Tunable diode laser measurements of hydrothermal/volcanic CO2 and implications for the global CO2 budget

Abstract

Quantifying the CO₂ flux sustained by low-temperature fumarolic fields in hydrothermal/volcanic environments has remained a challenge, to date. Here, we explored the potential of a commercial infrared tunable laser unit for quantifying such fumarolic volcanic/hydrothermal CO₂ fluxes. Our field tests were conducted between April 2013 and March 2014 at Nea Kameni (Santorini, Greece), Hekla and Krýsuvík (Iceland) and Vulcano (Aeolian Islands, Italy). At these sites, the tunable laser was used to measure the path-integrated CO₂ mixing ratios along cross sections of the fumaroles' atmospheric plumes. By using a tomographic post-processing routine, we then obtained, for each manifestation, the contour maps of CO₂ mixing ratios in the plumes and, from their integration, the CO₂ fluxes. The calculated CO₂ fluxes range from low (5.7 ± 0.9 t d^-1; Krýsuvík) to moderate (524 ± 108 t d^-1; La Fossa crater, Vulcano). Overall, we suggest that the cumulative CO₂ contribution from weakly degassing volcanoes in the hydrothermal stage of activity may be significant at the global scale.

Solid Earth, 5, 12091221, 2014 www.solid-earth.net/5/1209/2014/ doi:10.5194/se-5-1209-2014 Author(s) 2014. CC Attribution 3.0 License.

M. Pedone1,2, A. Aiuppa1,2, G. Giudice2, F. Grassa2, V. Francofonte2, B. Bergsson1,3, and E. Ilyinskaya4

1DiSTeM, Universit di Palermo, via Archira 36, 90123 Palermo, Italy

2Istituto Nazionale di Geosica e Vulcanologia, Sezione di Palermo, via Ugo La Malfa 153, 90146 Palermo, Italy

3Icelandic Meteorological Ofce, Bstaavegur 7, 150 Reykjavk, Iceland

4British Geological Survey, Murchison House, West Mains Road, Edinburgh, EH9 3LA, UK Correspondence to: M. Pedone ([email protected])

Received: 1 August 2014 Published in Solid Earth Discuss.: 27 August 2014 Revised: 21 October 2014 Accepted: 28 October 2014 Published: 2 December 2014

Abstract. Quantifying the CO2 ux sustained by low-temperature fumarolic elds in hydrothermal/volcanic environments has remained a challenge, to date. Here, we explored the potential of a commercial infrared tunable laser unit for quantifying such fumarolic volcanic/hydrothermal CO2 uxes. Our eld tests were conducted between

April 2013 and March 2014 at Nea Kameni (Santorini,

Greece), Hekla and Krsuvk (Iceland) and Vulcano (Aeolian Islands, Italy). At these sites, the tunable laser was used to measure the path-integrated CO2 mixing ratios along cross sections of the fumaroles atmospheric plumes. By using a tomographic post-processing routine, we then obtained, for each manifestation, the contour maps of CO2 mixing ratios in the plumes and, from their integration, the CO2 uxes. The calculated CO2 uxes range from low (5.7 [notdef] 0.9 t d1; Kr

suvk) to moderate (524 [notdef] 108 t d1; La Fossa crater, Vul

cano). Overall, we suggest that the cumulative CO2 contribution from weakly degassing volcanoes in the hydrothermal stage of activity may be signicant at the global scale.

Tunable diode laser measurements of hydrothermal/volcanic CO2

and implications for the global CO2 budget

1 Introduction

The chemical composition of volcanic gas emissions can provide hints concerning the mechanisms of magma ascent, de-gassing and eruption (Allard et al., 2005; Burton et al., 2007; Oppenheimer et al., 2009, 2011), and can add useful information for interpreting the dynamics of uid circulation at dormant volcanoes (Giggenbach, 1996; Chiodini et al., 2003, 2012).

Carbon dioxide (CO2) is, after water vapour, the main constituent of volcanic (Giggenbach, 1996) and hydrothermal (Chiodini et al., 2005) gases, and has attracted the attention of volcanologists because it can contribute to tracking magma ascent prior to eruption (Aiuppa et al., 2007, 2010). The volcanic/hydrothermal CO2 ux sustained by diffuse soil degassing can be measured relatively easily during surveys (Chiodini et al., 1996, 2005; Favara at al., 2001; Hernndez, 2001; Cardellini et al., 2003; Inguaggiato et al., 2005, 2012; Pecoraino et al., 2005; Mazot et al., 2011) or with permanent installations (Brusca et al., 2004; Carapezza et al., 2004; Werner and Cardellini, 2006; Inguaggiato et al., 2011). In contrast, the volcanic CO2 ux contributed by open vents and/or fumarolic elds is more difcult to measure, since the volcanic CO2 gas signal is diluted upon atmospheric transport into the overwhelming background air CO2 signal. Such volcanic CO2 ux emissions have been quantied for only 30 volcanic sources, based upon simultane

ous measurement of SO2 uxes (via UV spectroscopy) and

CO2/SO2 plume ratios (via direct sampling, Fourier transform infrared (FTIR) spectroscopy, or the Multi-GAS; see

Published by Copernicus Publications on behalf of the European Geosciences Union.

