from minor thermogenic contribution not evidenced by the δ13C–CH4 and δD–CH4 values. The Etnean gases, whose R/Ra and δ13C–CO2 values indicate a strong contribution from a hydrothermal fluid source related to the nearby volcanic system, show a significant enrichment of aromatics compounds (up to 36% of the non-methane organic gas fraction), which is likely produced by catalytic reforming processes, such as dehydrocyclization of alkanes.The thermogenic gases from the Apennines and southwestern Sicily are characterized by the presence of more than 20 different cyclic compounds with concentrations up to several μmol/mol. Cyclic compounds are likely formed by i) thermal cracking and ii) uncompleted aromatization of alkanes occurring at depth>3km and temperatures not exceeding 120–150°C.

Fluid geochemistry and geothermometry in the western sector of the Sabatini Volcanic District and the Tolfa Mountains (Central Italy)

A geochemical survey of 197 fluid discharges (cold and thermal waters and bubbling pools) and 15 gas emissions from the western sector of the Sabatini Volcanic District and the Tolfa Mountains (Latium, Central Italy) was carried out in 2007–2008. The chemical and isotopic compositions of the fluid discharges indicate the occurrence of two main sources: 1) relatively shallow aquifers with Ca(Na,K)–HCO3 and Ca(Mg)–HCO3 compositions when trapped in volcanic and sedimentary formations, respectively; and 2) a deep reservoir, which is hosted in the Mesozoic carbonate sequence, rich in CO2 and having a Ca–SO4(HCO3) composition. Dissolution of a CO2-rich gas phase into the shallow aquifers produces high-TDS and high-pCO2 cold waters, while oxidation of deep-derived H2S to SO42− generates low-pH (< 4) sulfate waters. The δ13C–CO2 values for gas emissions (from − 2.8 to + 2.7‰ vs. VPDB) suggest that the origin of CO2 associated with the deep fluids is mainly related to thermo-metamorphic reactions within the carbonate reservoir, although significant mantle contribution may also occur. However, R/Ra values (0.37–0.62) indicate that He is mainly produced by a crustal source, with a minor component from a crust-contaminated mantle. On the basis of the δ13C–CH4 and δD–CH4 values (from − 25.7 to − 19.5‰ vs. VPDB and from − 152 to − 93.4‰ vs. VSMOW, respectively) CH4 production is associated with thermogenic processes, possibly related to abiogenic CO2 reduction within the carbonate reservoir. The δ34S–H2S values (from + 9.3 to + 10.4‰ vs. VCDT) are consistent with the hypothesis of a sedimentary source of sulfur from thermogenic reduction of Triassic sulfates. Geothermometric evaluations based on chemical equilibria CO2–CH4 and, separately, H2S suggest that the reservoir equilibrium temperature is up to ~ 300 °C. The δD and δ18O data indicate that water recharging both the shallow and deep aquifers has a meteoric origin. Fluid geochemistry, coupled with gravimetric data and tectonic lineaments, supports the idea that significant contributions from a deep-seated geothermal brine are present in the Stigliano thermal fluid discharges. Exploration surveys investigated this area during 70's–90's for geothermal purposes. Nevertheless, presently the area is still under-exploited. The presence of thermal waters and anomalous heat flow together with the demographic growth of the last years, makes this site a suitable location for direct applications of the geothermal resource.

Application of stable isotope analysis for improved understanding of the methane budget: comparison of TROICA measurements with TM3 model simulations

Presented is a detailed comparison of CH4 and δ13C–CH4 measurements with simulations of the global transport model TM3. Experimental data were obtained during campaigns along the Trans-Siberian railroad in the framework of the TROICA project. Two summer (1999 and 2001) and one spring (2003) expeditions are evaluated. Model simulations include sensitivity tests to further investigate the isotopic composition of natural gas and emissions from Siberian wetlands. Comparison of the average mixing ratio of methane and its isotopic composition (δ13C) has been performed for different geographic zones, including the European part of Russia, Western Siberia and Central Siberia. Simulations are in reasonable agreement with the measurements for the European part of Russia and confirm a high contribution of natural gas to the observed methane levels. An increase of emission from bogs shifts the simulated methane isotopic composition closer to the observations. The relative importance of the Western Siberia emissions in current inventories is underestimated in comparison with other wetland regions in the former USSR. Simulated average mixing ratios are in a good agreement with the observations in Central Siberia, while 13C(CH4) values tend to be higher than measured in all considered scenarios. These results point to a bias in the modeled source mixture over Russia, which could be repaired by shifting emissions from isotopically heavy methane sources (e.g. coal, oil or biomass burning) to light sources (e.g. wetlands, ruminants, waste treatment). Alternatively, the average isotopic signature of Siberian wetlands may be lighter than expected.

