Maximum CH4 Concentration Research Articles

Real Time Measurement of Concentration and δ13C-CH4 in Water

A new, high temporal and spatial resolution, in-situ method for the fast and precise determination of methane (CH4) concentration and its δ13C isotope value (δ13C-CH4) in water was developed. Methane is extracted from water by the application of a vacuum to a membrane gas/liquid exchange module and analysed by a portable cavity ring-down spectroscopy (CRDS) analyser (Picarro® G2201-i). The system was calibrated for concentration and δ13C of CH4 using synthetic water standards. Reference samples were analysed with gas chromatography (GC) and mass spectroscopy (GC-C-IRMS) via the headspace method. All results show good correlations for CH4 concentration (R2 = 0.994) and a measuring accuracy of 0.002ppm (CH4 concentration) and 0.6‰ (δ13C-CH4). Calculated response times τ were averaged, resulting in 13.5 s. This allows much faster analyses compared to previously published methods. This method was successfully tested in a field campaign at Lake Stechlin (Germany), which has a pronounced CH4 peak at the thermocline in summer. Hypolimnic CH4 concentrations up to 0.6μmol/l indicate microbial methanogenesis in the well-oxygenated upper 10 m of the water column. Simultaneous measurements by the CRDS-membrane system (M-CRDS), a membrane combined with off-axis integrated cavity output spectrometer system (M-ICOS) and gas chromatography (GC) show similar results. 24-hour measurements were performed at the depth of maximum CH4 concentration and indicate short-term variations of 0.2μmol/l (CH4 concentration) and 7‰ (δ13C-CH4) within 24hours. The M-CRDS provides a high temporal and spatial data resolution for accurate detection, mapping and quantification of CH4 distribution in aquatic systems.

Effect of lithostatic stress on methane sorption by coal: Theory vs. experiment and implications for predicting in-situ coalbed methane content

Recent research has demonstrated that confining stresses applied to the solid framework of coal can reduce its gas sorption capacity by several percent to perhaps several tens of percent. To evaluate the magnitude of this effect more rigorously in relation to predicting in-situ coalbed methane (CBM) content, a better understanding of the effects of stress on methane sorption by coal is needed. In this paper, a previous thermodynamic model for the effects of stress on CO2 sorption by coal is revised and applied to CH4. The revised model predicts that in-situ CBM content is indeed determined not only by the geological factors generally considered, such as coal rank, coal composition, moisture content and temperature, but also by lithostatic or confining stress, which is usually ignored. This prediction is tested by means of experiments performed on a composite cylindrical sample of Brzeszcze 364 high volatile bituminous coal subjected to 10MPa methane pressure at a temperature of 40°C, varying the hydrostatic stress or confining pressure in the range 11-43MPa. In these experiments, we determined if CH4 was desorbed as confining pressure was increased by subtracting the poroelastic expulsion of CH4 from the total CH4 expelled, assuming the former to equal the gas volume expelled in control experiments performed using Helium. The experimental results show that the equilibrium sorption capacity for CH4 at 10MPa gas pressure and 11MPa confining pressure (1MPa Terzaghi effective stress) was 0.808mol/kgcoal. This was reduced by at least ~6% by increasing the confining pressure to 43MPa (33MPa effective stress), confirming the validity of our model. We apply our model to predict in-situ CBM concentration as a function of coal seam depth for dry, high volatile bituminous coal, assuming a geothermal gradient of 32°C/km. The results indicate a maximum CH4 concentration of ~0.76mol/kgcoal at a burial depth of ~900m, which is ~3% lower than conventional predictions. This reduction is minor but helps to explain why gas saturation is generally lower than expected from conventional sorption measurements on unconfined coal powders. More importantly, our results confirm that there is an intimate coupling between in-situ stress, strain and sorption in coal that needs to be considered in developing gas-enhanced CBM strategies.

