Dmitri Rouwet

Intense magmatic degassing through the lake of Copahue volcano, 2013-2014

Here we report on the first assessment of volatile fluxes from the hyperacid crater lake hosted within the summit crater of Copahue, a very active volcano on the Argentina-Chile border. Our observations were performed using a variety of in situ and remote sensing techniques during field campaigns in March 2013, when the crater hosted an active fumarole field, and in March 2014, when an acidic volcanic lake covered the fumarole field. In the latter campaign, we found that 566 to 1373 t d−1 of SO2 were being emitted from the lake in a plume that appeared largely invisible. This, combined with our derived bulk plume composition, was converted into flux of other volcanic species (H2O ~ 10989 t d−1, CO2 ~ 638 t d−1, HCl ~ 66 t d−1, H2 ~ 3.3 t d−1, and HBr ~ 0.05 t d−1). These levels of degassing, comparable to those seen at many open-vent degassing arc volcanoes, were surprisingly high for a volcano hosting a crater lake. Copahues unusual degassing regime was also confirmed by the chemical composition of the plume that, although issuing from a hot (65°C) lake, preserves a close-to-magmatic signature. EQ3/6 models of gas-water-rock interaction in the lake were able to match observed compositions and demonstrated that magmatic gases emitted to the atmosphere were virtually unaffected by scrubbing of soluble (S and Cl) species. Finally, the derived large H2O flux (10,988 t d−1) suggested a mechanism in which magmatic gas stripping drove enhanced lake water evaporation, a process likely common to many degassing volcanic lakes worldwide.

Recognizing and tracking volcanic hazards related to non-magmatic unrest: a review

Eruption forecasting is a major goal in volcanology. Logically, but unfortunately, forecasting hazards related to non-magmatic unrest is too often overshadowed by eruption forecasting, although many volcanoes often pass through states of non-eruptive and non-magmatic unrest for various and prolonged periods of time. Volcanic hazards related to non-magmatic unrest can be highly violent and/or destructive (e.g., phreatic eruptions, secondary lahars), can lead into magmatic and eventually eruptive unrest, and can be more difficult to forecast than magmatic unrest, for various reasons. The duration of a state of non-magmatic unrest and the cause, type and locus of hazardous events can be highly variable. Moreover, non-magmatic hazards can be related to factors external to the volcano (e.g., climate, earthquake). So far, monitoring networks are often limited to the usual seismic-ground deformation-gas network, whereas recognizing indicators for non-magmatic unrest requires additional approaches. In this study we summarize non-magmatic unrest processes and potential indicators for related hazards. We propose an event-tree to classify non-magmatic unrest, which aims to cover all major hazardous outcomes. This structure could become useful for future probabilistic non-magmatic hazard assessments, and might reveal clues for future monitoring strategies.

Dynamic fluid recycling at Laguna Caliente (Poás, Costa Rica) before and during the 2006–ongoing phreatic eruption cycle (2005–10)

Abstract Poás Volcano (Costa Rica) resumed phreatic activity on 24 March 2006 after twelve years of quiescence. From March 2006 to June 2010, the initial phase of the ongoing eruption cycle, 110 phreatic eruptions were reported. This study presents the temporal variations in the chemical and isotopic (δD and δ18O) compositions of Laguna Caliente crater lake for the period prior to the eruption until June 2010. No systematic relationship with the phreatic eruptive activity exists. A combined mass and Cl budget analysis enables quantification of the seepage rate (7 kg s−1), the input rate of the ‘volcanic fluid’ (Qf), and the Cl concentration (Cle) in the evaporation plume (13 300 mg l−1). A modelling procedure for variable seepage rates leads to more realistic estimates of 50–100 kg s−1, which better represent the observed lake water chemistry, hence suggesting dynamic fluid recycling between the lake and the underlying magmatic–hydrothermal system. The high Cl concentration in the evaporation plume and the dynamic fluid recycling at the lake bottom characterize Laguna Caliente as an ‘open-air’ fumarole, discrediting water chemistry as an efficient monitoring tool at the classic monitoring frequency. A conceptual model of phreatic eruptions is linked to our observations.

