Vol. 48, 01, 05ok.qxd 1 ANNALS OF GEOPHYSICS, VOL. 48, N. 1, February 2005 Key words methane – lithosphere degassing – mud volcanoes – greenhouse gas – geodynamics 1. Introduction Geodynamics and geophysical processes of lithosphere degassing are generally neglected in contemporary global climate change research. Nevertheless, recent studies have suggested that lithosphere carbon dioxide (CO2) and methane (CH4) outgassing is an important component of the natural greenhouse gas sources (Etiope and Klusman, 2002; Morner and Etiope, 2002). This is particularly evident for methane, whose geo- logical sources have been object of detailed in- vestigations during recent years. Methane is one of the main greenhouse gases playing a signifi- cant role in global climate changes, on geolo- gical, Quaternary and contemporary time scales. Natural sources of methane include wetlands (> 100 Mt yr–1), termites (20 Mt yr–1) and oceans (10 Mt yr–1). The Intergovernmental Panel on Cli- mate Change (IPCC, 2001) does not include in its official tables any geological source of methane, apart from hydrates (5-10 Mt yr–1). Only recent- ly, it has been suggested that several geologic processes may lead to the release of significant amounts of methane into the atmosphere, mainly from submarine seepage, mud volcanoes and mi- croseepage (Etiope and Klusman, 2002; Etiope et al., 2003, 2004a; Milkov et al., 2003; Etiope and Milkov, 2004). Today’s global estimates available for methane flux from these sources are probably underestimated and have a great potential of being increased. This work aims at evaluating this potential for mud volcanoes and mi- croseepage, discussing present limits and in- troducing new data. The global microseepage estimate is re-calculated on the basis of an up- graded experimental data set and on a new evaluation of the global microseepage area. Mailing address: Dr. Giuseppe Etiope, Istituto Nazio- nale di Geofisica e Vulcanologia, Via Vigna Murata, 605, 00143 Roma, Italy; e-mail: etiope@ingv.it Mud volcanoes and microseepage: the forgotten geophysical components of atmospheric methane budget Giuseppe Etiope Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy Abstract Mud volcanoes and microseepage are two important natural sources of atmospheric methane, controlled by neo- tectonics and seismicity. Petroleum and gas reservoirs are the deep sources, and faults and fractured rocks serve as main pathways of degassing to the atmosphere. Violent gas emissions or eruptions are generally related to seismic activity. The global emission of methane from onshore mud volcanoes has recently been improved thanks to new experimental data sets acquired in Europe and Azerbaijan. The global estimate of microseepage can be now improved on the basis of new flux data and a more precise assessment of the global area in which microseepage may occur. Despite the uncertainty of the various source strengths, the global geological methane flux is clearly comparable to or higher than other sources or sinks considered in the tables of the Intergovern- mental Panel on Climate Change. 2 Giuseppe Etiope 2. Mud volcano emissions Mud Volcanoes (MVs) are the largest sur- face expression of migration of hydrocarbon fluids through neotectonic faults in petroleum- bearing sedimentary basins (fig. 1). Geology and formation mechanisms are described in a wide literature (e.g., Milkov, 2000; Dimitrov, 2002; Revil, 2002). Methane flux from MVs is object of detailed studies only starting from 2001, when the main terrestrial MVs of Eu- rope, located in Romania and Italy were inves- tigated (Etiope et al., 2002, 2003, 2004a). More recently, gas flux has been measured in Azer- baijan, which