1210 M. Pedone et al.: Tunable diode laser measurements of hydrothermal/volcanic CO2

Figure 1. The study areas. (a) Nea Kameni summit crater (Greece), (b) Hekla summit (Iceland), (c) Krsuvk hydrothermal eld, and(d) La Fossa crater (Vulcano Island). In each picture, the positions of GasFinder and retro-reectors are shown with letters and numbers, respectively.

Burton et al., 2013). This methodology is however not applicable to the countless number of quiescent volcanoes with low-temperature (SO2-free) emissions (Aiuppa et al., 2013).

As a consequence, the available data set of volcanic CO2 uxes is still incomplete, making estimates of the global volcanic CO2 ux inaccurate (Burton et al., 2013).

In this paper, we discuss the use of tunable diode laser spectrometers (TDLSs) for estimating volcanic/hydrothermal CO2 uxes from quiescent volcanoes.

TDLSs are increasingly used in air monitoring (Gianfrani et al., 1997a) and, more recently, for volcanic gas observations (Gianfrani et al., 1997b, 2000; De Natale et al., 1998; Richter, 2002). Pedone et al. (2014) recently reported on the rst direct observation of the volcanic CO2 ux from the fumaroles of the Campi Flegrei (the Phlegraean Fields, southern Italy), by using a portable tunable diode laser (TDL) system. Here we extend this previous work, discussing the results of TDL observations at four additional quiescent volcanoes: Nea Kameni (Santorini, Greece), Hekla and Krsuvk (Iceland), and Vulcano Island (Aeolian Islands, Italy) (Fig. 1). We selected these volcanoes because they display a range of fumarolic activity from weak (Krsuvk, Hekla) to moderate (Vulcano Island). While there is strong argument for the global volcanic CO2 budget

being dominated by a relatively small number of strong emitters (Shinohara, 2013), weakly degassing volcanoes dominate at least in number the population of historically active volcanoes on Earth. This study contributes to better characterizing the typical levels of CO2 emission from such feeble volcanic point sources.

2 Background

Santorini, the site of the famous Minoan eruption 3600 yr

ago (Druitt et al., 1999), is an island located in the Aegean

Sea, part of the Cyclades Archipelago. Santorini has a surface of 75.8 km2 and is presently made up of ve islands (Thera, Therasia, Aspronisi, Palea Kameni and Nea Kameni) that constitute the active intra-caldera volcanic eld (Dominey-Howes and Minos-Minopulos, 2004). Four periods of unrest in the 20th century have culminated into small-scale eruptions in 19251926, 1928, 19391941 and 1950 (Fyticas et al., 1990). Outside the caldera, volcanic activity has been recorded in AD 16491650, in the Kolumbo submarine volcano (Vougioukalakis et al., 1994). Since the last eruption in 1950, the volcano has remained quiescent (Tsapanos et al., 1994; Papazakos et al., 2005; ISMOSAV, 2009). In early 2011, geodetic monitoring revealed a new

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M. Pedone et al.: Tunable diode laser measurements of hydrothermal/volcanic CO2 1211

Figure 2. Output of the tomographic algorithm, and example for the Nea Kameni campaign, 9 April 2013 is shown. (a) Geometric reconstruction of the eld experimental set-up and (b) tomographic matrix. The script uses a data inversion procedure to assign an averaged CO2 mixing ratio (in ppm) to each cell of the matrix. (c) CO2 mixing ratios (ppm) contour map. GasFinder and retro-reectors positions are shown with letters and numbers, respectively. Fum4, Fum5 and Fum6 are the positions of main degassing vents; blue triangles are the permanent INGV-PA stations; the red arrow depicts the principal direction of plume dispersal.

stage of caldera-wide uplift (Newman et al., 2012; Parks et al., 2012), accompanied by swarms of shallow earthquakes. This unrest lasted from January 2011 to April 2012 (Parks et al., 2013). Degassing activity at Santorini is currently concentrated in a small, hydrothermally altered area on top of Nea Kameni islet (Parks et al., 2013), where a number of weakly fuming fumaroles (mostly CO2, water vapour and air-derived gases; temperatures of 9397 C) are concentrated (Tassi et al., 2013). A recent survey carried out by

Parks et al. (2013) indicated increased diffuse CO2 emissions between September 2010 and January 2012; this period was characterized by a change in the degassing pattern, with an increase in soil CO2 emissions peaking at 38 [notdef] 6 t d1 in

January 2012 (Parks et al., 2013). Tassi et al. (2013) examined the response of fumarole composition to the 20112012 unrest, and reported increasing CO2 concentrations (and decreasing 13CCO2) from May 2011 to February 2012, suggesting increased mantle CO2 contribution. During the survey on 9 April 2013, we investigated the central portion

of the soil CO2 degassing structure identied by Parks et al. (2013), right on top of the most actively degassing Nea Kameni summit crater (Figs. 1a and 2).

