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An Overview of the Structure, Hazards, and Methods of Investigation of Nyos-Type Lakes from the Geochemical Perspective

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An Overview of the Structure, Hazards, and Methods of Investigation of Nyos-Type Lakes from the Geochemical Perspective J. Limnol., 2014; 73(1): 55-70 ORIGINAL ARTICLE DOI: 10.4081/jlimnol.2014.836 An overview of the structure, hazards, and methods of investigation of Nyos-type lakes from the geochemical perspective Franco TASSI,1,2* Dmitri ROUWET3 1Department of Earth Sciences, Via G. La Pira 4, 50121, University of Florence; 2CNR – Institute Geosciences and Earth Resources, Via G. La Pira, 4, 50121 Florence; 3Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Bologna, Via Donato Creti 12, 40128 Bologna, Italy *Corresponding author: [email protected] ABSTRACT Limnic eruptions represent a natural hazard in meromictic lakes hosted in volcanoes releasing CO2-rich magmatic gases. Biogeo- chemical 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 land- slides, 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 dramaticonly 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 carefullyuse 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 ap- plying 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. Key words: limnic eruption, volcanic hazard, dissolved gases, Nyos-type lake, meromictic lake, lake water geochemistry. Received: June 2013. Accepted: September 2013. INTRODUCTION Volcanic lakes exert a strong control on the eruptive style of active volcanoes, since lake water may interact with Volcanic lakes, a common feature of active and qui- i) magma, producing violent phreatic or phreatomag- escent volcanoes (SimkinNon-commercial and Sieber, 1994; Delmelle and matic eruptions that can generate hazardous base surges Bernard, 2000a), have been classified based on their and tephra emissions (from ash to ballistic); and/or ii) water physic-chemical characteristics, mainly depending non-juvenile volcanic products, which may lead to major on the input of heat and volcanic-hydrothermal fluids lahars and floods (Nairn et al., 1979; Badrudin, 1994; from the hosting system (Pasternack and Varekamp, Christenson, 2000; Mastin and Witter, 2000; Rodolfo, 1997). High-activity lakes commonly consist of warm- 2000; Matthews et al., 2002; Kilgour et al., 2010; Mor- to-hot acidic or hyperacidic brines, whose chemical com- rissey et al., 2010). In quiescent volcanoes, the stability position and volume is strictly controlled by the dynamic of the flanks may decrease after prolonged acid attack of balance between i) inputs of meteoric precipitation and infiltrating lake water, favouring landslide events (e.g., magmatic fluids from a shallow source, and ii) outputs, Pasternack and Varekamp, 1994; Rowe et al., 1995; San- related to evaporation, mineral precipitation and water ford et al., 1995; Delmelle and Bernard, 2000b; Kempter seepage through lake bottom (Varekamp, 2003; Taran and and Rowe, 2000; Varekamp et al. 2009). A second clas- Rouwet, 2008, Rouwet and Tassi, 2011). Low-activity sification system by Varekamp et al. (2000) reveals a lakes, i.e. those located in systems having a deep mag- contradiction as demonstrated for Lake Nyos: a CO2- matic source, are defined by typically low salinity and dominated, but rock-dominated lake, instead of gas-dom- near-neutral pH, with minor input of fluids from depth. inated. For these apparently low activity lakes a further 56 F. Tassi and D. Rouwet hazard is represented by the so-called limnic eruptions ORIGIN OF DISSOLVED GASES (Sabroux et al., 1987; Kusakabe, 1996; Halbwachs et al., IN NYOS-TYPE LAKES 2004; Kusakabe et al., 2008). This feature, which was in- Dissolved gases in meromictic volcanic lakes are typ- tensively studied after the two disasters at Monoun and ically dominated by CO2, followed by significant amounts Nyos Lakes (Cameroon, Fig. 1) in 1984 and 1986, re- of CH4 and N2, and minor concentrations of H2 and noble spectively (Kerr, 1986; Kling, 1987; Kling et al., 1987; gases. Sublacustrine vents discharge magmatic CO2 into Sigurdsson et al., 1987; Le Guern, 1989; Evans et al., the lake bottom layer (Evans et al., 1993; Aeschbach-Her- 1993), consists of a sudden outburst of massive amounts tig et al., 1996, 1999; Cioni et al., 2003; Caliro et al., of dissolved gases, mainly CO2, accumulated in deep lay- ers of meromictic lakes, i.e. those not subject to seasonal layer turnover. Despite the fact that Lake Kivu (DRC; Fig. 1) is not really a volcanic lake, it is also considered a potential site for limnic eruptions (Haberyan and Hecky, 1987; Schmid et al., 2004; Tassi et al., 2009) since a CO2- and CH4-rich gas reservoir up to three orders of magnitude larger than that in Lake Nyos is stored in its bottom waters (Tietze et al., 1980; Schoell et al., 1988; Schmid et al., 2004, 2005). These three African killer lakes (Fig. 1) are characterized by significant contribution of only CO2 of magmatic origin (Kling, 1987; Schoell et al., 1988; Evans et al., 2003; Kusakabe et al., 2000; Nayar, 2009), and CO2 accumulation in the hypolimnion (the deepest layer of stratified lakes; Fig. 2) is one of the main use pre-requisites for the occurrence of a gas outburst in case of lake strata perturbation. In recent times, a large num- ber of geochemical studies, aimed at evaluating the haz- ard related to a possible limnic eruption from other, apparently quiescent volcanic systems worldwide hosting meromictic lakes in France, Italy, Germany, Vanuatu Is- lands, and Chilean Andes, were carried out (Aeschbach- Herting et al., 1996, 1999; Aguilera et al., 2000; Anzidei et al., 2008; Caliro et al., 2008; Bani et al., 2009; Cara- causi et al., 2009; Gunkel et al., 2009). These studies pro- posed different sampling and analytical methods to Fig. 1. Map of Africa with the location of Nyos, Monoun provide data for their interpretative models. (Cameroon) and Kivu (DRC) lakes (stars). We believe that these low activity lakes could pose significant and peculiar risks to nearby human activity and therefore, if only for the sake of volcanicNon-commercial surveillance, de- serve a dedicated review paper. Moreover, a recent data base (VHub, CVL group page) surprisingly totalled 345 volcanic lakes worldwide, most poorly studied or even unknown and that represent a potential hazard for local population. This data contrasts with the earlier much, lower number of 114 volcanic lakes suggested by Delmelle and Bernard (2000a). The present study reports an up-to-date and compre- hensive overview of sampling and analytical methods and theoretical physical-chemical models proposed and adopted by different scientific groups in the framework of geochemical surveys of Nyos-type lakes. Different sci- entific approaches are evaluated and criticized to provide information of the most useful tools that can be used for Fig. 2. Sketch of the lake stratification of a meromictic lake, and future investigations aimed to study and mitigate the related processes to describe the origin of the dissolved gases in the various lake strata. limnic eruption hazard. Hazards and methods of investigation of Nyos-type lakes from the geochemical perspective 57 2008; Caracausi et al., 2009; Gunkel et al., 2009; Tassi et may be present in significant concentrations in lakes char- al., 2009), although significant CO2 contribution may also acterized by strong fluid contribution from an underlying derive from biotic respiration, anaerobic decomposition hydrothermal system, such as Lake Kivu (Tassi et al., of organic matter and microbial oxidation of CH4 often 2009), although it is generally accepted that in Nyos-type entering the lake by the inflow of sediment- and organic- lakes this gas has a dominant atmospheric origin. loaded rivers or surface run off (Rudd et al., 1974; Rich, The origin of He in volcanic lakes is related to that
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