Baciu, Calin

Mud volcanoes represent the largest expression of natural methane release into the atmosphere; however, the gas flux has never been investigated in detail. Methane output from vents and diffuse soil degassing is herewith reported for the first time. Measurements were carried out at 5 mud volcano fields around Sicily (Italy). Each mud volcano is characterized by tens of vents and bubbling pools. In the quiescent phase, methane emission from single vents ranges between 0.01 and 6.8 kg/day. Diffuse soil leakage around the vents is in the order of 102–104 mg m 2 d 1. An exceptional flux of 106 mg m 2 d 1 was recorded close to an everlasting fire. Soil CH4 flux is positive even at large distances from the mud volcano fields suggesting a diffuse microseepage over wider areas. A total of at least 400 tons CH4 per year can be estimated over the area investigated alone ( 1.5 km2).

On Earth, burial of fine-grained sediments in offshore passive margins (e.g., underwater fans and deltas) commonly results in fluid expulsion features including large-scale polygonal fractures, mud volcanoes, and pockmarks. Comparison of resulting offshore polygons and mud volcanoes with giant polygons and high-albedo mounds in the Chryse–Acidalia region of Mars shows the terrestrial and martian features to be similar in size, morphology, geologic context, and general co-occurrence within the same basin. These similarities suggest that the process of terrestrial fluid expulsion may provide an analog that could link the giant polygons and mounds in Chryse and Acidalia to a single process. Moreover, while the terrestrial offshore polygons and mud volcanoes commonly develop in the same basins, these features do not necessarily occur in exactly the same locations within those basins, as they are independent responses to compaction and dewatering. Thus, the fluid expulsion analog does not require that the martian giant polygons and mounds have identical distributions. This is the situation in Chryse and Acidalia where the giant polygons and mounds are extensively developed and generally have overlapping distributions, but where each set of features may occur in places without the other. This fluid expulsion analog is enhanced by the fact that giant polygons and mounds in Chryse and Acidalia cooccur in a regional sense and in a geologic setting that is consistent with a fluid expulsion model of formation. Implications of this analog may impact our view of the role of water in the depositional history of the martian lowlands.

Ciomadul is the youngest volcano in the Carpathian‐Pannonian Region, Eastern‐Central Europe, which last erupted 30 ka. This volcano is considered to be inactive, however, combined evidence from petrologic and magnetotelluric data, as well as seismic tomography studies, suggests the existence of a subvolcanic crystal mush with variable melt content. The volcanic area is characterized by high CO2 gas output rate, with a minimum of 8.7 × 103 t/year. We investigated 31 gas emissions at Ciomadul to constrain the origin of the volatiles. The δ13C–CO2 and 3He/4He compositions suggest the outgassing of a significant component of mantle‐derived fluids. The He isotope signature in the outgassing fluids (up to 3.10 Ra) is lower than the values in the peridotite xenoliths of the nearby alkaline basalt volcanic field (R/Ra 5.95 Ra ± 0.01), which are representative of a continental lithospheric mantle and significantly lower than MORB values. Considering the chemical characteristics of the Ciomadul dacite, including trace element and Sr–Nd and O isotope compositions, an upper crustal contamination is less probable, whereas the primary magmas could have been derived from an enriched mantle source. The low He isotopic ratios could indicate a strongly metasomatized mantle lithosphere. This could be due to infiltration of subduction‐related fluids and postmetasomatic ingrowth of radiogenic He. The metasomatic fluids are inferred to have contained subducted carbonate material resulting in a heavier carbon isotope composition (δ13C is in the range of −1.4‰ to −4.6‰) and an increase of CO2/3He ratio. Our study shows the magmatic contribution to the emitted gases.

Gas migration in the geosphere is a widespread process, that occurs in numerous geological environments. The most important gases taken into account are CO2, generally present in volcanic and geothermal areas, ans CH4, mainly related to hydrocarbon-prone areas. Diffusion and groundwater flow were traditionally considered as the main mechanisms responsible for the gas migration. However, this approach failed to explain the behaviour of gases in particular situations, such as the long distance transport of radon, or the rapid variations of hydrocarbon anomalies in soil. The "geogas" integrative theory represents a contribution of the last decades, proposing a re-evaluation of several concepts of gas migration in the Earth's crust. Various mechanism of gas migration are described in the present article. The importance of rapid advection and the capabilities of microbubble transport are highlighted.

