Gas seepage from Tokamachi mud volcanoes, onshore Niigata Basin (Japan): Origin, post-genetic alterations and CH4-CO2 fluxes

Methane and CO₂ emissions from the two most active mud volcanoes in central Japan, Murono and Kamou (Tokamachi City, Niigata Basin), were measured in from both craters or vents (macro-seepage) and invisible exhalation from the soil (mini- and microseepage). Molecular and isotopic compositions of the released gases were also determined. Gas is thermogenic (δ¹³C_CH4 from −32.9‰ to −36.2‰), likely associated with oil, and enrichments of ¹³C in CO₂ (δ¹³C_CO2 up to +28.3‰) and propane (δ¹³C_C3H8 up to −8.6‰) suggest subsurface petroleum biodegradation. Gas source and post-genetic alteration processes did not change from 2004 to 2010. Methane flux ranged within the orders of magnitude of 10¹-10⁴ g m⁻² d⁻¹ in macro-seeps, and up to 446 g m⁻² d⁻¹ from diffuse seepage. Positive CH₄ fluxes from dry soil were widespread throughout the investigated areas. Total CH₄ emission from Murono and Kamou were estimated to be at least 20 and 3.7 ton a⁻¹, respectively, of which more than half was from invisible seepage surrounding the mud volcano vents. At the macro-seeps, CO₂ fluxes were directly proportional to CH₄ fluxes, and the volumetric ratios between CH₄ flux and CO₂ flux were similar to the compositional CH₄/CO₂ volume ratio. Macro-seep flux data, in addition to those of other 13 mud volcanoes, supported the hypothesis that molecular fractionation (increase of the “Bernard ratio” C₁/(C₂ + C₃)) is inversely proportional to gas migration fluxes. The CH₄ “emission factor” (total measured output divided by investigated seepage area) was similar to that derived in other mud volcanoes of the same size and activity. The updated global “emission-factor” data-set, now including 27 mud volcanoes from different countries, suggests that previous estimates of global CH₄ emission from mud volcanoes may be significantly underestimated.     

A new estimate of global methane flux from onshore and shallow submarine mud volcanoes to the atmosphere

A new estimate of global methane emission into the atmosphere from mud volcanoes (MVs) on land and shallow seafloor is presented. The estimate, considered a lower limit, is based on 1) new direct measurements of flux, including both venting of methane and diffuse microseepage around craters and vents, and 2) a classification of MV sizes in terms of area (km²) based on a compilation of data from 120 MVs. The methane flux to the atmosphere is conservatively estimated between 6 and 9 Mt y^–1. This emission from MVs is 3–6% of the natural methane sources and is comparable with ocean and hydrate sources, officially considered in the atmospheric methane budget. The total geologic source, including MVs, seepage from seafloor, microseepage in hydrocarbon-prone areas and geothermal sources, would amount to 35–45 Mt y^–1. The authors believe it is time to add this parameter in the Intergovernmental Panel on Climate Change official tables of atmospheric methane sources.GEM     

Composition and exhalation flux of gases from mud volcanoes in Taiwan

Many mud volcanoes are distributed along the tectonic sutures in southern Taiwan and can be divided into five zones based on their relative positions in different tectonic domains. Most active mud volcanoes are exhaling methane-dominated gases. Nevertheless, some gases show unusual carbon dioxide-dominated and/or nitrogen-excess compositions. This implies that there are multiple sources for the gas compositions of mud volcanoes in Taiwan.For better understanding the total amount of exhalation gases and its flux, the gas flow and compositions were continuously measured in the interval of two minutes at Chung-lun (CL) bubbling mud pool for a few months. The major compositions of gases exhaling from this site were 75~90% of CO₂ and 5~12% of CH₄. The amount of gases exhaling from the mud pool can be estimated to be about 1.4 ton/year for CH₄ and 28 ton/year for CO₂, respectively. The preliminary results of exhaling gas flux from the major vents of representative active mud volcanoes, yielded an estimated total CH₄ output of the mud volcanoes in Taiwan of ca. 29 ton/year during quiescent period.     

