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Carbon dioxide (CO2) diffuse degassing structures (DDS) at Furnas volcano (São Miguel Island, Azores) are mostly associated with the main fumarolic fields, evidence that CO2 soil degassing is the surface expression of rising steam from the hydrothermal system. Locations with anomalous CO2 flux are mainly controlled by tectonic structures oriented WNW–ESE and NW–SE and by the geomorphology of the volcano, as evidenced by several DDS located in depressed areas associated with crater margins. Hydrothermal soil CO2 emissions in Furnas volcano are estimated to be ∼968 t d−1. Discrimination between biogenic and hydrothermal CO2 was determined using a statistical approach and the carbon isotope composition of the CO2 efflux. Different sampling densities were used to evaluate uncertainty in the estimation of the total CO2 flux and showed that a low density of points may not be adequate to quantify soil emanations from a relatively small DDS. Thermal energy release associated with diffuse degassing at Furnas caldera is about 118 MW (from an area of ∼4.8 km2) based on the H2O/CO2 ratio in fumarolic gas. The DDS also affect Furnas and Ribeira Quente villages, which are located inside the caldera and in the south flank of the volcano, respectively. At these sites, 58% and 98% of the houses are built over hydrothermal CO2 emanations, and the populations are at risk due to potential high concentrations of CO2 accumulating inside the dwellings.

Furnas volcano, in São Miguel island (Azores), being the surface expression of rising hydrothermal steam, is the site of intense carbon dioxide (CO 2 ) release by diffuse degassing and fumaroles. While the diffusive CO 2 output has long (since the early 1990s) been characterized by soil CO 2 surveys, no information is presently available on the fumarolic CO 2 output. Here, we performed (in August 2014) a study in which soil CO 2 degassing survey was combined for the first time with the measurement of the fumarolic CO 2 flux. The results were achieved by using a GasFinder 2.0 tunable diode laser. Our measurements were performed in two degassing sites at Furnas volcano (Furnas Lake and Furnas Village), with the aim of quantifying the total (fumarolic + soil diffuse) CO 2 output. We show that, within the main degassing (fumarolic) areas, the soil CO 2 flux contribution (9.2 t day −1 ) represents a minor (~15 %) fraction of the total CO 2 output (59 t day −1 ), which is dominated by the fumaroles (~50 t day −1 ). The same fumaroles contribute to ~0.25 t day −1 of H 2 S, based on a fumarole CO 2 /H 2 S ratio of 150 to 353 (measured with a portable Multi-GAS). However, we also find that the soil CO 2 contribution from a more distal wider degassing structure dominates the total Furnas volcano CO 2 budget, which we evaluate (summing up the CO 2 flux contributions for degassing soils, fumarolic emissions and springs) at ~1030 t day −1 .

Accurately locating and quantifying carbon dioxide (CO2) leakage to the atmosphere is important for diffuse degassing studies in volcanic / geothermal areas and for safety monitoring and/or carbon credit auditing of Carbon Capture and Storage (CCS) sites. This is typically conducted by measuring CO2 flux at numerous points over a large area and applying statistics or geostatistical interpolation. Accuracy of the results will depend on many factors related to survey/data-processing choices and site characteristics, and thus uncertainties can be difficult to quantify. To address this issue, we have developed a Monte Carlo-based program (MC-Flux) that repeatedly subsamples a high-resolution synthetic or real dataset using a choice of different sampling strategies (one random and four grid types) at multiple user-defined sample densities. The program keeps track of the anomalies found and estimates total flux using two statistical and two geostatistical approaches from the literature. This paper describes the use of MC-Flux to assess the potential impact of various sampling and interpretation decisions on the accuracy of the final results. Simulations show that an offset grid sample distribution yields the best results, however relatively dense sampling is required to obtain a high probability of an accurate flux estimate. For the test dataset used, ordinary kriging interpolation produces a range of flux estimates that are centered on the true value while sequential Gaussian simulation tends to slightly overestimate values at intermediate sample spacings and is sensitive to input parameters. These results point to the need for developing new approaches that decrease uncertainty, such as integration with high-resolution co-kriging datasets that complement the more accurate point flux measurements.

Background: Gaseous elemental mercury (Hg0 or GEM) is an atmospheric form of mercury (Hg)—a toxic heavy metal—that is naturally released in volcanic environments. Research with wild mice demonstrates that chronic exposure to a hydrothermal volcanic environment leads to the bioaccumulation of Hg in the lungs, but also in both the central (CNS) and peripheric (PNS) nervous systems, with marked indications of neurotoxicity. Studies addressing human exposure to volcanogenic Hg0 are scarce, hence its risks are still unknown. This study aims to evaluate the level of exposure to Hg0 in children living in a volcanically active environment. Methodology and main findings: Two groups of school-aged children (from 6 to 9 years old) were part of this study: one with children inhabiting a hydrothermal area (exposed group) and another with children inhabiting an area without volcanic activity (non-exposed group). Hair samples were collected from each individual for Hg level analysis. It was found that the levels of Hg in the hair of exposed children were 4.2 times higher than in that of non-exposed children (≈1797.84 ± 454.92 ppb vs. 430.69 ± 66.43 ppb, respectively). Conclusion: Given the vast health risks Hg poses, the need to monitor the health of populations inhabiting volcanically active areas is highlighted. Because little is known about the fate, modifications, and effects of Hg0 in the human body, particularly regarding its effects on the nervous system in children, the development of further research within the scope is strongly encouraged.

