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Preliminary estimate of CO2 output from Pantelleria Island volcano (Sicily, Italy): evidence of active mantle degassing
Applied Geochemistry 16 (2001) 883±894
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Preliminary estimate of CO2 output from Pantelleria Island volcano (Sicily, Italy): evidence of active mantle degassing Rocco Favara a, Salvatore Giammanco a,*, Salvatore Inguaggiato b,c, Giovannella Pecoraino a a
Istituto Geochimica dei Fluidi, CNR, via Ugo La Malfa 153-90146 Palermo, Italy b Istituto Nazionale di Geo®sica, via di Vigna Murata 605-00143 Rome, Italy c Poseidon System, Piazza Roma 2-95123 Catania, Italy Received 11 October 1999; accepted 12 June 2000 Editorial handling by H. AÂrmannsson
Abstract Total CO2 output from fumaroles, bubbling and water dissolved gases and soil gases was investigated at Pantelleria Island volcano, Italy. The preliminary results indicate an overall output of 0.39 Mt a 1 of CO2 from the island. The main contribution to the total output was from diuse soil degassing (about 0.32 Mt a 1), followed by dissolved CO2 (0.034 Mt a 1), focussed soil degassing (0.028 Mt a 1) and bubbling CO2 (0.013 Mt a 1). The contribution of CO2 from fumarole gases was found to be negligible (1.410 6 Mt a 1). Carbon-13 values for CO2 coupled with those for associated He in gases from fumaroles and sites of focussed soil degassing clearly rule out any signi®cant organic CO2 component and suggest a common mantle origin for these gas species. The inferred mantle source beneath Pantelleria would seem to have peculiar geochemical characteristics, quite distinct from those of mantle producing MORB but compatible with those of magmatic sources of central Mediterranean and central European volcanoes. These ®ndings indicate that the Pantelleria volcanic complex is a site of active mantle degassing that is worthy of attention for future geochemical surveillance of the island. # 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction Quantifying the CO2 budget from volcanic areas of the world can provide information useful for modeling the global carbon cycle. Although still far from having achieved this goal, studies on present-day CO2 degassing over the last years have shown the importance of CO2 contribution to the atmosphere not only from actively degassing volcanoes, such as Mt. Etna, Kilauea and Popocatepetl (Gerlach and Graeber, 1985; Allard et al., 1991; Delgado et al., 1998), but also from those in a quiescent state (e.g. Sorey et al., 1998; Etiope et al., 1999). Pantelleria Island (Fig. 1) is a quiescent volcano located in the Sicily Channel, 110 km south of Sicily and 70 km north of Tunisia. Its total emerged surface is 84
* Corresponding author. Fax: +39-91-680-9449. E-mail address: [email protected] (S. Giammanco).
km2 and its highest point is at Montagna Grande (836 m asl). Pantelleria Island represents the uppermost part of a large volcanic edi®ce that extends below sea level to a depth of about 1200 m and lies along an intra-plate rift system directed NW±SE in the median part of the Sicily Channel. The rift is still active and the NW±SEtrending strike-slip faults that form it are in many cases connected with N±S normal faults producing pull-apart basins (Orsi et al., 1991). The volcanic products that outcrop on the island are mainly pantellerites to trachytes, and only minor mildly alkalic basalts are found (e.g. Villari, 1974; Civetta et al., 1988). The oldest outcropping rocks date to 324.210.5 ka (Mahood and Hildreth, 1986) and the latest volcanic activity in the area took place during the 19th century with two submarine eruptions (1831 and 1891) in the vicinity of the island. The NW±SE and the N±S-trending regional faults seem to strongly control basaltic volcanic activity at
0883-2927/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0883-2927(00)00055-X
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Fig. 1. Structural map of Channel of Sicily and location of the island of Pantelleria (modi®ed from Orsi et al., 1991).
Pantelleria, as most of the volcanic eruptions of this type at Pantelleria occurred on the two structural trends mentioned above (Villari, 1974; Civetta et al., 1988). Subvertical inland dipping faults delimit the Montagna Grande block, which is a volcano-tectonic horst formed by caldera resurgence (Orsi et al., 1991). Such faults have a mean NW±SE direction on the northwestern and southeastern sides of the horst, and a mean NNE±SSW direction on its eastern side. The western side of the block is buried by recent volcanic products. Eruptive vents that formed around Montagna Grande after the volcano-tectonic event that produced it occurred along directions that follow the structural pattern of the uplifted block (Civetta et al., 1988; Orsi et al., 1991). At present, the only visible volcanic activity at Pantelleria is represented by some steam emissions, particularly in the areas of Favare, Passo del Vento and CuddõÁa di Mida, some gas manifestations in the Lake of Venus and the Gadir gulf (with gas bubbling into shallow water) and several hot water springs along the coastline (Fig. 2). A thermal aquifer is widely present in the island, with a maximum estimated temperature of 260 C (D'Alessandro et al., 1994; Squarci et al., 1994). CO2 manifestations are present at Pantelleria as separate gas phases in fumaroles, diuse emissions and bubbling gases and dissolved in ground water (divided among dissolved CO2, HCO3 and CO23 as a function of the pH of water).
The aim of the present work was to estimate the total output of CO2 from Pantelleria island, considering all the forms in which this gas is released to the surface. The authors carried out widespread measurements of CO2 concentrations from fumaroles, mofettes, bubbling gas manifestations and soils, and also measured the diffuse emission of CO2 through the soils over the entire surface of the island and the TDC content in local ground waters. The surface distribution of gas emissions on the island were also studied in order to highlight possible correlations with existing volcano-tectonic structural features. Lastly, it was intended to detect sites where anomalous gas emissions of probable magmatic origin may be monitored in the future with high-frequency sampling for the prediction of volcanic activity. 2. Sampling and analytical methods 2.1. Soil CO2 concentration and ¯ux Samples for CO2 determination were collected in a gas-tight syringe after inserting a te¯on probe in the soil to a depth of 50 cm. Each sample was analysed using a portable ®xed wavelength IR spectrophotometer (ADC Lim., UK, accuracy within 5%). The CO2 concentrations obtained are in % vol enrichment relative to ambient CO2. Carbon dioxide ¯ux measurements were
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performed using the method of Gurrieri and Valenza (1988). A special probe is inserted into the soil to a depth of 50 cm and connected to the IR spectrophotometer. By pumping at a constant ¯ow rate, the mixture of soil gas and air inside the probe reaches a constant concentration of CO2. Such values (called dynamic because of the way they are determined) are directly proportional to the ¯ux of CO2 through the soil according to the relation: k Cdyn
Fig. 2. (a) Structural map of Pantelleria Island with location of sampling sites for soil and fumarole gases. Solid lines indicate tectonic and volcano-tectonic structures (after Orsi et al., 1991); dashed curved lines indicate caldera rims; solid dots=soil CO2 measurements; solid triangles=free and bubbling gases; P=Passo del Vento fumaroles; F=Favare fumaroles; L=Lake of Venus; M=CuddõÁa di Mida. (b) Hydrogeological sketch map of Pantelleria Island with locations of sampling sites for rain and ground water. Dashed bold lines indicate the boundaries of the 3 main hydrogeological basins of the island (indicated with capital letters in italics); open circles=ground water points; solid rhombs=rain gauges; D=Daietti water well; L=Lake of Venus; N=NikaÁ water well.