Hekla is one of the most active volcanoes in Europe. Its historical volcanic activity, petrology and geochemistry of volcanic rocks have been the subject of several studies (e.g. Thorarinsson, 1967; Sigmarsson, 1992). Hekla (63.98 N,19.70 W; 1490 m a.s.l) is located in the southern part of Iceland at the intersection of the South Iceland Fracture Zone and the Eastern Volcanic Zone (Thordarsson and Larsen, 2007 and references therein). Five Plinian eruptions have been identied in the historical record, most recently in AD 1104 (Thorarinsson, 1967; Larsen et al., 1999). In recent decades, Hekla has erupted frequently, at an average rate of one eruption per decade, and most recently in 2000 (Hskuldsson et al., 2007). Gas information has long remained missing, because Hekla appears to be only degassing during eruptions. Very recently, Ilyinskaya et al. (2014) identied a weakly degassing, warm ground on the summit of

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1212 M. Pedone et al.: Tunable diode laser measurements of hydrothermal/volcanic CO2

Table 1. CO2 uxes (in t d1) and standard deviation (1) calculated in this study. The survey duration (in h), the number of CO2 readings (with R2 > 0.95), and the plume vertical transport speed (in m s1) are also given for each site.

Volcano Date Survey Number of Gas speed (m s1 ) CO2 ux (t d1) duration (h) readings ([notdef]1 ) ([notdef]1 )

Nea Kameni 9 April 2013 4 1070 1.20 [notdef] 0.4 63 [notdef] 22

Hekla 2 July 2013 1 985 1.00 [notdef] 0.5 15 [notdef] 7

Krsuvk 5 July 2013 1.5 1150 1.17 [notdef] 0.18 5.7 [notdef] 0.9

Vulcano 11 March 2014 2 1757 1.00 [notdef] 0.20 524 [notdef] 108

the Hekla 19801981 crater (Fig. 1b), and studied the composition of this gas using data from a permanent Multi-GAS instrument and eld campaigns using an accumulation chamber installed by INGV-PA (Istituto Nazionale di Geosica e Vulcanologia, Sezione di Palermo) and IMO (Icelandic Meteorological Ofce) in 2012. These authors provided evidence for this gas spot being the only current surface manifestation at Hekla. This degassing eld was therefore the site of our measurement survey with the TDL on 2 July 2013 (see Fig. 1b).

Krsuvk (Fig. 1c) is one of ve presently active geothermal areas on the Reykjanes Peninsula, in Iceland (Marksson and Stefansson, 2011). Geothermal activity at Krsuvk includes hot grounds, steaming vents, steam-heated hot springs and mud pots, and pervasive surface alteration. The most important surface manifestations are conned to the Sveiuhls area, including Austurengjahver and the small areas of Seltn and Hveradalur (Marksson et al., 2011). On 5 July 2013, we performed TDL observations in Hveradalur (63 53, 449[prime] N, 22 4, 190[prime] W; Fig. 1c). This area included two major fumarolic manifestations (indicated as FumA and FumB

in our study. The fumarolic vent FumA is monitored by a permanent Multi-GAS instrument deployed in a joint monitoring program led by the British Geological Survey (BGS), INGV and IMO.

Vulcano is a volcanic island belonging to the Aeolian Islands in the southern Tyrrhenian Sea (Italy). Since the last eruption in 18881990, this closed-conduit volcanic system has been characterized by intense fumarolic activity concentrated on the summit of La Fossa crater (Fig. 1d), a small (391 m a.s.l.; 2 km in diameter) < 5 ka old pyroclastic cone.Degassing activity has shown signs of intensication in the last few decades, including increased fumarole temperatures (Badalamenti et al., 1991; Chiodini et al., 1995; Capasso et al., 1997), and episodic variations of gas/steam ratios (Chiodini et al., 1996; Capasso et al., 1999; Paonita et al., 2002, 2013). The CO2 ux from the La Fossa fumarolic eld has been measured previously by Aiuppa et al. (2005, 2006), Mc-Gonigle et al. (2008), Tamburello et al. (2011) and Inguaggiato et al. (2012). On 11 March 2014, we measured the CO2 emissions from La Fossa using the measurement conguration of Fig. 1d.

3 Methods

The tunable diode laser spectroscopy technique (TDLS) relies on measuring the absorbance due to the absorption of IR radiation (at specic wavelengths) by a target gas. Like in previous work at Campi Flegrei (Pedone et al., 2014), we used a GasFinder 2.0 Tunable Diode Laser (produced by Boreal Laser Inc.), a transmitter/receiver unit that can measure CO2 mixing ratios over linear open paths of up to 1 km distance, operating in the 1.31.7 m wavelength range. Radiation emitted by the IR laser transmitter propagates to a gold plated retro-reector mirror, where it is reected back to the receiver and focused onto a photodiode detector. Incoming light is converted into an electrical waveform, and processed to determine in real-time the linear CO2 column amount (in ppm [notdef] m) along the optical path, using the pro

cedure described in Tulip (1997). CO2 column amounts are converted into average CO2 mixing ratios (in ppm) along the path by knowledge of path lengths (measured with an IR manual telemeter, 1 m resolution). A portable meteorological station was continuously recording (frequency = 1 Hz) dur

ing the measurements to restrict post-processing to sampling intervals characterized by similar meteorological conditions.Instrumental accuracy is evaluated using a correlation coefcient (R2), which is a measure of the similarity between the waveforms of the sample and reference signals. According to the manufacturers data sheets, an accuracy of [notdef]2 % is

achieved for R2 > 0.95 (Trottier et al., 2009).

