Layout 6 ANNALS OF GEOPHYSICS, 60, 6, S0663, 2017; doi: 10.4401/ag-7305 Temporal analysis of δ13CCO2 and CO2 efflux in soil gas emissions at Mt. Etna: a new tool for volcano monitoring Salvatore Giammanco1,*, Bor Krajnc2,3, Jože Kotnik3, Nives Ogrinc2,3 1 Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo, Catania, Italy 2 Jožef Stefan International Postgraduate School, Ljubljana, Slovenia 3 Jožef Stefan Institute, Department of Environmental Sciences, Ljubljana, Slovenia Article history Received November 11, 2016; accepted October 2, 2017. Subject classification: Mt. Etna; Carbon isotopes; CO2 efflux; Volcanic activity; Hydrothermal systems. S0660 ABSTRACT We monitored the soil gas emission of CO2 from selected sites of Mt. Etna volcano during the period February 2009 to December 2010 by measuring periodically the soil CO2 ef- flux together with the associated stable carbon isotope com- position of CO2. Correlation between the two parameters showed distinct behaviors depending on the sites as a re- flection of the different interactions between crustal and sub-crustal fluids. Where deep CO2 interacted with shallow cold ground water and/or with shallow biogenic CO2, a positive correlation between soil CO2 effluxes and carbon isotopes was evident and it depended strongly on the veloc- ity of gas through the soil. In these cases, the highest CO2 effluxes corresponded to δ13CCO2 values similar to those of the deep magmatic CO2 emitted from the crater and peri- crateric gas emissions at the summit. In areas where a shal- low hydrothermal system was presumed, then a similar correlation was less evident or even absent, suggesting strong control on C isotopes arising from the interactions between CO2 gas and dissolved HCO3- that occur in aquifers at T>120 °C. Marked temporal variations were observed in both parameters at all sites. No significant effect of me- teorological parameters was found, so the observed changes were reasonably attributed to variations in volcanic activity of Mt. Etna. In particular, the variations were attributed to increased degassing of CO2 from incoming new magma, possibly coupled with increased hydrothermal activity in at least some of the shallow aquifers of the volcano. The largest anomalies in the monitored parameters preceded the opening of the New Southeast Crater in late 2009 and there- fore they could represent a key to unveiling the dynamics of the volcano. 1. Introduction Mt. Etna volcano, one of the most active volca- noes in the world, is also known as a one of the main sources of magmatic CO2. The total emission rate has been recently estimated at about 60,000 t·d-1 [Hernán- dez et al. 2015]. Most of the release of CO2 occurs through the active summit vents of the volcano, but a remarkable fraction of this gas (about 10 % of the total amount according to D’Alessandro et al. 1997b and Hernández et al. 2015] is released from its flanks. In this case, CO2 emissions at the surface occur in diffuse form through volcano-tectonic faults [e.g., Giammanco et al. 1998, Aiuppa et al. 2004] and they are produced by magma outgassing from reservoirs located at interme- diate to great depths beneath the volcano [Bruno et al. 2001, Aiuppa et al. 2004, Giammanco et al. 2013]. Stud- ies on soil gas carried out in the last 17 years at Mt. Etna showed that the central eastern flank and the lower southwestern flank of the volcano are characterised by the strongest anomalies in both soil gas concentration and soil CO2 efflux and by the highest content of mag- matic CO2, which dissolves into local groundwater [Anzà et al. 1989, Giammanco et al. 1995, 1996, Allard et al. 1997, Brusca et al. 2001, Aiuppa et al. 2004, Gi- ammanco and Bonfanti 2009]. The aim of the present work was to study the tem- poral variations of soil CO2 efflux and the associated stable carbon isotope composition of CO2 over a time span of several months, covering the period between the end of the 2008-2009 flank eruption and the open- ing and eruption of the New Southeast Crater (NSEC), today the most active summit crater of Mt. Etna (Fig- ure 1). The main objectives of our study are: i) to better define the correlation between the above two parame- ters in selected high-degassing sites of the volcano; ii) to model the dynamics of geochemical interactions be- tween CO2 and the surrounding environment and iii) to recognize possible influences of volcanic activity on soil degassing. 2. Eruptive phenomena at Mt. Etna during the studied period Mt. Etna is a large Quaternary composite volcano, which grew to its present elevation of 3320 m by accu- mulation of lavas and tephra erupted during the last 200 kyr [Gillot et al. 1994]. At present, its activity is rep- resented by summit and/or flank eruptions, the former being mostly short-lived and characterized mainly by production of tephra with minor lavas, the latter being, on average, long-lived and mostly producing lava effu- sions that may emplace voluminous lava flow fields. Our study began during the final stages of the long- standing 2008-2009 flank eruption. Details on this erup- tion are given in Bonaccorso et al. [2011] and in Corsaro and Miraglia [2014]. The eruption started on May 13th from a fissure that developed from the SE Crater to- GIAMMANCO ET AL. 2 Figure 1. Map of the study area. The red circles indicate the location of soil CO2 sampling points. The three main rift zones (NE, W and S) are also shown. VB = Valle del Bove morphological depression. The blue squares indicate the location of the two SIAS meteorologi- cal stations used for the purposes of this study. The yellow star shows the location both of the SE Crater and of the nearby NSEC. 3 δ 13CCO2 AND CO2 EFFLux IN SOIL GASES OF MT. ETNA wards the Valle del Bove (VB in Figure 1), a morpho- logical depression that occupies a large part of the east flank of Mt. Etna. Lava flows from this fissure expanded inside the VB, reaching a maximum length of about 6 km and a total