Lakes Nyos and Monoun Gas Disasters (Cameroon)—Limnic Eruptions Caused By

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GEochemistry Monograph Series, Vol. 1, No. 1, pp. 1–50 (2017) www.terrapub.co.jp/onlinemonographs/gems/

Lakes Nyos and Monoun Gas Disasters (Cameroon)—Limnic Eruptions Caused by

Excessive Accumulation of Magmatic CO2 in Crater Lakes

Minoru Kusakabe

Department of Environmental Biology and Chemistry University of Toyama 3190 Gofuku, Toyama 930-8555, Japan e-mail: [email protected]

Citation: Kusakabe, M. (2017) Lakes Nyos and Monoun gas disasters (Cameroon)—Limnic eruptions caused by excessive accumulation of magmatic CO2 in crater lakes. GEochem. Monogr. Ser. 1, 1–50, doi:10.5047/gems.2017.00101.0001.

Abstract Received on December 5, 2015 This is a review paper on the Lakes Nyos and Monoun gas disasters that took place in the Accepted on May 11, 2016 mid-1980s in Cameroon, and on their related geochemistry. The paper describes: (i) the Online published on April 7, 2017 gas disasters (the event and testimonies); (ii) the unusual geochemical characters of the lakes, i.e., strong stratification with high concentrations of dissolved CO2; (iii) the evolu- Keywords tion of the CO2 content in the lakes during pre- and syn-degassing; (iv) the noble gas • Cameroon signatures and their implications; (v) a review of models of a limnic eruption; (vi) a • Lakes Nyos and Monoun revision of a spontaneous eruption hypothesis that explains the cyclic nature of a limnic • gas disaster eruption (Kusakabe 2015); (vii) a brief review of the origin of the Cameroon Volcanic • crater lakes Line (CVL) and the geochemistry of CVL magmas; (viii) a brief review of other CO - • magmatic CO2 2 • limnic eruption rich lakes in the world; and (ix) concluding remarks. • disaster mitigation Degassing of the two lakes has been successful. Most of the dissolved CO2 has been • degassing removed from Lake Monoun, resulting in the stoppage of the degassing system. How- • Cameroon Volcanic Line ever, the CO2 content in the lake started to increase in recent years due to the continuing •SATREPS supply of gas from the underlying magma, indicating the necessity of the continuous removal of gas from the lake. Lake Nyos will attain the same situation in several years when degassing will stop. Thus, a continuation of scientific monitoring of the lakes is essential. Since the transfer to Cameroonian scientists of monitoring techniques, including analytical equipment necessary for the monitoring, has been effected through the SATREPS project (Japan’s Official Development Aid), the responsibility is now theirs, and it is strongly hoped that the lake monitoring, the rehabilitation of displaced people, and the setting up of an infrastructure for them, etc., will be carried out by the Cameroonian Government and local scientists.

1. Introduction degassing of CO2 is the quiet discharge of gas often derived from a magmatic source, with varying degrees Volatiles in the deep interior of the Earth are brought of contamination by crustal or biological CO2. Crater to the surface mainly by volcanic activity. In terms of lakes usually sit on top of volcanic conduits and act as the present-day global carbon cycle, the CO2 discharge condensers or traps for magmatic volatiles. The Lake from subaerial volcanism including the passive dis- Nyos gas disaster in 1986, and a similar event in 1984 charge from the craters or flanks of volcanoes, is the at Lake Monoun, both in Cameroon, Central Africa, major non-anthropogenic contributor to atmospheric resulted from an excessive accumulation of magmatic CO2 (e.g., Kerrick, 2001; Gerlach, 2011). The passive CO2 in the bottom layers of the lakes. These volcanic

Fig. 1. Location of Lakes Nyos and Monoun (red circles) and volcanoes along the Cameroon Volcanic Line (solid black) in Cameroon, Central Africa. Modified from figure 1 of Environmental Monitoring and Assessment Journal, Hydrogeochemistry of surface- and groundwater in the vicinity of Lake Monoun, West Cameroon: Approach from multivariate statistical analysis and stable isotopic characterization, 2015, Kamtchueng, B. T., Fantong, W. Y., Takounjou, A. F., Tiodjio, E. R., Kusakabe, M., Mvondo, J. O., Zhang, J., Ohba, T., Tanyileke, G., Hell, J. V. and Ueda, A. „ Springer International Publishing Switzerland 2015 with permission of Springer.

crater lakes are considered to be the sites of passive geochemical investigations revealed that the gas was degassing of CO2. On 26th August, 1986, a large CO2 that originated from magma and had accumulated amount of CO2 was suddenly released from Lake Nyos passively in the deep part of these lakes. The physico- that asphyxiated 1746 people, and an unaccountable chemical characteristics of the lakes are unique and number of cattle, living near the lake (Sigvaldason, have evolved with time, even after the gas release, due 1989). A very similar gas event took place in August to the continuing supply of magmatic CO2. 1984 at Lake Monoun, with 37 casualties (Sigurdsson In the present paper, issues related to these gas dis- et al., 1987). Lake Monoun is located only 100 km charges are reviewed in the following sections; (III) south-east of Lake Nyos (Fig. 1). A term “limnic erup- what happened at the time of the Lakes Nyos and tion” was coined by J.-C. Sabroux to describe a gas Monoun gas disasters?; (IV) pre- and syn-degassing outburst from a lake (Halbwachs et al., 2004), and will chemical evolution of the lakes; (V) possible causes be used in this review. Given that this type of gas dis- of the disasters, the models and the repetitive nature aster had not been previously recorded (Sigurdsson, of a limnic eruption. In relation to the recurrence pre- 1987a), the Lakes Monoun and Nyos events attracted vention of a limnic eruption, a bilateral scientific a great deal of attention, not only from the media but project between Japan and Cameroon called SATREPS- also from a disaster science perspective. At that time, NyMo was carried out during 2011 and 2016, and is nobody imagined that the lakes had accumulated so outlined in Section 5. much lethal gas and that the gas was released into the The upper 40 m of Lake Nyos is bounded on the north atmosphere without any precursor. Subsequent by a narrow dam of poorly consolidated pyroclastic

