Monitoring the response of volcanic CO2 emissions to changes in the Los Humeros hydrothermal system

Carbon dioxide is the most abundant, non-condensable gas in volcanic systems, released into the atmosphere through either diffuse or advective fluid flow. The emission of substantial amounts of CO2 at Earth’s surface is not only controlled by volcanic plumes during periods of eruptive activity or fumaroles, but also by soil degassing along permeable structures in the subsurface. Monitoring of these processes is of utmost importance for volcanic hazard analyses, and is also relevant for managing geothermal resources. Fluid-bearing faults are key elements of economic value for geothermal power generation. Here, we describe for the first time how sensitively and quickly natural gas emissions react to changes within a deep hydrothermal system due to geothermal fluid reinjection. For this purpose, we deployed an automated, multi-chamber CO2 flux monitoring system within the damage zone of a deep-rooted major normal fault in the Los Humeros Volcanic Complex (LHVC) in Mexico and recorded data over a period of five months. After removing the atmospheric effects on variations in CO2 flux, we calculated correlation coefficients between residual CO2 emissions and reinjection rates, identifying an inverse correlation of ρ = − 0.51 to − 0.66. Our results indicate that gas emissions respond to changes in reinjection rates within 24 h, proving an active hydraulic communication between the hydrothermal system and Earth’s surface. This finding is a promising indication not only for geothermal reservoir monitoring but also for advanced long-term volcanic risk analysis. Response times allow for estimation of fluid migration velocities, which is a key constraint for conceptual and numerical modelling of fluid flow in fracture-dominated systems.

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

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 magma 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 before the paroxysmal activity. The CO2 flux started to increase in 2006, following a long-lasting positive trend, interrupted by short-lived high amplitude transients in 2016–2018 and 2018–2019. This increasing trend was recorded both in the summit and peripheral degassing areas of Stromboli, indicating that the magmatic gas release affected the whole volcanic edifice. These results suggest that Stromboli volcano is in a new critical phase, characterized by a great amount of volatiles exsolved by the shallow plumbing system, which could generate other energetic paroxysms in the future.

Soil Degassing From the Xianshuihe–Xiaojiang Fault System at the Eastern Boundary of the Chuan–Dian Rhombic Block, Southwest China

The Chuan–Dian region, situated in the middle part of the north-south seismic zone of mainland China in a highly deformed area of the eastern margin of the Tibetan Plateau, is one of the principal areas for monitoring earthquake activities in China. In this study, the geochemical characteristics of soil degassing (of CH4, H2, CO2, Rn, and Hg) and, the relationship between degassing and fault activity, were investigated in the Xianshuihe–Xiaojiang fault system (XXFS) at the eastern boundary of the Chuan–Dian rhombic block. The mean soil-gas concentrations of CH4, H2, CO2, Rn, and Hg in the XXFS were 8.1 ppm, 9.9 ppm, 0.5%, 15.1 kBq/m3 and 12.9 ng/m3, respectively. The δ13CCO2 and δ13CCH4 values of the hot-spring gases varied from −11.9‰ to −3.7‰ and −62.5‰ to 17‰, respectively. The He-C isotopic ratios indicate that the carbon in the northern and middle parts of the XXFS may have originated from deep fluids, whereas the carbon in the southern part of the XXFS is of organic origin. The high concentrations of soil gas were distributed near the faults, indicating that the faults could act as channels for gas migration. The distributions of the high soil-gas concentrations in the XXFS coincide with the highest stress and maximum strain rates, indicating that the fault activity enhanced permeability and increased the emission rates of the gases. The results of this study will be helpful for degassing in active fault zones and earthquake monitoring.

Diffuse H2 degassing studies: a useful geochemical tool for monitoring Cumbre Vieja volcano, La Palma, Canary Islands

<p>Hydrogen (H<sub>2</sub>) 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 H<sub>2</sub> can be produced in soils by N<sub>2</sub>-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, H<sub>2</sub> generated within the crust moves rapidly and escapes to the atmosphere. These characteristics make H<sub>2</sub> 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 km<sup>2</sup> 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 H<sub>2</sub> 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. H<sub>2</sub> content was analysed later by a VARIAN CP4900 micro-GC. A simple diffusive emission mechanism was applied to compute the emission rate of H<sub>2</sub> at each survey. Diffuse H<sub>2</sub> 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 H<sub>2</sub> emission rate was ∼2.5 kg·d<sup>−1</sup> and an increase of this value was observed between 2013-2017 (∼16.6 kg·d<sup>−1</sup>), reaching a value of 36 kg·d<sup>−1</sup> 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 H<sub>2</sub> emission related to this unrest had been observed reaching values up to ∼70 kg·d<sup>−1</sup>. Diffuse H<sub>2</sub> emission surveys have demonstrated to be sensitive and excellent precursors of magmatic processes occurring at depth in Cumbre Vieja. Periodic diffuse H<sub>2</sub> emission surveys provide valuable information to improve and optimize the detection of early warning signals of volcanic unrest at Cumbre Vieja volcano.</p>

