Michael J. Whiticar

Biogenic methane formation in marine and freshwater environments: CO2 reduction vs. acetate fermentation—Isotope evidence

Two primary methanogenic pathways can be distinguished using the carbon and hydrogen stable isotope composition of the methane as a function of the coexisting carbon dioxide and formation water precursors. Although both pathways may occur in both marine and freshwater sediments. CO2 reduction is dominant in the sulphate-free zone of the former, while acetate fermentation is the major pathway in freshwater sediments. Methane in marine sediments can be defined isotopically by δ13C −110 to −600/%., and δD −250 to −1700‰. In contrast, methane from freshwater sediments ranges from δ13C −65 to −500/%. and δD −400 to −2500/%.. Carbon isotope fractionations (αcCO2-CH4) are generally between 1.05 and 1.09 for marine sediments, while lower in freshwater sediments (1.04 to 1.06). The relationship of the methane to the formation water indicates the source of the hydrogen for CO2 reduction to be the water directly with an associated hydrogen fractionation of −180 ± 200/%.. The CH4-H2O hydrogen fractionation is larger for acetate fermentation due to the transfer of the methyl group during methanogenesis which is depleted in deuterium and accounts for 34 of the hydrogen in the methane. A model is presented showing that the fourth hydrogen via acetate fermentation may ultimately come from the formation water but is isotopically fractionated. Combination of the carbon and hydrogen isotope fractionations (αC, αD) from CH4 with CO2 and H2O respectively, can clearly delineate the CO2 reduction and acetate fermentation environments. Defining the character of the methanogenic types with carbon and hydrogen isotopes not only provides information about the environment of formation, it is also most useful in distinguishing biogenic from thermogenic methane gases.

Methane oxidation in sediment and water column environments ― isotope evidence

Abstract Microbial anaerobic oxidation of methane in sediments is a kinetic process associated with a carbon isotope effect which enriches the remaining methane in 13 C. Three, models: % residual methane, higher hydrocarbon enrichment, and CO 2 -CH 4 coexisting pairs are used to independently calculate fractionation factors (αc) in the range of 1.002–1.014, which overlap the range determined by culture studies, αc is smaller than that associated with methanogenesis by CO 2 reduction or by acetate-type fermentation, and comparison of the coexisting CO 2 -CH 4 pairs can distinguish between the formation and consumption processes. Methane oxidation in sediments continues to a threshold concentration of ca . 0.2 mM; the residual methane is either unavailable or unattractive to consumption. Minor amounts of methane may also be produced simultaneously in the methane consumption zone, influencing the apparent fractionation factor in this zone.

A geochemial perspective of natural gas and atmospheric methane

Natural gases are key components in the description of the global carbon cycle. Although the estimated 120 Gt C reserves of natural gas are minute compared to the total carbon reservoir (c. 1.4 × 108 Gt C), the variations in composition and distribution can provide information on the magnitude of carbon fluxes between specific geo-, hydro- and atmospheric reservoirs. Natural gases, and in particular methane, influence not only our economies, but our atmosphere and ultimately our climate as well. Volatile hydrocarbons also represent, in certain environments, a significant carbon nutrient source for specialized organisms. The magnitudes of natural gas reservoirs and fluxes are put into perspective. On a global basis, natural gas supplies c. 20% of mans primary, external energy needs; following the consumption of oil and coal (38 and 30%, respectively). Currently, worldwide production of natural gas amasses 1.8 × 1012m3y−1 or c. 2% y−1 of the proven geogas reserves (estimated total geogas reserves > 200 × 1012m3). Geochemical exploration for hydrocarbons relies heavily on the characterization of natural gases. As the volatile hydrocarbon phase, natural gases have physical and chemical properties which assist in their detection and application. They migrate readily, and are widely distributed in the sediment column. In addition, the significant range in molecular and isotope composition of natural gases provides interpretative information on the gases (e.g. conventional bacterial/diagenetic, thermogenic, geothermal or unconventional “deep gas”) and on the maturity/type of the precursor source material from which they are derived. Reliable interpretation of natural gas data requires that secondary effects (e.g. migration, mixing, oxidation) can be identified. Artifacts, such as hydrocarbons generated during drilling or analysis, and sampling contamination/alteration must also be considered. These various aspects are reviewed. Occasionally, anomalous natural gas data lead us to the recognition of new gas types or processes. Some recent geochemical enigmas are presented for both the adventurous and skeptics.