In the eld, the GasFinder was set to measure CO2 mixing ratios at a 1 Hz rate (Pedone et al., 2014). Alignment between the laser unit and the retro-reector mirror was optimized using a red visible aiming laser and a sighting scope. The size of the retro-reector mirror was chosen as to adjust the returning light level to a desired value, depending on the path length and the expected amount of absorbed radiation.

4 Results and discussions

4.1 Field operations

The GasFinder operated for more than 10 h during the four eld campaigns (more than 4 h at Nea Kameni on9 April 2013; 1 h at Hekla on 2 July 2013; 1.5 h at

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M. Pedone et al.: Tunable diode laser measurements of hydrothermal/volcanic CO2 1213

Figure 3. Time series of CO2 mixing ratios (ppm) and meteorological conditions for the Nea Kameni example ( 4 h of observa

tions). The meteorological measurements were acquired from the INGV-type station temporarily installed on the summit crater (except wind speed, which was measured using a portable weather station). T case: temperature inside the station ( C); T air: outside air temperature ( C); Wind dir: wind direction ( N); Wind

Speed: average wind speed (0.2 m s1, blue line) during eld operation; dashed black line: CO2 background average. CO2 measurements at plume margins, oscillating close to the background values, are plotted. In-plume cross sections (CO2 mixing ratios peaking at

600 ppm) are shown. CO2 average values (ppm) for each laser-mirror path are also given. The right vertical axis refers to the meteorological parameters (blue units, range of 01, for the wind speed; black units, range of 0200, for the air and case temperature, and wind direction).

Krsuvk on 5 July 2013; and more than 2 h at Vulcano on11 March 2014, Table 1). Measuring at 1 Hz, the GasFinder acquired more than 9000 readings of path-integrated CO2 mixing ratios. However, we concentrate here onto a subset of data (1070 readings for Nea Kameni; 985 readings for Hekla; 1150 readings for Krsuvk; and 1757 for Vulcano Island, Table 1), extracted from the original data set based on data quality criteria (the same described in Pedone et al., 2014): we selected readings characterized by high accuracy (R2 values > 0.95, optimal light values), and taken during phases of stable wind direction and speed.

The environmental parameters were monitored using a portable weather station, equipped with a data logger, that was temporarily installed in each of the survey sites. As an example, Fig. 3 shows time series of CO2 mixing ratios (acquired by the TDL) and ambient parameters (air temperature, temperature inside the station, and wind speed/direction) recorded by the meteorological station, during 4 h of acquisition at Nea Kameni (Fig. 3, Table 1). Wind was nearly absent and stable (mean speed, 0.2 m s1; blue line in

Fig. 3) during the time covered by TDL observations, making conditions ideal for TDL operations (especially for accurate quantication of plume transport speed; see below). Northern trending winds prevailed during the eld campaign at Nea Kameni (red arrow in Fig. 2); southern trending winds

Figure 4. Contour map of CO2 mixing ratios (ppm), Hekla campaign of 2 July 2013. GasFinder and retro-reector positions are shown with letters and numbers, respectively. Blue triangle: INGVPA/IMO station; red arrow: principal direction of plume dispersal.

at Hekla (red arrow in Fig. 4); and north-western trending winds at both Krsuvk (red arrows in Fig. 5) and La Fossa crater at Vulcano Island (red arrow in Fig. 6).

Figure 1 shows the GasFinder operational eld set-up at the four volcanoes. In each picture, the GasFinder unit positions are expressed by letters; while retro-reectors positions are expressed by numbers (Fig. 1). During each campaign, and at each of the degassing areas, the position of the GasFinder unit was sequentially moved (e.g. from positions A to F in Fig. 1a) so as to scan the plume from different viewing directions and angles. We acquired along each single GasFinderretro-reector path (e.g. path A-1 in Fig. 1a) for 45 min, before rotating the instrument head to mea

sure along the subsequent path (e.g. A-2). The number of operated paths ranged from 9 (Hekla) to 36 (Nea Kameni and Vulcano), and the entire measurement grid (i.e. the total number of possible GasFinderretro-reector paths) was covered in a few h at most.

4.2 CO2 mixing ratios and plume transport speed

The highest CO2 mixing ratios ( 1050 ppm) were measured

at Hekla (Fig. 4), while the lowest mixing ratios values were

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1214 M. Pedone et al.: Tunable diode laser measurements of hydrothermal/volcanic CO2

Figure 6. Contour map of CO2 mixing ratios (ppm), La Fossa campaign, Vulcano Island, 11 March 2014. GasFinder and retroreectors positions are shown with letters and numbers, respectively. Red arrow: principal direction of plume dispersal.

visible rising plume at 25 frames per second (see Aiuppa et al., 2013; Pedone et al., 2014). The sequences of frames were later post-processed to calculate the time-averaged plume transport speed, after converting camera pixels into distances (using a graduated pole, positioned close to the vent). Plume transport vertical speeds are reported in Table 1, and converge at 11.2 m s1 at all volcanoes.

