geosciences

Article The Monitoring of CO2 Soil Degassing as Indicator of Increasing Volcanic Activity: The Paroxysmal Activity at in 2019–2021

Salvatore Inguaggiato 1,* , Fabio Vita 1 , Marianna Cangemi 2 , Claudio Inguaggiato 3,4 and Lorenzo Calderone 1

1 Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, Via Ugo La Malfa, 90146 Palermo, ; [email protected] (F.V.); [email protected] (L.C.) 2 Dipartimento di Scienze della Terra e del Mare, Via Archirafi 36, 90123 Palermo, Italy; [email protected] 3 Departamento de Geología, Centro de Investigación Científica y de Educación Superior de Ensenada, Baja California (CICESE), Carretera Ensenada-Tijuana 3918, 22860 Ensenada, Mexico; [email protected] 4 Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Bologna, Via Donato Creti 12, 40128 Bologna, Italy * Correspondence: [email protected]; Tel.: +39-091-6809435

Abstract: Since 2016, Stromboli volcano has shown an increase of both frequency and energy of the volcanic activity; two strong paroxysms occurred on 3 July and 28 August 2019. The paroxysms were followed by a series of major explosions, which culminated on January 2021 with overflows   and lava flows along the Sciara del Fuoco. This activity was monitored by the soil CO2 flux network of Istituto Nazionale di Geofisica e Vulcanologia (INGV), which highlighted significant changes Citation: Inguaggiato, S.; Vita, F.; before the paroxysmal activity. The CO2 flux started to increase in 2006, following a long-lasting Cangemi, M.; Inguaggiato, C.; positive trend, interrupted by short-lived high amplitude transients in 2016–2018 and 2018–2019. Calderone, L. The Monitoring of CO2 Soil Degassing as Indicator of This increasing trend was recorded both in the summit and peripheral degassing areas of Stromboli, Increasing Volcanic Activity: The indicating that the magmatic gas release affected the whole volcanic edifice. These results suggest Paroxysmal Activity at Stromboli that Stromboli volcano is in a new critical phase, characterized by a great amount of volatiles exsolved Volcano in 2019–2021. Geosciences by the shallow plumbing system, which could generate other energetic paroxysms in the future. 2021, 11, 169. https://doi.org/

10.3390/geosciences11040169 Keywords: Stromboli volcano; paroxysmal activity; soil CO2 fluxes

Academic Editors: Roberto Moretti and Jesus Martinez-Frias 1. Introduction Received: 27 February 2021 Volatiles degassing from volcanic systems is a peculiar and useful tool to monitoring Accepted: 30 March 2021 the volcanic activity to the aims of characterizing the geochemistry of shallow plumbing Published: 8 April 2021 systems and forecasting and individuating the changes of the volcanic activity level. For this reason, many scientists have carried out investigations on shallow volatile degassing Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in to characterize the normal activity level and for identifying the main active degassing published maps and institutional affil- structures that are present on the studied volcanic systems [1–12]. iations. Consequently, many volcano observatories have been established thorough the world, including geochemical monitoring networks of extensive parameters, like CO2 and SO2 fluxes from soil and plume [13–21]. Such a geochemical tool has been successfully applied to Stromboli volcano, which in the last two decades has been characterized by an increasing frequency and intensity of Copyright: © 2021 by the authors. explosions, paroxysms, and effusive eruptions. Licensee MDPI, Basel, Switzerland. Among the three active volcanoes of the Aeolian archipelago, Stromboli, This article is an open access article distributed under the terms and and , the former is the most active (Figure1). Stromboli is an open conduit vol- conditions of the Creative Commons cano characterized by persistent explosive activity (Strombolian activity). During the Attribution (CC BY) license (https:// last decades, major explosions, paroxysms, and three effusive eruptions interrupted the creativecommons.org/licenses/by/ normal Strombolian activity. The effusive eruptions occurred in 2002–2003, 2007, and 2014; 4.0/). the 2002–2003 eruption affected the stability of the volcanic edifice, generating a tsunami

Geosciences 2021, 11, 169. https://doi.org/10.3390/geosciences11040169 https://www.mdpi.com/journal/geosciences Geosciences 2021, 11, 169 2 of 11

(30 December 2002) with a run-up of 11 m, caused by a sliding mass movement along the Sciara del Fuoco to the sea. Paroxysmal explosions occurred during the 2002–2003 (5 April 2003)[22] and 2007 (15 March) effusive eruptions [23], and twice in 2019 (3 July and 28 August) [24].