Fig. 2. (a) Victims near Lake (Stager and Suau, 1987). Reproduced with permission of Helimission (www.helimission.org). (b) Dead cow by the lake (photo taken by the author).

rocks. This dam is being affected by back erosion. A gional units which are differentiated by their geogra- warning was given that the collapse of the dam could phy, climate and vegetation characteristics as follows: cause a flood that would affect inhabited areas over a (1) The Sudano-Sahelian zone in the North is composed 220 km distance (Lockwood et al., 1988). An accurate of the Mandara mountains, Diamaré plains and the estimation of the rate of back erosion of the dam is Benue Valley. (2) The savanna zone is composed of critical for the safety of people living downstream. the Adamawa highlands, the Tikar plain, the low land Thus, the age of the dam formation (or Nyos maar for- savanna of the Center and East regions, and the high- mation) has been hotly debated using different age land of the West and Northwest regions. (3) The tropi- determination techniques. Recent progress on the age cal forest zone is composed of the degraded forests of of the dam is briefly reviewed in Section 6. the Central and Littoral regions, and the tropical rain- Thirty nine crater lakes including Lakes Nyos and forests of the Southwest and East regions. (4) The Monoun and numerous soda springs are located along coastal and marine zone spreads along the Gulf of the Cameroon Volcanic Line (CVL). An understand- Guinea. The country’s economy is driven by agro-in- ing of the origin and the geochemistry of CVL mag- dustry in the coastal, central and southern zones (Molua mas is essential. These subjects are reviewed in Sec- and Lambi, 2006). Because of the above geographic tion 6, which constitutes the basis on which CO2 accu- characteristics, its wide range of climatic types, and mulation in these lakes is scientifically interpreted. We its cultural diversity, Cameroon is often nicknamed also need to understand why CO2 becomes enriched in “Africa in miniature”. The population of Cameroon is magmatic volatiles as they leave the magma. The Lakes estimated to be ~23 million as of January 2015 (http:/ Nyos and Monoun events have stimulated geochemical /countrymeters.info/en/Cameroon). According to the interest in other CO2-rich volcanic lakes in the world Demographics of Cameroon (http://en.wikipedia.org/ for their gas hazard potential. This is reviewed in Sec- wiki/Demographics_of_Cameroon), the country com- tion 7. prises an estimated 250 distinct ethnic groups, which may be classified into five large regional-cultural di- 2. Gas disasters at lakes Nyos and Monoun, visions: (1) the western highlanders (Semi-Bantu or Cameroon grassfielders), including the Bamileke, Bamoun, and many smaller Tikar groups in the Northwest (~38% of 2-1. Cameroon: Location and physiography the total population); (2) the coastal tropical forest peoples, including the Bassa, Duala (or Douala), and Cameroon is a country in Central Africa located be- many smaller groups in the Southwest (12%); (3) the tween 2–13∞N latitude, and 8–16∞E longitude (Fig. 1). southern tropical forest peoples, including the Beti- It is bounded by 6 countries: Chad to the northeast, Pahuin with subgroups called Bulu, Fang, Maka, Njem, Nigeria to the west, Central African Republic to the and Bakapygmies (18%); (4) the predominantly Islamic east, Equatorial Guinea, Gabon and Congo to the south. peoples of the northern semi-arid regions (the Sahel) Cameroon can be divided into ten major ecological and central highlands, including the Fulani (or Fulbe) regions. These regions are classified under four re- (14%); and (5) the “Kirdi”, non-Islamic, or recently

Fig. 3. An aerial view of Lake Nyos taken 10 days after the limnic eruption (photo taken by the author). Debris of vegetation washed away from the shore was floating on the reddened lake surface.

Islamic, peoples of the northern desert and central high- international support by the Government of the Repub- lands (18%). Since people of different ethnic groups lic of Cameroon, the Japan International Cooperation speak different languages, French and English, inher- Agency (JICA) under the Ministry of Foreign Affairs ited from colonialism, are used for mutual communi- of Japan sent a Japan Disaster Relief Team (JDR) to cation, although they retain their original languages. the site. I was asked to join the team as a volcanic gas expert. It was my first visit to Cameroon. The JDR team 2-2. Cameroon lakes arrived at Douala on 28 August, 1986. A few days later the team was taken to Lake Nyos by helicopter because There are at least 39 lakes of volcanic origin that are of poor road conditions and heavy rains in the Nyos distributed along the CVL (Kling, 1988; Issa et al., area. We were shocked by the terrible scenes we wit- 2014a). Lake Nyos in the Northwest Region of nessed (Fig. 2), and the reddened surface water of the Cameroon (06∞26¢ N and 10∞17¢ E) is a meromictic lake (Fig. 3), which increased our anxiety concerning volcanic crater lake with a N-S length of ~2.0 km, E- the cause(s) of the disaster. Since the main purpose of W length of ~1.2 km, surface area of 1.58 km2, and a JDR was to provide relief supplies and medical care to maximum depth of 209.5 m. It lies in the Oku volcanic the refugees, we made an initial cursory scientific sur- field along the CVL, and was formed by a basaltic vey during this first visit. There was no indication of phreato-magmatic eruption (Lockwood and Rubin, the direct involvement of volcanic gases as initially 1989). The age of the lake, which has been a topic of suggested, for we did not find any trace of acid gases controversy, will be described later (Subsection 6-1). like SO2, H2S and HCl, which are major components Lake Monoun in the West region of Cameroon lies at of high-temperature volcanic gases. Later reports in- 05∞35¢ N and 10∞35¢ E, and is also a meromictic vol- dicated that the cause of the deaths was CO2 gas re- canic crater lake with a NEE-SWW length of ~1.6 km, leased from Lake Nyos on the evening of 21 August, a maximum NW-SE width of ~0.7 km, a surface area 1986, and that the gas killed 1746 people and ~8000 of 0.31 km2, and a maximum depth of 99 m. It belongs livestock by asphyxiation (Kling et al., 1987; Kusakabe to the Oku volcanic center along the CVL. The age of et al., 1989). Exactly 2 years prior to the Lake Nyos the lake is unknown. disaster, there had occurred a very similar gas release from Lake Monoun on 15 August, 1984, that killed 37 2-3. Lake Nyos disaster: The event and testimo- people also by asphyxiation by CO2 gas released from nies the lake. These extremely unusual disasters had never been recorded before, and therefore constituted a new Unusual news raced around the world in late August type of natural disaster (Sigurdsson, 1987a). 1986. The first news that reached Japan reported that August 21, 1986, was a Thursday, and a market day 40 local people had been killed by a “poisonous” gas at the Nyos village. Local people were selling and ex- released from a volcanic lake (Lake Nyos) in changing their agricultural products and articles for Cameroon. The number of casualties increased to daily use. At the time when the catastrophe occurred ~1200 in a later report. In response to a request for (8~9 p.m.), people must have been relaxed and chat-

