Total (fumarolic + diffuse soil) CO2 output from Furnas volcano
© Pedone et al. 2015
Received: 24 June 2015
Accepted: 15 October 2015
Published: 26 October 2015
Furnas volcano, in São Miguel island (Azores), being the surface expression of rising hydrothermal steam, is the site of intense carbon dioxide (CO2) release by diffuse degassing and fumaroles. While the diffusive CO2 output has long (since the early 1990s) been characterized by soil CO2 surveys, no information is presently available on the fumarolic CO2 output. Here, we performed (in August 2014) a study in which soil CO2 degassing survey was combined for the first time with the measurement of the fumarolic CO2 flux. The results were achieved by using a GasFinder 2.0 tunable diode laser. Our measurements were performed in two degassing sites at Furnas volcano (Furnas Lake and Furnas Village), with the aim of quantifying the total (fumarolic + soil diffuse) CO2 output. We show that, within the main degassing (fumarolic) areas, the soil CO2 flux contribution (9.2 t day−1) represents a minor (~15 %) fraction of the total CO2 output (59 t day−1), which is dominated by the fumaroles (~50 t day−1). The same fumaroles contribute to ~0.25 t day−1 of H2S, based on a fumarole CO2/H2S ratio of 150 to 353 (measured with a portable Multi-GAS). However, we also find that the soil CO2 contribution from a more distal wider degassing structure dominates the total Furnas volcano CO2 budget, which we evaluate (summing up the CO2 flux contributions for degassing soils, fumarolic emissions and springs) at ~1030 t day−1.
Volcano-hosted hydrothermal systems are the source of sizeable carbon dioxide (CO2) emissions, either vented by hydrothermal steam vents (Chiodini et al. 1998) or diffusively released by degassing soils (Chiodini et al. 1999; Rogie et al. 2001; Werner et al. 2008). While considerable work has been spent in the past to estimate the soil CO2 flux from hydrothermal areas (e.g., Chiodini et al. 1999, 2001a; Hernández et al. 2001; Werner et al. 2008; Viveiros et al. 2010), far less is known on the CO2 output from hydrothermal fumarolic vents (Werner et al. 2000; Fridriksson et al. 2006; Aiuppa et al. 2013; Pedone et al. 2014a, b), which are technically more difficult to study. Consequently, the total (fumarolic + soil degassing) budget remains unconstrained for most hydrothermal systems in our planet (with a few exceptions; Aiuppa et al. 2013).
The aim of the present work is to provide more information on the fumarolic vs. diffusive contribution to the hydrothermal CO2 output. Our test site is Furnas volcano, a quiescent polygenetic volcano located in the eastern part of São Miguel island, in the Azores, an archipelago of nine volcanic islands located in the North Atlantic Ocean at the triple junction between American, Eurasian, and Nubian plates (Searle 1980). Furnas volcano has frequently been active in the Holocene (the oldest volcanic products are dated back 100,000 years BP; Moore 1990). The last “magmatic” eruption occurred in 1630 (Cole et al. 1995). In recent times, hydrothermal explosions have re-occurred (in 1840–1841, 1944, and 1990) from the hydrothermal vent (named “Asmodeu”) belonging to the Furnas Village fumarolic field (Ferreira, T: Contribuição para o estudo das emanações gasosas associadas a processos de vulcanismo no arquipélago dos Açores, unpublished Master thesis). Hydrothermal activity is widespread on the island and includes soil diffuse degassing areas (Ferreira et al. 2005; Viveiros et al. 2010), steaming ground, thermal springs, cold CO2-rich springs, and low-temperature fumaroles (95–100 °C), mostly concentrated inside the Furnas caldera (where three main fumarolic fields are observed; Viveiros et al. 2010; Caliro et al. 2015). Further studies have been done since the early nineties to study CO2 diffuse emissions. The first soil CO2 surveys (Baubron et al. 1994; Baxter et al. 1999) in the Furnas caldera identified a CO2 degassing area in the proximity of Furnas village. Recently, Viveiros et al. (2010, 2012) estimated the soil CO2 fluxes emitted from the Furnas volcanic system using the accumulation chamber method (Chiodini et al. 1998); this led to identifying the presence of several diffuse degassing structures (DDS). The diffuse hydrothermal-volcanic CO2 output from Furnas volcano (Furnas caldera and the southern Ribeira Quente village area) was estimated at ~968 t day−1 (Viveiros et al. 2010), and the groundwater CO2 transport was evaluated at ~12 t day−1 (Cruz et al. 1999). As for the majority of the hydrothermal system worldwide, the fumarolic CO2 output is unknown.
