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Terrestrial mud volcanoes represent surface features of channels for subsurface methane transport and, therefore, constitute an important source of methane emission from natural environments. How microbial processes regulate methane emissions in terrestrial mud volcanoes has yet to be fully addressed. This study demonstrated the geochemical characteristics and microbial communities of four mud volcano and seep sites in two geological settings of Sicily, Italy. At sites within the accretionary wedge that exhibited higher methane and sulfate concentrations, the communities were dominated by members capable of catalyzing methane and sulfate metabolisms and organic degradation. In particular, both anaerobic and aerobic methanotrophs were abundant and their abundance distribution coincided with the geochemical transition. In contrast, the sites near Mount Etna were characterized by high fluid salinity, CO2, and low methane and sulfate concentrations, with communities consisting of halophilic organic degraders and sulfur metabolizers, along with a minor presence of aerobic methanotrophs. Substantial variations in community composition and geochemistry across spatial and vertical redox gradients suggest that physicochemical contexts imposed by the geology, fluid path, and source characteristics play a vital role in shaping community composition and cycling of methane, sulfur and organic carbon in Sicily mud volcanoes. [ABSTRACT FROM AUTHOR]

This document is a research article published in the journal Frontiers in Microbiology. It acknowledges the contributions of a researcher named Barny to the field of methanogen research and the broader microbial field. The article highlights Barny's work in clarifying the taxonomy of methanogens and recognizes his wider contributions as an editor. The article also includes information about the funding sources for the research and acknowledges the authors and reviewers who contributed to the research topic. It concludes with a statement about potential conflicts of interest and a note from the publisher. [Extracted from the article]

Microbial communities of terrestrial mud volcanoes are involved in aerobic and anaerobic methane oxidation, but the biological mechanisms of these processes are still understudied. We have investigated the taxonomic composition, rates of methane oxidation, and metabolic potential of microbial communities in five mud volcanoes of the Taman Peninsula, Russia. Methane oxidation rates measured by the radiotracer technique varied from 2.0 to 460 nmol CH4 cm−3 day−1 in different mud samples. This is the first measurement of high activity of microbial methane oxidation in terrestrial mud volcanos. 16S rRNA gene amplicon sequencing has shown that Bacteria accounted for 65–99% of prokaryotic diversity in all samples. The most abundant phyla were Pseudomonadota, Desulfobacterota, and Halobacterota. A total of 32 prokaryotic genera, which include methanotrophs, sulfur or iron reducers, and facultative anaerobes with broad metabolic capabilities, were detected in relative abundance >5%. The most highly represented genus of aerobic methanotrophs was Methyloprofundus reaching 36%. The most numerous group of anaerobic methanotrophs was ANME-2a-b (Ca. Methanocomedenaceae), identified in 60% of the samples and attaining relative abundance of 54%. The analysis of the metagenome-assembled genomes of a community with high methane oxidation rate indicates the importance of CO2 fixation, Fe(III) and nitrate reduction, and sulfide oxidation. This study expands current knowledge on the occurrence, distribution, and activity of microorganisms associated with methane cycle in terrestrial mud volcanoes. [ABSTRACT FROM AUTHOR]

Mud volcanoes (MVs) are surface structures typically created by episodic discharge of fluids and solids, often associated to onshore and offshore accretionary prisms on convergent plate boundaries. Detailed investigations of ongoing activity and its associated morphological changes, as well as a better understanding of the temporal evolution of these highly dynamic systems, may improve the estimations of material fluxes from MVs drastically. Until today, approximately 70 individual MVs were discovered in the northern Ionian Sea in the Calabrian Arc (Central Mediterranean Sea), but only a few have been analyzed and described in detail. In this study, new evidence for recent recurring eruptive activity of the ~45 m-high and 1 km-wide Sartori MV situated in the clastic wedge of the Calabrian Arc is presented. High-resolution seafloor mapping as well as sediment temperature, geochemical, and sedimentological data received from two research cruises in 2016 and 2020 are used. Bathymetric and seafloor backscatter data (1 m scale) indicate the presence of two active eruption centers at the flat-topped Sartori MV. Elevated sediment temperature gradients at both eruption centers show that currently heat is transferred to the surface sediments. Pore water analyses indicate that fluids rising below the eruption centers are CH4-rich, Cl--poor, and SO42--free. Stable C and H isotopic compositions of methane suggest that it originates from a mix of primary microbial, secondary microbial, and/or thermogenic sources. A relatively shallow position of the sulfate-methane interface at both eruption centers also indicates the presence of upward fluid migration in recent times. Pore water modeling suggests that seawater has penetrated the surface sediments to a greater extent within the last few years. In contrast, centimeter-thick layers of hemipelagic sediments overlying mud breccia in sediment cores taken from both eruption centers show that no solid material has been ejected in recent times. Sediment core analyses combined with high-resolution seafloor mapping show an absence of rim-passing mudflows over the past ~10 ka. It is concluded that Sartori MV is an episodically active MV from which fluids with a comparatively low flux were released into the bottom water in recent times. [ABSTRACT FROM AUTHOR]

The Guaymas Basin in the Gulf of California is characterized by active seafloor spreading, the rapid deposition of organic-rich sediments, steep geothermal gradients, and abundant methane of mixed thermogenic and microbial origin. Subsurface sediment samples from eight drilling sites with distinct geochemical and thermal profiles were selected for DNA extraction and PCR amplification to explore the diversity of methane-cycling archaea in the Guaymas Basin subsurface. We performed PCR amplifications with general (mcrIRD), and ANME-1 specific primers that target the alpha (α) subunit of methyl coenzyme M reductase (mcrA). Diverse ANME-1 lineages associated with anaerobic methane oxidation were detected in seven out of the eight drilling sites, preferentially around the methane-sulfate interface, and in several cases, showed preferences for specific sampling sites. Phylogenetically, most ANME-1 sequences from the Guaymas Basin subsurface were related to marine mud volcanoes, seep sites, and the shallow marine subsurface. The most frequently recovered methanogenic phylotypes were closely affiliated with the hyperthermophilic Methanocaldococcaceae, and found at the hydrothermally influenced Ringvent site. The coolest drilling site, in the northern axial trough of Guaymas Basin, yielded the greatest diversity in methanogen lineages. Our survey indicates the potential for extensive microbial methane cycling within subsurface sediments of Guaymas Basin. [ABSTRACT FROM AUTHOR]

The construction of a dam for the Irkutsk hydroelectric power plant caused the water level in Lake Baikal to increase by 80 cm between 1958 and 1961. This led to a downward migration of the base of the gas hydrate stability zone (BGHSZ) in the sediments of the lake, which was followed by a long transition process to a new state of equilibrium. In the first stage of the transition, new gas hydrates formed at the BGHSZ, which was accompanied and followed by a decrease in pore pressure, a decrease in methane transfer from the BGHSZ to the lake floor (via fast transfer pathways, such as faults or mud volcanoes), a decrease in the intensity of methane release from the lake floor into the water column, and a decrease in the methane concentrations in the water. In the second stage, which covers the last 10–15 years, methane concentrations in the water column have started to increase again, possibly in response to an uptick in methane flux from the lake floor. In this paper, we look at possible explanations. Mathematical modeling of the migration of the BGHSZ allowed us to estimate how long the transition process takes. The modeled transition times are different for different locations in the lake, depending mainly on the sedimentation rate and the gas hydrate content of the sediments. In the near future, Lake Baikal may reach a quasi-stationary state again similar to that before the construction of the dam. This stationary state likely involves much higher methane concentrations in the water column than what is observed today, as well as adverse effects on biota of pulsed expulsions of methane, sourced from the BGHSZ, into the water column by means of e.g. mud-volcano eruptions. Such effects may include events of mass deaths of the endemic deep-water fish, golomyanka, similar to what was reported to have occurred in the 19th and first half of the twentieth century, prior to the construction of the dam. This study also reemphasizes how variations in the dynamics of a natural gas hydrate system may have a profound impact on the water bodies in which they occur and on the ecosystems within these water bodies. It also highlights which effects can be expected in other hydrate-bearing marine basins where climate-induced sea-level rise will impact the dynamics of the hydrate reservoirs. [ABSTRACT FROM AUTHOR]