Influence of hydrate structure on continuous separation of coal bed methane gas: A thermodynamic simulation study

Influence of hydrate crystallographic structures on continuous CH4 + N2 gas separation is studied using thermodynamic simulations. A continuous separation process with discharge of hydrate crystals only (not as hydrate slurry; without liquid phases) is modelled. 35 mol% CH4/N2 and 50 mol% CH4/N2 gas separation simulations are performed for simple sI system, and isobutane sII, methylcyclohexane sH and neohexane sH promoted systems at pressures 1.0 MPa, 4.0 MPa and 7.0 MPa depending on hydrate structures. At the steady state, under 7.0 MPa, CH4 concentration in hydrate phase (xj=CH4,hyd) and percentage of CH4 recovery (S. Fr.) were in the increasing order of isobutane sII < neohexane sH ≈ methylcyclohexane sH < simple sI systems. In the sII and sH system studied, xj=CH4,hyd and S. Fr. at the steady state were lower during gas separation at higher pressures. The maximum CH4 concentration that can be recovered in hydrate for continuous 35 mol% CH4/N2 and 50 mol% CH4/N2 gas separations are calculated to be approximately 64 mol% CH4/N2 and 77 mol% CH4/N2 respectively.

Surface geochemical anomalies in the vicinity of the Wańkowa Oil Field (SE Polish Carpathians)

The oil accumulations discovered in the Skole Synclinorium (eastern part of Polish Outer Carpathians) are located in zones of pinch-out of the Kliva Sandstone. An example is the Wankowa Oil Field, which contains the largest oil reserves in the region. As the seismic identification of this type of hydrocarbon trap is ambiguous, a surface geochemical survey was carried out in the vicinity of the Wankowa Oil Field along an experimental line perpendicular to the fold axes. A traverse across the zones with anomalous seismic records indicated the presence of undiscovered lithological traps. During the surface geochemical survey, 94 samples of soil gas were collected from a depth of 1.2 m and then investigated chromatographically. The spacing of sampling sites was 100 m, which was reduced to 50 m in the Wankowa Oil Field area. The maximum concentrations of CH4 and total alkanes C 2 -C 5 detected in samples were: 4250.0 ppm (0.425 vol. %) and 0.43 ppm, respectively. The first of these was detected at measurement point no. 86, located over the Wankowa Field and the second at point no. 59, about 1,300 m south of the Wankowa Field. The chemical analyses also detected maximum values of total alkenes C 2 -C 4 , H 2 and CO 2 : 0.147 ppm, 0.042 vol. % and 4.4 vol. %, respectively. The results of the surface geochemical survey were integrated with observations on subsurface geological structures, which were interpreted on the basis of seismic data. This procedure permitted the documentation of anomalous concentrations of alkanes in the near-surface zone and contributed to an understanding of the tectonics of the hydrocarbon reservoirs in depth. The pattern of geochemical anomalies here is controlled by anticlines made up of Early Oligocene–Paleocene sediments and by overthrusts that displace these structures. The hydrocarbons migrated from condensate and/or oil accumulations located at various depths. The character of the anomalous zone discovered over the Wankowa Oil Field is related to the effective sealing of hydrocarbon traps and/or the relatively low pressure caused by the production of oil for 130 years. However, this anomaly also may be the result of hydrocarbon migration from deeper, as yet undiscovered gas or gas-condensate accumulations, hosted in older reservoirs forming the hinge of the Ropienka-Łodyna Mine Anticline. The most pronounced anomalies were detected over the Wankowa Village - Bandrow and Grabownica-Zaluz anticlines. The active hydrocarbon dispersion in this zone may have resulted from the presence of overthrusts displacing the structures. Moreover, these anomalies may indicate the presence of shallow, maybe poorly sealed hydrocarbon accumulations.

Hydrothermal plumes in the Gulf of Aden, as characterized by light transmission, Mn, Fe, CH4 and δ13C–CH4 anomalies

We conducted water column surveys to search for hydrothermal plumes over the spreading axes in the Gulf of Aden between 45°35′E and 52°42′E. We measured light transmission and chemical tracers Mn, Fe, CH4 and δ13C of CH4 in seawater taken using a CTD-Carrousel multi-sampling system at 12 locations including a control station in the Arabian Sea. We recognized three types of hydrothermal plumes at depths of ~650 to ~900m (shallow plumes), ~1000 to ~1200m (intermediate plumes), and >1500m (deep plumes). The shallow plumes were apparently originated from newly discovered twin seamounts (12°03–06′N and 45°35–41′E) at the westernmost survey area, where two-dimensional distributions of light transmission and Mn were mapped by tow-yo observations of the CTD-sampling system with an in situ auto-analyzer GAMOS. The maximum concentrations of Mn, Fe, and CH4 of 46nM, 251nM, and 15nM, respectively, were observed for collected seawater within the shallow plumes. The intermediate plumes were characterized by anomalies of light transmission, Mn, Fe, and δ13C of CH4, but by little CH4 anomalies, suggesting that CH4 had been consumed down to the background level during the aging of the plumes. Anomalies of δ3He already reported by the World Ocean Circulation Experiment (WOCE) program exhibited a hydrothermal plume-like peak at ~2000m depth in the Gulf of Aden, which seems to coincide with the deep plumes observed in this study. The endmember δ13C–CH4 values for the shallow and the deep plumes were estimated to be in a range between −10‰ and −15‰, demonstrating that the sources of CH4 are not biogenic but magmatic as similarly observed at sediment-starved mid-oceanic ridges.