1
where is the ¯ux of soil CO2 (g cm 2 s 1), Cdyn is the dynamic concentration of CO2 (ppm vol) and k is an empirical constant that depends on the geometry of the sampling system, the ¯ow of the pump, and soil permeability (e.g. Gurrieri and Valenza, 1988; Giammanco et al., 1995, 1997). Soil permeability, unlike the other parameters listed above which can easily be kept constant during measurements in the ®eld, can have dierent values for each area of measurement in the ®eld. Although Gurrieri and Valenza (1988) did not explicitly relate their multiplication factor in Eq. (1) to soil permeability, the authors think that the greater the dierence between the real soil permeability and that of the soil test piece used in the laboratory by Gurrieri and Valenza (1988), the greater the error in the calculation of the value of k and hence of the ¯ux of CO2 through the soil. The soil test piece used by Gurrieri and Valenza (1988) had a permeability of about 210 11 m2, and the corresponding value of k was 6.4410 11 g ppm 1 cm 2 s 1. Soils of Pantelleria show a substantial homogeneity, with an average permeability value of about 310 10 m2, calculated from an average transmissivity coecient of 0.145 measured during geothermal drilling on the island (Barbier, 1969). This permeability value is one order of magnitude higher than that of the soil used in the laboratory for testing the CO2 ¯ux system. In the authors' opinion, such a dierence signi®cantly aects the value of k in Eq. (1) when computing the ¯ux values. In order to calculate the multiplier constant in Eq. (1) relevant to the soils of Pantelleria the above calculated value of local soil permeability was used and also the calculated ratio between ``static'' (Cstat ) and ``dynamic'' (Cdyn ) CO2 concentrations (both expressed as molar fraction) measured in the soils of the island. In the soil test piece used by Gurrieri and Valenza (1988) the average value of this ratio was found to be about 29.5 at soil CO2 concentrations lower than 0.6. At Pantelleria the average Cstat =Cdyn value was about 5.8, and in this case soil CO2 concentrations did not exceed 0.6 in terms of molar fraction (Table 2). The average Cstat =Cdyn ratio at Pantelleria is even lower, i.e. close to 4, if the only value (28.7, from site labelled 10 in Fig. 2b and in Table 2) that is above the mean+1 standard
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deviation is discarded. In any case, the lower mean Cstat =Cdyn ratio calculated at Pantelleria compared to that from the Gurrieri and Valenza (1988) test is ascribed to higher values of dynamic concentration of soil CO2 due to the higher permeability of the local soil. Higher soil permeability actually allows proportionally more gas to ¯ow through a unit volume of ground, according to Darcy's law. The Cstat =Cdyn ratio at Pantelleria is about one order of magnitude lower than that of Gurrieri and Valenza (1988), and this agrees well with the higher soil permeability, about one order of magnitude, at Pantelleria. Assuming an average soil permeability value of 310 10 m2 and a Cstat =Cdyn ratio of about 4 for Pantelleria Island, the corresponding value of k in Eq. (1) is about 9.7510 12 g ppm 1 cm 2 s 1. New laboratory experiments are currently under way to test the method used to measure soil CO2 dynamic concentrations using soils with a broad range of permeabilities, in order to obtain experimental values of k for dierent types of ground, and hence to calculate more reliable values of soil CO2 ¯ux. Ninety-one CO2 concentration and ¯ux measurements were carried out over Pantelleria Island from 21± 23 September 1998, with a sampling grid of about 1 point/km2 (Fig. 2a). In addition to these, 35 CO2 ¯ux measurements were made in the Lake of Venus area (surface of about 0.16 km2) and 20 were made in the Favare area (surface of about 0.1 km2). In these two areas, sampling density was higher because of the greater soil degassing and the presence of free gas manifestations (see below). 2.2. CO2 in free gas manifestations CO2 output from the fumarolic emissions at Favare and Lake of Venus was estimated using direct measurements of steam output made with a specially designed device described by Italiano et al. (1998). Taking into account the H2O output and the fumarolic concentration ratios between vapor and CO2 gases, the CO2 output is calculated from the following relation: QCO2 QH2 O CCO2 =CH2 O
2
where Q indicates the output and C indicates the concentrations of CO2 and H2O. In order to determine the CO2 concentration, fumarolic gas samples were collected using both glass ¯asks equipped with vacuum stopcocks and pre-evacuated ¯asks containing an alkaline solution (4 M NaOH solution) in which steam condenses and CO2 is absorbed (Giggenbach, 1975). Gas samples were analysed in the laboratory using a Perkin-Elmer Sigma 8500 gas chromatograph, with Ar as carrier gas and equipped with Carbosieve S-II columns and HWD-FID detectors.
Concentration of CO2 dissolved in alkaline solution was determined by potentiometric titration. Measurements of CO2 output from bubbling gases into lake or sea water were made using the method described by Italiano and Nuccio (1991, 1994). An inverted stainless-steel funnel connected to a reversible bottle of known volume was placed on the degassing vent. Gas output was evaluated by measuring the time required for the emitted gas to displace water in the bottle, after correction of the volume of emitted gas for hydrostatic pressure. After each output measurement, the gas in the bottle was stored in glass ¯asks equipped with vacuum stopcocks and then analysed for CO2 content in the laboratory using the gas-chromatographic methods described above. 2.3. CO2 dissolved in ground water The amount of CO2 dissolved in ground water at Pantelleria was calculated using the measured values for dissolved TDC and estimates of ground water supply to the ground water system. To achieve this goal 15 thermal and cold groundwaters were sampled and analysed to evaluate the average amount of TDC (Fig. 2). An estimate of the groundwater supply to the aquifer examined was obtained assuming the following water budget: I P R E, where I is the amount of water that in®ltrates (in mm/a), P is the amount of precipitation (in mm/a), R is the rilling coecient of the basin (in mm/a) and E is the quantity of water returned to the atmosphere by evapotranspiration (in mm/a). In the case of Pantelleria the value of R was taken to be negligible. On the basis of the data from the local rain gauge network (Fig. 2b), precipitation was estimated using the method proposed by Thiessen (1911). The calculation of the evapotranspiration term was carried out using the equation proposed by Thornthwaite (1948). Determination of TDC (dissolved CO2, HCO3 and CO23 ) was carried Table 1 Isotopic values for carbon and oxygen in CO2 measured at the sample sites with the highest CO2 concentration at Pantelleria d18O CO2 Sampling sites Date d13C CO2 Permil vs. PDB Gadir Gadir Passo del Vento Passo del Vento P 72a P 71a P 60a
01/13/1998 02/03/1998 11/05/1996 03/13/1998 09/22/1998 09/22/1998 09/22/1998
4.8 5.6 4.0 4.6 5.2 4.1 5.8
Permil vs. V-SMOW 31.9 32.1 18.7 20.9 27.7 26.9 26.1
a Points sampled during the soil gas survey over the whole of Pantelleria.
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out using dierent analytical methods. Carbonates and bicarbonates were measured by titration with 0.1 N HCl. The amount of dissolved CO2 was determined by means of the method proposed by Capasso and
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Inguaggiato (1998) and Capasso et al. (1998a), which is based on the partitioning equilibrium of gaseous species between liquid and gas phase. Values of TDC are expressed in tons of CO2-equivalent a 1.
Table 2 Soil CO2 concentration and ¯ux values measured over the whole surface of Pantelleria and soil CO2 ¯ux values measured in the areas of Lake of Venus and Favare Sampling points Pantelleria 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 a
Soil CO2 concentration (% v/v)
Soil CO2 ¯ux (g cm 2 s 1)
Sampling points
Soil CO2 concentration (% v/v)
Soil CO2 ¯ux (g cm 2 s 1)
< LODa 0.04 0.12 < LOD < LOD 0.02 < LOD < LOD 0.02 0.56 < LOD < LOD < LOD 0.04 < LOD < LOD < LOD < LOD < LOD < LOD < LOD 0.08 < LOD < LOD < LOD < LOD < LOD 0.14 < LOD < LOD 0.04 < LOD < LOD < LOD < LOD < LOD < LOD 0.22 0.02 < LOD < LOD 0.08 < LOD < LOD < LOD
< LOD 3.7810 < LOD < LOD < LOD < LOD < LOD < LOD < LOD 1.7910 < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD 3.8010 < LOD < LOD < LOD < LOD < LOD < LOD < LOD
46 47 48 49 50 51 52 53 54 55 56 57 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91
< LOD 0.44 < LOD < LOD < LOD < LOD 0.12 < LOD < LOD < LOD 1.2 0.26 0.22 3.25 0.51 0.008 0.06 0.24 < LOD < LOD 0.16 < LOD 0.12 < LOD 2.4 3.61 < LOD < LOD < LOD < LOD 0.08 < LOD 0.08 < LOD 0.06 < LOD 0.08 0.24 0.04 0.65 0.02 < LOD < LOD 0.02 < LOD
< LOD < LOD < LOD < LOD < LOD < LOD 9.4910 < LOD < LOD < LOD 1.0410 1.9010 1.9010 3.0410 4.7510 < LOD < LOD 1.9010 < LOD < LOD < LOD < LOD < LOD < LOD 2.3210 3.3410 < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD 1.9010 < LOD < LOD 7.6010 < LOD < LOD < LOD < LOD < LOD
9
9
9
Sampling points
Soil CO2 ¯ux (g cm 2 s 1)
Lake of Venus 1 < LOD 2 < LOD 3 < LOD 4 3.9010 5 3.9010 6 4.3910 7 3.9010 8 5.8510 9 4.8810 10 6.3410 11 3.0210 12 1.4610 13 < LOD 14 1.5110 15 5.8510 16 1.4110 17 < LOD 18 < LOD 19 9.7510 20 8.7810 21 < LOD 22 3.5110 23 < LOD 24 < LOD 25 < LOD 26 < LOD 27 < LOD 28 9.2610 29 < LOD 30 < LOD 31 3.9010 32 3.9010 33 3.5110 34 3.9010 35 1.7610
9
7 9 8 7 8
9
7 7
9 9 8 8 7 7 8 7 6
7 9 7
9 8
Sampling points Favare 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
8
8
9 9 8 9 7
9
9
LOD (limit of detection)=0.01% v/v for CO2 concentrations; LOD=9.7510
10
g cm
2
s
1
for soil CO2 ¯uxes.
