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The Cape Verde Islands are a group of 10 intraplate oceanic islands of which 3 show significant levels of recent volcanic activity: Fogo, Santo Antao and Brava. Of these, Fogo is the only historically active volcano with intense activity up to 1725 AD followed by less frequent, mainly effusive eruptions in the last 280 years. The last eruption began on 23 November 2014, and continuing until 8 February 2015. This eruption occurred 19 years after the previous eruptive event, but on the contrary of the 1995 eruption, when there was not monitoring program for the Cape Verde volcanic surveillance, a simple and multidisciplinary volcanic monitoring program, based on a collaborative research volcano monitoring program between Cape Verde and Spanish scientists, was operative to face volcanic unrest such as the 2014 eruption. As part of this volcano monitoring program, from May 2007 to January 2015, forty eight diffuse CO2 emission surveys were performed at the summit crater of Pico do Fogo, covering homogeneously an area of about 0.142 km2. Measurements of diffuse CO2 emission were performed at the surface environment following the accumulation chamber method. The emission rate was calculated after the construction of spatial distribution maps following the sequential Gaussian simulation (sGs) algorithm. The emission rate showed a first anomalous peak in the diffuse CO2 emission, suggesting the occurrence of a first magmatic reactivation likely due to a deep magma intrusion beneath Pico do Fogo volcano between November 2008 and February 2009. Diffuse CO2 emission data suggest the occurrence of a second magmatic intrusion that trigger the eruption process as can be observed by the significant increase of CO2 emission on March 2014. The increment of the diffuse CO2 emission rate shows a good temporal correlation with the satellite-based long-wavelength infrared data reported by Girona et al., (2021). The diffuse CO2 emission data and satellite-based thermal infrared radiance measurements prior the 2014-15 Fogo eruption demonstrate the importance of measuring both parameters to identify and evaluate changes in the volcanic activity of Pico do Fogo volcano. Girona, T. et al., (2021). Nature Geoscience 14, 238–241.

Carbon dioxide (CO2) is a gas that is found in small amounts in Earth's atmosphere. It is released into the atmosphere by volcanoes, but it is typically diluted to low concentrations very quickly and is not harmful to people or animals. However, in certain stable atmospheric conditions, cold CO2 gas can flow into low-lying areas and reach higher concentrations. At these concentrations, CO2 can be dangerous. If the air contains more than 3% CO2, it can cause symptoms such as headaches, dizziness, increased heart rate, and difficulty breathing. At concentrations above 15%, CO2 can quichly lead to unconsciousness and death. The boundary between healthy air and lethal gas can be extremely sharp; even a single step upslope may be adequate to escape death.Since the Tajogaite eruption ended on December 13, 2021, high concentrations of CO2 up to 20% have been observed inside the buildings of La Bombilla and Puerto Naos (La Palma, Canary Islands). Anomalous concentrations of CO2 are detected mainly in the ground-floor and basement of the buildings in Puerto Naos, and their distribution is not homogeneous or uniform throughout the Puerto Naos area (Hernández P.A. et al, 2022). Because of the ultradilute nature of CO2 in the atmosphere, chemical sorbents with strong CO2-binding affinities are typically employed for CO2 capture. For this study, we set up a network of 45 alkaline traps stations located on the ground-floor (streel level) inside buildings of Puerto Naos. These chemical sorbents were made up KOH 1.2 M and replace weekly. The aim of this study was to distinguish between indoor areas that had high levels of CO2 in their alkaline traps and those with lower levels of CO2. This was done through weekly surveys conducted from May to October 2022 in order to identify and define areas with the greatest CO2 hazard. The statistical analysis of the results shows that 75% of the variance of the amounts of CO2 retained weekly in the alkaline traps seems to be related to an endogenous CO2 source while 25% of the variance observed is related to external variables. Hernández, P. A., Padrón, E., Melián, G. V., Pérez, N. M., Padilla, G., Asensio-Ramos, M., Di Nardo, D., Barrancos, J., Pacheco, J. M., and Smit, M.: Gas hazard assessment at Puerto Naos and La Bombilla inhabited areas, Cumbre Vieja volcano, La Palma, Canary Islands, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7705, https://doi.org/10.5194/egusphere-egu22-7705, 2022.

The eruption of the Tajogaite volcano occurred between September and December 2021, being the longest one recorded at La Palma Island. During the course of the eruption, the emission of gas and pyroclasts was constant, forming a cone of approximately 200m. Once the eruption ended, the gas emission has gradually decreased with fumaroles of low and high temperatures along craters. In situ sampling techniques, close as possible to the gas emission sources, are the most suitable tools to investigate the chemical composition of primary emitted volcanic gases. For this reason, we applied the sampling technique proposed by Noguchi & Kamita (1963) to monitor C/S, S/Cl, S/F and F/Cl ratios in the air inside the crater of Tajogaite by using alkaline traps (named JB1, JB3 and JB6) spatially distributed in order to access to different volcanic gas emission spots. This method gives the advantage of an integral monitoring of the acid gases during a certain sampling interval. Initially, two redundant sampling sites were use at three selected locations in April 2022 to study whether there were significant differences in the data from each pair of traps. The results indicated no significant differences in the results of the redundant sampling sites. The alkaline solutions, consisting of 2N KOH solutions, are analyzed using both volumetric and chromatographic methods. The carbon is analyzed by volumetric titrations, whereas Cl, S and F are analyzed by Ion Chromatography (IC). Samples are collected weekly for subsequent analysis.Measured C/S ratios varied between 0.003-0.02, 0.001-002 and 0.002-4.1 for JB1, JB3 and JB6, respectively. S/Cl ratios varied between 11.5-62.3, 64.3-241.3 and 0.1-482.0 for JB1, JB3 and JB6, respectively. S/F ratios varied between 0.8-82.1, 6.3-78.9 and 0.01-74.4 for JB1, JB3 and JB6, respectively. F/Cl ratios varied between 0.4-17.8, 1.8-22.6 and 0.6-169.7 for JB1, JB3 and JB6, respectively. Results show a progress increase on the C/S ratio during all sampling period (April-November 2022), except for the JPB1 and JB3 during October and November 2022, which is explained in base of a observed slight decreasing trend in S content, which is to be expected since the post-eruptive activity should decrease as well as the decrease in the fumarolic output. However, the variations observed in the S/Cl, S/F and F/Cl ratios show a more chaotic behavior, depending on the alkaline trap and therefore its location. The fumaroles that come into contact with the alkaline traps are not the same, and the possible effects of external variables such as wind, can considerably reduce the transport of minor fumarolic gases from their source to the traps producing these variations. S/Cl ratio seems to show higher values during the first period of monitoring, that may be related to variations in the fumarolic activity whereas S/F and F/Cl do not show in general a clear temporal tendency. Direct sampling of plume/fumarolic gases by alkaline traps is a cheap and easy technique to monitor the relationship between various chemical species and volcanic activity.Noguchi & Kamiya (1963). Bull. Volcanol., 24, 367-378.

