ALBERT

Description: 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.

Description: Published

Description: 89-96

Description: 4.5. Studi sul degassamento naturale e sui gas petroliferi

Description: JCR Journal

Description: reserved

Description: Methane and CO2 emissions from the two most active mud volcanoes in central Japan, Murono and Kamou (Tokamachi City, Niigata Basin), were measured in from both craters or vents (macro-seepage) and invisible exhalation from the soil (mini- and microseepage). Molecular and isotopic compositions of the released gases were also determined. Gas is thermogenic (d13CCH4 from 32.9‰ to 36.2‰), likely associated with oil, and enrichments of 13C in CO2 (d13CCO2 up to +28.3‰) and propane (d13CC3H8 up to 8.6‰) suggest subsurface petroleum biodegradation. Gas source and post-genetic alteration processes did not change from 2004 to 2010. Methane flux ranged within the orders of magnitude of 101–104 gmˉ2 dˉ1 in macro-seeps, and up to 446 g mˉ2 dˉ1 from diffuse seepage. Positive CH4 fluxes from dry soil were widespread throughout the investigated areas. Total CH4 emission from Murono and Kamou were estimated to be at least 20 and 3.7 ton aˉ1, respectively, of which more than half was from invisible seepage surrounding the mud volcano vents. At the macro-seeps, CO2 fluxes were directly proportional to CH4 fluxes, and the volumetric ratios between CH4 flux and CO2 flux were similar to the compositional CH4/CO2 volume ratio. Macro-seep flux data, in addition to those of other 13 mud volcanoes, supported the hypothesis that molecular fractionation (increase of the ‘‘Bernard ratio’’ C1/(C2 + C3)) is inversely proportional to gas migration fluxes. The CH4 ‘‘emission factor’’ (total measured output divided by investigated seepage area) was similar to that derived in other mud volcanoes of the same size and activity. The updated global ‘‘emission-factor’’ data-set, now including 27 mud volcanoes from different countries, suggests that previous estimates of global CH4 emission from mud volcanoes may be significantly underestimated.

Description: Published

Description: 348-359

Description: 4.5. Studi sul degassamento naturale e sui gas petroliferi

Description: JCR Journal

Description: restricted

Description: The “Santa Maria di Leuca” Cold-Water Coral (CWC) province (northern Ionian Sea) was investigated for the first time to detect eventual occurrence of methane anomalies as a possible indication of hydrocarbon seepage stimulating the coral growth. Most coral mounds have developed in correspondence with tectonic scarps and faults, orthogonal to the southern margin and trending NW-SE, which could be potential sites of gas escape. A visual and instrumental inspection was performed by using a new deep-sea probe equipped with video-cameras, sonar, CTD, methane sensors, and a water sampler. Eight areas were explored by 10 surveys, depths ranging from 380 to 1100 m, for a total of more than 26 h of continuous video and instrumental recording. Sediments were also sampled by gravity corers and analysed in laboratory. The images allowed to assess distribution, abundance and geometry of the colonies, most of which are developed on morphological highs often characterised by tectonic scarps. All data indicate however the lack of a significant occurrence of methane, both in seawater and sediments. No direct or indirect expressions of gas seepage were recognised on the seabed. Weak methane anomalies were detected only in seawater at the base of some fault-linked scarps, where more reducing conditions and bacterial methanogenesis are possibly enhanced by less water circulation. The faults are not fluid-bearing as previously suggested by high-resolution geophysical signatures. The development of the coral colonies thus cannot be attributed to seeping fluids, but to a favourable physiographic position with exposure to nutrient-rich currents.