Soil CO2 ¯ux (g cm 2 s 1) 7.7010 < LOD 2.3410 7.8010 3.9010 < LOD < LOD < LOD 3.9010 7.8010 2.9210 3.9010 4.2910 2.2410 2.3410 1.7510 2.5310 1.7510 2.9210 4.0010
8
8 9 8
9 9 8 9 7 7 8 6 8 7 7 7
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3. Results and discussion Table 1 shows the isotopic values for C and O in CO2 emitted from the highest degassing sites on the island. The d13C values for C in CO2 range from 4.0 to 5.8%. According to the classic interpretation (e.g. Taylor et al., 1967; Deines, 1970; Allard, 1986) such isotopic values would be compatible with a magmatic origin for the gas [considering a MORB-type magmatic component with a mean d13C CO2 value of 6.5%], but they would imply a variable contribution of heavier C from thermo-metamorphosed limestones. Using the method proposed by Sano and Marty (1995), based on the calculated CO2/3He ratios and on the measured d13C values in gas samples from volcanic areas, the authors tried to estimate the relevant contribution from dierent sources of CO2 and He (i.e. magma, sediments and limestones). According to the values available for He isotopes (R=Ra=7.0 and 7.2; R=8.5210 6 and 9.710 6) measured by D'Alessandro et al. (1994) in the highest degassing sites of Lake of Venus and Favare, respectively, the relevant values for the CO2/3He ratio have been calculated. These values (8.5109 and 7.1109, respectively) were plotted against the associated values of d13C CO2 [ 3.8 and 5.2, respectively, according to D'Alessandro et al. (1994)]. Fig. 3 shows that such values fall within the ®eld indicated by Sano and Marty (1995) for high-temperature volcanic gases, thus suggesting that both CO2 and He are indeed derived mainly from magma that has a composition close, but not equal, to MORB. The apparent contribution of CO2 from limestones in the two samples considered is signi®cant, ranging from about 40 to about 25%, respectively. This computation is based on the assumption that the magmatic source of CO2 and He from Pantelleria is MORB-like. However, recent geochemical studies (Allard et al., 1997; Capasso et al., 1997, 1998b, 1999; Giammanco et al., 1998; Inguaggiato and Italiano, 1998; Inguaggiato and Pecoraino, 1998; Inguaggiato et al., 2000) showed that volcanic gases from all the active Italian volcanoes are characterised by more positive values of d13C CO2 and lower values of He isotopic ratio relative to MORB (inferred ``mantle'' isotopic values in the range 3 to 0% for C and around 7.5 R=Ra for He). These authors hypothesised that such peculiar isotopic values do not re¯ect shallow contamination processes during uprise of the magmatic ¯uids (i.e. input of CO2 derived from thermal metamorphism of carbonate sediments, contamination of 4 He at shallow crustal levels), but rather suggest that the source of magmatic gas is not MORB-like. More generally, mantle beneath central Europe, including the Mediterranean, would be already contaminated at depth, in accord with the results of isotopic studies on ¯uid inclusions from Etna's lavas (Marty et al., 1994), on geothermal ¯uids discharged from the western Eger
Fig. 3. Correlation diagram between CO2/3He and d13C-value for low-temperature gases emitted at Lake of Venus mofette and Favare fumaroles (solid stars). Also plotted are values relevant to high-temperature volcanic gases from volcanic areas of the world (open squares; data from Sano and Marty, 1995). Model end-members of mid-ocean ridge basalt (MORB), limestone+slab carbonate and sediment rocks are also indicated. Lines show mixing processes among the 3 end-members.
rift system in central Europe (Weinlich et al., 1999) and on volcanic rocks of western and central Europe (Hoernle et al., 1995). Based on the above considerations, it is hypothesised that at Pantelleria also the measured C and He isotopic values mainly re¯ect their local magmatic sources, but are somewhat modi®ed by shallow processes between magmatic gases and crustal ¯uids and rocks. Preliminary results of a geochemical study on such processes at Pantelleria (Favara et al., 1999) indicate original C and He isotopic values in the same respective ranges as those inferred for the other central European volcanoes. Table 2 shows soil CO2 concentration and ¯ux values measured in the 91-point survey over the whole surface of Pantelleria, those of soil CO2 ¯ux measured around Lake of Venus and those of soil CO2 ¯ux and soil temperature measured in the Favare area. Soil CO2 concentration values from the 91-point survey ranged from 0.02 to 3.61% v/v, omitting values below the instrumental detection limit (0.01% v/v), with a mean value of 0.45% v/v. Soil CO2 ¯ux values from the same survey ranged from 1.7910 9 to 3.3410 7 g cm 2 s 1, omitting values below the instrumental detection limit (LOD=9.7510 10 g cm 2 s 1), with a mean value of 7.6710 8 g cm 2 s 1. In the Lake of Venus area, soil CO2 ¯ux values ranged from 3.9010 9 to 1.4610 6 g cm 2 s 1, omitting values below LOD, with a mean value of 1.8010 7 g cm 2 s 1. In the Favare area, soil CO2 ¯uxes ranged from 3.9010 9 to 1.7510 6 g cm 2
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s 1, omitting values below LOD, with a mean value of 2.2010 7 g cm 2 s 1. In order to assess the anomaly threshold for both concentration and ¯ux of soil CO2, log probability plots were used for all 147 soil gas sample points at Pantelleria (Fig. 4). The log probability plot for CO2 concentrations indicates two in¯ection points at 42 and 73%, indicating the presence of three distinct populations. Population A contains values below 0.08% v/v, which therefore can be regarded as the upper limit for background CO2 con-
Fig. 4. Log probability plots for (a) log soil CO2 concentrations and (b) log soil CO2 ¯uxes measured at Pantelleria. Values for all the areas surveyed have been considered in the calculations. Three statistically distinct populations (indicated with capital letters) have been recognised in plot (a), 2 in plot (b); population A includes background values for each parameter.
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centrations. Population C contains values above 0.4% v/v, mostly measured in or close to areas of focussed degassing and/or fumarolic emissions (Lake of Venus, Favare, Passo del Vento, CuddõÁa di Mida), which is interpreted as due to convective transport of CO2 in active geothermal systems. The log probability plot for CO2 ¯uxes indicates only one evident in¯ection point at 35%, indicating the presence of 2 distinct populations. Population A contains values below 3.9010 8 g cm 2 s 1, which therefore can be set as the upper limit for background CO2 ¯ux level. Both above threshold values were used to highlight anomalous levels in the surface maps of soil CO2 concentrations and ¯uxes, and the latter also to discriminate between ``organic'' and ``magmatic'' CO2 emissions in the estimate of the CO2 budget for the island. In order to obtain as accurate an estimate of the CO2 budget as possible from a given area a realistic approach is to acquire data from a sampling grid in which the distance between sampling points is comparable to the spatial extent of expected anomalies. In the present investigation, the main aim was to have an initial, broad idea of the existence of anomalous degassing areas other than those evident in the ®eld or known from literature (i.e. fumarolic ®elds and areas of focussed degassing). Therefore, a sampling grid of about 1 point km 2 was chosen. One way to process the acquired soil gas data was to perform surface mapping using the Kriging method (Swan and Sandilands, 1995), which was used to produce maps of concentration or ¯ux contour lines for all sampled areas. Fig. 5 shows the distribution of soil concentrations and soil ¯uxes of CO2 measured over the entire surface of Pantelleria using the Kriging method. Both parameters show a fairly similar surface distribution over the island. Due to the relatively low density of sampling points, no clear correlation is seen between anomalous soil degassing and the structural pattern of Pantelleria. However, the highest degassing areas (i.e. Lake of Venus, Favare, Passo del Vento and CuddõÁa di Mida) are located close to an old caldera rim, close to some important faults that bound the central resurgent block of Montagna Grande and on the summit of an eruptive center (CuddõÁa di Mida cone), whose age ranges between 10 and 8 ka (Civetta et al., 1988), respectively (Fig. 2a). This suggests a tectonic control on diuse degassing of magmatic origin. Only 5 of the values measured during the extensive survey at Pantelleria exceed the above threshold calculated for soil CO2 ¯uxes. The average value obtained from these anomalous ¯uxes was about 2.0410 7 g cm 2 s 1 and this corresponds to about 64,000 t km 2 a 1. By assigning an overall surface of about 10 km2 to the areas aected by such soil CO2 degassing, deduced from the surface bounded by the 3.910 8 g cm 2 s 1 contour line in Fig. 5b, a value of about 0.64 Mt a 1 was obtained. However, when the Kriging method is
890
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Fig. 5. Maps of areal distribution of (a) soil CO2 concentrations and (b) soil CO2 ¯uxes at Pantelleria. Contour lines have been obtained by the Kriging interpolation method (Swan and Sandilands, 1995). Shaded areas include values higher than thresholds obtained using the log probability plots of Fig. 4 (0.08% v/v for CO2 concentrations and 3.9010 8 g cm 2 s 1 for CO2 ¯uxes). The areas of Lake of Venus (LV) and Favare (F) were not considered in the calculations.