Microbial anaerobic oxidation of methane (AOM) substantially mitigates atmospheric methane emissions on Earth and is a process to consider for astrobiological targets where methane has been detected. The measurement of doubly substituted, or "clumped", methane isotopes has proven useful in tracing processes of methane formation and oxidation. Both near-equilibrium and extreme disequilibrium methane clumped isotope signatures can be attributed to AOM, but, to date, understanding the mechanistic and environmental controls on those signatures has been lacking. We report measurements of methane clumped isotope compositions of residual methane in AOM-active microbial incubations using sediment slurries from Svalbard and Santa Barbara Channel methane seeps. Incubation experiments of Svalbard sediment slurries resulted in residual methane with very high Δ13CH 3 D and Δ12CH 2 D 2 values up to 19.5‰ and 65.1‰, respectively. We found similarly high Δ13CH 3 D and Δ12CH 2 D 2 values in fluid samples from the Chamorro Seamount, a serpentinite mud volcano in the Mariana forearc, suggesting that minimal reversibility of AOM intracellular reactions leads to kinetic fractionation of clumped isotopologues. When conditions were consistent with a low thermodynamic drive for AOM, however, methane isotopologues approached intra-species quasi-equilibrium. This was clearly observed in isotope exchange experiments with methyl-coenzyme M reductase (Mcr) and in microbial incubations of the Santa Barbara Channel sediment slurries. Using an isotopologue fractionation model, we highlight the critical role of reversibility in controlling the trajectory of gases in Δ13CH 3 D vs. Δ12CH 2 D 2 space during AOM. The near-equilibrium methane isotopologue signatures are generalized as a result of the Mcr-catalyzed intracellular isotope exchange operating under near-threshold free energy conditions, as shown in the deep-biosphere incubations. Our results show that the reversibility of the Mcr-catalyzed reaction is central to understanding the meaning of methane isotopologue ratios affected by microbial production and oxidation. [ABSTRACT FROM AUTHOR]

The transitional zone of the marginal seas of the Asia‒Pacific region is a significant object for geological research, the large deposits of hydrocarbons (oil, gas, gas condensate, gas hydrates) have been discovered in the regions of the transition zone. The article gives an interpretation of the main gas-geochemical and geological tectonic regularities of the occurrence and distribution of methane and carbon dioxide fields of different types on the shelf of Sakhalin Island. A study of the gas-geochemical parameters of the emitted gases in the "continent‒shelf" transit zone was carried out based on comparison of field studies and satellite observation data. The data of satellite observation of Sakhalin Island and its shelf are presented to identify the relationship between seismic events and changes in the level of methane concentration in the atmospheric air (in particular, on the example of the methane discharge regime of the Yuzhno-Sakhalinsky mud volcano). Remote sensing data are compared with the results of the field studies, and an assessment is made of the potential using satellite observation methods for studying the gas geochemical composition of the methane and carbon dioxide field. The results obtained can make it possible to refine the assessment of the emission of climatically active gases, and also bring the significant incentive to the development of the Russian Federation project "Far Eastern Marine Carbon Monitoring Plots Consortium." [ABSTRACT FROM AUTHOR]

The key microbial group involved in anaerobic methane oxidation is anaerobic methanotrophic archaea (ANME). From a terrestrial mud volcano, we enriched a microbial community containing ANME-2a, using methane as an electron donor, Fe(III) oxide (ferrihydrite) as an electron acceptor, and anthraquinone-2,6-disulfonate as an electron shuttle. Ferrihydrite reduction led to the formation of a black, highly magnetic precipitate. A significant relative abundance of ANME-2a in batch cultures was observed over five subsequent transfers. Phylogenetic analysis revealed that, in addition to ANME-2a, two bacterial taxa belonging to uncultured Desulfobulbaceae and Anaerolineaceae were constantly present in all enrichments. Metagenome-assembled genomes (MAGs) of ANME-2a contained a complete set of genes for methanogenesis and numerous genes of multiheme c-type cytochromes (MHC), indicating the capability of methanotrophs to transfer electrons to metal oxides or to a bacterial partner. One of the ANME MAGs encoded respiratory arsenate reductase (Arr), suggesting the potential for a direct coupling of methane oxidation with As(V) reduction in the single microorganism. The same MAG also encoded uptake [NiFe] hydrogenase, which is uncommon for ANME-2. The MAG of uncultured Desulfobulbaceae contained genes of dissimilatory sulfate reduction, a Wood–Ljungdahl pathway for autotrophic CO2 fixation, hydrogenases, and 43 MHC. We hypothesize that uncultured Desulfobulbaceae is a bacterial partner of ANME-2a, which mediates extracellular electron transfer to Fe(III) oxide. [ABSTRACT FROM AUTHOR]

Gas hydrates stored in the continental margins of the world's oceans represent the largest global reservoirs of methane. Determining the source and history of methane from gas hydrate deposits informs the viability of sites as energy resources, and potential hazards from hydrate dissociation or intense methane degassing from ocean warming. Stable isotope ratios of methane (13C/12C, D/H) and the molecular ratio of methane over ethane plus propane (C 1 /C 2+3) have traditionally been applied to infer methane sources, but often yield ambiguous results when two or more sources are mixed, or when compositions were altered by physical (e.g., diffusion) or microbial (e.g., methanotrophy) processes. We measured the abundance of clumped methane isotopologue (13CH 3 D) alongside 13C/12C and D/H of methane, and C 1 /C 2+3 for 46 submarine gas hydrate specimens and associated vent gases from 11 regions of the world's oceans. These samples are associated with different seafloor seepage features (oil seeps, pockmarks, mud volcanoes, and other cold seeps). The average apparent equilibration temperatures of methane from the Δ13CH 3 D (the excess abundance of 13CH 3 D relative to the stochastic distribution) geothermometer increase from cold seeps (15–65 °C) and pockmarks (36–54 °C), to oil-associated gas hydrates (48–120 °C). These apparent temperatures are consistent with, or a few tens of degrees higher than, the temperature expected for putative microbial methane sources. Apparent methane generation depths were derived for cold seep, pockmark, and oil seep methane from isotopologue-based temperatures and the local geothermal gradients. Estimated methane generation depths ranged from 0.2 to 5.3 kmbsf, and are largely consistent with source rock information, and other chemical geothermometers based on clay mineralogy and fluid chemistry (e.g., Cl, B, and Li). Methane associated with mud volcanoes yielded a wide range of apparent temperatures (15–313 °C). Gas hydrates from mud volcanoes the Kumano Basin and Mediterranean Sea yielded δ13C-CH 4 values from −36.9 to −51.0‰, typical for thermogenic sources. Δ13CH 3 D values (3.8–6.0‰) from these sites, however, are consistent with prevailing microbial sources. These mud volcanoes are located at active convergent plate margins, where hydrogen may be supplied from basement rocks, and fuel methanogenesis to the point of substrate depletion. In contrast, gas hydrate from mud volcanoes located on km-thick sediments in tectonically less active or passive settings (Black Sea, North Atlantic) yielded microbial-like δ13C-CH 4 and C 1 /C 2+3 values, and low Δ13CH 3 D values (1.6–3.3‰), which may be due to kinetic isotope effects. Additionally, using samples from two sites, we found that Δ13CH 3 D values of hydrate-bound gas and vent gas agree within measurement error. This study is the first to document the link between methane isotopologue-based temperature estimates and key submarine gas hydrate seepage features, and validate previous models about their geologic driving forces. [ABSTRACT FROM AUTHOR]

