B. Sherwood Lollar

Abiogenic formation of alkanes in the Earth's crust as a minor source for global hydrocarbon reservoirs.

Natural hydrocarbons are largely formed by the thermal decomposition of organic matter (thermogenesis) or by microbial processes (bacteriogenesis). But the discovery of methane at an East Pacific Rise hydrothermal vent and in other crustal fluids supports the occurrence of an abiogenic source of hydrocarbons. These abiogenic hydrocarbons are generally formed by the reduction of carbon dioxide, a process which is thought to occur during magma cooling and—more commonly—in hydrothermal systems during water–rock interactions, for example involving Fischer–Tropsch reactions and the serpentinization of ultramafic rocks. Suggestions that abiogenic hydrocarbons make a significant contribution to economic hydrocarbon reservoirs have been difficult to resolve, in part owing to uncertainty in the carbon isotopic signatures for abiogenic versus thermogenic hydrocarbons. Here, using carbon and hydrogen isotope analyses of abiogenic methane and higher hydrocarbons in crystalline rocks of the Canadian shield, we show a clear distinction between abiogenic and thermogenic hydrocarbons. The progressive isotopic trends for the series of C1–C4 alkanes indicate that hydrocarbon formation occurs by way of polymerization of methane precursors. Given that these trends are not observed in the isotopic signatures of economic gas reservoirs, we can now rule out the presence of a globally significant abiogenic source of hydrocarbons.

Abiogenic methanogenesis in crystalline rocks

Isotopically anomalous CH4-rich gas deposits are found in mining sites on both the Canadian and Fennoscandian shields. With δ 13CCH4 values from −22.4 to −48.5% and δDCH4 values from −133 to −372%, these methane deposits cannot be accounted for by conventional processes of bacterial or thermogenic methanogenesis. Compositionally the gases are similar to other CH4-rich gas occurrences found in Canadian and Fennoscandian shield rocks (Sherwood Lollar et al., 1993). However, the isotopically anomalous gases of this study are characterized by unexpectedly high concentrations of H2 gas, ranging from several volume percent up to 30 vol%. The H2 gases are consistently depleted in the heavy isotope, with δDH2 values of −619 to −659‰ 3H/4He ratios in the range of 0.4 × 10−8 to 4.4 sx 10−8 indicate that there is no resolvable component of mantle-derived He in these deposits. Based on these results a mantle-derived source for the C-bearing gases is unlikely. Several lines of evidence support an alternative abiogenic origin for the gases from Sudbury, Canada, and Juuka and Pori, Finland. The D-depleted H2 gas and calculated ΔD(CH4−H2) equilibration temperatures of 110–170°C at all three sites are in good agreement with results obtained for abiogenically produced CH4 and H2 in ophiolite sequences in Oman and the Philippines. Serpentinization and the hydration of ultramafic rocks are the proposed mechanisms for CH4 and H2 production in these ophiolite sequences. The widespread occurrence of serpentinized and altered ultramafic rocks at Juuka and at a number of other sites in both Canada and Finland implies that similar mechanisms may be involved in gas production at at least three sites on the shields. The origin of the gases at the remaining shield sites is discussed. Alternative hypothesis include (1) production of the rest of the shield gases by mixing between abiogenic endmembers and bacterially generated hydrocarbon gas such as identified elsewhere on the Canadian and Fennoscandian shields (Sherwood Lollar et al., 1993); and (2) production of a series of isotopically distinct abiogenic CH4 endmembers at each site due to variability in the isotopic composition of available carbon sources. We cannot conclusively distinguish between the alternative scenarios based on existing data. However, the evidence for an abiogenic CH4 endmember at at least three sites emphasizes the need for substantial revision of current theories of methanogenesis. Abiogenic processes of CH4 production may be considerably more widespread than previously anticipated.

Contrasting carbon isotope fractionation during biodegradation of trichloroethylene and toluene: Implications for intrinsic bioremediation

Abstract In experiments involving anaerobic biodegradation of trichloroethylene (TCE), δ 13 C values for residual TCE changed from −30.4‰ to values more enriched than −16‰. All data exhibit a consistent correlation between δ 13 C value of the residual TCE and the extent of biodegradation of TCE, described by a fractionation factor ( α ) of 0.9929. In contrast, during aerobic biodegradation of toluene by two separate mixed consortia, no change in δ 13 C value of the residual toluene was observed within analytical uncertainty (0.5‰). Stable carbon isotopes have the potential to be a useful indicator for identification and monitoring of intrinsic bioremediation of chlorinated hydrocarbons such as TCE. Conversely, for aromatic hydrocarbons such as toluene, more conservative isotopic values may instead be more applicable as a means of source differentiation at sites with a history of multiple spills.