Figure 1. (a) Location of the study area. (b) Map of Stromboli island with location of wells (white

circle), Fulco (FUL), Saibbo (SAI), Zurro (ZUR), COA, Limoneto (LIM), Piscità (PIS); soil CO2 moni- toring stations STR01 and STR02 (red circles; photos in the insets); main tectonic lineaments of active fault N41◦ and N64◦ (red dashed lines). (c,d) Reddish ash deposits from the 16 November 2020 major explosion on Le Schicciole beach and along the streets of the Stromboli village. (e) Eruptive column of the 16 November major explosion. Photos courtesy of the association AttivaStromboli (www.attivastromboli.net, accessed on 30 March 2021).

The fluid manifestations, useful for a geochemical monitoring program, are present both in the crater area (plume, , and soil degassing) and in the peripheral NE area as testified by the presence of many thermal wells and anomalous soil degassing zones linked to the N64 and N41 active faults (Figure1)[23,25,26]. The recent increase of volcanic activity at Stromboli (since 2016) [20,27,28] has been accompanied by a strong increase of the summit soil degassing, culminating with the 2019 paroxysms [24]. Moreover, the volcanic activity of Stromboli showed a further increase in the 2020–2021 period as shown by major explosions (19 July; 16 August; 10 and 16 November (Figure1c–e ) of 2020; 24 January 2021, and magma overflows in the summit craters (18, 22, and 24 January 2021) with following lava effusions along the Sciara del Fuoco, which occa- sionally reached the sea (22 January 2021). The aim of this article is to investigate the strong increase in volcanic activity of Stromboli over the last five years, utilizing the significant geochemical changes observed both in the summit area (2016–2019 period, STR02 station) characterized by a continuous positive long trend of CO2 soil degassing [24,27], and in the NE peripheral anomalous soil CO2 degassing area recorded in the period of 2017–2021 (STR01 station, new data). In this way, we are able to assess the extent of geochemical anomalies and the areas of influence of volatiles throughout the volcanic edifice.

2. Methods: Soil CO2 Fluxes Geochemical Network A volcano surveillance program was carried out by the INGV of Italy, under the aegis of the Italian civil protection, which developed, installed, and maintained an interdisci- plinary monitoring network at Stromboli volcano. In particular, inside of this program a Geosciences 2021, 11, 169 3 of 11

geochemical network to monitoring a soil CO2 outgassing using an automated accumu- lation chamber system (manufactured by West Systems, Pontedera, Pisa, Italy) [29] was installed in 1999 [22,23,30,31] in the summit and peripheral areas of Stromboli (Figure1), located in Pizzo Sopra la Fossa and Scari anomalous degassing areas [6,21]. The continuous monitoring of CO2 fluxes is performed on an hourly basis (near real- time measurements), and the data are transmitted to the COA Civil Protection Volcano Observatory at Stromboli via Wi-Fi, from which it is sent to the INGV-Palermo geochemical monitoring center via a virtual private network [21]. The near-real acquisition of these data together with the high performance of this geochemical network and a very long acquired data set (two decades) provided scientific information on the shallow plumbing degassing system useful for the evaluation of the volcanic activity levels of Stromboli.

