An overview of the structure, hazards, and methods of investigation of Nyos-type lakes from the geochemical perspective

Limnic eruptions represent a natural hazard in meromictic lakes hosted in volcanoes releasing CO2-rich magmatic gases. Biogeochemical processes also contribute to dissolved gas reservoirs since they can produce significant amounts of gases, such as CH4 and N2. Dissolved gases may have a strong influence of the density gradient and the total dissolved gas pressure along the vertical profile of a volcanic lake. An external triggering event, possibly related to uncommon weather conditions, volcanic-seismic activity, or landslides, or spontaneous formation of gas bubbles related to the progressive attainment of saturation conditions at depth, may cause a lake rollover and the consequent release of dissolved gases. This phenomenon may have dramatic consequences due to i) the release of a toxic CO2-rich cloud able to flow long distances before being diluted in air, or ii) the contamination of the shallow water layer with poisonous deep waters. The experience carried out over the past twelve years at Lake Nyos, where a pumping system discharges CO2- rich deep water to the surface, has shown that controlled degassing of deep water layers is the best solution to mitigate such a hazard. However, the application of this type of intervention in other lakes must be carefully evaluated, since it may cause severe contamination of shallow lake water or create dangerous density instabilities. Monitoring of physical and chemical parameters controlling lake stability and the evolution in time of dissolved gas reservoirs can provide essential information for evaluating the risk associated with possible rollover phenomena. Conceptual models for the description of limnological, biogeochemical and volcanic processes regulating water lake stability have been constructed by interpreting compositional data of lake water and dissolved gas compositions obtained by applying different sampling and analytical techniques. This study provides a critical overview of the existing methodological approaches and discusses how future investigations of Nyos-type lakes, aimed at mitigating the hazard for limnic eruptions, can benefit from i) the development of new technical and theoretical approaches aimed to constrain the physical-chemical mechanisms controlling this natural phenomenon, and ii) information from different scientific disciplines, such as microbiology, fluid dynamics and sedimentology

A science & arts sensitization program in Chapultenango, 25 years after the 1982 El Chichόn eruptions (Chiapas, Mexico)

Volcanic risk perception may drastically decrease after eruptions and during periods of volcanic quiescence. Despite the fact that the adults in Chapultenango, a Zoque indigenous community near El Chichόn volcano (Chiapas, Mexico), lived through the 1982 Plinian eruptions, their awareness of present volcanic risk is low. In particular, children, adolescents and young adults (born after 1982) should be informed about the activity of El Chichόn, as they are more likely to be affected by possible future eruptions. This grass roots level sensitization project uses a novel approach to poll risk perception and to transmit knowledge of El Chichόn volcano among 6- to 11-year old children by combining scientific information sessions with arts workshops. Similar scientific sessions, although without the arts workshops, were less efficient for the older age group (Secondary School students). Moreover, the local Proteccion Civil and Gobierno Municipal was invited to participate in a basic monitoring of El Chichόn volcano. A lack in continuity in local political terms presented the major barrier for an effective and self-sufficient following-up of the volcanic surveillance. The entire population of Chapultenango was involved during informal meetings and semi-scientific projections of "their volcano", offering an alternative and more scientific view on El Chichόn's activity, often referenced in a more mystical-religious frame. It is experienced that the volcanologist is recognized as a highly trusted professional, bridging the gap between the official authorities and society

Volcanic Lakes in Africa: The VOLADA_Africa 2.0 Database, and Implications for Volcanic Hazard