In this study, we use a tunable diode laser spectrometer (TDLS) to estimate, for the first time, the fumarolic output of volcanic/hydrothermal CO2 at Furnas volcano. The TDLS technique is based on measuring the absorption of IR radiation (at specific wavelengths) by a target gas, and can suitably be adapted to measure the flux of volcanic CO2 from low-temperature hydrothermal manifestations (Pedone et al. 2014a, b), where the use of traditional UV spectroscopy remote-sensing techniques is prevented by the absence of SO2. TDLS employs a light source of very narrow line-width that is tunable over a narrow wavelength range. In other words, tunable diode laser (TDL) steams on absorption spectroscopy using a single isolated absorption line of the target species, allowing positive identification and unambiguous measurement of complex gas mixtures. A major disadvantage is that TDLS applications are better suited to accurate measurement of a specific target gas (known to be present in the atmosphere) than for identification of previously unidentified species (Pedone et al. 2014a). In addition, the quality of the measurements can be limited in highly condensed, optically thick fumarolic plumes.
The fumarole observations were complemented by simultaneous soil CO2 measurements with an accumulation chamber. This is the first time in which two methodologies are applied together to evaluate their relative CO2 contributions to the total CO2 output. Our results, while limited to only one single hydrothermal system, offer new information to understand the modes of hydrothermal carbon release. In addition, the sulfur output from the fumarolic system is quantified based on measurement of the fumarole CO2/H2S ratios (obtained with a Multi-GAS detector).
Tunable diode lasers are increasingly used in environmental monitoring applications (Gianfrani et al. 1997a) and for volcanic gas observations (Gianfrani et al. 1997b, 2000; De Natale et al. 1998; Richter et al. 2002). Pedone et al. (2014a, b) recently reported on the first direct observations of the volcanic CO2 flux by using a portable tunable diode laser (TDL) system.
Like in previous studies (Pedone et al. 2014a, b), we used a GasFinder 2.0 Tunable Diode Laser (produced by Boreal Laser Inc.), a transmitter/receiver unit operating in the 1.3–1.7 μm wavelength range. GasFinder 2.0 is designed to measure CO2 concentrations over linear open-paths of <1 km. In order to achieve this, radiation emitted by the IR laser transmitter propagates to a set of gold-plated retroreflector mirrors, where it is reflected back to the receiver and focused onto a photodiode detector. The CO2 column amount (in ppm∙m) along each optical path is calculated by the spectral analysis of reflected light, converted into electrical waveform, and processed by using the procedure described in Tulip (1997). CO2 column amounts are converted into average CO2 concentrations (in ppm) along the path by considering path lengths (measured with an IR manual telemeter, 1 m resolution).
In both days, the position of the GasFinder laser unit was sequentially moved (e.g., from positions A to E in Figs. 2 and 3) to scan the fumaroles’ atmospheric plumes from different angles and viewing directions. The positions of transmitter-receivers were limited by time-logistic constraints (e.g., morphology and accessibility of the degassing areas); given the geometry of the optical paths, we admit some heterogeneities in our CO2 maps (Figs. 4 and 5) may result from inhomogeneous, incomplete coverage of the degassing areas. During operations, the GasFinder was left to acquire data along each single GasFinder-retroreflector path for ~3–5 min, before being rotated to measure along the successive path (an additional documentation file shows more details of acquisition-paths (see A1 in Additional file 1).