Surface geochemical exploration for hydrocarbons in the area of prospective structures of the Lublin Trough (Eastern Poland)

The search for crude oil and natural gas deposits is dominated by expensive seismic methods. However, less expensive and faster surface methods are available, which may improve the effectiveness of petroleum exploration. One of these methods is the surface geochemical survey. The objective of this method is to detect, record and analyse the trace amounts of hydrocarbons in soil gas samples. Our paper presents the results of such research carried out over the prospective structures of the Lublin Trough in the Eastern Poland. Three hundred fifty five soil gas samples were collected from 1.2 m depth below the surface, which were analysed by gas chromatographic analysis for their molecular composition. Sampling sites were arranged along 5 sampling lines with 200 m spacing. Maximum concentrations of CH4, total alkanes C2–C4, total alkenes C2–C4, H2, CO2 and O2 detected in our samples were: 0.16 vol.%, 0.95 ppm, 6.79 ppm, 0.37 vol.%, 5.01 vol. % and 21.6 vol.% respectively. The results demonstrate that alkanes recorded in the soil gas which are heavier than methane, originated from the deeper hydrocarbon accumulations. These components migrated to the near-surface zone due to their active microseepage along the tectonic discontinuities and cracks in the overburden. Weak correlation of H2 and CO2 with alkanes were found, which precludes the application of both components as additional prospective indicators in the study area. Diverse activity of partial oxidation of alkanes to alkenes, perhaps due to different rate of microseepage of hydrocarbons, has been determined using the C2H6/C2H4 (C2/C2=) ratio. The presence of numerous tectonic dislocations intensifies the microseepage and increases the amplitudes of geochemical anomalies recorded at the surface. Normalization of concentrations of methane, total alkanes C2–C4 and total alkenes C2–C4 with their of background values, which were calculated for separate subsets of C2/C2= ratio values, significantly reducing the ranges and the amplitudes of hydrocarbons anomalies. It allowed for modification of the anomalous pattern due to diverse activity of partial oxidation of alkanes to alkenes. Statistical analysis of C2H6/C3H8 (C2/C3) ratio values showed that in the study area accumulations of gas condensate and dry gas dominate with the strongest surface geochemical signals.The surface geochemical results were integrated with the structural map of the top of Devonian formations as well as we when displayed as trend integrated with geological cross-sections. The surface alkane anomalies generally occur along the major, NW–SE-trending chain of structural uplifts located between Abramów, Glinnik and Nasutów. This zone is most prospective and indicates the highest petroleum potential. Changes in normalized concentrations of total alkanes C2–C4 allowed the identification of 9 anomalous zones, indicating the presence of deep hydrocarbon sources. The no. 2 anomaly, which was localized at the intersection of the B–B′ and the E–E′ sampling lines, seems to be the most prospective. The accumulations of dry gas or gas condensate likely occur in the anticline formed by Devonian and/or Carboniferous strata. This zone should be opened for confirmation by drilling. The geochemical lead should now be integrated with all available geological/seismic data for prospect development. Our study demonstrates that the results of surface geochemical surveys combined with the results of geological studies and geophysical surveys provide a valuable source of comprehensive information about the explored area.

