δ13CCH4 Values Research Articles

Spatial variation in shallow sediment methane sources and cycling on the Alaskan Beaufort Sea Shelf/Slope

The MITAS (Methane in the Arctic Shelf/Slope) expedition was conducted during September, 2009 onboard the U.S. Coast Guard Cutter (USCGC) Polar Sea (WAGB-11), on the Alaskan Shelf/Slope of the Beaufort Sea. Expedition goals were to investigate spatial variations in methane source(s), vertical methane flux in shallow sediments (<10 mbsf), and methane contributions to shallow sediment carbon cycling. Three nearshore to offshore transects were conducted across the slope at locations approximately 200 km apart in water column depths from 20 to 2100 m. Shallow sediments were collected by piston cores and vibracores and samples were analyzed for sediment headspace methane (CH4), porewater sulfate (SO42−), chloride (Cl−), and dissolved inorganic carbon (DIC) concentrations, and CH4 and DIC stable carbon isotope ratios (δ13C). Downward SO42− diffusion rates estimated from sediment porewater SO42− profiles were between −15.4 and −154.8 mmol m−2 a−1 and imply a large spatial variation in vertical CH4 flux between transects in the study region. Lowest inferred CH4 fluxes were estimated along the easternmost transect. Higher inferred CH4 flux rates were observed in the western transects. Sediment headspace δ13CCH4 values ranged from −138 to −48‰, suggesting strong differences in shallow sediment CH4 cycling within and among sample locations. Measured porewater DIC concentrations ranged from 2.53 mM to 79.39 mM with δ13CDIC values ranging from −36.4‰ to 5.1‰. Higher down-core DIC concentrations were observed to occur with lower δ13C where an increase in δ13CCH4 was measured, indicating locations with active anaerobic oxidation of methane. Shallow core CH4 production was inferred at the two western most transects (i.e. Thetis Island and Halkett) through observations of low δ13CCH4 coupled with elevated DIC concentrations. At the easternmost Hammerhead transect and offshore locations, δ13CCH4 and DIC concentrations were not coupled suggesting less rapid methane cycling. Results from the MITAS expedition represent one of the most comprehensive studies of methane source(s) and vertical methane flux in shallow sediments of the U.S. Alaskan Beaufort Shelf to date and show geospatially variable sediment methane flux that is highly influenced by the local geophysical environment.

Biogeochemical processes involving dissolved CO2 and CH4 at Albano, Averno, and Monticchio meromictic volcanic lakes (Central–Southern Italy)

This paper focuses on the chemical and isotopic features of dissolved gases (CH4 and CO2) from four meromictic lakes hosted in volcanic systems of Central–Southern Italy: Lake Albano (Alban Hills), Lake Averno (Phlegrean Fields), and Monticchio Grande and Piccolo lakes (Mt. Vulture). Deep waters in these lakes are characterized by the presence of a significant reservoir of extra-atmospheric dissolved gases mainly consisting of CH4 and CO2. The δ13C-CH4 and δD-CH4 values of dissolved gas samples from the maximum depths of the investigated lakes (from −66.8 to −55.6 ‰ V-PDB and from −279 to −195 ‰ V-SMOW, respectively) suggest that CH4 is mainly produced by microbial activity. The δ13C-CO2 values of Lake Grande, Lake Piccolo, and Lake Albano (ranging from −5.8 to −0.4 ‰ V-PDB) indicate a significant CO2 contribution from sublacustrine vents originating from (1) mantle degassing and (2) thermometamorphic reactions involving limestone, i.e., the same CO2 source feeding the regional thermal and cold CO2-rich fluid emissions. In contrast, the relatively low δ13C-CO2 values (from −13.4 to −8.2 ‰ V-PDB) of Lake Averno indicate a prevalent organic CO2. Chemical and isotopic compositions of dissolved CO2 and CH4 at different depths are mainly depending on (1) CO2 inputs from external sources (hydrothermal and/or anthropogenic); (2) CO2–CH4 isotopic exchange; and (3) methanogenic and methanotrophic activity. In the epilimnion, vertical water mixing, free oxygen availability, and photosynthesis cause the dramatic decrease of both CO2 and CH4 concentrations. In the hypolimnion, where the δ13C-CO2 values progressively increase with depth and the δ13C-CH4 values show an opposite trend, biogenic CO2 production from CH4 using different electron donor species, such as sulfate, tend to counteract the methanogenesis process whose efficiency achieves its climax at the water–bottom sediment interface. Theoretical values, calculated on the basis of δ13C-CO2 values, and measured δ13CTDIC values are not consistent, indicating that CO2 and the main carbon-bearing ion species (HCO3 −) are not in isotopic equilibrium, likely due to the fast kinetics of biochemical processes involving both CO2 and CH4. This study demonstrates that the vertical patterns of the CO2/CH4 ratio and of δ13C-CO2 and δ13C-CH4 are to be regarded as promising tools to detect perturbations, related to different causes, such as changes in the CO2 input from sublacustrine springs, that may affect aerobic and anaerobic layers of meromictic volcanic lakes.