Biotic and abiotic methane releases from Lake Biwa sediment slurry

To determine the rate and mechanism of CH4 production in Lake Biwa sediment, slurry was prepared and incubated. Surface sediment (sed) slurry (1.5–6 cm) showed a CH4 release rate (4.9–9.5 nmol g-dry-sed−1 day−1) higher than that observed in the 5- to 10-cm sediment slurry (0.2–2 nmol g-dry-sed−1 day−1). Methane release from the surface (1.5–6 cm) sediment slurry was biotic and was inhibited by addition of 2-bromoethanesulfonate (BES, an inhibitor of CH4 production), whereas that from 5- to 10-cm sediment slurry was abiotic. The addition of BES, HNO3, and O2 showed no effect on the CH4 release rate from the 5- to 10-cm sediment slurry. In addition, tracers (NaH13CO3, 13CH3COONa) were not incorporated into the released CH4. However, δ13C of CH4 released from the 5- to 10-cm sediment slurry (−74.0 ± 0.6‰) indicated that this CH4 was produced by bacterial metabolism in the past, stored by adsorption on the surface of clay minerals in the sediment, and then released abiotically by desorption from the sediment slurry as a result of a decrease in hydraulic pressure and CH4 concentration in the pore water. This CH4 stored by adsorption could be extracted by autoclaving. In the sediment below 5 cm, bacterial activity for CH4 production ceased, possibly because of the limitated availability of H2. To clarify the mechanism of CH4 production in the sediment, biotic CH4 production and the abiotic CH4 release found here should be estimated separately.

Discovery of a new hydrothermal venting site in the southernmost Mariana Arc: Al-rich hydrothermal plumes and white smoker activity associated with biogenic methane

This paper reports a series of studies leading to the discovery of a submarine hydrothermal field (called Nakayama Field) at an arc seamount (12°43′N, 143°32′E) in the southernmost part of the Mariana Trough, western Pacific Ocean. We first detected hydrothermal plumes characterized by water column anomalies of temperature, light transmission, Mn, Fe, Al, O2, CH4, and δ13C of CH4 above the summit caldera of the seamount. Then deep-tow camera surveys confirmed the existence of hydrothermal activity inside the caldera, and an ROV dive finally discovered white smoker-type fluid venting associated with vent fauna. A high concentration of aluminum in the plume and white smoker-type emissions imply acidic hydrothermal activity similar to that observed at the DESMOS Caldera in the eastern Manus Basin, Papua New Guinea. Anomalously low δ13C (CH4) of −38‰ of a vent fluid sample compared to other arc hydrothermal systems along the Izu-Bonin and Mariana Arcs suggests an incorporation of biogenic methane based on a subsurface microbial ecosystem.

On the contribution of anthropogenic Cl to the increase in δ13C of atmospheric methane

A two‐dimensional model is used to evaluate the impact of stratospheric chlorine on δ13C of CH4 at the surface and to analyze sensitivity to different chemical and transport processes. It is found that the impact of Cl depends on the relative strength of the lower and upper branches of the stratospheric circulation, which emphasize regions of differing rates of methane fractionation. It is estimated that the increase in concentrations of Cl associated with human activity has contributed at most 0.54‰ to the 2‰ increase of δ13C observed over the past century. It follows that growth in the relative importance of heavier sources of CH4 (associated, for example, with fossil fuels and biomass burning) has been largely responsible for the trend.

Behavior of gases in the Nojima Fault Zone revealed from the chemical composition and carbon isotope ratio of gases extracted from DPRI 1800 m drill core

Abstract An 1800 m borehole was drilled into the Nojima Fault Zone at Ogura, Awaji Island, Hyogo prefecture, Japan. The chemical compositions and isotope ratios of gases extracted from the drill core were investigated. Major components were carbon dioxide (CO2) and methane (CH4). Microcracks in granodiorite outside the fracture zone were occupied mainly by CO2, and this CO2 is interpreted to have generated biogenically at shallow depths based on the measured δ13C value of –17 to –22. The CO2 gas was probably transported with underground water to deeper portions to fill microcracks in the basement granodiorite with CO2. However, the pores in the fracture zone are occupied predominantly by CH4. The ratio of CH4 to ethane (C2H6), 80 to100, and δ13C of CH4, –40 to –52, suggest that CH4 and C2H6 formed by the thermal decomposition of organic materials at temperatures above 75°C. We interpret that they originated at depths from organic materials and migrated upwards through the fault zone. It is interpreted that the concentration of CO2 in the fracture zone has decreased by the replacement with CH4 and/or by the consumption of CO2 in fault clay minerals. Although hydrogen (H2) and helium (He) were minor components of the gases from cores, they increased in quantity in the fracture zone. High concentration of H2 in the fracture zones is consistent with the idea that H2 was generated by radical reactions on the fresh surface of fractured rocks during the earthquake. The 3He/4He ratio of 0.723 Ra in the fracture zones suggests that He is of radiogenic origin; that is, it is not from the mantle.