Analysis of process related factors to increase volumetric productivity and quality of biomethane with Methanothermobacter marburgensis

The biological conversion of H2 and CO2 into CH4, using methanogenic archaea is an interesting technology for CO2 conversion, energy storage and biogas upgrading. For an industrial application of this process however, the optimization of the volumetric productivity and the product quality is an important issue. Since the reactants in this fermentation process are, unlike in most microbial fermentations, solely gasses, the gas liquid mass transfer is supposed to play an important role on the way to a higher volumetric productivity.This work aimed at investigating the effects of the gassing rate, the reactor pressure, as well as reactor design issues on the performance of Methanothermobactermarburgensis by using continuous cultures. Our results show that biological methanogenesis with M. marburgensis is gas limited. Maximum physiological capacity is not reached yet. The gassing rate influenced mainly the volumetric methane production rate (MER), the reactor pressure influenced mainly the offgas quality. Based on this information, we demonstrated how a combination of increased gas flow rate and increased reactor pressure can be used to reach high volumetric productivity at high offgas quality. Maximum MER was 950mmolL−1h−1 at a CH4 concentration of 60Vol.-%, maximum CH4 concentration reached was 85Vol.-% at a MER of 255mmolL−1h−1.The reactor design currently limits further increase in gas flow rate and reactor pressure. Therefore Interdisciplinary bridges from bioprocessing to chemical reactor design must be followed in the future to boot this promising bioprocess to gain biomethane via CO2 fixation.

Observational Study of Surface O3, NOx, CH4 and Total NMHCs at Kannur, India

This paper presents the results of measurements of the surface ozone (O3), oxides of nitrogen (NOx), methane (CH4) and total non-methane hydrocarbons (TNMHCs) in a rural coastal location at Kannur (11.9°N, 75.4°E, 5 m asl), India from November 2009 to December 2011. The diurnal cycle for surface O3 had a peak in the afternoon and declined during nighttime. The maximum and minimum mixing ratio of surface O3 was observed in winter and monsoon seasons respectively. NOx concentration was high during mid-night to early morning and low during noontime. The diurnal variations of mixing ratios for NOx and O3 were anti-correlated. Monthly average maximum (2.26 ± 0.44 ppmv) and minimum (0.43 ± 0.19 ppmv) CH4 concentrations were observed in December and August respectively. The diurnal variations of CH4 were similar to that of NOx. A CH4 buildup was observed during early morning hours throughout the observational period. On an annual basis, the maximum, minimum and average total NMHCs were (25.45 ± 6.58 ppbv), (13.84 ± 4.31 ppbv), and (19.23 ± 5.56 ppbv) respectively at the observational site. Analysis of O3, NO, NO2, CH4 and NMHC have been carried out and the correlation between O3 and its precursors is discussed detailed. Further, the diurnal variation of O3 over a free tropospheric region at Ooty, a hill station lying in the Western Ghats region of south India on clear sky days in February 2011 is also reported for a comparison.

Influence of ozone precursors and PM10 on the variation of surface O3 over Kannur, India

This paper presents the results of the continuous measurements of surface ozone (O3), oxides of nitrogen (NOx) and methane (CH4) in a rural coastal location at Kannur (11.9°N, 75.4°E, 5m AMSL), India from November 2009 to December 2012. The maximum and minimum mixing ratios of surface O3 were observed in winter and monsoon seasons respectively. NOx concentration was high during mid-night to early morning and low during noontime. The diurnal variations of mixing ratios for NOx and O3 were anti-correlated. Monthly average maximum and minimum CH4 concentrations were observed in December and August respectively. The diurnal variations of CH4 were quite similar to that of NOx. A gradual build up of CH4 was observed during early morning hours of all days throughout the observational period. Further, the analyses of O3, NO, NO2 and NOx have been carried out and the correlation between O3 and its precursors is discussed in detail. The chemical analysis of air samples conducted to identify the main inorganic species present in air and their significance is presented. An attempt has been made to investigate the impact of concentration of Particulate Matter (PM10) present in the ambient air on the production of surface O3 at this location to explore the influence of PM10 on surface O3.

Estimating the global flux of methane into the atmosphere and its seasonal variations

A new method of estimating the global fluxes of methane into the Earth’s atmosphere is proposed. This method allows one to take into account the whole spectrum of methane sources irrespectively of their location and calculate seasonal variations in methane fluxes. The results of numerical calculations of the amount of methane emitted into the atmosphere on the basis of this method are supported by data obtained from field measurements. In the Northern Hemisphere (NH), during fall, maximum concentrations of CH4 are due to methane sources in the arctic region which have not been taken into account before. For the condition of balance between the emission and sink of methane to be fulfilled, the total capacity of its sources amounts to no less than ∼530 Tg/year for the NH and ∼470 Tg/year for the Southern Hemisphere (SH). The results of our calculations and an analysis of the behavior of the concentration and mass of methane in the Earth’s hemispheres show that the global flux of methane from the surfaces of the lithosphere and ocean into the atmosphere may amount to more than ∼1000 Tg/year.