Deep gases discharged from mud volcanoes of Azerbaijan: New geochemical evidence

This paper presents new geochemical data of hydrocarbon-rich gases released from some mud volcanoes of Azerbaijan. Methane is considerably the most abundant component of all the sampled gases, which show δD-CH4 and δ13C-CH4 values likely related to a dominant thermogenic source. These gases are characterized by the presence of more than 20 different cyclic compounds with concentrations up to several μmol/mol. A similar gas composition has recently been found to characterize many mud volcanoes of the Northern Apennines and Sicily (Italy). The data of the Azerbaijan mud volcanoes corroborate the notion that cyclic compounds can be considered reliable tracers for hydrocarbon gas production at considerable depths and temperatures up to 120–150 °C, which correspond to a 6.5–8.3 km depth range assuming an average geothermal gradient of 18 °C/km. This depth interval is consistent with both the depth of potential source rocks imaged seismically beneath some mud volcanoes, and the results of previous estimates that used the 13C/12C values of methane and ethane. Such deep-sourced gases and material (fluidized clayey mass and rock fragments) ascend into the core of anticlines and accumulate at shallower reservoirs, where fold-parallel outer-arc faults or fold-orthogonal fractures may penetrate and transfer the fluids to the surface. Finally, the basically equivalent composition of the different hydrocarbon groups (C2–C10 alkanes, aromatics and cyclic) determined in the gases sampled in both Azerbaijan and Italy manifests the lack of evident relationships between the chemistry of light hydrocarbons and the type of source rock.

A combustion setup to precisely reference δ&amp;lt;sup&amp;gt;13&amp;lt;/sup&amp;gt;C and δ&amp;lt;sup&amp;gt;2&amp;lt;/sup&amp;gt;H isotope ratios of pure CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; to produce isotope reference gases of δ&amp;lt;sup&amp;gt;13&amp;lt;/sup&amp;gt;C-CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; in synthetic air

Abstract. Isotope records of atmospheric CH4 can be used to infer changes in the biogeochemistry of CH4. One factor currently limiting the quantitative interpretation of such changes are uncertainties in the isotope measurements stemming from the lack of a unique isotope reference gas, certified for δ13C-CH4 or δ2H-CH4. We present a method to produce isotope reference gases for CH4 in synthetic air that are precisely anchored to the VPDB and VSMOW scales and have δ13C-CH4 values typical for the modern and glacial atmosphere. We quantitatively combusted two pure CH4 gases from fossil and biogenic sources and determined the δ13C and δ2H values of the produced CO2 and H2O relative to the VPDB and VSMOW scales within a very small analytical uncertainty of 0.04‰ and 0.7‰, respectively. We found isotope ratios of −39.56‰ and −56.37‰ for δ13C and −170.1‰ and −317.4‰ for δ2H in the fossil and biogenic CH4, respectively. We used both CH4 types as parental gases from which we mixed two filial CH4 gases. Their δ13C was determined to be −42.21‰ and −47.25‰ representing glacial and present atmospheric δ13C-CH4. The δ2H isotope ratios of the filial CH4 gases were found to be −193.1‰ and −237.1‰, respectively. Next, we mixed aliquots of the filial CH4 gases with ultrapure N2/O2 (CH4 &amp;amp;leq; 2 ppb) producing two isotope reference gases of synthetic air with CH4 mixing ratios near atmospheric values. We show that our method is reproducible and does not introduce isotopic fractionation for δ13C within the uncertainties of our detection limit (we cannot conclude this for δ2H because our system is currently not prepared for δ2H-CH4 measurements in air samples). The general principle of our method can be applied to produce synthetic isotope reference gases targeting δ2H-CH4 or other gas species.