doi:10.5047/gems.2017.00101.0001 © 2017 TERRAPUB, Tokyo. All rights reserved. M. Kusakabe / GEochem. Monogr. Ser. 1: 1–50, 2017 5 ting at home, or drinking beer outside. Some people may have already been in bed. Such a peaceful situation was disrupted by a sudden release of lethal gas from Lake Nyos. It was indeed a nightmare. It is obvious that the local people did not understand what happened. From the testimonies collected later from survivors by journalists and researchers, the event may have proceeded as follows. Some people heard faint rumbles and noises like a car coming from a distance. They went out of the house to look around, and then felt a tepid breeze with a smell of rotten eggs or gun powder. Most people fell down, lost consciousness and died (Fig. 2). At Nyos village, where 1200 people lived at that time, only a few people (4~6) survived. The survivors were stunned to find that they had lost most of their family members, relatives and friends. Sigurdsson (1987b) noted that “some survivors of the disaster attributed it to the wrath of their dead tribal chief, who, on his deathbed in 1983, ordered that his best cattle be driven off the sheer cliffs above Lake Nyos as a sacrifice to the spirit of the lake, Mami-Wa- ter. But the chief’s family failed to honor his last wish, and many today believe that the 1986 calamity was an expression of the chief’s posthumous displeasure”. Fig. 4. Photograph showing a white cloud still remaining Sigurdsson (1987b) also described that, four days be- along the valley downstream of Lake Nyos. Nyos village is fore the lethal event, local herdsmen noticed unusual seen at the bottom. The photo was taken 2 days after the bubbling on the lake’s surface, which prompted twenty eruption by a helicopter pilot carrying a Catholic mission five of them to move to a distant village. There were (supplied by G. Tanyileke). also unconfirmed reports claiming the emission of foaming water and vapor from the lake two to three weeks earlier. At about 4:00 p.m. on August 21, nearby the village at 11:00 p.m. (5) Minor events with an herdsmen heard strange bubbling sounds and observed upwelling of hydrothermal water and gas occurred in a slight emission of vapor from the lake. At about 8:00 Lakes Nyos and Njupi, a small and shallow lake 2 km p.m., villagers in Cha, a village about 7 km northwest east of Lake Nyos (Chevrier, 1990). (6) White cloud of the lake, heard two loud noises, followed by three was seen during the catastrophe on August 21. Based rumbles at about 9:00 p.m., when activity built up to on these testimonies and observations, Le Guern et al. the climactic disaster. (1992) preferred the interpretation that the Lake Nyos According to Aramaki et al. (1987) who interviewed catastrophe was caused by the input of hot hydrother- Mallam Jae, a local farmer who lived at a place 120 m mal fluids containing CO2 into the lake and surround- higher than the lake surface, the gas explosion took ing area. Their interpretation seemed to have been in- place at about 8:30 p.m. on August 21 and continued fluenced by their experience at Dieng volcano (Indo- until 1 a.m. the next day. This account of the time at nesia) where CO2 gas, originating from a phreatic erup- which the events took place may be reliable, since tion of the volcano, killed 142 local people who were Mallam Jae was wearing a nice wrist watch. Initially, fleeing from the site (Le Guern et al., 1982). How- Mallam Jae heard sounds like a murmur followed by ever, there is a view that the anecdotal evidence (such detonations. He also felt tremors and a smell of gun as “the smell of rotten eggs and gun powder, rumbling powder. The next morning he found the lake quite unu- noise heard at distance”, etc.), collected soon after the sual. Le Guern et al. (1992) published details of inter- disaster through interviews with local survivors by views with some local people who spoke in pidgin journalists and scientists, should be interpreted with English (which was translated into English) about what care because stories told by local people may have been they saw. Observations by local people included: (1) tailored to give answers to please the visiting inter- Minor upwelling of hydrothermal waters from the bot- viewers (Freeth, 1990). The author adopts this view tom of Lake Nyos on August 20, one day before the and believes that the phreatic hypothesis did not have event. (2) A small explosion that took place at 4:00 a firm scientific basis, because this interpretation of p.m. followed by a major explosion between 8:00 and the events was largely based on the testimonies and 8:30 p.m. on August 21. (3) A water jet accompanied anecdotal evidence. by white illuminations. (4) A detonation was heard in In March 1987, a Cameroon Government and