The fate of mantle-derived carbon in a continental sedimentary basin: Integration of CHe relationships and stable isotope signatures

Abstract The isotopic composition and abundances of the rare gases (He, Ne, and Ar) and active gases (CO2, CH4) have been determined in a series of commercial gas reservoirs in the Pannonian and Vienna basins of Hungary and Austria, respectively. In these zones of continental extension, significant components of mantle-derived 4He (up to 39.8%) and 21Ne (up to 58%) are identified. The results of this study indicate a major component of mantle-derived carbon in these systems as well. Ranging in composition from close to 100% CO2, to CH4-dominated reservoirs with trace concentrations (ppm) of CO2, these gas reservoirs provide a unique opportunity to examine the relationship of the conservative rare gases to the active gas components and to examine the sources and sinks for mantle- and crustal-derived carbon phases. With the exception of Kismarja gas field (which shows evidence of addition of 3He-depleted crustal CO2), all gas fields exhibit a trend whereby the most CO2-rich samples approach CO 2 3 He mntl values similar to MORB (2 × 109 to 7 × 109), while CO2-depleted samples extend to CO 2 3 He mntl values as low as 105. The lack of any correlation of CO 2 3 He mntl ratios with R/Ra values indicates the observed trends are not a function of mixing between crustal- and mantle-derived endmembers, but instead reflect progressive loss of the mantle-derived CO2 carrier phase during volatile transport and emplacement in the continental crust. Based on stable isotopic signatures from the Kismarja field, a crustal CO2 endmember with an isotopic composition of −6.8‰ and a mantle CO2 endmember with an isotopic composition no more depleted in 13C than −5.0‰ can be identified, one of the few instances where the isotopic signatures of mantle- and crustal-derived CO2 can be reliably distinguished in a continental setting. Covariation in δ13CCO2 and δ13CCH4 values in the gas fields is used to place constraints on two alternative models whereby the trends in percent CO2 and CO 2 3 He mntl ratios can be accounted for by (1) loss of the mantle-derived CO2 carrier phase; (2) addition of crustal-derived CO2; (3) addition of crustal-derived (thermogenic) CH4; and potentially (4) conversion of the CO2mntl carrier phase to mantle-derived CH4. Based on an estimated total mantle 4He flux for the Pannonian Basin of 4.2 × 108 atoms m−2 s−1, mantle carbon flux estimates for this basin alone range from 3 × 108 g C/yr to 1 × 109 g C/yr, which over the lifetime of the basin is only 4–5 orders of magnitude less than flux estimates based on the total area of the spreading ridges. Clearly the addition of mantle-derived carbon to the crust in areas of continental extension is significant and may have been previously underestimated. We demonstrate here that integration of δ13C data with rare gas isotopic tracers provides an important tool in constraining models of mantle carbon sources and sinks in continental settings.

Evidence for bacterially generated hydrocarbon gas in Canadian shield and fennoscandian shield rocks

Hydrocarbon-rich gases found in crystalline rocks on the Canadian and Fennoscandian shields are isotopically and compositionally similar, suggesting that such gases are a characteristic feature of Precambrian Shield rocks. Gases occure in association with saline groundwaters and brines in pressurized “pockets” formed by sealed fracture systems within the host rocks. When released by drilling activities, gas pressures as high as 5000 kPa have been recorded. Typical gas flow rates for individual boreholes range from 0.25 L/min to 4 L/min. The highest concentrations of CH4 are found in the deepest levels of the boreholes associated with CaNaCl (and NaCaCl) brines. N2 is the second major component of the gases and with CH4 accounts for up to 80 to >90 vol%. Higher hydrocarbon (C2+) concentrations range from <1 to 10 vol.%, with C1/(C2 = C3) ratios from 10−1000. Isotopically the gases show a wide range of values overall (σ13C = −57.5 to −41.1%; σD = −245 to −470‰) but a relatively tight cluster of values within each sampling locality. The Enonkoski Mine methanes are unique with σ13C values between −65.4 and −67.3‰ and σD values between −297 and −347‰. The shield gases are not readily reconcilable with conventional theories of methanogenesis. The range of C1/(C2 + C3) ratios for the shield gases is too low to be consistent with an entirely bacterial origin. In addition, σDCH4 values are in general too depleted in the heavy isotope to be produced by thermogenic methanogenesis or by secondary alteration processes such as bacterial oxidation or migration. However, isotopic and compositional evidence indicates that bacterially derived gas can account for a significant component of the gas at all shield sites. Conventional bacterial gas accounts for 75–94 vol% of the occurrences at Enonkoski Mine in Finland. At each of the other shield sites, bacterial gas can account for up to 30–50 vol% of the total gas accumulation. This study and other recent evidence of active bacterial communities in deep hydrogeological environments emphasize the need for more comprehensive investigation of the role of microorganisms in the deep subsurface.