Terrestrial mud volcanoes usually develop in overpressured, hydrocarbon-bearing sedimentary basins where argillaceous, viscous fluids and gases find passages to the surface. In this study mud samples collected at the Beciu mud volcano area (Romania) were examined using 16S rRNA gene amplicon sequencing technique. Bacterial communities were dominated by Proteobacteria, Patescibaceria, Bacteriodota, and Campilobacterota while Archaea by Halobacterota and Nanoarchaeota. All the most abundant taxa can be connected to the characteristic methane production of the onshore mud volcanism and/or the presence of a deep subsurface salt layer. The discharged methane might be utilized by the detected ubiquitous anaerobic methane oxidizers (ANME-1, ANME-2a-2b, ANME-3) coupling the reaction with the sulfate or sulfur reduction by representatives of genera Sulfurovum, Sulfurimonas, Desulfobacterales, Desulfuromonadales, Halanaeroarchaeum, and Halodesulfurarchaeum. In addition to methanotrophs, methylotrophic and acetoclastic methanogens were also detected. Most of the identified OTUs were associated with halophilic taxa. The results suggest that prokaryotes inhabiting the mud volcanoes can be participated in the closely interconnected methane and sulfur cycles. [ABSTRACT FROM AUTHOR]

Submarine mud volcanoes (MVs) are one of the most spectacular methane expulsion features at the seafloor and they represent a significant geohazard worldwide. In this work, we focus on the physical processes controlling the initiation and early evolution of the Absheron mud volcano (AMV). Our analyses were carried out based on basin modeling calibrated thanks to existing seismic interpretation of the AMV, analysis of sediment samples from seabed, and data from two exploration wells. Acquired laboratory geotechnical data allowed us to derive laws considering the impact of gas exsolution on host sediment behaviors. In this study, we identified key geological and physical conditions that led to MV formation: by coupling diffusion processes with hydrofracturing and fluid advection, we were able to simulate the conditions required to generate mud 3.5 km below the AMV. Mud remobilization up to the seabed was reproduced by using Navier‐Stokes equations modified to account for the impact of gas expansion on mud density. Considering density inversion only, simulations indicate that mud would be extruded at the seabed 100 years after its generation, an ascent rate similar to extrusion rates measured at the active Kotyrdag MV in Azerbaijan. Plain Language Summary: Mud volcanoes (MVs) build‐up at the Earth surface from liquidized sediments remobilized from depths reaching several kilometers. They are distributed globally, onshore and offshore, in different geological backgrounds and represent serious geohazards for people and infrastructures. Moreover, they are also among the most spectacular methane expulsion features at the surface, participating to the global budget of green‐house gases emissions. Their understanding is thus paramount to prevent natural catastrophes such as Lusi eruption in Indonesia in 2006 and to quantify the human impact on global warming. If MVs are known for centuries (Pline the Elder) and studied for decades, little is known about the processes controlling mud generation from stiff deep sediments and its vertical migration toward the surface. The methane is often considered as being a driving element in this geological process and bubble formation in compacted sediments during laboratory testing led to strong damage of sediment properties. Here we develop two independent numerical models, based on simple physical processes and relying on in situ observations of the Absheron mud volcano (AMV, South Caspian Basin) and laboratory testing. They allow to simulate mud generation at depth and its remobilization to the surface. Our research work provides reliable information to describe the initiation and early evolution of an active MV. Key Points: Simulation of mud generation is possible through 2D basin modeling including the effect of gasMud ascent through gas‐expansion‐driven density inversion is possibleA semi‐quantitative formation model for the AMV is derived from modeling results [ABSTRACT FROM AUTHOR]

Earth's hydrocarbon degassing through gas-oil seeps, mud volcanoes and diffuse microseepage is a major natural source of methane (CH4) to the atmosphere. While carbon dioxide degassing is typically associated with extensional tectonics, volcanoes, and geothermal areas, CH4 seepage mostly occurs in petroleum-bearing sedimentary basins, but the role of tectonics in degassing is known only for some case studies at local scale. Here, we perform a global scale geospatial analysis to assess how the presence of hydrocarbon fields, basin geodynamics and the type of faults control CH4 seepage. Combining georeferenced data of global inventories of onshore seeps, faults, sedimentary basins, petroleum fields and heat flow, we find that hydrocarbon seeps prevail in petroleum fields within convergent basins with heat flow ≤ 98 mW m−2, and along any type of brittle tectonic structure, mostly in reverse fault settings. Areas potentially hosting additional seeps and microseepage are identified through a global seepage favourability model. CH4 seepage mostly occurs in petroleum-bearing sedimentary basins, but the role of tectonics in degassing is mostly only known at a local scale. Here, the authors conduct a global scale analysis of seeps, faults, sedimentary basins, petroleum fields and heat flow, and find that geological CH4 seepage preferably develops in convergent basins, while gas seeps can occur along any brittle tectonic structure. [ABSTRACT FROM AUTHOR]

Serpentinitic systems are potential habitats for microbial life due to frequently high concentrations of microbial energy substrates, such as hydrogen (H2), methane (CH4), and short-chain organic acids (SCOAs). Yet, many serpentinitic systems are also physiologically challenging environments due to highly alkaline conditions (pH > 10) and elevated temperatures (>80°C). To elucidate the possibility of microbial life in deep serpentinitic crustal environments, International Ocean Discovery Program (IODP) Expedition 366 drilled into the Yinazao, Fantangisña, and Asùt Tesoru serpentinite mud volcanoes on the Mariana Forearc. These mud volcanoes differ in temperature (80, 150, 250°C, respectively) of the underlying subducting slab, and in the porewater pH (11.0, 11.2, 12.5, respectively) of the serpentinite mud. Increases in formate and acetate concentrations across the three mud volcanoes, which are positively correlated with temperature in the subducting slab and coincide with strong increases in H2 concentrations, indicate a serpentinization-related origin. Thermodynamic calculations suggest that formate is produced by equilibrium reactions with dissolved inorganic carbon (DIC) + H2, and that equilibration continues during fluid ascent at temperatures below 80°C. By contrast, the mechanism(s) of acetate production are not clear. Besides formate, acetate, and H2 data, we present concentrations of other SCOAs, methane, carbon monoxide, and sulfate, δ13C-data on bulk carbon pools, and microbial cell counts. Even though calculations indicate a wide range of microbial catabolic reactions to be thermodynamically favorable, concentration profiles of potential energy substrates, and very low cell numbers suggest that microbial life is scarce or absent. We discuss the potential roles of temperature, pH, pressure, and dispersal in limiting the occurrence of microbial life in deep serpentinitic environments. [ABSTRACT FROM AUTHOR]

The concentrations, isotope ratios, and fluxes of CH4 and CO2 from mud volcanoes measured during a field survey on the southern margin of the Junggar Basin are reported. The origins and outputs of CH4 and CO2 from the four mud volcanoes are assessed, and the relationship between the characteristics of degassing and regional tectonics is discussed. The geochemical data indicate that thermogenic gases derived from the hydrocarbon reservoirs beneath anticlines could be the main source of the CH4 emitted from the mud volcanoes and that hydrocarbon degradation by microbes and carbonate decomposition could be the primary contributors to CO2 emissions from the soil in and around the mud volcanoes. Microseepage was the main mode of release for CH4 and CO2 from the mud volcanoes. The total CH4 output from the mud volcanoes was 147.83 ton/year, comparable to that from mud volcanoes elsewhere. The total output of CO2 from the mud volcanoes was 603.96 ton/year, which suggests that more attention should be paid to CO2 from mud volcanoes. The CH4 and CO2 outputs from the mud volcanoes, and from the Xihu fold in the four anticlines, decreased from south to north. This observation is consistent with the intensity of regional tectonic activity, which weakens from south to north. The results suggest that regional compression is the major triggering mechanism for CH4 and CO2 emissions from the mud volcanoes. Key Points: The geochemistry of the gases and fluxes of CH4 and CO2 from mud volcanoes in the southern margin of the Junggar Basin were investigatedMicroseepage is the main mode for CH4 and CO2 emissions from the mud volcanoes; methane output from the mud volcanoes is comparable to that from other mud volcanoes worldwide; the high output of CO2 suggests that CO2 production by mud volcanoes requires more attentionRegional compression might be the major triggering mechanism for CH4 and CO2 emissions from mud volcanoes [ABSTRACT FROM AUTHOR]