along low-lying areas, such as valleys, before mixing with air. Costa and Chiodini (2015) simulated the gas flow, using a computer code TWODEE-2, for 4 different scenarios that considered different gas masses and fluxes from Lake Nyos in 1986. The simulations, indicating how far and fast the cloud dispersed after the limnic eruption, are useful for making up a hazard map of the area. Figure 4 is a photograph taken two days after the eruption by a Helimission helicopter pilot (G. Tanyileke, pers. commun.) and shows that the white cloud was still present along a valley downstream of the lake. Figure 5 (modified from Sigurdsson et al., 1987a) shows the gas flow path estimated from the distribution of victims around Lake Nyos. The gas cloud traveled more than 20 km, asphyxiating people on its way before dissipating into air. The number of victims was 1200 at Nyos village, 300 at Cha village and 52 at Subum village. More than 8000 cattle were Fig. 5. Distribution of localities where victims were found also killed. Survivors were evacuated to 7 resettlement around Lake Nyos (red circles), and estimated flow paths of camps, namely, Kimbi, Buabua, Kumfoutu, Yemge, the gas (arrows). Modified from Sigurdsson (1987a). Ipalim, Esu and Upkwa (around Lake Wum). As of July 2015, these people were still cut off from the general population, as neither the national radio, electricity grid, nor television signals reached them. UNESCO-sponsored international conference on the Since the victims were asphyxiated almost instantly, Lake Nyos Disaster was held in Yaoundé, the capital the oxygen concentration in the gas must have been of Cameroon. More than 200 scientists participated and extremely low compared to normal the atmosphere, or presented the results of their initial studies on the geo- the CO2 concentration was very high. Table 1 shows logical, geochemical, physical, medical and socio-an- the effect of some gases on human health (Kusakabe thropological aspects of the disaster (Sigvaldason, et al., 1989). Mammals, including human beings, live 1989). Regarding the cause of the gas burst, there was on a normal atmosphere that contains 21 vol % of O2. a sharp confrontation between a group of scientists who If air is breathed containing less than half of this nor- believed that the lake played a key role in the accumu- mal air concentration of O2, a coma, fainting, cyano- lation of the CO2 which was subsequently released (this sis, syncope, respiratory arrest, and ultimately, cardiac interpretation was later named “limnic eruption hypoth- arrest can result. If we breathe air containing high con- esis”) and another group of scientists who believed that centrations of CO2 (e.g., >10 vol %), a coma, and even- the cause of the Nyos catastrophe was due to a vol- tually, death can result. Some survivors of the Lake canic (phreatic) eruption from the bottom of the lake Nyos disaster were found to suffer from pulmonary (volcanological or phreatic hypothesis) (Tazieff, 1989; edema, respiratory distress, conjunctivitis, and skin Barberi et al., 1989; Le Guern et al., 1992). Disagree- lesions or “burns” (Baxter et al., 1989). The skin burns ment between the two scientific views resulted in a were taken by the phreatic hypothesis supporters as compromise of the resolutions of the Yaoundé Confer- evidence that the gas was at a high temperature and ence (Sigvaldason, 1989), and encouraged the need for contained some acidic, corrosive components, such as follow-up investigations, which clearly indicated a SO2 (which turned to sulfuric acid later) and HCl that steady supply of magmatic CO2 from the lake bottom are commonly contained in high-temperature volcanic and its accumulation in the lake. This gave strong sup- gases. This interpretation was highly unlikely, since port to the limnic eruption hypothesis (Kling et al., vegetation, and clothes on the victims stayed intact and 2005; Kusakabe et al., 2000, 2008). This hypothesis no appreciable level of sulfur and chlorine components will be described in detail in Sections 3 and 4. were found in the lake water (Kusakabe et al., 1989). The gas released from Lake Nyos was almost pure The medical interpretation of the skin damage or blis- CO2 (Kling et al., 1987; Kusakabe et al., 1989). Since ters was that the body’s metabolic rate was drastically CO2 gas is 1.5 times denser than air at room tempera- reduced in a state of deep coma, inducing a severely ture, and since it may have been cooled due to adi- restricted circulation of blood. As a consequence, the abatic expansion when released from the pressurized capillary vessels of the skin lacked circulation, result- deep part of the lake, the density of the gas was likely ing in necrosis and the development of skin lesions on to have been significantly greater than that of the am- approximately 5% of the patients (Baxter et al., 1989). bient atmosphere. This would have facilitated its flow

Table 1. Effect of some gases on human health*.

Concentration in atmosphere Stage Response and symptoms

O2 (%) 21 1 Normal 16-12 2 Lowered concentration, headache 14-9 3 Disorientation, unstable gait, headache, nausea, vomiting, facial pallor, somnolence 10-6 4 Coma, fainting, damage of central nervous system, cyanosis, convulsion <6 5 Syncope, coma, bradypnea, respiratory arrest, cardiac arrest

CO2 (%) 0.04 1 Normal 1.5 2 Changes in physiological ranges (techypnea etc.) 5 3 Shortness of breath, headache, coma 10 4 Coma in 10-15 min. exposure >40 5 Sudden death

*Reproduced from table 6 of J. Volcanol. Geotherm. Res. 39, Kusakabe, M., Ohsumi, T. and Aramaki, S., The Lake Nyos gas disaster: chemical and isotopic evidence in waters and dissolved gases from three Cameroonian crater lakes, Nyos, Monoun and Wum, 167–185, Copyright 1989, with permission from Elsevier.

2-4. Lake Monoun disaster several months after the gas burst, upon a request by the Cameroonian Government (Sigurdsson et al., Lake Monoun experienced a gas burst on 15 August, 1987). They found unusual chemical characteristics. 1984, that killed 37 people by asphyxiation. A recon- Waters below 50 m were anoxic, dominated by high 2+ – struction of the event (Sigurdsson et al., 1987) showed Fe (~200 mg/l) and HCO3 (~1000 mg/1), and su- that at almost midnight of that day, people in Njindoun, persaturated with siderite, a major component of the a village about 5 km north of the lake, heard a loud crater floor sediments. The unusually high Fe2+ levels noise in the vicinity of the lake. They informed the were attributed to the reduction of laterite-derived fer- nearby police early next morning. A policeman together ric iron that was gradually brought into the lake as loess with a medical doctor went to the site where they saw and in river input. Sulfur compounds were below de- a whitish, smoke-like cloud that covered the ground to tection limits in both water and gas. Table 2 shows the a height of ~3 m. Since they became nauseous and chemical composition of Lake Monoun water samples dizzy, they left the site and moved to Njindoun vil- collected between 27 February and 16 March, 1985 lage. After the cloud dissipated, they came back to the (Sigurdsson et al., 1987). It includes data for samples site and found dead people lying on the road. The vic- collected in 1986 (Kusakabe et al., 1989) and 1993 tims had skin lesions or blisters. Clothes were not af- (Kusakabe et al., 2008). Analysis of the 1985 and 1986 fected. Domestic and wild animals were also found samples showed lower gas and ionic contents than the dead. Altogether 37 people were killed by this event. original solution. This was interpreted to be due to (i) A survivor described the smell of the gas cloud as loss of CO2 from the waters during retrieval of the “sulfurous like a car battery”. It was found in a later Niskin sampler from the lake, (ii) loss of CO2 from survey that vegetation at the east end of the lake was waters collected in the sample containers, and/or (iii) flattened, indicating that the water wave locally reached oxidation and precipitation of iron prior to the analy- up to 5 m high, and that the color of the lake water sis. During the early stages of investigation of the Lakes changed to a reddish brown. From the above state- Monoun and Nyos disasters, these problems high- ments, the Lake Monoun event was very similar to the lighted the difficulty of sampling and the analysis of Lake Nyos event. For this reason, it seems appropriate CO2-rich waters. The data for the 1993 samples were to describe and compare, at the same time, the results more reliable (Kusakabe et al., 2008). Based on the of the geochemical surveys made after the gas bursts data obtained in 1985, however, Sigurdsson et al. at both lakes. (1987) had come to the important conclusion that accumulation of CO2(aq) in the lake was attributed to the 2-5. Unusual geochemical characters of Lakes long-term emission of magmatic CO2(aq) from vents Nyos and Monoun within the crater, which led to a gradual build-up of – CO2(aq) and HCO3 in the lake, i.e., an essential con- A scientific survey of Lake Monoun was undertaken cept of the “limnic eruption hypothesis”. It is noted by the Office of Foreign Disaster Assistance (USAID) that the manuscript of the paper by Sigurdsson et al.