3. Results 3.1. STR02 Soil CO2 Fluxes The area of Pizzo Sopra La Fossa, in the summit area of Stromboli Island (Figure1), is characterized by an anomalous soil degassing with areal CO2 fluxes ranging between −1 26 and 55 t d [6,20] and represents the higher soil CO2 degassing zone with respect to the entire volcanic edifice [6]. The STR02 soil CO2 flux monitoring station was installed at Pizzo Sopra La Fossa in 1999 and produced a large and unique dataset of continuous soil CO2 degassing. STR02 station worked from July 1999 to July 2019. Unfortunately, the paroxysmal event of 3 July 2019 destroyed the STR02 instrumentation, interrupting the acquisition of CO2 monitoring data from soils after 20 years of hard and productive work [20,24,27], which well supported the geochemical surveillance for the evaluation of the volcanic activity level of Stromboli. The near real-time measurements of soil CO2 flux carried out at the STR02 station (Figure2a) showed, after an effusive/paroxysmal period occurred in 2002–2004, a slow and continuous increase in the period from 2005 to 2018 [11], with values ranging from 2000 g m−2 d−1 to 23,000 g m−2 d−1 (Figure2a), interrupted by several abrupt increases of CO2 linked to the input of magmatic volatiles (Figure2b,c). This long-lasting modification of CO2 flux (Figure2a) was explained by the slow but continuous increases of the total volatile pressure in the shallow plumbing system of Stromboli [9,11], due to a continuous refill from the depth. In the last years of life of the STR02 monitoring station (2016–2019), two new strong −2 −1 and abrupt increases of shallow CO2 soil degassing (up to 23,000 g m d ), with the highest degassing rate (24 and 32 g m−2 d−2;[24,27]) ever documented, have been recorded (Figure2b,c). This strong increase, both in degassing CO2 rate and CO2 fluxes, with values similar to those observed in the 2000–2004 strong effusive period, indicated that in the shallow plumb- ing system a very high volatile content was restored. This suggests a new critical phase of degassing with a high paroxysmal potential, as already occurred in 2003 [20,24,27,32], and further confirmed by the occurrence of the highest energetic paroxysms recorded in the last 20 years on 3 July and 28 August 2019 [24]. After the 3 July 2019 paroxysm, due to the destruction of the STR02 station, the geochemical monitoring of soil CO2 degassing continued with the STR01 station located in the peripheral area. GeosciencesGeosciences 20212021, ,1111, ,x 169 FOR PEER REVIEW 4 4of of 11 11

FigureFigure 2. 2. (a()a )Daily Daily average average CO CO2 fluxes2 fluxes at atSTR02 STR02 station, station, 2000–2019 2000–2019 period. period. The The effusive effusive eruptions eruptions ofof 2000, 2000, 2007, 2007, and and 2014 2014 have have been been in insertedserted (vertical (vertical green green bar) bar) together together with with the the paroxysmal paroxysmal events events(vertical (vertical red lines) red lines) that occurred that occurred in 5 April in 5 Ap 2003;ril 152003; March 15 March 2007; 2007; 3 July; 3 andJuly; 28 and August 28 August 2019. 2019. Green Greenand orange and orange horizontal horizontal dashed dash linesed lines have have been been plotted plotted indicating, indicating, respectively, respectively, the baseline the baseline values values and threshold value for the CO2 fluxes at STR02 (see [5,21,23]). The insets of the higher de- and threshold value for the CO2 fluxes at STR02 (see [5,21,23]). The insets of the higher degassing gassing rate periods have been plotted together with the major explosions (vertical blue lines) oc- rate periods have been plotted together with the major explosions (vertical blue lines) occurred. (b) curred. (b) Daily average CO2 fluxes at STR02 station, June 2010–December 2013 period, with the Daily average CO fluxes at STR02 station, June 2010–December 2013 period, with the abrupt CO abrupt CO2 degassing2 rate of 16 g m−2 d−2 . (c) Daily average CO2 fluxes at STR02 station, 2016–2018 2 −2 −2 period,degassing with rate the ofabrupt 16 g mCO2 degassingd .(c) Daily rate of average 24 g m CO−2 d2−2fluxes. (d) Daily at STR02 average station, CO2 fluxes 2016–2018 at STR02 period, −2 −2 station,with the October abrupt 2018–June CO2 degassing 2019 period rate of 24with g mthe abruptd .(d CO) Daily2 degassing average rate CO of2 fluxes 32 g m at−2 STR02d−2. station, −2 −2 October 2018–June 2019 period with the abrupt CO2 degassing rate of 32 g m d . 3.2. STR01 Soil CO2 fluxes 3.2. STR01 Soil CO2 fluxes The area of Scari, in the northeast side of the Stromboli Island [6], is characterized by The area of Scari, in the northeast side of the Stromboli Island [6], is characterized an anomalous soil degassing well documented in the past [6] with CO2 fluxes ranging by an anomalous soil degassing well documented in the past [6] with CO fluxes ranging from 0.1 to 10 t d−1 [20]; the higher value was reached during the effusive eruption2 period from 0.1 to 10 t d−1 [20]; the higher value was reached during the effusive eruption period in 2014. in 2014. The soil degassing in this area is driven by the N64 active fault (Figure 1) that receives The soil degassing in this area is driven by the N64 active fault (Figure1) that receives deeper fluids exsolved by shallow magma. The peripheral soil degassing is controlled by deeper fluids exsolved by shallow magma. The peripheral soil degassing is controlled by the underlying thermal aquifer that modulates and buffers the fluid inputs linked to the the underlying thermal aquifer that modulates and buffers the fluid inputs linked to the volcanicvolcanic activity activity [33]. [33]. Moreover, Moreover, in in the the shallo shalloww portion portion of of the the soil soil the the climatic climatic conditions conditions (mainly(mainly temperature) temperature) influence influence and and mo modulatedulate the the degassing degassing process process [33]. [33]. The daily average CO2 fluxes recorded in the period of 2017–2021 at STR01, plotted The daily average CO2 fluxes recorded in the period of 2017–2021 at STR01, plotted togethertogether with with the the daily daily air air temperature temperature (Figur (Figuree 3),3), showed two peculi peculiarar behaviors: (i) (i) a cyclic increasing and decreasing of CO2 flux values during summer and winter seasons, cyclic increasing and decreasing of CO2 flux values during summer and winter seasons,