Soil gas changes at Terre Calde di Medolla during and after 2012 seismic sequence

<p>Several soil gas surveys were performed from 2008 to 2015 in Medolla (Northern Italy) within a farming area characterized by macroseeps, absence of vegetation and anomalous temperatures of soil to investigate the soil gas migration mechanism and verify the presence of a buried fault intersecting the macroseeps. In this work, we show results of soil gas measurements of radon and thoron activities, and helium and carbon dioxide concentrations, which have been carried out in the area struck of the 2012 seismic sequence.</p><p>We found that the seismic sequence sensibly influenced the soil gas distribution in the area. Indeed, soil gas anomalies are useful to recognize influences of surface features on natural gas migration. The study of the association of different gases with different origin and physical/chemical behaviour, the collection of a large number of samples during the dry season and the use of proper data analysis are fundamental in the comprehension of gas migration mechanism. The study of spatial distribution of soil gas anomalies can give information on the origin and processes involving deep and superficial gas species. In particular, the study of the spatial distribution of radon, often together with other soil gases, appears to be a suitable tool for identifying active tectonic structures in faulted areas.</p><p><sup>222</sup>Rn and <sup>220</sup>Rn were recorded starting from 2012, early after the mainshock of 20<sup>th</sup> May. The May 2012 distribution map shows a broad sector of the area with anomalous values approximately aligned NW-SE. Radon vs thoron distribution data highlighted two different circulation mechanisms. After an initial perturbation of the system in May, a deep fluid migration is prevalent in September 2012. From 2013, the soil degassing returned to the main shallow origin. Over time, the anomalous high values of all the investigated species were always measured in correspondence of macroseeps supporting the hypothesis of a hidden fault. However, <sup>222</sup>Rn values collected early after the mainshocks have ubiquitous distribution, likely due to perturbation of the system which enhanced the degassing of surficial layers and masked the deep contribution. The shallow and deep contributions presumably coexist for the other data, located at the intersection of the two trends. Over time <sup>222</sup>Rn is better related to CO<sub>2</sub> concentrations than CH<sub>4</sub>, in particular for the May 2012, 2013 and 2015 surveys (0.43 < r > 0.60) and, to a lesser degree, for Sept 2012 (r = 0.25). This relationship suggests that CO<sub>2</sub> likely acts as a carrier for <sup>222</sup>Rn allowing it to quickly reach the surface. Although, generally, radon concentrations increase with flow, elevated mass flux due to high flows can dilute the <sup>222</sup>Rn activities and its values recorded at the surface. This phenomenon could justify the slightly anomalous values in correspondence of macroseeps.</p><p>Geochemical surveys highlight the importance to carry out a discrete monitoring that can help to study the stress/strain changes related to seismic activity that may force crustal fluid to migrate up, thereby altering the geochemical characteristics of the fault zone at surface before and after earthquakes.</p>

Time-Lapse Landform Monitoring in the Pisciarelli (Campi Flegrei-Italy) Fumarole Field Using UAV Photogrammetry

The utility of new imaging technologies to better understand hazardous geological environments cannot be overstated. The combined use of unmanned aerial vehicles (UAV) and digital photogrammetry (DP) represents a rapidly evolving technique that permits geoscientists to obtain detailed spatial data. This can aid in rapid mapping and analyses of dynamic processes that are modifying contemporary landscapes, particularly through the creation of a time series of digital data to help monitor the geomorphological evolution of volcanic structures. Our study comprises a short-term (in geological terms) monitoring program of the dynamic and diffuse Pisciarelli degassing structure caused by the interplay between intensive rainfall and hydrothermal activity. This area, an unstable fumarole field located several hundred meters east of the Solfatara Crater of the Campi Flegrei caldera (southern Italy), is characterized by consistent soil degassing, fluid emission from ephemeral vents, and hot mud pools. This degassing activity is episodically accompanied by seismic swarms and macroscopic morphology changes such as the appearance of vigorously degassing vents, collapsing landslides, and bubbling mud. In late-2019 and 2020, we performed repeated photogrammetric UAV surveys using the Structure from Motion (SfM) technique. This approach allowed us to create dense 3D point clouds and digital orthophotos spanning one year of surveys. The results highlight the benefits of photogrammetry data using UAV for the accurate remote monitoring and mapping of active volcanoes and craters in harsh and dangerous environments.