Table 2. Representative analyses of water samples collected in 1985, 1986 and 1993 from Lake Monoun.

+ + + 2+ 2+ 2+ 2- - - Depth pH Na K NH4 Mg Ca Fe SO4 Cl SiO2 HCO3 CO2(aq) m mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l February 1985*1 0 6.9 9 2.2 <0.1 6.0 8.7 <0.02 <1 <1 19 88 26 15 5.8 17 4.7 6.2 22 20 0.03 <1 1.8 40 265 952 61 6.3 25 5.6 15 29 41 200 <1 3.2 35 1050 1086 90 6.4 24 5.7 13 30 42 220 <1 3.4 45 1050 890 90 6.1 24 5.7 15 30 41 170 <1 3.3 50 775 1311 90 6.3 25 5.8 18 30 43 260 <1 3.2 48 805 851 90 6.4 24 5.9 13 30 42 190 <1 3 41 805 680 90 6.0 26 5.5 15 29 42 290 <1 3.4 50 1000 2112 90 5.9 26 5.5 12 30 41 190 <1 3.2 51 885 2357

October 1986*2 16 0 æ 11 3 æ 4.2 4 1.7 0.1 0.4 17 57 202 15 æ 14 4.2 6 13 10 110 0.3 1.0 40 421 542 25 æ 17 5.3 11 17 12 190 0.2 1.4 37 657 2859 50 æ 17 5.1 17 13 10 340 0.4 2.5 38 1087 2385 75 æ 21 7.2 26 22 18 540 0.4 2.5 42 1520 2922 95 æ 22 7.2 26 23 19 590 0.2 2.6 44 1660

January 1993*3 10 6.58 13 3.5 10 8 13 100 0.1 1.1 34 82 4 20 6.46 15 4.3 12 15 21 259 0.1 1.5 58 105 18 30 6.00 18 4.6 18 24 30 464 0.1 1.5 86 1352 1809 40 5.91 19 5.0 19 24 33 505 0.1 1.6 91 1448 2271 50 5.80 21 5.3 22 23 38 533 0.1 1.8 95 1546 3217 55 5.60 23 5.5 28 27 46 641 0.1 2.3 110 1823 6760 75 5.58 24 5.8 28 27 46 646 0.1 2.4 114 1862 6848 90 5.60 24 5.4 30 28 48 682 0.1 2.4 114 1961 6813 95 5.66 25 5.9 37 29 50 804 0.1 2.6 123 2272 6778 100 5.72 26 7.4 39 29 51 918 0.5 2.8 129 2523 6029

*1Sigurdsson et al. (1987). *2Reproduced from table 2 of J. Volcanol. Geotherm. Res. 39, Kusakabe, M., Ohsumi, T. and Aramaki, S., The Lake Nyos gas disaster: chemical and isotopic evidence in waters and dissolved gases from three Cameroonian crater lakes, Nyos, Monoun and Wum, 167–185, Copyright 1989, with permission from Elsevier. *3Reproduced from table 1 in Kusakabe et al. (2008). Numbers in italics were calculated assuming carbonate equilinria.

(1987) had been prepared prior to the 1987 Interna- enon (Evans et al., 1993; Kusakabe et al., 2000). This tional Conference on the Lake Nyos disaster in situation led scientists working on Lakes Nyos and Yaoundé, so the American team who started an initial Monoun to warn of the possible recurrence of a limnic scientific survey at Lake Nyos must have had a gen- eruption in the near future and to recommend the arti- eral idea of the cause of the disaster. ficial removal of dissolved gases from the lakes (Freeth Lake Nyos also has unusual chemical and physical et al., 1990; Tietze, 1992; Kling et al., 1994; Kusakabe characteristics similar to Lake Monoun. Dissolved spe- et al., 2000). To achieve this goal, the Nyos-Monoun cies are overwhelmingly dominated by CO2(aq) fol- Degassing Program (NMDP) was set up by scientists – 2+ 2+ 2+ + + lowed by HCO3 , Fe , Mg , Ca , SiO2, NH4 , Na , who were deeply involved in the disaster mitigation K+ in decreasing order. The concentration of the dis- issues of the limnic eruption. After experimental solved species in the water column increases with depth degassing at Lake Monoun (Halbwachs et al., 1993) with maximum values reached at the bottom (210 m). and Lake Nyos (Halbwachs and Sabroux, 2001), a per- Follow-up studies of Lakes Nyos and Monoun clearly manent degassing apparatus was installed at Lake Nyos indicated that the CO2 content in the lakes was increas- in 2001 and at Lake Monoun in 2003 under the NMDP, ing at an unusually high rate for a geological phenom- funded by the U.S. Office of Foreign Disaster Assist-