Tenerife is the largest (2,034 km2) and highest (3,718 m) island of the Canarian archipelago and is the only one hosting an active stratovolcano (Teide-Pico Viejo volcanic system). Its structure is controlled by a volcanotectonic rift-system with NW, NE and NS directions, with the Teide-Pico Viejo volcanic system located in the intersection. The last eruption Teide-Pico Viejo volcanic system occurred in 1798 through an adventive cone. Although Teide volcano shows a weak fumarolic system, volcanic gas emissions observed in the summit cone consist mostly of diffuse CO2 degassing (Hernández et al., 1998; Mori et al., 2001).More than 200 diffuse CO2 efflux surveys have been performed in the summit crater of Teide Volcano during the period 1999-2023. During each survey, diffuse CO2 emission was estimated in 38 sampling sites, homogeneously distributed inside the crater, by means of a portable non dispersive infrared (NDIR) CO2 fluxmeter using the accumulation chamber method. CO2 emission rates was estimated after spatial distribution maps constructed by sequential Gaussian simulation (sGs) algorithm. During 23 years of the studied period, CO2 emissions ranged from 2.0 to 346 t/d. The most remarkable feature of the temporal evolution of diffuse CO2 emission rate was an important increase that began few weeks after the occurrence of a seismic swarm of long period events was recorded on Tenerife in 2 October 2, 2016, a followed by a general increase of the seismic activity in and around the island (D’Auria et al., 2019). Several geochemical parameters showed significant changes during ∼June–August of 2016 and 1–2 months before the occurrence of the October 2, 2016, long-period seismic swarm (Padrón et al., 2021). Since then, anomalously high diffuse CO2 emission rates has remained in the crater of Teide. This change might be explained as an input of magmatic fluids triggered by an injection of fresh magma and convective mixing after the 2 October 2016 seismic swarm (D'Auria et al., 2019; Padrón et al., 2021). This work reflects how useful are the diffuse studies to understand the behaviour of the volcanic system and to forecast future volcanic activity. Monitoring the diffuse degassing rates has demonstrated to be an essential tool for the prediction of future seismic–volcanic unrest and has become an important monitoring tool to reduce volcanic risk in Tenerife.D'Auria, L., et al. (2019). J. Geophys. Res.124,8739-8752Hernández, P., et al. (1998). Geophys. Res. Lett. 25, (17) 3311-3314Mori, T., et al. (2001). Chem. Geol. 177, 85–99Padrón, E., et al. (2021). J. Geophys. Res.126,e2020JB020318

The most common types of survey in known geothermal areas are water and gas sampling of natural discharges. However, where wáter and gas discharge features are few and where the extent of the field is not known, soil and soil-gas surveys can prove useful. These surveys can identify areas in a field that are permeable and potential upflow or boiling zones. They can also outline the boundaries of a geothermal system, often in conjunction with geophysical surveys to provide more comprehensive understanding, especially when geophysical data is difficult to interpret. For all of the above, a soil and soil-gas survey for geothermal exploration had been carried out at Cumbre Vieja volcano that is characterized by neither having visible manifestations of volcanic gases nor zones of hydrothermal alteration on its surface. We have divided the soil and soil-gas survey for geotermal exploration into four categories based on the number of sampling sites per square kilometer: 1) Regional exploration survey (1 sampling site/km2), 2) Regular exploration survey (about 5 sample sites/km2), 3) Detailed I exploration survey (between 10 and 100 sampling sites/km2) and 4) Detailed II exploration survey (more than 500 sampling sites/km2). We have classified our soil and soil-gas survey for geothermal exploration at Cumbre Vieja volcano as a regular exploration survey due to the fact that 1200 observation or measurement points were selected for its 220 km2 area, resulting in approximately 5.5 sampling sites per km2. Similar soil gas surveys had been performed in other study áreas of the Canarian archipelago (Rodríguez et al. 2021). At every sampling site, we performed in-situ measurements of soil CO2 efflux and 222Rn activity in the soil gas atmosphere. In addition, soil gas samples were collected at a depth of 40 cm for further chemical and isotopic analysis in our geochem lab. Simultaneously with the soil gas survey, soil samples were collected from the base of the B Horizon-Subsoil (30–50 cm) which were subsequently dried and sieved (80 mesh) for subsequent leaching in order to analyze those volátiles (Hg0, As, B and NH3) of interest that were fixed in the organic-clay fraction of the soils. Some preliminary results of this soil and soil-gas survey for geothermal exploration at Cumbre Vieja are presented.Rodríguez F., Pérez N. M., Melián G. V., Padrón E., Hernández P. A., Asensio-Ramos M., Padilla G. D., Barrancos J. and D’Auria L. (2021). Exploration of deep-seated geothermal reservoirs in the Canary Islands by means of soil CO2 degassing surveys. Renewable Energy, https://doi.org/10.1016/j.renene.2020.09.065

During and after the end of the 2021 Tajogaite eruption (La Palma, Canary Islands), anomalous CO2 degassing has been detected in the neighborhoods of La Bombilla and Puerto Naos, located around 5 km distance southwestern of the 2021 Tajogaite eruption vents. The aim of this study is to determine the indoor air quality of the houses of the aforementioned neighborhoods. For that purpose, from August 11 to October 24, 2022, air samples were taken, for further analysis, from indoors of 10 locations in Puerto Naos, on a weekly basis. In addition, on September 22, 2022, a discrete survey of the indoor ambient air was carried out in 10 houses of La Bombilla, consisting on in-situ measurements and gas sampling for further analysis.Gas samples were taken for a complete geochemical characterization (i.e., He, Ar, Ne, H2, N2, O2, CH4, CO contents) by micro-gas chromatography (micro-GC) and quadrupole mass spectrometry (QMS) and, as well as for carbon isotopic analysis of the CO2 (δ13C-CO2) by isotopic ratio mass spectrometry (IRMS). In-situ measurements of CO2, O2, 222Rn, 220Rn, H2S and Hg0 were conducted in La Bombilla with and without natural ventilation.National Health Systems in the European Union reflect that the upper limit of the acceptable CO2 concentration range for long-term exposure in the indoor ambient air of buildings for residential use should be of the order of 1,000-1,200 ppm to guarantee people health. The concentrations of CO2 registered in the indoor ambient air of the 10 houses of La Bombilla determined by the in-situ measurements showed relatively high values -above 5,000 ppm- even reaching a maximum of 183,900 ppm in conditions without natural ventilation. In these conditions of absence of ventilation, a certain displacement of O2 was observed, which dropped to 18.7% in the worst case. Under conditions with natural ventilation for a period of 2 hours, the range of CO2 concentration fell to a range between 1,050 and 14,200 ppm and the O2 concentration registered was 20.9%. These results reflect that natural ventilation, and even more forced ventilation, would contribute to reduce CO2 concentration in the ambient air inside buildings. Regarding the results of the indoor gas samples analysis from Puerto Naos, the CO2 concentration and the δ13C-CO2 mean values ranged from 1,190 to 230,952 ppm and -7.9 to -4.8‰ vs. VPDB, respectively. These results of the chemical and isotopic composition of the indoor ambient air of Puerto Naos and La Bombilla demonstrate the importance of these studies to monitor and manage these silent hazards that pose a threat to the population and restrict access to their houses.

Monitoring the volcanic activity of Teide, the only active stratovolcano in Tenerife, the largest island of the Canary Islands, is extremely important for the prevention and reduction of the volcanic disasters of the island. As part of the geochemical monitoring of the Teide volcanic activity, during the last three decades the volcano has been the subject of a geochemical monitoring of the fumarole discharges, characterized by low flux emission of fluids with temperatures of ∼83°C, located at the Teide summit crater (Pérez et al., 1992 and 1996; Melián et al., 2012). Teide fumaroles show chemical compositions typical of hydrothermal fluids, i.e., meteoric steam dominates the gas composition, followed by CO2, N2, H2, H2S, HCl, Ar, CH4, He, and CO (Pérez et al., 1992). The temporal variations in fumarole gas chemistry at Teide volcano was useful to detect significant changes in the chemical composition of the Teide fumarole, including the appearance of SO2, and increases in the HCl and CO concentrations, one year before a seismic crisis that occurred in Tenerife Island between April and June 2004, what suggested that the associated temporal changes in seismic activity and magmatic degassing indicate that geophysical and fluid geochemistry signals in this system are unequivocally related (Melián et al., 2012). The average of the air-corrected 3He/4He ratio during the period 1991-2022 was 6.80 RA (being RA the atmospheric ratio), with the maximum value of the time series (7.57 RA) measured in August 2016, when an input of magmatic fluids triggered by an injection of fresh magma and convective mixing took place beneath Teide volcano (Padrón et al., 2021). After such input of magmatic fluids, increases in the CO2/H2O, C/S and He/CO2 ratios, a decrease in the CO/CO2 ratio were observed together with a significant increase in the seismic activity recorded in the island of Tenerife. This work highlights the important role of volcanic gases in the monitoring of volcanic activity, paying attention to different chemical and isotopic species in the fumarolic discharges. Melián G.V. et al., (2012), Bull. VolcanoL. 74, 1465–1483.Padrón E. et al., (2021), J. Geophys. Res. 126, e2020JB020318.Pérez N.M. et al., (1992), Actas de las sesiones científicas. III Congreso Geológico de España, 1, 463–467.Pérez N.M. et al., (1996), Geophys. Res. Lett. 23(24), 3531–3534.