The morphology of mud volcanoes (MVs) has been extensively studied over the last few decades. Although recent research has begun to focus on deep processes and structures, little is known about mud generation mechanisms. This study aims to investigate the feeder system and formation of an active kilometer‐scale MV by relying on a 3‐D seismic survey and an in situ data set on the Absheron anticline (South Caspian Basin). Seismic data show a depleted area in the Anhydritic Surakhany Formation (ASF), whose mineralogical composition fits with surface mud. Well data show that the ASF is a succession of evaporitic beds and low‐temperature shales near its fracture pressure. Biostratigraphic analysis confirms a Pliocene origin for the mud, suggesting that the ASF may be the source. Numerical modeling of sedimentation coupled with laboratory test results and well sonic logs fairly reproduces the observed in situ overpressure trend. Two‐dimensional methane diffusion coupled with overpressure caused by rapid sedimentation highlights the superposition of critical fracturing conditions with methane‐saturated sediments at the base of the studied MV. The present study demonstrates the predominant role of fluid overpressure due to sedimentation and gas saturation in the formation of the Absheron MV, and this is shown to occur as follows: (1) methane migration through the thrust‐related faults reaching the ASF, accompanied by (2) lateral overpressure, caused by rapid sedimentation, diffusing along the ASF leading to (3) hydro‐fracturing of overpressured and methane‐saturated sediments resulting in an important decrease in overpressure, causing (4) gas exsolution and expansion triggering sediment remobilization. Plain Language Summary: Mud volcanoes have proved to be a real risk for people living near them (e.g., Lusi catastrophe, Indonesia) and for infrastructures. While their surface morphology is well understood and their plumbing system is correctly imaged by high‐resolution seismic technology, their formation mechanisms and their trigger is yet to be understood. The South Caspian Basin is known for the presence of a large number of active structures. On the Absheron anticline, a giant active mud volcano is surrounded and covered by a large data set from seismic imaging to sediment cores. From the analysis of the seismic data, and from diverse measurements on sediments, we were able to locate the source of the mud, which is shallower than the source for many mud volcanoes in the region. We developed a numerical model able to reproduce the pore pressure trends recorded at the wells, and it shows that the interaction between methane‐saturated areas and potential rupture zones is able to explain the mud volcano location. Finally, based on the different results and observations, a formation model is proposed where hydrofracturing is the initial trigger for the subsequent gas exsolution and expansion that disaggregate and remobilize weak sedimentary layers. Key Points: The Absheron mud volcano, unlike many mud volcanoes in the South Caspian Basin, is not sourced from the deep Maykop Formation but from the Upper Productive SeriesThe developed numerical model shows that an interaction between methane‐saturated areas and potential rupture zones allows locating the mud volcano formationA formation model is proposed where hydrofracturing is the initial trigger for the subsequent gas exsolution and expansion that disaggregate and remobilize weak sedimentary layers [ABSTRACT FROM AUTHOR]

In this study, we report lipid biomarker patterns and phylogenetic identities of key microbial communities mediating anaerobic oxidation of methane (AOM) in active mud volcanoes (MVs) on the continental slope of the Canadian Beaufort Sea. The carbon isotopic compositions (δ13C) of sn -2- and sn -3-hydroxyarchaeol showed the highly 13C -depleted values (- 114 ‰ to - 82 ‰) associated with a steep depletion in sulfate concentrations within 0.7 m of sediment depths. This suggested the presence of methanotrophic archaea involved in sulfate-dependent AOM, albeit in a small amount. The ratio of sn -2-hydroxyarchaeol to archaeol (> 1) and operational taxonomic units (OTUs) indicated that the anaerobic methanotrophic archaea (ANME) clades ANME-2c and ANME-3 were involved in AOM. Higher δ13C values of archaeol and biphytanes (BPs; -55.2±10.0 ‰ and -39.3±13.0 ‰, respectively) suggested that archaeal communities were also assimilating AOM-derived inorganic carbon. Furthermore, the distinct distribution patterns of methanotrophs in the three MVs appears to be associated with varying intensities of ascending gas fluids. Consequently, our results suggest that the niche diversification of active mud volcanoes has shaped distinct archaeal communities that play important roles in AOM in the Beaufort Sea. [ABSTRACT FROM AUTHOR]

Highlights • 13CDH 3 and 12CD 2 H 2 were measured independently on a Panorama mass spectrometer. • 13CDH 3 and 12CD 2 H 2 geothermometers give CH 4 formation temperature of 150 °C. • Thermogenic CH 4 is not in thermal equilibrium with hosting liquid mud. Abstract Resolved measurements of the relative abundances of 13CDH 3 and 12CD 2 H 2 from a Taiwan mud volcano confirm that thermogenic CH 4 with a formation temperature of 150 °C is bubbling from a pool of liquid mud. Analysis for both doubly-substituted molecules provides confirmation that isotopic exchange equilibrium was achieved between methane isotopologues. Methane extracted from the headspace of core samples taken from sediments deposited proximal to the bubbling pool of mud shows small departures from isotopic equilibrium. [ABSTRACT FROM AUTHOR]

In the present study, some mud volcanoes in Pingtung and Kaohsiung areas were selected. The selected mud volcanoes are located above the mud diaper system in southern Taiwan. Three different sampling techniques were used to collect samples in and around the said mud volcanoes. The results show that river water samples from different places fall on the meteoric line. Mud samples are found to be heavier in oxygen isotope while hydrogen isotope remains the same. Soil gas samples were also collected at the depth of 1 m and dissolved gases from the mud volcanic site (or nearby) were collected for gas chromatography analyses. Soil gas samples and dissolved gas from mud pool and groundwater were analyzed for radon concentration using RAD7 (solid-state nuclear track detector) bi-weekly. Long-term investigation will be needed to understand the relationship of mud volcanoes the eruption cycle with gas composition variations. It will also help to understand their relationship with tectonic activities in the region. [ABSTRACT FROM AUTHOR]

Mud volcanoes represent related formations, which have structural and functional similarities and can be found on land and in undersea zones. Mud volcanoes are different from other geological forms by their capacity to expell different forms of materia (gas, water and sediment) and the relatively violent kind of performing it. The sizes of mud volcanoes go from very small up to kilometers, shapes go from caldera to cones and they can exhibit from almost inaction to rising columns of fire. The mud consistency also differ, from aqueous to high viscosity, and the emissions can involve high quantities of methane and carbon dioxide. Volcanoes releasing thermogenic methane have the reservoirs at profound depths, while the ones with biogenic gas have the reservoirs no deeper than 2000 m. This review provides informations regarding genesis and distribution of mud volcanoes, and also the description of morphology and topology of the mud volcano and the adjacent area. The classification types are set out, as well as the phenomenon of mud volcanism. In the end the expelled products are identified, different post-genetic processes being established, thus it can be assessed if the expelled products are of thermogenic or biogenic nature. Microbiological processes within mud volcanoes were also described from anaerobic oxidations, aerobic oxidations to methanogenesis. Both, Archaea and Bacteria are involved in these prcesses. Common patterns were identified for mud volcanoes which harbour a large array of bacterial and archaeal phyla. Mud volcanoes have a consistent share in the natural gass emissions into the atmosphere. They have a global occurance and give us a different way to investigate the Earth's interior, yet are largely unresearched. [ABSTRACT FROM AUTHOR]