Description: Published

Description: 431-440

Description: 4.5. Studi sul degassamento naturale e sui gas petroliferi

Description: JCR Journal

Description: restricted

Description: The presence of methane on Mars is of great interest, since one possibility for its origin is that it derives from living microbes. However, CH4 in the martian atmosphere also could be attributable to geologic emissions released through pathways similar to those occurring on Earth. Using recent data on methane degassing of the Earth, we have estimated the relative terrestrial contributions of fossil geologic methane vs. modern methane from living methanogens, and have examined the significance that various geologic sources might have for Mars. Geologic degassing includes microbial methane (produced by ancient methanogens), thermogenic methane (from maturation of sedimentary organic matter), and subordinately geothermal and volcanic methane (mainly produced abiogenically). Our analysis suggests that not, vert, similar80% of the “natural” emission to the terrestrial atmosphere originates from modern microbial activity and not, vert, similar20% originates from geologic degassing, for a total CH4 emission of not, vert, similar28.0×107 tonnes year−1. Estimates of methane emission on Mars range from 12.6×101 to 57.0×104 tonnes year−1 and are 3–6 orders of magnitude lower than that estimated for Earth. Nevertheless, the recently detected martian, Northern-Summer-2003 CH4 plume could be compared with methane expulsion from large mud volcanoes or from the integrated emission of a few hundred gas seeps, such as many of those located in Europe, USA, Mid-East or Asia. Methane could also be released by diffuse microseepage from martian soil, even if macro-seeps or mud volcanoes were lacking or inactive. We calculated that a weak microseepage spread over a few tens of km2, as frequently occurs on Earth, may be sufficient to generate the lower estimate of methane emission in the martian atmosphere. At least 65% of Earth’s degassing is provided by kerogen thermogenesis. A similar process may exist on Mars, where kerogen might include abiogenic organics (delivered by meteorites and comets) and remnants of possible, past martian life. The remainder of terrestrial degassed methane is attributed to fossil microbial gas (not, vert, similar25%) and geothermal-volcanic emissions (not, vert, similar10%). Global abiogenic emissions from serpentinization are negligible on Earth, but, on Mars, individual seeps from serpentinization could be significant. Gas discharge from clathrate-permafrost destabilization should also be considered. Finally, we have shown examples of potential degassing pathways on Mars, including mud volcano-like structures, fault and fracture systems, and major volcanic edifices. All these types of structures could provide avenues for extensive gas expulsion, as on Earth. Future investigations of martian methane should be focused on such potential pathways.