Cumbre Vieja is the most active volcano of the Canary Islands since it has been the scenario of 8 of 17 historical eruptions in this archipelago. A recent magmatic reactivation started at Cumbre Vieja volcano on October 2017, and 9 additional seismic swarms occurred until the recent eruption which started on September 19, 2021, and ended on December 13, 2022 after 85 days of eruption. Since the first day of the eruption, extending to current days, INVOLCAN performed the monitoring of SO2 realesed by this eruption using a miniDOAS on terrestrial (car), sea (ship) and air (helicopter) mobile position. More than 360 measurements of SO2 emission rates were carried out daily. The standard deviation of the estimated values obtained daily was ~ 20%. During the first days of the eruption, estimated SO2 emission rates reached more than 30,000 tons/day, and maintaining weekly average values above 10,000 tons/day until the end of the eruption. After a final paroxysmal phase with an eruptive column of 8,500 m altitude, decreased significantly to averages values of 250 tons/day. Estimated SO2 emission rates from the 2021 Cumbre Vieja eruption became a powerful tool to contribute to the understanding of eruptive dynamics.

The oceanic active volcanic island of Tenerife (2,034 km2) is the largest of the Canarian archipelago. There are more than 1,000 galleries (horizontal drillings) in the island, which are used for groundwater exploitation and allow reaching the aquifer at different depths and elevations. This work presents the first extensive study on the temporal variation of dissolved gases in groundwaters from Fuente del Valle and San Fernando galleries (Tenerife, Spain) since April 2016 to June 2020. This investigation is focused on the chemical and isotopic content of several dissolved gas species (CO2, He, O2, N2 and CH4) present in the groundwaters and its relationship with the seismic activity registered in the island. The results show CO2 as the major dissolved gas specie in the groundwater from both galleries presenting a mean value of 260 cm3STP·L-1 and 69 cm3STP·L-1 for Fuente del Valle and San Fernando, respectively. The average δ13C-CO2 data (-3.9‰ for Fuente del Valle and -6.4‰ for San Fernando) suggest a clear endogenous origin as result of interaction of them with deep-origin fluid. A bubbling gas sample from Fuente del Valle gallery was analysed, obtaining a CO2 rich gas (87 Vol.%) with a considerable He enrichment (7.3 ppm). The isotopic data of both components in the bubbling gas support the results obtained in the dissolved gases, showing an endogenous component that could be affected by the different activity of the hydrothermal system. During the study period, an important seismic swarm occurred on October 2, 2016, followed by an increase of the seismic activity in and around Tenerife. After this event, important geochemical variations were registered in the dissolved gas species, such as dissolved CO2 and He content and the CO2/O2, He/CO2, He/N2 and CH4/CO2 ratios. These findings suggest an injection of fluids into the hydrothermal system during October 2016, a fact that evidences the connection between the groundwaters and the hydrothermal system. The present work demonstrates the importance of dissolved gases studies in groundwater for volcanic surveillance.

On September 19, 2021, a new eruption began at the west flank of Cumbre Vieja volcano (La Palma, Canary Islands), after an inter-eruptive period of 50 years from the previous eruption (Teneguía, October 1971). The 2021 event was a fissure and powerful strombolian eruption with a magnitude VEI=3 and it has been considered as the most important eruption of Europe during the last 75 years in terms of the significant amount of SO2 released and the serious damage caused by the lava flows.In this work we report the leachate analyses of volcanic ash from the beginning of the eruption, focused on determining the relationship between chemical composition of water‐soluble components adhering to volcanic ash and the volcano’s activity episodes. A total of 5 main control sites or ash-collecting stations were established and ash was picked up in a daily basis. These were located around the main eruptive vents at different distances. Water-extractable concentrations of the samples leached at 1:25 for 2 hours were analyzed by ion chromatography (Cl-, SO42-, F-, Br-, NO3-, Na+, Ca2+, Mg2+, K+) and ICP-MS (Li, B, Al, Si, Sr, Ba, Fe, Ti, Cu, Sb, Rb, Ni, Co, Cd, V). The most abundant components in the leachates were SO42- for the anions and Na+ for the cations, with mean concentrations of 854 and 455 mg/kg, respectively. The results showed the following trend, in decreasing order of abundance: SO4-2>Na+>Cl->F->Ca2+>Al3+>K+>Mg2+. Fluoride, an element of primary concern for human and animal health, showed a range of 16 and 733 mg/kg and an average of 239 mg/kg, which is relatively higher than global median value (129 mg/kg). The S/Cl molar ratio in the ash leachate presented a ranged of values from 0.16 to 5.9 and the observed values > 3.0 seems to be related to ash-rich phases of the eruption. Preliminary results show significant temporal variations in ash leachate compositions, revealing changes in the eruption dynamics.

A recent volcanic eruption has occurred in Cumbre Vieja volcano located in the west of the La Palma Island (at the northwest of the Canary Islands) from September 19 to December 13, 2021. A total of 85 days of eruption makes it the longest volcanic event since historical data have been recorder on La Palma. This volcanic eruption is part of the last stage in the geological evolution of La Palma Island, the fifth in extension (706 km2) and the second in elevation (2,423 m a.s.l.) of the Canarian archipelago. Cumbre Vieja volcano, where the volcanic activity has taken place exclusively in the last 123 ka, forms the southern part of the island. As a response to the occurrence of several seismic swarms and to strengthen the volcanic monitoring of Cumbre Vieja, a regular sampling of groundwater started in October 2017. Three sampling points have been selected, Las Salinas well and two horizontal galleries: Peña Horeb and Trasvase Oeste. Temperature (ºC), pH and electrical conductivity (EC, µS·cm 1) were measured in situ. Water samples were taken to measure the chemical and isotopic composition of the groundwaters in the laboratory. The temperature values showed mean values of 22.1 ºC, 23.7 ºC and 19.6 ºC for Las Salinas, Peña Horeb and Trasvase Oeste, respectively. The mean pH values were 6.50 for Las Salinas, 7.33 for Peña Horeb and 6.81 for Trasvase Oeste, while the mean E.C. values were 41,566 µS·cm-1, 1,684 µS·cm-1 and 426 µS·cm-1 for Las Salinas, Peña Horeb and Trasvase Oeste, respectively. The total alkalinity mean value of groundwater from Las Salinas well was 8.75 mEq·L-1 HCO3-, while that from Peña Horeb was 18.8 mEq·L-1 HCO3- and 4.4 mEq·L-1 HCO3- for Trasvase Oeste. The isotopic composition of O and H (δ18O and δD) showed a meteoric origin, with mean values of 0.8‰ and 7‰ vs. VSMOW for Las Salinas, -4.0‰ and -15‰ vs. VSMOW for Peña Horeb and -4.5‰ and -17 ‰ vs. VSMOW for Trasvase Oeste. Significant changes were observed during the eruptive period, likely related to interaction with endogenous gases such as CO2, H2S and H2. Regarding the isotopic composition of total dissolved carbon, expressed as δ13C-CO2, the average values were -4.6‰, -8.1‰ and -10.8‰ for Las Salinas, Peña Horeb and Trasvase Oeste, respectively, what suggests an endogenous origin for the CO2. Such deep-seated interaction seems stronger at Las Salinas. At this point, the isotopic composition of total dissolved carbon became more magmatic along the studied period, changing from an average of -3.79 in 2017-2018 to -5.17 from 2019 to 2021.Temporal variations were observed in the total alkalinity, δ13C-CO2 andδ18O and δD during the eruption period on La Palma Island. These changes observed in both chemical and isotopic composition were related to interaction between deep volcanic fluids and the groundwaters. Groundwaters studies associated to volcanic aquifers can provide important information about the magmatic gas input in the aquifer, to model groundwater circulations edifice and to strengthen the volcano monitoring.