Methane release from the seafloor is commonly associated with chemosynthesis-based cold seep ecosystems that facilitate the precipitation of authigenic carbonates. It has been proposed that carbonate growth results in self-sealing, but little is known regarding the evolution of cold seep structures in relation to fluid migration pathways. This study investigates structures resulting from gas seepage along ring faults peripheral to Venere mud volcano (1600 m water depth), based on multibeam bathymetry and seafloor backscatter data collected by an autonomous underwater vehicle, together with photomosaics, video observations, and samples obtained by a remotely operated vehicle. Sites of focused fluid flow are identified by gas bubble streams rising from the seafloor while anaerobic oxidation of methane (AOM) over wider areas is indicated by the occurrence of chemosynthesis-based organisms (microbial mats, vesicomyid clams, vestimentiferan tube worms). At some sites, flakes of gas hydrate were observed in the water column during sampling. A range of carbonate structures exists at these sites: 1) flat and extensive pavements; 2) mounds with disseminated nodules or centimeter-thick crusts; 3) fractured mounds with exposed, decimeter-thick crusts; and 4) seafloor depressions lined by decimeter-thick crusts. The mineralogy and stable carbon isotopic compositions of the carbonates are consistent with anaerobic oxidation of methane from thermogenic sources, and possible near-seabed influence of gas hydrate formation and dissolution. The seafloor expressions of seepage are inferred to be controlled by the interaction of fluid flow due to carbonate precipitation and gas hydrate formation. A conceptual model for mound development is proposed in the context of the known timescales of seep-colonization and rates of carbonate precipitation: (A) the onset of hydrocarbon-rich fluid seepage through hemipelagic sediments leads to (B) the establishment of microbial mats on flat seafloor over decadal timescales and is followed by (C) the growth of pavements cemented by carbonates that seal the seafloor; over longer timescales (centuries to millennia), carbonate growth and subsurface gas hydrate formation/dissolution lead to (D) upward doming and fracturing of carbonate mounds, re-sealing and stacking of carbonates and in some cases to (E) their collapse to form seafloor depressions. Gas migration through fractures in the carbonates allows re-sealing and fuels AOM to provide habitats for chemosynthesis-based fauna. This evolutionary scenario is argued to be broadly applicable to the development of ruptured mounds and collapse features described at other seepage sites in the Mediterranean Sea and elsewhere. [ABSTRACT FROM AUTHOR]

In this study, we report lipid biomarker patterns and phylogenetic identities of key microbes mediating anaerobic oxidation of methane (AOM) communities in active mud volcanos (MVs) on the continental slope of the Canadian Beaufort Sea. The enriched δ13C values of total organic carbon (TOC) as well as lipid biomarkers such as archaeol and biphytanes (BPs) relative to δ13CCH4 values suggested that the contribution of AOM-related biomass to sedimentary TOC was in general negligible in the Beaufort Sea MVs investigated. However, the δ13C values of sn-2- and sn-3-hydroxyarchaeol were more negative than CH4, indicating the presence of AOM communities, albeit in a small amount. The ratio of sn-2-hydroxyarchaeol to archaeol and the 16S rRNA results indeed indicated that archaea of the ANME-2c and ANME-3 clades were involved in AOM. Further studies are needed to investigate the diversity and distribution of AOM communities and to characterize their habitats in the uppermost surface sediments of Beaufort Sea MV systems. [ABSTRACT FROM AUTHOR]

Methane emissions out of the seabed could seriously affect Earth's climate and are usually associated with the dissociation of gas hydrates stored in marine sediments on the continental margins. Spatially, gas emissions out of the seafloor are not evenly distributed in continental margins. Gas emissions out of the seabed generally occur through submarine mud volcanoes and gas seeps. To understand the seabed gas emissions off SW Taiwan, we investigate the distributions of active submarine mud volcanoes, gas seeps, and gas plumes off SW Taiwan. We examine all of the available sub-bottom profiler and EK echo sounder data. We identified 19 submarine mud volcanoes, 220 gas seeps, and 295 gas plumes. The gas emissions are generally distributed at the crests of mud diapiric ridges. Most of the active mud volcanoes and gas seeps cluster at the KASMVG (Kaoping submarine mud volcanoes group) area. We speculate that the intensive mud volcanism and gas seepage at the KASMVG area are ascribed to submarine channel erosion along the continental slope base. The erosion causes a deep V-shaped channel and a steep BSR (Bottom-Simulating Reflector) slope curve across the continental margin. The upward migration rate of free gas beneath the BSR is thus increased and intensifies mud volcanism and gas seepage at the KASMVG area. The gas seeps can reduce the slope stability and generate small-scale slides. The development of mud volcanoes in an area could effectively disturb the seabed morphology so that large-scale submarine landslides cannot easily happen. [ABSTRACT FROM AUTHOR]

Natural hydrocarbon seepage is a widespread phenomenon in sedimentary basins, with important implications in petroleum exploration and emission of greenhouse gases to the atmosphere. China has vast petroleum (oil and gas) bearing sedimentary basins, but hydrocarbon seepage has rarely been the object of systematic studies and measurements. Based on the available Chinese literature, we report a first inventory of 932 hydrocarbon seeps or seepage zones (710 onshore seeps and 222 offshore seeps), including 81 mud volcanoes, 449 oil seeps, 215 gas seeps, and 187 solid seeps (bitumen outcrops). The seeps are located within the main 20 Mesozoic-Cenozoic petroliferous sedimentary basins, especially along the marginal, regional and local faults. The type of manifestations (oil, gas or mud volcano) reflects the type and maturity of the subsurface petroleum system and the sedimentary conditions of the basin. Oil seeps are particularly abundant in the Junggar Basin. Gas seeps mostly developed in the Lunpola Basin, in smaller basins of the eastern Guizhou and Yunnan provinces, onshore Taiwan and in the offshore Yinggehai Basin. Mud volcanoes developed in basins (Junggar, Qaidam, Qiangtang, onshore and offshore Taiwan) that experienced rapid sedimentation, which induced gravitative instability of shales and diapirism. In comparison to available global onshore seep data-bases, China results to be the country with the highest number of seeps in the world. The massive gas seepage in China could represent a considerable natural source of methane to the atmosphere, and a key process that may drive future hydrocarbon exploration. [ABSTRACT FROM AUTHOR]

Romania is one of the countries with the largest number of surface hydrocarbon seeps in the world. Seeps may be an important tool for petroleum exploration as they can provide useful information regarding source rock maturity, reservoir quality, and secondary gas alterations. Seeps also represent an important source of methane, ethane, and propane to the atmosphere. To date, the genetic characterization of natural gas in Romania has only been based on molecular composition, without isotopic information. Here, we present an overview of investigations performed over the past 15 years for the main Romanian hydrocarbon seeps, and report the molecular and isotopic compositions of gas, and the fluxes of methane, ethane and propane to the atmosphere. We assessed gas origin and secondary alterations in 17 seeps from several Romanian petroleum systems, and potential source rock types and maturity have been evaluated. As previously inferred, gas within the Transylvanian Basin is largely microbial, but also displays indications of a minor thermogenic component that is likely related to a deep petroleum system. Carpathian Flysch and Foredeep petroleum systems contain thermogenic gas, with clear evidence of biodegradation in some cases. Thermogenic gas generation modelling and maturity plots suggest that most Romanian gases originate from mature type II and III kerogen (%R o : 2–3). For cases of high flux seeps, gas has the same hydrocarbon molecular composition as the reservoir, while in weaker seeps and some mud volcanoes gas is altered by molecular fractionation (a loss of C 2 and C 3 during gas migration). Gas seep geochemistry, in general, reflects the different geological and maturity conditions of basins where seeps are located. A vertical sequence of petroleum systems has been suggested in some basins by seeps displaying different maturity and secondary alterations. Measurements of methane flux to the atmosphere from 94 seeps display a wide range of emissions (kilograms to hundreds of tons per year), with a total, conservative estimated methane emission of approximately 3000 t y −1 . Microseepage may also release a similar quantity of methane. Consequently, seepage is a substantial contributor to natural emissions of methane on a national level. [ABSTRACT FROM AUTHOR]