volume of about 70 ± 20 × 106 m3 [Branca S. quoted in Corsaro and Miraglia 2014]. The eruption ended on July 7th, 2009, 419 days after its onset. Just a few months later, on November 6th, 2009, a new vent opened on the lower eastern slopes of the Southeast Crater, one of the four summit craters of Mt. Etna and the most active one in the last thirty years [e.g., Behncke et al. 2014]. This new vent, named New Southeast Crater (NSEC), appeared as a relatively small open pit emitting hot pressurised gas. Its birth marked the end of the eruptive activity at the SE Crater, thus virtually “replacing” it as a new opening in the conduit of the previous SE cone. The vigorous degassing activ- ity of NSEC continued until the end of 2010, without appreciable changes. In the meantime, since August 25th, 2010, some isolated explosive episodes occurred at the Bocca Nuova crater (BN), the westernmost of the four sum- mit craters of Etna (for details on this and on the following volcanological information, see http://www.ct.ingv.it/en/rapporti.html). The strong gas and ash explosion of August 25th, 2010 at the BN was followed by several smaller explo- sive events that produced short-lived emissions of vol- canic ash. Seven major explosions occurred at BN until August, 29th, although with less intensity than the first one. Other significant explosive events took place at BN on October 7th and on November 1st, both producing minor ash plumes. After more than one month of rel- ative quiescence at Etna, a new strong explosion oc- curred at BN on December 22nd, 2010. This explosion was probably less powerful than the one on August 25th, but certainly stronger than the numerous events between late August and November 2010. On the afternoon of December 23rd, 2010, Strom- bolian activity apparently started for the first time inside the NSEC since its formation. However, bad weather conditions prevented clear observation of the summit area, so this activity could not be followed in detail. On the late afternoon of January 2nd, 2011 mild Strombolian activity appeared again within the pit of NSEC, preceded by several days of repeated emissions of hot gas. Strom- bolian explosions, although with variable intensity, con- tinued through January 6th and then ceased. Mild explosive activity resumed on January 11th, and its inten- sity began to increase on January 12th until producing a small lava outflow. Finally, a paroxysmal eruption oc- curred during the night between January 12 and 13, with lava fountains and voluminous lava flows. The event ceased completely after about two hours. This was the first of a long and complex sequence of eruptive events that characterized the evolution and growth of the NSEC until 2015 [Behncke et al. 2014, De Beni et al. 2015]. 3. Materials and methods 3.1. Sampling sites For the purposes of this study, we selected the sites most representative of soil degassing (Figure 1), as they are all characterized by anomalous high emissions of soil CO2. In particular, site P39 is located on the southwest- ern flank at an altitude of 115 m a.s.l., ~2 km SW of the town of Paternò, in an area with visible absence of veg- etation due to the intense gas emissions from the soil. The other three sites (P78, ZAF06 and ZAF08) are lo- cated on the eastern flank of Mt. Etna, at altitudes be- tween 320 m and 510 m a.s.l.. Apart from CO2, the chemical composition of the gas released from all sites, but especially at P39, shows trace amounts of CH4, He and sometimes H2 and CO [Giammanco et al. 1998]. Previous geochemical studies on the gases emitted at sites P39 and P78 indicated that the origin of these gases is dominantly magmatic [D’A- lessandro et al. 1997a, Giammanco et al. 1998, Pecoraino and Giammanco 2005]. In particular, the chemical and isotope features of gases emanating at site P39 show ev- idence of their direct origin from the mantle source of Mt. Etna basalts [Giammanco et al. 1998a, Caracausi et al. 2003a, 2003b]. This site is located on a NE-SW-di- rected regional fault that is thought, based mainly on geochemical data, to be part of the deep feeder system of Mt. Etna [>10 km; Caracausi et al. 2003a, 2003b]. Sites P78, ZAF06 and ZAF08 are part of an anomalous de- gassing zone that is aligned on a WNW-ESE fault sys- tem [Anzà et al. 1993, Giammanco et al. 1995]. A recent study on soil CO2 emissions in the central eastern flank of Mt. Etna [Giammanco and Bonfanti 2009], recognized this line as a major fault characterized by clear anoma- lous degassing whose changes in time are linked to vol- canic activity. Gases from this site are assumed to derive from a shallower portion (5 - 10 km) of the magma feeder system of Mt. Etna [Giammanco et al. 1998, Bruno et al. 2001]. 3.2. Soil CO2 efflux Soil CO2 effluxes (expressed in g m -2 d-1) were mea- sured at all selected sampling sites using the method of the dynamic accumulation chamber [e.g., Parkinson 1981, Chiodini et al. 1998, Farrar et al. 1995]. CO2 ef- fluxes were measured in at least duplicate during each survey, and the arithmetic average of the values was con- sidered for the temporal analysis of data. The sampling frequency was variable but, on average, once every 20-30 days. The data cover the period from February 2nd, 2009 to December 12th, 2010. In order to evaluate the possible influence of the main meteorological parameters on the temporal variations of CO2 efflux, values of air temper- ature, total daily rainfall, minimum and maximum rela- tive humidity and wind speed at a height of 2 m above the ground were also acquired thanks to the monitoring network of the Servizio Informativo Agrometeoro- logico Siciliano (SIAS). In particular, we chose the sta- tion of Paternò, representative of the weather conditions of the low SW flank of Mt. Etna, and the station of Ri- posto, representative of the weather conditions of the low E flank of Mt. Etna (see Figure 1). Because the CO2 efflux data are log-normally dis- tributed [Ahrens 1954, Giammanco et al. 2010], all graphic analyses of these data in the present paper have been performed on a log10 scale. 