La Palma Island (708 km2) is located at the north-west and is one of the youngest (~2.0My) of the Canarian Archipelago. On September 19, 2021, a new volcanic eruption occurred at Cumbre Vieja volcanic system at the southern part of the island, the most active basaltic volcano in the Canaries. The erupting fissure (~1.0 km-length) is characterized by lava effusion, strombolian activity, lava fountaining, ash venting and gas jetting. After 85 days of eruption finished on December 13, 2021. We report herein the results of an intensive soil gas study, focused on non-reactive and/or highly mobile gases such as helium (He) and hydrogen (H2), in Cumbre Vieja. He has unique characteristics as a geochemical tracer: it is chemically inert and radioactively stable, non-biogenic, highly mobile and relatively insoluble in water. H2 is one of the most abundant trace species in volcano-hydrothermal systems and is a key participant in many redox reactions occurring in the hydrothermal reservoir gas. Since 2002, soil gas samples were regularly collected at ~40 cm depth using a metallic probe at 600 sites for each survey. He content was analysed by means of a quadrupole mass spectrometer (QMS; Pfeiffer Omnistar 422 and HIDEN QGA) and H2 concentrations by a micro-gas chromatograph (microGC; VARIAN CP490). Spatial distribution maps have been constructed following the sequential Gaussian simulation (sGs) procedure to quantify the diffuse He and H2 emission from the studied area. The time series of both diffuse He and H2 emission show significant increases before and during the occurrence of seismic swarms that took place in the period 2017-2021. During the eruptive period, significant increases in diffuse He and H2 emission were also observed with good temporal agreement with the increase of the volcanic tremor. These increases in diffuse He and H2 emission preceded the peak of diffuse CO2 emission as expected by the characteristics of these gases. The absence of visible volcanic gas emissions (fumaroles, hot springs, etc.) at the surface environment of Cumbre Vieja, makes this type of studies in an essential tool for volcanic surveillance purposes.

The oceanic active volcanic island of Tenerife (2034 km2) is the largest of the Canarian archipelago. There are more than 1,000 galleries (horizontal drillings) in the island, which are used for groundwater exploitation and allow reaching the aquifer at different depths and elevations. During a two-year period (July 2016 to July 2018), a hydrogeochemical study was carried out in two galleries on Tenerife (Fuente del Valle and San Fernando) for volcanic monitoring purposes with weekly sampling. Physicochemical parameter of water, such us temperature (ºC), pH and electrical conductivity (E.C., µS·cm-1), were measured in-situ at each sampling point and chemical/isotopic composition of the water determined later in the laboratory.Temperature values showed mean values of 28.1 ºC and 19.0 ºC for Fuente del Valle and San Fernando galleries, respectively. According to the average pH values, which were 6.30 for Fuente del Valle and 7.13 for San Fernando, and based on the chemical composition, both galleries are sodium-bicarbonate (Na-HCO3) type. E.C. values in both galleries presented high ranges, with mean values of 975 and 1648 µS·cm-1 for Fuente del Valle and San Fernando, respectively. The total alkalinity mean value of groundwater from Fuente del Valle gallery was 11.3 mEq·L-1 HCO3-, while that from San Fernando was 17.3 mEq·L-1 HCO3-. The SO42-/Cl molar ratio was 0.59 and 3.4 for the samples from Fuente del Valle and San Fernando galleries, respectively.The δ18O and δD isotopic analyses showed a meteoric origin of groundwaters, with mean values of -6.2‰ and -26‰ vs. VSMOW for Fuente del Valle and -6.2‰ and -21 ‰ vs. VSMOW for San Fernando. The isotopic data showed a strong interaction with endogenous gases such as CO2, H2S, H2, etc. Regarding the isotopic composition of total dissolved carbon species, expressed as δ13CTDIC, average values of -0.17‰ and 0.26‰ were obtained for Fuente del Valle and San Fernando galleries, respectively. These results show an endogenous origin CO2 signature, heavier for Fuente del Valle gallery groundwater compared to that of San Fernando.Groundwater physicochemical parameters exhibited stable values throughout the study period, while significant temporal variations were observed in the total alkalinity, SO42-/Cl- molar ratio, δ18O and δD. Changes in isotopic ratios coincided with variations observed in the alkalinity and the SO42-/Cl- molar ratio. On October 2, 2016, a seismic swarm of long-period events was recorded on Tenerife followed by a general increase of the seismic activity in and around the island. A correlation was observed between some hydrogeochemical parameters in the groundwaters of the galleries, related to observed changes of the seismic activity. This study demonstrates the suitability of monitoring the chemical and isotopic composition of groundwater from Fuente del Valle and San Fernando galleries, as they are sensitive to changes in volcanic activity on Tenerife island. The study of groundwaters associated to a volcanic system can provide information about the magmatic gas input in the aquifer, modelling how the groundwaters flow through the edifice, and offer important geochemical information that could herald a future eruption.

The island of Tenerife (Canary Islands, Spain) relies on basalt-hosted aquifers to provide 90% of water for agriculture and human consumption. The island is characterised by a low-permeability core, overlain by permeable materials which are cut by impermeable dykes. The effect is a compartmentalised aquifer, which is exploited sequentially as each “pocket” of water is exhausted. The island is home to ~1 million people (with an additional 5 million visiting tourists per year), and although rain/snowfall can be heavy in winter storms, it is unpredictable from year to year, and rapid surface water run off occurs due to the steep geography. While net recharge into the upper zones of the Tenerife aquifer have been quantified (around 2 months between intense rainfall and water table fluctuations), water must then follow a tortuous path to recharge lower zones and aquifer “pockets”. Water recharge to the coastal aquifers is also interrupted and extracted during its journey. Human and agricultural pressure is highest near the coast, and has led to intensive exploitation of existing wells and horizontal galleries. In response to the intensification of water extraction and slow recharge rates, marine intrusions into the coastal aquifers of Tenerife have occurred, traditionally recorded by rising chloride levels and resulting in well/gallery closures as well as increased pressure on other extraction sites. However, in a volcanic ocean island setting, natural processes can mimic the appearance of salinisation in a coastal aquifer. Management of aquifer resources require careful consideration of seawater incursions vs. volcanic degassing contributions vs. ocean island rainfall. Full hydrochemical breakdown of 43 coastal aquifer extraction sites reveal seawater intrusion is affecting the western coastal aquifer, with the agreement of multiple parameters. The strontium isotopic signature of well samples was also measured, because it is not subject to the biological or physical fractionation processes of other isotopic systems, thereby forming distinct reservoirs for groundwater (87Sr/86Sr of host rock), and seawater. 87Sr/86Sr signatures suggest the northern coastal aquifers are also subject to seawater incursions. This parameter may be a more sensitive indicator than chlorides and conductivity markers for salinisation, especially in an ocean island environment where coastal aquifers are subject to intensive land use practices, seawater spray, and affected by diffuse volcanic degassing.

Cape Verde archipelago is a cluster of several volcanic islands arranged in a westward opening horseshoe shape located in the Atlantic Ocean, between 550 and 800 km-west of the coast of Senegal (Africa). Fogo Island is located in the southwest of the archipelago, and as main feature is a 9-km-north to south wide collapse caldera opened toward the east, within Pico do Fogo volcano rises 2,829 m.a.s.l. Pico do Fogo crater has an area of 0.142 km2 and its characterized by a fumarolic field composed by low and moderate temperature fumaroles, with temperatures around 95ºC and reaching 400ºC respectively. The last eruption of Fogo volcanic system took place between November 2014 and February 2015, when four new eruptive vents were formed, and destroyed partially the villages of Portela and Bangaeira (Silva et. al., 2015) forcing the evacuation of 1,300 inhabitants. In this work we present the temporal evolution of 3He/4He isotopic ratio, 3He and 4He emission and thermal energy released data measured from March 2007 to November 2018 in the crater of Pico do Fogo. In all the studied temporal evolutions, we can observe two main increases in the above parameters, the first in early 2010, suggesting a magmatic intrusion, and the second several months before the eruption onset. We have also observed that changes in the 3He emission might be accompanied by a significant increase in thermal output if the system is in an eruptive cycle. Our results confirm 3He emission studies are highly reliable indicator of imminent volcanic eruption and constitute a powerful tool to monitor the activity of volcanic areas around the world.Silva et al., (2015), Geophysical Research Abstracts Vol. 17, EGU2015-13378, EGU General Assembly.

