High-spectral-resolution Fabry-Perot interferometers overcome fundamental limitations of present volcanic gas remote sensing techniques

Remote sensing (RS) of volcanic gases has become a central tool for studying volcanic activity. For instance, ultraviolet (UV) skylight spectroscopy with grating spectrographs (GS) enables SO2 (and, under favourable conditions, BrO) quantification in volcanic plumes from autonomous platforms at safe distances. These measurements can serve volcanic monitoring and they cover all stages of volcanic activity in long measurement time series, which substantially contributes to the refinement of theories on volcanic degassing. Infrared (IR) remote sensing techniques are able to measure further volcanic gases (e.g., HF, HCl, CO2, CO). However, the employed Fourier transform spectrometers (FTSs) are intrinsically intricate and, due to limited resolving power or light throughput, mostly rely on either lamps, direct sun, or hot lava as light source, usually limiting measurements to individual field campaigns. We show that many limitations of grating spectrographs and Fourier transform spectrometer measurements can be overcome by Fabry-Perot interferometer (FPI) based spectrograph implementations. Compared to grating spectrographs and Fourier transform spectrometers, Fabry-Perot interferometer spectrographs reach a 1-3 orders of magnitude higher spectral resolution and superior light throughput with compact and stable set-ups. This leads to 1) enhanced sensitivity and selectivity of the spectral trace gas detection, 2) enables the measurement of so far undetected volcanic plume constituents [e.g., hydroxyl (OH) or sulfanyl (SH)], and 3) extends the range of gases that can be measured continuously using the sky as light source. Here, we present measurements with a shoe-box-size Fabry-Perot interferometer spectrograph (resolving power of ca. 150000), performed in the crater of Nyiragongo volcano. By analysing the light of a ultraviolet light emitting diode that is sent through the hot gas emission of an active lava flow, we reach an OH detection limit of about 20 ppb, which is orders of magnitude lower than the mixing ratios predicted by high-temperature chemical models. Furthermore, we introduce example calculations that demonstrate the feasibility of skylight-based remote sensing of HF and HCl in the short-wave infrared with Fabry-Perot interferometer spectrographs, which opens the path to continuous monitoring and data acquisition during all stages of volcanic activity. This is only one among many further potential applications of remote sensing of volcanic gases with high spectral resolution

Small-scale spatial variability of soil CO2 flux: Implication for monitoring strategy

In recent decades, soil CO2 flux measurements have been often used in both volcanic and seismically active areas to investigate the interconnections between temporal and spatial anomalies in degassing and telluric activities. In this study, we focus on a narrow degassing area of the Piton de la Fournaise volcano, that has been chosen for its proximity and link with the frequently active volcanic area. Our aim is to constrain the degassing in this narrow area and identify the potential processes involved in both spatial and temporal soil CO2 variations in order to provide an enhanced monitoring strategy for soil CO2 flux. We performed a geophysical survey (self-potential measurements: SP; electrical resistivity tomography: ERT) to provide a high-resolution description of the subsurface. We identified one main SP negative anomaly dividing the area in two zones. Based on these results, we set ten control points, from the site of the main SP negative anomaly up to 230 m away, where soil CO2 fluxes were weekly measured during one year of intense eruptive activity at Piton de la Fournaise. Our findings show that lateral and vertical soil heterogeneities and structures exert a strong control on the degassing pattern. We find that temporal soil CO2 flux series at control points close to the main SP negative anomaly better record variations linked to the volcanic activity. We also show that the synchronicity between the increase of soil CO2 flux and deep seismicity can be best explained by a pulsed process pushing out the CO2 already stored and fractionated in the system. Importantly, our findings show that low soil CO2 fluxes and low carbon isotopic signature are able to track variations of volcanic activity in the same way as high fluxes and high carbon isotopic signature do. This result gives important insights in terms of monitoring strategy of volcanic and seismotectonic areas in geodynamics contexts characterized by difficult environmental operational conditions as commonly met in tropical areaPublished13-264A. Oceanografia e climaJCR Journa

Recommendations and Protocols for the Use of the Isotope RatioInfrared Spectrometer (Delta Ray) to Measure StableIsotopes from CO2: An Application to Volcanic Emissions atMount Etna and Stromboli (Sicily, Italy)

Co-auteur étrangerInternational audienceAmong major volatiles released from the Earth’s interior, CO2 is an important target for the international community. The interestis keenly motivated by the contribution of CO2 in the Earth’s carbon budget and its role on past, current, and future climatedynamics. In particular, the isotopic signature of CO2 is fundamental to characterize the source of this gas and its evolution upto the atmosphere. The recent development of new laser-based techniques has marked an important milestone for the scientificcommunity by favoring both high-frequency and in situ stable isotope measurements. Among them, the Delta Ray IRIS(Thermo Scientific Inc., Waltham, USA) is one of the most promising instruments thanks to its high precision, its limitedinterferences with other gaseous species (such as H2S and/or SO2), and its internal calibration procedure. These characteristicsand the relative easiness to transport the Delta Ray IRIS have encouraged its use on the field to analyze volcanic CO2 emissionsin recent years but often with distinct customized protocols of measurements. In this study, various tests in the laboratory andon the field have been performed to study the dependence of CO2 isotope measurements on analytical, instrumental, andenvironmental conditions. We emphasize the exceptional ability of the Delta Ray IRIS to perform isotope measurements for alarge range of CO2 concentration (200 ppm–100%) thanks to a dilution system and to get a reliable estimation of the real CO2content from the diluted one. These tests lead to point out major recommendations on the use of Delta Ray IRIS and allow thedevelopment of adapted protocols to analyze CO2 emissions like in volcanic environments

