Carbon Isotopic Composition Of Methane Research Articles

Spatial distribution of methane in the Lake Biwa sediments and its carbon isotopic compositions.

The spatial variation of methane contents and its stable carbon isotopic composition of sediments from a mesotrophic lake (Lake Biwa) were determined. Methane concentrations in the sediments (0–10 cm), ranging from 0.04 to 2.41 μmol ml-1, were weakly correlated with the amounts of organic carbon in the surface sediments (0–2 cm), but were poorly correlated with the nitrogen isotopic composition of the bulk sediments used as an indicator of the origin of the organic matter (autochthonous vs. allochthonous). The carbon isotopic composition of methane ranged from −61 to −80‰ and exhibited positive correlation with methane concentration and the carbon isotopic composition of total carbonate (ΣCO2). The apparent fractionation factor (αcCH4-CO2) between methane and ΣCO2 was in the range between 1.07 and 1.085, which were rather high compared to previously reported values in freshwater sediments. The results suggested that major parts of methane were produced via CO2-H2 in the Lake Biwa sediments.

Methane exchange between coal-bearing basins and the atmosphere: the Ruhr Basin and the Lower Rhine Embayment, Germany

A precise knowledge of methane exchange processes is required to fully understand the recent rise of atmospheric methane concentration. Three of these processes take place at the lithosphere/atmosphere boundary: bacterial consumption of methane and emission of bacterial or thermogenic methane. This study was initiated to quantify these processes on a regional scale in the Ruhr Basin and the Lower Rhine Embayment. Since these areas are subject to bituminous coal and lignite mining, natural and anthropogenically-induced methane exchange processes could be studied. The methane emission and consumption rates and their carbon isotope signal were measured at the lithosphere/atmosphere boundary using flux chambers. On most of the soils studied, methane consumption by bacteria was identified. Thermogenic methane was released only at some of the natural faults examined. In active and abandoned bituminous coal mining areas methane emissions were restricted to small areas, where high emission rates were measured. The carbon isotope composition of methane at natural faults and in mining subsidence troughs was typical of thermogenic methane (−45 to −32 ‰ δ13C). Methane exchange balancing revealed that natural methane emissions from these two basins represent no source of atmospheric importance. However, methane release by upcast mining shafts dominates the methane exchange processes and is by about two orders of magnitude greater than methane consumption by bacterial oxidation in the soils.

Methane Generation from Methylated Aromatics: Kinetic Study and Carbon Isotope Modeling

The aim of this work was to elaborate a mathematical model that accounts for the carbon isotopic composition of methane generated during the thermal cracking of two model compounds: 9-methylphenanthrene (9-MPh) and 1-methylpyrene (1-MPyr). Pyrolysis experiments were carried out in an anhydrous closed system (gold vessels) during times ranging from 1 to 120 h under isothermal conditions (400−475 °C) at a constant pressure of 150 bar. Global rate constants were determined for methane generation from 1-methylpyrene decomposition, similar to those determined by Behar et al. (see ref 36 in the text) for 9-MPh thermal cracking. Two main processes of methane formation were recognized: one related to the loss of the methyl group and the second corresponding to the opening of the aromatic rings, the second of which is hydrogen pressure dependent. The derived apparent first-order kinetic parameters were determined only for the first process: E = 55.6 kcal/mol and A = 4.2 × 1012 s-1. These parameters are in the same range as those found for methane generated from 9-MPh (ref 36). When these results are extrapolated to geological conditions, methane generation occurs at temperatures lower than 200 °C and, thus, constitutes a significant source for natural gas accumulations. This source of natural gas can compete with late methane generation from kerogen. Based on the global kinetic scheme proposed for methane generation, a model of carbon isotopic fractionation was elaborated for predicting the isotopic composition of methane. Results show that very high isotopic fractionation can take place when the methylated aromatics are thermally degraded: the demethylation reaction leads to an isotopic fractionation between the generated methane and its source which is significantly dependent upon the isotopic heterogeneity of the aromatic compound. This study shows that specific isotopic signatures in natural gas might fingerprint the secondary cracking of aromatics in deep reservoirs.