3.3. Stable carbon isotope composition of CO2 Samples for stable isotope analysis were collected in the soil at a depth of 50 cm using a 5 mm ID teflon tube connected to a syringe. The gas aliquots were immedi- ately injected into pre-evacuated 12-ml glass serum vials through a double-holed needle. To avoid possible con- tamination with gas from the sampler tube, the system was purged twice with the soil gas. The septum pene- tration needle allows direct delivery of the gas sample into the pre-evacuated vial thereby minimizing possible contamination with atmospheric air [Torn et al. 2003]. The stable isotope composition of CO2 was determined according to the methods of Knohl et al. [2004] and Spötl [2004]. These methods have been applied successfully in various forest soils in Slovenia [Čater and Ogrinc 2011, Krajnc et al. 2016, Krajnc et al. 2017]. Measurements were made directly from vials using a Europa Scientific 20-20 continuous flow isotope ratio mass spectrometer (IRMS) coupled to an ANCA-TG preparation module for gas samples. Stable isotope results are reported using the conventional delta-notation (δ13CCO2), in per mil (‰) relative to the VPDB reference standard - δ13CCO2. The accuracy of the measurements was checked using “CO2 ISO-TOP, High” CO2 standard with δ 13CCO2 value of −4.3 ± 0.2‰. The precision of measurements was ±0.2‰. 4. Results and discussion 4.1. Geochemical characterization of sites The results of the measurements carried out dur- ing our study are shown in Table 1. Figure 2 shows the correlation plot between the δ13CCO2 values and the cor- responding log10 values of CO2 efflux measured at all sites. Site P39 displays the highest levels of CO2 efflux (up to about 32,000 g m-2 d-1) and the most positive val- ues of δ13CCO2 (between +1.6 and +3.1‰). Further- more, the isotope composition of carbon showed very small variation with time, despite the wide range of CO2 effluxes measured. This is in line with previous mea- surements of both parameters at this site [Giammanco et al. 1998, Pecoraino and Giammanco 2005]. The iso- topic shift of carbon observed at this site was explained as being due to strong interaction between deep mag- matic fluids and a hydrothermal aquifer at T° > 120 °C because, in this case, the fractionation factor in water be- tween gaseous CO2 and dissolved HCO3 - favors enrich- ment of the heavier carbon isotope in the residual gas phase that passes through the ground water [Mook et al. 1974]. Higher water temperature causes a larger positive isotopic shift in δ13CCO2 values. In fact, the presence of a geothermal aquifer beneath the area of Paternò has al- ready been postulated by Chiodini et al. [1996], who cal- culated a temperature of water of up to about 150 °C, based on liquid and gaseous geothermometers. In our case, the deep CO2 that interacts with the hydrothermal aquifer at about 150 °C should undergo an isotopic shift of about +2‰ [Mook et al. 1974]. Therefore, the esti- mated δ13CCO2 values of the pristine deep CO2 before interaction with thermal water would be very similar to those measured in the high-temperature fumaroles near the summit of Mt. Etna [Giammanco et al. 1998, Aiuppa et al. 2004], assumed as representative of the C-isotope composition of magmatic CO2 of this volcano (grey hor- izontal band in Figure 2). However, the C isotope values found in this study at site P39 are slightly more positive than those found at the same site in previous work [Gi- ammanco et al. 1998, Pecoraino and Giammanco 2005]. If we assume a pristine C-isotope composition in the range from -2.5 to -1‰ as representative of the mag- matic CO2 emitted from Etna [Giammanco et al. 1998, Pecoraino and Giammanco 2005], then the range of val- ues that we observed would be compatible with an iso- topic shift of +3 to +4‰. According to Mook et al. [1974], such a fractionation factor would be due to in- teraction between CO2 and the hydrothermal system at a water temperature of 180-200 °C. A summary of the above geochemical interactions for site P39 is shown in the graphic model of Figure 3a. The other monitored sites showed a distinct and generally coherent behavior. Figure 2 highlights that, dif- ferently from site P39, δ13CCO2 values at sites P78, ZAF06 and ZAF08 showed a wider range of variations GIAMMANCO ET AL. 4 5 δ 13CCO2 AND CO2 EFFLux IN SOIL GASES OF MT. ETNA with time (from about -18‰ to about +1‰), although with a smaller deviation (from about -6‰ to about +1‰) at site ZAF06. The most negative δ13CCO2 value clearly points to a marked influence of an organic com- ponent in the emitted CO2, produced from a shallow biogenic source of gas in the thick and well-structured soil in these sites [e.g., Giammanco et al. 1998]. It must be considered, however, that a shift towards more nega- tive values of δ13CCO2 may also occur as consequence of fractionation of C-isotopes between gaseous CO2 and dissolved HCO3 - when deep CO2 reacts with cold ground water for a long time. The eastern flank of Mt. Etna is characterized by the ubiquitous presence of large unconfined aquifers. Therefore, deep CO2 rising at high pressure (>> 100 kPa) through faults inevitably encoun- ters ground water and at least in part reacts with it, both chemically and isotopically. The entity of these reactions is more or less marked as a function of the contact time between gas and water. A long reaction time occurs be- cause of long pathways of CO2 gas through a thick aquifer or more generally because of low gas flux con- ditions. Given enough contact time between high-pres- sure dissolved CO2 and water, the fractionation factor ranges from -9.6 to -8.5‰ in the temperature interval between 10° and 25°C [Mook et al. 1974]. The above geochemical interactions for sites ZAF06, ZAF08 and P78 are summarized in the graphic model of Figure 3b. The