Fig. 6. Fountains from the degassing pipes. The fountain heights were 45 m at Lake Nyos, February 2001 (a) and 8 m at Lake Monoun, January 2004 (b). The tapping depth of the pipes was 203 m and 73 m, respectively. ance (USAID) and the Cameroonian Government. Con- until the early 2000s when degassing started. After the trolled degassing is continuing successfully at both initiation of gas removal, the lake structure was obvi- lakes. Figure 6 shows the amazingly beautiful ously affected by degassing. For this reason, the evo- fountains in the initial phase of degassing at Lakes lution is better described separately as “pre-degassing” Nyos and Monoun, a 45 m high fountain at Lake Nyos and “syn-degassing”. (Feb. 2001) and a 8 m high fountain at Lake Monoun (Jan. 2004). The degassing system and the construc- 3. Pre- and syn-degassing evolution of Lakes Nyos tion of the degassing pipes have been described in and Monoun Halbwachs et al. (1993, 2004). There was concern that artificial degassing might trigger another limnic erup- 3-1. Pre-degassing evolution tion, since degassing could bring deep water to the surface, which will become cooler due to adiabatic The first scientific reports on the 1986 Lake Nyos cooling, and therefore may sink and destabilize the lake gas disaster were published by Freeth and Kay (1987), (e.g., Freeth, 1994). However, numerical modeling of Kling et al. (1987) and Tietze (1987). Of these, Kling the evolution of CO2 in the lake under different input et al. (1987), which is easily accessible, gave the most conditions (McCord and Schladow, 1998; Kusakabe et comprehensive results of the initial survey of the dis- al., 2000; Schmid et al., 2003, 2006) suggested that aster, which included the geology of the region, the destabilization of the water column due to controlled geochemistry of water and gas from the lake, and the degassing could not pose an immediate threat from a pathology of hospitalized people and victims. They “man-made” limnic eruption. However, the possibil- concluded that (i) the gas released was CO2 that had ity of thermal instability of the water column between been stored in the lake’s hypolimnion (bottom layer), 50–70 m, which could become a trigger for a limnic (ii) the victims died of CO2 asphyxiation, (iii) CO2 was eruption, was suggested by Schmid et al. (2004), for derived from magmatic sources, and (iv) there was no they found double-diffusive convection at that depth direct volcanic activity involved. Kusakabe et al. range. In agreement with the results of the numerical (1989) reached similar conclusions on the basis of modeling, the observed chemical structure of the lakes water chemistry and carbon and noble gas isotopic after the initiation of the controlled degassing opera- compositions of the gases dissolved in Lakes Nyos and tion indicated that a stable stratification was estab- Monoun. They noted that the H2S concentration in the lished, which remained basically the same as the pre- released gas must have been far below a lethal level, a degassing situations at both lakes (Kling et al., 2005; point that precluded the phreatic eruption hypothesis Kusakabe et al., 2008). (see above). The same authors also reported the first As stated above, the chemical and physical structure petrochemical data of ejecta around Lakes Nyos and of Lakes Nyos and Monoun evolved steadily with time Monoun, which indicated that the lavas were transi-

Fig. 7. Profiles of electric conductivity normalized at 25∞C (abbreviated as C25) at Lake Monoun, January 2003 (left) and Lake Nyos, January 2001 (right). The water column of each lake can be divided into 4 layers, each separated by a chemocline. Reproduced from Fig. 1 and Fig. 7 of Kusakabe et al. (2008).

Fig. 8. Evolution of pre-degassing temperature profiles at Lake Monoun (a) (1986–2003) and (b) Lake Nyos (1986–2001). Reproduced from figures 4 and 9 of Kusakabe et al. (2008), but colored.

tional to slightly alkaline in composition (see below). ized by four distinct layers. At Lake Monoun, layer I A detailed temporal variation of the chemical struc- is the shallowest, is well-mixed, and contains low con- ture of Lakes Nyos and Monoun since the limnic erup- ductivity water. A sharp chemocline separates layers I tions at both lakes was reported by Kusakabe et al. and II at 23 m in January 2003. Layer II extends down (2008). This paper presented the most comprehensive to a 51 m depth, where a second chemocline develops. data set of chemical composition, conductivity, tem- A well-mixed layer III continues down to ca. 85 m. perature, pH and CO2 profiles obtained from measure- Below this depth, conductivity increases steadily to- ments taken almost every 2–3 years from 1986 to 2006, ward the bottom (layer IV). Lake Nyos has basically which enabled an evaluation of the evolution of CO2 the same structure as Lake Monoun in January 2001: in the lakes over a period of about 20 years and which layer I is the shallowest, is well mixed, and contains encompassed pre- and syn-controlled degassing peri- low conductivity water. A sharp chemocline at about a ods. The chemical structure of the lakes is best repre- 50 m depth separates layers I and II, the latter extend- sented by a conductivity profile (Fig. 7). Both lakes ing down to about a 180 m depth. A lower chemocline have a similar chemical structure which is character- develops around this depth, below which a well-mixed

Fig. 9. Evolution of pre-degassing conductivity profiles at Lake Monoun (a) (1986–2003) and (b) Lake Nyos (1986–2001). Reproduced from figures 4 and 9 of Kusakabe et al. (2008), but colored.