Gases in Taiwan mud volcanoes: Chemical composition,methane carbon isotopes,and gas fluxes

Mud volcanoes are important pathways for CH₄ emission from deep buried sediments; however, the importance of gas fluxes have hitherto been neglected in atmospheric source budget considerations. In this study, gas fluxes have been monitored to examine the stability of their chemical compositions and fluxes spatially, and stable C isotopic ratios of CH₄ were determined, for several mud volcanoes on land in Taiwan. The major gas components are CH₄ (>90%), “air” (i.e. N₂ + O₂ + Ar, 1–5%) and CO₂ (1–5%) and these associated gas fluxes varied slightly at different mud volcanoes in southwestern Taiwan. The Hsiao-kun-shui (HKS) mud volcano emits the highest CH₄ concentration (CH₄ > 97%). On the other hand, the Chung-lun mud volcano (CL) shows CO₂ up to 85%, and much lower CH₄ content (<37%). High CH₄ content (>90%) with low CO₂ (<0.2%) are detected in the mud volcano gases collected in eastern Taiwan. It is suggestive that these gases are mostly of thermogenic origin based on C₁ (methane)/C₂ (ethane) + C₃ (propane) and δ¹³C_CH4 results, with the exception of mud volcanoes situated along the Gu-ting-keng (GTK) anticline axis showing unique biogenic characteristics. Only small CH₄ concentration variations, <2%, were detected in four on-site short term field-monitoring experiments, at Yue-shi-jie A, B, Kun-shui-ping and Lo-shan A. Preliminary estimation of CH₄ emission fluxes for mud volcanoes on land in Taiwan fall in a range between 980 and 2010 tons annually. If soil diffusion were taken into account, the total amount of mud volcano CH₄ could contribute up to 10% of total natural CH₄ emissions in Taiwan.     

Eddy covariance based methane flux in Sundarbans mangroves,India

We report the initial results of the methane flux measured using eddy covariance method during summer months from the world’s largest mangrove ecosystem, Sundarbans of India. Mangrove ecosystems are known sources for methane (CH₄) having very high global warming potential. In order to quantify the methane flux in mangroves, an eddy covariance flux tower was recently erected in the largest unpolluted and undisturbed mangrove ecosystem in Sundarbans (India). The tower is equipped with eddy covariance flux tower instruments to continuously measure methane fluxes besides the mass and energy fluxes. This paper presents the preliminary results of methane flux variations during summer months (i.e., April and May 2012) in Sundarbans mangrove ecosystem. The mean concentrations of CH₄ emission over the study period was 1682 ± 956 ppb. The measured CH₄ fluxes computed from eddy covariance technique showed that the study area acts as a net source for CH₄ with daily mean flux of 150.22 ± 248.87 mg m^?2 day^?1. The methane emission as well as its flux showed very high variability diurnally. Though the environmental conditions controlling methane emission is not yet fully understood, an attempt has been made in the present study to analyse the relationships of methane efflux with tidal activity. This present study is part of Indian Space Research Organisation–Geosphere Biosphere Program (ISRO–GBP) initiative under ‘National Carbon Project’.     

Mud volcanic natural phenomena in the South Caspian Basin: geology,fluid dynamics and environmental impact