Geochemical characteristics of biogenic gases in China

A number of contributions have demonstrated that the stable carbon and hydrogen isotopic compositions together with molecular compositions have a great significance on determination for gas origin and gas generation pathways of biogenic gases. Together with some data from published work, a total of 143 gas samples were compiled from different biogenic gas producing locations in China, to investigate the gas origin and gas generation pathways by the molecular composition, stable carbon and hydrogen isotopes. The results demonstrate that biogenic gases in China are dominated by methane with low level of C2+3 (generally less than 0.5%) and low δ13CCH4 values (generally≤55‰). The relatively high level of C2+3 (up to 11.79%) in the biogenic gases from the Baise and Songliao basins indicates the possible mixture of low-mature thermogenic gases. Biogenic gases from the Jiangzhe coastal area (including Shanghai, Zhejiang, and Jiangsu provinces) and Yingqiong basin have δDCH4 values greater than −190‰, indicating marine salt water depositional environments. However, all other gases have δDCH4 values less than −190‰, possibly indicating a terrestrial freshwater or freshwater to slightly saline water depositional environment. The methanogenic pathway of biogenic gases in China is dominated by biogenic carbonate reduction, though a number of these biogenic gases were generated in a terrestrial freshwater or freshwater to slightly saline water depositional environments. Compared to the bacterial carbonate reduction methanognic pathway in the Luliang basin, gas formation mechanism in the Baoshan basin was more toward bacterial methyl-type fermentation, likely due to the relatively higher geothermal gradient in the Baoshan basin.

Geochemistry of a continental site of serpentinization, the Tablelands Ophiolite, Gros Morne National Park: A Mars analogue

The presence of aqueously altered, olivine-rich rocks along with carbonate on Mars suggest that serpentinization may have occurred in the past and may be occurring presently in the subsurface, and possibly contributing methane (CH4) to the martian atmosphere. Serpentinization, the hydration of olivine in ultramafic rocks, yields ultra-basic fluids (pH⩾10) with unique chemistry (i.e. Ca2+–OH− waters) and hydrogen gas, which can support abiogenic production of hydrocarbons (i.e. Fischer–Tropsch Type synthesis) and subsurface chemosynthetic metabolisms. Mars analogue sites of present-day serpentinization can be used to determine what geochemical measurements are required for determining the source methane at sites of serpentinization on Earth and possibly on Mars. The Tablelands Ophiolite is a continental site of present-day serpentinization and a Mars analogue due to the presence of altered olivine-rich ultramafic rocks with both carbonate and serpentine signatures. This study describes the geochemical indicators of present-day serpentinization as evidenced by meteoric ultra-basic reducing groundwater discharging from ultramafic rocks, and travertine and calcium carbonate sediment, which form at the discharge points of the springs. Dissolved hydrogen concentrations (0.06–1.20mg/L) and methane (0.04–0.30mg/L) with δ13CCH4 values (-28.5‰ to -15.6‰) were measured in the spring fluids. Molecular and isotopic analyses of CH4, ethane, propane, butane, pentane and hexane suggest a non-microbial source of methane, and attribute the origin of methane and higher hydrocarbon gases to either thermogenic or abiogenic pathways.

Impact of hot fluid advection on hydrocarbon gas production and seepage in mud volcano sediments of thick Cenozoic deltas