slightly more positive values observed at site ZAF06 may be due at least to partial interaction between deep magmatic CO2 and thermal ground water, similar to that observed at site P39. It should be noted that this site is lo- cated very close to a water well (“Petrulli” well) with Figure 2. Correlation plot between δ13CCO2 values and the corresponding log10 values of CO2 efflux measured at all sites. The hori- zontal bands indicate the ranges of δ13CCO2 values related to the main sources of CO2 found at Mt. Etna (i.e., biogenic, magmatic and hydrothermal). water temperature constantly a little higher than the av- erage temperature of Mt. Etna ground water and the highest among the ground waters of the east flank of the volcano [Bonfanti et al. 1996, Brusca et al. 2001, GIAMMANCO ET AL. 6 Date ZAF06 ZAF08 P78 Tair Rain RH min RH max Wind CO2 flux δ 13CCO2 CO2 flux δ 13CCO2 CO2 flux δ 13CCO2 °C mm % % m/s 02/03/09 3753 -5.10 233 -7.37 514 -12.93 14.70 0.00 55.00 100.00 1.09 02/16/09 12.4 -3.94 9.10 -17.69 5.30 -15.12 7.87 0.00 33.00 72.00 0.88 02/26/09 28.2 -6.17 7.60 -14.98 7.80 -14.43 9.20 0.20 30.00 73.00 1.95 03/12/09 77.5 -4.72 82.3 -16.21 68.5 -14.08 12.06 0.00 31.00 92.00 3.21 03/24/09 115 -4.42 39.1 -17.17 169 -16.18 10.96 0.00 38.00 79.00 0.98 04/09/09 2369 -5.10 62.3 -11.92 47.8 -11.03 13.23 2.80 66.00 100.00 0.53 04/20/09 n.d. n.d. 493 -6.92 406 -10.58 14.38 9.00 78.00 100.00 0.69 04/29/09 798 -5.00 280 -5.83 145 -12.43 15.98 0.00 39.00 98.00 0.75 05/13/09 709 -3.00 1480 -4.13 909 -9.02 18.22 0.00 40.00 92.00 0.87 06/01/09 587 -2.10 882 -3.62 1075 -6.00 21.14 0.00 64.00 100.00 0.80 06/15/09 2534 -2.62 548 -3.34 676 n.d. 23.69 0.00 27.00 69.00 0.89 06/26/09 1212 n.d. 515 -3.81 427 -4.80 21.80 0.00 41.00 79.00 0.81 07/13/09 2017 -2.37 991 -4.41 856 n.d. 24.00 0.00 37.00 84.00 0.98 07/30/09 920 -2.39 968 -3.14 1118 -4.39 26.46 0.00 34.00 78.00 0.97 08/10/09 736 -2.16 1493 -3.44 1188 -4.36 25.95 0.00 52.00 94.00 0.74 08/25/09 603 -2.31 764 -3.16 1104 -4.31 26.07 0.00 55.00 100.00 0.65 09/07/09 667 -1.90 619 -3.22 702 -4.22 23.46 6.40 38.00 100.00 1.17 10/22/09 2604 -1.91 2138 -3.12 1887 -6.52 20.15 8.00 83.00 100.00 0.42 11/01/09 1921 -1.11 232 -2.52 491 -4.95 13.53 0.00 43.00 82.00 0.77 11/16/09 757 -1.04 2104 -2.54 396 -5.06 17.65 0.00 25.00 74.00 0.80 12/10/09 534 -0.55 52.2 -7.06 338 -4.37 13.13 0.00 39.00 76.00 2.88 12/30/09 601 -1.00 2722 -2.35 678 -4.71 15.34 0.00 48.00 89.00 0.64 01/19/10 272 -1.03 37.3 -15.60 198 -5.44 9.99 1.60 42.00 100.00 1.20 01/29/10 3200 -1.76 408 -3.18 727 -6.67 9.51 0.00 59.00 90.00 0.68 02/12/10 193 -0.08 10.8 -16.91 29.0 -4.05 9.19 0.00 16.00 100.00 2.49 03/01/10 1540 -1.31 895 -2.57 740 -4.82 14.53 0.00 52.00 100.00 0.63 03/19/10 220 -0.53 18.8 -13.05 111 -5.90 11.28 0.00 40.00 81.00 1.05 04/09/10 1487 -0.87 336 -2.45 697 -4.09 14.10 0.00 50.00 85.00 0.56 04/26/10 4638 -0.74 670 -2.77 556 -3.31 16.83 0.00 55.00 100.00 0.72 06/08/10 2158 1.16 661 -2.96 949 -1.32 21.25 0.00 42.00 100.00 0.48 06/22/10 1479 1.01 312 -4.52 1223 -0.94 19.87 0.00 43.00 100.00 0.15 07/14/10 608 0.83 685 -2.16 2506 -0.85 25.93 0.00 48.00 100.00 0.93 07/29/10 521 0.72 422 -1.74 1362 -0.92 23.98 0.00 53.00 83.00 0.90 08/26/10 614 0.71 1156 -1.49 2415 -1.08 26.45 0.00 44.00 100.00 0.78 09/09/10 3037 0.55 550 -2.69 2723 -1.02 23.68 104.20 53.00 100.00 1.09 09/23/10 281 0.13 321 -2.29 605 -1.28 20.33 0.40 61.00 100.00 0.57 10/06/10 2777 -0.30 1971 -1.45 4606 -1.79 21.67 0.00 72.00 100.00 0.59 10/22/10 838 -0.30 8.40 -5.91 24.2 -3.85 16.95 0.80 75.00 100.00 0.75 11/19/10 176 -0.08 34.9 -6.92 224 -1.83 14.74 0.00 52.00 95.00 0.76 12/07/10 167 -0.80 281 -2.98 709 -3.04 13.19 0.00 63.00 100.00 0.86 Table 1. Values of soil CO2 efflux (in g m -2 d-1) and δ13CCO2 (in ‰ vs. VPDB) measured at sites ZAF06, ZAF08 and P78 during the stud- ied period. Also shown are the corresponding meteorological data measured at the station located at Riposto. n.d. = not determined. 7 Aiuppa et al. 2004]. A general positive correlation exists between the δ 13CCO2 values measured at sites P78, ZAF06 and ZAF08 and the corresponding values of soil CO2 efflux (on a log10 scale) (Figure 2). In detail, assuming a log10 fit of data on the plots, the correlation coefficients (R2) between the pairs of parameters for each site were cal- culated as +0.40 for site P78, +0.07 for site ZAF06 and +0.66 for site ZAF08. These results also confirm pre- liminary results based on previous measurements per- formed in the same area. A similar positive correlation was found at site P78 by Giammanco et al. [1998] and by Pecoraino and Giammanco [2005]. According to these authors, when CO2 effluxes from site P78 were the high- est, the corresponding values of δ13CCO2 became very similar to the pristine magmatic ones assumed for Mt. Etna. More negative values of δ13CCO2 would then in- dicate a higher degree of mixing between the magmatic component and the biogenic source in the soil gas, par- ticularly evident at low CO2 efflux values. This indicates that when CO2 efflux becomes higher, magmatic CO2 can reach the surface more efficiently, thus surmount- δ 13CCO2 AND CO2 EFFLux IN SOIL GASES OF MT. ETNA Date P78 Tair °C Rain mm RH min % RH max % Wind m/s CO2 flux δ 13CCO2 05/13/09 568 2.79 18.59 0.00 13.00 91.00 0.48 06/01/09 31608 2.53 22.09 0.00 15.00 87.00 1.77 06/15/09 11384 n.d. 23.15 0.00 12.00 77.00 1.08 06/26/09 24468 2.50 22.58 0.00 12.00 86.00 1.07 07/13/09 22838 2.30 24.57 0.00 12.00 90.00 1.04 07/30/09 15143 2.33 26.73 0.00 18.00 82.00 1.49 08/10/09 10446 n.d. 26.78 0.00 26.00 82.00 1.49 08/25/09 7213 2.16 26.97 0.00 19.00 94.00 1.26 09/07/09 5288 2.13 24.38 0.00 26.00 94.00 1.24 11/16/09 1378 2.47 15.53 0.00 22.00 98.00 0.50 12/10/09 3076 2.44 13.36 0.00 38.00 80.00 1.90 12/30/09 3054 2.58 15.77 0.00 25.00 95.00 0.63 01/07/10 1777 2.94 12.81 0.00 43.00 96.00 1.64 01/29/10 1160 2.65 9.57 0.00 53.00 90.00 0.69 02/12/10 307 2.90 7.05 1.00 42.00 97.00 0.84 03/01/10 904 3.10 14.88 0.00 41.00 97.00 1.00 