4.3 Contouring of in-plume CO2 mixing ratios

At each of the four volcanoes, we combined the available set of path-integrated mixing ratio data to derive a two-dimensional reconstruction of CO2 distribution (in ppm) in the plume cross section, between the GasFinder position(s) and the retro-reectors.

In order to achieve this, we used a Matlab script (released by the authors, and available on request; see Pedone et al., 2014 for more details), to (i) create a matrix containing information on the geometry of the experimental setup (an example is given in Fig. 2 for Nea Kameni) and (ii) use this matrix

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Figure 5. CO2 Contour map of CO2 mixing ratios (ppm), Krsuvk campaign of 5 July 2013. GasFinder and retro-reectors positions are shown with letters and numbers, respectively. FumA and FumB: positions of main degassing vents; blue triangle: INGVPA/IMO station; red arrow: principal direction of plume dispersal.

detected at Nea Kameni and Krsuvk (peaking at 590 ppm and < 500 ppm, respectively, see Figs. 2 and 5). Intermediate CO2 mixing ratios ( 900 ppm) were detected at La Fossa

crater at Vulcano Island (Fig. 6), reecting gas contributions from fumarolic vents located on the rim and in the inner wall of the crater.

Background readings were obtained in each of the measurement sites by pointing the laser beam toward a mirror, positioned upwind of the fumarolic area (Pedone et al., 2014). Background values of < 400 ppm were observed in all the analysed areas (Figs. 26).

During each campaign, the vertical plume transport speed was measured by a video camera pointing toward the fumarolic vents, and acquiring sequences of images of each

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to obtain a bi-dimensional reconstruction of CO2 concentrations in a cross section of the atmospheric plumes, starting from the raw GasFinder data set. In order to start the calculations, the Matlab script was initialized with the coordinates of the laser and retro-reectors positions. The additional input data was a column vector, containing the mean CO2 column amount (in ppm [notdef] m) obtained for the different

GasFinderretro-reector paths. With these inputs, the script performed a data inversion using a least-squares method, previously described by Pedone et al. (2014). The geometric matrix (Fig. 2a) generated by the Matlab algorithm, is a geometric reconstruction of the experimental set-up (the explored space was divided into 16 equally sized cells; the cells separated by the red lines in Fig. 2a). The scripts used the data inversion procedure to assign an averaged CO2 mixing ratio (in ppm) to each cell of the 4 [notdef] 4 matrix (the same 16 cells as

Fig. 2a). Using sets of synthetic data to test the algorithm, we estimated an error of 3 % associated with these individual

cell mixing ratios.

The so-called tomographic matrix (Fig. 2b) was then interpolated with the Surfer software to obtain the contour maps of Figs. 2c and 46. We used the point Kriging geo-statistical method to interpolate the available data and produce an interpolated grid (Isaaks and Srivastava, 1989). Figure 2c is the contour map of CO2 mixing ratios obtained at Nea Kameni.

This map (obtained by interpolation of the tomographic matrix of Fig. 2b) shows the distribution of CO2 mixing ratios in the roughly horizontal atmospheric cross section, covering the area between the GasFinder (AF) and retro-reector (16) positions (Fig. 1a). The gure shows that, in spite of the feeble degassing activity present, a CO2 plume is imaged by our observations on the eastern, inner rim of the Nea Kameni crater. Low CO2 mixing ratios ( 390 ppm) are outputted by

the Matlab routine on the north-western portion of the investigated area, while higher CO2 mixing ratios (from 490 to 540 ppm) are identied on the east, where the main gas

emission vents are located. The peak CO2 mixing ratio of

590 ppm is located in correspondence to one principal gas vent (marked as Fum6 in Fig. 2c).

Similar results have been obtained at Hekla, Krsuvk and Vulcano. Figure 4 is a contour map of CO2 mixing ratios at the Hekla measurement site (Fig. 1b). Given the positioning of GasFinder and retro-reectors, the Matlab-derived contour map is here relative to an hypothetical horizontal cross section, taken at about 1 m height above the warm degassing ground identied by Ilyinskaya et al. (2014) on the rim of the 19801981 summit crater of Hekla (Figs. 1b and 4). In this area, the background CO2 mixing ratio was evaluated at around 400 ppm. The peak CO2 mixing ratio ( 1050 ppm)

was detected in the central portion of the investigated area, in the same sector where the highest soil CO2 uxes have been observed (Ilyinskaya et al., 2014).

The CO2 contour map obtained at Krsuvk is shown in Fig. 5. In this area, CO2 mixing ratios ranged from 350380 ppm at the periphery of the exhaling area, and up

Figure 7. Our TDL CO2 map for Nea Kameni volcano (same as Fig. 2) compared with the soil CO2 ux map of Parks et al. (2013).

The study of Parks et al. (2013) covered a wider exhaling area that contributes a diffuse CO2 output of 38 [notdef] 6 t d

1 (in January 2012),

1 fumarolic CO2 output. Left, (modied from Parks et al., 2013): red square: survey area investigated from Parks et al. (2013); blue square: our survey area; red dots: position of fumaroles (from Tassi et al., 2013); yellow box: the area with elevated diffuse degassing (Parks et al., 2013 and Chiodini et al., 1998); black dashed circles: summit craters.

to 500 ppm near the two main fumarolic vents (FumA

and FumB in Fig. 5).