Hydrocarbon seeps are ubiquitous at gas-prone Cenozoic deltas such as the Nile Deep Sea Fan (NDSF22NDSF:Nile Deep Sea Fan.) where seepage into the bottom water has been observed at several mud volcanoes (MVs33MV: mud volcano.) including North Alex MV (NAMV44NAMV: North Alex mud volcano.). Here we investigated the sources of hydrocarbon gases and sedimentary organic matter together with biomarkers of microbial activity at four locations of NAMV to constrain how venting at the seafloor relates to the generation of hydrocarbon gases in deeper sediments. At the centre, high upward flux of hot (70°C) hydrocarbon-rich fluids is indicated by an absence of biomarkers of Anaerobic Oxidation of Methane (AOM) and nearly constant methane (CH4) concentration depth-profile. The presence of lipids of incompatible thermal maturities points to mixing between early-mature petroleum and immature organic matter, indicating that shallow mud has been mobilized by the influx of deep-sourced hydrocarbon-rich fluids. Methane is enriched in the heavier isotopes, with values of δ13C∼−46.6‰VPDB and δD ∼−228‰VSMOW, and is associated with high amounts of heavier homologues (C2+) suggesting a co-genetic origin with the petroleum.On the contrary at the periphery, a lower but sustained CH4 flux is indicated by deeper sulphate–methane transition zones and the presence of 13C-depleted biomarkers of AOM, consistent with predominantly immature organic matter. Values of δ13C-CH4∼−60‰VPDB and decreased concentrations of 13C-enriched C2+ are typical of mixed microbial CH4 and biodegraded thermogenic gas from Plio-Pleistocene reservoirs of the region. The maturity of gas condensate migrated from pre-Miocene sources into Miocene reservoirs of the Western NDSF is higher than that of the gas vented at the centre of NAMV, supporting the hypothesis that it is rather released from the degradation of oil in Neogene reservoirs. Combined with the finding of hot pore water and petroleum at the centre, our results suggest that clay mineral dehydration of Neogene sediments, which takes place posterior to reservoir filling, may contribute to intense gas generation at high sedimentation rate deltas.

Redefining the isotopic boundaries of biogenic methane: Methane from endoevaporites

The recent reports of methane in the atmosphere of Mars, as well as the findings of hypersaline paleoenvironments on that planet, have underscored the need to evaluate the importance of biological (as opposed to geological) trace gas production and consumption, particularly in hypersaline environments. Methane in the atmosphere of Mars may be an indication of extant life, but it may also be a consequence of geologic activity and/or the thermal alteration of ancient organic matter. On Earth these methane sources can be distinguished using stable isotopic analyses and the ratio of methane (C1) to C2 and C3 alkanes present in the gas source (C1/(C2+C3)). We report here that methane produced in hypersaline environments on Earth has an isotopic composition and alkane content outside the values presently considered to indicate a biogenic origin. Methane-rich bubbles released from sub-aqueous substrates contained δ13CCH4 and δ2HCH4 values ranging from −65‰ to −35‰ and −350‰ to −140‰ respectively. Higher salinity endoevaporites yielded what would be considered non-biogenic methane based upon stable isotopic and alkane content, however incubation of crustal and algal mat samples resulted in methane production with similar isotopic values. Radiocarbon analysis indicated that the production of the methane was from recently fixed carbon. An extension of the isotopic boundaries of biogenic methane is necessary in order to avoid the possibility of false negatives returned from measurements of methane on Mars and other planetary bodies.

Methanogenesis pathways in a stratified eutrophic alpine lake (Lake Bled, Slovenia)

The production pathway and carbon source of CH4 in the surface sediment of a eutrophic alpine lake (Lake Bled, northwest Slovenia), in which the hypolimnion is anoxic for most of the year, were determined from molecular and biogeochemical studies. The average δ13CCH4 value of ‐69.5‰ ± 1.2‰, associated with low acetate concentrations, suggested that CH4 should be formed, predominantly, hydrogenotrophically. The proportion of “fresh” autochthonous lipids in total extractable lipids in sediment decreased from 62% at the surface to 41% at a depth of 20 cm. The contribution of lipids of bacterial origin was more pronounced at the surface, comprising 13%. Terminal restriction fragment length polymorphism (T‐RFLP) analysis of 16S ribosomal ribonucleic acid (rRNA) sequences of bacterial and archaeal community members suggested that larger sediment‐depth‐dependent changes occurred in the latter. The majority of archaeal sequences belonged to Euryarchaeota. The methanogenic population accounted for 73% and 38% of the archaeal community at depths of 0‐2 cm and 10‐12 cm, respectively. In the upper 2 cm, hydrogenotrophs, mostly Methanomicrobiaceae, were dominant. In the deeper sediment, archaeal sequences were mostly those of unknown affiliation with Euryarchaeota, Thermoplasmatales, and related linkages, and only 21% of the hydrogenotrophic methanogenic archaea were detected. Somewhat lower percentages (&lt; 18%) of sequences representing acetotrophic archaea (Methanosaetaceae) were present in the two layers. The biogeochemical processes and structure of the archaeal community support the hypothesis that hydrogenotrophic methanogenesis is the dominant pathway in the sediment of alpine Lake Bled, despite low temperature and the prevalence of “fresh” autochthonous‐derived organic matter.