Seepage of methane at Jaco Scar, a slide caused by seamount subduction offshore Costa Rica

Methane (CH4) concentrations and CH4 stable carbon isotopic composition (d13CCH4 ) were investigated in the water column within Jaco Scar. It is one of several scars formed by massive slides resulting from the subduction of seamounts offshore Costa Rica, a process that can open up structural and stratigraphical pathways for migrating CH4. The release of large amounts of CH4 into the adjacent water column was discovered at the outcropping lowermost sedimentary sequence of the hanging wall in the northwest corner of Jaco Scar, where concentrations reached up to 1,500 nmol L-1. There CH4-rich fluids seeping from the sedimentary sequence stimulate both growth and activity of a dense chemosynthetic community. Additional point sources supplying CH4 at lower concentrations were identified in density layers above and below the main plume from light carbon isotope ratios. The injected CH4 is most likely a mixture of microbial and thermogenic CH4 as suggested by d13CCH4 values between -50 and -62 % Vienna Pee Dee Belemnite. This CH4 spreads along isopycnal surfaces throughout the whole area of the scar, and the concentrations decrease due to mixing with ocean water and microbial oxidation. The supply of CH4 appears to be persistent as repeatedly high CH4 concentrations were found within the scar over 6 years. The maximum CH4 concentration and average excess CH4 concentration at Jaco Scar indicate that CH4 seepage from scars might be as significant as seepage from other tectonic structures in the marine realm. Hence, taking into account the global abundance of scars, such structures might constitute a substantial, hitherto unconsidered contribution to natural CH4 sources at the seafloor.

Dry matter losses in combination with gaseous emissions during the storage of forest residues

Past published research on the storage of fresh woody biomass has rarely presented observations of gaseous emissions in combination with the related dry matter losses. The objectives of this study are to determine dry matter losses and gaseous emissions from stored logging residues. Lab-scale vessels were set up to study the concentration of off gases at 15°C and 35°C. Results showed that the maximum concentrations of CO2, CO and CH4 were 13.8%, 0.16%, and 0.15%, respectively over a period of 35days. The oxygen level decreased to 0% at the end of storage. Volatile organic compounds (VOCs) were qualitatively detected by GC/MS technique. The major chemical compounds identified were alcohols, terpenes, aldehydes, acids, acetone, benzene, ethers and esters. The total VOC concentration reached 85ppm at 35°C storage temperature at the end of the storage period. Dry matter loss ranged from 0.78% to 2.0% increasing with storage temperature.

Dissolved methane distribution in the South Pacific and the Southern Ocean in austral summer

Oceanic methane (CH4) was examined in the South Pacific and the Southern Ocean from December 2001 to February 2002. From the oligotrophic South Pacific (10°S) to the Subtropical Front (STF), we observed the maximum concentrations of CH4 in the subsurface layer which ranged from 2.7 to 3.9 nmol kg−1. South of the STF, higher levels of CH4 were often detected in both the surface and the subsurface layers. Elevated surface CH4 concentrations (3.4–6.1 nmol kg−1) south of the STF tended to be higher than those north of the STF. Higher CH4 were often concomitant with an increase in chlorophyll a levels in the Seasonal Ice Zone (SIZ). Considering that phytoplankton does not generate methane directly, the high CH4 production probably resulted from the grazing processes of Antarctic krill and/or zooplankton fed on phytoplankton and the subsequent microbial methanogenesis in addition to the aerobic CH4 production in the water column. Present results showed a good relation between surface CH4 data (&lt;50 m) and σθ between 10°S and the Polar Front (PF), which allow us to provide interpolation/extrapolation schemes for CH4 concentration and sea‐air CH4 flux in the wide area ([CH4] = 0.15 σθ − 0.98 (RMS = 0.21 nmol kg−1, r = 0.68, n = 49, p &lt; 0.001). The sea‐air fluxes estimated during our survey were from 2.4 to 4.9 mol CH4 km−2 d−1 between 10°S and the PF (54°S), and from 0.8 to 2.3 mol CH4 km−2 d−1 south of the PF, where the sea‐air CH4 flux was largely affected by the wind speed.