HCl degassing from extremely acidic crater lakes: preliminary results from experimental determinations and implications for geochemical monitoring

Abstract Crater lakes are monitored to detect volcanic unrest starting from the assumption that they behave as condensers for magmatic gases. A further assumption is that acidic gases such as HCl are conservative once dissolved in water. This is not true for extremely acidic crater lakes, whose H+ activity is high enough to induce Cl− hydrolysis and consequently HCl degassing. This study presents the results of experimental determinations at 40–45°C demonstrating that HCl degassing from acidic water depends on pH and Cl− concentration. HCl degassing starts at pH values c. 0.05–0.1 with a rate of 5–10 mg min−1 l−1, increasing up to c. 70 mg min−1 l−1 at pH<−0.2. This implies that the rate of HCl removal from a crater lake with a volume of 104–105 m3 and a seawater-like Cl− concentration ranges from 5 to 50 t h−1. The estimated HCl/H2O ratio in the separated vapour phase (0.01–0.2) is coherent with HCl/H2O ratios of fumaroles. Our experiments imply that: (i) the presence of very acidic gas species in fumaroles can be associated with a liquid-dominated feeding system, and (ii) dissolved in extremely acidic crater lakes, Cl− behaves as a non-conservative component.

Biogeochemical processes involving dissolved CO2 and CH4 at Albano, Averno, and Monticchio meromictic volcanic lakes (Central–Southern Italy)

This paper focuses on the chemical and isotopic features of dissolved gases (CH4 and CO2) from four meromictic lakes hosted in volcanic systems of Central–Southern Italy: Lake Albano (Alban Hills), Lake Averno (Phlegrean Fields), and Monticchio Grande and Piccolo lakes (Mt. Vulture). Deep waters in these lakes are characterized by the presence of a significant reservoir of extra-atmospheric dissolved gases mainly consisting of CH4 and CO2. The δ13C-CH4 and δD-CH4 values of dissolved gas samples from the maximum depths of the investigated lakes (from −66.8 to −55.6 ‰ V-PDB and from −279 to −195 ‰ V-SMOW, respectively) suggest that CH4 is mainly produced by microbial activity. The δ13C-CO2 values of Lake Grande, Lake Piccolo, and Lake Albano (ranging from −5.8 to −0.4 ‰ V-PDB) indicate a significant CO2 contribution from sublacustrine vents originating from (1) mantle degassing and (2) thermometamorphic reactions involving limestone, i.e., the same CO2 source feeding the regional thermal and cold CO2-rich fluid emissions. In contrast, the relatively low δ13C-CO2 values (from −13.4 to −8.2 ‰ V-PDB) of Lake Averno indicate a prevalent organic CO2. Chemical and isotopic compositions of dissolved CO2 and CH4 at different depths are mainly depending on (1) CO2 inputs from external sources (hydrothermal and/or anthropogenic); (2) CO2–CH4 isotopic exchange; and (3) methanogenic and methanotrophic activity. In the epilimnion, vertical water mixing, free oxygen availability, and photosynthesis cause the dramatic decrease of both CO2 and CH4 concentrations. In the hypolimnion, where the δ13C-CO2 values progressively increase with depth and the δ13C-CH4 values show an opposite trend, biogenic CO2 production from CH4 using different electron donor species, such as sulfate, tend to counteract the methanogenesis process whose efficiency achieves its climax at the water–bottom sediment interface. Theoretical values, calculated on the basis of δ13C-CO2 values, and measured δ13CTDIC values are not consistent, indicating that CO2 and the main carbon-bearing ion species (HCO3−) are not in isotopic equilibrium, likely due to the fast kinetics of biochemical processes involving both CO2 and CH4. This study demonstrates that the vertical patterns of the CO2/CH4 ratio and of δ13C-CO2 and δ13C-CH4 are to be regarded as promising tools to detect perturbations, related to different causes, such as changes in the CO2 input from sublacustrine springs, that may affect aerobic and anaerobic layers of meromictic volcanic lakes.