The CO2 distribution map of La Fossa crater at Vulcano Island is shown in Fig. 6. The highest CO2 mixing ratios (up to 880 ppm; Fig. 6) were detected in correspondence of the principal fumaroles (F0, F5 and F11) of the crater rim and the FA fumarolic eld in the inner wall of the crater.

4.4 Calculation of the CO2 ux

The ability of the TDL to contour CO2 mixing ratios in a volcanic gas plume cross section (Figs. 26) opens the way to quantication of the fumarolic CO2 output from each of the studied areas. In order to calculate the CO2 output from each fumarolic area, we integrated each set of CO2 mixing ratio values in each CO2 contour map (Figs. 2, and 46), to obtain a CO2 integrated column amount (ICA) over the entire plume cross section. This ICA was then multiplied by the vertical plume transport speed, yielding a CO2 ux. The calculated

CO2 uxes are listed, for each site and each campaign, in Table 1. The accuracy (1) of the mean ux estimates are calculated from error propagation theory applied to both ICA and plume transport vertical speed.

Applying this procedure to the contour map of Fig. 2, we estimate a CO2 ux from Nea Kameni fumaroles of 63 [notdef] 22 t d1. This fumarolic output is 4 times higher than

the total diffuse discharge from the soils of 15.4 t d1 reported by Chiodini et al. (1998), and 1.5 times higher

than the soil CO2 output of 38 [notdef] 6 t d1 estimated (in Jan

uary 2012) by Parks et al. (2013). One of the advantages of using the Tunable Diode Laser is the possibility to capture simultaneously the CO2 contributions from both diffuse soil degassing and concentrated emissions (fumaroles). Our CO2 concentration map of Fig. 2, and the CO2 output we derive

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or 60 % of our 63 [notdef] 22 t d

1216 M. Pedone et al.: Tunable diode laser measurements of hydrothermal/volcanic CO2

Figure 8. Time series of CO2 ux values (t d1) for La Fossa crater (Vulcano Island). Previous studies shown are: Aiuppa et al. (2005, 2006), Tamburello et al. (2011) and Inguaggiato et al. (2012). The ux value of 524 [notdef] 108 t d

1, obtained in this study, is also shown.

from it, allows us to capture a cross section through whole areas of CO2 degassing, that includes both diffuse and concentrated (fumaroles) forms of emission. To the make the case more clear, we compare in Fig. 7 the spatial distribution of our CO2 anomaly with that detected (in January 2012) in the diffuse CO2 ux map of Parks et al. (2013). The study of Parks et al. (2013) covered a wider (than studied here) exhaling area that nonetheless contributed diffusively only a fraction ( 60 %) of the diffuse and fumarolic CO2 output we

estimate in our study. From this comparison, we argue that persistent fumarolic activity on top of Nea Kamenis central crater dominates the CO2 degassing budget over more peripheral weakly degassing soils.

For Hekla, we estimated a CO2 ux of about 15 [notdef] 7 t d1

(Table 1). The large error in our ux estimate ([notdef]46 %) re

ects the poor quality of our plume transport speed measurement, the determination of which was complicated by the strong winds blowing across the top of Hekla at the time of our measurements. We still observe, however, that our 15 [notdef] 7 t d1 estimate matches closely the recently reported

CO2 ux for Hekla summit (13.7 [notdef] 3.7 t d1), obtained using

conventional (accumulation chamber) soil survey techniques (Ilyinskaya et al., 2014).

For the Hveradalur fumarolic eld of Krsuvk, we estimate a CO2 ux of 5.7 [notdef] 0.9 t d1 (Table 1). This is the rst

CO2 output estimate for this area, at least to our knowledge.

Finally, on March 2014 we evaluate the CO2 ux at La Fossa crater at 524 [notdef] 108 t d1 which is in the same range

of those estimated in previous studies, obtained with other techniques (e.g. CO2 / SO2 ratio + SO2 ux), by Aiuppa

et al. (2005) (420 [notdef] 250 t d1), Tamburello et al. (2011)

(488 t d1, average of two campaigns in 2009), and Inguaggiato et al. (2012) (453 t d1) (see Fig. 8).

4.5 Implications for the global volcanic CO2 ux

Our CO2 observations were taken at four volcanoes displaying a range of fumarolic activity, from weak (Hekla) to moderately strong (La Fossa of Vulcano). As such, our results add novel information on the CO2 degassing regime of quiescent volcanoes in the solfatara stage of activity (for which the fumarolic CO2 contribution was undetermined until the present study), and on their potential contribution to the global volcanic CO2 budget.

The current state-of-the-art of volcanic CO2 ux research has recently been summarized in Burton et al. (2013). The authors (2013) presented a compilation of 33 subaerial volcanoes for which CO2 ux observations were available at that time. These measured emissions totalled a cumulative CO2 output of 59.7 Mt yr1. The same authors used linear extrapolation, from the measured 33 to the 150 plume-creating, passively degassing volcanoes on the GVP catalogue (Siebert and Simkin, 2002), to obtain an extrapolated global volcanic CO2 ux of 271 Mt yr1.