Sources and flux of natural gases from Mono Lake, California

Abstract The ability to identify a formation mechanism for natural gas in a particular environment requires consideration of several geochemical factors when there are multiple sources present. Four primary sources of methane have been identified in Mono Lake. Two of these sources were associated with numerous natural gas seeps which occur at various locations in the lake and extend beyond its present boundary; the two other gas sources result from current microbiological processes. In the natural gas seeps, we observed flow rates as high as 160 moles CH4 day−1, and estimate total lakewide annual seep flux to be 2.1 × 106 moles CH4. Geochemical parameters (δ13CH4,δDCH4,CH4/[C2H6+ C3H8]) andδ14CH4measurements revealed that most of the seeps originate from a paleo-biogenic (δ13CH4 = about −70%.). natural gas deposit of Pleistocene age which underlies the current and former lakebed. Gas seeps in the vicinity of hot springs had, in combination with the biogenic gas, a prominent thermogenic gas component resulting from hydrothermal alteration of buried organic matter. Current microbiological processes responsible for sources of natural gas in the lake included pelagic meth- anogenesis and decomposition of terrestrial grasses in the littoral zone. Methanogenesis in the pelagic sediments resulted in methane saturation (2–3 mM at 50 cm; δ13CH4 = about −85%.). Interstitial sulfate decreased from 133 mM at the surface to 35 mM by 110 cm depth, indicating that sulfate-reduction and methanogenesis operated concurrently. Methane diffused out of the sediments resulting in concentrations of about 50 μM in the anoxic bottom waters. Methane oxidation in the oxic/anoxic boundry lowered the concentration by >98%, but values in surface waters (0.1–1.3μM) were supersaturated with respect to the atmosphere. The δ13CH4 (range = −21.8 to −71.8%.) of this unoxidized residual methane was enriched in 13C relative to methane in the bottom water and sediments. Average outward flux of this methane was 2.77 × 107 moles yr−1. A fourth, but minor source of methane (δ13CH4 = −55.2%.) was associated with the decomposition of terrestrial grasses taking place in the lakes recently expanded littoral zone.

Methane-derived CO2 in pore fluids expelled from the Oregon subduction zone

Pore fluids extracted from near-surface sediments of the deformation front along the Oregon subduction zone have, in general, the dissolved nutrient pattern characteristic of bacterial sulfate reduction. However, in certain locations there are peculiar ammonium distributions and anomalously 13C-depleted dissolved ΣCO2. These carbon isotope and nutrient patterns are attributed to the concurrent microbially-mediated oxidation of sedimentary organic matter (POC) and methane (CH4) originating from depth. In contrast to the oxidation of sedimentary organic matter in the sulfate zone, utilization of methane as the carbon source by sulfate-reducing bacteria would generate only half as much total carbon dioxide for each mole of sulfate consumed and would not generate any dissolved ammonium. The isotopically light ΣCO2 released from methane oxidation depletes the total metabolic carbon dioxide pool. Therefore, NH4+, ΣCO2 and δ13C of interstitial carbon dioxide in these pore fluids distintcly reflect the combined contributions of each of the two carbon substrates undergoing mineralization; i.e. methane and sedimentary organic matter. By appropriately partitioning the nutrient and substrate relationships, we calculate that in the area of the marginal ridge of the Oregon subduction zone as much as 30% of the ΣCO2 in pore fluids may result from methane oxidation. The calculation also predicts that the carbon isotope signature of the carbon dioxide derived from methane is between −35‰ and −63‰ PDB. Such an isotopically light gas generated from within the accretionary complex could be the residue of a biogenic methane pool. Fluid advection is required to carry such methane from depth to the present near-surface sediments. This mechanism is consistent with large-scale, tectonically-induced fluid transport envisioned for accreted sediments of the worlds convergent plate boundaries.

Bacterial ethane formation from reduced, ethylated sulfur compounds in anoxic sediments☆

Trace levels of ethane were produced biologically in anoxic sediment slurries from five chemically different aquatic environments. Gases from these locations displayed biogenic characteristics, having 12C-enriched values of δ13CH4 (−62 to −86%.), δ13C2H6 (−35 to −55%.) and high ratios (720 to 140,000) of CH4[C2H6 + C3H8]. Endogenous production of ethane by slurries was inhibited by autoclaving or by addition of the inhibitor of methanogenic bacteria, 2-bromoethanesulfonic acid (BES). Ethane formation was stimulated markedly by ethanethiol (ESH), and, to a lesser extent, by diethylsulfide (DES). Formation of methane and ethane in ESH- or DES-amended slurries was blocked by BES. Experiments showed that ethionine (or an analogous compound) could be a precursor of ESH. Ethylamine or ethanol additions to slurries caused only a minor stimulation of ethane formation. Similarly, propanethiol additions resulted in only a minor enhancement of propane formation. Cell suspensions of a methyltrophic methanogen produced traces of ethane when incubated in the presence of DES, although the organism did not grow on this compound. These results indicate that methanogenic bacteria produce ethane from the traces of ethylated sulfur compounds present in recent sediments. Preliminary estimates of stable carbon isotope fractionation associated with sediment methane formation from dimethylsulfide was about 40%., while ethane formation from DES and ESH was only 4. 6 and 6.5%., respectively.