Volcanic lakes pose specific hazards inherent to the presence of water: phreatic and phreatomagmatic eruptions, lahars, limnic gas bursts and dispersion of brines in the hydrological network. Here we introduce the updated, interactive and open-access database for African volcanic lakes, country by country. The previous database VOLADA (VOlcanic LAke DAta Base, Rouwet et al., Journal of Volcanology and Geothermal Research, 2014, 272, 78–97) reported 96 volcanic lakes for Africa. This number is now revised and established at 220, converting VOLADA_Africa 2.0 in the most comprehensive resource for African volcanic lakes: 81 in Uganda, 37 in Kenya, 33 in Cameroon, 28 in Madagascar, 19 in Ethiopia, 6 in Tanzania, 2 in Rwanda, 2 in Sudan, 2 in D.R. Congo, 1 in Libya, and 9 on the minor islands around Africa. We present the current state-of-the-art of arguably all the African volcanic lakes that the global experts and regional research teams are aware of, and provide hints for future research directions, with a special focus on the volcanic hazard assessment. All lakes in the updated database are classified for their genetic origin and their physical and chemical characteristics, and level of study. The predominant rift-related volcanism in Africa favors basaltic eruptive products, leading to volcanoes with highly permeable edifices, and hence less-developed hydrothermal systems. Basal aquifers accumulate under large volcanoes and in rift depressions providing a potential scenario for phreatomagmatic volcanism. This hypothesis, based on a morphometric analysis and volcanological research from literature, conveys the predominance of maar lakes in large monogenetic fields in Africa (e.g. Uganda, Cameroon, Ethiopia), and the absence of peak-activity crater lakes, generally found at polygenetic arc-volcanoes. Considering the large number of maar lakes in Africa (172), within similar geotectonic settings and meteoric conditions as in Cameroon, it is somewhat surprising that “only” from Lake Monoun and Lake Nyos fatal CO2 bursts have been recorded. Explaining why other maars did not experience limnic gas bursts is a question that can only be answered by enhancing insights into physical limnology and fluid geochemistry of the so far poorly studied lakes. From a hazard perspective, there is an urgent need to tackle this task as a community

The effect of three large Mw¿7.3 subduction earth-quakes (August-November 2012) on volcanic unrest in Central America

¿Was the volcanic eruption triggered by the earthquake?¿ The answer to this question usually is ¿maybe¿ or ¿a coincidence¿. A region like Central America, is an adequate area to find hints to answer this question because have the necessary ingredients: the frequent occurrence of large earthquakes (M5+) and dozens of active volcanoes. This research focuses on whether the uncommon occurrence of three large earthquakes in the subduction zone of Central America, within a time span of ten weeks in 2012, promoted enhanced volcanic activity. The time window analyzed is from 2000 to 2019, which includes a total of 50 volcanic eruptions with a VEI¿2. Before the 2012 earthquakes, 22 eruptions occurred. The Monte Carlo statistical simulation method allowed to demonstrate that this increase in the number of volcanic eruptions after the three large earthquakes of 2012 it is not a temporal coincidence. We analyzed the characteristics of each earthquake and described how they could disturb the volcanic systems. Although Central America hosts 24 volcanoes with historical eruptions, only 11 of them erupted after the 2012 earthquakes. Why did only these volcanoes erupt? To answer this question, we calculated the dynamic and static stress in each volcano and the level of volcanic unrest (the change in volcanic activity beyond background behavior to worrisome levels) prior to the earthquakes. We found that volcanoes in a unrest stage before the earthquakes but, without experiencing explosive eruptions before, erupted after receiving the seismic shocks. This fact suggests that the earthquakes by themselves did not transfer enough energy to generate the volcanic eruptions when volcanoes were not ready to erupt. However, earthquakes could promote volcanic eruptions when volcanoes were already at unrest. This research offers a tool for forecasting volcanic activity when a large earthquake hits a region, if the volcanic activity is previously monitored

The crater lake of Ilamatepec (Santa Ana) volcano, El Salvador: insights into lake gas composition and implications for monitoring

We here present the first chemical characterization of the volcanic gas plume issuing from the Santa Ana crater lake, a hyper-acidic crater lake (pH of -0.2 to 2.5) in north-western El Salvador. Our results, obtained during regular surveys in 2017 and 2018 using a Multi-GAS instrument, demonstrate an hydrous gas composition (H2O/SO2 ratios from 32 to 205), and SO2 as the main sulphur species (H2S/SO2 = 0.03-0.1). We also find that gas composition evolved during our investigated period, with the CO2/SO2 decreasing by one order of magnitude from March 2017 (37.2 ± 9.7) to 2018 (<3). This compositional evolution toward more magmatic (SO2-rich) compositions is interpreted in the context of the long-term evolution of the volcano following its 2005 and 2007 eruptions. We find that, in spite of reduced (background-level) seismicity, the magmatic gas supply into the lake was one order of magnitude higher in March 2017 (Total Volatile Flux: 20,200-30,200 t/day; the total volatile flux is the sum of H2O+CO2+SO2+H2 fluxes in our specific case) than in the following periods (Total Volatile Flux: 615-4112 t/day). We propose that the elevated magmatic/hydrothermal transport in March 2017, combined with a 15% reduction in precipitation, caused the volume of the lake to decrease, ultimately reducing its sulfur absorbing and scrubbing capacity, and hence causing the gas plume CO2/SO2 ratio to decrease. The recently observed increases in temperature, acidity and salinity of the lake are consistent with this hypothesis. The small volume of Santa Ana lake, compared to other crater lakes such as the pre-2017 Poás (Costa Rica), Yugama (Japan), Ruapehu (New Zealand) and Kawah Ijen (Indonesia), makes it sensitive to variations in the underlying magmatic-hydrothermal system. We conclude that the installation of a continuous, fully-automated Multi-GAS is highly desirable to monitor any future change in lake plume chemistry, and hence state of volcanic activity.Publishedid 664V. Processi pre-eruttiviJCR Journa

