Title: Reply to comment by A. Kopf on “Methane emission from the mud volcanoes of Sicily (Italy)”, and notice on CH<sub>4</sub> flux data from European mud volcanoes
Abstract: [1] 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. [4] The number (“at least 700”) of MVs mentioned in Etiope et al. [2002] refers only to subaerial structures, coherently to the subject of the paper. This value, reported also by Guliyev and Feizullayev [1997], should be considered a lower limit. A more recent review [Dimitrov, 2002] suggests the existence of more than 900 on-shore MVs. These numbers, however, are not based on an exact definition of the units counted, so that in many areas “mud volcanoes” can refer to single or groups of vents (craters or gryphons), morphologically distinguished or not within a given area (mud volcano field). Just for an example Dimitrov [2002] reports >10 mud volcanoes NE of Ploiesti town in Romania; in that region 4 major MV fields are known including several morphological units where, it is often impossible to recognise single “volcanic” structures or to distinguish a summit crater from a flank gryphon. Detailed structural and morphological examination [Hovland et al., 1997] should be made to exactly define these items. In any case we counted at least 145 vents (craters, gryphons and pools; 2001 survey, paper submitted). Therefore caution is needed to define the number of MVs in a given region. [5] About submarine MVs, Dimitrov [2002] reports >268 confirmed offshore structures and >572 as inferred. In a previous review Milkov [2000] estimated a range of 103 to 105 for the total number of submarine MVs worldwide. [6] The considerations by Kopf [2003] on the global significance of mud volcanic CH4 emission are right, in fact mud volcanoes may contribute significantly to global methane emissions. The global significance of MVs as CH4 sources has already been examined by Etiope and Klusman [2002], who discussed also geothermal sources and proposed a first, conservative global estimate of the CH4 output from subaerial MVs in the range of 2 to 10 Mt y−1. [7] A better and more constrained estimate might be achieved by verifying the specific CH4 flux (amount of gas released per year per square km) in other MV areas, including microseepage, and estimating the global area (km2) covered by MV fields. As discussed above, all the surveyed subaerial MVs present so far a specific flux on the order of 102–103 t km−2 y−1. It is likely that this range is valid for small to medium size mud volcanoes. Since giant structures, like those in Azerbaijan, present higher flux from vents, they should provide intense microseepage from the degassing soil. [8] The total geologic methane source (mud volcanoes, microseepage, submarine seeps, geothermal emissions) is estimated by Etiope and Klusman [2002] to be on the order of 30–70 Mt y−1, which is about 8–18% of the anthropogenic sources, and comparable with the natural source strength from oceans and termites. These conservative estimates lead to a total methane source, which more effectively balances the total sink. Even considering the high uncertainty in these estimates, it is clear that geologic sources are more than enough to provide the amount of CH4 required to account for the suspected missing source of fossil CH4. The geologic sources should therefore be added in the official CH4 budget. [9] New methane flux data have been collected in 2001 and 2002 on the largest MVs of Europe, in Eastern Romania, and on very small mud volcanoes in Transylvania (Central Romania) and Abruzzo-Marche (Central Italy). We summarise herewith the data from the limited areas in Central Italy and Transylvania (Table 1). In May 2002 we measured CH4 flux from small MVs (max. height 1.5 m), mostly inactive, in the Abruzzo and Marche regions, close to the Adriatic Sea coast. They are in the same geologic framework characterising the submarine MVs discovered in the Adriatic off-shore [Hovland and Curzi, 1989]. Only one MV is active (Pineto), displaying a weak bubbling in the central pool. We found soil CH4 fluxes ranging from −3 to 480 mg m−2 d−1. Generally the highest values (>100 mg m−2 d−1) occur in correspondence with or just around the MV cones (even if inactive), where there is no vegetation. [10] In June 2002 we investigated in Transylvania (Central Romania) mainly two areas: the first with everlasting fires (Sarmasel), the second with small mud cones and pools (Homorod). The Transylvanian basin corresponds to a Neogene subsidence area, superimposed on a Cretaceous nappe system and surrounded by the Romanian Carpathian chain. The methane emissions are related to natural gas fields, widespread in this region, which is considered to be one of the most important gas producing areas of Europe [Cranganu and Deming, 1996]. The everlasting fire of Sarmasel showed soil degassing fluxes from 110 to 450,000 mg m−2 d−1, over an area of about 7500 m2. The Homorod MV has 4 small pools (with very weak bubbling) and a main cone (inactive) distributed over an area of 5000 m2. The soil flux survey however indicated that a significant degassing is pervasive throughout the area, with fluxes from 80 to 700 mg m−2 d−1. Table 1 summarises the specific flux and the total output from all the MVs investigated so far, including the biggest Eastern Romania MVs. All the obtained results confirm that MVs typically display a specific flux in the range 102 and 103 t km−2 y−1 and that the diffuse and pervasive microseepage around MV craters, gryphons and pools is a fundamental component of the total methane output. The soil CH4 positive flux (on the order of 102–105 mg m−2 d−1) is significantly high, even at large distances (some kms) from the MV fields, suggesting that microseepage exists over wide areas. [11] The main concern is the short-distance spatial variability, and the short-term temporal variability in the flux rates, resulting in poorly constrained estimates of average CH4 fluxes in a survey area and in the absence of long-term monitoring activity. [12] It is absolutely clear how the crustal permeability, the tectonic structures and their activity may play a significant role in enhancing the CH4 escape toward the atmosphere. Since the crustal permeability varies with the time, for example during a seismogenic process, the temporal variations of the CH4 output might have an important role also in tracing an impending earthquake. Moreover we consider that the assessment of the origin of the released CH4 combined with its output is also a powerful tool to investigate the tectonics of a MV area. We measured the 3He/4He isotopic ratio of the helium associated to CH4 at all the investigated MVs (Figures 1 and 2) and the results have shown how MVs can release CH4 coming from both crustal and mantle-related environments. The 3He/4He ratio, providing genetic information on the released gases, has clearly shown how the tectonic environment influences both the origin and the release of CH4. In turn the existence itself of the MVs. [13] These simple considerations lead to the same conclusion achieved by A. Kopf, that is much more work has to be done to better constrain the significance and the role of the submarine and subaerial MVs around the world.