Gas Emission Sites Research Articles

Genomic Insights into the Landfill Microbial Community: Denitrifying Activity Supporting One-Carbon Utilization.

In spite of the developments in understanding of denitrifying methylotrophy in the recent years, challenges still exist in unravelling the overall biochemistry of nitrate-dependent methane oxidation in novel or poorly characterized/not-yet-cultured bacteria. In the present study, landfill site was mined for novel C1-carbon-metabolizing bacteria which can use nitrate/nitrite as an electron acceptor. A high-throughput rapid plate assay identified three bacterial isolates with eminent ability for nitrate-dependent methane metabolism under anaerobic conditions. Taxonomic identification by whole-genome sequence-based overall genome relatedness indices accurately assigned the isolates AAK_M13, AAK_M29, and AAK_M39 at the species level to Enterobacter cloacae, Bacillus subtilis, and Bacillus halotolerans, respectively. Several genes encoding sub-components involved in alcohol utilization and denitrification pathways, such as adh, fdh, fdo, nar, nir, and nor, were identified in all the genomes. Though no gene clusters encoding MMO/AMO were annotated, sequencing of PCR amplicons revealed similarity with pMMO/AMO gene using translated nucleotide sequence of strains AAK_M29 and AAK_M39, while strain AAK_M13 showed similarity with XRE family transcriptional regulator. This suggests the horizontal gene transfer and/or presence of a truncated version of a housekeeping enzyme encoded by genes exhibiting partial sequence similarity with pMMO genes that mimicked its function at greenhouse gas emission sites. Owing to lack of conclusive evidence for presence of methane metabolism genes in the selected isolates, further experiment was performed to validate their nitrate-dependent methane oxidation capacities. Bacillus subtilis AAK_M29, Bacillus halotolerans AAK_M39, and Enterobacter cloacae AAK_M13 could oxidize 60%, 75%, and 85% of the added methane respectively accompanied by high nitrate reduction (56-62%) thus supporting the correlation between these two activities. The remarkable ability of these isolates for nitrate-dependent methane metabolism has highlighted their role in ecological contribution and biotechnological potential to serve as methane and nitrate sinks in the landfill sites.

Use of the Gas Emission Site Type Method in the Evaluation of the CO2 Emissions in Raised Bogs

Peatbogs are important in global greenhouse gas budget estimates. The main aim of the work was to assess the amount of greenhouse gas emissions based on the Greenhouse Gas Emission Site Type (GEST) method and compare them with actual field measurements. The research was carried out in selected peatbog areas in the coastal zone of northern Poland. The proposed method allowed us to assess whether the restoration treatments carried out in peatbogs contributed to the emission of these gases. The results of this research using the GEST method indicate that, in the studied peatbog areas, the reduction in CO2 emissions was approximately 12%. These results were compared with actual measurements of greenhouse gas emissions made using the chamber method in 2018. The average CO2 emission for the entire peatbog was 16,338.7 t CO2-eq./yr. Comparing this result with the result obtained using the GEST method, it should be stated that it is lower by 2.464.1 t CO2-eq./year, which gives an approximately 13% overestimation of the result by the Gas Emission Site Type method.

Neogene and Quaternary dikes and related joints as conduits for recent juvenile degassing: case studies from the seismically active region of NW-Bohemia, Czech Republic

The contemporary occurrence of juvenile gas emissions at springs or mofettes in active volcanic areas can be observed worldwide. This correlation is less frequent in areas with dormant or post-volcanic activity. The NW-Bohemia/Vogtland region located at the Eger rift zone (Czech Republic) provides an intriguing example of this correlation between numerous Neogene-aged post-volcanism locations and juvenile fluid emission sites. Newly discovered dikes and diatremes are located close to vents with the highest helium isotope ratios (3He/4He) of about 6 Ra. Our results lend support to the hypothesis that feeder dikes and their fractured wall rocks should be considered the predominant conduits for the continuous upwelling of juvenile fluids. Moreover, 77% of the CO2 gas emission sites are located close to dikes or diatremes, i.e., within a distance of 4 km. Our studies reveal a spatial dependence of the Ra values at the spring sites with the estimated distances to the basaltic bodies. The results confirm the negative trend known from the literature. That is, Ra values decrease with increasing distance from the feeder dikes.