Geochemical characteristics and source rock of Ordovician gas reservoir, Changqing Gasfield

Carbon isotopic composition of methane in Ordovician gas reservoir in the Changqing Gasfield can reflect the dominance of Upper Paleozoic coaltype gas despite its high mature degree. The12C-rich ethane in the Ordovician reservoir (δ13C2 < −29%), known as a marker as oiltype gas, does not indicate the Ordovician marine carbonate as main source rock, it is because of the relative less ethane content in coaltype gas than in oiltyped gas due to generation and migration. The way of Upper Paleozoic coaltype gas migrating into the Ordovician reservoir is widely through the unconformlty instead of only through the erosion groove in the weathering crust.

Spatial and temporal patterns in diurnal variations of carbon isotope ratios of early-diagenetic methane from freshwater sediments

Methane was collected from the sediments of two freshwater lakes in SW Poland, Lake Sulistrowiczki and Lake Nowa Cerekiew, on three separate occasions in 1994, 1995, and 1997. Samples were collected from sediments at various shallow depths in the lakes (0.15 to 0.8 m) over the course of the day, and analyzed for carbon isotope ratios. A diurnal variation in the carbon isotopic composition of the methane was evidenced by changes in δ13C ratios of up to 14.5‰, with the most 13C-depleted gases collected at night and the least 13C-depleted gases collected in the early afternoon. The magnitude of the diurnal variation decreases with increasing water column depth as well as with a decrease in the period of daylight. The lowest δ13C values are believed to result from increased production of methane from carbon dioxide and hydrogen. The highest δ13C values are believed to occur when the pool of carbon dioxide available as a substrate is most enriched in 13C and when acetic acid fermentation becomes a more important methanogenic pathway. The observed changes in diurnal carbon isotope composition suggest that increased photosynthetic activity accompanied by carbon dioxide exchange between the sediment pore waters, overlying water column, and the atmosphere are more important factors than variations in temperature or oxidation of methane. Seasonal variations in the carbon isotopic composition of sediment methane and seasonal variations in the magnitude of the observed diurnal variation in isotopic composition may be related to changes in the light/dark cycle and latitude. However, the observed magnitudes of the diurnal variations are as large or larger than the seasonal changes and/or spatial variability that have previously been reported for the carbon isotopic composition of methane from wetlands. Thus, existing isotopic data on biogenic methanogenesis may need to be re-evaluated in light of such diurnal variations.

Methane oxidation by termite mounds estimated by the carbon isotopic composition of methane

Emission rates and carbon isotope ratios of CH4, emitted by workers of termites, and of CH4, emitted from their mounds, were observed in a dry evergreen forest in Thailand to estimate the proportion of CH4 oxidized during emission through the mound. The δ13C of CH4 emitted from a termite mound (−70.9 to −82.4‰) was higher than that of CH4 emitted by workers in the mound (−85.4 to −97. l‰). Using a fractionation factor (a = 0.987) for oxidation of CH4 which was obtained in the incubation experiment, an emission factor defined as (CH4 emitted from a termite mound/CH4 produced by termites) was calculated. The emission factor obtained in each termite mound was nearly zero for Macrotermes (fungus‐growing termites), of which the nest has a thick soil wall and subterrannean termites, and 0.17 to 0.47 for Termitinae (small‐mound‐making termites). Global CH4 emission by termites was estimated on the basis of the CH4 emission rates by workers and termite biomass with the emission factors. The calculated result was 1.5 to 7.4 Tg/y (0.3 to 1.3% of total source), which is considerably smaller than the estimate by the IPCC [1994].