hosts the world’s biggest MVs and densest MV population (Etiope et al., 2004b). Thanks to these studies, it has been possible to elaborate a first estimate of global emission of methane from MVs to the atmosphere, that is at least 6-9 Mt yr –1 (Etiope and Milkov, 2004). This is the same level of the estimates today considered for ocean and hydrates sources. Methane emission from MVs (fig. 2) in- cludes not only the gas flux from localised vents (craters, gryphons, bubbling pools and salses) but also the diffuse exhalation from soil, known as microseepage, whose mechanisms are dis- cussed in the next section. The data collected from 2001 to 2003 in Europe and Azerbaijan (Etiope et al., 2002, 2003, 2004a,b) refer to the quiescent degassing. It is known however that many MVs, especially those in Azerbaijan, can erupt violently, generally in relation to seismic activity, injecting huge amounts of gas into the atmosphere within a few hours. So far, however, only some rough estimates of the eruptive flux of MVs in Azerbaijan have been reported, gen- erally based on subjective visual observations. For example, it has been reported that during the eruption of the Touragai mud volcano (Azerbai- jan) in 1946, about 0.36 Mt of CH4 were emit- ted, and more than 40 000 t of CH4 emitted from the Duvannyi Island volcano in 1961. Bolshoi Maraza erupted for three days in 1902 injecting more than 80 000 t of CH4 into the atmosphere (Guliyev and Feyzullayev, 1997). From 1810 until the present, about 250 eruptions of 60 mud volcanoes have been observed in Azerbaijan. Sokolov et al. (1969) described violent erup- tions of mud-volcanoes in the southern Caspian Basin, which have released hundreds of millions of cubic meters of gas and estimated that mud volcanoes in Azerbaijan have produced 106 Mt of gas in the last million years. Most of these eruptions followed large earthquakes. In their global estimation of gas flux from mud volca- noes, Milkov et al. (2003) concluded that the global eruptive degassing may be approximate- ly equal to the global quiescent degassing. In contrast, Dimitrov (2002) suggests that gas flux from quiescent periods is significantly (by a fac- tor of up to 30) less than the gas flux during eruptions. Direct measurements of methane flux from submarine MVs have rarely been performed (Linke et al., 2005), and only in a few active points. Some rough estimates, generally based on the volumes of mud extruded, are available as reviewed by Kopf (2002). On the basis of available data, including MVs dimensions, depth and gas dissolution models, Etiope and Milkov (2004) have estimated that at least 0.5 Mt of methane are injected into the atmosphere from MVs occurring at depths less than 200 m (shelf MVs). However, recent discoveries (e.g., Holland et al., 2003) suggest that shelf MVs are Fig. 1. Sketch of methane origin and emission in hydrocarbon-prone basins. 3 Mud volcanoes and microseepage: the forgotten geophysical components of atmospheric methane budget more abundant than previously assumed and that many of them release significant amounts of gas bubble plumes, which may easily cross the water column and enter the atmosphere. Therefore detailed studies and measure- ments of gas flux during eruption, and direct measurements of gas flux from submarine mud volcanoes appear to be critical to further con- strain the global gas flux from MVs. 3. Microseepage Etiope and Klusman (2002) defined mi- croseepage as the slow, continual loss of CH4 and light alkanes from depths of 2-5 km in sed- imentary basins where thermal degradation of indigenous organic matter is occurring. Mi- croseepage is basically a pervasive, diffuse exhalation of methane