Investigation of hot spring gas components and soil gas fluxes in Arxan Holocene volcanic field, Inner Mongolia, NE China

The latest research results show that there is a unified magma system and heating channel beneath the Arxan volcanic field, indicating a potential risk of eruption. The Arxan volcanic field features multiple gas emission sites (e.g., Jinjianggou hot springs and Yinjianggou hot springs) and exhibits strong hydrothermal activity. In this study, measurements of the hot spring gas composition and soil CO2 flux in the Arxan Holocene volcanic field were conducted, and the results were combined with previous research results to analyze the degassing characteristics of this region. The results show that the volcanic gases in the Arxan volcanic field are composed of 0.07%–1.09% CO2, 0.33–12 ppm CH4, 1.57–53 ppm H2, 800–30,241 ppm He, and 1.14%–1.86% Ar. The He content in this area is notably higher than that in other dormant volcanoes in China. This difference is possibly caused by U–Th decay in the Mesozoic granodiorite and acidic volcanic rocks in the study area, which can produce substantial radiogenic He. The soil gas concentrations near the Jinjianggou and Yinjianggou hot springs are higher than those of two Holocene volcanoes. The peak CO2 concentration in the soil near the Jinjianggou hot spring can reach 35,161 ppm. The single-site soil microseepage CO2 flux in the Arxan volcanic field is 4.66–107.18 g m−2 d−1, and the estimated annual CO2 emission flux from the volcanic field to the atmosphere is 0.63 × 105 t, which also demonstrates that soil CO2 flux of Arxan volcano is comparable to the soil CO2 emission level of the Iwojima volcano.

Environment-Forming Effect of Bubble Gas Emissions in the Golubaya Bay, Black Sea: Oxygen Regime and Bacterial Mats

Gas seep and fluid flows from the seabed are an environment-forming factor of the aquatic environment, mainly due to their influence on the dissolved gases in the water, including dissolved oxygen. During the summer seasons from 2019 to 2021 in the area of shallow water gas emission site off the southern coast of the Heracles Peninsula, series of vertical probing profiles were carried out to determine hydrological parameters of the water: dissolved oxygen concentration (O2), temperature (T), salinity (S), and flow velocity (U). The study area is an underwater ledge with faults in the form of three canyons composed of dense limestones, two of which contained bubble gas emissions. Significant variability in O2 was identified in canyons where gas emissions are observed: from 1 to 80% saturation in the bottom layer, in contrast to normoxia at the background sites. Hypoxia was observed in the bottom layer above the emission sites in the absence of turbulence at temperature stratification. The values S decreased with depth, and the maximum difference reached 0.4‰. The bubble gas was dominated by methane (68.5–75.5%), and the carbon isotope composition of the bubble methane gas varied from –67.9 to –59.8‰ VPDB in 2019 and 2020, respectively. This generally indicates that the CH4 is of predominantly microbial genesis, was formed under different conditions, and matured in various periods of research during the monitoring period. Bacterial mats (mostly sulfur-oxidizing bacteria) were found in the areas of gas emissions.

Monitoring of Shallow-Water Methane Seeps at Cape Fiolent (Black Sea)

During the period from 2019 to 2021, complex studies of new shallow-water methane bubble gas emission sites were carried out in the coastal zone near Cape Fiolent (Southwest Coast of Crimea). The studies included determining the hydrocarbon and isotopic composition of bubble gas, measuring the concentration of methane and nutrients in the water in the areas of gas emissions, estimating the value of bubble flows, and measuring hydrophysical parameters over the sip sites compared to background areas. The seasonal type of Cape Fiolent methane seeps was noted, its active phases of gas emissions differed in duration in different years. The increased pore water silica concentration at the seep sites and their localization in the vicinity of freshwater slope springs may indicate its association with submarine freshwater discharge in the area. However, no significant desalination of both pore water and the bottom water layer above the siphons was recorded. Dissolved methane concentrations in pore water at seep sites were two orders of magnitude higher compared to background areas and reached 448 μmol/L. Also high values were obtained for surface water directly above the bubble gas emission points (maximum 353 nmol/L). Multi-hour monitoring of hydrophysical parameters above the active seeps showed a dissolved oxygen decrease compared to the background sites. The maximum difference in O2 concentrations was 3 mg/l. The carbon isotopic composition of bubble gas methane δ13C-CH4 (–62.84…38.27‰) and сarbon dioxide δ13C-CO2 (–16.83…–10.17‰) was corresponded to a mixture of isotopically heavy gas and near-surface isotopically light gas of microbial origin. The question remains open: what are the reasons for the change in the summer active and the cold season passive gas emission phases?