Origins of methane discharging from volcanic-hydrothermal, geothermal and cold emissions in Italy

We have addressed the origins of methane in fumarolic, thermal and cold emissions discharging from a diversity of geological settings spread all over the Italian Peninsula and at Panarea and Pantelleria islands. Fluid discharges from the main geothermal systems of the peri-Tyrrhenian area (including Mt. Amiata, Larderello, Latera and Manziana) show δ13C-CH4 and δD-CH4 values >−28‰ V-PDB and >−160‰ V-SMOW, respectively, and C1/C2+ concentration ratios >1000. These patterns suggest that CH4 discharged from these systems is primarily generated by abiogenic reduction of CO2 and/or CO. Methane seems to have, at least partially, a different origin than the higher hydrocarbons that might be entirely related to the thermal decomposition of organic matter or, alternatively, to methane polymerization. Fumaroles from the volcanic systems of Solfatara, Vesuvio, Panarea and Pantelleria have C1/C2+ concentration ratios in a wide range (from 17 to 4300), but δ13C-CH4 and δD-CH4 values >−21‰ V-PDB and >−120‰ V-SMOW, respectively. Such isotopic values are very similar to those reported for CH4 discharging from ultramafic hydrothermal systems. In agreement with recent evidence provided for Solfatara and Pantelleria we, therefore, infer that the CH4 in the volcanic fumaroles is mainly abiogenic, too. The δ13C-CH4 (from −21.6 to −36.9‰ V-PDB) and δD-CH4 (from −113 to −164‰ V-SMOW) values, as well as the C1/C2+ concentration ratios (from 19 to 2240), in the CO2-rich fluid discharges located between the geothermal-volcanic systems and the Apennine sedimentary chain tend to decrease eastwards as the temperatures of the fluid reservoirs progressively diminish, suggesting that the CH4 production is mainly related to thermogenic processes. According to this general trend, the CH4-dominated fluids from mud volcanoes located eastwards of the CO2-rich emissions are fed by an almost pure thermogenic source. The easternmost fluid discharges from the Po valley and the Adriatic coast, where organic matter evolves at relatively shallow depths in the presence of a low geothermal gradient, record the lowest δ13C-CH4 and δD-CH4 values, i.e. as low as −69‰ V-PDB and <−190‰ V-SMOW, respectively, and C1/C2+ concentration ratios up to 5500, suggesting that the CH4 production is mainly from microbially driven processes.

Characterization of the relationship between microbial degradation processes at a hydrocarbon contaminated site using isotopic methods

Decisions to employ monitored natural attenuation (MNA) as a remediation strategy at contaminated field sites require a comprehensive characterization of the site-specific biodegradation processes. In the present study, compound-specific carbon and hydrogen isotope analysis (CSIA) was used to investigate intrinsic biodegradation of benzene and ethylbenzene in an aquifer with high levels of aromatic and aliphatic hydrocarbon contamination. Hydrochemical data and isotope fractionation analysis of sulfate and methane was used complementarily to elucidate microbial degradation processes over the course of a three year period, consisting of six sampling campaigns, in the industrial area of Weißandt-Gölzau (Saxony-Anhalt, Germany).Enrichment of 13C and 2H isotopes in the residual benzene and ethylbenzene pool downgradient from the pollution sources provided evidence of biodegradation of BTEX compounds at this site, targeting both compounds as the key contaminants of concern. The enrichment of heavy sulfur isotopes accompanied by decreasing sulfate concentrations and the accumulation of isotopically light methane suggested that sulfate-reducing and methanogenic processes are the major contributors to overall biodegradation in this aquifer. Along the contaminant plume, the oxidation of methane with δ13CCH4 values of up to +17.5‰ was detected. This demonstrates that methane formed in the contaminant source can be transported along groundwater flow paths and be oxidized in areas with higher redox potentials, thereby competing directly with the pollutants for electron acceptors. Hydrochemical and isotope data was summarized in a conceptual model to assess whether MNA can be used as viable remediation strategy in Weißandt-Gölzau. The presented results demonstrate the benefits of combining different isotopic methods and hydrochemical approaches to evaluate the fate of organic pollutants in contaminated aquifers.

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.

