Evidence for a new shallow magma intrusion at La Soufrière of Guadeloupe (Lesser Antilles). Insights from long-term geochemical monitoring of halogen- rich hydrothermal fluids

International audienceMore than three decades of geochemical monitoring of hot springs and fumaroles of La Soufrière of Guadeloupe allows the construction of a working model of the shallow hydrothermal system. This system is delimited by the nested caldera structures inherited from the repeated flank collapse events and the present dome built during the last magmatic eruption (1530 AD) and which has been highly fractured by the subsequent phreatic or phreatomagmatic eruptions. Because it is confined into the low volume, highly compartmented and partially sealed upper edifice structure, the hydrothermal system is highly reactive to perturbations in the volcanic activity (input of deep magmatic fluids), the edifice structure (sealing and fracturing) and meteorology (wet tropical regime). The current unrest, which began with a mild reactivation of fumarolic activity in 1990, increased markedly in 1992 with seismic swarms and an increase of degassing from the summit of the dome. In 1997 seismic activity increased further and was accompanied by a sudden high-flux HCl-rich gas from summit fumaroles. We focus on the interpretation of the time-series of the chemistry and temperature of fumarolic gases and hot springs as well as the relative behaviours of halogens (F, Cl, Br and I). This extensive geochemical time-series shows that the deep magmatic fluids have undergone large changes in composition due to condensation and chemical interaction with shallow groundwater (scrubbing). It is possible to trace back these processes and the potential contribution of a deep magmatic source using a limited set of geochemical time series: T, CO2 and total S content in fumaroles, T and Cl- in hot springs and the relative fractionations between F, Cl, Br and I in both fluids. Coupling 35 years of geochemical data with meteorological rainfall data and models of ion transport in the hydrothermal aquifers has allowed us to identify a series of magmatic gas pulses into the hydrothermal system since the 1976-1977 crisis. The contrasting behaviours of S- and Cl- bearing species in fumarolic gas and in thermal springs suggests that the current activity is the result of a new magma intrusion which was progressively emplaced at shallow depth since ~1992. Although it might still be evolving, the characteristics of this new intrusion indicate that it hasalready reached a magnitude similar to the intrusion that was emplaced during the 1976-1977 eruptive crisis. The assessment of potential hazards associated with evolution of the current unrest must consider the implications of recurrent intrusion and further pressurization of the hydrothermal system on the likelihood of renewed phreatic explosive activity. Moreover, the role of hydrothermal pressurization on the basal friction along low-strength layers within the upper part of the edifice must be evaluated with regards to partial flank collapse. At this stage enhanced monitoring, research, and data analysis is required to quantify the uncertainties related to future scenarios of renewed eruptive activity and magmatic evolution

Noeud A VOLCANO de RESIF : bilan et perspectives

Knot A VOLCANO is part of the RESIF research infrastructure information system and concentrates seismic data from the three volcanological and seismological observatories of the Institut de Physique du Globe de Paris. These observatories operate short-seismological stations-analogical period, short-digital three-component period, medium-digital band and digital wideband. These data are used for monitoring regional volcanic and land-based activity, as well as tsunami warning. The poster presents the results at the end of 2017 and the prospects for this knot A.The French Seismological and Geodetic Network RESIF is a national research infrastructure dedicated to the observation and understanding of the structure and dynamics of the Internal Earth. RESIF is based on high-tech observation networks, composed of seismological, geodetic and gravimetric instruments deployed in a dense manner throughout France. The data collected make it possible to study with high spatial and temporal resolution the deformation of the ground, surface and deep structures, seismicity on a local and global scale and natural hazards, and more particularly seismic events, on French territory. RESIF is integrated into European (EPOS - European Plate Observing System) and global systems of instruments for imaging the Earth's interior as a whole and studying many natural phenomena.Le nœud A VOLCANO fait partie du système d'information de l'infrastructure de recherche RESIF et concentre les données sismiques des trois observatoires volcanologiques et sismologiques de l'Institut de Physique du Globe de Paris. Ces observatoires opèrent des stations sismologiques courte-période analogique, courte-période numérique trois composantes, moyenne- bande numérique et large-bande numérique. Ces données sont utilisées pour le suivi de l’activité volcanique et tellurique régionale, ainsi que l’alerte aux tsunamis. Le poster présente le bilan fin 2017 et les perspectives pour ce noeud A.Le Réseau sismologique et géodésique français RESIF est une infrastructure de recherche nationale dédiée à l’observation et la compréhension de la structure et de la dynamique Terre interne. RESIF se base sur des réseaux d’observation de haut niveau technologique, composés d’instruments sismologiques, géodésiques et gravimétriques déployés de manière dense sur tout le territoire français. Les données recueillies permettent d’étudier avec une haute résolution spatio-temporelle la déformation du sol, les structures superficielles et profondes, la sismicité à l’échelle locale et globale et les aléas naturels, et plus particulièrement sismiques, sur le territoire français. RESIF s’intègre aux dispositifs européens (EPOS - European Plate Observing System) et mondiaux d’instruments permettant d’imager l’intérieur de la Terre dans sa globalité et d’étudier de nombreux phénomènes naturels