Effect of Pressure Perturbations on CO2 Degassing in a Mofette System: The Case of Hartoušov, Czech Republic

Mofettes are gas emission sites where high concentrations of CO2 ascend through conduits from as deep as the mantle to the Earth’s surface and as such provide direct windows to processes at depth. The Hartoušov mofette, located at the western margin of the Eger Graben, is a key site to study interactions between fluids and swarm earthquakes. The mofette field (10 mofettes within an area of 100 m × 500 m and three wells of 28, 108, and 239 m depth) is characterized by high CO2 emission rates (up to 100 t/d) and helium signatures with (3He/4He)c up to 5.8 Ra, indicating mantle origin. We compiled geological, geophysical, geochemical, and isotopic data to describe the mofette system. Fluids in the Cheb basin are mixtures between shallow groundwater and brine (>40 g/L at a depth of 235 m) located at the deepest parts of the basin fillings. Overpressured CO2-rich mineral waters are trapped below the mudstones and clays of the sealing Cypris formation. Drilling through this sealing layer led to blow-outs in different compartments of the basin. Pressure transients were observed related to natural disturbances as well as human activities. External (rain) and internal (earthquakes) events can cause pressure transients in the fluid system within hours or several days, lasting from days to years and leading to changes in gas flux rates. The 2014 earthquake swarm triggered an estimated excess release of 175,000 tons of CO2 during the following four years. Pressure oscillations were observed at a wellhead lasting 24 h with increasing amplitudes (from 10 to 40 kPa) and increasing frequencies reaching five cycles per hour. These oscillations are described for the first time as a potential natural analog to a two-phase pipe–relief valve system known from industrial applications.

Bubble vent localization for marine hydrocarbon seep surveys

Commercial success of marine seep hunting exploration campaigns involves acquisition of high-quality bathymetry and backscatter along with targeted coring of shallow geochemical sampling of seep sediments. The sharp lateral chemical gradient encompassing seafloor seeps requires accurate identification of seep sites from high-resolution acoustic data. Active seafloor seeps featuring plumes of gas bubbles and oil droplets rising into the water column can be imaged with modern multibeam echosounders providing an effective approach to remotely characterizing seafloor seeps. Interpreting the seafloor position of gas plume emissions in multibeam data using existing mapping methodology is hindered by slow processing due to large files sizes, a manual “by eye” qualitative assessment of each sonar ping searching for plume anomalies, skill and fatigue of the geoscientist, and environmental or acquisition artifacts that can mask the precise location of gas emission on the seafloor. These limitations of midwater backscatter mapping create a qualitative data set with varying inherent positional errors that can lead to missed or incorrect observations about seep-related seafloor features and processes. By vertically integrating midwater multibeam amplitude samples, a 2D midwater backscatter raster can be generated and draped over seafloor morphology, providing a quantitative synoptic overview of the spatial distribution of gas plume emission sites for more refined seafloor interpretation. We reprocess multibeam midwater data set from NOAA Cruise EX1402L2 in the northwestern Gulf of Mexico using a vertical amplitude stacking technique. Constructed midwater backscatter surfaces are compared with digitized plume positions collected during the survey for a comparison into assessing uncertainty in mapping approaches. Our results show that the accuracy of manually digitizing gas emission sites varies considerably when compared with the midwater backscatter amplitude maps. This quantitative plume mapping technique offers multiple advantages over traditional geopicking from cost effectiveness, offshore efficiency, repeatability, and higher accuracy, ultimately improving the detectability and sampling of active seafloor seeps through precisely located cores.