used with large data spacing such as that of the 91-point survey the computed areas are inevitably aected by a signi®cant error. Delineating the area of anomalies at Pantelleria in a more accurate way, for example by increasing the density of measurement points, is actually very dicult due to site inaccessibility (presence of steep rocky walls and inaccessible mountains, presence of
fenced private land, etc.). Therefore, an alternative way to Kriging in order to estimate the output of diuse CO2 from Pantelleria was attempted. First the 5 soil CO2 ¯ux values from the 91-point survey above the organic threshold were considered. Values were assigned to each relevant square kilometer area with reference to the sampling grid used. The resulting 5 values for annual CO2 output were then summed. The total output thus obtained is about 0.32 Mt/a, that is half that obtained with the Kriging method. However, the Kriging method certainly overestimates degassing areas because of its limitations when applied to the small number of measurements over the Pantelleria surface, and because diffuse degassing tends to occur through relatively small areas usually corresponding to tectonic structures, as suggested above. Therefore, the authors prefer to use the output value obtained with the second method to that obtained using the Kriging method. In areas of focussed degassing, such as Lake of Venus and Favare, the Kriging method can be applied more safely, because of the higher density of sampling points compared to the total area surveyed. In these cases, the results describe the distribution of anomalous soil degassing more realistically. In the Lake of Venus area (Fig. 6) the highest soil CO2 emissions were detected on the western and southwestern sides of the lake, close to the caldera rim. In this area, the average soil CO2 ¯ux calculated for the points whose measured ¯ux values exceeded the imposed threshold was 2.9810 7 g cm 2 s 1, which gives an estimated output of about 0.015 Mt a 1 from a surface of 0.16 km2. In the Favare area, the highest CO2 emissions are located in the southeastern part (Fig. 7a) and seem to be oriented roughly NE±SW, in the direction of one of the faults that bound the Montagna Grande horst to the south (see Fig. 2a). In this area, anomalous soil CO2 degassing had an average value of 4.2410 7 g cm 2 s 1. This value gives a soil CO2 output of 0.013 Mt a 1 from a surface of 0.1 km2, which is similar to that calculated for the Lake of Venus area. In the Favare area, a survey of soil temperatures at a depth of 50 cm was also carried out, in order to de®ne the spatial extent of the fumarolic ®eld and to acquire further information about the physical processes leading to gas release through soil. Soil temperature data were also processed by the Kriging method. The results (Fig. 7b) show a signi®cant correlation between the surface distribution of shallow soil temperatures and that of soil CO2 ¯uxes, thus suggesting that soil degassing is produced by condensation of steam. The total contribution from the Lake of Venus and Favare areas to the diuse CO2 output from Pantelleria (0.028 Mt a 1) is negligible compared to that from the 91-point survey. Moreover, considering that it is from areas of focussed degassing, it was not considered in the ®nal computation of diuse degassing. Therefore, the
R. Favara et al. / Applied Geochemistry 16 (2001) 883±894
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Fig. 6. Map of areal distribution of soil CO2 ¯ux (expressed in g cm 2 s 1) in the Lake of Venus area. Contour lines have been obtained by the Kriging interpolation method. Shaded areas include values higher than thresholds obtained using the log probability plot of Fig. 4b (3.9010 8 g cm 2 s 1). Solid stars indicate sites of bubbling gas sampling. The heavy dark line around the Lake of Venus is an access road. Altitude in m asl.
value of about 0.32 Mt a 1 is assumed as a rough, provisional estimate of the amount of diuse CO2 emitted by Pantelleria. Considering by comparison other volcanic areas of Sicily, this value is one order of magnitude lower than that obtained for the soils of Mt. Etna's ¯anks (1 to 12 Mt a 1; Allard et al., 1991; D'Alessandro et al., 1997), but it is comparable to that estimated for Vulcano Island, which is part of an active volcanic arc (Aeolian Islands) located just o the NE coast of Sicily (Chiodini et al., 1996), and that estimated for Ustica Island, an inactive volcano located o the NW coast of Sicily (Etiope et al., 1999). The value for CO2 output obtained from the Favare fumaroles, 1.42 t a 1 (about 110 6 Mt a 1), is based on a measured steam ¯ux of 0.712 cm3 s 1, a measured water/gas ratio of 0.94 and a measured CO2 concentration close to 100% in the dry gas emitted. A direct measure of CO2 output from bubbling gases was carried out in the Lake of Venus and Gadir gas manifestations. The values obtained are 6716 and 6022 t a 1, respectively, thus giving a total CO2 output of about 0.013 Mt a 1 from such manifestations. The contribution of CO2 from TDC has been estimated in the following way: ®rst a calculated total annual rainfall of about 55106 m3 was considered, of which 24106 m3 eectively in®ltrate, Pantelleria's surface was divided into 3 sub-areas (Fig. 2b and Table 3) that correspond to the 3 main hydrogeological basins of
Fig. 7. Map of areal distribution of (a) soil CO2 ¯ux (expressed in g cm 2 s 1) and (b) soil temperature (measured at 50 cm depth and expressed in C) in the Favare area (not inclusive of fumarole emissions). Contour lines have been obtained by the Kriging interpolation method. Shaded areas include values higher than thresholds obtained using the log probability plot of Fig. 4b (3.9010 8 g cm 2 s 1). F=Favare fumaroles; P=Passo del Vento fumaroles. Altitude in m asl.
the island (R. Favara, S. Inguaggiato and G. Pecoraino, in prep.). For each sub-area 1 ground water sampling site was selected, where the total dissolved C content, expressed as equivalent CO2, is thought to match closely that in the deep geothermal ¯uids that interact with shallow waters (R. Favara, S. Inguaggiato and G. Pecoraino, in prep.). At the sampling sites chosen mean annual values of concentration of equivalent dissolved CO2 range between 16 and 40 mmols l 1 (Table 3). Therefore, a total amount of about 0.034 Mt a 1 of equivalent dissolved CO2 was obtained. In this calculation it was assumed that the contribution of bicarbonate from rain water due to its equilibration with atmospheric
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Table 3 Hydrogeological data and dissolved CO2 contents used to estimate the contribution of aquifers to the total budget of CO2 at Pantelleria Hydrogeological basins
Surface of each basin (km2)
Ground water sampling sitesa
Equivalent dissolved CO2 content (mmols/l 1)
CO2 output (Mt a 1)
A B C
17 37 30
D L N
16 35 40
0.0033 0.016 0.015
a
See Fig. 2b for location of sampling sites.
Table 4 Summary of the estimated contribution of CO2 from the different types of gas emissions on Pantelleria island Type of CO2 emission
Number of sampling points
Output (Mt a 1)
Surface (km2)
Soil gas Focussed degassing Fumarole gas Bubbling gas Dissolved
91 56 1 7 15
0.32 0.028 1.410 0.013 0.034
84 0.26 0.001 0.01 84
6
Table 5 CO2 output from some volcanic and geothermal areas of the world Mt/a
Ref.
Pantelleria Etna
0.39 13±25
Vulcanoa
0.13±0.14
Solfatarab Usticab Mid-Oceanic Volcanic System Popocatepetlc Mammoth Mountain White Islandc Mt. Erebusc New Zealand Geothermal Systems
0.048 0.26 30±65
This work Allard et al., 1991; D'Alessandro et al., 1997 Baubron et al., 1991; Bonfanti et al., 1993; Italiano and Nuccio, 1994; Chiodini et al., 1996, 1998 Chiodini et al., 1998 Etiope et al., 1999 Gerlach, 1991
14.5±36.5 0.2
Delgado et al., 1998 Sorey et al., 1998
0.95 0.66 0.002±0.048
Wardell and Kyle, 1998 Wardell and Kyle, 1998 Seward and Kerrick, 1996
a b c
Based on soil gas and fumarolic gas emissions. Based on soil gas emissions. Based on crater plume emissions.
CO2 is negligible. However, large volumes of sea water, whose content of equivalent dissolved CO2 is 2.5 mmols l 1, that intrude into Pantelleria's aquifers and dilute the
C species in ground water need to be considered. Therefore, the amount of equivalent dissolved CO2 indicated above is almost certainly underestimated. 4. Conclusions The investigation of CO2 emission at Pantelleria Island has shown that important degassing of this gas takes place on this currently quiescent volcano. The total CO2 output from all types of ¯uids at Pantelleria is preliminarily estimated as about 0.39 Mt a 1, the largest contribution by far being from diuse degassing (Table 4). Results of C and He isotope measurements on the gas collected from the sites with highest degassing suggest that the source of the emitted CO2 is magma with a composition quite dierent from MORB, but compatible with the mantle source individuated for the central Mediterranean and central European volcanoes. The estimated total amount of CO2 emitted from Pantelleria, although lower than that from other active volcanoes of the world (Table 5), indicates that Pantelleria is a site of active mantle degassing. This evidence suggests that this volcanic area is far from being extinct and deserves more careful volcanological monitoring. Acknowledgements We wish to thank F. Italiano and S. Francofonte for their help during the ®eld work, and S. ArnoÂrsson and W.C. Evans for their critical reviews that helped improve this manuscript. This work was funded by the National Group for Volcanology, Italy.
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Mahood, G.A., Hildreth, W., 1986. Geology of the peralkaline volcano at Pantelleria, Strait of Sicily. Bull. Volcanol. 48, 143±172. Marty, B., Trull, T., Lussiez, P., Basile, I., Tanguy, J.C., 1994. He, Ar, O, Sr and Nd isotope constraints on the origin and evolution of Mount Etna magmatism. Earth Planet. Sci. Lett. 126, 23±39. Orsi, G., Gallo, G., Zanchi, A., 1991. Simple-shearing block resurgence in caldera depressions. A model from Pantelleria and Ischia. J. Volcanol. Geotherm. Res. 47, 1±11. Sano, Y., Marty, B., 1995. Origin of carbon in fumarolic gas from island arcs. Chem. Geol. 119, 265±274. Seward, T.M., Kerrick, D.M., 1996. Hydrothermal CO2 emission from the Taupo Volcanic Zone, New Zealand. Earth Planet. Sci. Let. 139, 105±113. Sorey, M.L., Evans, W.C., Kennedy, B.M., Farrar, C.D., Hainsworth, L.J., Hausback, B., 1998. Carbon dioxide and helium emissions from a reservoir of magmatic gas beneath Mammoth Mountain, California. J. Geophys. Res. 103, 15,303±15,323. Squarci, P., Gianelli, G., Grassi, S., Mussi, M., D'Amore, F., 1994. Preliminary results of geothermal prospecting on the island of Pantelleria (Italy). Acta Vulcanol. 5, 117±123.