New insights into Kawah Ijen's volcanic system from the wet volcano workshop experiment

Abstract Volcanoes with crater lakes and/or extensive hydrothermal systems pose significant challenges with respect to monitoring and forecasting eruptions, but they also provide new opportunities to enhance our understanding of magmatic–hydrothermal processes. Their lakes and hydrothermal systems serve as reservoirs for magmatic heat and fluid emissions, filtering and delaying the surface expressions of magmatic unrest and eruption, yet they also enable sampling and monitoring of geochemical tracers. Here, we describe the outcomes of a highly focused international experimental campaign and workshop carried out at Kawah Ijen volcano, Indonesia, in September 2014, designed to answer fundamental questions about how to improve monitoring and eruption forecasting at wet volcanoes.

A new Bayesian Event Tree tool to track and quantify volcanic unrest and its application to Kawah Ijen volcano

Although most of volcanic hazard studies focus on magmatic eruptions, volcanic hazardous events can also occur when no migration of magma can be recognized. Examples are tectonic and hydrothermal unrest that may lead to phreatic eruptions. Recent events (e.g., Ontake eruption on September 2014) have demonstrated that phreatic eruptions are still hard to forecast, despite being potentially very hazardous. For these reasons, it is of paramount importance to identify indicators that define the condition of nonmagmatic unrest, in particular for hydrothermal systems. Often, this type of unrest is driven by movement of fluids, requiring alternative monitoring setups, beyond the classical seismic-geodetic-geochemical architectures. Here we present a new version of the probabilistic BET (Bayesian Event Tree) model, specifically developed to include the forecasting of nonmagmatic unrest and related hazards. The structure of the new event tree differs from the previous schemes by adding a specific branch to detail nonmagmatic unrest outcomes. A further goal of this work consists in providing a user-friendly, open-access, and straightforward tool to handle the probabilistic forecast and visualize the results as possible support during a volcanic crisis. The new event tree and tool are here applied to Kawah Ijen stratovolcano, Indonesia, as exemplificative application. In particular, the tool is set on the basis of monitoring data for the learning period 2000-2010, and is then blindly applied to the test period 2010-2012, during which significant unrest phases occurred.

Geosphere-Biosphere Interactions in Bio-Activity Volcanic Lakes: Evidences from Hule and Rìo Cuarto (Costa Rica)

Hule and Río Cuarto are maar lakes located 11 and 18 km N of Poás volcano along a 27 km long fracture zone, in the Central Volcanic Range of Costa Rica. Both lakes are characterized by a stable thermic and chemical stratification and recently they were affected by fish killing events likely related to the uprising of deep anoxic waters to the surface caused by rollover phenomena. The vertical profiles of temperature, pH, redox potential, chemical and isotopic compositions of water and dissolved gases, as well as prokaryotic diversity estimated by DNA fingerprinting and massive 16S rRNA pyrosequencing along the water column of the two lakes, have highlighted that different bio-geochemical processes occur in these meromictic lakes. Although the two lakes host different bacterial and archaeal phylogenetic groups, water and gas chemistry in both lakes is controlled by the same prokaryotic functions, especially regarding the CO2-CH4 cycle. Addition of hydrothermal CO2 through the bottom of the lakes plays a fundamental priming role in developing a stable water stratification and fuelling anoxic bacterial and archaeal populations. Methanogens and methane oxidizers as well as autotrophic and heterotrophic aerobic bacteria responsible of organic carbon recycling resulted to be stratified with depth and strictly related to the chemical-physical conditions and availability of free oxygen, affecting both the CO2 and CH4 chemical concentrations and their isotopic compositions along the water column. Hule and Río Cuarto lakes were demonstrated to contain a CO2 (CH4, N2)-rich gas reservoir mainly controlled by the interactions occurring between geosphere and biosphere. Thus, we introduced the term of bio-activity volcanic lakes to distinguish these lakes, which have analogues worldwide (e.g. Kivu: D.R.C.-Rwanda; Albano, Monticchio and Averno: Italy; Pavin: France) from volcanic lakes only characterized by geogenic CO2 reservoir such as Nyos and Monoun (Cameroon).