Methane Plume Emissions Associated With Puget Sound Faults in the Cascadia Forearc

AbstractMethane gas plumes have been discovered to issue from the seafloor in the Puget Sound estuary. These gas emission sites are co‐located over traces of three major fault zones that fracture the entire forearc crust of the Cascadia Subduction Zone. Multibeam and single‐beam sonar data from cruises conducted in years 2011, 2018, 2019, 2020, and 2021 identified the acoustic signature of 349 individual bubble plumes. Dissolved CH4 gas from the plumes combines to elevate seawater methane concentrations of the entire Puget Sound estuary. Fluid samples from adjacent terrestrial hot springs and deep‐water wells surrounding the estuary contain a helium‐3 isotope signature, suggesting a deep fluid source located near the underlying Cascadia Subduction Zone plate interface. However, limited data from this pilot study suggest that Puget Sound seawater emission sites lack both similar chemical isotope signatures and elevated thermal anomalies that would be expected from association with a deep plate‐interface reservoir. A shallow reservoir within the Holocene sediments that cover the older Puget Sound basement with horizontal transfer to the thinly covered Alki Point and Kingston Arch anticlines is also a possibility, as has been suggested for other methane seep areas. The existence of vigorous marine methane plumes arising from areas of thin sediment cover associated with deeply penetrating forearc fault zones but presenting no thermal or chemical anomalies found in other similar forearc environments, remains an unresolved paradox.

Биогеохимические характеристики мелководных струйных метановых газовыделений в прибрежных районах Крыма в сравнении с глубоководными сипами Чёрного моря

Струйные метановые газовыделения (сипы) — широко распространённый феномен в Мировом океане, в том числе в бассейне Чёрного моря. Актуальность исследований метановых сипов обусловлена их важной ролью в качестве источника метана как парникового и средообразующего газа для водной толщи и атмосферы. В работе представлен сравнительный анализ данных собственных биогеохимических исследований мелководных сипов Крымского полуострова, охватывающих последние десять лет, и материалов, посвящённых глубоководным газовыделениям Чёрного моря. В этот период были проведены поисковые гидроакустические исследования, выявлен компонентный состав пузырькового газа, измерен изотопный состав углерода метана, а также молекулярно-биологическими методами определена структура микробного сообщества бактериальных матов, покрывающих площадки газовыделений, и выполнена оценка газовых потоков от отдельных сипов. В течение многолетнего мониторинга обнаружено и описано 14 отдельных газовыделяющих площадок в прибрежных районах Крыма, которые располагались от мыса Тарханкут на западе полуострова до бухты Двуякорная на юго-востоке. Превалирующая часть прибрежных сипов Крыма имела биогенную природу и сезонный характер газовыделений. К глубинному газу термокаталитического генезиса отнесены сипы в бухте Ласпи. Наблюдался значительный разброс величин изотопного состава углерода метана δ13C-CH4 пузырькового газа прибрежных мелководных районов (−94…−34 ‰), что указывает на разные условия его генерации и созревания в донных отложениях. Так же, как и глубоководным сипам, прибрежным струйным газовыделениям сопутствовали бактериальные маты разной структуры с различными доминирующими видами. Показано, что для формирования устойчивой бактериальной биомассы, основу которой составляли, как правило, сульфид- и сероокисляющие бактерии, необходим флюидный поток восстановленных растворённых газов, тогда как точечная разгрузка пузырькового газа не обеспечивает достаточных градиентов концентрации и может механически разрушать структуру образующегося сообщества. Различными методами сделаны оценки размерных спектров пузырьков и потоков от отдельных струй. Диапазон значений газового потока варьировал от 1,8 л·сут−1 (бухта Мартынова) до 40 л·сут−1 (бухта Ласпи). Проанализированы средообразующие эффекты, связанные с выделением пузырькового газа в прибрежных районах: влияние сипов на кислородный режим в донных осадках и в толще воды́ над точками газовыделений; вертикальное перемешивание вод за счёт газлифтового эффекта; флюидная разгрузка на площадках струйных газовыделений.