Carbon isotopic fractionation in the process of Fischer-Tropsch reaction in primitive solar nebula

Carbon isotopic fractionation of low-weight molecules of the products of Fischer-Tropsch reaction under the conditions of Earth's accumulation region in primitive solar nebula has been experimentally studied. Among the reaction products, carbon dioxide has the heaviest isotopic composition; carbon isotopic composition of methane has a large variation; different from biogenic natural gases, carbon isotopic compositions among ethane, propane and butane yield a reverse distribution pattern. It suggests that primordial hydrocarbons that were captured in the interior of the Earth during its accretion possess a reverse carbon isotopic distribution pattern.

Genetic and temporal relations between formation waters and biogenic methane: Upper Devonian Antrim Shale, Michigan Basin, USA

Controversy remains regarding how well geochemical criteria can distinguish microbial from thermogenic methane. Natural gas in most conventional deposits has migrated from a source rock to a reservoir, rarely remaining associated with the original or cogenetic formation waters. We investigated an unusual gas reservoir, the Late Devonian Antrim Shale, in which large volumes of variably saline water are coproduced with gas. The Antrim Shale is organic-rich, of relatively low thermal maturity, extensively fractured, and is both source and reservoir for methane that is generated dominantly by microbial activity. This hydrogeologic setting permits integration of chemical and isotopic compositions of coproduced water and gas, providing a unique opportunity to characterize methane generating mechanisms. The well-developed fracture network provides a conduit for gas and water mass transport within the Antrim Shale and allows invasion of meteoric water from overlying aquifers in the glacial drift. Steep regional concentration gradients in chemical and isotopic data are observed for formation waters and gases; dilute waters grade into dense brines (300,000 ppm) over lateral distances of less than 30 km. Radiogenic ( 14C and 3H) and stable isotope ( 18O and D) analyses of shallow Antrim Shale formation waters and glacial drift groundwaters indicate recharge times from modern to 20,000 yr bp. Carbon isotope compositions of methane from Antrim Shale wells are typical of the established range for thermogenic or mixed gas (δ 13C = −47 to −56‰). However, the unusually high δ 13C values of CO 2 coproduced with methane (∼+22‰) and dissolved inorganic carbon (DIC) in formation waters (∼+28‰) require bacterial mediation. The δD values of methane and coproduced formation water provide the strongest evidence of bacterial methanogenesis. Methane/[ethane + propane] ratios and δ 13C values for ethane indicate: (1) the presence of a thermogenic gas component that increases basinward and (2) progressive bacterial oxidation of ethane as the Antrim Shale subcrop is approached. Multiple episodes of Pleistocene glaciation over northern Michigan appear critical to the development of these gas deposits. Loading of thick ice sheets may have provided hydraulic head that enhanced dilation of preexisting fractures and influx of meteoric water. The physical erosion cycle of repeated glacial advances and retreats exhumed the Antrim Shale around the northern margin of the Michigan Basin, subjecting it to near-surface physiochemical and biochemical processes. The chemical and hydrologic relations demonstrated in the Antrim Shale reservoir suggest a dynamic connection between Pleistocene glacial history of the midcontinent region and development of recoverable, microbially generated natural gas reserves.

Chemical and isotopic evidence of gas‐influenced flow at a transform plate boundary: Monterey Bay, California

Chemical and isotopic compositions of pore fluids document upward flow through communities of vesicomyid clams in Monterey Bay, California. Within the clam communities, the sulfate reduction zone is only 10 cm thick, and Ca and Mg concentrations decrease to values as low as 2.2 mM and 34.5 mM, respectively, at depths less than 30 cm below the sediment‐water interface. Less than 5 m outside the communities, the base of the sulfate reduction zone is deeper than the greatest penetration of the cores (∼30 cm), and Ca and Mg exhibit only minor changes from seawater values. The sediment exhibits no significant variation in grain size, mineralogy, organic carbon, nitrogen, or carbonate content throughout the region. The composition of pore fluid within clam communities results from upward flow of altered fluid rather than different diagenetic reactions within and outside the communities. Isotopically light dissolved inorganic carbon (DIC), with δ13C values ranging from −3.2 to −54.1‰, could reflect carbon sources from either oxidized thermogenic methane and/or a mixture of oxidized microbial methane and solid organic carbon. The C1/(C2+C3) ratios (ranging from 34 to 1142) and the hydrogen and carbon isotopic compositions of methane (δD values of −109 to −156‰; δ13C values of −30.6 to −86.6‰) suggest that methane is primarily microbial but that a minor component could be thermally generated. Any thermogenic methane would have migrated from great depths, possibly &gt;2 km. The presence of methane is likely to contribute to fluid flow by reducing the density of the fluids. Past fluid migration and venting are reflected by widespread carbonate mineralization at the sediment‐water interface. This mineralization and the geographic distribution and proportions of microbial and thermogenic methane suggest that vent sites migrate when permeability is reduced during carbonate cementation. These results demonstrate that along with convergent and divergent plate boundaries, transform plate boundaries are characterized by fluid flow and that the flow may be widespread, occurring at sites away from fault zones.