Changes in CO2 Soil Degassing Style as a Possible Precursor to Volcanic Activity: The 2019 Case of Stromboli Paroxysmal Eruptions

Paroxysmal explosions are some of the most spectacular evidence of volcanism on Earth and are triggered by the rapid ascent of volatile-rich magma. These explosions often occur in persistently erupting basaltic volcanoes located in subduction zones and represent a major hazard due to the sudden occurrence and wide impact on the neighboring populations. However, the recognition of signals that forecast these blasts remains challenging even in the best-monitored volcanoes. Here, we present the results of the regular monitoring of soil CO2 flux from a fumarole field at the summit of Stromboli (Italy), highlighting that the 2016–2019 period was characterized by two important phases of strong increases of volatile output rate degassing (24 g m2 d−2 and 32 g m2 d−2, respectively) and moreover by significant changes in the degassing style few months before the last paroxysmal explosions occurred in the summer 2019 (3 July and 28 August). Establish that the deep portions of a volcano plumbing system are refilled by new volatiles-rich magma intruding from the mantle is therefore a key factor for forecasting eruptions and helping in recognizing possible precursors of paroxysmal explosions and could be highlighted by the monitoring of soil CO2 flux. The abrupt increase of degassing rate coupled with the strong increase of fluctuating signal (daily natural deviation) recorded during 2019 at Stromboli could be the key to predicting the occurrence of paroxysmal events.

Clustering analysis of soil gas chemical ratios as a potential geochemical tool for surface geothermal exploration

<p>Geochemistry is a fundamental tool in surface geothermal exploration. In particular, the analysis of the composition of the soil atmosphere, the measurement of diffuse CO<sub>2</sub> flux and of the gas <sup>222</sup>Rn activity are important parameters to detect and characterize the contribution of volcanic/hydrothermal sources in the diffuse soil degassing.</p><p>The analysis of the soil atmosphere usually consists of determining the chemical and isotopic composition of the gases, including concentrations and molar ratios of multiple chemical species (e.g. He, H<sub>2</sub>, N<sub>2</sub>, Ar, Ne, O<sub>2</sub>, CH<sub>4</sub>, CO<sub>2</sub>), as well as the C isotopic ratios (<sup>13</sup>C/<sup>12</sup>C). In practice a single geochemical survey provides tens of different parameters for each sampling point. Taking into account that a typical survey is composed of hundreds of sampling points, the huge amount of collected data requires effective data mining tools to perform analyses going beyond the simple mapping of concentrations and/or ratios and to detect hidden patterns in the dataset.</p><p>Among the most effective multivariate statistical tools is clustering analysis. This technique allows determining the presence of groups of points showing a given degree of similarity. In this work we used and compared two different clustering techniques: the K-means and the DBSCAN algorithms, applying them to a geochemical dataset related to surveys realized in 2010 in the southern part of the island of Tenerife (Canary Islands Spain) with the aim of geothermal exploration. We show how the clustering analysis allows determining the presence of areas characterized by a similar chemical and isotopic composition. The use of standard geochemical tools allows interpreting the nature of these areal groups in terms of their relevance for the purposes of surface geothermal exploration.</p>