The linear extrapolation approach of Burton et al. (2013) is based on the implicit assumption that the measured 33 volcanoes represent a statistically signicant sub-set of the volcanic CO2 ux population. However, we argue that past volcanic CO2 observations have been prioritized at strongly degassing volcanoes during periods of unrest; therefore, the 33 volcanoes population may be biased towards the category of top gas emitter, implying the linear extrapolation technique may be incorrect. The low CO2 output associated with quiet volcanoes, as reported in our present work, corroborates this conclusion.

The alternative extrapolation approach used to quantify CO2 emissions from unmeasured volcanoes assumes that the distribution of volcanic CO2 uxes obeys a power law (Brantley and Koepenick, 1995), as other geophysical parameters do (Marret and Allmendinger, 1991; Turcotte, 1992). If volcanic emissions follow a power-law distribution, then the number of volcanoes (N) with an emission rate f are given

by:

N = af c, (1) where a and c are constants that can be derived from linear regression on measured CO2 emission data sets. In the power-law assumption, the global volcanic CO2 ux (ftot) was extrapolated to 88132 Mt yr1 (Brantley and

Koepenick, 1995) using the relation:

ftot = f1 + f2 + f3 + fN

[bracketleftBigg]

1c

c1 c

(N + 1)

[parenleftbigg]

NN + 1

#, (2)

where fN refers to the Nth largest measured ux. This 88132 Mt yr1 estimate is a factor of 23 lower than that obtained with the linear extrapolation technique (Burton et al., 2013). On the same basis, the volcanic plus metamorphic

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M. Pedone et al.: Tunable diode laser measurements of hydrothermal/volcanic CO2 1217

formation on weakly fuming, quiescent volcanoes. The case of Hekla is emblematic in this context: the volcano has remained in a very active state in the last century (it violently erupted only fourteen years ago; Hskuldsson et al., 2007), but shows today a very weak diffuse gas emission. Yet our data suggest the volcano may contribute daily 15 t of CO2

to the atmosphere in a not distinctly visible, but probably persistent form. Similarly, no plume is distinctly seen on top of Nea Kameni in Santorini, nonetheless its weak fumaroles release 63 [notdef] 22 t of CO2 every day (in addition to a sizeable

diffuse contribution from the soil), and 5.7 [notdef] 0.9 t of CO2

are released daily by quiet hydrothermal activity at Krsuvk (whose most recent activity probably dates back the 14th century; Smithsonian Institute, 2013). While the individual contribution of each of the above volcanoes is negligible globally, the cumulative contribution of all feebly degassing volcanoes on Earth may not be, and may in fact impact the global CO2 ux distribution of Fig. 9.

To explore the latter argument further, we consider that, of the 1549 volcanic structures listed in the GVP catalogue, around 500 are considered to have been active in the Holocene (Smithsonian Institution, 2013), and thus still potentially degassing. For the sake of illustration, we assume that all such 500 volcanoes have a CO2 ux equal to or higher than 10 t d1 (the mean of our measured Krsuvk and Hekla uxes). This yields a new point on Fig. 9, with coordinates log f = 1 (CO2 ux = 10 t d1) and log N = 2.69 (500 vol

canoes), which lies right above the linear regression line of the high CO2 ux (log f > 2.5) population (see dashed line

H in Fig. 9). The regression line (line H1; R2 = 0.98) ob

tained considering the high CO2 ux volcanoes (log f 2.5)

plus this new log f = 1 point has slope c = 0.72. Using

this value in Eq. (1), and with N = 500, we calculated an

extrapolated CO2 ux of 67 Mt yr1.

From these preliminary calculations, we conclude that the power-law distribution may be an appropriate representation of the population of CO2 ux data, provided the output of the several hundreds of weakly degassing, quiescent/hydrothermal/dormant volcanoes is considered. We caution, however, that a large number of potentially strong volcanic CO2 emitters remain to be measured (specically, in poorly explored areas such as Papua New Guinea, Indonesia and nearby countries), and that these have the potential to strongly impact the distribution and regression shown in Fig. 9.

5 Conclusions

We have investigated the fumarolic CO2 output from four quiescent volcanoes in a hydrothermal state of activity, using an infrared TDL. At each of the studied volcanoes, the acquired TDL results have been used to produce contour maps of CO2 mixing ratios in the plumes cross sections, and consequently to quantify the fumarolic CO2 output.

www.solid-earth.net/5/1209/2014/ Solid Earth, 5, 12091221, 2014

Figure 9. Cumulative frequency of the number of volcanoes (N) emitting CO2 ux (logarithmic scale). The diagram is based on the data set of Burton et al. (2013), implemented with new results from this study and additional data (see text). The red point, with coordinates log f = 1 (CO2 ux = 10 t d

1) and log N = 2.69 (500

volcanoes), lies right above the linear regression line of the high CO2 ux (log f > 2.5) population (dashed line H). The regression line (line H1; R2 = 0.98) is obtained considering the high CO2 ux

volcanoes (log f 2.5) plus this new log f = 1 point.