CO2 fluxes (in t day−1) and standard deviation (±1 σ) calculated in the investigated areas based on TDL and accumulation chamber surveys. The plume transport vertical speed (in m s−1) is also given for each site. Fumarolic CO2 outputs are given for each fumarole emissions (WJ and ST at Furnas Lake; CdA and CG at Furnas Village). Soil CO2 flux values are given for the entire areas (Lake and Village, respectively). Total CO2 emissions (fumarolic + soil) are also given
0.98 ± 0.07
17.6 ± 5.3
6.0 ± 0.2
1.00 ± 0.02
11.4 ± 4.7
0.90 ± 0.18
17.8 ± 4.1
3.2 ± 0.2
1.80 ± 0.19
3.00 ± 0.8
To convert the CO2 maps into a CO2 flux, we performed 2D integration inside the areas covered by the fumarolic plumes—the boxes delimited by black dashed lines in Figs. 4 and 5. These integration areas were delimited based on visual (field) observations of plume transport direction, and were mapped so as to include concentration data above fixed concentration thresholds (~600 ppm at lake and ~650 ppm at village). No integration was performed outside these areas, where the CO2 contribution was due either to the degassing soils or smaller fumaroles (for which manifestations—as for mud pool “MP”—the gas speed is more difficult to assess). The so-obtained CO2 ICAs were multiplied by vertical gas transport speed to obtain a CO2 flux (Table 1). Plume vertical transport speed was estimated from recordings of a video camera, pointing toward the fumarolic vents, and acquiring sequences of images of the atmospheric plume at 25 frames per second. The sequences of frames were later post-processed to calculate the time-averaged transport speed of the plume, after converting camera pixels into distances (using a graduated pole, positioned close to the vent).
During the field campaigns, we used a Multi-GAS (Aiuppa et al. 2011, 2012 for information about setup and performance of the instrument) to measure the compositions of the main gas manifestations. In detail, the Multi-GAS was exposed to the atmospheric plumes of the main fumaroles (WJ at Furnas Lake and CG at Furnas Village; blue triangles in Figs. 2, 3, 4, and 5) to measure the concentrations of CO2 (by NDIR spectroscopy), SO2, and H2S (by specific electrochemical sensors). The specific sensors mounted onboard the Multi-GAS were as follows: a Gascard Edinburgh Instruments infrared spectrometer for CO2 (0-3000 ppmv range, with a resolution of 0.8 ppmv), a 0- to 200-ppmv SO2 electrochemical sensor, and a 0- to 50-ppmv H2S electrochemical sensor (all from City Technology, and the manufacturer quoted a resolution of 0.5 ppmv). We also used a temperature-humidity Galltec sensor (T range, −30 to 70 °C; Rh range, 0–100 %). The Multi-GAS sensors were calibrated in the laboratory by using standard gas cylinders of concentrations within the sensor ranges (all in nitrogen matrixes). Laboratory tests indicate a typical measurement error in the CO2/H2S ratios of ≤20 %.
Results and discussion
TDL-based CO2 distribution maps
Figure 4 is a distribution map of atmospheric CO2 concentrations, calculated for a horizontal air cross-section taken at a 1.20-m height above the degassing soil and fumaroles of Furnas Lake (see Fig. 4). The calculated CO2 concentration values range from ~390 to >850 ppm. In detail, the lowest CO2 concentrations are detected in the eastern portion of the investigated area, far from the fumarolic field (close to laser positions B–E in Fig. 4). In contrast, two main clusters of higher CO2 concentrations (>580) are detected on the northern, northwestern, and western portion of the map, in the proximity of the main degassing fumarolic vents (crosses). Peak CO2 concentrations (>850 ppm), in particular, are observed close to the fumaroles WJ, MP, and ST (Fig. 4). It is worth noting that the CO2 peaks appear in the map as shifted toward the east-southeast relative to the fumaroles’ positions, in agreement with the prevalent plume transport direction during the observations (see wind rose diagram in Fig. 4).
Figure 5 is a similar map derived from interpolation of the GasFinder dataset acquired during operation field at Furnas Village. Again, the CO2 concentration anomalies are fairly consistent with the location of the main visible degassing areas. CO2 concentrations (~700–800 ppm) peak in a wide degassing area downwind (west of) fumarole CdA (the dominant plume transport directions during the observations were toward NW and SW; Fig. 5). A secondary, more moderate CO2 anomaly is observed north of CG fumarole. In contrast, the lowest CO2 concentrations are detected along paths A4 and E6, respectively, upwind and/or more remote from the fumaroles (Fig. 5; Additional file 1: Table S2).
Soil CO2 flux degassing
Comparison between soil CO2 fluxes at Furnas Lake and Furnas Village during 19 and 22 August 2014 (this study) and soil CO2 fluxes calculated in previous study (March 2008 at Furnas Lake and June–July 2009 at Furnas Village). Areas (in m2), soil CO2 flux, mean, median, minimum, maximum values (expressed in g m−2 day−1), and skewness are shown. Sp sampling period (year), Np number of points
Multi-GAS in-plume measurements
The Multi-GAS detected strong volcanic CO2 and H2S signals in both areas. SO2 was undetected (<0.05 ppmv) at both sites, and the relative humidity in the plumes ranged 30–50 % and 40–60 % at Furnas Lake and Village, respectively. The relative humidity variations in the plumes did not systematically correlate with changes in either CO2 or H2S, precluding the volcanic H2O signal to be clearly resolved.