from soil resulting from natural gas migration from underground hydrocarbon reservoirs. It is assumed that mi- croseepage is a general phenomenon driven by buoyancy of the gas phase relative to con- nate waters (Price, 1986; Klusman, 1993; Klusman and Saeed, 1996; Matthews, 1996); frequently, gas migration can be considered in terms of microbubbles, bubbles and slug flows along faults and fractured rocks (Etiope and Martinelli, 2002). It is evident that mi- croseepage is enhanced along faults, especial- ly those produced by neotectonics (Klusman, 1993; Etiope, 1999). In dry lands, methane flux is generally neg- ative, from the atmosphere to the soil, due to methanotrophic oxidation by CH4-consuming Fig. 2. Typical mud volcano morphology and methane emission structures: (top) single crater MV, Trinidad (from the Geological Society of Trinidad and Tobago); (bottom) multi-crater (gryphons) MV, Paclele (Eastern Romania). 4 Giuseppe Etiope bacteria in the soil. Due to this biological activ- ity, dry lands are considered a net sink of at- mospheric methane, on global scale (around 30 Mt yr–1), with fluxes generally in the order of –5 to –1 mgm–2d–1 (Dong et al., 1998). Microseep- age is instead responsible for less negative or positive fluxes of methane, indicating that soil consumption can be lower than the input from underground sources. The positive fluxes are typically of a few units or tens of mgm–2d–1, but may be at the hundreds level over wide tec- tonised and faulted areas in the most active mi- croseeping regions. These values are compara- ble with the CH4 emission in wet, anaerobic ecosystems, which are typically in the range 1- 500 mgm–2d–1 (Batjes and Bridges, 1994). In MV areas microseepage may easily reach flux- es in the order of 103-105 mgm–2d–1. The highest microseepage flux ever reported has been found close to the fire of Yanardag, in Azerbaijan: > 560 000 mgm–2d–1 (Etiope et al., 2004b). A re- view is made by Etiope and Klusman (2002), and data on microseepage linked to MVs are in Etiope et al. (2002, 2003, 2004a,b). The global coverage of microseepage is un- known. Potentially, microseeping areas are all the sedimentary basins in a dry climate, with petrole- um and gas generation processes at depth: this area has been estimated to be around 43 366 000 km2 (Klusman et al., 1998). Preliminary models suggest that this area can produce a mean mi- croseepage flux of 4.42 mg CH4 m–2d–1 (Klusman et al., 1998, 2000) and 90% of methanotrophic consumption leading to a global emission of methane of about 7 Mt yr–1. This is only a first, rough estimate, very likely quite conservative. Today it is possible to suggest another esti- mate, based directly on experimental values and on the area of the tectonic zones (faulted) actually hosting gas reservoirs. We have first to distinguish microseepage close to MVs (MV microseepage) and microseepage far from MVs or in sedimentary basins without MVs (simply microseepage). Global emission of MV microseepage has already been estimated by Etiope and Milkov (2004), who considered the diffuse flux occurring within the MV mor- phologic structure (hill, muddy cover, and ex- ternal bound of 250 m); this MV microseepage is at least 1-2.4 Mt yr–1. 