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respectively,respectively, and and (ii) (ii) a continuousa continuous positive positive degassing degassing trend, trend, with with relative relative yearly yearly maximum maxi- −2 −1 valuesmum values from ∼ from240 to⁓240∼400 to g⁓400 m g dm−2 .d−1.

FigureFigure 3.3. DailyDaily average average STR01 STR01 CO CO2 flux2 flux (blue (blue column) column) vs. time vs.time (2017–2021), (2017–2021), and STR01 and STR01 daily aver- daily averageage air temperature air temperature (red (redline). line). The cyclic The cyclic behavior behavior of CO of2 fluxes CO2 fluxes and air and temperature air temperature is veryis clear, very clear,with maximum with maximum and minimum and minimum values values recorded recorded for both for parameters both parameters during during the hot the and hot cold and sea- cold son, respectively. season, respectively.

2 TheThe cyclic increasing of of CO CO 2fluxesfluxes highlights highlights that that the the yearly yearly flux flux maxima maxima were were rec- recordedorded in inAugust, August, which which is ischaracterized characterized by by the the higher higher yearly valuesvalues ofof temperaturetemperature (around(around 30–3230–32 ◦°C),C), whereaswhereas thethe fluxflux minimaminima valuesvalues werewere recordedrecorded inin February,February, whichwhich isis characterizedcharacterized byby thethe lowerlower yearlyyearly temperaturetemperature valuesvalues (around(around 11–14 11–14◦ °C).C). TheThe dailydaily COCO22 fluxesfluxes in the STR01 station, measured during this last period of obser- vationvation (2017–2021),(2017–2021), rangerange fromfrom about about 30 30 to to 400 400 g g m m−2−2d d−−11, with anan averageaverage valuevalue aroundaround 150150 gg mm−−2 dd−−1.1 . TheThe cumulatedcumulated probability probability plot plot of of log log CO CO2 shows2 shows multimodal multimodal distribution distribution highlighting highlight- threeing three log-normal log-normal families families of degassing of degassing of 30, of 100,30, 100, and and 250 250 g m g− m2 d−2− d1−,1, respectively respectively named named biologicalbiological soilsoil respiration respiration (B), (B), hydrothermal hydrothermal low low degassing degassing (HL), (HL), and and hydrothermal hydrothermal high degassinghigh degassing (HH) (Figure(HH) (Figure4). Family 4). Fa B,mily representing B, representing the lower the CO lower2 flux CO measured2 flux measured at STR01 at duringSTR01 during the period the ofperiod observation, of observation, suggests sugges that itts represents that it represents the background the background level, mainly level, controlledmainly controlled by biological by biological CO2 production CO2 production in the soil. in the Families soil. Families HL and HL HH and are HH related are re- to thelated highest to the COhighest2 fluxes, CO2 suggestingfluxes, suggesting and representing and representi theng hydrothermal the hydrothermal degassing degassing both duringboth during normal normal (hydrothermal (hydrothermal low degassing)low degassing) and highand high volcanic volcanic activity activity (hydrothermal (hydrother- highmal high degassing). degassing). Figure5 shows a great positive correlation between air temperature and soil CO 2 fluxes with a high positive coefficient of correlation (R2 = 0.88) and a correlation equation equal to: CO2 fluxes = (11.22 × AirT) − 103.52 (1)