Quantifying Methane Oxidation from Landfills Using Stable Isotope Analysis of Downwind Plumes

Landfills are major contributors to the atmospheric CH4 budget. A major uncertainty in estimating CH4 flux from landfills is determining the attenuation of CH4 emission by methanotrophic bacteria in the aerobic outer portions of the cover soil. These bacteria intercept the gas as it migrates toward the atmosphere. To estimate cover soil oxidation, we made seasonal measurements of the difference in the δ13C of CH4 within the anoxic zone and CH4 released from landfills and captured downwind at two landfills in the Northeastern United States. Anoxic zone CH4 at the Nashua Landfill averaged −54.6 ± 0.7‰, n = 205, and displayed no significant seasonal pattern. Methane was in excess over ambient air concentrations in the downwind plume ranging from 2.13 to 3.41 ppmv. The δ13C of excess CH4, as determined by mass balance with correction for ambient air CH4, varied from −49.35 to −54.28‰. We used these values to calculate soil CH4 oxidation, which ranged from 0 to 23.6%. Oxidation was greatest in the summer and in the fall, with an annual value of 12 ± 8%. At a second landfill, plume CH4 ranged from 1.96 to 2.92 ppmv with excess δ13C values of −52.17 to −58.06‰. Oxidation at this landfill ranged from 0 to 14%.

Controls on CH4 emissions from a northern peatland

We examined the controls on summer CH4 emission from five sites in a peatland complex near Thompson, Manitoba, Canada, representing a minerotrophic gradient from bog to rich fen at wet sites, where the water table positions ranged from −10 to −1 cm. Average CH4 flux, determined by static chambers on collars, ranged from 22 to 239 mg CH4−C m−2 d−1 and was related to peat temperature. There was an inverse relationship between water table position and CH4 flux: higher water tables led to smaller fluxes. The determination of anaerobic CH4 production and aerobic CH4 consumption potentials in laboratory incubations of peat samples was unable to explain much of the variation in CH4 flux. Average net ecosystem exchange of CO2 ranged from 1.4 to 2.5 g CO2−C m−2 d−1 and was strongly correlated with CH4 flux; CH4 emission averaged 4% of CO2 uptake. End‐of‐season sedge biomass was also strongly related to CH4 flux, indicating the important role that vascular plants play in regulating CH4 flux. Determination of isotopic signatures in peat pore water CH4 revealed average δ13C values of between −50 and −73‰ and δD of between −368 and −388‰. Sites with large CH4 emission rates also had high CO2 exchange rates and enriched δ13C CH4 signatures, suggesting the importance of the acetate fermentation pathway of methanogenesis. Comparison of δD and δ13C signatures in pore water CH4 revealed a slope shallow enough to suggest that oxidation is not an important overall control on CH4 emissions at these sites, though it appeared to be important at one site. Analysis of 14C in pore water CH4 showed that most of the CH4 was of recent origin with percent of modern carbon values of between 112 and 128%. The study has shown the importance of vascular plant activities in controlling CH4 emissions from these wetland sites through influences on the availability of fresh plant material for methanogenesis, rhizospheric oxidation, and plant transport of CH4.

Methane and hydrogen production in a termite‐symbiont system

Methane and hydrogen emission rates and the δ13C of CH4 were observed for various termites in Australia, Thailand and Japan. Combined with the already reported emission rates of CH4 in the literature, the phylogenetic trend was examined. Emission rates of the observed termites were categorized into five groups: group I with high CH4 and low H2 emission rates with a CH4/H2 ratio of typically 10/1; group II with high CH4 and high H2 emissions with a CH4/H2 ratio of 4/1–1/2; group III with low emission rates of CH4 and H2; group IV with high H2 and insignificant CH4 emissions; and group V with insignificant emissions for both CH4 and H2. In lower termites, there are both colonies infected and uninfected with methanogens even in the same species, and no specific trend in CH4 and H2 emissions was observed within a genus. Whether protozoa in the hindgut of termites are infected with methanogens or not and the differences in species compositions of protozoa are possibly responsible for the inter‐colonial variations. The proportions of infected colonies were possibly small for the family Kalotermitidae (dry wood feeders), and relatively large for families of wet or damp wood feeders. The hydrogen emission rate possibly depends on the locality of methanogens: namely, whether they are intracellular symbionts of protozoa or whether they are attached to the hindgut wall. Emission rates of higher termites were classified into groups according to genera and the diet. Most species of soil or wood/soil interface feeders classified into group I, while the soil feeders Dicuspiditermes in Thailand and Amitermes in Australia were classified into groups with high H2 emission rates. Typical wood‐feeding termites and fungus‐growing termites were classified into group III. The results indicate that higher termites tend to increase the CH4 emission rate during dietary evolution from wood‐ to soil‐feeding, and two types of the system with different efficiencies of interspecies transfer of H2 have been formed. The δ13C of CH4 was discernible with a difference in the decomposition process in the termite–symbiont system among lower termites, fungus‐growing termites and other higher termites.