03/19/10 733 2.87 10.46 0.00 30.00 97.00 0.88 04/09/10 5124 2.92 13.87 0.00 40.00 93.00 1.72 04/26/10 5027 2.68 16.19 3.00 27.00 97.00 1.04 06/08/10 2374 2.99 22.44 0.00 26.00 91.00 1.83 06/23/10 3297 1.60 22.65 0.00 25.00 66.00 1.46 07/14/10 4227 2.79 27.23 0.00 18.00 79.00 1.80 07/29/10 10535 2.64 24.88 0.00 23.00 93.00 1.23 08/26/10 7871 2.65 27.30 0.00 20.00 87.00 1.74 09/09/10 4362 2.66 24.50 5.60 47.00 94.00 1.64 09/23/10 3699 2.47 19.85 1.20 52.00 95.00 1.38 10/06/10 5419 2.55 21.49 0.00 47.00 94.00 0.50 10/22/10 3044 2.31 16.12 1.40 61.00 93.00 0.95 11/19/10 1991 2.29 13.44 0.00 39.00 99.00 0.40 12/07/10 2339 2.31 13.59 0.00 42.00 99.00 0.45 Table 2.Values of soil CO2 efflux (in g m -2 d-1) and δ13CCO2 (in per ml vs. VPDB) measured at site P39 during the studied period. Also shown are the corresponding meteorological data measured at the station located at Paternò. n.d. = not determined. ing the shallow biogenic component of the gas. This in turn confirms the transport of free gas (mostly CO2) along preferential pathways in the crust that corre- spond to tectonic faults, where the local permeability is much higher than in the surrounding volcanic rocks (values of permeability in Etna rocks are in the range from 2.5 × 10-11 to 2.9 × 10-10 m2, according to Aureli 1973 and Ferrara 1975) and where local ground water quickly becomes saturated with CO2. This behavior appears to be valid, more or less, for all three sites of the east flank of Mt. Etna investigated during this study. Interestingly, our findings are also in agreement with those of similar studies carried out at the Solfatara crater (Phlaegrean Fields, Italy) by Chio- dini et al. [2008], who found a similar positive correla- tion between CO2 efflux and the δ 13CCO2 values of the corresponding soil CO2 efflux (i.e., C isotopes were not measured in soil gas samples, but at the outlet of the CO2 efflux measurement device) from fumaroles lo- cated inside the crater and from diffuse emissions oc- curring both inside and outside of the crater. They explained the correlation as being due to a combina- tion of three distinct statistical populations of samples: i) a high-flux population with a mean δ13C value of - 2.3±0.9‰, representative of pure hydrothermal de- gassing of CO2 (this value was actually very similar to that of local fumarolic CO2, that is -1.48‰); ii) a low flux population with a mean δ13C value of about - 19.4±2.1‰, representative of biogenic degassing of CO2 (from microbial decomposition of organic matter in the soil, from plant residues or from root respira- tion); iii) an intermediate efflux population with a mean δ 13C value of about -9.8±3.7‰, representative of a mixture of the two above. 4.2. Temporal changes of the monitored parameters The temporal patterns of the soil CO2 efflux val- ues for each site monitored, plotted on a log10 scale (Figure 4), show some common features between sites. A marked and sharp decrease was observed in mid- February 2009 simultaneously at sites P78, ZAF06 and ZAF08, to then give way to a steady increase since the end of March 2009. Further decreases were recorded, in GIAMMANCO ET AL. 8 P39 Tair Rain RH min RH max Wind P39 1.00 0.51 -0.13 -0.55 -0.31 0.27 T° air 1.00 0.02 -0.56 -0.48 0.45 Rain 1.00 0.34 0.21 0.11 RH min 1.00 0.41 -0.15 RH max 1.00 -0.49 Wind 1.00 Table 3. Correlation matrix between the soil CO2 efflux values measured at site P39 and the corresponding meteorological pa- rameters measured at the station of Paternò. Figure 3. Schematic graphic models of the geochemical interactions between magmatic CO2 gas and shallow fluids in the monitored sites: a) interaction between magmatic CO2 and hydrothermal fluids (depicted as an orange horizontal layer) at site P39, with consequent posi- tive shift of δ13C values due to partition of 12C into the liquid phase as HCO3 -; b) interaction between magmatic CO2 and cold ground water (depicted as a blue horizontal layer) at sites ZAF06, ZAF08 and P78, causing partition of 13C into the liquid phase, followed by mixing with shallow biogenic CO2 produced in the soil (depicted as a green horizontal layer) and consequent enrichment in 12C in the gas phase. 9 a more scattered way, from November 2009 to March 2010 and finally, following a sudden drop, in late Octo- ber-December 2010, the latter observed only at sites ZAF08 and P78. The highest effluxes were observed dur- ing June-July 2009 at most of the sites, more evidently at P39. High effluxes were also recorded in April 2010 at site ZAF06, in July 2010 at P39 and, during September- October 2010, at more or less all sites. Site ZAF06 showed slightly more stable CO2 effluxes after the above- mentioned period of very low values from January to March 2009. In order to determine whether the above changes were at least in part caused by variations in the main meteorological parameters, we performed a linear cross-correlation analysis of all the data, divided into areas (SE flank and E flank). The resulting correlation matrixes (Tables 3 and 4 for the area near Paternò and for that near Riposto, respectively), show no significant correlation between the considered parameters. All cor- relation coefficients were in the range between -0.56 and +0.7 and, apart from a very weak direct correlation be- tween CO2 efflux and air temperature at site P78 (R = +0.61), all other coefficients between soil CO2 efflux and meteorological parameters had values between -0.56 and +0.51. This clearly suggests a non-environmental, prob- ably volcanic, cause for the observed temporal changes in soil CO2 efflux values (and by reflection also in δ 13CCO2 values) at all monitored sites. The temporal patterns of δ13CCO2 values (Figure 5) show dissimilar behavior between site P39 and the other three sites. As noted above, the δ13CCO2 values at site P39 in general varied within a much narrower range (only about 1‰) than at the other sites and showed no appreciable correlation with the CO2 efflux