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Figure 4. (a) Cumulative probability diagram of log CO2 flux; the data distribution highlights three main degassing families with respectively 10 (B), 30 (HL), and 60% (HH) of cumulated frequency representing respectively the biological soil respiration, hydrothermal low degassing, and hydro- thermal high degassing. (b) Histogram of log CO2 flux frequency.

Figure 5 shows a great positive correlation between air temperature and soil CO2 2 Figure 4. (a) CumulativeFigure 4. ( probabilitya)fluxes Cumulative with diagram aprobability high of logpositive CO diagram2 flux; coefficient theof log data CO of distribution2 flux;correlation the data highlights (R distribu = 0.88) threetion andmain highlights a degassingcorrelation three families equation with respectivelymain 10 degassing (B), 30equal (HL), families to: and 60%with (HH)respectively of cumulated 10 (B), 30 frequency (HL), and representing 60% (HH) of respectively cumulated frequency the biological soil respiration, hydrothermalrepresenting low respectively degassing, the and biological hydrothermal soil re highspiration, degassing. hydrothermal (b) Histogram low degassing, of log CO andflux hydro- frequency. 2 2 thermal high degassing. (b) Histogram of logCO CO fluxes2 flux frequency. = (11.22 × AirT) − 103.52 (1)

Figure 5 shows a great positive correlation between air temperature and soil CO2 fluxes with a high positive coefficient of correlation (R2 = 0.88) and a correlation equation equal to:

CO2 fluxes = (11.22 × AirT) − 103.52 (1)

2 FigureFigure 5. 5. CorrelationCorrelation graphic graphic between between daily CO 2 fluxesfluxes and air temperatures.temperatures. A highhigh correlationcorrelation has 2 hasbeen been found, found, with with R2 equalR equal to 0.88.to 0.88.

To analyze in detail the behavior of CO2 flux and air temperature during the period of 2017–2020, we calculated their yearly cumulated probability (Figure6a,b). There is a

progressive increase of CO2 fluxes (Figure6a) during the period mentioned above, while Figure 5. Correlationthe range graphic between between winter daily minima CO2 fluxes and and summer air temperat maximaures. A air high temperatures correlation (Figure6b) 2 has been found,contemporarily with R equal decreasesto 0.88. (from 22 to 16 ◦C), due to both higher winter minima and lower summer maxima air temperatures.

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To analyze in detail the behavior of CO2 flux and air temperature during the period of 2017–2020, we calculated their yearly cumulated probability (Figure 6a,b). There is a progressive increase of CO2 fluxes (Figure 6a) during the period mentioned above, while Geosciences 2021, 11the, 169 range between winter minima and summer maxima air temperatures (Figure 6b) con- 7 of 11 temporarily decreases (from 22 to 16 °C), due to both higher winter minima and lower summer maxima air temperatures.