CO2 ux was evaluated at 264 Mt yr1 (Brantley and

Koepenick, 1995).

The power-law distribution assumption has extensively been used to extrapolate volcanic gas uxes at both the global and individual-arc scale (Hilton et al., 2002). However, concerns have recently been raised on its validity. For example, Mori et al. (2013) demonstrated that the SO2 ux distribution of Japanese volcanoes noticeably diverges from a simple power law distribution. The case of the global volcanic CO2 ux population is illustrated in Fig. 9. The gure is a loglog plot of the cumulative number of volcanoes (N) having measured CO2 ux of f . The diagram is based upon the data

set of Burton et al. (2013), implemented with new results from this study (Table 1) and additional data for Turrialba (1140 t d1; Conde et al., 2014) and Pos (24.7 t d1; Aiuppa et al., 2014) in Costa Rica, Telica (132 t d1; Conde et al., 2014) and San Cristbal (523 t d1; Aiuppa et al., 2014) in

Nicaragua, Lastarria (973 t d1) and Lscar (534 t d1) in Chile (Tamburello et al., 2013, 2014), and La Soufriere volcano in Guadeloupe, Lesser Antilles (14.9 t d1; Allard et al., 2014). This implemented CO2 ux population (43 volcanoes in total) clearly departs from a linear trend, as would be expected for a power-law distribution (see Eq. 1). The ob-served distribution shows, instead, a clear inection point at log f 2.52.8 (i.e. a CO2 ux of 300600 t d1), which

appears to divide high (> 600 t d1) from low (< 300 t d1) CO2 ux volcanoes (L and H regression lines in Fig. 9).

In view of our novel results (listed in Table 1), we propose that the non-linear behaviour of the volcanic CO2 ux population may (at least in part) reect the scarcity of CO2 ux in-

1218 M. Pedone et al.: Tunable diode laser measurements of hydrothermal/volcanic CO2

The highest output (524 [notdef] 108 t d1) is obtained at La Fossa

of Vulcano Island, the only volcano of the four where a persistent atmospheric plume is observed. The lowest CO2 output (5.7 [notdef] 0.9 t d1) is associated with hydrothermal ac

tivity at Krsuvk, with intermediate emissions at Hekla (15 [notdef] 7 t d1) and Nea Kameni (63 [notdef] 22 t d1). The latter

three volcanoes all currently display weak exhalative activity rather than predominant plume emission. We therefore suggest that a 5.763 t d1 CO2 output range may be characteristic of many of the 500 volcanoes active in the Holocene,

in spite of the majority lacking obvious surface manifestations of degassing. Assuming a representative CO2 output of 10 t d1 for such 500 Holocene volcanoes, we argue that the global population of CO2 emissions may approach a simple power-law distribution. This conclusion will remain somewhat speculative, however, until new measurements become available for the several potentially strong volcanic CO2 point sources (e.g. Papua New Guinea, Indonesia) that are missing from the global CO2 data set.

Our results here suggest that the TDL technique can assist CO2 output determinations at volcanoes covering a range of activities and surface degassing manifestations. Compared to other more consolidated gas sensing techniques (e.g. FTIR), the TDL has the disadvantages that only one species (CO2 in our case) can be measured at same time (against multi-species simultaneous detection by FTIR), and that no passive measurement is possible (FTIR uses passive sources such as the sun or hot rocks/magma). Advantages include, however, lower cost (a commercial TDL is a factor of 23 cheaper than FTIR), user-friendly operation and processing, and robustness for use in harsh/aggressive volcanic environments. Our results and that of Pedone et al. (2014) indicate, in particular, that the GasFinder can operate in a variety of volcanic conditions, provided the plume is not condensing and/or optically thick (fog and/or other obstacles within the laser-mirror path can reduce its functionality during eld operations). Such versatility and robustness, and the availability on the market of pan-tilt units that can be interfaced to the GasFinder to rapidly scan a target gas emission from a xed position, open new prospects for semi-continuous, automatic CO2 ux observations. We suggest that, although measurements will remain restricted to periods with stable meteorological conditions and good visibility, semi-permanent TDL volcano installations may pave the way to acquisition of volcanic CO2 output time series with temporal resolution of 10 s of minutes.

pated and provided technical support during eld campaigns in Iceland. All authors read and approved the nal manuscript.

Acknowledgements. The research leading to these results has received funding from the European Research Council under the European Unions Seventh Framework Programme (FP7/2007/2013)/ERC grant agreement no. 1305377, and from the FP7 grant Futurevolc. The handling topical editor, Albert Galy, and the reviewers, T. A. Mather and G. Williams-Jones, are acknowledged for their constructive reviews. The authors would like to acknowledge technical assistance from Boreal Laser Inc., in particular Michael Sosef. The authors also acknowledge IMO (Icelandic Meteorological Ofce) staff, in particular Melissa Pfeffer and Richard Yeo for support during eld work. Nicolas Cristou is thanked for technical assistance during the eld campaign at Santorini Island. Dario Gharehbaghian, a student at the University of Bologna, and Lorenza Li Vigni, a student at the University of Palermo, are acknowledged for their support during eld work at Vulcano Island.

Edited by: A. Galy

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