Swan, A.R.H., Sandilands, M., 1995. Introduction to geological data analysis. Blackwell Science, Oxford. Taylor, H.P., Frechen, J., Degens, E.T., 1967. Oxygen and carbon isotope studies of carbonatites from the Laacher See district, West Germany and the AlnoÈ district, Sweden. Geochim. Cosmochim. Acta 31, 407±430. Thiessen, A.H., 1911. Precipitation average for large areas. Monthly Weather Rev. 3, 45±81. Thornthwaite, C.W., 1948. An approach towards a rational classi®cation of climate. Geogr. Rev. Am. Geophys. Soc. 55, 94. Villari, L., 1974. The island of Pantelleria. Bull. Volcanol. 38, 680±724. Wardell, L. J., Kyle, P. R., 1998. Volcanic carbon dioxide emission rates: White Island, New Zealand and Mt. Erebus, Antarctica (abstract). EOS Trans., AGU, 79(45), Fall Meeting Suppl., p. 927. Weinlich, F.H., BraÈuer, K., KaÈmpf, H., Strauch, G., Tesar, J., Weise, S.M., 1999. An active subcontinental mantle volatile system in the western Eger rift, Central Europe: gas ¯ux, isotopic (He, C, and N) and compositional ®ngerprints. Geochim. Cosmochim. Acta 63, 3653±3671.
www.elsevier.com/locate/apgeochem
Preliminary estimate of CO2 output from Pantelleria Island volcano (Sicily, Italy): evidence of active mantle degassing Rocco Favara a, Salvatore Giammanco a,*, Salvatore Inguaggiato b,c, Giovannella Pecoraino a a
Istituto Geochimica dei Fluidi, CNR, via Ugo La Malfa 153-90146 Palermo, Italy b Istituto Nazionale di Geo®sica, via di Vigna Murata 605-00143 Rome, Italy c Poseidon System, Piazza Roma 2-95123 Catania, Italy Received 11 October 1999; accepted 12 June 2000 Editorial handling by H. AÂrmannsson
Abstract Total CO2 output from fumaroles, bubbling and water dissolved gases and soil gases was investigated at Pantelleria Island volcano, Italy. The preliminary results indicate an overall output of 0.39 Mt a 1 of CO2 from the island. The main contribution to the total output was from diuse soil degassing (about 0.32 Mt a 1), followed by dissolved CO2 (0.034 Mt a 1), focussed soil degassing (0.028 Mt a 1) and bubbling CO2 (0.013 Mt a 1). The contribution of CO2 from fumarole gases was found to be negligible (1.410 6 Mt a 1). Carbon-13 values for CO2 coupled with those for associated He in gases from fumaroles and sites of focussed soil degassing clearly rule out any signi®cant organic CO2 component and suggest a common mantle origin for these gas species. The inferred mantle source beneath Pantelleria would seem to have peculiar geochemical characteristics, quite distinct from those of mantle producing MORB but compatible with those of magmatic sources of central Mediterranean and central European volcanoes. These ®ndings indicate that the Pantelleria volcanic complex is a site of active mantle degassing that is worthy of attention for future geochemical surveillance of the island. # 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction Quantifying the CO2 budget from volcanic areas of the world can provide information useful for modeling the global carbon cycle. Although still far from having achieved this goal, studies on present-day CO2 degassing over the last years have shown the importance of CO2 contribution to the atmosphere not only from actively degassing volcanoes, such as Mt. Etna, Kilauea and Popocatepetl (Gerlach and Graeber, 1985; Allard et al., 1991; Delgado et al., 1998), but also from those in a quiescent state (e.g. Sorey et al., 1998; Etiope et al., 1999). Pantelleria Island (Fig. 1) is a quiescent volcano located in the Sicily Channel, 110 km south of Sicily and 70 km north of Tunisia. Its total emerged surface is 84
* Corresponding author. Fax: +39-91-680-9449. E-mail address: [email protected] (S. Giammanco).
km2 and its highest point is at Montagna Grande (836 m asl). Pantelleria Island represents the uppermost part of a large volcanic edi®ce that extends below sea level to a depth of about 1200 m and lies along an intra-plate rift system directed NW±SE in the median part of the Sicily Channel. The rift is still active and the NW±SEtrending strike-slip faults that form it are in many cases connected with N±S normal faults producing pull-apart basins (Orsi et al., 1991). The volcanic products that outcrop on the island are mainly pantellerites to trachytes, and only minor mildly alkalic basalts are found (e.g. Villari, 1974; Civetta et al., 1988). The oldest outcropping rocks date to 324.210.5 ka (Mahood and Hildreth, 1986) and the latest volcanic activity in the area took place during the 19th century with two submarine eruptions (1831 and 1891) in the vicinity of the island. The NW±SE and the N±S-trending regional faults seem to strongly control basaltic volcanic activity at
0883-2927/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0883-2927(00)00055-X
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Fig. 1. Structural map of Channel of Sicily and location of the island of Pantelleria (modi®ed from Orsi et al., 1991).
Pantelleria, as most of the volcanic eruptions of this type at Pantelleria occurred on the two structural trends mentioned above (Villari, 1974; Civetta et al., 1988). Subvertical inland dipping faults delimit the Montagna Grande block, which is a volcano-tectonic horst formed by caldera resurgence (Orsi et al., 1991). Such faults have a mean NW±SE direction on the northwestern and southeastern sides of the horst, and a mean NNE±SSW direction on its eastern side. The western side of the block is buried by recent volcanic products. Eruptive vents that formed around Montagna Grande after the volcano-tectonic event that produced it occurred along directions that follow the structural pattern of the uplifted block (Civetta et al., 1988; Orsi et al., 1991). At present, the only visible volcanic activity at Pantelleria is represented by some steam emissions, particularly in the areas of Favare, Passo del Vento and CuddõÁa di Mida, some gas manifestations in the Lake of Venus and the Gadir gulf (with gas bubbling into shallow water) and several hot water springs along the coastline (Fig. 2). A thermal aquifer is widely present in the island, with a maximum estimated temperature of 260 C (D'Alessandro et al., 1994; Squarci et al., 1994). CO2 manifestations are present at Pantelleria as separate gas phases in fumaroles, diuse emissions and bubbling gases and dissolved in ground water (divided among dissolved CO2, HCO3 and CO23 as a function of the pH of water).
The aim of the present work was to estimate the total output of CO2 from Pantelleria island, considering all the forms in which this gas is released to the surface. The authors carried out widespread measurements of CO2 concentrations from fumaroles, mofettes, bubbling gas manifestations and soils, and also measured the diffuse emission of CO2 through the soils over the entire surface of the island and the TDC content in local ground waters. The surface distribution of gas emissions on the island were also studied in order to highlight possible correlations with existing volcano-tectonic structural features. Lastly, it was intended to detect sites where anomalous gas emissions of probable magmatic origin may be monitored in the future with high-frequency sampling for the prediction of volcanic activity. 2. Sampling and analytical methods 2.1. Soil CO2 concentration and ¯ux Samples for CO2 determination were collected in a gas-tight syringe after inserting a te¯on probe in the soil to a depth of 50 cm. Each sample was analysed using a portable ®xed wavelength IR spectrophotometer (ADC Lim., UK, accuracy within 5%). The CO2 concentrations obtained are in % vol enrichment relative to ambient CO2. Carbon dioxide ¯ux measurements were
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performed using the method of Gurrieri and Valenza (1988). A special probe is inserted into the soil to a depth of 50 cm and connected to the IR spectrophotometer. By pumping at a constant ¯ow rate, the mixture of soil gas and air inside the probe reaches a constant concentration of CO2. Such values (called dynamic because of the way they are determined) are directly proportional to the ¯ux of CO2 through the soil according to the relation: k Cdyn
Fig. 2. (a) Structural map of Pantelleria Island with location of sampling sites for soil and fumarole gases. Solid lines indicate tectonic and volcano-tectonic structures (after Orsi et al., 1991); dashed curved lines indicate caldera rims; solid dots=soil CO2 measurements; solid triangles=free and bubbling gases; P=Passo del Vento fumaroles; F=Favare fumaroles; L=Lake of Venus; M=CuddõÁa di Mida. (b) Hydrogeological sketch map of Pantelleria Island with locations of sampling sites for rain and ground water. Dashed bold lines indicate the boundaries of the 3 main hydrogeological basins of the island (indicated with capital letters in italics); open circles=ground water points; solid rhombs=rain gauges; D=Daietti water well; L=Lake of Venus; N=NikaÁ water well.