3.1. Upgraded microseepage data-set In order to estimate the global non-MV mi- croseepage it is possible to refer to an upgraded data-set, including microseepage from United States (Klusman et al., 2000), former Soviet Union (Voitov, 1975; Balakin et al., 1981) and new data from reconnaissance surveys, carried out in 2002, in non-MV zones of Transylvania, cen- tral Romania and along the Adriatic coast of cen- tral Italy. These are two of the most important gas producing areas of Europe (Schlumberger, 1987; Cranganu and Deming, 1996). In these areas, 40 soil-atmosphere flux measurements were carried out in soils hosting wheat and grass communities, typical of temperate climates, by closed-chamber method; gas was analysed in duplicate by portable micro-GC (Etiope et al., 2002). The flux values ranged from – 5 to 142 mgm–2d–1, with a mean of 20 mgm–2d–1. Only 6 flux values were negative (from –5 to –1.5 mgm–2d–1); the highest values (from 90 to 142 mgm–2d–1) were measured in the «Cupello» gas reservoir (Vasto) on the Italian Adriatic coast. Here biogenic gas is exploited from sandy reser- voirs at depths between 800 and 1100 m and thermogenic gas occurs in deeper carbonate reservoirs (Schlumberger, 1987). The average microseepage value derived from the surveys cit- ed in table I (excluding the higher values of Great Caucasus and Azerbaijan) is around 10 mgm–2d–1. 3.2. New estimate of global microseepage area The flux data available today suggest that microseepage corresponds closely to the spatial distribution of underground petroleum reser- voirs. Instead of considering the whole area cov- ered by sedimentary basins, as made by Klus- man et al. (2000), it is today possible to estimate the global area of the onshore petroleum reser- voirs. This has been made elaborating the data from the last US Geological Survey World Pe- troleum Assessment (USGS, 2000). This work named and mapped 159 of the largest total pe- troleum systems (TPS’s) in the world using ge- ographic information system. The TPS’s are the hydrocarbon-fluid systems in the lithosphere in- cluding the essential elements and processes Table I. Microseepage in hydrocarbon-prone (no mud volcanos) areas. Reference No. of sites Flux range (mean) mgm–2d–1 Denver-Julesburg Basin (Colorado) Klusman et al. (2000) 84 – 41 to 43.1 (0.57) Piceance (Colorado) Klusman et al. (2000) 60 – 6.0 to 3.1 (– 1.1) Powder River (Wyoming) Klusman et al. (2000) 78 – 14.9 to 19.1 (0.02) Railroad Valley (Nevada) Klusman et al. (2000) 120 – 6.1 to 4.8 (– 0.2) Great Caucasus Balakin et al. (1981) Unknown 430 Lesser Caucasus Balakin et al. (1981) Unknown 12 Kura depression Balakin et al. (1981) Unknown 8 Azerbaijan Voitov (1975) Unknown 28-200 Transylvania (Central Romania) Tarnaveni-Bazna This work 5 2 to 64 (24) Abruzzo Adriatic coast (Central Italy) Vasto This work 30 – 5 to 142 (22) Pescara This work 5 – 4 to 13 (3.5) 5 Mud volcanoes and microseepage: the forgotten geophysical components of atmospheric methane budget needed for oil and gas accumulations, migration and seeps. It is assumed, therefore, that mi- croseepage occurs throughout the onshore TPS areas. Based on a careful analysis of TPS map and GIS data-sets, the global microseepage area can be estimated in the order of 8 × 106 km2. Assuming conservatively a mean microseep- age in the range 5-10 mgm–2d–1, a simple scaling- up would give a global emission of 14-28 Mt yr–1. 4. Conclusions Mud volcanoes and microseepage are close- ly related to neotectonic and seismic processes, and represent two important natural sources of atmospheric methane. The estimate of global emission of methane from onshore mud volca- noes has recently been refined thanks to new ex- perimental data sets acquired in Europe and Azerbaijan. Global microseepage has been esti- mated with less accuracy due to the few meas- urements available. A refinement is here pro- posed considering new data from hydrocarbon areas in U.S.A., former Soviet Union, Romania, Italy, and a more accurate assessment of the global area in which microseepage may occur. Potentially, the resulting global microseepage output can be in order of 14-28 MT yr–1. This