The South Caspian sedimentary basin is a unique area with thick Mesozoic-Cenozoic sediments (up to 30–32 km) characterized by an extremely high fluid generation potential. The large amount of active mud volcanoes and the volumes of their gas emissions prove the vast scale of fluid generation. Onshore and offshore mud volcanoes annually erupt more than 10⁹ cubic meters of gases consisting of CH₄ (79–98%), and a small admixture of C₂H₆, C₃H₈, C₄H₁₀, C₅H₁₂, CO₂, N, H₂S, Ar, He. Mud volcanism is closely connected to the processes occurring in the South Caspian depression, its seismicity, fluctuations of the Caspian Sea level, solar activity and hydrocarbon generation.The large accumulations of gas hydrates are confined to the bottom sediments of the Caspian Sea, mud volcanoes crater fields (interval 0–0.4 m, sea depth 480 m) and to the volcanoes body at the depth of 480–800 from the sea bottom. Resources of HC gases in hydrates saturated sediments up to a depth of 100 m and are estimated at 0.2×10¹⁵–8×10¹⁵ m³. The amount of HC gases concentrated in them is 10¹¹–10¹² m³.The Caspian Sea, being an inland closed basin is very sensitive to climatic and tectonic events expressed in sea level fluctuations. During regressive stages as a result of sea level fall and the reducing of hydrostatic pressure the decomposition of gas hydrates and the releasing of a great volume of HC gases consisting mainly of methane are observed.From the data of deep drilling, seismoacoustics, and deep seismic mud volcanic activity in the South Caspian Basin started in the Lower Miocene. Activity reached its highest intensity at the boundary between the Miocene and Pliocene and was associated with dramatic Caspian Sea level fall in the Lower Pliocene of up to 600 m, which led to the isolation of the PaleoCaspian from the Eastern ParaTethys. Catastrophic reduction of PaleoCaspian size combined with the increasing scale of mud volcanic activity caused the oversaturation and intoxication of water by methane and led to the mass extinction of mollusks, fishes and other groups of sea inhabitants. In the Upper Pliocene and Quaternary mud volcanism occurred under the conditions of a semi-closed sea periodically connected with the Pontian and Mediterranean Basins. Those stages of Caspian Sea history are characterized by the revival of the Caspian organic world.Monitoring of mud volcanoes onshore of the South Caspian demonstrated that any eruption is predicted by seismic activation in the region (South-Eastern Caucasus) and intensive fluid dynamics on the volcanoes.     

Modelling the effects of climate change on methane emission from a northern ombrotrophic bog in Canada

Peatlands are a large potential source of methane (CH₄) to the atmosphere. In order to investigate the effects of climate change on CH₄ emission from northern ombrotrophic peatlands, a simulation model coupling water table dynamics with methane emission was developed for the Mer Bleue Bog in Ontario, Canada. The model was validated against reported values of CH₄ flux from field measurements and the model outputs exhibited high sensitivity to acrotelm thickness, leaf area index, transmissivity and slope of water table. With a 2–4°C temperature rise over the 4-year simulation period, the rate of CH₄ release dropped significantly to under 0.1 mg m⁻² day⁻¹. On the other hand, mean CH₄ emission increased by >26-fold when the increase in precipitation was >15%. When looking at the combined effects, the highest CH₄ release (13.3 mg m⁻² day⁻¹) was attained under the scenario of 2°C temperature rise and 25% precipitation increase. Results obtained in this study highlight the importance of avoiding more extreme climate change, which would otherwise lead to enhanced methane release from peatlands and further atmospheric warming through positive feedback.     

Methane and carbon dioxide flux from a macrotidal salt marsh,Bay of Fundy,New Brunswick

Fluxes of methane (CH₄) and carbon dioxide (CO₂) to the atmosphere at 52 sites within a salt marsh were measured by a dark static chamber technique from mid July to mid September. Mean CH₄ fluxes ranged from 0.2 mg m^?2 d^?1 to 11.0 mg m^?2 d^?1, with an overall average of 1.6 mg m^?2 d^?1. Flux of CH₄ was inversely correlated (r²=0.23, p = 0.001) with salinity of the upper porewater at the site, suggesting the dominant role of SO₄ ^2? in inhibiting methanogenesis in salt-marsh sediments. The combination of salinity and water table position was able to explain only 29% of the variance in CH₄ emission. Mean soil flux of CO₂ ranged from 0.3 g m^?2 d^?1 to 3.7 g m^?2 d^?1, with an overall average of 2.5 g m^?2 d^?1; it was correlated with aboveground biomass (positive, r²=0.38, p = 0.001) and position of the water table (negative, r² = 0.55, p = 0.001). The combination of biomass and water table position accounted for 63% of the variance in CO₂ flux. There were high variations in gas flux within the six plant communities. The sequences were CH₄: upland edge > panne > pool > middle marsh > low marsh > high marsh, and CO₂: middle marsh > low marsh > upland edge > high marsh > panne > pool. Compared to other salt-marsh systems, this Bay of Fundy marsh emits small amounts of CH₄ and CO₂.     