1
where is the ¯ux of soil CO2 (g cm 2 s 1), Cdyn is the dynamic concentration of CO2 (ppm vol) and k is an empirical constant that depends on the geometry of the sampling system, the ¯ow of the pump, and soil permeability (e.g. Gurrieri and Valenza, 1988; Giammanco et al., 1995, 1997). Soil permeability, unlike the other parameters listed above which can easily be kept constant during measurements in the ®eld, can have dierent values for each area of measurement in the ®eld. Although Gurrieri and Valenza (1988) did not explicitly relate their multiplication factor in Eq. (1) to soil permeability, the authors think that the greater the dierence between the real soil permeability and that of the soil test piece used in the laboratory by Gurrieri and Valenza (1988), the greater the error in the calculation of the value of k and hence of the ¯ux of CO2 through the soil. The soil test piece used by Gurrieri and Valenza (1988) had a permeability of about 210 11 m2, and the corresponding value of k was 6.4410 11 g ppm 1 cm 2 s 1. Soils of Pantelleria show a substantial homogeneity, with an average permeability value of about 310 10 m2, calculated from an average transmissivity coecient of 0.145 measured during geothermal drilling on the island (Barbier, 1969). This permeability value is one order of magnitude higher than that of the soil used in the laboratory for testing the CO2 ¯ux system. In the authors' opinion, such a dierence signi®cantly aects the value of k in Eq. (1) when computing the ¯ux values. In order to calculate the multiplier constant in Eq. (1) relevant to the soils of Pantelleria the above calculated value of local soil permeability was used and also the calculated ratio between ``static'' (Cstat ) and ``dynamic'' (Cdyn ) CO2 concentrations (both expressed as molar fraction) measured in the soils of the island. In the soil test piece used by Gurrieri and Valenza (1988) the average value of this ratio was found to be about 29.5 at soil CO2 concentrations lower than 0.6. At Pantelleria the average Cstat =Cdyn value was about 5.8, and in this case soil CO2 concentrations did not exceed 0.6 in terms of molar fraction (Table 2). The average Cstat =Cdyn ratio at Pantelleria is even lower, i.e. close to 4, if the only value (28.7, from site labelled 10 in Fig. 2b and in Table 2) that is above the mean+1 standard
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deviation is discarded. In any case, the lower mean Cstat =Cdyn ratio calculated at Pantelleria compared to that from the Gurrieri and Valenza (1988) test is ascribed to higher values of dynamic concentration of soil CO2 due to the higher permeability of the local soil. Higher soil permeability actually allows proportionally more gas to ¯ow through a unit volume of ground, according to Darcy's law. The Cstat =Cdyn ratio at Pantelleria is about one order of magnitude lower than that of Gurrieri and Valenza (1988), and this agrees well with the higher soil permeability, about one order of magnitude, at Pantelleria. Assuming an average soil permeability value of 310 10 m2 and a Cstat =Cdyn ratio of about 4 for Pantelleria Island, the corresponding value of k in Eq. (1) is about 9.7510 12 g ppm 1 cm 2 s 1. New laboratory experiments are currently under way to test the method used to measure soil CO2 dynamic concentrations using soils with a broad range of permeabilities, in order to obtain experimental values of k for dierent types of ground, and hence to calculate more reliable values of soil CO2 ¯ux. Ninety-one CO2 concentration and ¯ux measurements were carried out over Pantelleria Island from 21± 23 September 1998, with a sampling grid of about 1 point/km2 (Fig. 2a). In addition to these, 35 CO2 ¯ux measurements were made in the Lake of Venus area (surface of about 0.16 km2) and 20 were made in the Favare area (surface of about 0.1 km2). In these two areas, sampling density was higher because of the greater soil degassing and the presence of free gas manifestations (see below). 2.2. CO2 in free gas manifestations CO2 output from the fumarolic emissions at Favare and Lake of Venus was estimated using direct measurements of steam output made with a specially designed device described by Italiano et al. (1998). Taking into account the H2O output and the fumarolic concentration ratios between vapor and CO2 gases, the CO2 output is calculated from the following relation: QCO2 QH2 O CCO2 =CH2 O
2
where Q indicates the output and C indicates the concentrations of CO2 and H2O. In order to determine the CO2 concentration, fumarolic gas samples were collected using both glass ¯asks equipped with vacuum stopcocks and pre-evacuated ¯asks containing an alkaline solution (4 M NaOH solution) in which steam condenses and CO2 is absorbed (Giggenbach, 1975). Gas samples were analysed in the laboratory using a Perkin-Elmer Sigma 8500 gas chromatograph, with Ar as carrier gas and equipped with Carbosieve S-II columns and HWD-FID detectors.
Concentration of CO2 dissolved in alkaline solution was determined by potentiometric titration. Measurements of CO2 output from bubbling gases into lake or sea water were made using the method described by Italiano and Nuccio (1991, 1994). An inverted stainless-steel funnel connected to a reversible bottle of known volume was placed on the degassing vent. Gas output was evaluated by measuring the time required for the emitted gas to displace water in the bottle, after correction of the volume of emitted gas for hydrostatic pressure. After each output measurement, the gas in the bottle was stored in glass ¯asks equipped with vacuum stopcocks and then analysed for CO2 content in the laboratory using the gas-chromatographic methods described above. 2.3. CO2 dissolved in ground water The amount of CO2 dissolved in ground water at Pantelleria was calculated using the measured values for dissolved TDC and estimates of ground water supply to the ground water system. To achieve this goal 15 thermal and cold groundwaters were sampled and analysed to evaluate the average amount of TDC (Fig. 2). An estimate of the groundwater supply to the aquifer examined was obtained assuming the following water budget: I P R E, where I is the amount of water that in®ltrates (in mm/a), P is the amount of precipitation (in mm/a), R is the rilling coecient of the basin (in mm/a) and E is the quantity of water returned to the atmosphere by evapotranspiration (in mm/a). In the case of Pantelleria the value of R was taken to be negligible. On the basis of the data from the local rain gauge network (Fig. 2b), precipitation was estimated using the method proposed by Thiessen (1911). The calculation of the evapotranspiration term was carried out using the equation proposed by Thornthwaite (1948). Determination of TDC (dissolved CO2, HCO3 and CO23 ) was carried Table 1 Isotopic values for carbon and oxygen in CO2 measured at the sample sites with the highest CO2 concentration at Pantelleria d18O CO2 Sampling sites Date d13C CO2 Permil vs. PDB Gadir Gadir Passo del Vento Passo del Vento P 72a P 71a P 60a
01/13/1998 02/03/1998 11/05/1996 03/13/1998 09/22/1998 09/22/1998 09/22/1998
4.8 5.6 4.0 4.6 5.2 4.1 5.8
Permil vs. V-SMOW 31.9 32.1 18.7 20.9 27.7 26.9 26.1
a Points sampled during the soil gas survey over the whole of Pantelleria.
R. Favara et al. / Applied Geochemistry 16 (2001) 883±894
out using dierent analytical methods. Carbonates and bicarbonates were measured by titration with 0.1 N HCl. The amount of dissolved CO2 was determined by means of the method proposed by Capasso and
887
Inguaggiato (1998) and Capasso et al. (1998a), which is based on the partitioning equilibrium of gaseous species between liquid and gas phase. Values of TDC are expressed in tons of CO2-equivalent a 1.
Table 2 Soil CO2 concentration and ¯ux values measured over the whole surface of Pantelleria and soil CO2 ¯ux values measured in the areas of Lake of Venus and Favare Sampling points Pantelleria 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 a
Soil CO2 concentration (% v/v)
Soil CO2 ¯ux (g cm 2 s 1)
Sampling points
Soil CO2 concentration (% v/v)
Soil CO2 ¯ux (g cm 2 s 1)
< LODa 0.04 0.12 < LOD < LOD 0.02 < LOD < LOD 0.02 0.56 < LOD < LOD < LOD 0.04 < LOD < LOD < LOD < LOD < LOD < LOD < LOD 0.08 < LOD < LOD < LOD < LOD < LOD 0.14 < LOD < LOD 0.04 < LOD < LOD < LOD < LOD < LOD < LOD 0.22 0.02 < LOD < LOD 0.08 < LOD < LOD < LOD
< LOD 3.7810 < LOD < LOD < LOD < LOD < LOD < LOD < LOD 1.7910 < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD 3.8010 < LOD < LOD < LOD < LOD < LOD < LOD < LOD
46 47 48 49 50 51 52 53 54 55 56 57 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91
< LOD 0.44 < LOD < LOD < LOD < LOD 0.12 < LOD < LOD < LOD 1.2 0.26 0.22 3.25 0.51 0.008 0.06 0.24 < LOD < LOD 0.16 < LOD 0.12 < LOD 2.4 3.61 < LOD < LOD < LOD < LOD 0.08 < LOD 0.08 < LOD 0.06 < LOD 0.08 0.24 0.04 0.65 0.02 < LOD < LOD 0.02 < LOD
< LOD < LOD < LOD < LOD < LOD < LOD 9.4910 < LOD < LOD < LOD 1.0410 1.9010 1.9010 3.0410 4.7510 < LOD < LOD 1.9010 < LOD < LOD < LOD < LOD < LOD < LOD 2.3210 3.3410 < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD < LOD 1.9010 < LOD < LOD 7.6010 < LOD < LOD < LOD < LOD < LOD
9
9
9
Sampling points
Soil CO2 ¯ux (g cm 2 s 1)
Lake of Venus 1 < LOD 2 < LOD 3 < LOD 4 3.9010 5 3.9010 6 4.3910 7 3.9010 8 5.8510 9 4.8810 10 6.3410 11 3.0210 12 1.4610 13 < LOD 14 1.5110 15 5.8510 16 1.4110 17 < LOD 18 < LOD 19 9.7510 20 8.7810 21 < LOD 22 3.5110 23 < LOD 24 < LOD 25 < LOD 26 < LOD 27 < LOD 28 9.2610 29 < LOD 30 < LOD 31 3.9010 32 3.9010 33 3.5110 34 3.9010 35 1.7610
9
7 9 8 7 8
9
7 7
9 9 8 8 7 7 8 7 6
7 9 7
9 8
Sampling points Favare 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
8
8
9 9 8 9 7
9
9
LOD (limit of detection)=0.01% v/v for CO2 concentrations; LOD=9.7510
10
g cm
2
s
1
for soil CO2 ¯uxes.
