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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, B09207, doi:10.1029/2006JB004307, 2006


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Rates of carbon dioxide plume degassing from Mount Etna volcano A. Aiuppa,1 C. Federico,2 G. Giudice,2 S. Gurrieri,2 M. Liuzzo,2 H. Shinohara,3 R. Favara,2 and M. Valenza1 Received 24 January 2006; revised 19 May 2006; accepted 14 June 2006; published 26 September 2006.

[1] We report here on the real-time measurement of CO2 and SO2 concentrations in the

near-vent volcanic gas plume of Mount Etna, acquired by the use of a field portable gas analyzer during a series of periodic field surveys on the volcano’s summit. During the investigated period (September 2004 to September 2005), the plume CO2/SO2 ratio ranged from 1.9 to 10.8, with contrasting composition for Northeast and Voragine crater plumes. Scaling the above CO2/SO2 ratios by UV spectroscopy determined SO2 emission rates, we estimate CO2 emission rates from the volcano in the range 0.9–67.5 kt d1 (average, 9 kt d1). About 2 kt of CO2 were emitted daily on average during quiescent passive degassing, whereas CO2 emission rates from Etna’s summit were 10–40 times larger during the 2004–2005 effusive event (with a cumulative CO2 release of 3800 kt during the 6 months of the eruption). Such a syneruptive increase, ascribed to the replenishment of the shallow (100). The very weak plumes released by these closed vents (Figure 1) were fed by several low-temperature fumaroles (from which reactive magmatic volatiles, such as SO2 and HCl, are probably scrubbed by gaswater-rock interactions in the fumarole conduit system; Figure 4). [9] The determined range (3.7 – 6.6) for time-averaged CO2/SO2 ratios of Voragine and Northeast craters is consistent with the representative Etna’s CO2/SO2 plume ratio (4.8) quoted by Allard et al. [1991]. The upper range of our measurements (10.8) is even higher than the spectroscopically determined CO2/SO2 ratios (7.3– 9.9) for the magmatic gas phase released during lava fountain events at Etna [Allard et al., 2005] (Figure 4). This finding disagrees with the idea that the high CO2/SO2 ratios (>7) are an exclusive feature of paroxystic explosive events on the volcano. Our measurements also confirm the high CO2/SO2 signature of magmatic volatiles emitted at Etna; among the basaltic and/or alkaline volcanoes whose magmatic gas composition is reported (Figure 4), Etna is the most CO2-rich, with only a few exceptions, including the recent (1995 to 1999) gas emissions from summit Kilauea Caldera [Gerlach et al., 2002] and gas emissions from Oldoinyo Lengai volcano [Koepenick et al., 1996]. Since summit emissions of both these two volcanoes are currently determined by low-temperature fumaroles

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Figure 5. Time trends of (a) CO2/SO2 molar ratios in Voragine (dots) and Northeast (squares) crater plumes; (b) remotely sensed SO2 emission rates (t d1) from the bulk plume (this study and T. Caltabiano, INGV-Catania, personal communication, 2005); (c) bulk CO2 emission rates from Etna’s plume (black line and dots), computed by cross correlating SO2 output rates with CO2/SO2 ratios in Etna’s ‘‘bulk’’ plume. The latter is computed assuming equal emissions from Voragine and Northeast plumes and negligible contributions from Southeast and Bocca Nuova craters. Upper and lower ranges for CO2 emission rates (given by the grey shaded area in Figure 5c are based on compositions of CO2-rich Voragine and CO2-poor Northeast plumes, respectively. The timing and duration of the 2004 –2005 effusive event are also shown. (T  300°C), scrubbing of degassed SO2 can be suspected [Gerlach et al., 2002]. 3.2. CO2 Emissions From Mount Etna Volcano [10] We use here the derived data set of CO2/SO2 ratios in Etna’s summit plume, combined with remotely sensed SO2 emission rates from the summit craters (INGV-CT, T. Caltabiano, personal communication, 2005), for an assessment of CO2 emission rates during the study period. The complete data set is presented in Figure 5, where the time variation of SO2 and CO2 emission rates are compared. Since the remotely sensed SO2 emission rates are from determinations carried out several km downwind the volcano’s summit, thus being representative of the ‘‘bulk’’ Etna’s plume, while our acquired CO2/SO2 ratios are from individual vents only, we make use CO2-rich Voragine and CO2-poor Northeast plume compositions to compute upper and lower limits of CO2 emissions from the volcano, ranging 1.7–67.5 and 0.9–39.8 kt d1 over the whole investigated period (Figure 5c), respectively. The ‘‘bulk’’ CO2 emission from the volcano will be intermediate between the two above estimates, since CO2 contributions from weakly degassing

Bocca Nuova and Southeast craters are trivial. If we assume an equal SO2 emission from Voragine and Northeast, as supported by our DOAS measurements on the summit crater area, the time-averaged (2004–2005 period) bulk CO2 emission is evaluated at 9 kt d1 (range, 1.4–53.6; see Figure 5c). This is four times lower than the previous in 1977–1984 [Allard et al., estimate (35 ± 8 kt d1 1991]). The contrast in the CO2 emission rate is consistent with the variation in the SO2 emission rates; the SO2 emission rates in 2004–2005 period (2 kt d1) were also lower than the averaged SO2 emission rates for the 1987–2000 period (5 kt d1 [Caltabiano et al., 2004]). 3.3. What Caused the Variations of CO2 Output? [11] The CO2 emission rates from the volcano varied by a factor 40 during only one year of observations (Figure 5). Such significant changes in a rather limited time span emphasize the need of a more prudent extrapolation of spot measurements to the derivation of long-term time averages of CO2 emission rates from a given volcano. Since the global volcanic emission rates are substantially controlled by the Etna’s emissions [Gerlach, 1991; Williams et al., 1992], more

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observations in the near future are needed for better constraining the volcanic contribution to the global CO2 cycle [Berner and Lasaga, 1989]. [12] From the perspective of volcano monitoring the existence of such short-term variations is encouraging, provided it can be demonstrated that the CO2 emission rates varied in response to changes in degassing dynamics within the subvolcanic plumbing system [e.g., Harris and Rose, 1996]. In principle, the fluctuations in the estimated CO2 emission rates can reflect changes in (1) CO2/SO2 ratios at constant SO2 emission rates, (2) SO2 emission rates at constant CO2/SO2, or (3) both CO2/SO2 ratios and SO2 emission rates. As seen on Figure 5, hypothesis 3 is the most reasonable in this case. At the onset of the 2004–2005 effusive event, the SO2 emission rates were low (

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