Description: In press

Description: 4.5. Studi sul degassamento naturale e sui gas petroliferi

Description: JCR Journal

Description: restricted

Description: Natural emissions of CH4 are not only produced by contemporary biochemical sources such as wetlands, termites, oceans, wildfires and wild animals, and fossil CH4 (that is geologically ancient, radiocarbon-free CH4) is not emitted only by the fossil fuel industry. Beyond CH4 from the biosphere and CH4 from anthropogenic sources, a third CH4 ‘breath’ exists – earth’s degassing. The term ‘degassing’, in general, makes one think of volcanoes and geothermal manifestations (eruptions, fumaroles, mofettes, hydrothermal springs, either on land or on the seafloor) that release carbon dioxide, water vapour and sulphur gases, but this is only a partial vision of earth’s degassing. Earth also exhales hydrocarbons, especially in geologically ‘cold’ areas, such as sedimentary basins where large quantities of natural gas migrate from shallow or deep rocks and reservoirs to the surface along faults and fractured rocks. The phenomenon is called ‘seepage’ and the gas is almost totally CH4, with low quantities (hundreds of ppmv to a few per cent) of other hydrocarbons (mainly ethane and propane) and non-hydrocarbon gases (CO2, N2, H2S, Ar and He). Gaseous hydrocarbons are produced by geologically ancient microbial activity, in shallow and low-temperature sedimentary rocks, and by thermogenic processes in deeper, warmer rocks. Therefore, seepage is a natural source of fossil CH4. Until recently, geological seepage has generally been neglected or considered a ‘minor source’ for CH4 in the scientific literature (for example Lelieveld et al, 1998). The Second and Third Assessment Reports of the Intergovernmental Panel on Climate Change (Schimel et al, 1996; Prather et al, 2001) only considered gas hydrates as geological sources of methane. Gas hydrates, or CH4 clathrates as they are sometimes called, are ice-like mixtures of water and CH4 trapped in oceanic sediments (for example Kvenvolden, 1988). The majority of this gas escaping from melting deep-sea hydrates is dissolved in the seawater column and does not enter the atmosphere. However, global emissions of CH4 to the atmosphere from hydrates have been reported to be roughly 3Tg y–1 (Kvenvolden, 1988) to 10Tg y–1 (Lelieveld et al, 1998),highly speculative values since they result from misquotations not supported by direct measurements. Studies conducted during the last ten years have made it clear that other geological CH4 sources, much more important than gas hydrates, exist; and there has been a growing consensus regarding the importance of marine (offshore) seepage, independent from gas hydrates, as a global contributor of CH4 to the atmosphere (for example Judd et al, 2002; Judd and Hovland, 2007). Experimental flux data, acquired since 2001, have provided more and more evidence for large emissions from continental (onshore) gas manifestations, including macroseeps and diffuse microseepage from soils (Etiope et al, 2008; Etiope, 2009, and references therein). Geothermal emissions are subordinate, but worth considering globally, while volcanoes appear not to be substantial CH4 contributors (Etiope et al, 2007a). At present, it is clear and unambiguously understood that geological emissions are a significant global source of CH4; and today, earth’s degassing is considered the second highest natural source for CH4 emissions after wetlands (for example Etiope, 2004; Kvenvolden and Rogers, 2005; Etiope et al, 2008). A new global estimate for geological sources has finally been acknowledged by the IPCC in its Fourth Assessment Report (Denman et al, 2007). Also, geological seepage has been considered as a new source for natural CH4 in the Emission Inventory Guidebook of the European Environment Agency (EMEP/EEA, 2009) and in the new US Environmental Protection Agency report on Natural Emissions of Methane (US EPA, 2010).

Description: Published

Description: 42-61

Description: 4.5. Studi sul degassamento naturale e sui gas petroliferi

Description: restricted

Description: The role of mud volcanoes (MVs) as a source of methane(CH4) flux to the atmosphere and the ocean has been increasingly recognised in the last several years (Milkov 2000; Dimitrov 2002, 2003; Etiope and Klusman 2002; Kopf 2002, 2003; Milkov et al. 2003; Etiope and Milkov 2004). In one of the most recent papers, Kopf (2003) claims to report a reliable estimate of the global CH4 emission from MVs. However, the significance and usefulness of the estimate presented by Kopf (2003) are rather poor. The used dataset is smaller than in previous studies (although the author makes a reverse claim), and some previously published works are misquoted and misinterpreted. Numerous arithmetic mistakes made during simple calculations and data manipulations lead to confusing results and conclusions. In this comment, we highlight some of the most significant problems with the estimates published by Kopf (2003).