CO2 and H2S output
We estimated the fumarolic CO2 output from Furnas volcano by 2D integration of the TDL-based CO2 concentration values. This integration was restricted to the shaded box areas of Figs. 4 and 5, downwind the main fumaroles. These integrated CO2 amounts within the defined areas were finally multiplied by the vertical gas transport speeds to calculate the fluxes (Table 1). The so-estimated fumarolic CO2 fluxes were ~29 t day−1 at Furnas Lake (17.6 ± 5.3 t day−1 emitted from WJ and 11.4 ± 4.7 t day−1 from ST) and ~21 t day−1 at Furnas Village (17.8 ± 4.1 t day−1 emitted from CdA and 3 ± 0.8 t day−1 from CG) (Table 1). The cumulative fumarolic output from the two areas is therefore ~50 t day−1.
We additionally infer, using this fumarolic CO2 flux (Table 1) and the Multi-GAS-derived CO2/H2S ratios, fumarolic H2S fluxes of 0.08 t day−1 (Furnas Lake) to 0.17 t day−1 (Furnas Village), respectively. Our results indicate that Furnas fumaroles are weak, but yet not negligible, sources of S. The present-day H2S flux of 0.25 t day−1 (0.08 + 0.17 t day−1) at Furnas is nearly 1 order of magnitude lower than typical emissions from Campi Flegrei volcano (1.5–2.2 t day−1; Aiuppa et al. 2013), which similarly display a H2S-dominated flux and more than 1 order of magnitude lower than emissions from Vulcano Island (6–9 t day−1; Tamburello et al. 2011), whose fumarolic field hosts hot (>400 °C) magmatic fumaroles. For comparison, the H2S flux sustained by Etna, the largest source of volcanic gases worldwide, is 200 to 400 times larger (50 to 113 t day−1; Aiuppa et al. 2005) (note Etna additionally emits thousands of tons of SO2 every day; Caltabiano et al. 2004).
On the basis of the integration of the average of 100 sequential Gaussian simulations (Deutsch and Journel 1998; Cardellini et al. 2003), over the sampled area, mean soil diffuse CO2 emissions of ~6 ± 0.2 t day−1 and 3.2 ± 0.2 t day−1 were estimated for Furnas Lake and Furnas Village degassing areas, respectively. We then obtain a cumulative soil CO2 release of ~9.2 t day−1. Detailed soil CO2 surveys in the Furnas Lake and Furnas Village fumarolic fields (same as Fig. 7 in this study) have previously been performed by Viveiros et al. (2012). For the sake of comparison, we extracted from these earlier surveys (made between 2008 and 2009) the results obtained in the same areas as those investigated in the present study (2014) (Fig. 7). This comparison shows that the Furnas Lake area emitted in 2008 about 3 t day−1 of CO2, or about half of what was measured in 2014. Similarly, the Furnas Village area was found in 2009 to emit nearly half (1.7 t day−1) of the amount of CO2 released in 2014 (3.2 ± 0.2 t day−1). Increased CO2 emissivity in 2014 is consistent with the different average CO2 flux values observed in the two different surveys (Table 2) but is not supported by records of the permanent soil CO2 flux station GFUR2 (where lower soil CO2 flux values—about 250 g m−2 day−1—were recorded during 2014 compared to 2008 and 2009 surveys—values higher than 350 g m−2 day−1). We caution that differences in sampling grid/density (that was significantly higher in the present surveys, as can be also observed in the number of points of Table 2) can partially explain the CO2 flux diversity between the two campaigns (Viveiros et al. 2010).
Our results here suggest that, in the actively degassing fumarolic areas of Furnas, fumarolic vents, with their cumulative fumarolic output of ~50 t day−1, dominate the total CO2 degassing budget (~59 t day−1) and overwhelm the relatively marginal contribution (~15 %) of soil diffuse degassing. While the fumarolic output can be locally important, however, its contribution to the total CO2 degassing output becomes marginal at the scale of the entire volcano. Earlier soil CO2 flux measurements, in fact, have demonstrated a total diffusive hydrothermal CO2 release at Furnas, from an area (6.2 km2) far larger than that studied here, as high as 968 t day−1 (Viveiros et al. 2010). From this comparison, we conclude that—at Furnas—the most actively degassing areas, although featuring the most visible (e.g., fumaroles, hot pools) and spectacular manifestations of thermal activity, in no way correspond to the areas of largest CO2 output: in total, the actively degassing areas contribute only about 5 % to the total CO2 output.