The Dongsha Basin, circling Dongsha Island that is amid the northern margin of the South China Sea, is characterized by thin (∼0.5 km) Cenozoic sediments veneering on thick (up to 5 km) Mesozoic strata. Recently, several geophysical and geological surveys, including multiple channel reflection seismic, sub-bottom profiling and benthic dredging, have been conducted on the slope southwest to the Dongsha Island, where the water depth varies from 400 m to 2000 m. A novel discovery is numerous submarine mud volcanoes of various sizes over there, typically 50–200 m high and 0.5–5 km wide. Geophysical profiles document their unusual features, e.g., roughly undulating seafloor, high-amplitude seabed reflectivity, foggy hyperbolic diffractions up to 50 m in water column above seabed, and internal reflection chaos and wipe-out down to 2–3 km level or deeper below the seabed. Benthic dredging from the mud volcanoes gives abundant faunas of high diversity, e.g., scleractinian (stony coral), gorgonian, black coral, thiophil tubeworm, glass sponge, bryozoan etc., indicating booming chemosynthetic community, among which the Lophelia pertusa -like coral and the Euretidae -like glass sponges are the first reports in the South China Sea. Concomitantly with them, there are also abundant authigenic carbonate nodules and slabs, raw, brecciated and breccias with bio-clasts congregation. Besides, there coexist massive mudflows and allogenic coarse-grained quartz, feldspar and tourmaline most likely brought out by mud volcanism. Geochemical analysis of the bottom water samples give dissolved methane concentration up to 4 times higher than the background average. These results lend comprehensive evidences for the ongoing and historical mud volcanism. The escaping methane gas is inferred to source mainly from the Mesozoic strata. Occupying a large province of the deep water slope, ca. 1000 km 2 or more, the mud volcanoes is prospective for gas hydrate and natural gas for the Dongsha Basin. [ABSTRACT FROM AUTHOR]

There are many mud volcanoes in the southern margin of the Junggar Basin, northwest China, of which the Dushanzi area is the most typical and active one, emitting large amount of greenhouse gases associated with water and mud. The emitted gas is dominated by methane (average 90.1%), together with other gases, such as ethane (4.84–5.46%), propane (0.06–0.90%), CO 2 (0.67–1.0%), and N 2 (2.8–3.3%). The carbon (δ 13 C 1 ) and hydrogen (δD) isotopic ratios of methane are in the ranges of −40.6‰ to −45.0‰ and −221‰ to −249‰, respectively, whereas carbon isotope ratios of ethane (δ 13 C 2 ) are −25.2‰ to −27.6‰. Based on δ 13 C values, the released gas is characterized as a thermogenic coal-type and possibly originated from the middle-low Jurassic coal-bearing sequences according to the gas-source correlation and regional geology. Helium isotopes show a crustal source. The methane flux of Dushanzi mud volcanoes from both macro-seepage (craters/vents) and micro-seepage (ground soil exhalation) ranged over the orders of magnitude, from 0.4–2.7 kg d −1 and 4950 mg m −2 d −1 on average, respectively. Positive CH 4 fluxes from dry soil were widespread throughout the investigated areas. The total CH 4 emission from Dushanzi mud volcanoes is estimated to be at least 22.6 tons a −1 , of which about 89% is from micro-seepage surrounding the mud volcano vents. [ABSTRACT FROM AUTHOR]

Shikhzarli is a typical active mud volcano (MV) in Azerbaijan with extremely high percentages of fire eruptions reaching 74%. The strong eruption of Shikhzarli MV on 13 March 2011 (Shamakhi-Gobustan region, central Azerbaijan, at 40°29′12.5′′N; 49°02′03.4′′E) was accompanied by gas ignition with a straight-flow vertical gas flare. A large (60 m height) short-living (30 min) gas flare and small (1.5 m height) gas spouts with longer lifetimes (5 h) were simulated in SigmaFlow, and the model was used to evaluate methane flux and the amount of combusted methane. Geological and mineralogical data, field observations, as well as eye witness reports and photographs in mass media were used to choose the boundary conditions for the gas fire simulation. This study provides the first quantitative evaluation for methane emission during an MV catastrophic eruption. As simulation predicts, 14,000 m of methane was consumed for the life time (30 min) of the main 60-m high flare, which is equivalent to nine tons of cold gas at ambient conditions; the volume of erupted methane for the 5-h lifetime of a small spout (1.5 m high) was 72.6 m (45 kg) of cold CH. The amount of methane emitted during a single strong flame eruption is commensurate with the annual output of permanently active individual MV vents or may be ten to hundred times as high. [ABSTRACT FROM AUTHOR]

Submarine mud volcanoes (SMVs) are formed by muddy sediments and breccias extruded to the seafloor from a source in the deep subseafloor and are characterized by the discharge of methane and other hydrocarbon gasses and deep-sourced fluids into the overlying seawater. Although SMVs act as a natural pipeline connecting the Earth's surface and subsurface biospheres, the dispersal of deep-biosphere microorganisms and their ecological roles remain largely unknown. In this study, we investigated the microbial communities in sediment and overlying seawater at two SMVs located on the Ryukyu Trench off Tanegashima Island, southern Japan. The microbial communities in mud volcano sediments were generally distinct from those in the overlying seawaters and in the well-stratified Pacific margin sediments collected at the Peru Margin, the Juan de Fuca Ridge flank off Oregon, and offshore of Shimokita Peninsula, northeastern Japan. Nevertheless, in-depth analysis of different taxonomic groups at the sub-species level revealed that the taxon affiliated with Atribacteria, heterotrophic anaerobic bacteria that typically occur in organic-rich anoxic subseafloor sediments, were commonly found not only in SMV sediments but also in the overlying seawater. We designed a new oligonucleotide probe for detecting Atribacteria using the catalyzed reporter deposition-fluorescence in situ hybridization (CARD-FISH). CARD-FISH, digital PCR and sequencing analysis of 16S rRNA genes consistently showed that Atribacteria are abundant in the methane plumes of the two SMVs (0.58 and 1.5 × 104 cells/mL, respectively) but not in surrounding waters, suggesting that microbial cells in subseafloor sediments are dispersed as "deep-biosphere seeds" into the ocean. These findings may have important implications for the microbial transmigration between the deep subseafloor biosphere and the hydrosphere. [ABSTRACT FROM AUTHOR]

Seeps are the expression of the migration of hydrocarbons from subsurface accumulations to the surface in sedimentary basins. They may represent an important indication of the presence of petroleum (gas and oil) reservoirs and faults, and are a natural source of greenhouse gas (methane) and atmospheric pollutants (ethane, propane) to the atmosphere. Romania is one of the countries with the largest number of seeps in the world, due to the high petroleum potential and active tectonics. Based on a review of the available literature, and on the field surveys performed by the authors during the last 17 years, we report the first comprehensive GIS-based inventory of 470 seeps in Romania (HYSED-RO), including gas seeps (10.4% of the total), oil seeps (11.7%), mud volcanoes (50.4%), gas-rich springs (12.6%), asphalt (solid) seeps (4.3%), unclassified manifestations (4.0%), and uncertain seeps (6.6%). Seeps are typically located in correspondence with major faults and vertical and fractured stratigraphic contacts associated to petroleum reservoirs (anticlines) in low heat flow areas, and their gas-geochemistry reflects that of the subsurface reservoirs. The largest and most active seeps occur in the Carpathian Foredeep, where they release thermogenic gas, and subordinately in the Transylvanian Basin, where gas is mainly microbial. HYSED-RO may represent a key reference for baseline characterization prior to subsurface petroleum extraction, for environmental studies, and atmospheric greenhouse gas emission estimates in Romania. [ABSTRACT FROM AUTHOR]