The paper ‘‘Methane emission from the mud volcanoes of Sicily (Italy)’’ by Etiope et al. [2002] represents the first report ever done on experimental CH4 output data from subaerial mud volcanoes (MV). A review of available CH4 flux data and detailed discussion about the global implications of mud volcanic CH4 emission has been made elsewhere [Etiope and Klusman, 2002; Morner and Etiope, 2002]. [2] The comment by Kopf [2003] contributes to open discussions and to make the readership aware on how important this subject is. In this reply we wish to clarify that precise data of CH4 flux from geologic sources are beginning to be available only now. It would be opportune that the MV-expert community could agree in using a common unit for the gas flux. We propose t y 1 and Mt y 1, and not metres cubed, consistently with the data reported for the methane sources/sinks budget by the IPCC. [3] Sicilian MVs, the first to be measured in detail, are considerably much smaller than the Azeri Ashgil MV, mentioned by Kopf [2003], and it is therefore obvious to expect a lower gas flux. Anyway the Dashgil mud volcano flux data are not based on exact measurements but only on visual estimates of the bubbles [Hovland et al., 1997]. In order to fully reply to Kopf [2003], hereafter we briefly discuss the problem of how to estimate the total number of MVs in the world and present new data from other European MVs, recently investigated. Finally, we outline the global importance of mud volcanic CH4 emission, as Kopf [2003] and recent literature is stressing.

The paper describes a case of a natural emission of methane from soil in an urban development area, generating a significant risk for the local population and buildings, due to gas explosiveness and asphyxiation potential. The site is located on the south-western margin of the East-European Platform in eastern Romania, in a hydrocarbon-prone area crossed by the Pericarpathian lineament and regional faults. Molecular composition of gas and stable isotopic analyses of methane (CH4>90%, δ to the power of 13 C1: -49.4‰, δD1: -173.4‰) indicate a dominant thermogenic origin, with significant amounts of C2-C5 alkanes (~5%), likely migrating through faults from a deep reservoir. Possible candidates are the Saucesti and Secuieni gas fields, located in the same petroleum system. Two surface geochemical surveys, based on closed-chamber flux measurements, were performed to assess the degassing intensity and the extent of the affected area. Methane fluxes from soil reach orders of 10 to the power of 4 mg m to the power of -2 day to the power of -1. Gas seepage mainly occurs in one zone 30 000 m2 wide, and it is likely controlled by channeling along a fault and gas accumulation in permeable sediments and shallow subsoil. The estimated total CH4 emission is about 40 t year to the power of -1 CH4, of which 8–9 t year to the power of -1 are naturally released from soil and 30–35 t year to the power of -1 are emitted from shallow boreholes. These wells have likely channeled the gas accumulated in shallow alluvial sediment but gas flux from soil is still high and mitigation measures are needed to reduce the risk for humans and buildings.