Multi-Level Gas Monitoring: A New Approach in Earthquake Research

Fluid anomalies were often considered as possible precursors before earthquakes. However, fluid properties at the surface can change for a variety of reasons, including environmental changes near the surface, the response of the superficial fluid system to loads associated with the mechanical nucleation of earthquake fractures, or as a result of transients in fluid flow from the depths. A key problem is to understand the origin of the anomaly and to distinguish between different causes. We present a new approach to monitor geochemical and geophysical fluid properties along a vertical profile in a set of drillings from a depth of a few hundred metres to the surface. This setup can provide hints on the origin of temporal variations, as the migration direction and speed of properties can be measured. In addition, potential admixtures of fluids from a deep crustal or mantle origin with meteoric fluids can be better quantified. A prototype of a multi-level gas monitoring system comprising flow and pressure probes, as well as monitoring of fluid-geochemical properties and stable isotopes is being implemented in a mofette field with massive CO2 (up to 97 tons per day) degassing. The mofette is believed a gas emission site where CO2 ascends through crustal-scale conduits from as deep as the upper mantle, and may therefore provide a natural window to ongoing magmatic processes at mantle depth. Fluids from three adjacent boreholes - 30 m, 70 m, and 230 m deep - will be continuously monitored at high sampling rates.

Characteristics and hydrocarbon seepage at the Challenger Knoll in the Sigsbee Basin, Gulf of Mexico

Active seafloor hydrocarbon seepage from three distinct, positive seafloor features, termed knolls, in the Sigsbee Knolls area, Gulf of Mexico (GOM), was investigated in March 2015. The study sites included the Challenger Knoll, which was drilled by the Deep Sea Drilling Program (DSDP) in 1969 and showed the influence of salt tectonics on its evolution and the presence of subsurface petroleum. This study used gas flare mapping in the water column, detailed seafloor bathymetry, backscatter mapping, sub-bottom profiling, and remote sensing of surface water to locate seafloor areas of hydrocarbon emissions. Flares of gas bubbles were detected in the water column above all three knolls, making them the deepest reported gas emission sites in the GOM. Although surface oil slicks above the Sigsbee Knolls have been detected by satellite imaging in the past; in this study, neither satellite imagery nor ship-based observations detected floating oil, this suggests that oil seepage in the region is intermittent. High-resolution video surveys of the seafloor at Challenger Knoll, using a camera system mounted on a lowered tow-vehicle, documented the presence of seep-related chemosynthetic fauna including clam fields and microbial mats at the western part of the knoll. These findings are consistent with previous investigations that link hydrocarbon seepage at Challenger Knoll to salt tectonics. During visual seafloor inspections, no asphalt deposits, such as those known from sites in the Campeche Knoll area in the southern GOM, were observed. These findings indicate that hydrocarbon seepage is less pronounced in the Sigsbee Knolls region, central GOM, than is the case in hydrocarbon systems in the northern or southern GOM.

Features in REE and Methane Anomalies Distribution in the East China Sea Water Column: a Comparison with the South China Sea

This paper presents review of dissolved Rare Earth Elements (REE) and methane anomalies distribution in the East China Sea water column. In general, the REE concentrations of the shelf water are markedly higher than those of Okinawa Trough water in the East China Sea. Rare earth elemental concentrations in the Okinawa Trough water mainly governed by the Kuroshio Current, along with particle scavenging processes and REE released from lateral transport. Hydrothermal vent activities also have their influence at some sites in the Okinawa Trough intermediate water. The result of comparison between dissolved REE in the East China Sea water with the South China Sea (known as Bien Dong or East Vietnam Sea) water shows that the dissolved REE contents of the South China Sea water are relatively similar to those of the slope water and the Okinawa Trough water and significantly lower than those of the continental shelf water in the East China Sea, except for the case of Ce. The South China Sea surface water may have the effects of fluvial and aeolian input, and coastal water masses at the deep-sea basin but less influenced than at the continental shelf. Both the East China Sea and the South China Sea contain methane gas emission sites, indicating deep fluid input into the seawater that influences the chemical composition of water masses, especially near the bottom water layer.