Methane flux from miniseepage in mud volcanoes of SW Taiwan: Comparison with the data from Italy,Romania, and Azerbaijan

Mud volcanoes (MVs) are considered important methane (CH₄) sources for the atmosphere; gas is not only released from macro-seepage, i.e., from craters and visible gas bubbling manifestations, but also from invisible and pervasive exhalation from the ground, named miniseepage. CH₄ flux related to miniseepage was measured only in a few MVs, in Azerbaijan, Italy, Japan, Romania and Taiwan. This study examines in detail the flux data acquired in 5 MVs and 1 “dry” seep in SW Taiwan, and further compares with other 23 MVs in Italy, Romania and Azerbaijan. Miniseepage from the six manifestations in SW Taiwan MVs and seeps annually contribute at least 110 tons of methane directly to the atmosphere, and represents about ∼80% of total degassing during a quiescent period. Combining miniseepage flux and geo-electrical data from the Wu-shan-ding MV revealed a possible link between gas flux and electrical resistivity of the vadose zone. This suggests that unsaturated subsoil is a preferential zone for shallow gas accumulation and seepage to the atmosphere. Besides, miniseepage flux in Chu-huo everlasting fire decreases by increasing the distance from the main gas channeling zone and molecular fractionation (methane/ethane ratio) is higher for lower flux seepage, consistently with what observed in other MVs worldwide. Measurements from Azerbaijan, Italy, Romania, and Taiwan converge to indicate that miniseepage is directly proportional to the vent output and it is a significant component of the total methane budget of a MV. A miniseepage vs. macro-seepage flux equation has been statistically assessed and it can be used to estimate theoretically at least the order of magnitude of the flux of miniseepage for MVs of which only the flux from vents was evaluated, or will be evaluated in future. This will allow a more complete and objective quantification of gas emission in MVs, thus also refining the estimate of the global methane emission from geological sources.     

Methane flux from mangrove sediments along the Southwestern coast of Puerto Rico

Although the sediments of coastal marine mangrove forests have been considered a minor source of atmospheric methane, these estimates have been based on sparse data from similar areas. We have gathered evidence that shows that external nutrient and freshwater loading in mangrove sediments may have a significant effect on methane flux. Experiments were performed to examine methane fluxes from anaerobic sediments in a mangrove forest subjected to secondary sewage effluents on the southwestern coast of Puerto Rico. Emission rates were measured in situ using a static chamber technique, and subsequent laboratory analysis of samples was by gas chromatography using a flame ionization detector. Results indicate that methane flux rates were lowest at the landward fringe nearest to the effluent discharge, higher in the seaward fringe occupied by red mangroves, and highest in the transition zone between black and red mangrove communities, with average values of 4 mg CH₄ m^?2 d^?1, 42 mg CH₄ m^?2 d^?1, and 82 mg CH₄ m^?2 d^?1, respectively. Overall mean values show these sediments may emit as much as 40 times more methane than unimpacted pristine areas. Pneumatophores ofAviciennia germinans have been found to serve as conduits to the atmosphere for this gas. Fluctuating water level overlying the mangrove sediment is an important environmental factor controlling seasonal and interannual CH₄ flux variations. Environmental controls such as freshwater inputs and increased nutrient loading influence in situ methane emissions from these environments.     

Methane formation in soil–plant systems treating wastewater as influenced by microbial populations

The present study evaluated the effect of plant species on methane (CH₄) emission and microbial populations in three types of soil–plant systems. Results showed large variation of CH₄ flux rate ranging from 1.35 to 212.61 mg CH₄ m^?2 h^?1. Emission peak of CH₄ occurred in July. No significant difference was found in the non-vegetation system spanning 2 years. Compared with non-vegetation, vegetation systems had much higher flux of CH₄, and obvious seasonal variation was observed. The polyculture system planted with Zizania latifolia (Z. latifolia) and Phragmites australis (P. australis) released higher CH₄ fluxes than the mono system (P. australis), reflecting that Z. latifolia growth could simulate CH₄ emission. The fluorescence in situ hybridization (FISH) results support the characteristics of CH₄ fluxes. Much higher methanotrophs amount and lower methanogens amount from the mono system than those from the polyculture system was observed indicating that Z. latifolia growth may limit the oxygen transportation resulting in higher CH₄ emission. The polyculture system has the highest potential of CH₄ emission.     