values. This points to a marked geochemical stability of the hydrothermal system beneath site P39, likely due to its large volume that makes it less sensitive, at least in terms of isotopic composition of the escaping CO2, to changes in the input of high-enthalpy mag- δ 13CCO2 AND CO2 EFFLux IN SOIL GASES OF MT. ETNA Figure 4. Temporal pattern of the soil CO2 effluxes measured in the monitored sites during the study period: a) P39; b) P78; c) ZAF08; d) ZAF06. The pink area indicates the period of the 2008- 2009 flank eruption. The vertical red line indicates the moment of the opening of the NSEC. Figure 5. Temporal pattern of the δ13CCO2 values measured in the monitored sites during the studied period: a) P39; b) P78; c) ZAF08; d) ZAF06. The pink area indicates the period of the 2008-2009 flank eruption. The vertical red line indicates the moment of the opening of the NSEC. matic fluids coming from depth. The other sites show, instead, a consistent behavior, marked by a more or less sharp positive shift of δ13C val- ues in April-May 2009, followed by a slower trend to- wards even more positive values. Carbon isotope values remained quite stable or decreased slightly from June to October 2010 to show a fairly more marked shift towards negative values in October-November 2010. The overall picture arising from the above data in- dicates the significant arrival of deep magmatic CO2 just before the end of the 2008-2009 flank eruption, reason- ably related to emplacement of new gas-rich magma within relatively deep portions of the volcano and along conduits that were not directly connected with those feeding the ongoing flank eruption. This interpretation is strongly supported by other geochemical and geo- physical parameters that were measured during the same period by the monitoring network of the Istituto Nazionale di Geofisica e Vulcanologia [Mattia et al. 2015]. In particular, the behavior observed in the tem- poral pattern of soil CO2 efflux in early 2009 can be in- terpreted according to geochemical studies made both on soil CO2 emissions from Mt. Etna [Giammanco et al. 1995, Bruno et al. 2001], and on crater fumarole emis- sions at Vulcano Island [Nuccio and Valenza 1992, 1998]. Strong decreases in the CO2 output from soils or from fumaroles accompany early stages of magma motion to- wards the surface. In those cases, magma undergoes sud- den pressure decreases, which causes marked exsolution of a separate vapor phase in the volatile-saturated melt and consequent partition of volatiles into the newly formed bubbles according to the different gas solubili- ties. Because of its low solubility in magma, CO2 will be highly enriched in the separate vapor phase. The conse- quent decrease in the concentration of residual dissolved CO2 in the uppermost portion of a magma body pro- duces a kinetic diffusion effect that translates into an in- ward and upward migration of dissolved gas. This process results in an initial decrease of volatile fluxes re- leased from the deep magma towards the surface. Sub- sequently, when CO2-rich gas bubbles are able to escape from magma, outward advective fluxes of magmatic volatiles through the main pathways across the structure of the volcano (i.e., volcanic conduits, degassing faults) will markedly increase. The sequence observed in our data in early 2009, therefore, would represent a complete cycle of fresh magma transfer from deeper to shallower levels in the volcano. The simultaneous change in C iso- tope values would simply reflect the isotopic shift related with the above mentioned changes in the degree of mix- ing between the magmatic component and the biogenic source as function of CO2 efflux. After the end of the 2008-2009 eruption a further slow increase in magmatic CO2 degassing was in gen- eral observed, suggesting that deep-coming magma moved further upward and accumulated into shallower portions of the volcanic system, thus releasing higher amounts of fluids. This in turn produced a higher input of high-enthalpy fluids into the shallow aquifers of Mt. Etna, thus raising the temperature of water in the sub- surface hydrothermal systems within the volcano. This process would explain the further isotopic shift towards positive δ13C values observed during the summer of 2010. Besides, the progressive pressurization of the shallow volcanic system as consequence of magma ac- cumulation also reasonably led to the opening of the NSEC in early November 2009 and prepared for its fol- lowing eruptive activity. The sharp decrease in CO2 efflux, combined with the slight negative shift of δ13C values, observed since October 2010 at sites P78, ZAF08 and, to a lesser extent, also at ZAF06, might be the consequence of a sudden upward migration of magma. Marked transient de- creases in soil CO2 emissions at Mt. Etna were inter- preted by Giammanco et al. [1995] as an indication of rapid upward migration of the gas source (i.e., a CO2- oversaturated magma). Shallow magma up-rise pro- duces a greater gas pressure gradient between the GIAMMANCO ET AL. 10 ZAF06 ZAF08 P78 Tair Rain RH min RH max Wind ZAF06 1.00 0.18 0.32 0.16 0.26 0.29 0.19 -0.31 ZAF08 1.00 0.51 0.45 0.00 0.18 0.07 -0.34 P78 1.00 0.61 0.33 0.35 0.31 -0.28 T° air 1.00 0.18 0.14 0.18 -0.35 Rain 1.00 0.11 0.17 0.02 RH min 1.00 0.56 -0.49 RH max 1.00 -0.24 Wind 1.00 Table 4. Correlation matrix between the soil CO2 efflux values measured at sites ZAF06, ZAF08 and P78 and the corresponding meteo- rological parameters measured at the station of Riposto. 11 source of gas and the summit of the volcano, which greatly overcomes the gradient existing between the gas source and the peripheral areas. This large contrast fa- vors the release of magmatic gas along the direction source-summit of the volcano, thus significantly damp- ening diffuse flank degassing. This hypothesis is also strongly supported by the following occurrence both of Strombolian activity at NSEC in late December 2010 and, mostly, of the long sequence of eruptive episodes since January 12, 2011; the latter episodes would testify the final arrival of magma at the surface. 