The origin and age of biogeochemical trends in deep fracture water of the Witwatersrand Basin, South Africa

Water residing within crustal fractures encountered during mining at depths greater than 500 meters in the Witwatersrand basin of South Africa represents a mixture of paleo-meteoric water and 2.0–2.3 Ga hydrothermal fluid. The hydrothermal fluid is highly saline, contains abiogenic CH 4 and hydrocarbon, occasionally N 2 , originally formed at ∼ 250–300°C and during cooling isotopically exchanged O and H with minerals and accrued H 2 , 4 He and other radiogenic gases. The paleo-meteoric water ranges in age from ∼ 10 Ka to > 1.5 Ma, is of low salinity, falls along the global meteoric water line (GMWL) and is CO 2 and atmospheric noble gas-rich. The hydrothermal fluid, which should be completely sterile, has probably been mixing with paleo-meteoric water for at least the past ∼100 Myr, a process which inoculates previously sterile environments at depths > 2.0 to 2.5 km. Free energy flux calculations suggest that sulfate reduction is the dominant electron acceptor microbial process for the high salinity fracture water and that it is 10 7 times that normally required for cell maintenance in lab cultures. Flux calculations also indicate that the potential bioavailable chemical energy increases with salinity, but because the fluence of bioavailable C, N and P also increase with salinity, the environment remains energy-limited. The 4 He concentrations and theoretical calculations indicate that the H 2 that is sustaining the subsurface microbial communities (e.g. H 2 -utilizing SRB and methanogens) is produced by water radiolysis at a rate of ∼1 nM yr −1 . Microbial CH 4 mixes with abiogenic CH 4 to produce the observed isotopic signatures and indicates that the rate of methanogenesis diminishes with depth from ∼ 100 at < 1 kmbls, to < 0.01 nM yr −1 at > 3 kmbls. Microbial Fe(III) reduction is limited due to the elevated pH. The δ13C of dissolved inorganic carbon is consistent with heterotrophy rather than autotrophy dominating the deeper, more saline environments. One potential source of the organic carbon may be microfilms present on the mineral surfaces.

Hydrogeologic Controls on Episodic H2 Release from Precambrian Fractured Rocks—Energy for Deep Subsurface Life on Earth and Mars

Dissolved H(2) concentrations up to the mM range and H(2) levels up to 9-58% by volume in the free gas phase are reported for groundwaters at sites in the Precambrian shields of Canada and Finland. Along with previously reported dissolved H(2) concentrations up to 7.4 mM for groundwaters from the Witwatersrand Basin, South Africa, these findings indicate that deep Precambrian Shield fracture waters contain some of the highest levels of dissolved H(2) ever reported and represent a potentially important energy-rich environment for subsurface microbial life. The delta (2)H isotope signatures of H(2) gas from Canada, Finland, and South Africa are consistent with a range of H(2)-producing water-rock reactions, depending on the geologic setting, which include both serpentinization and radiolysis. In Canada and Finland, several of the sites are in Archean greenstone belts characterized by ultramafic rocks that have under-gone serpentinization and may be ancient analogues for serpentinite-hosted gases recently reported at the Lost City Hydrothermal Field and other hydrothermal seafloor deposits. The hydrogeologically isolated nature of these fracture-controlled groundwater systems provides a mechanism whereby the products of water-rock interaction accumulate over geologic timescales, which produces correlations between high H(2) levels, abiogenic hydrocarbon signatures, and the high salinities and highly altered delta (18)O and delta (2)H values of these groundwaters. A conceptual model is presented that demonstrates how periodic opening of fractures and resultant mixing control the distribution and supply of H(2) and support a microbial community of H(2)-utilizing sulfate reducers and methanogens.