Figure 6. (a) Cumulated probability of CO2 fluxes calculated and plotted for each year of acquisi- Figure 6. (a) Cumulated probability of CO2 fluxes calculated and plotted for each year of acquisition. The increase of the higher andtion. lower The family’s increase values of the for higher each year and from lower 2017 family’s to 2020 values is visible. for each (b) Theyear cumulated from 2017 probability to 2020 is visi- of air temperature ble. (b) The cumulated probability of air temperature plotted for each year shows a clear decrease plotted for each year shows a clear decrease of higher family’s values (hot temperature) from 2017 to 2021 and an increase of of higher family’s values (hot temperature) from 2017 to 2021 and an increase of lower family’s lower family’s values (cold temperature). values (cold temperature). Summer maxima of air temperature exhibit a decreasing trend, in counter tendency Summer maxima of air temperature exhibit a decreasing trend, in counter tendency with the systematic increase of CO2 flux observed during the last four years, thus excluding with the systematic increase of CO2 flux observed during the last four years, thus exclud- its climatic forcing. Conversely, higher soil CO2 flux can be attributed to a higher release of ing its climatic volatilesforcing. Conversely, exsolved from higher the magmatic soil CO2 flux plumbing can be system.attributed to a higher re- lease of volatiles exsolved from the magmatic plumbing system. 4. Discussion and Conclusions 4. Discussion and ConclusionsThe dynamic model of degassing at Stromboli volcano is based on the delicate equilib- The dynamicrium model between of degassing the continuous at Stromboli refilling volcano from the is based deep ofon volatiles the delicate exsolved equi- by magma and librium betweenthe the output continuous from the refilling shallow from system the [deep23,27 of]. volatiles exsolved by magma and the output fromDuring the shallow the “normal” system [23,27]. Strombolian activity, the deep volatiles interact with the shallow During theplumbing “normal” system Strombolian and outflow activity, to the the deep atmosphere, volatiles preservinginteract with the the dynamic shal- equilibrium low plumbing (Volatiles’system and pressure outflow IN to≈ theVolatiles’ atmosphere, pressure preserving OUT, in Figurethe dynamic7a). equilib- rium (Volatiles’ pressureWhen IN the ≈ deepVolatiles’ input pressure considerably OUT, increases in Figure (IN 7a). >>> OUT in Figure7b), the dynamic When the deepequilibrium input considerably is altered, increasing increases the(IN volatiles’>>> OUT pressurein Figure in 7b), the the shallow dynamic plumbing system. equilibrium is altered,Therefore, increasing the shallow the volatile systems’ tries pressure to restore in the the shallow equilibrium plumbing by increasing system. the output to Therefore, the shallowthe atmosphere system tries (soil to and restore plume the degassing) equilibrium and by to increasing the aquifer, the also output through to the increase of the atmosphereboth (soil frequency and plume and degassing) energy of and the to explosions, the aquifer, overflows also through of magma the increase confined in the crater of both frequencyterrace, and energy or lava of flow the explosions along Sciara, overflows del Fuoco of (Figure magma7b). confined in the crater terrace, or lava flow Thealong above Sciara described del Fuoco events (Figure have 7b). characterized the recent activity (2016–2021) of The aboveStromboli described [ 24events,27,28 ,have34]. characterized the recent activity (2016–2021) of Stromboli [24,27,28,34].Based on the above described dynamic degassing model, we founded our observa- tions on the monitoring of soil CO2 fluxes in the summit area, recorded by the STR02 station in the period of 2016–2019. During the abovementioned period, the CO2 fluxes of STR02 showed a clear pressurization process of the shallow plumbing system [27], with a proposed mechanism in three distinct phases: (i) long positive degassing trend of soil CO2 (pressurization); (ii) sharp variations of soil CO2 fluxes (inputs transient); and finally (iii) strong instability degassing process that culminated in the paroxysmal events occurred on 3 July 2021 [24]. This paroxysmal event destroyed the STR02 equipment for the CO2 flux measurements and interrupted the acquisition of this key extensive parameter utilized for monitoring the volcanic activity of Stromboli. Considering the logistical difficulties (use of the helicopter) and the lack of safety to operate in the summit area of Stromboli in this very active period, we were unable to proceed with replacement of the new monitoring station. Geosciences 2021, 11, 169 8 of 11

For the prosecution of the geochemical monitoring of the volcanic activity of Stromboli, we Geosciences 2021, 11, x FOR PEER REVIEW 8 of 11 utilize the STR01 station located in the peripheral area of Stromboli using data of soil CO2 fluxes acquired in the period of 2017–2021.