Methane and sulfide fluxes in permanent anoxia: In situ studies at the Dvurechenskii mud volcano (Sorokin Trough, Black Sea)

The Dvurechenskii mud volcano (DMV) is located in permanently anoxic waters at 2060 m depth (Sorokin Trough, Black Sea). The DMV was studied during the RV Meteor expedition M72/2 as an example of an active mud volcano system, to investigate the significance of submarine mud volcanism for the methane and sulfide budget of the anoxic Black Sea hydrosphere. Our studies included benthic fluxes of methane and sulfide, as well as the factors controlling transport, consumption and production of both compounds within the sediment. The pie-shaped mud volcano showed temperature anomalies as well as solute and gas fluxes indicating high fluid flow at its summit north of the geographical center. The anaerobic oxidation of methane (AOM) coupled to sulfate reduction (SR) was repressed in this zone due to the upward flow of sulfate-depleted fluids through recently deposited subsurface muds, apparently limiting microbial methanotrophic activity. Consequently, the emission of dissolved methane into the water column was high, with an estimated rate of 0.46 mol m⁻² d⁻¹. On the wide plateau and edge of the mud volcano surrounding the summit, fluid flow and total methane flux were lower, allowing higher SR and AOM rates correlated with an increase in sulfate penetration into the sediment. Here, between 50% and 70% of the methane flux (0.07-0.1 mol m⁻² d⁻¹) was consumed within the upper 10 cm of the sediment. The overall amount of dissolved methane released from the entire mud volcano structure into the water column was significant with a discharge of 1.3 × 10⁷ mol yr⁻¹. The DMV maintains also high areal rates of methane-fueled sulfide production and emission of on average 0.05 mol m⁻² d⁻¹. This is a difference to mud volcanoes in oxic waters, which emit similar amounts of methane, but not sulfide. However, based on a comparison of this and other mud volcanoes of the Black Sea, we conclude that sulfide and methane emission into the hydrosphere from deep-water mud volcanoes does not significantly contribute to the sulfide and methane inventory of the Black Sea.     

Quantification of methane emission from bacterial mat sites at Quepos Slide offshore Costa Rica

Seafloor methane emission from the Quepos Slide on the submarine segment of the Costa Rica fore-arc margin was estimated by extrapolating flux measurements from individual seeps to the total area covered by bacterial mats. This approach is based on the combination of detailed mapping to determine the abundance of seeps and the application of a numerical model to estimate the amount of benthic methane fluxes. Model results suggest that the majority of the studied seeps transport rather limited amount of methane (on average: ~177 μmol cm^?2 a^?1) into the water column due to moderate upward advection, allowing for intense anaerobic oxidation of methane (AOM; on average: 53 % of the methane flux is consumed). Depth-integrated AOM rates (56–1,538 μmol CH₄ cm^?2 a^?1) are comparable with values reported from other active seep sites. The overall amount of dissolved methane released into the water column from the entire area covered by bacterial mats on the Quepos Slide is estimated to be about 0.28 × 10⁶ mol a^?1. This conservative estimate which relies on rather accurate determinations of seafloor methane fluxes emphasizes the potential importance of submarine slides as sites of natural methane seepage; however, at present the global extent of methane seepage from submarine slides is largely unknown.     

若尔盖高原沼泽湿地CH4排放研究

若尔盖高原沼泽湿地海拔 3400 m，面积 4038km²，是我国面积最大的高原沼泽湿地分布区。2001年 5～9月的非冰冻期，其主要沼泽类型木里苔草沼泽的CH₄排放通量范围是 0.51～ 8.20 m g/（m ²· h），平均值为 2.87 m g/（m ²· h）；乌拉苔草沼泽CH₄排放通量范围是 0.36～10.04 m g/（m ²· h），平均值为 4.51 m g/（m ²· h）。在空间分布上，不同沼泽类型之间CH₄排放通量具有一定的差异。在季节变化上，没有明显的排放高峰。根据代表性观测点的CH₄平均排放量、日数和沼泽总面积推算，非冰冻期若尔盖高原沼泽湿地CH₄的排放量为 0.052Tg/a。     

Interaction of Biogeochemical Sulfur and Carbon Cycles in Marine Basins by the Example of the Black Sea

The interaction of biogeochemical sulfur and carbon cycles in water bodies has been studied by the different authors and methods, including the use of ³⁵S and ¹⁴C radioactive tracers. The main reactions of sulfur and carbon interaction by the example of the Black Sea (sulfate reduction, methanogenesis, aerobic and anaerobic methane oxidation) are considered. Based on the experimental radioisotope data, the annual microbial production of H₂S and CH₄ in the water column of the Black Sea is estimated. The average annual production of CH₄ is 3.8% of the annual production of H₂S. The share of migration methane from the bottom sediments (seeps, mud volcanoes, etc.) is approximately 2% of the total dissolved methane pool.