Tenerife (2,034 km2) is the largest of the Canary Islands. Its structure is controlled by a volcano-tectonic rift-system with NW, NE and NS directions, with the volcanic system Teide-Pico Viejo located in the intersection. Teide is 3,718 m.a.s.l. high and its last eruption occurred in 1798 through an adventive cone of Teide-Pico Viejo volcanic complex. Persistent degassing activity, both visible and diffuse, takes place at the summit cone of the volcano, being the diffuse degassing the principle mechanism.During the period 1999-2020, more than 200 diffuse CO2 efflux surveys have been performed in the summit crater of Teide Volcano. For each survey, 38 sampling sites homogeneously distributed inside the crater covering an area of 6,972 m2 were selected. Diffuse CO2 emission was estimated in each point by means of a portable non dispersive infrared (NDIR) CO2 fluxmeter using the accumulation chamber method. Additionally, soil gas samples were taken at 40 cm depth and analyzed later in the lab for the He and H2 content by means of quadrupole mass spectrometry and micro-gas chromatography, respectively. To estimate the He and H2 emission rates at each sampling point, the diffusive component was estimated following the Fick’s law and the convective emission component model was estimated following the Darcy’s law. In all cases, spatial distribution maps were constructed averaging the results of 100 simulations following the sequential Gaussian simulation (sGs) algorithm, in order to determine CO2, He and H2 emission rates.During the study period, CO2 emissions ranged from 2.2 to 176.1 t/d, He emissions between 0.013 and 4.1 kg/d and H2 between 1.3 and 35.6 kg/d. On October 2, 2016, a seismic swarm of long-period events was recorded on Tenerife followed by a general increase of the seismic activity in and around the island (D’Auria et al., 2019). Since then, relatively high values have been obtained in the diffuse CO2, He and H2 emission rate the crater of Teide. This increase reflects a process of pressurization of the volcanic-hydrothermal system.The variations in CO2, He and H2 emissions indicate changes in the activity of the system and can be useful to understand the behaviour of the volcanic system and to forecast future volcanic activity. Monitoring the diffuse degassing rates at Teide volcano has demonstrated to be an essential tool for predicting future seismic–volcanic unrest, and has become important to reduce volcanic risk in Tenerife (Melián et al., 2012; Pérez et al., 2013).D'Auria .L, Barrancos J., Padilla G.D., Pérez N.M., Hernández P.A., Melián G., Padron E., Asensio-Ramos M., García‐Hernández R. (2019). J. Geophys. Res. 124, 8739-8752Pérez N. M., Hernández P. A., Padrón E., Melián G., Nolasco D., Barrancos J., Padilla G., Calvo D., Rodríguez F., Dionis S. and Chiodini G. (2013). J. Geol. Soc., 170(4), 585-592.Melián G., Tassi F., Pérez N. M., Hernández P., Sortino F., Vaselli O., Padrón E., Nolasco D., Barrancos J., Padilla G., Rodriguez F., Dionis S., Calvo D., Notsu K., Sumino H. (2012). Bull. Volcanol, 74(6), 1465-1483.

Hydrogen (H2) is one of the most abundant trace species in volcano-hydrothermal systems and is a key participant in many redox reactions occurring in the hydrothermal reservoir gas. Although H2 can be produced in soils by N2-fixing and fertilizing bacteria, soils are considered nowadays as sinks of molecular hydrogen (Smith-Downey et al. 2006). Because of its chemical and physical characteristics, H2 generated within the crust moves rapidly and escapes to the atmosphere. These characteristics make H2 one of the best geochemical indicators of magmatic and geothermal activity at depth. Cumbre Vieja volcano (La Palma, Canary Islands) is the most active basaltic volcano in the Canaries with seven historical eruptions being Teneguía eruption (1971) the most recent one. Cumbre Vieja volcano is characterized by a main north–south rift zone 20 km long, up to 1950 m in elevation and covering an area of 220 km2 with vents located at the northwest and northeast. Cumbre Vieja does not show any visible degassing (fumaroles, etc.). For that reason, the geochemical volcano monitoring program at Cumbre Vieja volcano has been focused on soil degassing surveys. Here we show the results of soil H2 emission surveys that have been carried out regularly since 2001. Soil gas samples were collected in about 600 sampling sites selected to obtain a homogeneous distribution at about 40 cm depth using a metallic probe and 60 cc hypodermic syringes and stored in 10 cc glass vials. H2 content was analysed later by a VARIAN CP4900 micro-GC. A simple diffusive emission mechanism was applied to compute the emission rate of H2 at each survey. Diffuse H2 emission values were used to construct spatial distribution maps by using sequential Gaussian simulation (sGs) algorithm, allowing the estimation of the emission rate from the volcano. Between 2001-2003, the average diffuse H2 emission rate was ∼2.5 kg·d−1 and an increase of this value was observed between 2013-2017 (∼16.6 kg·d−1), reaching a value of 36 kg·d−1 on June 2017, 4 month before the first recent seismic swarm in October, 2017 at Cumbre Vieja volcano. Six additional seismic swarms had occurred at Cumbre Vieja volcano (February 2018, July-August 2020; October 8-10, 2020; October 17-19, 2020, November 21, 2020 and December 23-26, 2020) and changes of diffuse H2 emission related to this unrest had been observed reaching values up to ∼70 kg·d−1. Diffuse H2 emission surveys have demonstrated to be sensitive and excellent precursors of magmatic processes occurring at depth in Cumbre Vieja. Periodic diffuse H2 emission surveys provide valuable information to improve and optimize the detection of early warning signals of volcanic unrest at Cumbre Vieja volcano.

There is no evidence of hydrothermal fluid discharges in the surficial environment of the Canary Islands, the only Spanish territory with potential high enthalpy geothermal resource, with the exception of the Teide fumaroles.In 2011 and 2014 several geochemical and geophysical studies were carried out across 4 mining licenses on the island of Tenerife (Abeque, Berolo, Guayafanta, Garehagua) for geothermal exploration purposes. The geothermal exploration licenses known as Garehagua and Abeque, located on the south ridge and on the northwest ridge of the island of Tenerife respectively, showed the highest geothermal potential of the studied areas. It was decided to carry out several more detailed studies in the areas with the most significant anomalies to better characterize the potential for economic exploitation.Three surface geochemical surveys each with an average measurement spacing of ~40 m were conducted, and allowed for a mesh resolution of 10 m x 10 m: a mesh size of a much higher spatial resolution than typical grids for surveys of potentially prospective or known geothermal areas. An area called ‘’Madre del Agua’’, located in the northern zone of the Gareagua mining license was prioritised for attention due to i) observed geochemical anomalies at the soil surface; ii) the prominent low-resistivity structure interpreted as a clay alteration cap, and; iii) the positive correlation between thickness of clay alteration cap and helium emission. This study area covers ~0.7 km2. The second area selected for detailed study is located inside the Abeque mining license. The spatial correlation between the helium enrichment and the endogenous CO2 component of Abeque motivated the selection of this study area called ‘’Abeque Detalle’’, which covers ⁓0.8 km2. The third study area called ‘’Fuente del Valle’’ (⁓0.6 km2) was motivated by the observation of significant values ​​of helium anomalies in the Garehagua study area that were measured on the surface in the vertical of a bubbling of endogenous gases at depth, ~2,850 m from the entrance of the Fuente del Valle gallery.The studies were completed from July to September 2018 (Madre del Agua and Abeque Detalle) and from January to March 2019 (Fuente del Valle). During the survey 1065 sampling sites were made, distributed among the three surveys: 362 points in Madre del Agua, 377 in Abeque Detalle and 326 in Fuente del Valle. At each sampling site the soil CO2 efflux and 222Rn activity were measured in-situ and He and H2 were sampled at 40 cm depth and analysed in the lab. The spatial distribution of soil gases of the three study areas confirm the presence of relative enrichment of non-reactive and/or highly mobile gases in the soil gas atmosphere such as He and CO2, that suggests the existence of a significant contribution of deep-seated gases.