FigureFigure 7. Sketch 7. Sketch of the of thedynamic dynamic equilibrium equilibrium model, model, modified modified from from [20,23]. [20,23 (a].) (Normala) Normal Strombo- Strombolian lianactivity. activity. Volatiles Volatiles exsolved exsolved from from the the magma magma standing standing in in the the deeper deeper plumbing plumbing system system interact interact with with shallow fluids, feeding the shallow hydrothermal activity and diffuse soil degassing, whose shallow fluids, feeding the shallow hydrothermal activity and diffuse soil degassing, whose output to output to the atmosphere maintains the dynamic equilibrium. (b) When the deep input considera- the atmosphere maintains the dynamic equilibrium. (b) When the deep input considerably increases, bly increases, the dynamic equilibrium is altered and the shallow system tries to restore it, increas- ingthe the dynamic outputs. equilibrium is altered and the shallow system tries to restore it, increasing the outputs. The CO fluxes emitted by soils in this peripheral area (STR01) are strongly influenced Based on the2 above described dynamic degassing model, we founded our observa- by environmental factors (mainly air temperature; [33]); thus, we proceeded with a filtering tions on the monitoring of soil CO2 fluxes in the summit area, recorded by the STR02 sta- of the raw CO2 flux data to eliminate the influence of temperature and to consider only the tion in the period of 2016–2019. During the abovementioned period, the CO2 fluxes of variations of CO flux due to the changes of the volcanic activity. Based on the correlation STR02 showed a clear2 pressurization process of the shallow plumbing system [27], with a equation between the CO2 flux and air temperature (Equation (1)), raw data measured proposed mechanism in three distinct phases: (i) long positive degassing trend of soil CO2 at STR01 station were filtered subtracting the estimated environmental component of the (pressurization); (ii) sharp variations of soil CO2 fluxes (inputs transient); and finally (iii) CO flux at the measured temperature. Furthermore, we normalized the filtered CO strong2 instability degassing process that culminated in the paroxysmal events occurred 2 flux data (Equation (2)) to the average annual temperature recorded over the last 4 years on 3 July 2021 [24]. This paroxysmal event destroyed the STR02 equipment for the CO2 (approximately 25 ◦C): flux measurements and interrupted the acquisition of this key extensive parameter uti- lized for monitoring the volcanic activity of Stromboli. Considering the logistical difficul- Normalized residual CO2 flux ◦ = (CO2 fluxmeas − ((11.22*AirTmeas) − 103.5)) + CO2 flux ◦ )/CO2 flux ◦ (2) ties (use25 ofC the helicopter) and the lack of safety to operate in the summit25 C area of Stromboli25 C in this very active period, we were unable to proceed with replacement of the new moni- The filtered and normalized data of daily average CO2 fluxes (residuals) have been toringplotted station. in Figure For the8, togetherprosecution with of the the main geoc volcanichemical eventsmonitoring recorded of the in volcanic the last years.activity It is of Stromboli,very interesting we utilize to observe the STR01 the station variations loca ofted CO in 2theflux peripheral degassing area from of Stromboli 2017 to 2021, using with dataits of progressive soil CO2 fluxes and massiveacquired increasein the period that exceedsof 2017–2021. 100% over the entire period and with a sharpThe CO acceleration2 fluxes emitted in 2020–21. by soils In in particular, this peripheral the normalized area (STR01) residual are strongly CO2 fluxes influ- vary encedover by the environmental entire period (2017–2021)factors (mainly between air temp 75 anderature; 175 g[33]); m−2 thus,d−1, we with proceeded an average with annual a filteringincrease of the of degassing raw CO2 flux rate data of 33% to eliminate and with the an influence abrupt increase of temperature in the last and year to of consider 75%. The onlylast the sharp variations and substantial of CO2 flux increase due to ofthe CO changes2 in the of 2nd the halfvolcanic of 2020 activity. culminated Based on with the the correlationmagma overflowequation frombetween the summit the CO craters2 flux and and air with temperature the small and (Equation short-time (1)), lava raw effusion data measured(18–24 January at STR01 2021) station on thewere Sciara filtered del su Fuoco,btracting which the reached estimated the environmental sea on 22 January compo- 2021. nent of the CO2 flux at the measured temperature. Furthermore, we normalized the fil- tered CO2 flux data (Equation (2)) to the average annual temperature recorded over the last 4 years (approximately 25 °C):

Normalized residual CO2 flux25 °C = (CO2 fluxmeas − ((11,22*AirTmeas) − 103,5)) + CO2 (2) flux25 °C)/CO2 flux25 °C

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The filtered and normalized data of daily average CO2 fluxes (residuals) have been plotted in Figure 8, together with the main volcanic events recorded in the last years. It is very interesting to observe the variations of CO2 flux degassing from 2017 to 2021, with its progressive and massive increase that exceeds 100% over the entire period and with a sharp acceleration in 2020–21. In particular, the normalized residual CO2 fluxes vary over the entire period (2017–2021) between 75 and 175 g m−2 d−1, with an average annual in- crease of degassing rate of 33% and with an abrupt increase in the last year of 75%. The Geosciences 2021, 11, 169 last sharp and substantial increase of CO2 in the 2nd half of 2020 culminated with9 of the 11 magma overflow from the summit craters and with the small and short-time lava effusion (18–24 January 2021) on the Sciara del Fuoco, which reached the sea on 22 January 2021.