A statistical approach to relationships between fluid emissions and faults: The Sea of Marmara case

The Sea of Marmara is traversed by the North Anatolian Fault system and also presents abundant emission sites of methane gas into the water column. In order to assess the spatial relationship between gas emissions and active faults, the distribution of distances between gas emission sites and the nearest fault is calculated and compared with the distribution of distances between a uniform random distribution of points (Poisson process representing the null hypothesis of an absence of relationship between gas emissions and faults) and the nearest fault. Interestingly, the distance distribution for the Poisson process is nearly exponential, indicating that the fault map does not have a characteristic scale other than that representing the intensity of the fault network. The distance distribution for the observed gas emissions is significantly narrower than that arising from the Poisson process, with a Kolmogorov distance of 0.25 ± 0.02. The crossing point between the two distributions defines the characteristic half-width of the swath of gas emission sites around the mapped active faults. For the whole Sea of Marmara data set a characteristic half-width of 900–1000 m is found which matches the half-width of the seafloor deformation zone observed around the main active fault. When the same analysis is applied to zones covering the Western High and the Central High it is found that the swath of gas emissions is wider on the Central High (2 km half-width), and not clearly related to the seafloor deformation zone there. This difference is put in perspective with recent work showing that creep is occurring on the western segment of the Main Marmara Fault (this also causing microseismicity) while the central Istanbul-Silivri segment may have remained locked since the 1766 magnitude 7+ earthquake. This suggests that aseismic slip (and not only earthquake occurrence) effectively maintains high permeability conduits in fault zones in sediments.

Multidisciplinary investigation on cold seeps with vigorous gas emissions in the Sea of Marmara (MarsiteCruise): Strategy for site detection and sampling and first scientific outcome

MarsiteCruise was undertaken in October/November 2014 in the Sea of Marmara to gain detailed insight into the fate of fluids migrating within the sedimentary column and partially released into the water column. The overall objective of the project was to achieve a more global understanding of cold-seep dynamics in the context of a major active strike-slip fault. Five remotely operated vehicle (ROV) dives were performed at selected areas along the North Anatolian Fault and inherited faults.To efficiently detect, select and sample the gas seeps, we applied an original procedure. It combines sequentially (1) the acquisition of ship-borne multibeam acoustic data from the water column prior to each dive to detect gas emission sites and to design the tracks of the ROV dives, (2) in situ and real-time Raman spectroscopy analysis of the gas stream, and (3) onboard determination of molecular and isotopic compositions of the collected gas bubbles. The in situ Raman spectroscopy was used as a decision-making tool to evaluate the need for continuing with the sampling of gases from the discovered seep, or to move to another one. Push cores were gathered to study buried carbonates and pore waters at the surficial sediment, while CTD-Rosette allowed collecting samples to measure dissolved-methane concentration within the water column followed by a comparison with measurements from samples collected with the submersible Nautile during the Marnaut cruise in 2007.Overall, the visited sites were characterized by a wide diversity of seeps. CO2- and oil-rich seeps were found at the westernmost part of the sea in the Tekirdag Basin, while amphipods, anemones and coral populated the sites visited at the easternmost part in the Cinarcik Basin. Methane-derived authigenic carbonates and bacterial mats were widespread on the seafloor at all sites with variable size and distributions. The measured methane concentrations in the water column were up to 377 μmol, and the dissolved pore-water profiles indicated the occurrence of sulfate depleting processes accompanied with carbonate precipitation. The pore-water profiles display evidence of biogeochemical transformations leading to the fast depletion of seawater sulfate within the first 25-cm depth of the sediment. These results show that the North Anatolian Fault and inherited faults are important migration paths for fluids for which a significant part is discharged into the water column, contributing to the increase of methane concentration at the bottom seawater and favoring the development of specific ecosystems.