is a provisional estimate based on the assumption «microseepage area = TPS area». A large num- ber of data over wide areas, from different TPS, and more accurate scaling-up procedures are necessary to reach a more constrained estimate. Given these uncertainties, the global emission of methane from geological sources, including MVs (6-9 Mt yr–1), marine seepage (20 Mt yr–1), geothermal flux (2.5-6.3 Mt yr–1) and microseep- age in petroliferous basins (14-28 Mt yr–1) would amount at least to 40-60 Mt yr–1. The previous es- timate was 35-45 Mt yr–1 (Etiope and Milkov, 2004). These numbers are of the same level of or higher than other sources or sinks considered in the tables of the Intergovernmental Panel on Cli- mate Change (IPCC, 2001), such as biomass burning (40 Mt yr–1), termites (20 Mt yr–1), oceans (10 Mt yr–1) and soil uptake (30 Mt yr–1). These results show clearly that geologic methane sources, strictly controlled by geodynamic and tectonic processes, have a primary role in the at- mospheric greenhouse gas budget. Acknowledgements Thanks are due to Feliks Persits (USGS, Central Energy Resources Team) for providing the GIS data-set for TPS. 6 Giuseppe Etiope Microseepage data of Transylvania are from preliminary surveys carried out with Calin L. Baciu (Babes-Bolyay University of Cluj-Napoca), Franco Italiano and Antonio Caracausi (INGV, Palermo Section), in the framework of a NATO project (contract EST.CLG.977422). REFERENCES BALAKIN, V.A, G.A. GABRIELANTS, I.S. GULIYEV, F.G. DADASHEV, V.M. KOLOBASHKIN, A.I. POPOV and A.A. FEYZULLAYEV (1981): Test of experimental study of hy- drocarbon degassing of lithosphere of South Caspian Basin and adjacent mountains systems, using laser gas- analyzer «Iskatel-2», Dokl. Akad. Nauk SSSR, 260 (1), 154-156 (in Russian). BATJES, N.H. and E.M. BRIDGES (1994): Potential emissions of radiatively active gases from soil to atmosphere with spe- cial reference to methane: development of a global data- base (WISE), J. Geophys. Res., 99 (D8), 16,479-16,489. CRANGANU, C. and D. DEMING (1996): Heat flow and hy- drocarbon generation in the Transylvanian Basin, Ro- mania, AAPG Bull., 80 (10), 1641-1653. DIMITROV, L. (2002): Mud volcanoes – the most important pathway for degassing deeply buried sediments, Earth- Sci. Rev., 59, 49-76. DONG, Y., D. SCHARFFE, J.M. LOBERT, P.J. CRUTZEN and E. SANHUEZA (1998): Fluxes of CO2, CH4 and N2O from temperate forest soil: the effect of leaves and humus layers, Tellus, 50B, 243-252. ETIOPE, G. (1999): Subsoil CO2 and CH4 and their advective transfer from faulted grassland to the atmosphere, J. Geophys. Res., 104 (D14), 16,889-16,895. ETIOPE, G. and R.W. KLUSMAN (2002): Geologic emis- sions of methane to the atmosphere, Chemosphere, 49, 777-789. ETIOPE, G. and G. MARTINELLI (2002): Migration of car- rier and trace gases in the geosphere: an overview, Phys. Earth Planet. Int., 129, 3-4, 185-204. ETIOPE, G. and A.V. MILKOV (2004): A new estimate of global methane flux from onshore and shallow sub- marine mud volcanoes to the atmosphere, Environ. Geol., 46 (8), 997-1002. ETIOPE, G., A. CARACAUSI, R. FAVARA, F. ITALIANO and C. BACIU (2002): Methane emission from the mud vol- canoes of Sicily (Italy), Geophys. Res. Lett., 29 (8), doi:10.1029/2001GL014340. ETIOPE, G., A. CARACAUSI, R. FAVARA, F. ITALIANO and C. BACIU (2003): Reply to comment by A. KOPF on «Methane emission from the mud volcanoes of Sicily (Italy)», and notice on CH4 flux data from European mud volcanoes, Geophys. Res. Lett., 30 (2), 1094, doi:10.1029/2002GL016287. ETIOPE, G., C. BACIU, A. CARACAUSI, F. ITALIANO and C. COSMA (2004a): Gas flux to the atmosphere from mud volcanoes in Eastern Romania, Terra Nova, 16 (4), 179-184. ETIOPE, G., A. FEYZULLAYEV, C. BACIU and A.V. MILKOV (2004b): Methane emission from mud volcanoes in the Eastern Azerbaijan, Geology, 32 (6), 465-468. GULIYEV, I.S. and A. FEYZULLAYEV (1997): All About Mud Volcanoes (Baku Pub. House, NAFTA-Press), pp. 120. HOLLAND, C.W., G. ETIOPE, A.V. MILKOV, E. MICHELOZZI and P. FAVALI (2003): Mud volcanos discovered off- shore Sicily, Mar. Geol., 199, 1-6. IPCC (Intergovernmental Panel on Climate Change) (2001): Climate Change 2001: the Scientific Basis, edited by J.T. HOUGHTON, Y. DING, D.J. GRIGGS, M. NOGUER, P.J. VAN DER LINDEN, X. DAI, K. MASKELL and C.A. JOHNSON (Cambridge Univ. Press., Cam- bridge, U.K.), pp. 881. KLUSMAN, R.W. (1993): Soil Gas and Related Methods for Natural Resource Exploration (John Wiley and Sons, Chichester, U.K.), p. 483. KLUSMAN, R.W. and M.A. SAEED (1996): Comparison of light hydrocarbon microseepage mechanisms, in Hy- drocarbon Migration and Its Near-Surface Expression, edited by D. SCHUMACHER and M.A. ABRAMS, Am. As- soc. Petrol. Geol. Mem. 66 (Tulsa, OK), 157-168. KLUSMAN, R.W., M.E. JAKEL and M.P. LEROY (1998): Does microseepage of methane and light hydrocar- bons contribute to the atmospheric budget of methane and to global climate change?, Assoc. Petrol. Geochem. Explor. Bull., 11, 1-55. KLUSMAN, R.W., M.E. LEOPOLD and M.P. LEROY (2000): Seasonal variation in methane fluxes from sedimentary basins to the atmosphere: Results from chamber meas- urements and modeling of transport from deep sources, J. Geophys. Res., 105 (D20), 24,661-24,670. KOPF, A.J. (2002): Significance of mud volcanism, Rev. Geophys., 40 (2), 1005, doi:10.1029/2000RG000093. LINKE, P., K. WALLMANN, E. SUESS, C. HENSEN and G. REHDER (2005): In-situ benthic fluxes from an intermittently active mud volcano at the Costa Rica convergent margin, Earth Planet. Sci. Lett. (in press). MATTHEWS, M.D. (1996): Hydrocarbon migration-A view from the top, in Hydrocarbon Migration and Its Near- Surface Expression, edited by D. SCHUMACHER and M.A. ABRAMS, Am. Assoc. Petrol. Geol. Mem. 66, (Tul- sa, OK), 139-155. MILKOV, A.V. (2000): Worldwide distribution of subma- rine mud volcanoes and associated gas hydrates, Mar. Geol., 167 (1-2), 29-42. MILKOV, A.V., R. SASSEN, T.V. APANASOVICH and F.G. DADASHEV (2003): Global gas flux from mud volca- noes: a significant source of fossil methane in the at- mosphere and the ocean, Geophys. Res. Lett., 30 (2), 1037, doi:10.1029/2002GL016358. MORNER, N.A. and G. ETIOPE (2002): Carbon degassing from the lithosphere, Global Planet. Change, 33 (1/2), 185-203. PRICE, L.C. (1986): A critical overview and proposed work- ing model of surface geochemical exploration, in Un- conventional Methods in Exploration for Petroleum and Natural Gas-IV, edited by M.J. DAVIDSON (South- ern Methodist University, Dallas, TX), 245-309. REVIL, A. (2002): Genesis of mud volcanoes in sedimenta- ry basins: a solitary wave-based mechanism, Geophys. Res. Lett., 29 (12), 1574, doi:10.1029/2001GL014465. SCHLUMBERGER (1987): Well Evaluation Conference, WEC 1987, Schlumberger. 7 Mud volcanoes and microseepage: the forgotten geophysical components of atmospheric methane budget SOKOLOV, V.A., Z.A. BUNIAT-ZADE, A.A. GEODEKIAN and F.D. DADASHEV (1969): The origin of gases and mud volcanoes and the regularities of their powerful erup- tions, in Advances in Organic Geochemistry edited by P.A. SCHENCK and I. HAVENAAR (Oxford, Pergamon Press), 473-484. USGS WORLD ENERGY ASSESSMENT TEAM (2000): US Geological Survey World Petroleum Assessment 2000, Description and Results, Digital Data Series – DDS-60 (US Dept. of the Interior, USGS). VOITOV, G.I. (1975): Gas breath of Earth, Nature, 3, 91- 98 (in Russian).