Paroxysmal eruptions tracked by variations of helium isotopes: inferences from Piton de la Fournaise (La Réunion island)

Co-auteur étrangerInternational audienceHelium (He) with its isotopes (3He, 4He) is a key tracer enabling the Earth’s mantle and dynamics to be characterized. Enrichment in primordial helium (3He) has been detected in volcanic gases of numerous magmatic systems in different geodynamic settings. Despite past use to monitor volcanotectonic unrest, temporal 3He/4He variability in volcanic emissions is still poorly constrained. Here, we investigate noble gas chemistry of Piton de la Fournaise hotspot volcano, where temporal fluctuations of 3He/4He in response to the eruptive activity have never been studied. We compare the 3He/4He signature of volcanic gases and fluid inclusions and we highlight analogous evolution of the 3He/4He signature in both during the last decades of eruptive activity (1990–2017), even during the same eruption. We show that the maximum enrichment in 3He is found in magmatic fluids that fed the most voluminous eruptions which culminated in caldera collapse events. We argue that this enrichment in 3He mostly reflects a greater contribution of magmatic fluids from a primitive component of the mantle plume. These results emphasize that He isotopes may provide warnings of increases in deep magmatic contributions that potentially herald paroxysmal eruptions, as documented here at Piton dela Fournaise (2007) and also at Kilauea (2018)

Recent Activity of Nyiragongo (Democratic Republic of Congo): New Insights From Field Observations and Numerical Modeling

Nyiragongo volcano is known for its active lava lake and for socioeconomic issues arising from future possible eruptive events having major impacts on the community living in the Virunga region. The 2020 field expedition inside the summit crater has allowed the collection of unprecedented field observations to state on the current activity. Since the February 2016 intracrater event, the crater floor level has been rising much faster than during the 2010–2016 period. The current activity is reminiscent of the 1970–1972 and 1994–1995 periods preceding the lava lake drainage events in 1977 and 2002. Numerical simulations, successfully validated with data over the past 30 years, show that the rising of the crater floor could slow down in the next months/years and reach a critical equilibrium. Based on the past eruptive history and on the current activity, a flank eruption in the March 2024 to November 2027 interval could be a possible scenario

The SoilExp software: An open-source Graphical User Interface (GUI) for post-processing spatial and temporal soil surveys

Co-auteur étrangerInternational audiencePreliminary interpretation of geological processes during field measurement campaigns require fast data analysisto adapt ongoing target strategies. It is the case of soil investigations where coupling geochemical and geophysicalrecords favors a better understanding of subsurface processes. This task requires (i) statistical analysisto identify areas of interest during spatial surveys and (ii) signal processing to analyze temporal series. Here wepresent SoilExp, an open-source Python-based Graphical User Interface (GUI) that permits to process spatial andtemporal surveys of soil gases (e.g. soil CO2 flux) combined with common physical parameters (e.g. self-potential,temperature) that are synchronously recorded on the field. SoilExp mixes innovative algorithms with the morecommon tools used for the analysis of both spatial surveys or temporal series. It offers the possibility to displaydistribution plots, maps, comparative plots, spectra and spectrograms, as well as data statistical analysis, in orderto deal efficiently with datasets acquired on the field. Field measurements performed at Stromboli (Italy) supportsthat such software solution facilitates a quick visualization of the data output and is a powerful tool on thegeochemical and geophysical analysis

Unconventional filling dynamics of a pit crater

co-auteur étrangerInternational audienceThe rise and fall of magma columns is a process commonly observed in volcanoes hosting lava lakes and serves as proxy of magmatic reservoir pressure and warning for potential eruptions. This dynamic process typically involves the filling and emptying of a “pit crater” -a cylindrical depression usually formed by one or more collapses of a structure lying above an emptied surficial reservoir. Between 2016 and 2020, the 300 m deep pit crater located within Nyamulagira volcano (Democratic Republic of Congo) underwent an unusual filling dynamic. In May 2019 while filled to about 90% the crater floor collapsed by 90 m within two months, followed by the rapid emergence of a sub-circular solidified block overlooking the crater rim in less than 6 months. Using numerical simulations of models based on thermal energy and constrained by multiparametric data, we account for the incremental filling of the crater by successive intra-crateric lava flows and the subsequent collapse of the crater floor. We further characterize this unconventional filling mode based on thermal budget considerations