Methane (CH4) in terrestrial environments, whether microbial, thermogenic, or abiogenic, exhibits a large variance in C and H stable isotope ratios due to primary processes of formation. Isotopic variability can be broadened through secondary, post-genetic processes, such as mixing and isotopic fractionation by oxidation. The highest and lowest 13C and 2H (or D, deuterium) concentrations in CH4 found in various geologic environments to date, are defined as “natural” terrestrial extremes. We have discovered a new extreme in a natural gas seep with values of deuterium concentrations, δDCH4, up to+124‰that far exceed those reported for any terrestrial gas. The gas, seeping from the small Homorod mud volcano in Transylvania (Romania), also has extremely high concentrations of nitrogen (N92 vol.%) and helium (up to 1.4 vol.%). Carbon isotopes in CH4, C2H6 and CO2, and nitrogen isotopes in N2 indicate a primary organic sedimentary origin for the gas (a minor mantle component is suggested by the 3He/4He ratio, R/Ra~0.39). Both thermogenic gas formation modeling and Rayleigh fractionation modeling suggest that the extreme deuterium enrichment could be explained by an oxidation process characterised by a δDCH4 and δ13CCH4 enrichment ratio (ΔH/ΔC) of about 20, and may be accounted for by abiogenic oxidation mediated by metal oxides. All favourable conditions for such a process exist in the Homorod area, where increased heat flow during Pliocene–Quaternary volcanism may have played a key role. Finally we observed rapid variations (within 1 h) in C and H isotope ratios of CH4, and in the H2S concentrations which are likely caused by mixing of the deep oxidized CH4–N2–H2S–He rich gas with a microbial methane generated in the mud pool of one of the seeps. We hypothesize that the unusual features of Homorod gas can be the result of a rare combination of factors induced by the proximity of sedimentary organic matter, mafic, metal-rich volcanic rocks and salt diapirs,leading to the following processes: a) primary thermogenic generation of gas at temperatures between 130 and 175 °C; b) secondary alteration through abiogenic oxidation, likely triggered by the Neogene–Quaternary volcanism of the eastern Transylvanian margin; and c) mixing at the surface with microbial methane that formed through fermentation in the mud volcano water pool. The Homorod gas seep is a rare example that demonstrates how post-genetic processes can produce extreme gas isotope signatures (thus far only theorized), and that extremely positive δDCH4 values cannot be used to unambiguously distinguish between biotic and abiotic origin.

A global database of gas composition and methane stable isotopes of 143 terrestrial mud volcanoes from 12 countries and 60 seeps independent from mud volcanism from eight countries, was compiled and examined in order to provide the first worldwide statistics on the origin of methane seeping at the earth’s surface. Sixteen seep data were coupled with their associated subsurface reservoirs. The surface seepage data indicate that at least 76% of the mud volcanoes release thermogenic gas, with only 4% biogenic and 20% with mixed character. The average (201 data) of methane concentration and methane carbon isotope ratios (δ to the power of 13 C1) of mud volcanoes are 90% v/v and -46.4‰, respectively. The other types of seeps, which are independent from mud volcanism, have an average δ to the power of 13 C1 value that is slightly higher (-42.9‰). Gases from mud volcanoes are generally lighter (more methane, less ethane and propane) than their associated reservoir gases, suggesting a molecular fractionation during advective fluid migration. Other types of seeps, especially "dry" seeps, maintain the reservoir C1/(C2 + C3) "Bernard" ratio. Mud volcanoes behave like a "natural refinery" and the origin of gas more isotopically enriched than -50% and with C1/(C2 + C3) >500 should be attributed to a thermogenic source, rather than partial oxidation of biogenic gas. Some data that appear biogenic in the "Bernard diagram" can be explained by molecular fractionation of mixed gas. Consequently, the "Bernard" parameter may be misleading when applied to mud volcanoes since it does not always reflect the original gas composition. The mechanisms of the molecular advective segregation should be studied quantitatively by specific models and experiments.

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 ofmbiodegradation in some cases. Thermogenic gas generation modelling and maturity plots suggest thatmmost Romanian gases originate from mature type II and III kerogen (%Ro: 2e3). 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 C2 and C3 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.

Methane (CH4) flux to the atmosphere was measured from gas vents and, for the first time, from soil microseepage at four quiescent mud volcanoes and one ‘‘everlasting fire’’ in eastern Azerbaijan. Mud volcanoes show different activity of venting craters, gryphons, and bubbling pools, with CH4 fluxes ranging from less than one to hundreds of tons per year. Microseepage CH4 flux is generally on the order of hundreds of milligrams per square meter per day, even far away from the active centers. The CH4 flux near the everlasting fires (on the order of 105 mg·m22·d21) represents the highest natural CH4 emission from soil ever measured. The specific CH4 flux to the atmosphere, between 102 and 103 t·km22·yr21, was similar to specific flux from other mud volcanoes in Europe. At least 1400 tons of CH4 per year are released from the investigated areas. It is conservatively estimated that all onshore mud volcanoes of Azerbaijan, during quiescent activity, may still emit ;0.3–0.9 3 106 t of CH4 per year into the atmosphere. The new data fill a significant gap in the worldwide data set and confirm the importance of geologic sources of greenhouse CH4, although they are not yet considered in the climate-study budgets of atmospheric CH4 sources and sinks.