     

The crucial role of deep-sourced methane in maintaining the subseafloor sulfate budget

Methane (CH₄) is a powerful greenhouse gas and its largest reservoir on Earth is held in marine sediments. CH₄ in marine sediments is mainly stored in gas-hydrate reservoirs and deep sedimentary strata along continental margins, where large amounts of deep-sourced CH₄ ascend to different degrees toward the seafloor. However, the amount of deep-sourced CH₄ and its role in subseafloor carbon and sulfur cycling remains poorly constrained. We analyzed sulfate (SO₄^2?) profiles of 157 sites along with previous published 85 sites to determine the regional distribution and amount of SO₄^2? reduction for an area of 1.23 × 10⁵ km² of the northern South China Sea. Then we compared these obtained results with estimates based on sedimentation rates from the same area. Significantly higher regional SO₄^2? flux estimates based on SO₄^2? profiles (4.26 × 10^?3 Tmol a^?1), compared to lower estimates based on sedimentation rates (1.23 × 10^?3 Tmol a^?1), reflect abundant ascending deep-sourced CH₄. The difference of the regional SO₄^2? flux estimates (3.03 × 10^?3 Tmol a^?1) represents the amount of SO₄^2? reduced by CH₄ through the anaerobic oxidation of CH₄ (AOM). Deep-sourced CH₄ contributes 71% to total SO₄^2? consumption in the study area, largely exceeding SO₄^2? consumption by organoclastic sulfate reduction. Our findings substantiate that deep-sourced CH₄ governs subseafloor carbon and sulfur cycling to a previously underrated extent, fueling extensive chemosynthesis-based ecosystems along continental slope and rise.     

Microbial methane turnover at mud volcanoes of the Gulf of Cadiz

The Gulf of Cadiz is a tectonically active area of the European continental margin and characterised by a high abundance of mud volcanoes, diapirs, pockmarks and carbonate chimneys. During the R/V SONNE expedition “GAP-Gibraltar Arc Processes (SO-175)” in December 2003, several mud volcanoes were surveyed for gas seepage and associated microbial methane turnover. Pore water analyses and methane oxidation measurements on sediment cores recovered from the centres of the mud volcanoes Captain Arutyunov, Bonjardim, Ginsburg, Gemini and a newly discovered, mud volcano-like structure called “No Name” show that thermogenic methane and associated higher hydrocarbons rising from deeper sediment strata are completely consumed within the seabed. The presence of a distinct sulphate-methane transition zone (SMT) overlapping with high sulphide concentrations suggests that methane oxidation is mediated under anaerobic conditions with sulphate as the electron acceptor. Anaerobic oxidation of methane (AOM) and sulphate reduction (SR) rates show maxima at the SMT, which was found between 20 and 200 cm below seafloor at the different mud volcanoes. In comparison to other methane seeps, AOM activity (<383 mmol m⁻² year⁻¹) and diffusive methane fluxes (<321 mmol m⁻² year⁻¹) in mud volcano sediments of the Gulf of Cadiz are low to mid range. Corresponding lipid biomarker and 16S rDNA clone library analysis give evidence that AOM is mediated by a mixed community of anaerobic methanotrophic archaea and associated sulphate reducing bacteria (SRB) in the studied mud volcanoes. Little is known about the variability of methane fluxes in this environment. Carbonate crusts littering the seafloor of mud volcanoes in the northern part of the Gulf of Cadiz had strongly ¹³C-depleted lipid signatures indicative of higher seepage activities in the past. However, actual seafloor video observations showed only scarce traces of methane seepage and associated biological processes at the seafloor. No active fluid or free gas escape to the hydrosphere was observed visually at any of the surveyed mud volcanoes, and biogeochemical measurements indicate a complete methane consumption in the seafloor. Our observations suggest that the emission of methane to the hydrosphere from the mud volcano structures studied here may be insignificant at present.     