Isotope Fractionation and HCl Partitioning During Evaporative Degassing from Active Crater Lakes

This chapter provides the theoretical background and necessary practical tools to study one of the most spectacular natural features: vigorous evaporation from active crater lakes. We will give qualitative insights (lake water chemical—Cl content, and isotopic composition) rather than quantify evaporation fluxes from lakes. A major problem is that, with the current methods, we are only able to sample the lake water, while the input fluid rising into the lake (sublacustrine) and evaporation plume coming off the lake remain “inaccessible”. This means that the lake behaves as a “black box”, being the result of incoming and outgoing fluids of unknown chemical and isotopic composition. As visually demonstrated at many active crater lakes, evaporation is a major process. Strong evaporation from the lake surface will affect the isotopic composition of the remnant lake water, and the “steam devils” (evaporation plume) swirling over the lake. It is found that the kinetic (diffusion) isotope fractionation overshadows the equilibrium isotope fractionation effect, as a dynamic crater lake is intuitively hard to imagine as an equilibrated system. Besides a hot water mass in evaporation, water of active crater lakes is generally a hyper-saline (total salinity >100,000 mg l−1) and hyper-acidic brine (pH as low as −0.5). Although “small scale” equilibrium fractionation effects, the “isotope salt effect” and “isotope acid effect” lead to isotopically heavier evaporation plumes, with respect to vapor coming off pure neutral water. Besides isotope fractionation of the water itself under such extreme lake conditions, HClgas (and HF) will partition between the liquid and vapor phases. HCl degassing is enhanced when pH is continuously lowered by the input of acidic gases (SO2, HCl, HF), lake temperature is higher, and evaporation is physically favored by wind or lake convection. It is empirically deduced that HCl partitioning into the vapor phase is chemically controlled by the lake water temperature and density, rather than the Cl content or pH. A better quantification of the chemical and isotopic composition of evaporative gas plumes from active crater lakes will be of importance for volcano monitoring when we aim to deduce the flux and composition of the “hot magmatic end member”, through chemical and isotope budget analyses. A major challenge for the future is to develop field methods to enable to sample the evaporation plume coming off lake surfaces, so we can directly determine its chemical and isotopic composition and compare them with the theoretical approach presented in this review chapter.

How Steep Is My Seep? Seepage in Volcanic Lakes, Hints from Numerical Simulations

The existence and survival of volcanic lakes require the accomplishment of a delicate balance between meteoric recharge, evaporation, and water loss by infiltration within the volcanic edifice, commonly referred to as seepage. A deep-seated, volcanic component may participate to a variable extent to the lake’s evolution, depending on volcanic activity. In this work, we apply a numerical model of hydrothermal fluid circulation to study the interaction between the hot volcanic gases and the shallow lake water. We focus on the conceptual model developed for Poas volcano (Costa Rica), where a shallow magma intrusion drives the hydrothermal activity underneath and around the crater lake. Numerical simulations are carried out to assess the role of relevant system properties, including rock permeability, reservoir conditions, lake geometry, and meteoric recharge. Our results suggest that vertical seepage can be severely hindered by the ascent of volcanic gases, whereas horizontal infiltration through the vertical lake walls may ensure a long-term water loss. Our simulations also show that the permeability distribution, especially around the lake, determines the overall pattern of circulation affecting the development and spatial distribution of hot springs and fumaroles, and ultimately controlling the evolution of the lake.