Characterization of gases and solid residues from closed system pyrolysis of peat and coals at two heating rates

Closed system gold-tube pyrolysis experiments were performed on a peat and two coals (TY: Ro=0.51%; SX: Ro=0.94%) at temperatures ranging from 337 to 600°C and a pressure of 50MPa with heating rates of 2 and/or 20°C/h. Solid reaction residues were analyzed microscopically. Yields and chemical and isotopic compositions of the generated gases were also determined. All three samples had similar thermal evolution pathways. With increasing heating temperature, vitrinite reflectances (VRr) of the residues increased linearly from 0.72% to 4.50%. This increase was lesser for the sample with a higher hydrocarbon generation potential and at faster heating rates. Gas compositions are dominated by CO2 and CH4 throughout the experimental process. Total gas and CH4 yields gradually increase with pyrolysis temperature for all samples. The carbon isotopic compositions of CH4 generated from the peat are lighter than those from the coals. The δ13CCH4 values exhibit a generic evolution pattern which the initial CH4 is isotopically heavy, then becomes lighter at moderate temperatures, and finally becomes heavier again. Methane produced from the samples at low heating rate has higher transformation ratio than that at high heating rate under the same temperature, so tends to be isotopically heavy after pyrolysis temperature of more than 408°C.

Gas hydrates in shallow deposits of the Amsterdam mud volcano, Anaximander Mountains, Northeastern Mediterranean Sea

We investigated gas hydrate in situ inventories as well as the composition and principal transport mechanisms of fluids expelled at the Amsterdam mud volcano (AMV; 2,025 m water depth) in the Eastern Mediterranean Sea. Pressure coring (the only technique preventing hydrates from decomposition during recovery) was used for the quantification of light hydrocarbons in near-surface deposits. The cores (up to 2.5 m in length) were retrieved with an autoclave piston corer, and served for analyses of gas quantities and compositions, and pore-water chemistry. For comparison, gravity cores from sites at the summit and beyond the AMV were analyzed. A prevalence of thermogenic light hydrocarbons was inferred from average C1/C2+ ratios <35 and δ13C-CH4 values of −50.6‰. Gas venting from the seafloor indicated methane oversaturation, and volumetric gas–sediment ratios of up to 17.0 in pressure cores taken from the center demonstrated hydrate presence at the time of sampling. Relative enrichments in ethane, propane, and iso-butane in gas released from pressure cores, and from an intact hydrate piece compared to venting gas suggest incipient crystallization of hydrate structure II (sII). Nonetheless, the co-existence of sI hydrate can not be excluded from our dataset. Hydrates fill up to 16.7% of pore volume within the sediment interval between the base of the sulfate zone and the maximum sampling depth at the summit. The concave-down shapes of pore-water concentration profiles recorded in the center indicate the influence of upward-directed advection of low-salinity fluids/fluidized mud. Furthermore, the SO42− and Ba2+ pore-water profiles in the central part of the AMV demonstrate that sulfate reduction driven by the anaerobic oxidation of methane is complete at depths between 30 cm and 70 cm below seafloor. Our results indicate that methane oversaturation, high hydrostatic pressure, and elevated pore-water activity caused by low salinity promote fixing of considerable proportions of light hydrocarbons in shallow hydrates even at the summit of the AMV, and possibly also of other MVs in the region. Depending on their crystallographic structure, however, hydrates will already decompose and release hydrocarbon masses if sediment temperatures exceed ca. 19.3°C and 21.0°C, respectively. Based on observations from other mud volcanoes, the common occurrence of such temperatures induced by heat flux from below into the immediate subsurface appears likely for the AMV.

Biogenic origin of coalbed gas in the northern Gulf of Mexico Coastal Plain, U.S.A.