Soil CO2 ¯ux (g cm 2 s 1) 7.7010 < LOD 2.3410 7.8010 3.9010 < LOD < LOD < LOD 3.9010 7.8010 2.9210 3.9010 4.2910 2.2410 2.3410 1.7510 2.5310 1.7510 2.9210 4.0010
8
8 9 8
9 9 8 9 7 7 8 6 8 7 7 7
888
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3. Results and discussion Table 1 shows the isotopic values for C and O in CO2 emitted from the highest degassing sites on the island. The d13C values for C in CO2 range from 4.0 to 5.8%. According to the classic interpretation (e.g. Taylor et al., 1967; Deines, 1970; Allard, 1986) such isotopic values would be compatible with a magmatic origin for the gas [considering a MORB-type magmatic component with a mean d13C CO2 value of 6.5%], but they would imply a variable contribution of heavier C from thermo-metamorphosed limestones. Using the method proposed by Sano and Marty (1995), based on the calculated CO2/3He ratios and on the measured d13C values in gas samples from volcanic areas, the authors tried to estimate the relevant contribution from dierent sources of CO2 and He (i.e. magma, sediments and limestones). According to the values available for He isotopes (R=Ra=7.0 and 7.2; R=8.5210 6 and 9.710 6) measured by D'Alessandro et al. (1994) in the highest degassing sites of Lake of Venus and Favare, respectively, the relevant values for the CO2/3He ratio have been calculated. These values (8.5109 and 7.1109, respectively) were plotted against the associated values of d13C CO2 [ 3.8 and 5.2, respectively, according to D'Alessandro et al. (1994)]. Fig. 3 shows that such values fall within the ®eld indicated by Sano and Marty (1995) for high-temperature volcanic gases, thus suggesting that both CO2 and He are indeed derived mainly from magma that has a composition close, but not equal, to MORB. The apparent contribution of CO2 from limestones in the two samples considered is signi®cant, ranging from about 40 to about 25%, respectively. This computation is based on the assumption that the magmatic source of CO2 and He from Pantelleria is MORB-like. However, recent geochemical studies (Allard et al., 1997; Capasso et al., 1997, 1998b, 1999; Giammanco et al., 1998; Inguaggiato and Italiano, 1998; Inguaggiato and Pecoraino, 1998; Inguaggiato et al., 2000) showed that volcanic gases from all the active Italian volcanoes are characterised by more positive values of d13C CO2 and lower values of He isotopic ratio relative to MORB (inferred ``mantle'' isotopic values in the range 3 to 0% for C and around 7.5 R=Ra for He). These authors hypothesised that such peculiar isotopic values do not re¯ect shallow contamination processes during uprise of the magmatic ¯uids (i.e. input of CO2 derived from thermal metamorphism of carbonate sediments, contamination of 4 He at shallow crustal levels), but rather suggest that the source of magmatic gas is not MORB-like. More generally, mantle beneath central Europe, including the Mediterranean, would be already contaminated at depth, in accord with the results of isotopic studies on ¯uid inclusions from Etna's lavas (Marty et al., 1994), on geothermal ¯uids discharged from the western Eger
Fig. 3. Correlation diagram between CO2/3He and d13C-value for low-temperature gases emitted at Lake of Venus mofette and Favare fumaroles (solid stars). Also plotted are values relevant to high-temperature volcanic gases from volcanic areas of the world (open squares; data from Sano and Marty, 1995). Model end-members of mid-ocean ridge basalt (MORB), limestone+slab carbonate and sediment rocks are also indicated. Lines show mixing processes among the 3 end-members.
rift system in central Europe (Weinlich et al., 1999) and on volcanic rocks of western and central Europe (Hoernle et al., 1995). Based on the above considerations, it is hypothesised that at Pantelleria also the measured C and He isotopic values mainly re¯ect their local magmatic sources, but are somewhat modi®ed by shallow processes between magmatic gases and crustal ¯uids and rocks. Preliminary results of a geochemical study on such processes at Pantelleria (Favara et al., 1999) indicate original C and He isotopic values in the same respective ranges as those inferred for the other central European volcanoes. Table 2 shows soil CO2 concentration and ¯ux values measured in the 91-point survey over the whole surface of Pantelleria, those of soil CO2 ¯ux measured around Lake of Venus and those of soil CO2 ¯ux and soil temperature measured in the Favare area. Soil CO2 concentration values from the 91-point survey ranged from 0.02 to 3.61% v/v, omitting values below the instrumental detection limit (0.01% v/v), with a mean value of 0.45% v/v. Soil CO2 ¯ux values from the same survey ranged from 1.7910 9 to 3.3410 7 g cm 2 s 1, omitting values below the instrumental detection limit (LOD=9.7510 10 g cm 2 s 1), with a mean value of 7.6710 8 g cm 2 s 1. In the Lake of Venus area, soil CO2 ¯ux values ranged from 3.9010 9 to 1.4610 6 g cm 2 s 1, omitting values below LOD, with a mean value of 1.8010 7 g cm 2 s 1. In the Favare area, soil CO2 ¯uxes ranged from 3.9010 9 to 1.7510 6 g cm 2
R. Favara et al. / Applied Geochemistry 16 (2001) 883±894
s 1, omitting values below LOD, with a mean value of 2.2010 7 g cm 2 s 1. In order to assess the anomaly threshold for both concentration and ¯ux of soil CO2, log probability plots were used for all 147 soil gas sample points at Pantelleria (Fig. 4). The log probability plot for CO2 concentrations indicates two in¯ection points at 42 and 73%, indicating the presence of three distinct populations. Population A contains values below 0.08% v/v, which therefore can be regarded as the upper limit for background CO2 con-
Fig. 4. Log probability plots for (a) log soil CO2 concentrations and (b) log soil CO2 ¯uxes measured at Pantelleria. Values for all the areas surveyed have been considered in the calculations. Three statistically distinct populations (indicated with capital letters) have been recognised in plot (a), 2 in plot (b); population A includes background values for each parameter.
889
centrations. Population C contains values above 0.4% v/v, mostly measured in or close to areas of focussed degassing and/or fumarolic emissions (Lake of Venus, Favare, Passo del Vento, CuddõÁa di Mida), which is interpreted as due to convective transport of CO2 in active geothermal systems. The log probability plot for CO2 ¯uxes indicates only one evident in¯ection point at 35%, indicating the presence of 2 distinct populations. Population A contains values below 3.9010 8 g cm 2 s 1, which therefore can be set as the upper limit for background CO2 ¯ux level. Both above threshold values were used to highlight anomalous levels in the surface maps of soil CO2 concentrations and ¯uxes, and the latter also to discriminate between ``organic'' and ``magmatic'' CO2 emissions in the estimate of the CO2 budget for the island. In order to obtain as accurate an estimate of the CO2 budget as possible from a given area a realistic approach is to acquire data from a sampling grid in which the distance between sampling points is comparable to the spatial extent of expected anomalies. In the present investigation, the main aim was to have an initial, broad idea of the existence of anomalous degassing areas other than those evident in the ®eld or known from literature (i.e. fumarolic ®elds and areas of focussed degassing). Therefore, a sampling grid of about 1 point km 2 was chosen. One way to process the acquired soil gas data was to perform surface mapping using the Kriging method (Swan and Sandilands, 1995), which was used to produce maps of concentration or ¯ux contour lines for all sampled areas. Fig. 5 shows the distribution of soil concentrations and soil ¯uxes of CO2 measured over the entire surface of Pantelleria using the Kriging method. Both parameters show a fairly similar surface distribution over the island. Due to the relatively low density of sampling points, no clear correlation is seen between anomalous soil degassing and the structural pattern of Pantelleria. However, the highest degassing areas (i.e. Lake of Venus, Favare, Passo del Vento and CuddõÁa di Mida) are located close to an old caldera rim, close to some important faults that bound the central resurgent block of Montagna Grande and on the summit of an eruptive center (CuddõÁa di Mida cone), whose age ranges between 10 and 8 ka (Civetta et al., 1988), respectively (Fig. 2a). This suggests a tectonic control on diuse degassing of magmatic origin. Only 5 of the values measured during the extensive survey at Pantelleria exceed the above threshold calculated for soil CO2 ¯uxes. The average value obtained from these anomalous ¯uxes was about 2.0410 7 g cm 2 s 1 and this corresponds to about 64,000 t km 2 a 1. By assigning an overall surface of about 10 km2 to the areas aected by such soil CO2 degassing, deduced from the surface bounded by the 3.910 8 g cm 2 s 1 contour line in Fig. 5b, a value of about 0.64 Mt a 1 was obtained. However, when the Kriging method is
890
R. Favara et al. / Applied Geochemistry 16 (2001) 883±894
Fig. 5. Maps of areal distribution of (a) soil CO2 concentrations and (b) soil CO2 ¯uxes at Pantelleria. Contour lines have been obtained by the Kriging interpolation method (Swan and Sandilands, 1995). Shaded areas include values higher than thresholds obtained using the log probability plots of Fig. 4 (0.08% v/v for CO2 concentrations and 3.9010 8 g cm 2 s 1 for CO2 ¯uxes). The areas of Lake of Venus (LV) and Favare (F) were not considered in the calculations.