To what extent this conclusion can be generalized to other hydrothermal-volcanic systems remains unknown, given the paucity of information we have in hand. A small contribution of fumarolic gas vents to the total CO2 budget was also suggested by Fridriksson et al. (2006) for the Reykjanes geothermal area, SW Iceland, where the contribution of steam vents/mud pools is only about 3 % of the soil CO2 degassing output. In other systems, in contrast, the contribution of fumaroles is manifestly more significant. At Mud volcano (Yellowstone, USA), for example, Werner et al. (2000) estimated that fumarolic vent emissions contribute to more than 32 % of the total degassing; based on statistical approach, the authors even suggested that the hydrothermal-focused emissions can be responsible for up to 63 % of the total degassing in that thermal area. At Campi Flegrei, the CO2 fumarolic output of ~500 t day−1 (range, 460–507 t day−1, Aiuppa et al. 2013; Pedone et al. 2014a) makes a substantial contribution to the total CO2 output, which is still dominated by soil diffuse degassing (~1100 ± 120 t day−1; Chiodini et al. 2010). From these examples, we argue that much remains to be done to fully understand the fumarolic vs. diffusive gas contribution at volcano-hydrothermal systems. The relative significance of the fumarolic output can differ significantly from one volcano to another. A general conclusion is that the relative significance of the above two forms of gas dissipation will likely be dependent upon the scale at which the comparison is made. At the local scale of an active hydrothermal manifestation, the fumarolic output will likely overwhelm the soil diffuse flux, while the latter will most likely dominate at a larger scale (at the scale of an entire volcanic complex). For example, based on results from 20 analyzed hydrothermal areas, Harvey et al. (2015) concluded that the contribution of focused venting to the total CO2 emission is typically less than 10 %. The TDL, with the ability to characterize the fumarolic CO2 output, promises to contribute a substantial advancement in this field in the following years.
We estimated the total CO2 emissions from the main thermal manifestations of Furnas volcano by jointly using two different techniques: the GasFinder 2.0 tunable diode laser and the accumulation chamber method. We find that, in the most vigorously degassing areas, the soil CO2 flux contribution (approximately 9.2 t day−1) represents a minor (~18 %) contribution to the total CO2 output, which is dominated by the fumaroles (about ~50 t day−1). The CO2 output contributed by the fumaroles is larger than that contributed by Furnas springs (~12 t day−1, Cruz et al. 1999), but far lower than the total hydrothermal diffuse degassing flux (~968 t day−1) at the scale of the entire volcano. This observation supports the conclusions that although fumaroles are the most visible surface manifestations of thermal activity, they are not necessarily the biggest contributors to the total CO2 output from quiescent, Solfatara-stage volcanoes, where CO2 is mainly released in silent, invisible form through soil emissions.
Summing up the CO2 flux contributions for the fumarolic emissions, the degassing soils and the springs (12 t day−1, Cruz et al. 1999), the total volcanic/hydrothermal CO2 output for Furnas volcano is estimated to be ~1030 t day−1. These results show once more the importance of taking into consideration both soil degassing and gas vent emissions to estimate the CO2 emission in hydrothermal areas, since their relative contribution seems to be quite different depending on the study sites. Using a portable Multi-GAS, we also obtained the CO2/H2S ratio signature for the investigated fumaroles, and concluded that the Furnas fumaroles are, in their present state of activity, weak, but yet not negligible, sources of H2S (~0.25 t day−1).
The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007/2013)/ERC grant agreement n 305377, and from the FP7 grant “Futurevolc.” F. Viveiros is supported by a post-doctoral grant from Fundação para a Ciência e a Tecnologia (FCT). The authors would like to acknowledge the technical assistance from the staff of Centro de Vulcanologia e Avaliação de Riscos Geológicos, University of the Azores, and the staff of INGV (Istituto Nazionale di Geofisica e Vulcanologia) of Palermo, Italy. The authors would also like to thank the two reviewers, Dr. C. Werner and Dr. H. Shinohara, for the valuable contribution to improve the early version of the manuscript.
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