Variability in Concentrations and Fluxes of Methane in the Indian Estuaries

In order to examine the fluxes of methane (CH₄) from the Indian estuaries, measurements were carried out by collecting samples from 26 estuaries along the Indian coast during high discharge (wet) and low water discharge (dry) periods. The CH₄ concentrations in the estuaries located along the west coast of India were significantly higher (113?±?40 nM) compared to the east coast of India (27?±?6 nM) during wet and dry periods (88?±?15 and 63?±?12 nM, respectively). Supersaturation of CH₄ was observed in the Indian estuaries during both periods ((0.18 to 22.3?×?10³ %). The concentrations of CH₄ showed inverse relation with salinity indicating that freshwater is a significant source. Spatial variations in CH₄ saturation were associated with the organic matter load suggesting that its decomposition may be another source in the Indian estuaries. Fluxes of CH₄ ranged from 0.01 to 298 μmol m^?2 day^?1 (mean 13.4?±?5 μmol m^?2 day^?1) which is ~30 times lower compared to European estuaries (414 μmol m^?2 day^?1). The annual emission from Indian estuaries, including Pulicat and Adyar, amounted to 0.39?×?10¹⁰ g CH₄?year^?1 with the surface area of 0.027?×?10⁶ km² which is significantly lower than that in European estuaries (2.7?±?6.8?×?10¹⁰ g CH₄?year^?1 with the surface area of 0.03?×?10⁶ km²). This study suggests that Indian estuaries are a weak source for atmospheric CH₄ than European estuaries and such low fluxes were attributed to low residence time of water and low decomposition of organic matter within the estuary. The CH₄ fluxes from the Indian estuaries are higher than those from Indian mangroves (0.01?×?10¹⁰ g CH₄?year^?1) but lower than those from Indian inland waters (210?×?10¹⁰ g CH₄?year^?1).     

Stable isotopic evidence in support of active microbial methane cycling in low-temperature diffuse flow vents at 9°50′N East Pacific Rise

A unique dataset from paired low- and high-temperature vents at 9°50′N East Pacific Rise provides insight into the microbiological activity in low-temperature diffuse fluids. The stable carbon isotopic composition of CH₄ and CO₂ in 9°50′N hydrothermal fluids indicates microbial methane production, perhaps coupled with microbial methane consumption. Diffuse fluids are depleted in ¹³C by ∼10‰ in values of δ¹³C of CH₄, and by ∼0.55‰ in values of δ¹³C of CO₂, relative to the values of the high-temperature source fluid (δ¹³C of CH₄ =−20.1 ± 1.2‰, δ¹³C of CO₂ =−4.08 ± 0.15‰). Mixing of seawater or thermogenic sources cannot account for the depletions in ¹³C of both CH₄ and CO₂ at diffuse vents relative to adjacent high-temperature vents. The substrate utilization and ¹³C fractionation associated with the microbiological processes of methanogenesis and methane oxidation can explain observed steady-state CH₄ and CO₂ concentrations and carbon isotopic compositions. A mass-isotope numerical box model of these paired vent systems is consistent with the hypothesis that microbial methane cycling is active at diffuse vents at 9°50′N. The detectable ¹³C modification of fluid geochemistry by microbial metabolisms may provide a useful tool for detecting active methanogenesis.     

Isotopic-geochemical peculiarities of gases in mud volcanoes of eastern Georgia

Isotopic-geochemical study revealed the presence of mantle He (³He/⁴He up to 223 × 10^?8) in gases from mud volcanoes of eastern Georgia. This fact confirms that the Middle Kura basin fill encloses an intrusive body previously distinguished from geophysical data. Wide variations in the carbon isotopic composition δ¹³C of CH₄ and CO₂ and the chemical composition of gas and water at a temporally constant ³He/⁴He ratio indicate their relation with crustal processes. Unusual direct correlations of the ³He/⁴He ratio with the contents of He and CH₄ and the ⁴⁰Ar/³⁶Ar ratio can be explained by the generation of gas in the Cenozoic sequence of the Middle Kura basin.