5. Concluding remarks The results of this study will help to better under- stand and define the physico-chemical mechanisms that rule the interactions occurring between deep CO2 de- riving from magma degassing inside Mt. Etna feeder conduit and the shallow fluids within the volcano. The observed temporal variations of the studied parameters are explained in terms of variable degree of geochemi- cal interaction between high-enthalpy magmatic fluids and shallow ground water, as a function of the supply of magmatic gas from depth. At high soil CO2 effluxes, δ 13CCO2 values in the pe- ripheral sites are similar to those found in the fumarole gases emitted close to the active summit craters of Mt. Etna. When this occurs, we can reasonably hypothesize that new gas-rich magma is accumulating in relatively deep (5-10 km) volumes within the volcano, causing a strong pressure increase in the whole volcanic system. Conversely, when flank CO2 emission is low, values of δ 13CCO2 become more negative, pointing to a greater interaction with shallow non-magmatic fluids, in our case cold ground water and biogenic CO2 [Giammanco et al. 1998, Pecoraino and Giammanco 2005]. Our results are encouraging for future monitoring of the eruptive activity of Mt. Etna. The sampling sites chosen are always easy to access under most weather conditions. Further, the sampling procedure is simple and the analytical methods are precise and reliable. A higher sampling frequency will certainly improve the temporal resolution on the studied parameters, thus permitting their use also as short-term indicators of the dynamics of Mt. Etna. Acknowledgements. We wish to thank S. Consoli, F. Cal- vagna and M. Lopez for their help during the field work. We also thank the “Regione Siciliana - SIAS - Servizio Informativo Agromete- orologico Siciliano” for kindly providing the meteorological data used in this work. We acknowledge the financial support of Slovenian Re- search Programme “Cycling of substances in the environment, mass balances, modelling of environmental processes and risk assessment” (P1-0143) and bilateral Slovenian-Italian Cooperation “Mercury emis- sions, its influence and correlation with Rn on Etna area”. Special thanks are given to Prof Roger H. Pain for linguistic corrections. References Aiuppa, A., Allard, P., D’Alessandro, W., Giammanco, S., Parello, F., and M. Valenza (2004). Magmatic gas leakage at Mount Etna (Sicily, Italy): relationships with the volcano-tectonic structures, the hydrolog- ical pattern and the eruptive activity, in Mt. Etna: Volcano Laboratory, A.G.u. Geophysical Mono- graph Series 143, 129-145, doi: 10.1029/143GM09. Allard, A., Jean-Baptiste, P., D’alessandro, W., Parello, F., Parisi, B., and C. Flehoc (1997). Mantle-derived he- lium and carbon in groundwaters and gases of Mount Etna, Italy, Earth Planet, Sci. Let., 148, 501-516. Anzà, S., Dongarrà, G., Giammanco, S., Gottini, V., Hauser, S. and M. Valenza (1989). Geochimica dei flu- idi dell'Etna: Le acque sotterranee, Miner. Petrogr. Acta, 32, 231-251 (in Italian). Ahrens, L.H. (1954). The lognormal distribution of the elements (A fundamental law of geochemistry and its subsidiary). Geochim. Cosmochim. Acta, 5, 49-73. Aureli, A. (1973). Idrogeologia del fianco occidentale etneo, proc. 2nd Int.l Congress on underground Wa- ters, Palermo, Italy, 425-487 (in Italian). Behncke, B., Branca, S., Corsaro, R.A., De Beni, E., Miraglia, L. and C. Proietti (2014). The 2011-2012 summit activity of Mount Etna: birth, growth and products of the new SE crater. J. Volcanol. Geotherm. Res., 270, 10-21. http://dx.doi.org/10.1016/j.jvolgeores.2013.11.012. De Beni, E., Behncke, B., Branca, S., Nicolosi, I., Carluc- cio, R., D'Ajello Caracciolo, F. and M. Chiappini (2015). The continuing story of Etna's New South- east Crater (2012-2014): Evolution and volume cal- culations based on field surveys and aerophotogrammetry. J. Volcanol. Geotherm. Res., 303, 175-186. http://dx.doi.org/10.1016/j.jvolgeo- res.2015.07.021. Bonaccorso, A., Bonforte, A., Calvari, S., Del Negro, C., Di Grazia, G., Ganci, G., Neri, M., Vicari, A. and E. Boschi (2011). The initial phases of the 2008-2009 Mount Etna eruption: A multidisciplinary approach for hazard assessment. J. Geophys. Res., 116, B03203, doi:10.1029/2010JB007906. Bonfanti, P., D'Alessandro, W., Dongarrà, G., Parello, F. and M. Valenza (1996). Medium-term anomalies in groundwater temperature before 1991-1993 Mt. Etna Eruption, J. Volcanol. Geotherm. Res., 73, 303-308. Bruno N, Caltabiano T, Giammanco S and R. Romano (2001). Degassing of SO2 and CO2 at Mount Etna (Sicily) as an indicator of pre-eruptive ascent and shal- low emplacement of magma, J. Volcanol. Geotherm. Res., 110, 137-153. δ 13CCO2 AND CO2 EFFLux IN SOIL GASES OF MT. ETNA Brusca, L., Aiuppa, A., D’alessandro, W., Parello, F., Al- lard, P., and A. Michel (2001). Geochemical mapping of magmatic gas-water-rock interactions in the aquifer of Mount Etna volcano, J. Volcanol. Geotherm. Res., 108, 199-218. Caracausi, A., Favara, R., Giammanco, S., Italiano, F., Nuccio, P.M., Paonita, A., Pecoraino, G. and A. Rizzo (2003a). Mount Etna: Geochemical signals of magma ascent and unusually extensive plumbing system, Geophys. Res. Lett., 30, 1057-1060, doi:10.1029/2002GL015463. Caracausi, A., Italiano, F., Nuccio, P.M., Paonita, A. and A. Rizzo (2003b). Evidence of deep magma degassing and ascent by geochemistry of peripheral gas emis- sions at Mount Etna (Italy): Assessment of the mag- matic reservoir pressure, J. Geophys. Res., 108, B10, 2463, doi: 10.1029/2002JB002095. Chiodini, G., D’Alessandro, W. And F. Parello (1996). Geochemistry of gases and waters discharged by the mud volcanoes at Paternò, Mt. Etna (Italy), Bull. Vol- canol., 58, 51-58. Chiodini, G., Cioni, R., Guidi, M., Raco, B. and L. Marini (1998). Soil CO2 flux measurements in volcanic and geothermal areas, App. Geochem., 13, 543- 552. Chiodini, G., Caliro S., Cardellini C., Avino R., Granieri D. and A. Schmidt (2008). Carbon isotopic composi- tion of soil CO2 efflux, a powerful method to dis- criminate different sources feeding soil CO2 degassing in volcanic-hydrothermal areas, Earth Planet. Sci. Lett., 274, 372-379, doi:10.1016/j.epsl.2008.07.051. Corsaro, R.A. and L. Miraglia (2014). The transition from summit to flank activity at Mt. Etna, Sicily (Italy): Inferences from the petrology of products erupted in 2007-2009, J. Volcanol. Geotherm. Res., 275, 51-60, doi:10.1016/j.jvolgeores.2014.02.009. Čater M. and N. Ogrinc (2011). Soil respiration rates and δ 13CCO2 in natural beech forest (Fagus sylvatica L.) in relation to stand structure, Isot. Environ. Health, S 47, 221-237. D’Alessandro, W., De Gregorio, S., Dongarrà, G., Gurri- eri, S., Parello, F. and B. Parisi (1997a). Chemical and isotopic characterization of the gases of Mount Etna (Italy), J. Volcanol. Geotherm. Res., 78, 65-76. D’Alessandro, W., Giammanco, S., Parello, F. and M. Valenza (1997b). CO2 output and δ 13(CCO2)from Mount Etna as indicators of degassing of shallow as- thenosphere, Bull. Volcanol., 58, 455-458. Farrar, C.D., Sorey, M.L., Evans, W.C., Howle, J.F., Kerr, B.D., Kennedy, B.M., King, C.Y. and J.R Southon (1995). Forest-killing diffuse CO2 emission at Mam- moth Mountain as a sign of magmatic unrest, Na- ture, 376, 675-678. Ferrara, V. (1975). Idrogeologia del versante orientale dell'Etna, proc. 3rd Int.l Congress on underground Waters, Palermo, Italy, 91-144 (in Italian). Giammanco, S., Gurrieri, S., and M. Valenza (1995). Soil CO2 degassing on Mt. Etna (Sicily) during the period 1989-1993: discrimination between climatic and vol- canic influences, Bull. Volcanol., 57, 52-60. Giammanco, S., Inguaggiato, S. and M. Valenza (1998). Soil and fumarole gases of Mount Etna: Geochem- istry and relations with volcanic activity, J. Volcanol. Geotherm. Res., 81, 297-310. Giammanco, S. and P. Bonfanti (2009). Cluster analysis of soil CO2 data from Mt. Etna (Italy) reveals volcanic influences on temporal and spatial patterns of de- gassing, Bull. Volcanol., 71, 201-218, doi: 10.1007/s00445-008-0218-x. Giammanco, S., Bellotti, F., Groppelli, G. and A. Pinton (2010). Statistical analysis reveals spatial and temporal anomalies of soil CO2 efflux on Mount Etna volcano (Italy), J. Volcanol. Geotherm. Res., 194, 1-14, doi:10.1016/j.jvolgeores.2010.04.006. Giammanco, S., Neri, M., Salerno, G.G., Caltabiano, T., Burton, M.R and V. Longo (2013). Evidence for a re- cent change in the shallow plumbing system of Mt. Etna (Italy): gas geochemistry and structural data during 2001-2005, J. Volcanol. Geotherm. Res., 251, 90-97, doi:10.1016/j.jvolgeores.2012.06.001. Gillot., P.-Y., Kieffer, G. and R. Romano (1994). The evo- lution of Mount Etna in the light of potassium-argon dating, Acta Vulcanol., 5, 81-87. Hernández, P.A., Melián, G., Giammanco, S., Sortino, F., Barrancos, J., Pérez, N.M., Padrón, E., López, M., Donovan, A., Mori, T. and K. Notsu (2015). Contri- bution of CO2 and H2S emitted to the atmosphere by visible and non-visible degassing from volcanoes: The Etna Volcano case study, Surv. Geophys., 36 (3), 327-349, doi: 10.1007/s10712-015-9321-7. Knohl, A., Werner, R.A., Geilmann, H. and W.A. Brand (2004). Kel-F discs improve storage time of canopy air samples in 10-mL vials for CCO2 δ 13 analysis, Rapid Commun. Mass. Spectrom., 14, 1663-1665, doi: 10.1002/rcm.1528. Krajnc, B., Fujiyoshi, R., Vaupotič, J., Amano, H., Sakuta, Y., Gregorič, A. and N. Ogrinc (2016). using 222Rn and carbon isotopes (12C, 13C and 14C) to determine CO2 sources in forest soils developed on contrasting geology in Slovenia, Environ. Earth. Sci., 75, 1068 doi: 10.1007/s12665-016-5866-0. Krajnc, B., Ferlan, M. and N. Ogrinc (2017). Soil CO2 sources above a subterranean cave-Pisani rov (Posto- jna Cave, Slovenia), J. Soils Sed., 17, 1883-1892. doi: GIAMMANCO ET AL. 12 13 10.1007/s11368-016-1543-x. Mattia, M., Bruno, V., Caltabiano, T., Cannata, A., Can- navò, F., D’Alessandro, W., Di Grazia, W., Federico, C., Giammanco, S., La Spina, A., Liuzzo, M., Longo, M., Monaco, C., Patanè, D. and Salerno, G. (2015). A comprehensive interpretative model of slow slip events on Mt. Etna’s eastern flank, Geochem. Geo- phys. Geosyst., 16, 635-658, doi:10.1002/2014GC005585. Mook, W.G., Bommerson, J.C., and W.H. Staverman (1974). Carbon isotope fractionation between dis- solved bicarbonate and gaseous carbon dioxide, Earth Planet. Sci. Lett., 22, 169-176. Nuccio, P.M. and Valenza, M. (1992). Modification of geochemical parametres during magma ascent: The case of Vulcano, Aeolian Islands. Publ. I.G.F. - CNR, Palermo, Italy, 7, 1-23. Nuccio, P.M. and Valenza, M. (1998). Magma degassing and geochemical detection of its ascent, in Water- Rock Interaction, Balkema Rotterdam, 475-478. Parkinson, K.J. (1981). An improved method for mea- suring soil respiration in the field, J. App. Ecol., 18, 221-228. Pecoraino, G. and S. Giammanco (2005). Geochemical Characterization and Temporal Changes in Parietal Gas Emissions at Mt. Etna (Italy) During the Period July 2000 - July 2003, Terr. Atmosph. Ocean. Sci., 16, 805-841. Spötl C. (2004). A simple method of soil gas stable car- bon isotope analysis, Rapid Commun. Mass Spec- trom., 18, 1239-1242, doi: 10.1002/rcm.1468. Torn, M.S., Davis, S., Bird, J.A., Shaw, M.R. and M.E. Conrad (2003). Automated analysis of C-13/C-12 ra- tios in CO2 and dissolved inorganic carbon for eco- logical and environmental applications, Rapid Commun. Mass Spectrom., 17, 2675-2682. doi:10.1002/rcm.1246. *Corresponding author: Salvatore Giammanco, Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo, Catania, Italy; email: salvatore.giammanco@ingv.it. © 2017 by the Istituto Nazionale di Geofisica e Vulcanologia. All rights reserved. δ 13CCO2 AND CO2 EFFLux IN SOIL GASES OF MT. ETNA