Emission flux and carbon isotopic composition of methane from rice paddies in Guizhou Province

Methane emission flux from rice paddies in Guizhou Province shows obvious three-stage diurnal variations: it is relatively low from the morning to the noon, then successively increases in the afternoon and at night, and finally follows a gradual decrease. The δ13C values of trace methane emitted from rice paddies in Guizhou Province range from −64.5‰ to −54. 1‰ with an average of −60.7‰, very close to the reported values from America or Kenya.

Hydrogen and carbon isotopic composition of methane as evidence for biogenic origin of natural gases from the Green Tuff Basin, Japan

Hydrogen and carbon isotopic analyses of methane were carried out to investigate the origin of natural gases commercially produced from volcanic rocks in the Green Tuff Basin of northeast Japan, whose high 3He/ 4He ratios have been interpreted as an indication of their abiogenic origin. Methane isotopic signatures (δ 13C ≈ −32 to −67‰, δD ≈ −163 to −202‰, respectively) suggest that these are thermogenic gases or mixtures of thermogenic gases with bacterial gases. The correlations of methane δD vs. δ 13C for the gases from specific fields are consistent with a mixing model between bacterial and thermogenic end-members, which was previously proposed on the basis of 13C contents of C 1 to C 4 hydrocarbons (Sakata, 1991; Berner and Faber, 1996). The Green Tuff Basin gases are isotopically distinct from well-known abiogenic methanes, e.g. geothermal methane from the East Pacific Rise. A significant contribution of abiogenic methane would, therefore, be unlikely for these gases.

Seasonal variations in the 13C-CH4 of Amazon floodplain waters

Measurements of the stable carbon isotopic composition of methane (CH4) trapped in bubbles in the sediments and CH4 emitted to the atmosphere from wetland areas of the Amazon River floodplain are r...

Measurement of the Carbon Isotopic Composition of Methane Using Helicoidal Laser Eigenstates

The spatially generalized Jones matrix formalism is used to design a laser cavity to make intracavity measurements of the carbon isotopic composition of methane. The method is based on a double optical lever effect for helicoidally polarized eigenstates, permitting to measure successively the ~~CH4 and ~~CH4 concentrations. To choose the probed isotope, one simply tunes the frequency of the laser by Zeeman effect. The experiment exhibits a good agreement with the predictions and permits to measure the ~~CH4/~~CH4 composition ratio of methane with an uncertainty of the order of +0.07% for a sample containing only 6 X 10~~ mole of methane.

Factors that control the stable carbon isotopic composition of methane produced in an anoxic marine sediment