One of the main volcano-structural and geomorphological feature in Tenerife (2,034 km2) is the triple rift system, formed by aligned of hundreds of monogenetic eruptive products of shield basaltic volcanism. At the intersection of this triple rift system rises the Teide-Pico Viejo volcanic complex. These volcanic rifts are considered as active volcanic edifices. The North East volcanic Rift Zone (NERZ, 210 km2) form a main NE-SW structure. The North West volcanic Rift Zone (NWRZ, 72 km2) is oriented in NW-SE direction and the North South volcanic Rift Zone (NSRZ, 325 km2) comprises a more scattered area on the south of these monogenetic cones. The most recent eruptive activity of Tenerife has taken place in these rift systems. NERZ host the fissural eruption of Arafo-Fasnia-Siete Fuentes (1704-1705). NWRZ host two historical eruptions: Arenas Negras in 1706 and Chinyero in 1909. Recently the eruption of Boca Cangrejo (1492) has been added to the historical register through 14C dating. NSRZ does not host historical volcanism, although it is recent, up to 10,000 years old.In order to provide a multidisciplinary approach to monitor potential volcanic activity changes at the NERZ, NWRZ and NSRZ, diffuse CO2 emission surveys have been undertaken since 2000, in general in a yearly basis, but with a higher frequency when seismic swarms have occurred in and around NWRZ volcano. Each study area for NERZ, NWRZ and NSRZ comprises hundreds of sampling sites homogenously distributed. Soil CO2 efflux measurements at each sampling site were conducted at the surface environment by means of a portable non-dispersive infrared spectrophotometer (NDIR) LICOR Li820 following the accumulation chamber method. To quantify the CO2 emission rate from the NERZ, NWRZ and NSRZ a sequential Gaussian simulation (sGs) was used as interpolation method.The diffuse CO2 emission rate for the NERZ ranged from 532 up to 2823 t d-1 between 2001 and 2020, with the highest value measured in 2020. In the case of NWRZ, the diffuse CO2 emission rate ranged from 52 up to 867 t d-1 between 2000 and 2020, with the highest value measured in one of the surveys of 2005. Finally, and for NSRZ, the diffuse CO2 emission rate ranged from 78 up to 819 t d-1 between 2002 and 2020, with the highest value measured in 2019. The temporal evolution of diffuse CO2 emission at the NERZ, NWRZ and NSRZ shows a nice and clear relationship with the volcanic seismicity in and around Tenerife Island, which started to take place from the end of 2016. The good temporal correlation between the volcanic seismicity and the increase trend observed in the time series of diffuse CO2 emission rates at NERZ, NWRZ and NSRZ is also coincident with the observed increase of diffuse CO2 emission rate at the summit crater of Teide. This work demonstrates the importance of performing soil CO2 efflux surveys at active rift systems in volcanic oceanic islands as an effective geochemical monitoring tool.

Significant temporal variations in the chemical and isotopic composition of Taal fumarolic gas as well as in diffuse CO2 emission from Taal Main Crater Lake (TMLC) have been observed across the ~12 years of geochemical monitoring (Arpa et al., 2013; Hernández et a., 2017), with significant high CO2 degassing rates, typical of plume degassing volcanoes, measured in 2011 and 2017. In addition to these CO2 surveys at the TCML, soil CO2 efflux continuous monitoring was implemented at Taal volcano since 2016 and a clear increasing trend of the soil CO2 efflux in 2017 was also observed. Increasing trends on the fumarolic CO2/St, He/CO2, CO/CO2 and CO2/CH4 ratios were recorded during the period 2010-2011 whereas increasing SO2/H2S, H2/CO2 ratios were recorded during the period 2017-2018. A decreasing on the CO2/CH4 and CO2/St ratios was observed for 2017-2018. These changes are attributed to an increased contribution of magmatic fluids to the hydrothermal system in both periods. Observed changes in H2 and CO contents suggest increases in temperature and pressure in the upper parts of the hydrothermal system of Taal volcano. The 3He/4He ratios corrected (Rc/Ra), and δ13C of fumarolic gases also increased during the periods 2010-2011 and 2017-2018 before the eruption onset. During this study, diffuse CO2 emission values measured at TMCL showed a wide range of values from >0.5 g m−2 d−1 up to 84,902 g m−2 d−1. The observed relatively high and anomalous diffuse CO2 emission rate across the ~12 years reached values of 4,670 ± 159 t d-1 on March 24, 2011, and 3,858 ± 584 t d-1 on November 11, 2017. The average value of the soil CO2 efflux data measured by the geochemical station showed oscillations around background values until 14 March, 2017. Since then at 22:00 hours, a sharp increase of soil CO2 efflux from ~0.1 up to 1.1 kg m-2 d-1 was measured in 9 hours and continued to show a sustained increase in time up to 2.9 kg m-2 d-1 in 2 November, that represents the main long-term variation of the soil CO2 emission time series. All the above variations might be produced by two episodes of magmatic intrusion which favored degassing of a gas-rich magma at depth. During the 2010-2011 the magmatic intrusion of volatile-rich magma might have occurred from the mid-crustal storage region at shallower depths producing important changes in pressure and temperature conditions, whereas a new injection of more degassed magma into the deepest zone of the hydrothermal system occurring in 2017-2018 might have favored the accumulation of gases in the subsurface, promoting conditions leading to a phreatic eruption. These geochemical observations are most simply explained by magma recharge to the system, and represent the earliest warning precursor signals to the January 2020 eruptive activity.Arpa, M.C., et al., 2013. Bull. Volcanol. 75, 747. https://doi.org/10.1007/s00445-013-0747-9.Hernández, P.A., et al., 2017. Geol. Soc. Lond. Spec. Publ. 437:131–152. https://doi.org/10.1144/SP437.17.

Taal Volcano produces powerful eruptions and is the largest volcanic threat to the Phillipines. Six of the 24 known eruptions since 1572 have resulted in fatalities, and today several million people live with a 20-km radius. Since 2008, our volcano research group has conducted a collaborative research program with Phillipine scientists on applied geochemistry for volcano monitoring. One of the outcomes of this collaborative research was to observed precursor signals to the January 2020 eruptive activity.Significant temporal variations in diffuse CO2 emission at the Taal Crater Lake (TLC) was observed across the ~12 years. Two periods are especially noteworthy. From March 2010 to March 2011 the diffuse CO2 emission rate increased from 763 ± 18 to 4.670 ± 159 tons per day. This anomalous increase coincided with the occurrence of a volcano-seismic unrest characterized mainly by a significant increase in the frequency of volcanic earthquakes, which was interpreted as indicating a new magma intrusion (Arpa et al., 2013; Hernández et al., 2017). A second anomalous diffuse CO2 degassing at the TCL, from 860 ± 42 to 3.858 ± 584 tons per day during the period October 2016 to November 2017, was observed.In addition to the geochemical surveys of diffuse CO2 emission from the TCL, an automatic geochemical station for continuous monitoring of soil CO2 efflux at the northern sector of the Taal Volcano Island crater rim was installed on January 2016. Although short-temp fluctuations in the diffuse CO2 emission time series have been partially driven by meteorological parameters, the major CO2 efflux changes were not driven by such external fluctuations. The major long-term variation of the CO2 emission was an increase trend of the moving average of soil CO2 efflux measurements (168 values) in 2017. Since 14 March, 2017, the station measured a sharp increase of CO2 emission from ~0.1 up to 1.1 kg m-2 d-1 in 9 hours and continued to show a sustained increase in time up to 2.9 kg m-2 d-1 in November 2017. These combined geochemical and geophysical observations are most simply explained by magma recharge to the system, and represent precursor signals to the January 2020 eruptive activity.Taal Volcano BackgroundTaal Volcano is one of the most active volcanoes in the Philippines and has produced some of its most powerful historical eruptions. Located on the southwestern part of Luzon Island, Taal consists of a 15-22-km prehistoric caldera, occupied by the Taal Lake and the active vent complex of Taal Volcano Island with its Crater Lake (TCL).Arpa M. C. et al (2013). Geochemical evidence of magma intrusion inferred from diffuse CO2 emissions and fumarole plume chemistry: the 2010–2011 volcanic unrest at Taal Volcano, Philippines. Bulletin of Volcanology, DOI: 10.1007/s00445-013-0747-9.Hernández P. A. et al (2017). The acid crater lake of Taal Volcano, Philippines: hydrogeochemical and hydroacoustic data related to the 2010–11 volcanic unrest. Geological Society, London, Special Publications, 437, DOI:10.1144/SP437.17