Profitability of Direct Greenhouse Gas Measurements in Carbon Credit Schemes of Peatland Rewetting

MoorFutures® is the world's first carbon credit scheme from peatland rewetting. Thus far, MoorFutures® rely on proxies (greenhouse gas emission site types or GESTs) to estimate emission reductions. Here, we tested the profitability of including direct greenhouse gas (GHG) measurements of project emissions for a range of rewetting costs and vegetation scenarios based on a hypothetical MoorFutures® project. In almost all scenarios GEST assessments underestimated emission reductions compared with direct measurements. Including direct measurements was lucrative in >50% of all vegetation scenario/rewetting cost combinations with net profits ranging from EUR −8.18 to 26.31 per certificate. Profitability was achieved at rewetting costs of ~EUR 5400ha−1 upward. More sophisticated GHG measurements became profitable at twice the rewetting costs. In cases where direct flux measurements do not generate a profit they can strengthen reliability and buyers' trust and thus support higher prices of the certificates.

Tidally controlled gas bubble emissions: A comprehensive study using long-term monitoring data from the NEPTUNE cabled observatory offshore Vancouver Island

Long-term monitoring over one year revealed high temporal variability of gas emissions at a cold seep in 1250 m water depth offshore Vancouver Island, British Columbia. Data from the North East Pacific Time series Underwater Networked Experiment observatory operated by Ocean Networks Canada were used. The site is equipped with a 260 kHz Imagenex sonar collecting hourly data, conductivity-temperature-depth sensors, bottom pressure recorders, current meter, and an ocean bottom seismograph. This enables correlation of the data and analyzing trigger mechanisms and regulating criteria of gas discharge activity. Three periods of gas emission activity were observed: (a) short activity phases of few hours lasting several months, (b) alternating activity and inactivity of up to several day-long phases each, and (c) a period of several weeks of permanent activity. These periods can neither be explained by oceanographic conditions nor initiated by earthquakes. However, we found a clear correlation of gas emission with bottom pressure changes controlled by tides. Gas bubbles start emanating during decreasing tidal pressure. Tidally induced pressure changes also influence the subbottom fluid system by shifting the methane solubility resulting in exsolution of gas during falling tides. These pressure changes affect the equilibrium of forces allowing free gas in sediments to emanate into the water column at decreased hydrostatic load. We propose a model for the fluid system at the seep, fueled by a constant sub-surface methane flux and a frequent tidally controlled discharge of gas bubbles into the ocean, transferable to other gas emission sites in the world's oceans.

Seasonal methane accumulation and release from a gas emission site in the central North Sea

Abstract. We investigated dissolved methane distributions along a 6 km transect crossing active seep sites at 40 m water depth in the central North Sea. These investigations were done under conditions of thermal stratification in summer (July 2013) and homogenous water column in winter (January 2014). Dissolved methane accumulated below the seasonal thermocline in summer with a median concentration of 390 nM, whereas during winter, methane concentrations were typically much lower (median concentration of 22 nM). High-resolution methane analysis using an underwater mass-spectrometer confirmed our summer results and was used to document prevailing stratification over the tidal cycle. We contrast estimates of methane oxidation rates (from 0.1 to 4.0 nM day−1) using the traditional approach scaled to methane concentrations with microbial turnover time values and suggest that the scaling to concentration may obscure the ecosystem microbial activity when comparing systems with different methane concentrations. Our measured and averaged rate constants (k') were on the order of 0.01 day−1, equivalent to a turnover time of 100 days, even when summer stratification led to enhanced methane concentrations in the bottom water. Consistent with these observations, we could not detect known methanotrophs and pmoA genes in water samples collected during both seasons. Estimated methane fluxes indicate that horizontal transport is the dominant process dispersing the methane plume. During periods of high wind speed (winter), more methane is lost to the atmosphere than oxidized in the water. Microbial oxidation seems of minor importance throughout the year.