Escalating CO2 degassing at the Pisciarelli fumarolic system, and implications for the ongoing Campi Flegrei unrest

This short communication aims at providing an updated report on degassing activity and ground deformation variations observed during the ongoing (2012–2019) Campi Flegrei caldera unrest, with a particular focus on Pisciarelli, currently its most active fumarolic field.We show that the CO2 flux fromthe main Pisciarelli fumarolic vent (referred as “Soffione”) has increased by a factor N 3 since 2012, reaching in 2018–2019 levels (N600 tons/ day) that are comparable to those typical of a medium-sized erupting arc volcano. A substantial widening ofthe degassing vents and bubbling pools, and a further increase in CO2 concentrations in ambient air (up to 6000 ppm), have also been detected since mid-2018. We interpret this escalating CO2 degassing activity using a multidisciplinary dataset that includes thermodynamically estimated pressures for the source hydrothermal system, seismic and ground deformation data. From this analysis, we show that degassing, deformation and seis- micity have all reached in 2018–2019 levels never observed since the onset ofthe unrest in 2005, with an overall uplift of~57 cmand ~448 seismic events in the last year. The calculated pressure ofthe Campi Flegrei hydrother- mal system has reached ~44 bar and is rapidly increasing. Our results raise concern on the possible evolution of the Campi Flegrei unrest and reinforce the need for careful monitoring of the degassing activity at Pisciarelli, hopefully with the deployment of additional permanent gas monitoring units.Published151-1574V. Processi pre-eruttiviJCR Journa

Author Correction: Increment in the volcanic unrest and number of eruptions after the 2012 large earthquakes sequence in Central America

Correction to: Scientific Reports https://doi.org/10.1038/s41598-021-01725-1, published online 17 November 2021.The original version of this Article contained a repeated error in the Introduction, in Figure 1 and its accompanying legend, in the Results section under the subheading ‘Stress changes caused by the earthquakes’, in the Discussion and conclusions section under the subheading ‘Volcanic eruptions long after the earthquakes’, and in the Supplementary Information file, where the earthquake that occurred on November 7, 2012 was incorrectly mentioned as having occurred on November 11, 2012. The original Fig. 1 and accompanying legend appear below. The original Article and the Supplementary Information file that accompanies the original Article have been corrected.Peer reviewe

Increment in the volcanic unrest and number of eruptions after the 2012 large earthquakes sequence in Central America.

Understanding the relationship cause/effect between tectonic earthquakes and volcanic eruptions is a striking topic in Earth Sciences. Volcanoes erupt with variable reaction times as a consequence of the impact of seismic waves (i.e. dynamic stress) and changes in the stress field (i.e. static stress).In2012, three large(Mw≥7.3)subductionearthquakes struck Central Americawithin a period of 10 weeks;subsequently, some volcanoesin the regionerupted a few days after, whileotherstook months or even years to erupt. Here, we show thatthese threeearthquakes contributed to the increase in the number of volcanic eruptionsduring the 7 years that followed these seismic events. We found that onlythosevolcanoes that were already in a critical state ofunrest eventually erupted, which indicates that the earthquakes only prompted the eruptions. Therefore, we recommend the permanent monitoring of active volcanoes toreveal which are more susceptible to culminate into eruption in the aftermath of the next large-magnitude earthquake hits a region.Funding was provided by Japan International Cooperation Agency (JICA) and Istitute Nazionale di Geofisica e Vulcanologia (INGV).Peer reviewe