The occurrence of hydrothermal emissions implies the existence of heat sources related to magma reservoirs both in convergent margins (Bransfield-South Shetland) and in mid-ocean ridge and intra-plate settings (Azores). The importance of these systems lies in (a) producing important mineralizations, (b) favouring extremophilic ecosystems, (c) being precursors of underwater volcanic eruptions, (d) playing a major role they play in the matter and energy exchange between the geosphere and the hydrosphere and (d) their impact on the geochemistry of the oceans. In subduction margins, rifts, transforming faults or volcanic buildings in hot spots, emissions of hot fluids related to magmas and/or circulation in hydrothermal systems can occur. The fluids associated with magmas are fundamentally gases (CO2, H2O, H2, SO2, H2S, He, etc.). Hydrothermal fluids constitute a complex system where seawater percolates through fissures and fractures in sediments and rocks at different depths and heats up upon contact with magmas and hot volcanic rocks, leaching a large amount of chemical elements. The identification of acoustic plumes in the water column is the first step in the exploration of unknown underwater emissions. The new acoustic detection technologies, which operate with a wide frequency range, are one of the most innovative tools for detecting gas plumes and other fluids in the water column, especially in deep waters. Once detected, physical-chemical parameters (temperature, salinity, turbidity, cations, anions, dissolved gases, isotopic signature, etc.) that allow their characterization and classification will be determined. This type of studies is particularly useful when it is not possible to collect free gases, fumarolic and/or bubbling gases, as in the case of submarine activity. In this work, we show the results obtained regarding the chemical composition of dissolved gases (He, H2, CO2 (aq), O2, N2, CH4 and He) and isotopic signature of the dissolved CO2 (δ13C-CO2) in sea water sampled in sites of hydrothermal interest. With this purpose, we carried out two oceanographic surveys (EXPLOSEA1 and EXPLOSEA2) in 2019: the first in Antarctica aboard the Spanish Research Vessel (RV) Hespérides and the second in North Atlantic Ocean aboard the Spanish RV Sarmiento de Gamboa. To do so, 13 and 10 water vertical profiles were studied in the RV Hespérides and the RV Sarmiento de Gamboa, respectively, using a SBE 911plus CTD system where there was evidence of acoustic plumes or where appropriate, emission buildings of fluids were present. Water samples were kept in glass bottles for subsequent analysis. The establishment of the physicochemical characteristics of volcanic hydrothermal fluids and the characterization of the nature and origin of the different types of fluid emissions will help to classify the hydrothermal fluids in order to understand the phenomena that take place in them and their surroundings.

Tenerife (2034 km2), the largest island of the Canarian archipelago, is characterized by three volcanic rifts oriented NW-SE, NE-SW and N-S with a central volcanic complex, Las Cañadas Caldera, hosting Teide-Pico Viejo volcanoes. The North West volcanic Rift Zone (NWRZ, 72 km2) of Tenerife is one of the youngest and most active volcanic systems of the island, where two historical eruptions have occurred: Arenas Negras in 1706 and Chinyero in 1909. Diffuse degassing studies has become an important volcanic surveillance tool at those volcanic areas where visible manifestations of volcanic gases are absent, as in the case of NWRZ. Mapping soil gas emission along volcanic structures can provide a better understanding of the processes occurring at depth and allows monitoring the spatial distribution, magnitude and temporal evolution of the surface gas emissions. The geochemical properties of He, minimize the interaction of this noble gas on its movement toward the earth’s surface, and make this gas an almost ideal geochemical indicator of changes occurring in the magmatic plumbing system of the volcano (Padrón et al., 2013, Geology 41(5):539–542). Since 2014, surface He emission surveys have been performed once a year as an additional geochemical tool to monitor the volcanic activity of NWRZ. At 345 sampling sites soil gas samples were collected at 40 cm depth and analyzed for He concentration within 24 hours by means of QMS, model Pfeiffer Omnistar 422. The soil helium concentration data were used to estimate the diffusive helium flux at each point, to construct spatial distribution maps by sequential Gaussian simulation and then to estimate the total helium emission in the NWRZ. Helium emission ranged between non-detected values up to 7.2 mgm-2d-1, and the emission rate of the entire area was in the range ~1 – 45 kg d-1. An increasing trend was observed in the period 2016-2018, showing a good temporal coincidence with a significant increase in seismic activity recorded in Tenerife. The promising results observed in the NWRZ and in other volcanic systems (Padrón et al., 2013) indicate that soil helium emission monitoring could be an excellent early warning geochemical precursory signal for future volcanic unrest.

Carbon dioxide (CO2) is one of the first gases to escape from the magmatic environment due to its low solubility in basaltic magmas at low pressures. Monitoring of volcanic gases in Tenerife Island (2,304 km2) has been focused mainly on diffuse CO2 degassing and other volatiles due to the absence of visible gas manifestations except fumaroles at the summit of Teide volcano. An inexpensive method to determine CO2 fluxes based in the absorption of CO2 through an alkaline medium followed by titration analysis has been used with the aim of contributing to the volcanic surveillance of Tenerife. During summer 2016, a network of 31 closed alkaline traps was deployed along the three volcanic rifts of Tenerife (NE, NW and NS) and at Cañadas Caldera. To do so, an aliquot of 50 mL of 0.1N KOH solution is placed inside the chamber at each station to absorb the CO2 released from the soil. The solution is replaced in a weekly basis and the trapped CO2 is later analyzed at the laboratory by titration. Values are expressed as weekly integrated CO2 efflux. We present herein the results of one year CO2 efflux estimated by closed alkaline traps. The CO2 efflux values ranged from 1.0 to 14.5 g·m-2·d-1, with average values of 8.5 g·m-2·d-1 for the NE rift-zone, 5.2 g·m-2·d-1 for Cañadas Caldera, 6.4 g·m-2·d-1 for NW rift-zone and 6.1 g·m-2·d-1 for NS rift-zone. The estimated CO2 efflux values were of the same order than the observed ones in 2016. Relatively high CO2 efflux values were observed at the NE rift-zone, where maximum values were measured. The temporal evolution of CO2 efflux estimated by closed alkaline traps did not show significant variations during 2019. However, small seasonal variations are observed during the period 2016 – 2019. To investigate the origin of the soil CO2, soil gas samples were weekly sampled on the head space of the closed chambers. Chemical and isotopic composition of C in the CO2 were analysed in the gas samples. The concentration of CO2 on the head space of the closed chambers showed a range of 355-50,464 ppm, with an average value of 1,850 ppmV, while the isotopic composition expressed as d13C-CO2 showed a range from -5.03 to -30.44 ‰, with an average value of -15.9 ‰. The heaviest values of d13C-CO2 are in the NW rift-zone. The systematics of closed static chambers alkaline traps can be a simple and economical tool with volcanic surveillance purposes in system where visible volcanic gases manifestations are absence.