used with large data spacing such as that of the 91-point survey the computed areas are inevitably aected by a signi®cant error. Delineating the area of anomalies at Pantelleria in a more accurate way, for example by increasing the density of measurement points, is actually very dicult due to site inaccessibility (presence of steep rocky walls and inaccessible mountains, presence of
fenced private land, etc.). Therefore, an alternative way to Kriging in order to estimate the output of diuse CO2 from Pantelleria was attempted. First the 5 soil CO2 ¯ux values from the 91-point survey above the organic threshold were considered. Values were assigned to each relevant square kilometer area with reference to the sampling grid used. The resulting 5 values for annual CO2 output were then summed. The total output thus obtained is about 0.32 Mt/a, that is half that obtained with the Kriging method. However, the Kriging method certainly overestimates degassing areas because of its limitations when applied to the small number of measurements over the Pantelleria surface, and because diffuse degassing tends to occur through relatively small areas usually corresponding to tectonic structures, as suggested above. Therefore, the authors prefer to use the output value obtained with the second method to that obtained using the Kriging method. In areas of focussed degassing, such as Lake of Venus and Favare, the Kriging method can be applied more safely, because of the higher density of sampling points compared to the total area surveyed. In these cases, the results describe the distribution of anomalous soil degassing more realistically. In the Lake of Venus area (Fig. 6) the highest soil CO2 emissions were detected on the western and southwestern sides of the lake, close to the caldera rim. In this area, the average soil CO2 ¯ux calculated for the points whose measured ¯ux values exceeded the imposed threshold was 2.9810 7 g cm 2 s 1, which gives an estimated output of about 0.015 Mt a 1 from a surface of 0.16 km2. In the Favare area, the highest CO2 emissions are located in the southeastern part (Fig. 7a) and seem to be oriented roughly NE±SW, in the direction of one of the faults that bound the Montagna Grande horst to the south (see Fig. 2a). In this area, anomalous soil CO2 degassing had an average value of 4.2410 7 g cm 2 s 1. This value gives a soil CO2 output of 0.013 Mt a 1 from a surface of 0.1 km2, which is similar to that calculated for the Lake of Venus area. In the Favare area, a survey of soil temperatures at a depth of 50 cm was also carried out, in order to de®ne the spatial extent of the fumarolic ®eld and to acquire further information about the physical processes leading to gas release through soil. Soil temperature data were also processed by the Kriging method. The results (Fig. 7b) show a signi®cant correlation between the surface distribution of shallow soil temperatures and that of soil CO2 ¯uxes, thus suggesting that soil degassing is produced by condensation of steam. The total contribution from the Lake of Venus and Favare areas to the diuse CO2 output from Pantelleria (0.028 Mt a 1) is negligible compared to that from the 91-point survey. Moreover, considering that it is from areas of focussed degassing, it was not considered in the ®nal computation of diuse degassing. Therefore, the
R. Favara et al. / Applied Geochemistry 16 (2001) 883±894
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Fig. 6. Map of areal distribution of soil CO2 ¯ux (expressed in g cm 2 s 1) in the Lake of Venus area. Contour lines have been obtained by the Kriging interpolation method. Shaded areas include values higher than thresholds obtained using the log probability plot of Fig. 4b (3.9010 8 g cm 2 s 1). Solid stars indicate sites of bubbling gas sampling. The heavy dark line around the Lake of Venus is an access road. Altitude in m asl.
value of about 0.32 Mt a 1 is assumed as a rough, provisional estimate of the amount of diuse CO2 emitted by Pantelleria. Considering by comparison other volcanic areas of Sicily, this value is one order of magnitude lower than that obtained for the soils of Mt. Etna's ¯anks (1 to 12 Mt a 1; Allard et al., 1991; D'Alessandro et al., 1997), but it is comparable to that estimated for Vulcano Island, which is part of an active volcanic arc (Aeolian Islands) located just o the NE coast of Sicily (Chiodini et al., 1996), and that estimated for Ustica Island, an inactive volcano located o the NW coast of Sicily (Etiope et al., 1999). The value for CO2 output obtained from the Favare fumaroles, 1.42 t a 1 (about 110 6 Mt a 1), is based on a measured steam ¯ux of 0.712 cm3 s 1, a measured water/gas ratio of 0.94 and a measured CO2 concentration close to 100% in the dry gas emitted. A direct measure of CO2 output from bubbling gases was carried out in the Lake of Venus and Gadir gas manifestations. The values obtained are 6716 and 6022 t a 1, respectively, thus giving a total CO2 output of about 0.013 Mt a 1 from such manifestations. The contribution of CO2 from TDC has been estimated in the following way: ®rst a calculated total annual rainfall of about 55106 m3 was considered, of which 24106 m3 eectively in®ltrate, Pantelleria's surface was divided into 3 sub-areas (Fig. 2b and Table 3) that correspond to the 3 main hydrogeological basins of
Fig. 7. Map of areal distribution of (a) soil CO2 ¯ux (expressed in g cm 2 s 1) and (b) soil temperature (measured at 50 cm depth and expressed in C) in the Favare area (not inclusive of fumarole emissions). Contour lines have been obtained by the Kriging interpolation method. Shaded areas include values higher than thresholds obtained using the log probability plot of Fig. 4b (3.9010 8 g cm 2 s 1). F=Favare fumaroles; P=Passo del Vento fumaroles. Altitude in m asl.
the island (R. Favara, S. Inguaggiato and G. Pecoraino, in prep.). For each sub-area 1 ground water sampling site was selected, where the total dissolved C content, expressed as equivalent CO2, is thought to match closely that in the deep geothermal ¯uids that interact with shallow waters (R. Favara, S. Inguaggiato and G. Pecoraino, in prep.). At the sampling sites chosen mean annual values of concentration of equivalent dissolved CO2 range between 16 and 40 mmols l 1 (Table 3). Therefore, a total amount of about 0.034 Mt a 1 of equivalent dissolved CO2 was obtained. In this calculation it was assumed that the contribution of bicarbonate from rain water due to its equilibration with atmospheric
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R. Favara et al. / Applied Geochemistry 16 (2001) 883±894
Table 3 Hydrogeological data and dissolved CO2 contents used to estimate the contribution of aquifers to the total budget of CO2 at Pantelleria Hydrogeological basins
Surface of each basin (km2)
Ground water sampling sitesa
Equivalent dissolved CO2 content (mmols/l 1)
CO2 output (Mt a 1)
A B C
17 37 30
D L N
16 35 40
0.0033 0.016 0.015
a
See Fig. 2b for location of sampling sites.
Table 4 Summary of the estimated contribution of CO2 from the different types of gas emissions on Pantelleria island Type of CO2 emission
Number of sampling points
Output (Mt a 1)
Surface (km2)
Soil gas Focussed degassing Fumarole gas Bubbling gas Dissolved
91 56 1 7 15
0.32 0.028 1.410 0.013 0.034
84 0.26 0.001 0.01 84
6
Table 5 CO2 output from some volcanic and geothermal areas of the world Mt/a
Ref.
Pantelleria Etna
0.39 13±25
Vulcanoa
0.13±0.14
Solfatarab Usticab Mid-Oceanic Volcanic System Popocatepetlc Mammoth Mountain White Islandc Mt. Erebusc New Zealand Geothermal Systems
0.048 0.26 30±65
This work Allard et al., 1991; D'Alessandro et al., 1997 Baubron et al., 1991; Bonfanti et al., 1993; Italiano and Nuccio, 1994; Chiodini et al., 1996, 1998 Chiodini et al., 1998 Etiope et al., 1999 Gerlach, 1991
14.5±36.5 0.2
Delgado et al., 1998 Sorey et al., 1998
0.95 0.66 0.002±0.048
Wardell and Kyle, 1998 Wardell and Kyle, 1998 Seward and Kerrick, 1996
a b c
Based on soil gas and fumarolic gas emissions. Based on soil gas emissions. Based on crater plume emissions.
CO2 is negligible. However, large volumes of sea water, whose content of equivalent dissolved CO2 is 2.5 mmols l 1, that intrude into Pantelleria's aquifers and dilute the
C species in ground water need to be considered. Therefore, the amount of equivalent dissolved CO2 indicated above is almost certainly underestimated. 4. Conclusions The investigation of CO2 emission at Pantelleria Island has shown that important degassing of this gas takes place on this currently quiescent volcano. The total CO2 output from all types of ¯uids at Pantelleria is preliminarily estimated as about 0.39 Mt a 1, the largest contribution by far being from diuse degassing (Table 4). Results of C and He isotope measurements on the gas collected from the sites with highest degassing suggest that the source of the emitted CO2 is magma with a composition quite dierent from MORB, but compatible with the mantle source individuated for the central Mediterranean and central European volcanoes. The estimated total amount of CO2 emitted from Pantelleria, although lower than that from other active volcanoes of the world (Table 5), indicates that Pantelleria is a site of active mantle degassing. This evidence suggests that this volcanic area is far from being extinct and deserves more careful volcanological monitoring. Acknowledgements We wish to thank F. Italiano and S. Francofonte for their help during the ®eld work, and S. ArnoÂrsson and W.C. Evans for their critical reviews that helped improve this manuscript. This work was funded by the National Group for Volcanology, Italy.
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