Carbon dioxide flux from the soil is regularly monitored in selected areas of Vesuvio and Solfatara (Campi Flegrei, Pozzuoli) with the twofold aim of i) monitoring spatial and temporal variations of the degassing process and ii) investigating if the surface phenomena could provide information about the processes occurring at depth. At present, the surveyed areas include 15 fixed points around the rim of Vesuvio and 71 fixed points in the floor of Solfatara crater. Soil CO 2 flux has been measured since 1998, at least once a month, in both areas. In addition, two automatic permanent stations, located at Vesuvio and Solfatara, measure the CO 2 flux and some environmental parameters that can potentially influence the CO 2 diffuse degassing. Series acquired by continuous stations are characterized by an annual periodicity that is related to the typical periodicities of some meteorological parameters. Conversely, series of CO 2 flux data arising from periodic measurements over the arrays of Vesuvio and Solfatara are less dependent on external factors such as meteorological parameters, local soil properties (porosity, hydraulic conductivity) and topographic effects (high or low ground). Therefore we argue that the long-term trend of this signal contains the “best” possible representation of the endogenous signal related to the upflow of deep hydrothermal fluids.

Diffuse degassing through the soil is commonly observed in volcanic areas and monitoring of carbon dioxide flux at the surface can provide a safe and effective way to infer the state of activity of the volcanic system. Continuous measurement stations are often installed on active volcanoes such as Furnas (Azores archipelago), which features low temperature fumaroles, hot and cold CO2 rich springs, and several diffuse degassing areas. As in other volcanoes, fluxes measured at Furnas are often correlated with environmental variables, such as air temperature or barometric pressure, with daily and seasonal cycles that become more evident when gas emission is low. In this work, we study how changes in air temperature and barometric pressure may affect the gas emission through the soil. The TOUGH2 geothermal simulator was used to simulate the gas propagation through the soil as a function of fluctuating atmospheric conditions. Then, a dual parameters study was performed to assess how the rock permeability and the gas source properties affect the resulting fluxes. Numerical results are in good agreement with the observed data at Furnas, and show that atmospheric variables may cause the observed daily cycles in CO2 fluxes. The observed changes depend on soil permeability and on the pressure driving the upward flux. Key Points Modeling the effects of atmospheric variables on surface diffuse degassing Correlation between atmospheric variables and carbon dioxide fluxes Atmospheric effects as possible indicators of changes in system properties

We report herein the results of 13 soil CO 2 efflux surveys at Cumbre Vieja volcano, La Palma Island, the most active basaltic volcano in the Canary Islands. The CO 2 efflux measurements were undertaken using the accumulation chamber method between 2001 and 2013 to constrain the total CO 2 output from the studied area and to evaluate occasional CO 2 efflux surveys as a volcanic surveillance tool for Cumbre Vieja. Soil CO 2 efflux values ranged from non-detectable up to 2442 g m −2  days −1 , with the highest values observed in the south, where the last volcanic eruption took place (Teneguía, 1971). Isotopic analyses of soil gas carbon dioxide suggest an organic origin as the main contribution to the CO 2 efflux, with a very small magmatic gas component observed at the southern part of the volcano. Total biogenic and magmatic combined CO 2 emission rates showed a high temporal variability, ranging between 320 and 1544 t days −1 and averaging 1147 t days −1 over the 220-km 2 region. Two significant increases in the CO 2 emission observed in 2011 and 2013 were likely caused by an enhanced magmatic endogenous contribution revealed by significant changes in the 3 He/ 4 He ratio in a CO 2 -rich cold spring. The relatively stable emission rate presented in this work defines the background CO 2 emission range for Cumbre Vieja during a volcanic quiescence period.