Atmospheric contribution to cations cycling in highly weathered catchment, Guadeloupe

International audienc

Modeling the effect of aging on Deccan Traps weathering and CO2 consumption

International audienc

Shield effect on continental weathering: Implication for climatic evolution of the Earth at the geological timescale

International audienc

Atmospheric contribution to cations cycling in highly weathered catchment, Guadeloupe (Lesser Antilles)

International audienceThe important fertilizing role of atmospheric dust, and particularly African dust, in tropical rainforests is increasingly recognized but still poorly quantified. To better evaluate dust input into the Caribbean basin, we sampled critical zone compartments of a small forested volcanic catchment in Guadeloupe (soils, parent rock, atmospheric dust, plants, soil solutions, stream and rain waters). The aims of this study are to track sources of cation nutrients (Ca, Mg, K, Sr) developed on highly weathered soil in the rainforest of Guadeloupe, to quantify plant recycling of these nutrients, and to identify constraints on regolith development and its associated nutrient pool.In the Quiock Creek catchment, a large isotopic range of 87Sr/86Sr and εNd values was observed despite the small scale of observation. Sr isotopic composition of the dissolved load varied from 0.7084 in rainfall to 0.7110 in soil solution, whereas it ranges between 0.7068 and 0.7153 for soil samples and between 0.7096 and 0.7102 for plants. The Nd isotopic composition varied between -8.39 in near-surface soil samples to 2.71 in deeper soil. All samples had an intermediate signature between that of the bedrock endmember (87Sr/86Sr = 0.7038; εNd = 4.8) and the atmospheric endmember (sea salt: 87Sr/86Sr = 0.7092 and Saharan dust: 87Sr/86Sr = 0.7187, εNd=-11.5).The regolith was built on pyroclastic deposits, but, because of extreme leaching, the regolith has lost its original bedrock signature and inherited an exogenous atmospheric signature. Our results show that only the chemical weathering of the fresh near-surface minerals can provide nutrients to the ecosystem (first 30 cm). However, this dust weathering is too low to sustain the tropical forest ecosystem on its own. The cationic mass balance at the catchment scale, as well as the Sr isotopic signature, show that cation and Sr fluxes are of atmospheric origin only and that original bedrock no longer participates in nutrient cycles. The vegetation reflects the 87Sr/86Sr of the dissolved pool of atmospheric Sr.At the soil-plant scale, the cation-nutrient fluxes provided by vegetation (litter fall + leaf excretion) are major compared to input and output fluxes. The annual Ca, K, Sr and Mg fluxes within the vegetation are, respectively, 31, 28, 20 and 3 times greater than the exported fluxes at the outlet of the basin. The residence time of nutrients in the vegetation is 16 years for K and close to 45 years for Sr, Ca and Mg. These results emphasize the highly efficient vegetative turnover that dominates the nutrient cycle in the Quiock Creek catchment.This first characterization of biogeochemical cycles in the Guadeloupean rainforest suggests that the forest community of Quiock Creek is sustained by a small near-surface nutrient pool disconnected from the deep volcanic bedrock. We also demonstrated that, even with efficient nutrient recycling, Saharan dust plays a significant role in maintaining ecosystem productivity in Guadeloupe over long-time scales

DIC concentration and

International audienc

Chlorine isotopes of thermal springs in arc volcanoes for tracing shallow magmatic activity