New coal-gas exploration and production in northern Louisiana and south-central Mississippi, Gulf of Mexico Basin, is focused on the Wilcox Group (Paleocene–Eocene), where the depth to targeted subbituminous C to high volatile C bituminous coal beds ranges from 300 to 1680 m, and individual coal beds have a maximum thickness of about 6 m. Total gas content (generally excluding residual gas) of the coal beds ranges from less than 0.37 cm3/g (as-analyzed or raw basis; 1.2 cm3/g, dry, ash free basis, daf) at depths less than 400 m, to greater than 7.3 cm3/g (as-analyzed basis; 8.76 cm3/g, daf) in deeper (> 1,500 m) parts of the basin. About 20 Wilcox coal-gas wells in northern Louisiana produce from 200 to 6485 m3 of gas/day and cumulative gas production from these wells is approximately 25 million m3 (as of December, 2006). U.S. Geological Survey assessment of undiscovered, technically recoverable gas resources in the Gulf of Mexico Coastal Plain, including northern and south-central Mississippi, indicates that coal beds of the Wilcox Group contain an estimated mean total 109.3 million m3 (3.86 trillion ft3) of producible natural gas. To determine the origin of the Wilcox Group coal gases in northern Louisiana, samples of gas, water, and oil were collected from Wilcox coal and sandstone reservoirs and from under- and overlying Late Cretaceous and Eocene carbonate and sandstone reservoirs. Isotopic data from Wilcox coal-gas samples have an average δ13CCH4 value of − 62.6‰ VPDB (relative to Vienna Peedee Belemnite) and an average δDCH4 value of − 199.9‰ VSMOW (relative to Vienna Standard Mean Ocean Water). Values of δ13CCO2 range from − 25.4 to 3.42‰ VPDB. Produced Wilcox saline water collected from oil, conventional gas, and coalbed gas wells have δDH2O values that range from − 27.3 to − 18.0‰ VSMOW. These data suggest that the coal gases primarily are generated in saline formation water by bacterial reduction of CO2. Shallow (< 150 m) Wilcox coal beds containing freshwater have little or no biogenic gas. Molecular and isotopic analyses of gas samples collected from conventional gas and oil wells suggests that both biogenic and thermogenic gases are present in and adjacent to the Wilcox intervals that contain biogenic coal gases. Oil, probably sourced from thermally mature, down-structural-dip parts of the Wilcox Group, is produced from sandstones within the coal-bearing interval. Gas chromatograms of C10+ saturated hydrocarbons from Wilcox oils show a depletion of n-alkanes probably resulting from biodegradation of the oil. Isotopic composition of the gases associated with the oils is of mixed themogenic and biogenic origin (average δ13CCH4 = − 44.4‰ VPDB, and average δDCH4 = − 182.4‰ VSMOW).

Spatial and seasonal variations in the stable carbon isotopic composition of methane in stream sediments of eastern Amazonia

The stable carbon isotopic composition of methane (δ13C-CH4) gas bubbles formed in the sediments of three Amazonian streamswas determined over a 5-yr period. The study sites were two ‘várzea’ floodplain (A¸cu and Maicá) and one ‘terrafirme’ (Jamaraquá) streams. The δ13C of sedimentary organic matter (SOM) from the surrounding vegetation and bottom sediments were also determined. The mean δ13C value ofSOMwas lower in the terra-firme (-29.6‰) than in the várzea stream (-23.8‰) as a result of less C4 plant deposition in the former. The δ13C-CH4 values varied systematically both seasonally and spatially among the sites during all 5 yr of the study, in association with changes during hydrologic cycle. Overall, the variation in values of δ13C-CH4 during the high water phase covered a narrower range of values, -63 to -56‰. Contrastively, during the low water phase the δ13C-CH4 values for várzea and terra-firme streams are different and are in direct opposition. At this phase, the δ13C-CH4 at terra-firme stream is at least 20‰ depleted of 13C compared to várzea streams. Changes in organic matter sources, water levels and associated microbial degradation processes control the observed seasonal and spatial variations in net stable carbon isotopic composition of methane.

Seismically triggered microbial methane production relating to the Vogtland NW Bohemia earthquake swarm period 2000, Central Europe

Long-term radiometric and hydrological investigations at the Wettinquelle mineral spring in Bad Brambach demonstrated the existence of a fluidal connection to the currently most frequent earthquake-swarm hypocentre at Novy Kostel, 10 km east of Bad Brambach. The gas composition and δ13CCH4 values of this mineral spring were monitored from May 2000 until October 2003, i.e., before, during and after the protracted swarm earthquake period from late August until late December 2000. About eight weeks after the beginning of the seismically active period, we observed an increase in the methane concentration (from ≈40 up to ≈250 ppmv) accompanied by a decrease in the methane δ13C values from ≈-50 to ≈-70‰. For more than two years, such periods of variations were repeatedly observed before returning to the baseline signature. It is assumed that this additional methane was microbially produced in the granite-enclosed aquifer using H2, which was released (seismically triggered) from the fissured granite in which the Wettinquelle spring capture is located. The additional methane production might have started as a co-seismic event, with only the migration from the deep granite to the surface being responsible for the eight-week delay.