layer III continues down to ~203 m. Below this depth, pears high judging from the change in the temperature conductivity sharply increases towards the bottom profiles (Fig. 8a). (layer IV). Similar to Lake Monoun, the temperature of the Lake Pre-degassing temperature variations at Lakes Nyos bottom water increased continuously after the Monoun (October 1986 to January 2003) and Nyos limnic eruption in 1986 (Fig. 8b), indicating a heat in- (November 1986 to January 2001) are shown in Figs. put into the lake. The heat input to layers III and IV 8a,b, respectively, (reproduced from Kusakabe et al., was reported to decrease from an initially high value 2008). Temperature profiles at Lake Monoun show a of 0.93 MW (August 1986 to May 1987) to 0.43 MW minimum at the 5–21 m range (layer I); followed by (November 1986 to December 1988, Nojiri et al., 1993) (i) an increasing temperature to about 23∞C down to to 0.32 MW (May 1987 to September 1990, Evans et the lower chemocline at depths of 50–63 m (layer II), al., 1993). (ii) constant values down to around 90 m (layer III), Figure 9 shows the temporal change in the pre- and (iii) a second increase to >23∞C towards the bot- degassing conductivity profiles at both lakes. As pre- tom (layer IV). It is worth noting that the temperature viously stated, Lake Monoun profiles have a “shoul- of the layer III water increased significantly between der” between layers II and III. The shoulder became 1986 and 1999, and that, at the same time, layer III shallower and sharper, and layer III widened with time (thermally homogeneous zone) widened, forming a and its conductivity increased (Fig. 9a). Vertical con- “shoulder” at a depth of 51 m. This widening suggests ductivity profiles in layer III suggest that the layer is that warmer water was added to layer IV, and the pro- well mixed. The rise of the shoulder indicates an addi- files were pushed upward. A simple heat balance indi- tion of recharge water from the bottom, pushing bot- cates that the heat accumulated in layers III and IV tom water upward. This is consistent with the changes during 15 years (October 1986 to January 2003) is 7.8 in the temperature (Fig. 8a). By combining the con- 12 ¥ 10 J, supplying heat at an average rate of 5.1 ¥ ductivity profiles from October 1986 to January 2003 1011 J/yr (~0.02 MW). The incremental upward move- (15 years) with the bathymetry used in Kling et al. 3 ment of the lower thermocline (Fig. 8a) indicates the (2005), we calculated an overall increase of 2.7 ¥ 10 addition of water to layer IV, most likely from the bot- tons of Total Dissolved Solids (TDS) in layers III and 8 tom. If 4.1 ¥ 10 tons of water having a temperature of IV. This translates into an average annual TDS input 2 27∞C were added, it would account for the heat accurate of 1.7 ¥ 10 tons/yr. The sharp conductivity rise mulation during that period. Diffusive and conductive toward the bottom in layer IV may be related to disso- heat loss to layer II and above was not taken into con- lution and reduction of particles containing ferric com- 2+ – sideration in this simple heat balance calculation, thus pounds to release Fe and HCO3 in the sediments that giving a minimum heat supply. Note that the rate of are rich in organic material. The concentration of Fe2+, – + heat and water supply to layers III and IV initially ap- HCO3 and NH4 increased significantly with depth

Fig. 11. Comparison of the CO2 concentrations at Lake Nyos measured by the pH method (solid curve) in March 1995, those obtained by the syringe technique (red open circles, November 1993) and the cylinder technique (blue open squares, April 1992, Evans et al., 1994). Dotted curves along

the pH-based CO2 profile indicate possible errors due to an uncertainty in the pH measurement of ±0.05. Modified from figure 2 of J. Volcanol. Geotherm. Res. 97, Kusakabe, M., Tanyileke, G., McCord, S. A. and Schladow, S. G., Recent

pH and CO2 profiles at Lakes Nyos and Monoun, Cameroon: implications for the degassing strategy and its numerical simulation, 241–260, Copyright 2000, with permission from Elsevier. Fig. 10. Schematic presentation of the “MK sampler”. Re- printed from figure 2 of J. Volcanol. Geotherm. Res. 97, Kusakabe, M., Tanyileke, G., McCord, S. A. and Schladow, earlier high conductivity in layer IV, indicating the ini- S. G., Recent pH and CO2 profiles at Lakes Nyos and Monoun, Cameroon: implications for the degassing strategy tiation of mixing in the deepest zone. This tendency and its numerical simulation, 241–260, Copyright 2000, with had started in 1998, although the 1998 profile is par- permission from Elsevier. tially obscured behind the 2001 profile in Fig. 9b. From the depth of 205 m to the bottom, the conductivity increased sharply. This trend is the same as observed at the bottom water of Lake Monoun. The pre-degassing only in layer IV whereas the other ions such as Na+, increase of the conductivity in layers III and IV from K+, Mg2+, Ca2+ showed a steady increase with depth November 1986 to January 2001 (14 years) corresponds (Kusakabe et al., 2008). to an increase of 7700 tons of TDS, with the average At Lake Nyos, shallow water in November 1986 had annual input of 540 ton/yr. Initially, the input rate was a higher conductivity, even at about 7 m (Fig. 9b), than relatively high, but later decreased by at least a factor that in later years, indicating that deep, TDS-rich wa- of two as shown by the close spacing of the conductiv- ter was brought to the surface during the limnic erup- ity profiles (Fig. 9b). This temporal trend was similar tion, the effect still remaining 3 months after the limnic to that of the water temperature. eruption. This upper chemocline in November 1986 Before describing the temporal variation of CO2 pro- deepened gradually with time down to 30 m in 1988, files in the lakes, it may be worthwhile mentioning the 47 m in 1993 and 50 m in 2001. Conductivity profiles analytical methods used to determine the dissolved at mid-depths (70–160 m) stayed almost unchanged for CO2. As stated before, during the early days of our 15 years after the eruption, suggesting that transport observations, we encountered difficulties in measur- of dissolved chemical species through layer II was lim- ing CO2 from deep water. The partial pressure of dis- ited. The conductivity in layers III and IV (170–210 solved gases in Lake Nyos was so high (~1.1 MPa in m) increased notably with time (Fig. 9b). In January 1990, Evans et al., 1993) that we could not use a Niskin 2001, the conductivity profile between 185 m and 202 water sampler to collect water and gases, since the lid m became steep, with an associated slight reduction of of the sampler was forced open before retrieval due to

Fig. 12. (a) Cylinder sampler used by Evans et al. (1993). A pre-evacuated cylinder is deployed to a desired depth, and a check valve is opened to sample water. (b) Gas pressure probe used by Evans et al. (1993). Dissolved gas molecules except water diffuse through the membrane unit consisting of multiple silicone rubber tubing. The total gas pressure inside the collection chamber is measured at the surface. Reprinted from figures 3 and 4 of Appl. Geochem. 8, Evans, W. C., Kling, G. W., Tuttle, M. L., Tanyileke, G. and White, L. D., Gas buildup in Lake Nyos, Cameroon: The recharge process and its consequences, 207–221, Copyright 1993, with permission from Elsevier.