The carbon isotopic composition of methane produced in anoxic marine sediment is controlled by four factors: (1) the pathway of methane formation, (2) the isotopic composition of the methanogenic precursors, (3) the isotope fractionation factors for methane production, and (4) the isotope fractionation associated with methane oxidation. The importance of each factor was evaluated by monitoring stable carbon isotope ratios in methane produced by a sediment microcosm. Methane did not accumulate during the initial 42-day period when sediment contained sulfate, indicating little methane production from 'noncompetitive' substrates. Following sulfate depletion, methane accumulation proceeded in three distinct phases. First, CO2 reduction was the dominant methanogenic pathway and the isotopic composition of the methane produced ranged from -80 to -94 per thousand. The acetate concentration increased during this phase, suggesting that acetoclastic methanogenic bacteria were unable to keep pace with acetate production. Second, acetate fermentation became the dominant methanogenic pathway as bacteria responded to elevated acetate concentrations. The methane produced during this phase was progressively enriched in C-13, reaching a maximum delta(C-13) value of -42 per thousand. Third, the acetate pool experienced a precipitous decline from greater than 5 mM to less than 20 micro-M and methane production was again dominated by CO2 reduction. The delta(C-13) of methane produced during this final phase ranged from -46 to -58 per thousand. Methane oxidation concurrent with methane production was detected throughout the period of methane accumulation, at rates equivalent to 1 to 8 percent of the gross methane production rate. Thus methane oxidation was too slow to have significantly modified the isotopic signature of methane. A comparison of microcosm and field data suggests that similar microbial interactions may control seasonal variability in the isotopic composition of methane emitted from undisturbed Cape Lookout Bight sediment.

Genetic classification of natural gases in the oil-gas zone and its application in the Sichuan basin

On the basis of the carbon isotopic compositions of methane (CH4) and its homologues and the differences in isotopic values for CH4 and ethane (C2H6) and the correlation and compositional characteristics of hydrocarbon gases, the author has proposed a genetic classification of natural gases in the oil-gas zone. They are classified as biogenetic and abiogenetic gases in terms of the types of hydrocarbon-generating precursors (or parent materials) and their thermal evolution stages. Biogenetic gases can also be further divided into two series: biochemical and thermochemical gases, with the latter formed at different evolution stages. Gases generated from type-I and -II1 organic matter are called oil-series gases, those from type-III, coal-series gases, and those from type-II2, mixture-type gases. Gases generated from two or more than two types of precursors are called mixture-source gases.

Microbiological Processes in Landfills

Large landfills in the Soviet Union cover more than 140 thousand hectares. The gas emission intensities are extremely disproportional over the surface of these landfills. According to our data the rates of streams of the biogas components vary from 0 till 20, 46, 1.2 and 0.75 (×10−4m3 × hour −1/m2) for CH4, CO2, H2, CO, respectively. The stable carbon isotope composition of methane and carbon dioxide in the biogas of deep landfills layers is typical for methanogenesis from organic wastes, but it depends on the concentration of organic substance in the landfill ground and on the age of these landfills. In the upper layer methane becomes heavier and carbon dioxide lighter due to the microbiological oxidation processes. The most intensive methanogenesis is usually observed in the upper part of the anaerobic zone where the organic substance concentration is relatively high. The methanogenic microflora is represented by mesophilic methanobacteria, species of Methanosarcina and Methanobacterium genera are prevalent forms. At the depth of 10-100 cm from the surface intensive oxidation of methane, hydrogen and carbon monoxide is observed. The number of gas oxidizing bacteria is up to 10 cells per gram of soil. 14 species of methane oxidizing bacteria, which belong to 5 genera were iden tified. Hydrogen oxidizing bacteria belong to Alcaligenes,Pseudomonas, Paracoccus, Mycobacteriun genera. Between them psychrotrophic forms were found. It has been shown that in small landfills methane, produced in anaerobic zone, can be oxidized completely in the upper ground layer. The biogas extraction from landfills is important not only as an additional fuel source, but as a means of preventing pollution of the Earth's atmosphere.