Global methane flux into the atmosphere and its seasonal variations

We have analyzed the regularities in the spatiotemporal behavior of the methane concentration and mass, recorded in different regions of the globe. In the southern hemisphere (SH), the methane concentration does not depend on latitude. In the northern hemisphere (NH), the methane concentration increases towards the Arctic zone. The maximum CH4 concentration in the fall is due to the action of the additional methane sources in the NH Arctic region. The methane flux calculations showed that in the SH the methane flux into the Earth’s atmosphere is barely season dependent. In the NH during the fall season, the methane flux considerably increases, and the difference between the maximum flux in August-September and the flux in December-January reaches three. The additional, still disregarded sources, sustaining high methane emission in the spring season at high SH latitudes are estimated to be as strong as 100–120 Tg per year. To equilibrate the methane supply to the atmosphere and methane sink in SH, additional, yet unidentified sources of over 300 Tg/yr are required. The global methane flux into the Earth’s atmosphere should be no less than 1050–1100 Tg yearly.

Indications of a link between seismotectonics and CH4 release from seeps off Costa Rica

Measurements of CH4 concentrations in the bottom water during two discrete sampling periods in subsequent years above different cold seeps at the Pacific margin off Costa Rica indicate large‐scale variations of CH4 release. CH4 is emitted from mud extrusions and a slide scar at 1000–2300 m water depth. Maximum CH4 concentrations were found to be lower above all investigated sites in autumn 2003 than in autumn 2002 although seep sites are up to 300 km apart. Tidal and current changes were observed but found to apply only to individual seep sites. Increased seismic activity connected to the moment magnitude (MW) 6.4 earthquake offshore Costa Rica in June 2002 could have had an impact on all seep sites and thereby caused an increase in CH4 emission. This is supported by the largest variations of CH4 concentration found above mud extrusions located above faults likely more strongly affected by tectonic movements. Even though our data indicate a relation between seismicity and CH4 seepage, the relation is not proven, and future work is needed to comprehensively test this hypothesis.

Methane seepage and its relation to slumping and gas hydrate at the Hikurangi margin, New Zealand

Dissolved methane and high resolution bathymetry surveys were conducted over the Rock Garden region of Ritchie Ridge, along the Hikurangi margin, eastern New Zealand. Multibeam bathymetry reveals two prominent, northeast trending ridges, parallel to subduction along the margin, that are steep sided and extensively slumped. Elevated concentrations of methane (up to 10 nM, 10× background) within the water column are associated with a slump structure at the southern end of Eastern Rock Garden. The anomalous methane concentrations were detected by a methane sensor (METS) attached to a conductivity‐temperature‐depth‐optical backscatter device (CTDO) and are associated with elevated light scattering and flare‐shaped backscatter signals revealed by the ship's echo sounder. Increased particulate matter in the water column, possibly related to the seepage and/or higher rates of erosion near slump structures, is considered to be the cause of the increased light scattering, rather than bubbles in the water column. Methane concentrations calculated from the METS are in good agreement with concentrations measured by gas chromatography in water samples collected at the same time. However, there is a c. 20 min (c. 900 m) delay in the METS signal reaching maximum CH4 concentrations. The maximum methane concentration occurs near the plateau of Eastern Rock Garden close to the edge of a slump, at 610 m below sea level (mbsl). This is close to the depth (c. 630 mbsl) where a bottom simulating reflector (BSR) pinches out at the seafloor. Fluctuating water temperatures observed in previous studies indicate that the stability zone for pure methane hydrate in the ocean varies between 630 and 710 mbsl. However, based on calculations of the geothermal gradients from BSRs, we suggest gas hydrate in the study area to be more stable than hydrate from pure methane in sea water, moving the phase boundary in the ocean upward. Small fractions of additional higher order hydrocarbon gases are the most likely cause for increased hydrate stability. Relatively high methane concentrations have been measured down to c. 1000 mbsl, most likely in response to sediment slumping caused by gas hydrate destabilisation of the sediments and/or marking seepage through the gas hydrate zone.