International audienceThe evaluation of the status of shallow magma body (i.e., from the final intrusion stage, to quiescence, and back to activity), one of the key parameters that trigger and sustain volcanic eruptions, has been challenging in modern volcanology. Among volatile tracers, chlorine (Cl) uniquely exsolves at shallow depths and is highly hydrophilic. Consequently, Cl enrichment in volcanic gases and thermal springs has been proposed as a sign for shallow magmatic activities. However, such enrichment could also result from numerous other processes (e.g., water evaporation, dissolution of old chloride mineral deposits, seawater contamination) that are unrelated to magmatic activity. Here, based on stable isotope compositions of chloride and dissolved inorganic carbon, as well as previous published 3He/4He data obtained in thermal springs from two recently erupted volcanoes (La Soufrière in Guadeloupe and Montagne Pelée in Martinique) in the Lesser Antilles Arc, we show that the magmatic Cl efficiently trapped in thermal springs displays negative δ37Cl values (≤−0.65‰≤−0.65‰), consistent with a slab-derived origin but distinct from the isotope compositions of chloride in surface reservoirs (e.g. seawater, local meteoric waters, rivers and cold springs) displaying common δ37Cl values of around 0‰. Using this δ37Cl difference as an index of magmatic Cl, we further examined thermal spring samples including a 30-year archive from two thermal springs in Guadeloupe covering samples from its last eruption in 1976–1977 to 2008 and an island-wide sampling event in Martinique in 2008 to trace the evolution of magmatic Cl in the volcanic hydrothermal systems over time. The results show that magmatic Cl can be rapidly flushed out of the hydrothermal systems within <30 to 80 years after the eruption, much quicker than other volatile tracers such as CO2 and noble gases, which can exsolve at greater depths and constantly migrate to the surface. Because arc volcanoes often have well developed hydrothermal systems where magmatic Cl is easily transferred to the surface following its exsolution from shallow magma body, we suggest that δ37Cl has great potential to be a unique proxy to monitor the cessation and revival of infrequent arc volcanoes, particularly at centennial time scales

Determination of the energy budget of the Tarissan Pit, Soufrière de Guadeloupe

International audienceThe Tarissan pit located on top of La Soufriere de Guadeloupe volcano is the major vent of La Soufriere very active geothermal system, system that had its last phreatic eruption in 1976. This pit is both the most important heat exchanger in the actual geothermal machine and the center of the activity during the last 1976 eruption. The pit whose section is approximately 120 square meters is filled by an acid lake and is a major source of fumaroles and acid vapor on the Lava dome. It shows a constant increase of acidity for the last ten years. In a general effort to better constraint the energy budget of la Soufriere geothermal system, several experiments have been conductedon the Tarissan pit to evaluate the average energy that is released by this vent. Despite the extremely difficult field conditions, the energy release of Tarissan has been estimated between 1 and 2 MW, using lake level and temperature measurements, vapor collector, etc... The presentation shows the results of these measurements and a simple model to estimate the average energy release

Saharan dusts inputs to north-western atlantic ocean with three years time series

International audienceNorth Africa, the largest dust source worldwide, accounts for 55% of global continental dust emission [e.g. Muhs et al., 1990]. These dusts can be transported over long distance, and significantly impact ocean biogeochemistry in the North Atlantic and the ecosystems of the North Tropical Atlantic Islands, after deposition through biogeochemical processes [Clergue et al., 2015]. Yet, the inputs of Saharan dust to the North-Western Atlantic are not precisely measured.In this work, we present time series of dust deposition performed in Guadeloupe. Atmospheric total deposition was continuously sampled on a weekly basis during three years (2015-2018). Airborne aerosol samples were simultaneously collected during the last 18 months of deposition sampling.Deposition and aerosol samples were analyzed for major and trace elements including rare earth elements (REEs), together with Nd, Pb and Sr isotopes. Compositional analyses mathematical tool was used to study the elemental composition of trace elements and REEs. It showed (i) seasonal and interannual variations of deposition fluxes of major and trace elements, (ii) samples collected during different years present different trace and REEs compositions and also different Nd isotopic signatures, (iii) Saharan dust and sea-salt depositions can represent significant sources of nutritive trace metals (like Mo and Sr) which are important for ecology systems in North-Western Atlantic and Caribbean Islands, (iv) differences were also observed on Pb isotopic ratios between airborne aerosol and deposition samples collected simultaneously, which are likely due to human activities.This long time series will help us to better assess the contribution and impact of Saharan dust to the biogeochemical cycle of trace metals in Western North Atlantic