Mud volcanism, or sedimentary volcanism, represents one of the most intriguing phenomena of the Earth's crust, with important implications in energy resource exploration, seismicity, geo-hazard and atmospheric budget of greenhouse gases. Since the first review papers were issued at the beginning of 2000s, a large amount of new geological, geophysical and geochemical data has been acquired, which clarified ambiguous concepts and significantly improved our knowledge of mud volcanism. Here, we offer an updated review of the knowledge and implications of mud volcanoes, with emphasis on: the terminology used to describe different processes and structures; the physical, chemical and morphological characteristics of the several fluid emission structures; the chemical properties of the released fluids, in particular the molecular and isotopic composition of the gas; the mud volcano formation dynamics; and the several implications for petroleum exploration, geo-hazards and global atmospheric methane budget. This review integrates new fluids data collected in Azerbaijan and is complemented with field observations from various mud volcano provinces worldwide. Although the total number of mud volcanoes on Earth is still uncertain, > 600 main onshore structures, with a large variety in shapes and sizes, are documented in recent global data-sets, and several thousand are assumed to exist in the oceans. It is clear that: (a) mud volcanoes are broadly distributed throughout the globe in active margins, compressional zones of accretionary complexes, thrust and overthrust belts, passive margins, deep sedimentary basins related to active plate boundaries, as well as delta regions; (b) they are specifically located in hydrocarbon bearing basins, along anticline axes, strike slips and normal faults, and fault-related folds in Petroleum Systems; (c) they represent a specific category of natural gas/oil seepage manifestation, often related to deep and pressurised reservoirs; (d) the main engine driving mud volcanism is given by a combination of gravitative instability of shales and fluid overpressure build-up, followed by hydrofracturing; (e) hydrocarbons are generally of thermogenic origin, while microbial gas is released in only a few cases. Mud volcanism on other planets (e.g. Mars and Titan), and microbial activity associated with gas seepage represent emerging issues and opportunities for future research. [ABSTRACT FROM AUTHOR]

This paper reports the recent progress on mud volcanism data accumulation in the case of the Gulf of Cadiz area. The discovery of giant mud volcanoes, deep coral reefs, and gas hydrates in 1999, from the Guadalquivir Diapiric Ridge to the Larache Moroccan margin, launched a dynamic expansion of new projects (GeNesis, MoundForce, HERMES) and international oceanographic campaigns (R/V Sonne, Marion-Dufresne). The present monitoring of this Ibero-Moroccan oceanic zone is in need of a comprehensive database available in one site to make online search possible from a single interface. The database would constitute a reference point for a focused full scope collection.

Romania is one of the European countries with the most vigorous natural seepage of methane, uprising from pressurised natural gas and petroleum reservoirs through deep faults. The largest seepage zone is represented by large mud volcanoes, with CH4 >80% v/v, occurring on the Berca-Arbanasi hydrocarbon-bearing faulted anticline, in the Carpathian Foredeep. Smaller mud volcanoes have been identified in other areas of the Carpathian Foredeep, in the Transylvanian Depression and on the Moldavian Platform. New surveys carried out in Transylvania allowed us to discover the richest N2 mud volcano zone in the world (N2>90% v/v), with a remarkably high He content and a helium isotopic signature which highlights a contribution of mantle-derived source. The large mud volcanoes are generally quiescent, with rare explosive episodes and provide a methane flux in the order of 102-103 t km?2 y?1. Independently from mud volcanism, a remarkable dry macroseepage, however, has been found, with a degassing rate up to three orders of magnitude higher than that of mud volcanoes (i.e. 103-105 t km?2 y?1). The total gas flux from all investigated macroseepage zones in Romania is estimated in the range of 1500-2500 t y?1. The emission from microseepage, pervasively occurring throughout the hydrocarbon-prone basins, has yet to be assessed and added to the total gas output to the atmosphere.

Seeps are the expression of the migration of hydrocarbons from subsurface accumulations to the surface in sedimentary basins. They may represent an important indication of the presence of petroleum (gas and oil) reservoirs and faults, and are a natural source of greenhouse gas (methane) and atmospheric pollutants (ethane, propane) to the atmosphere. Romania is one of the countries with the largest number of seeps in the world, due to the high petroleum potential and active tectonics. Based on a review of the available literature, and on the field surveys performed by the authors during the last 17 years, we report the first comprehensive GIS-based inventory of 470 seeps in Romania (HYSED-RO), including gas seeps (10.4% of the total), oil seeps (11.7%), mud volcanoes (50.4%), gas-rich springs (12.6%), asphalt (solid) seeps (4.3%), unclassified manifestations (4.0%), and uncertain seeps (6.6%). Seeps are typically located in correspondence with major faults and vertical and fractured stratigraphic contacts associated to petroleum reservoirs (anticlines) in low heat flow areas, and their gas-geochemistry reflects that of the subsurface reservoirs. The largest and most active seeps occur in the Carpathian Foredeep, where they release thermogenic gas, and subordinately in the Transylvanian Basin, where gas is mainly microbial. HYSED-RO may represent a key reference for baseline characterization prior to subsurface petroleum extraction, for environmental studies, and atmospheric greenhouse gas emission estimates in Romania.

The Asian-Pacific - continental margins have been the object of intensive geochemical research in recent years, as areas within this zone have been found to contain large hydrocarbon deposits in a variety of forms including: oil, natural gas, gas condensate, and gas hydrates. One such area is Sakhalin Island, located in Russia's Far Eastern Federal District. Genetically, the territory of Sakhalin Island is a part of the shelf of the Hokkaido-Sakhalin fold area, therefore, the nature of the sub-vertical gas migration is controlled by deep faults, along which the concentrations of natural gases change, and is also influenced by high seismotectonic activity of the Okhotsk region. This paper presents data regarding the distribution of helium, hydrogen, carbon dioxide and methane from mud volcanoes, thermal springs, mineral waters and other seep types on Sakhalin island and the surrounding marine margins, along with an interpretation of the geological and tectonic factors that have likely led to the present distribution of these gases. In general, the results of geothermometry equations based on the concentration and stable isotopic composition of carbon dioxide indicate much higher temperatures than equations involving the saturation concentrations of hydrogen gas. We interpret this to indicate that deeply-sourced carbon dioxide does not reequilibrate at near-surface conditions, whereas the equilibration time for hydrogen gas is relatively rapid. Areas on the island which are observed to have active degassing, coupled with high apparent reservoir temperatures also provide indicators of geodynamics along the regional tectonic structures. Mud volcanoes to the south of the island are predominantly situated at the juncture between NW-SE trending faults which extend the entire length of Sakhalin and continue as far south as Hokkaido Island, and more recently developed NE-SW trending faults. Despite the relatively low water temperatures of these mud volcanoes, the high reservoir temperatures indicated by gas chemistry suggest a source of fluids and gas at several km from the surface. • Distribution of helium, hydrogen, carbon dioxide and methane on Sakhalin Island and the surrounding marine margins. • Geological and tectonic factors leading to helium, hydrogen, carbon dioxide and methane distribution on Sakhalin Island. • Areas of active degassing present high apparent reservoir temperatures as a consequence of regional tectonic structures. • The high reservoir temperatures in Sakhalin mud volcanoes suggest a source several km from the surface. [ABSTRACT FROM AUTHOR]