Description: Published

Description: 490-492

Description: 4.5. Degassamento naturale

Description: JCR Journal

Description: reserved

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

Description: Published

Description: 997-1002

Description: 4.5. Degassamento naturale

Description: JCR Journal

Description: reserved

Description: The assessment of gas origin in mud volcanoes and related petroleum systems must consider postgenetic processes which may alter the original molecular and isotopic composition of reservoir gas. Beyond eventual molecular and isotopic fractionation due to gas migration and microbial oxidation, investigated in previous studies, we now demonstrate that mud volcanoes can show signals of anaerobic biodegradation of natural gas and oil in the subsurface. A large set of gas geochemical data from more than 150 terrestrial mud volcanoes worldwide has been examined. Due to the very low amount of C2+ in mud volcanoes, isotopic ratios of ethane, propane and butane (generally the best tracers of anaerobic biodegradation) are only available in a few cases. However, it is observed that 13C-enriched propane is always associated with positive б13 CCO2 values, which are known indicators of secondary methanogenesis following anaerobic biodegradation of petroleum. Data from carbon isotopic ratio of CO2 are available for 134 onshore mud volcanoes from 9 countries (Azerbaijan, Georgia, Ukraine, Russia, Turkmenistan, Trinidad, Italy, Japan and Taiwan). Exactly 50% of mud volcanoes, all releasing thermogenic or mixed methane, show at least one sample with б13 CCO2〉+5‰ (PDB). Thermogenic CH4 associated with positive carbon isotopic ratio of CO2 generally maintains its б13C-enriched signature, which is therefore not perturbed by the lighter secondary microbial gas. There is, however, high variability in the б13 CCO2 values within the same mud volcanoes, so that positive б13 CCO2 values can be found in some vents and not in others, or not continuously in the same vent. This can be due to high sensitivity of б13 CCO2 to gas–water–rock interactions or to the presence of differently biodegraded seepage systems in the same mud volcano. However, finding a positive б13 CCO2 value should be considered highly indicative of anaerobic biodegradation and further analyses should be made, especially if mud volcanoes are to be used as pathfinders of the conditions indicative of subsurface hydrocarbon accumulations in unexplored areas.

Description: Published

Description: 1692-1703

Description: 4.5. Studi sul degassamento naturale e sui gas petroliferi

Description: JCR Journal

Description: reserved

Description: 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.

Description: Published

Description: 465-468

Description: reserved

Description: The presence of methane on Mars is of great interest, since one possibility for its origin is that it derives from living microbes. However, CH4 in the martian atmosphere also could be attributable to geologic emissions released through pathways similar to those occurring on Earth. Using recent data on methane degassing of the Earth, we have estimated the relative terrestrial contributions of fossil geologic methane vs. modern methane from living methanogens, and have examined the significance that various geologic sources might have for Mars. Geologic degassing includes microbial methane (produced by ancient methanogens), thermogenic methane (from maturation of sedimentary organic matter), and subordinately geothermal and volcanic methane (mainly produced abiogenically). Our analysis suggests that not, vert, similar80% of the “natural” emission to the terrestrial atmosphere originates from modern microbial activity and not, vert, similar20% originates from geologic degassing, for a total CH4 emission of not, vert, similar28.0×107 tonnes year−1. Estimates of methane emission on Mars range from 12.6×101 to 57.0×104 tonnes year−1 and are 3–6 orders of magnitude lower than that estimated for Earth. Nevertheless, the recently detected martian, Northern-Summer-2003 CH4 plume could be compared with methane expulsion from large mud volcanoes or from the integrated emission of a few hundred gas seeps, such as many of those located in Europe, USA, Mid-East or Asia. Methane could also be released by diffuse microseepage from martian soil, even if macro-seeps or mud volcanoes were lacking or inactive. We calculated that a weak microseepage spread over a few tens of km2, as frequently occurs on Earth, may be sufficient to generate the lower estimate of methane emission in the martian atmosphere. At least 65% of Earth’s degassing is provided by kerogen thermogenesis. A similar process may exist on Mars, where kerogen might include abiogenic organics (delivered by meteorites and comets) and remnants of possible, past martian life. The remainder of terrestrial degassed methane is attributed to fossil microbial gas (not, vert, similar25%) and geothermal-volcanic emissions (not, vert, similar10%). Global abiogenic emissions from serpentinization are negligible on Earth, but, on Mars, individual seeps from serpentinization could be significant. Gas discharge from clathrate-permafrost destabilization should also be considered. Finally, we have shown examples of potential degassing pathways on Mars, including mud volcano-like structures, fault and fracture systems, and major volcanic edifices. All these types of structures could provide avenues for extensive gas expulsion, as on Earth. Future investigations of martian methane should be focused on such potential pathways.

Description: Published

Description: 182-195

Description: 4.5. Studi sul degassamento naturale e sui gas petroliferi

Description: JCR Journal

Description: restricted