Correction to “Temporal variations in the stable carbon isotopic composition of methane emitted from Minnesota peatlands” by Cheryl A. Kelley, Nancy B. Dise, and Christopher S. Martens

Global Biogeochemical CyclesVolume 6, Issue 4 p. 347-347 CorrectionFree Access Correction to “Temporal variations in the stable carbon isotopic composition of methane emitted from Minnesota peatlands” by Cheryl A. Kelley, Nancy B. Dise, and Christopher S. Martens First published: December 1992 https://doi.org/10.1029/92GB02548AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume6, Issue4December 1992Pages 347-347 RelatedInformation

Methane transport mechanisms and isotopic fractionation in emergent macrophytes of an Alaskan tundra lake

The carbon isotopic composition of methane emitted by the Alaskan emergent aquatic plantsArctophila fulva, a tundra mid‐lake macrophyte, andCarex rostrata, a tundra lake margin macrophyte, was −58.6 ± 0.5 (n=2) and −66.6±2.5 (n= 6) ‰ respectively. The methane emitted by these species was found to be depleted in13C by 12‰ and 18‰, relative to methane withdrawn from plant stems 1 to 2 cm below the waterline. As the macrophyte‐mediated methane flux represented approximately 97% of the flux from these sites, these results suggest the more rapid transport of12CH4relative to13CH4through plants to the atmosphere. This preferential release of the light isotope of methane, possibly combined with CH4oxidation, caused the buildup of the heavy isotope within plant stems. Plant stem methane concentrations ranged from 0.2 to 4.0% (, 1.4; standard deviation (sd), 0.9;n=28) inArctophila, with an isotopic composition of −46.1±4.3 ‰ (n= 8).Carexstem methane concentrations were lower, ranging from 150 to 1200 ppm (, 500; standard deviation, 360; n = 8), with an isotopic composition of −48.3±1.4‰ (n=3). Comparisons of the observed isotopic fractionations with those predicted from gas phase effusion and diffusion coefficients suggest a combination of one or both of these gas transport mechanisms with bulk (non‐fractionationating) flow.

Factors that control the stable carbon isotopic composition of methane produced in an anoxic marine sediment

The carbon isotopic composition of methane produced in anoxic marine sediment is controlled by four factors: (1) the pathway of methane formation, (2) the isotopic composition of the methanogenic precursors, (3) the isotope fractionation factors for methane production, and (4) the isotope fractionation associated with methane oxidation. The importance of each factor was evaluated by monitoring stable carbon isotope ratios in methane produced by a sediment microcosm. Methane did not accumulate during the initial 42‐day period when sediment contained sulfate, indicating little methane production from “noncompetitive” substrates. Following sulfate depletion, methane accumulation proceeded in three distinct phases. First, CO2 reduction was the dominant methanogenic pathway and the isotopic composition of the methane produced ranged from −80 to −94‰. The acetate concentration increased during this phase, suggesting that acetoclastic methanogenic bacteria were unable to keep pace with acetate production. Second, acetate fermentation became the dominant methanogenic pathway as bacteria responded to elevated acetate concentrations. The methane produced during this phase was progressively enriched in 13C, reaching a maximum δ13C value of −42‰. Third, the acetate pool experienced a precipitous decline from &gt;5 mM to &lt;20 μM and methane production was again dominated by CO2 reduction. The δ13C of methane produced during this final phase ranged from −46 to −58‰. Methane oxidation concurrent with methane production was detected throughout the period of methane accumulation, at rates equivalent to 1 to 8% of the gross methane production rate. Thus methane oxidation was too slow to have significantly modified the isotopic signature of methane. A comparison of microcosm and field data suggests that similar microbial interactions may control seasonal variability in the isotopic composition of methane emitted from undisturbed Cape Lookout Bight sediment.

Temporal variations in the stable carbon isotopic composition of methane emitted from Minnesota peatlands

The stable carbon isotopic composition of methane (δ13C) emitted from two peatland sites in the Marcell Experimental Forest in northern Minnesota was investigated during the snow‐free season of 1989–1990. A seasonal range in δ13C values of 13‰ was seen for a forested bog with heavier (13C enriched) methane emitted during the wanner summer months. This shift was correlated with water table level suggesting control by microbial oxidation. Methane from a nearby poor fen transitional to bog dominated byCarex oligospermashowed a similar temporal trend but with a much smaller range of 5‰ during the same time period and with no water table level correlation. The methane emitted from the fen was consistently heavier than that emitted by the bog.