El Hierro (278 km2), the youngest, smallest and westernmost island of the Canarian archipelago, is a 5-km-high edifice constructed by rapid constructive and destructive processes in ~1.12 Ma, with a truncated trihedral shape and three convergent ridges of volcanic cones. It experienced a submarine eruption from 12 October, 2011 to 5 March 2012, off its southern coast that was the first one to be monitored from the beginning in the Canary Islands. As no visible emanations occur at the surface environment of El Hierro, diffuse degassing studies are a useful geochemical tool to monitor the volcanic activity in this volcanic island. Diffuse CO2 emission surveys have been performed at El Hierro Island since 1998 in a yearly basis, with much higher frequency during the period 2011-2012. At each survey, about 600 sampling sites are selected to obtain a homogeneous distribution. Measurements of soil CO2 efflux are performed in situ following the accumulation chamber method. During pre-eruptive and eruptive periods, the diffuse CO2 emission released by the whole island experienced significant increases before the onset of the submarine eruption and the most energetic seismic events of the volcanic-seismic unrest (Melián et al., 2014. J. Geophys. Res. Solid Earth, 119, 6976–6991). The most recent diffuse CO2 efflux survey was carried out in July 2019. Values ranged from non-detectable to 28.9 g m−2 d−1. Statistical-graphical analysis of the data shows two different geochemical populations; Background (B) and Peak (P) represented by 97.5% and 0.5% of the total data, respectively, with geometric means of 1.2 and 23.6 g m−2 d−1, respectively. Most of the area showed B values while the P values were mainly observed at the interception center of the three convergent ridges and the north-east of the island. To estimate the diffuse CO2 emission for the 2019 survey, we ran about 100 sGs simulations. The estimated 2019 diffuse CO2 output released to atmosphere by El Hierro was 214 ± 10 t d-1, value lower than the background average of CO2 emission estimated on 412 t d-1 and slightly higher than the background range of 181 t d-1 (−1σ) and 930 t d-1 (+1σ) estimated at El Hierro volcano during the quiescence period 1998-2010 (Melián et al., 2014, JGR). Monitoring the diffuse CO2 emission has proven to be a very effective tool to detect early warning signals of volcanic unrest at El Hierro.

The exploration of geothermal resources on the island of La Palma, Canary Islands, was first conducted by the Spanish Geological Survey (IGME) from 1982 to 1984. These studies were focused exclusively on the southern part, where the last historical eruption, Teneguía, took place in 1971. This area still shows some geothermal features such us relatively high ground water temperatures (about 40ºC) and soil CO2 efflux values. Recent studies carried out at Cumbre Vieja volcano, the southern part of the island, on diffuse degassing, 3D gravimetry and Audio-MT probes point to promising results, although more studies are needed. We continue applying a multidisciplinary approach to obtain additional information about the geothermal system underlying at Palma island using novel techniques as well as tools which are appropriate to evaluate this system. For this reason, during summer 2019 a soil diffuse degassing research started at Cumbre Vieja volcano (220 km2) for geothermal exploration purposes. In this first phase of the diffuse degassing study about 1,200 sampling sites, with an average distance between sites of approximately 250 m were selected after taking into consideration the volcano-structural features and accessibility. In each sampling site in-situ soil CO2 efflux measurements were performed, and soil gas samples were collected at 40 cm depth for chemical and isotopic analysis. Spatial distribution of CO2 efflux, statistical-graphical analysis of CO2 efflux, and δ13C-CO2 isotopic data to calculate and map the volcano-hydrothermal contribution of CO2 were combined and used for geothermal exploration. The statistical-graphic analysis of the diffuse CO2 efflux values confirms the existence of different geochemical populations showing two log-normal geochemical populations, a fact that suggests the addition of deep-seated CO2. Relatively low CO2 efflux values were measured ranging from non-detected up to 72.8 g m-2 d-1, with an average value of 4.6 g m-2 d-1. The highest CO2 efflux values were measured at the north end of Cumbre Vieja, around the surface contact with Cumbre Nueva ridge. The CO2 isotopic composition, expressed as δ13C- CO2 showed the contribution of three different end-members: biogenic, atmospheric and deep-seated CO2. The results indicate that most of the sampling sites exhibited CO2 composed by different mixtures between atmospheric and biogenic CO2 with slight inputs of deep-seated CO2, with a mean value of -15.3‰, being the maximum and the minimum -2.8‰ and -25.4‰ respectively. The results showed here can help to identify the existence of zones where deep-seated actively degassing from geothermal reservoirs occurs, particularly where the interpretation and application of geophysical data might be difficult.

La Palma Island (708.32 km2) is located at the north-western end of the Canary Archipelago and is one of the youngest of the archipelago. In the last 123 ka, volcanic activity has taken place exclusively at Cumbre Vieja, the most active basaltic volcano in the Canaries, which is located at the southern part of the island. Since no visible geothermal manifestations occur at the surface environment of this volcano, during the last 20 years there has been considerable interest in the study of diffuse degassing as a powerful tool in the volcano monitoring program. In this study we have used two different geochemical approaches for volcano monitoring from October 2017 to November 2019. First, we have developed a network of 21 closed static chambers to determine soil CO2 effluxes. Additionally, we have monitored physical-chemical parameters (temperature, pH, electrical conductivity -EC-) and chemical/isotopic composition and dissolved gases in the water of two galleries (Peña Horeb and Trasvase Oeste) and one water well (Las Salinas). Soil CO2 effluxes for the alkaline traps showed an average value of 7.4 g·m-2·d-1 for the entire Cumbre Vieja volcano. The gas sampled on the head space of the traps can be considered as CO2-enriched air, showing an average value of 1,942 ppmV of CO2. Regarding the CO2 isotopic composition (δ13C-CO2), most of the stations exhibited CO2 composed by different mixing degrees between atmospheric and biogenic CO2 with slight contributions of deep-seated CO2, with an average value of -19.3‰. The results of the physical-chemical parameters measured in waters showed mean temperature values of 23.7ºC, 19.6ºC and 22.1ºC, 7.40, 6.27 and 6.60 for the pH and 1,710 µS·cm-1, 411 µS·cm-1 and 41,100 µS·cm-1 for the EC, for Peña Horeb, Trasvase Oeste and Las Salinas, respectively. The δ13C-CO2 composition of the dissolved gas has a mean value of -7.8‰, -10.2‰ and -3.8‰ vs. VPDB for Peña Horeb, Trasvase Oeste and Salinas, respectively. The highest values of CO2 efflux coincided with the stations showing highest CO2 concentration values located at the southern end of Cumbre Vieja, where the most recent volcanic eruption took place, and also on the northwest flank. This is in accordance with the results obtained for Las Salinas well, located in the south of the island, which show a high concentration of dissolved CO2 and δ13C-CO2 values with a strong deep-seated CO2 contribution. This study represents an interesting contribution to detect early warning signals of future unrest episodes at Cumbre Vieja.

Significant changes in the chemical and isotopic composition of Taal fumarolic gas have been recorded observed before the January 2020 eruption, during the periods 2011 and 2016–2018. Increasing CO2/St, He/CO2, CO/CO2 and CO2/CH4 ratios were recorded during 2011 whereas increasing SO2/H2S, H2/CO2 ratios were recorded during the period 2017–2018. A decreasing on the CO2/CH4 and CO2/St ratios was observed for 2017–2018. These changes are attributed to an increased contribution of magmatic fluids to the hydrothermal system in both periods. Observed changes in H2 and CO contents suggest increases in temperature and pressure in the upper parts of the hydrothermal system of Taal volcano. The 3He/4He ratios corrected (Rc/Ra), and δ13C of fumarolic gases also increased during the periods 2010–2011 and 2017–2018 before the eruption onset. These variations were produced by two episodes of magmatic intrusion which favored degassing of a gas-rich magma at depth. During the 2010–2011 the magmatic intrusion of volatile-rich magma might have occurred from the mid-crustal storage region at shallower depths producing important he observed changes in pressure and temperature conditions, whereas a new injection of more degassed magma into the deepest zone of the hydrothermal system occurring in 2017–2018 might have favored the accumulation of gases in the subsurface, promoting conditions leading to a phreatic eruption.