Hydrothermal hydrocarbon gases in the sediments of the King George Basin, Bransfield Strait, Antarctica

Thermogenic hydrocarbons, formed by the thermal alteration of organic matter, are encountered in several piston core stations in the King George Basin, Anatarctica. These hemipelagic sediments are being deposited in an area of active hydrothermalism, associated with the back-arc spreading in the Bransfield Strait. The lateral extent of sediments infiltrated by the hydrothermally influenced interstitial fluids is characterized by basalt diapirix intrusions and is delineated by an acoustically turbid zone in the sediments of the eastern part of the basin. Iron-sulphide-bearing veins and fractures cut across the sediment in several cores; they appear to be conduits for flow of hydrothermally altered fluids. These zones have the highest C2+ and ethene contents. The thermogenic hydrocarbons have molecular C1/(C2 + C3) ratios typically < 50 and δ13CH4 values between −38% and −48%, indicating an organic source which has undergone strong thermal stress. Several sediment cores also have mixed gas signatures, which indicate the presence of substantial amounts of bacterial gas, predominantly methane. Hydrocarbon generation in the King George Basin is thought to be a local phenomenon, resulting from submarine volcanism with temperatures in the range 70–150°C. There are no apparent seepages of hydrocarbons into the water column, and it is not believed that significant accumulation of thermogenic hydrocarbons reside in the basin.

Extreme isotope fractionation of hydrocarbon gases in Permian salts

Hydrocarbon gases are entrapped in the north german Zechstein evaporites. Methane is the predominant hydrocarbon present (2.2–1243.3 μmol/kg). These relatively low gas concentrations are believed to occupy the intracrystalline sites; the intercrystalline gases were probably lost during sampling and storage. The gases can be differentiated by molecular and stable carbon and hydrogen isotopic composition into two major groups: Gases from halites, anhydrites and salt clays have a signature of thermogenic, associated hydrocarbons which are derived from the marine Kupferschiefer source rock (T1) at the base of the Zechstein formation. Isotope maturity estimates from the gas data (1.0–1.2% Rr) correspond to measured vitrinite reflectance on T1 (1.0% Rr). Gases from the potash layers are enriched in methane and depleted in C2+, relative to the other group. Methane and ethane are extremely 13C-rich (δ13C1 up to + 12.7‰, δ13C2 up to + 10.7‰) presenting a most unusual isotope signature for natural hydrocarbon gases. Hydrogen isotopes in methane are also anomalous with deuterium depletions as great as −495‰. Alteration effects due to bacterial oxidation, degassing etc. are unlikely to cause these unusual values, whereas the isotopic effects associated with diffusion and migration in evaporites are currently unknown. Indigeneous precursor organic source matter with such as anomalous isotope signature has not been identified. A Gorleben well (Go 1004) penetrated a continuous section through the various evaporite types, clearly showing the effects of mixing between the two gas groups. Additionally, there is evidence suggesting that the anomalous gases in the potash layers are created at stratigraphic boundaries, perhaps due to mechanical stress during salt tectonics.

Authigenic carbonates in sediments from the Gulf of Mexico

Abstract The carbon isotopic composition of diagenetic dolomite and calcite in some sediments of the Gulf of Mexico varies between “normal-marine” ( δ 13 C ca. 0‰) and −14.6‰ which suggests that biogenic CO 2 contributed to the carbonate formation. The δ 13 O values of dolomite and coexisting calcite are very similar but variable down-core. Dolomite and calcite precipitated early from pore water where SO 4 2− was not reduced. However, during (and after?) SO 4 2− reduction dolomite and calcite still formed and there are at least two generations of carbonate minerals present.

Hydrogen and Carbon Isotopes of C1 to C5 Alkanes in Natural Gases: ABSTRACT

A technique has been developed to determine C12/C13 and D/H isotopic ratios on small quantities of methane through pentane hydrocarbons and has been applied to natural gases from various genetic sources (i.e., early-diagenetic, oil-associated, late-catagenic and mixed-gas sources). Carbon isotopes measured from 27 natural gases have ^dgr13C range of 23, 15, and 20^pmil for the C1 to C3 alkanes and maximum ^dgr13C values of -35.1, -26.8, and -20.8 ^pmil respectively. With a smaller sample base, butane and pentane vary within 16 and 4 ^pmil, respectively, for those samples with the most positive ^dgr13C = -26.5 and -26.9 ^pmil. Deuterium isotopes exhibit greater isotopic variation than the corresponding carbon values. D/H variations clearly decrease toward the higher homologues with ^dgrD ranges of 182, 110, 75, 43, and 29 ^pmil for C1 through C5. Most negative D/H measurements also decrease with carbon number from ^dgrD = -311 ^pmil for methane to -128 ^pmil for pentane. These relative changes in carbon and hydrogen isotopic contents for the higher homologues are useful in the classification of natural gases, particularly those of mixed origin. End_of_Article - Last_Page 316------------