Methane along the western Mexican margin

We investigated the processes controlling the water‐column and sediment methane distributions at 14 stations along the western Mexican margin, in and around the Gulf of California. Stations were grouped into two categories: coastal basins and open margins. Diffusive methane fluxes from the sediment at all sites, as estimated from sediment methane gradients, were 0.24–5.5 µmol m−2 d−1, with the highest fluxes observed on the Pacific margin of Baja California at both basin and open‐margin sites. These high rates occur despite the lack of significant terrestrial input to these sediments, reflecting the importance of upwelling‐induced productivity. Methane concentrations in the upper water column were supersaturated with respect to the present atmosphere at all sites, with sea‐air fluxes of methane of 0.5–5.9 µmol m−2 d−1. Four of the open‐margin sites had seafloor depths extending below the oxygen minimum zone (~400–800 m) and contained low methane concentrations below the subsurface methane maximum. The remaining margin site was shallow (593 m), with a seafloor that intersected the oxygen minimum zone, and had elevated methane concentrations throughout the water column; this indicates that such sediments may be a significant source of methane to the eastern tropical North Pacific (ETNP). The seawater within silled basin sites also had supersaturated methane concentrations, reflecting the anoxic conditions within the basins. However, methane levels were low at the sill depth, indicating that the silled basins were unlikely to be significant sources of methane to the ETNP. We observed an inverse relationship between methane concentration and δ13C‐CH4 value in the basin waters, consistent with biological aerobic oxidation of methane being released from the sediment; an apparent kinetic isotopic fractionation factor of 1.0100–0.0038 was calculated for this process. Isotopically heavy methane resulting from similar oxidation of seafloor‐derived methane may be the source of large pools of heavy methane previously observed offshore in the ETNP.

MERCER COMPLETES MILL FINANCING

Decisions to employ monitored natural attenuation (MNA) as a remediation strategy at contaminated field sites require a comprehensive characterization of the site-specific biodegradation processes. In the present study, compound-specific carbon and hydrogen isotope analysis (CSIA) was used to investigate intrinsic biodegradation of benzene and ethylbenzene in an aquifer with high levels of aromatic and aliphatic hydrocarbon contamination. Hydrochemical data and isotope fractionation analysis of sulfate and methane was used complementarily to elucidate microbial degradation processes over the course of a three year period, consisting of six sampling campaigns, in the industrial area of Weißandt-Gölzau (Saxony-Anhalt, Germany).Enrichment of 13C and 2H isotopes in the residual benzene and ethylbenzene pool downgradient from the pollution sources provided evidence of biodegradation of BTEX compounds at this site, targeting both compounds as the key contaminants of concern. The enrichment of heavy sulfur isotopes accompanied by decreasing sulfate concentrations and the accumulation of isotopically light methane suggested that sulfate-reducing and methanogenic processes are the major contributors to overall biodegradation in this aquifer. Along the contaminant plume, the oxidation of methane with δ13CCH4 values of up to + 17.5‰ was detected. This demonstrates that methane formed in the contaminant source can be transported along groundwater flow paths and be oxidized in areas with higher redox potentials, thereby competing directly with the pollutants for electron acceptors. Hydrochemical and isotope data was summarized in a conceptual model to assess whether MNA can be used as viable remediation strategy in Weißandt-Gölzau. The presented results demonstrate the benefits of combining different isotopic methods and hydrochemical approaches to evaluate the fate of organic pollutants in contaminated aquifers.

Multi-origin alkanes related to CO2-rich, mantle-derived fluid in Dongying Sag, Bohai Bay Basin

With the newly obtained carbon isotope data for the natural gas, a pilot study of multiple-sourced alkanes related to the mantled-derived fluid is presented. The carbon isotope values of alkanes in the Dongying Sag possess the features indicating a general organic origin. However, there are two sub-populations in the isotopic data set, which reflect two specific types of origins. In general, the sub-population with high δ13CCH4 values is related to the CO2-rich, mantle-derived fluid, and it is distributed in the belts where mantle-derived fluid flow and basic volcanic activities have occurred. Geological and geochemical studies demonstrate that this variation of methane carbon isotope values in the Dongying Sag is unrelated with the basin bury and thermal histories, types of source rocks, and reactions between basin fluid and rocks. Mixing of mantle-derived fluid and organic sourced hydrocarbons is probably the cause for the variation.