Gas emissions at the continental margin west of Svalbard: mapping, sampling, and quantification

Abstract. We mapped, sampled, and quantified gas emissions at the continental margin west of Svalbard during R/V Heincke cruise He-387 in late summer 2012. Hydroacoustic mapping revealed that gas emissions were not limited to a zone just above 396 m water depth. Flares from this depth have gained significant attention in the scientific community in recent years because they may be caused by bottom-water warming-induced hydrate dissolution in the course of global warming and/or by recurring seasonal hydrate formation and decay. We found that gas emissions occurred widespread between about 80 and 415 m water depth, which indicates that hydrate dissolution might only be one of several triggers for active hydrocarbon seepage in that area. Gas emissions were remarkably intensive at the main ridge of the Forlandet moraine complex in 80 to 90 m water depths, and may be related to thawing permafrost. Focused seafloor investigations were performed with the remotely operated vehicle (ROV) "Cherokee". Geochemical analyses of gas bubbles sampled at about 240 m water depth as well as at the 396 m gas emission sites revealed that the vent gas is primarily composed of methane (> 99.70%) of microbial origin (average δ13C = −55.7‰ V-PDB). Estimates of the regional gas bubble flux from the seafloor to the water column in the area of possible hydrate decomposition were achieved by combining flare mapping using multibeam and single-beam echosounder data, bubble stream mapping using a ROV-mounted horizontally looking sonar, and quantification of individual bubble streams using ROV imagery and bubble counting. We estimated that about 53 × 106 mol methane were annually emitted at the two areas and allow for a large range of uncertainty due to our method (9 to 118 × 106 mol yr−1). First, these amounts show that gas emissions at the continental margin west of Svalbard were on the same order of magnitude as bubble emissions at other geological settings; second, they may be used to calibrate models predicting hydrate dissolution at present and in the future; and third, they may serve as a baseline (year 2012) estimate of the bubble flux that will potentially increase in the future due to ever-increasing global-warming-induced bottom water warming and hydrate dissociation.

Origins of gas discharging from the Qiangtang Basin in the northern Qinghai–Tibet Plateau, China: Evidence from gas compositions, helium, and carbon isotopes

Gas compositions, helium isotopic ratios, and carbon isotopic compositions of CO2 and CH4 from six gas emission sites in the Qiangtang Basin and its adjacent areas were measured in order to unravel their origins. Gas samples from the Beiluhe, the Bucha Lake, and the eastern Tuotuo River are N2-rich with N2 over 75vol.%, while the contents of CO2 and CH4 are only 3.45–20.91vol.% and 0–3.58vol.%, respectively. Relatively high CO2/3He (~9.95×1011) and CH4/3He (5.9×109–1.65×1010), and low δ13C values of CO2 (~−15.4‰) and CH4 (~−29.7‰) suggest that both CO2 and CH4 from the Bucha Lake are biotic. The S–M–L (S, M, and L represent sedimentary organic carbon, mantle, and limestone, respectively.) three-component mixing model suggests that no mantle-derived CO2 exists in these samples, which is consistent with the result that He in these gas samples is all crust-derived. CO2 in gas samples from the Beiluhe and the eastern Tuotuo River are relatively enriched in 13C of CO2 (δ13C: −8.4‰ and −5.5‰) and low CO2/3He (2.3×109 and 2.4×109) as well as CH4/3He (5.9×107 and 3.4×107) ratios. These patterns suggest that both CO2 and CH4 are abiotic, demonstrated by the S–M–L three-component mixing model displaying that more than 50% CO2 in these two sites are mantle-derived. CO2-rich gases are discovered in the southern Erdaogou Depot, the branch of Tuotuo River, and the Tanggula Mountain Depot with CO2 exceeding 96vol.%. The relatively high δ13C of CO2 (−4.2 to −7.7‰), low CO2/3He ratios (7.9×108–9.3×109), as well as the S–M–L three-component mixing model reveal that CO2 at least partially originates from the mantle source. Only one gas sample from the Tanggula Mountain Depot contains a trace amount of abiotic CH4 (CH4/3He=1.2×107) generated by abiotic reduction of mantle-derived CO2. However, the source of He is dominantly crust-derived with only 2.15–5.66% mantle-derived He. The emission of large quantity of mantle-derived CO2 is likely due to the presence of the Hoh Xil–Jinsha River suture and the Cenozoic volcanism in northern Tibet. Nevertheless, the small fraction of mantle-derived He may result from the limited degree of openness at the bottom of fault zones and possibly reflect a geological setting of strong extrusion and crustal thickening.