Deep fracture fluids isolated in the crust since the Precambrian era

Fluids trapped as inclusions within minerals can be billions of years old and preserve a record of the fluid chemistry and environment at the time of mineralization. Aqueous fluids that have had a similar residence time at mineral interfaces and in fractures (fracture fluids) have not been previously identified. Expulsion of fracture fluids from basement systems with low connectivity occurs through deformation and fracturing of the brittle crust. The fractal nature of this process must, at some scale, preserve pockets of interconnected fluid from the earliest crustal history. In one such system, 2.8 kilometres below the surface in a South African gold mine, extant chemoautotrophic microbes have been identified in fluids isolated from the photosphere on timescales of tens of millions of years. Deep fracture fluids with similar chemistry have been found in a mine in the Timmins, Ontario, area of the Canadian Precambrian Shield. Here we show that excesses of 124Xe, 126Xe and 128Xe in the Timmins mine fluids can be linked to xenon isotope changes in the ancient atmosphere and used to calculate a minimum mean residence time for this fluid of about 1.5 billion years. Further evidence of an ancient fluid system is found in 129Xe excesses that, owing to the absence of any identifiable mantle input, are probably sourced in sediments and extracted by fluid migration processes operating during or shortly after mineralization at around 2.64 billion years ago. We also provide closed-system radiogenic noble-gas (4He, 21Ne, 40Ar, 136Xe) residence times. Together, the different noble gases show that ancient pockets of water can survive the crustal fracturing process and remain in the crust for billions of years.

Isotopic fractionation during reductive dechlorination of trichloroethene by zero-valent iron: influence of surface treatment

During reductive dechlorination of trichloroethene (TCE) by zero-valent iron, stable carbon isotopic values of residual TCE fractionate significantly and can be described by a Rayleigh model. This study investigated the effect of observed reaction rate, surface oxidation and iron type on isotopic fractionation of TCE during reductive dechlorination. Variation of observed reaction rate did not produce significant differences in isotopic fractionation in degradation experiments. However, a small influence on isotopic fractionation was observed for experiments using acid-cleaned electrolytic iron versus experiments using autoclaved electrolytic iron, acid-cleaned Peerless cast iron or autoclaved Peerless cast iron. A consistent isotopic enrichment factor of epsilon = -16.7/1000 was determined for all experiments using cast iron, and for the experiments with autoclaved electrolytic iron. Column experiments using 100% cast iron and a 28% cast iron/72% aquifer matrix mixture also resulted in an enrichment factor of -16.9/1000. The consistency in enrichment factors between batch and column systems suggests that isotopic trends observed in batch systems may be extrapolated to flowing systems such as field sites. The fact that significant isotopic fractionation was observed in all experiments implies that isotopic analysis can provide a direct qualitative indication of whether or not reductive dechlorination of TCE by Fe0 is occurring. This evidence may be useful in answering questions which arise at field sites, such as determining whether TCE observed down-gradient of an iron wall remediation scheme is the result of incomplete degradation within the wall, or of the dissolved TCE plume by passing the wall.

Microbial precipitation of a strontium calcite phase at a groundwater discharge zone near rock Creek, British Columbia, Canada

Mineralogical, chemical, and microscopic analyses demonstrate an intimate relationship between epilithic cyanobacteria and the precipitation of a strontium calcite phase at a groundwater discharge zone located near Rock Creek, British Columbia, Canada. The groundwater flows out of a serpentinite bedrock outcrop that provides a hard surface for the accretion of a coherent calcareous crust. The mean pH of water samples collected ∼2.0 m above the base of the outcrop was 8.5, whereas a value of 8.8 was recorded for samples taken near the outcrop base. This increase in pH can be attributed to the growth of cyanobacteria that carry out a HCO3 ‐/OH‐ exchange process during photosynthesis. Calcium was present in the water samples at levels of 32–36 ppm, whereas strontium occurred at lower concentrations (5.8–6.6 ppm). In each case, the lowest calcium and strontium values occurred in samples taken near the base of the outcrop, as expected for carbonate mineral precipitation. The crust itself is a porous thrombolit...