The Big Tarkhan cold spring is located within the Bulganak zone of mud volcanism in the northern Kerch Peninsula (Crimea). The spring waters mainly have Cl–HCO 3 –Na–Ca chemistry and temperatures from 18 to 23 °C. Active travertine deposition at the site produces abundant calcite and minor amounts of siderite, halite, tincalconite, trona, gaylussite, northupite and amorphous iron hydroxides. Calcite contains the impurities of 0.26–2.16 wt.% MgO, up to 0.87 wt.% FeO, 0.15–0.73 wt.% SrO, 0.28–0.98 BaO, up to 0.43 wt.% MnO, and 0.09–0.60 Na 2 O. Active travertines are depleted in REE (ΣREE = 2.6–4.8 ppm) compared to the inactive ones. The Kerch travertine calcite shows unusual carbon and oxygen isotope compositions (+ 8.1 ÷ + 12.5‰ VPDB δ 13 C and + 10.1 ÷ + 12.9‰ VPDB δ 18 O). Their isotopic and trace element signatures (including REE patterns) suggest their relation to basinal waters and origin from the organic-rich clayey Maikop Fm., which is the principal source rock of the area. The Big Tarkhan travertine deposits are associated with thermogenic methane production. [ABSTRACT FROM AUTHOR]

The Shenhu Area of the northern South China Sea (SCS) has been regarded as a promising area for gas hydrate exploration. In this research, the petrography and stable isotopic composition of authigenic carbonates within a core of shallow sediment from Site 08CF7 in the Shenhu area have been studied. Based on the results of grain sizes, magnetic susceptibility, carbonate contents, textural characteristics, the age constrains of the core and the tectonic setting of the study area, it can be inferred that the core may represent the subsurface sediment in a mud volcano area within the gas hydrate system. Core sediments below 188 cm in the core may be associated with the in situ mud volcano activity. The authigenic carbonates (calcite, aragonite and dolomite) in fine size fraction below 188 cm indicate the mud volcano-induced methane seepage and anaerobic oxidation of methane (AOM), and their carbon source was mainly biogenic methane. In addition, the variation in magnetic susceptibility corresponds with in situ mud volcano sediments and associated methane seepage. These signals provide new methods of identifying AOM and application prospect for the exploration of seepage gas hydrates in SCS. [ABSTRACT FROM AUTHOR]

The Menes caldera is a fault-controlled depression (~8 km in diameter) at ~3,000 m water depth in the western province of the Nile deep-sea fan off NW Egypt, comprising seven mud volcanoes (MVs) of which two are active. Based on multichannel and chirp seismic data, temperature profiles, and high-resolution bathymetric data collected during the 2000 Fanil, 2004 Mimes and 2007 Medeco2 expeditions, the present study investigates factors controlling MV morphology, the geometry of feeder channels, and the origin of emitted fluids. The active Cheops and Chephren MVs are 1,500 m wide with subcircular craters at their summits, about 250 m in diameter, generally a few tens of metres deep, and filled with methane-rich muddy brines with temperatures reaching 42 °C and 57 °C respectively. Deployments of CTDs and corers with attached temperature sensors tracked these warm temperatures down to almost 0.5 km depth below the brine lake surface at the Cheops MV, in a feeder channel probably only a few tens of metres wide. Thermogenic processes involve the dissolution of Messinian evaporites by warm fluids likely sourced even deeper, i.e. 1.7 and 2.6 km below the seabed at the Cheops and Chephren MVs respectively, and which ascend along listric faults. Seepage activity appears broadly persistent since the initiation of mud volcanism in the Early Pliocene, possibly accompanied by lateral migration of feeder channels. [ABSTRACT FROM AUTHOR]

On the passive margin of the Nile deep-sea fan, the active Cheops mud volcano (MV; ca. 1,500 m diameter, ~20-30 m above seafloor, 3,010-3,020 m water depth) comprises a crater lake with hot (up to ca. 42 °C) methane-rich muddy brines in places overflowing down the MV flanks. During the Medeco2 cruise in fall 2007, ROV dives enabled detailed sampling of the brine fluid, bottom lake sediments at ca. 450 m lake depth, sub-surface sediments from the MV flanks, and carbonate crusts at the MV foot. Based on mineralogical, elemental and stable isotope analyses, this study aims at exploring the origin of the brine fluid and the key biogeochemical processes controlling the formation of these deep-sea authigenic carbonates. In addition to their patchy occurrence in crusts outcropping at the seafloor, authigenic carbonates occur as small concretions disseminated within sub-seafloor sediments, as well as in the bottom sediments and muddy brine of the crater lake. Aragonite and Mg-calcite dominate in the carbonate crusts and in sub-seafloor concretions at the MV foot, whereas Mg-calcite, dolomite and ankerite dominate in the muddy brine lake and in sub-seafloor concretions near the crater rim. The carbonate crusts and sub-seafloor concretions at the MV foot precipitated in isotopic equilibrium with bottom seawater temperature; their low δC values (-42.6 to -24.5‰) indicate that anaerobic oxidation of methane was the main driver of carbonate precipitation. By contrast, carbonates from the muddy lake brine, bottom lake concretions and crater rim concretions display much higher δC (up to -5.2‰) and low δO values (down to -2.8‰); this is consistent with their formation in warm fluids of deep origin characterized by C-rich CO and, as confirmed by independent evidence, slightly higher heavy rare earth element signatures, the main driver of carbonate precipitation being methanogenesis. Moreover, the benthic activity within the seafloor sediment enhances aerobic oxidation of methane and of sulphide that promotes carbonate dissolution and gypsum precipitation. These findings imply that the coupling of carbon and sulphur microbial reactions represents the major link for the transfer of elements and for carbon isotope fractionation between fluids and authigenic minerals. A new challenge awaiting future studies in cold seep environments is to expand this work to oxidized and reduced sulphur authigenic minerals. [ABSTRACT FROM AUTHOR]

Two major groups of the lithosphere sources of methane are distinguished, i.e., the lithosphere sources proper (metamorphism, and serpentinization and spreading zones) and organic fuel deposits in the Earth's crust. Hydrocarbon deposits, mud volcanoes, geothermal systems, and mineral sources are located on Sakhalin Island. All investigated objects are associated with large tectonic faults: the Yuzhno-Sakhalinsky and Main Pugachiovsky mud volcanoes are controlled by the Central Sakhalin fault; the Shakhterskoe coal deposit is confined to the West Sakhalin fault, while the Daginskaya geothermal system and the studied oil- and-gas fields are associated with the Hokkaido-Sakhalin fault. The authors' studies of natural gases allow allocating two basic zones on the Sakhalin island with different types of hydrocarbon-carbon dioxide sources: Oil-and-gas (mostly methane; north-east) area and Coal-Gas area (carbon-dioxide-methane; west, south-west). Those areas differ in both chemical and isotope composition of natural gases. Some gashazardous areas have also been identified along tectonic faults, where they are due to gas release on the land surface. The populated localities on Sakhalin are shown to be situated near fault zones; therefore, the problem of gas hazard control is of critical importance. [ABSTRACT FROM AUTHOR]

Small mud volcanoes (cold seeps), which are common in the floodplains of northern rivers, are potentially important (although poorly studied) sources of atmospheric methane. Field research on the cold seeps of the Mukhrina River (Khanty-Mansiysk Autonomous okrug, Russia) revealed methane fluxes from these structures to be orders of magnitude higher than from equivalent areas of the mid-taiga bogs. Microbial communities developing around the seeps were formed under conditions of high methane concentrations, low temperatures (3-5°C), and near-neutral pH. Molecular identification of methane-oxidizing bacteria from this community by analysis of the pmoA gene encoding particulate methane monooxygenase revealed both type I and type II methanotrophs (classes Gammaproteobacteria and Alphaproteobacteria, respectively), with prevalence of type I methanotrophs. Among the latter, microorganisms related to Methylobacter psychrophilus and Methylobacter tundripaludum, Crenothrix polyspora (a stagnant water dweller), and a number of methanotrophs belonging to unknown taxa were detected. Growth characteristics of two methanotrophic isolates were determined. Methylobacter sp. CMS7 exhibited active growth at 4-10°C, while Methylocystis sp. SB12 grew better at 20°C. Experimental results confirmed the major role of methanotrophic gammaproteobacteria in controlling the methane emission from cold river seeps. [ABSTRACT FROM AUTHOR]