Biogeochemistry of methane in the permanently ice-covered Lake Untersee, central Dronning Maud Land, East Antarctica

We found unprecedentedly high abundances of microbially produced CH4 in the anoxic deep waters of Lake Untersee, an oligotrophic, perennially ice-covered Antarctic freshwater lake. The maximum CH4 concentration (approaching 21.8 ± 1.4 mmol L−1) is one of the highest observed so far in a natural aquatic ecosystem. Although surficial lake sediments are the predominant source of CH4 in Lake Untersee, methanogenesis occurs also within the anoxic waters. Radiocarbon labeling experiments show that H2/CO2 reduction is the predominant methanogenic pathway (90–100%) both in the sediments and the water column, whereas acetate is only a minor CH4 precursor. This result is consistent with the stable carbon isotope fractionation between coexisting CH4 and CO2. In the water column, CH4 is partly consumed by both aerobic and anaerobic microbial oxidation as evidenced by CH4 concentration patterns, stable isotope analyses (13C, 2H), and 14C-CH4 assays. Dissimilatory sulfate reduction also occurs and peaks at 84 m water depth (1.83 µmol SO4 L−1 d−1). Intense methanogenesis in surficial lake sediments, diffusion of CH4 from sediments to the water column, additional CH4 production in the water column, gross CH4 production higher than CH4 consumption, and lack of mixing because of the permanent ice cover cause the exceptionally high CH4 concentration in the lake. Our studies demonstrate that H2/CO2 reduction may sometimes be the major pathway of methanogenesis in low-sulfate freshwater environments even at low temperatures. This pathway is obviously more important in Antarctic lakes than hitherto assumed.

Atmospheric Methane and Nitrous Oxide of the Late Pleistocene from Antarctic Ice Cores

The European Project for Ice Coring in Antarctica Dome C ice core enables us to extend existing records of atmospheric methane (CH4) and nitrous oxide (N2O) back to 650,000 years before the present. A combined record of CH4 measured along the Dome C and the Vostok ice cores demonstrates, within the resolution of our measurements, that preindustrial concentrations over Antarctica have not exceeded 773 +/- 15 ppbv (parts per billion by volume) during the past 650,000 years. Before 420,000 years ago, when interglacials were cooler, maximum CH4 concentrations were only about 600 ppbv, similar to lower Holocene values. In contrast, the N2O record shows maximum concentrations of 278 +/- 7 ppbv, slightly higher than early Holocene values.

A Methodology to Estimate Odors around Landfill Sites: The Use of Methane as an Odor Index and Its Utility in Landfill Siting

The estimation of odor production and dispersion from landfill sites is a very complicated task because of the different chemical species that exist in biogas. To site a new landfill, it is necessary to know the distance that odors can travel around the landfill under atmospheric conditions that increase the concentration of pollutants. Although CH4 is an odorless gas, it can be used as an index to determine the dispersion of low-reactivity odorous species around a landfill site. Methane production rates, estimated by biogas production models, were used by an air dispersion model to determine the spatial distribution of CH4 around landfill sites. By utilizing dispersion models under extreme atmospheric conditions, a maximum CH4 concentration around the landfills was determined. Based on the ratio between CH4 and odorous chemical species, the spatial distribution of the concentration of an odorous species was determined for those species with low reactivity in the atmosphere. For odorous species with high reactivity in the atmosphere, a dispersion-reaction model must be used. In this way, the acceptable distance between new landfills and residential areas can be determined. The proposed methodology could be used as a design tool for those who are interested in landfill siting.

Microbial processes of carbon and sulfur cycles in lake Mogil'noe

In the beginning of summer 1999, complex microbiological and biogeochemical investigations of meromictic Lake Mogil'noe (Kil'din Island, Barents Sea) were carried out. The analysis of the results shows clearly pronounced vertical zonality of the microbial processes occurring in the water column of the lake. To a depth of 8 m, the total number and activity of microorganisms was limited by the relatively low content of organic matter (OM). In the upper part of the hydrogen-sulfide zone of the lake (beginning at a depth of 8.25 m), the content of particulate OM and the microbial number sharply increased. In this zone, the daily production of OM during anaerobic photosynthesis at the expense of massive development of colored sulfur bacteria reached 620 mg C/m2, which was twofold greater than the daily production of phytoplankton photosynthesis and led to a considerable change in the isotopic composition (delta 13 C) of the particulate OM. In the same intermediate layer, the highest rates of sulfate reduction were recorded, and fractionation of stable sulfur isotopes occurred. Below 10 m was the third hydrochemical zone, characterized by maximum concentrations of H2S and CH4 and by a relatively high rate of autotrophic methanogenesis. The comparison of the results obtained with the results of investigations of previous years, performed in the end of summer, shows a decrease in the intensity of all microbial processes inspected. An exception was anoxygenic photosynthesis, which can utilize not only the de novo formed H2S but also the H2S accumulated in the lake during the winter period.