Duvalo (North Macedonia): A “volcano” without volcanic activity

<p>The Duvalo locality is located in the SW of the Republic of North Macedonia, in the Ohrid region, near the village of Kosel. It is an area of strong soil degassing, called “volcano” by the local people despite volcanic activity has never been documented in the recent geologic history of the area [1]. A large area (thousands of sqm) shows signs of strong alteration and is devoid of vegetation. Until the 19<sup>th</sup>century sulphur was mined from this area [1].</p><p>In August 2019, a campaign of soil CO<sub>2 </sub>flux measurements and soil gas sampling was made. Duvalo is sometimes referred to as an active geothermal feature but no signs of enhanced geothermal gradient were found and the soil temperatures at 50 cm depth in this campaign were always within the range of local mean air temperatures. Soil CO<sub>2 </sub>flux values ranged from 1.3 to 59,000 g/m<sup>2</sup>/d and can be modelled with the overlapping of 3 or 4 flux populations. A possible biological background is estimated in 6.8±1.8 g/m<sup>2</sup>/d while the other populations are characterized by an anomalous average flux ranging from 180 to 33,000 g/m<sup>2</sup>/d. The CO<sub>2 </sub>total emission, estimated both with a statistical and geostatistical approach, provided similar values in the order of 50 t/d. This has to be considered as a minimum value because only areas with evident signs of alteration have been investigated. Nevertheless, the estimated output is quite high for an area unrelated with recent volcanism or geothermal activity.</p><p>The chemical composition of soil gases shows: CO<sub>2 </sub>(96.6%), N<sub>2 </sub>(1.8%), H<sub>2</sub>S (0.6%) and CH<sub>4 </sub>(0.3%) as the main gases. The present composition is almost indistinguishable from previous analyses made in 1957 and 1977 [1] pointing to a stability of the system in last decades. The isotope compositions indicate for CO<sub>2 </sub>(δ<sup>13</sup>C -0.2 ‰) a pure carbonate rock origin, for CH<sub>4 </sub>(δ<sup>13</sup>C -34.4 ‰ and δ<sup>2</sup>H -166 ‰) a thermogenic origin and for He (R/R<sub>A </sub>0.10) a pure crustal origin.</p><p>The H<sub>2</sub>S released at Duvalo may be produced by either microbial or thermochemical sulphate reduction favoured by hydrocarbons whose presence can be inferred by the uprise of thermogenic methane. Partial oxidation of H<sub>2</sub>S during its upflow, producing sulphuric acid, may be responsible of the production of abundant CO<sub>2 </sub>through dissolution of carbonate rocks. Similar processes have been evidenced also in other parts of North Macedonia [2]. These gases rise up through the N–S trending normal faults bordering the seismically active Ohrid basin graben [3] being released to the atmosphere through the soils of Duvalo “volcano”.</p><p>This research was funded by: DCO Grant n. 10881-TDB “Improving the estimation of tectonic carbon flux”; GINOP-2.3.2-15-2016-00009 ‘ICER’ project and PO-FSE Sicilia 2014–2020 (CUP: G77B17000200009).</p><p>References</p><p>[1] Markovski B. et al., 2018. Duvalo a geological phenomenon near Ohrid. DOI: 10.18509/AGB.2020.05</p><p>[2] Temovski M., 2017. Hypogene Karst in Macedonia. In: Klimchouk et al. (eds.), Hypogene Karst Regions and Caves of the World, Springer</p><p>[3] Hoffmann N. et al., 2010. Biogeosciences, 7, 3377–3386</p>

Nonvolcanic Carbon Dioxide Emission at Continental Rifts: The Bublak Mofette Area, Western Eger Rift, Czech Republic

This study presents the results of gas flux measurements of cold, mantle-derived CO2 release at the Bublák mofette field (BMF), located inside of the N-S directed Počátky Plesná fault zone (PPFZ). The PPFZ is presently seismically active, located in the eastern part of the Cheb Basin, western Eger Rift, Central Europe. The goal of the work was to identify the linkage between tectonics and gas flux. The investigated area has a size of 0,43 km2 in which 1.115 locations have been measured. Besides classical soil CO2 gas flux measurements using the closed chamber method (West Systems), drone-based orthophotos were used in combination with knowledge of plant zonation to find zones of high degassing in the agriculturally unused part of the BMF. The highest observed soil CO2 gas flux is 177.926,17 g m-2 d-1, and the lowest is 0,28 g m-2 d-1. Three statistical methods were used for the calculation of the gas flux: arithmetic mean, kriging, and trans-Gaussian kriging. The average CO2 soil degassing of the BMF is 30 t d-1 for an area of 0,43 km2. Since the CO2 soil degassing of the Hartoušov mofette field (HMF) amounts to 23 t d-1 for an area of 0,35 km2, the average dry degassing values of the BMF and HMF are in the same magnitude of order. The amount of CO2 flux from wet mofettes is 3 t d-1 for the BMF and 0,6 t d-1 for the HMF. It was found that the degassing in the BMF and HMF is not in accordance with the pull-apart basin interpretation, based on the direction of degassing as well as topography and sediment fill of the suggested basins. En-echelon faults inside of the PPFZ act as fluid channels to depth (CO2 conduits). These structures inside the PPFZ show beginning faulting and act as tectonic control of CO2 degassing.