FigureFigure 8.8. DailyDaily averageaverage COCO22 fluxesfluxes (residuals) filtered filtered and no normalizedrmalized to to the the average average temperature temperature (25(25 ◦°C)C) of of the the 2017–2021 2017–2021 period. period. The The 30 day 30 day running running average average (white (white line) line)and the and 3rd the degree 3rd degree poly- polynomialnomial fit (green fit (green line) line) have have been been plotted. plotted. The The main main events events of volcanic of volcanic activity activity (major (major explosions, explosions, paroxysm, and lava flow) have been reported too. paroxysm, and lava flow) have been reported too.

2 InIn conclusion,conclusion, wewe cancan affirmaffirm thatthat thethe soilsoil COCO2 degassingdegassing wellwell representsrepresents thethe dynamicdynamic equilibriumequilibrium balancebalance modelmodel [[23]23] forfor monitoringmonitoring thethe StrombolianStrombolian activityactivity andand forecastingforecasting andand highlightinghighlighting the the changes changes of of volcanic volcanic activity activity level level recorded recorded in thein the last last years. years. Moreover, Moreo- thever, continuous the continuous increase increase of soil COof soil2 degassing CO2 degassing observed observed in the peripheral in the peripheral area of Stromboli area of inStromboli the 2017–2021 in the 2017–2021 period, which period, follows which thefollows long-lasting the long-lasting increase increase observed observed during dur- the 2006–2019ing the 2006–2019 period in period the summit in the area,summit suggests area, su thatggests the recharge that the inputrecharge of volatiles input of fromvolatiles the magmafrom the did magma not end did with not theend summer with the 2019 summer paroxysms 2019 paroxysms and still continues and still continues today affecting today theaffecting entire the volcanic entire edifice. volcanic The edifice. volcanic The activity volcan levelic activity of Stromboli level of is Stromboli still very highis still today very withhigh atoday high volatileswith a high pressure volatiles in thepressure shallow in plumbingthe shallow systems, plumbing which systems, can lead which to new can energeticlead to new volcanic energetic events volcanic in the events future. in The the scientific future. The volcanic scientific community volcanic iscommunity required to is keeprequired a high to keep attention a high level attention for further level potential for further signs potential of impending signs of paroxysm.impending paroxysm.

AuthorAuthor Contributions: Conceptualization,Conceptualization, S.I. S.I. and and C.I.; C.I.; data data acquisition acquisition andand methodology, methodology, F.V. F.V.and andL.C.; L.C.; graphical graphical editing, editing, M.C. M.C.and S.I.; and data S.I.; curati dataon, curation, M.C. and M.C. L.C.; and writin L.C.;g—original writing—original draft prepara- draft preparation,tion, S.I., C.I.; S.I., M.C.; C.I.; F.V.; M.C.; writing—review F.V.; writing—review and editin andg, editing, S.I. All S.I. authors All authors have read have and read agreed and agreed to the topublished the published version version of the ofmanuscript. the manuscript. Funding:Funding: ThisThis researchresearch waswas fundedfunded byby thethe INGV-DPCNINGV-DPCN (Italian(Italian NationalNational InstituteInstitute ofof GeophysicsGeophysics andand -ItalianVolcanology-Italian NationalNational DepartmentDepartment forfor CivilCivil Protection)Protection) volcanicvolcanic surveillancesurveillance programprogram ofof StromboliStromboli volcano. ObFu ObFu 0304.010. 0304.010 and by the Project FIRST-ForecastIng eRuptive activity at Stromboli volcano: Timing, eruptive style, size, intensity, and duration, INGV-Progetto Strategico DipartimentoVulcani 2019, (Delibera n. 144/2020). Acknowledgments: The authors wish to thank their colleagues at the Istituto Nazionale di Geofisica e Vulcanologia of Palermo for their help in acquiring and processing data.

Conflicts of Interest: The authors declare no conflict of interest.

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