Active subaerial volcanoes often discharge large amounts of CO 2 and H 2 S to the atmosphere, not only during eruptions but also during periods of quiescence. These gases are discharged through focused (plumes, fumaroles, etc.) and diffuse emissions. Several studies have been carried out to estimate the global contribution of CO 2 and H 2 S emitted to the atmosphere by subaerial volcanism, but additional volcanic degassing studies will help to improve the current estimates of both CO 2 and H 2 S discharges. In October 2008, a wide-scale survey was carried out at Mt. Etna volcano, one the world’s most actively degassing volcanoes on Earth, for the assessment of the total budget of volcanic/hydrothermal discharges of CO 2 and H 2 S, both from plume and diffuse emissions. Surface CO 2 and H 2 S effluxes were measured by means of the accumulation chamber method at 4075 sites, covering an area of about 972.5 km 2 . Concurrently, plume SO 2 emission at Mt. Etna was remotely measured by a car-borne Differential Optical Absorption Spectrometry (DOAS) instrument. Crater emissions of H 2 O, CO 2 and H 2 S were estimated by multiplying the plume SO 2 emission times the H 2 O/SO 2 , CO 2 /SO 2 and H 2 S/SO 2 gas plume mass ratios measured in situ using a portable multisensor. The total output of diffuse CO 2 emission from Mt. Etna was estimated to be 20,000 ± 400 t day −1 with 4520 t day −1 of deep-seated CO 2 . Diffuse H 2 S output was estimated to be 400 ± 20 kg day −1 , covering an area of 9.1 km 2 around the summit craters of the volcano. Diffuse H 2 S emission on the volcano flanks was either negligible or null, probably due to scrubbing of this gas before reaching the surface. During this study, the average crater SO 2 emission rate was ~2100 t day −1 . Based on measured SO 2 emission rates, the estimated H 2 O, CO 2 and H 2 S emission rates from Etna’s crater degassing were 220,000 ± 100,000, 35,000 ± 16,000 and 510 ± 240 t day −1 , respectively. These high values are explained in terms of intense volcanic activity at the time of this survey. The diffuse/plume CO 2 emission mass ratio at Mt. Etna was ~0.57, that is typical of erupting volcanoes (mass ratio <1). The average CO 2 /SO 2 molar ratio measured in the plume was 11.5, which is typical of magmatic degassing at great depth beneath the volcano, and the CO 2 /H 2 S mass ratio in total diffuse gas emissions was much higher (~11,000) than in plume gas emissions (~68). These results will provide important implications for estimates of volcanic total carbon and sulfur budget from subaerial volcanoes.

We report the first detailed study of diffuse emission of carbon dioxide (CO 2 ), hydrogen sulfide (H 2 S), helium (He), and hydrogen (H 2 ) from the summit crater of Pico do Fogo volcano, Cape Verde. Diffuse CO 2 , H 2 S, He, and H 2 gas fluxes were measured at 57 sampling sites and ranged up to 12,800, 13, 1, and 6 g m −2  day −1 , respectively. Soil temperature measurements at each sampling site were used to evaluate the heat flux. Most of the summit crater shows relatively high CO 2 efflux, with highest values close to the fumarolic area, suggesting a structural control of the degassing process. In contrast, H 2 S effluxes were negligible or very low at the summit crater, except close to the fumarolic area where anomalously high CO 2 efflux and soil temperatures were also measured. We estimate total CO 2 , H 2 S, He, and H 2 diffuse gas fluxes of 219 t day −1 , 25, 4, and 33 kg day −1 , respectively. Based on a H 2 O/CO 2 mass ratio of 1.52 measured at the fumaroles, we estimate a diffuse steam flux from the summit crater of approximately 330 t day −1 . The enthalpy of this steam is equivalent to a heat flux of about 10.3 MW. The diffuse gas emission and thermal energy released from the summit crater of Pico do Fogo volcano are comparable to those observed at other volcanoes. Sustained surveillance of Pico do Fogo using these methods will be valuable for monitoring the activity of one of the most active volcanoes in the Atlantic Ocean.

Cerro Negro volcano is a young basaltic cinder cone which is part of the Nicaraguan volcanic arc. Eruptive activity at Cerro Negro is characterized by explosive strombolian to subplinian eruptions driven by volatile-rich basaltic magma ascending rapidly from various crustal depths (>15 to 6 km) resulting in the onset of precursory activity only ∼30 min before an eruption. In this paper, we present a comprehensive degassing characterization of the volcano over a 4-year period aimed at improving our understanding of the magmatic plumbing network and its relationship with regional tectonics. A total of 124 individual soil gas samples were collected between 2010 and 2013 and analyzed for stable carbon isotopes (δ 13 C) from CO 2 . High temperature fumaroles were sampled for δ 18 O, δD, and 3 He/ 4 He isotope analysis, and major degassing zones were mapped using soil CO 2 flux measurements. Gases at Cerro Negro are characterized by a strong 3 He/ 4 He mantle signature (6.3 to 7.3 R A ), magmatic δ 13 C ratios (−2.3 to −3.0 ‰), meteoric δ 18 O and δD ratios, and stable CO 2 fluxes (31 t d −1 ). The lack of δ 13 C fractionation and an increase in the relative mantle component from 2002 to 2012 suggest that the volatile flux at Cerro Negro originates from the mantle and ascends to the surface via a series of crustal fractures that act as permeable conduits. Despite the lack of new eruptions, the hydrothermal system of Cerro Negro continues to evolve due to seasonal inputs of meteoric water, slope failures that expose and bury sites of active degassing, and bursts of regional seismicity that have the potential to open up new conduits for gas release as well as magma. Continuing geophysical and geochemical monitoring of the main edifice and the recently formed south zone is essential, as the volcano remains overdue to erupt.