the exsolution of high-pressure dissolved gases. This bonate equilibria, it is possible to determine the H2CO3 difficulty was partially solved by attaching a gas bag (or CO2,aq) concentration from pH values (measured – to the Niskin sampler, which enabled us to collect wa- by CTD) if the HCO3 concentration at a correspond- – ter samples (Kusakabe et al., 1989), but it was still ing depth is known (Kusakabe et al., 2000). The HCO3 difficult to measure the dissolved CO2 accurately. To concentration is closely related to the electric conduc- overcome this difficulty we developed a new method tivity, so we can calculate the concentration of H2CO3 called the “the MK or syringe method” (Kusakabe et (or CO2,aq) of a water column using the CTD data. It is al., 2000). The sampler in the MK method is schemat- a big advantage that we can obtain a continuous CO2 ically shown in Fig. 10. In this technique, the total dis- profile, although very careful calibration of the pH – 2– solved carbonate species (H2CO3 + HCO3 + CO3 ) at sensor is an important prerequisite. This method is a given depth is fixed in situ in a 50-ml plastic syringe called “the pH method”. Figure 11 compares CO2 pro- containing concentrated (5 M) KOH solution. The to- files at Lake Nyos in 1995 obtained by the pH and sy- tal carbonate concentration in the alkaline solution is ringe methods. In the figure, the results obtained by determined later in the laboratory by a classical mi- the “cylinder method” are included. In the cylinder cro-diffusion method (Conway, 1958). Subtraction of method (Fig. 12a), deep water was sucked into a re- – the HCO3 concentration and the blank carbonate in motely-operated evacuated stainless steel cylinder and the KOH solution from the total carbonate concentra- the exsolved total CO2 was later analyzed in the labo- tion gives the H2CO3 (or CO2,aq) concentration. ratory (Evans et al., 1993). They also introduced an Since the total carbonate species dissolved in Lakes interesting device called a “gas-probe” (Fig. 12b) with Nyos and Monoun are essentially controlled by car- which the total gas pressure at a given depth of a water

Fig. 13. Analytical system for measuring CO2 concentrations in gassy lakes (copied from Yoshida et al., 2010). Two-phase flow (gas and water) from a plastic hose deployed to a desired depth in the lake is introduced into a separator and a gas flow meter. The amount of water accumulated in the separator and the volume of gas that goes through the flow meter in a given time are measured, from which the CO2 concentration is calculated.

Fig. 14. Evolution of pre-degassing CO2 profiles at (a) Lake Monoun (1986–2003) and (b) Lake Nyos (1986–2001). The saturation of CO2 in water at 25∞C is shown by a dashed line. Reproduced from figures 4 and 9 of Kusakabe et al. (2008), but colored. Note that the CO2 concentrations at a depth of ~55 m in 2001–2003 at Lake Monoun are close to saturation. column was measured. Figure 11 shows the satisfac- rator. The liquid phase accumulates in the separator tory agreement between the CO2 concentration ob- and is collected as the water sample, while the dry gas tained by the different methods. flows through a volumetric gas meter to measure the Recently, “the plastic hose method” (Yoshida et al., gas volume at the sampling time (Fig. 13). A gas sam- 2010) for CO2 determination has also been used. This ple collected directly from the dry gas line is perfectly is based on a self-gas-lifting principle in a plastic hose free from air contamination which is essential for no- that is deployed into the deep water of the lake. A mix- ble gas analysis (Nagao et al., 2010). A similar method ture of gas and water spouting from the hose is sepa- has been reported by Tassi et al. (2009) for gas collec- rated into liquid and gas phases by using a plastic sepa- tion from Lake Kivu.

Table 3. Change with time in CO2 content at Lakes Monoun and Nyos during the last 28 years.

Date Time aftereruption Total CO2 CO2 below layer II CO2 accumulation rate CO2 removal rate year giga mol giga mol giga-mol/yr giga-mol/yr Lake Monoun: Pre-degassing 0.38 0.38 October 1986 2.17 ææ November 1993 9.25 0.53 0.53 ææ 0.59 0.59 April 1996 11.67 ææ November 1999 15.25 0.60 0.60 ææ December 2001 17.33 0.61 0.61 ææ January 2003 18.42 0.61 0.61 0.0084 (1993-2003) æ

Lake Monoun: During-degassing January 2004 19.42 0.53 0.52 ææ January 2005 20.42 0.42 0.42 ææ June 2006 21.92 0.43 0.42 ææ January 2007 22.42 0.22 0.21 ææ December 2007 23.33 0.11 0.10 æ 0.098 (2003-2007) January 2009 24.42 0.071 0.055 ææ January 2011 26.42 0.041 0.036 æ 0.005 (2009-2011) March 2012 27.59 0.066 0.051 ææ March 2013 28.59 0.074 0.059 ææ Apr 2014 29.84 0.079 0.065 0.0048 (2011-2014) æ

Lake Nyos: Pre-degassing 13.1 12.9 November 1986 0.17 ææ 13.3 13.3 December 1988 2.33 ææ November 1993 7.25 13.6 13.6 ææ April 1998 11.67 14.1 14.0 ææ November 1999 13.25 14.4 14.0 ææ January 2001 14.42 14.8 14.6 0.12 (1986-2001) æ

Lake Nyos: During-degassing December 2001 15.33 14.2 14.0 ææ January 2003 16.42 13.1 13.1 ææ January 2004 17.42 13.2 13.0 ææ January 2005 18.42 12.3 12.6 ææ January 2006 19.42 11.8 11.7 ææ January 2007 20.42 11.6 11.4 ææ January 2009 22.42 11.2 11.1 ææ January 2011 24.42 10.0 9.7 æ 0.46 (2001-2011) March 2012 25.59 7.8 7.7 ææ March 2013 26.59 6.6 6.5 ææ March 2014 27.59 5.9 5.8 æ 1.2 (2011-2014)

This table was revised from table 1 in Kusakabe et al. (2008).

CO2 removal rate was calculated using the CO2 content below layer II during the period shown in parentheses. Data after 2011 were supplied by T. Ohba.

More recently, a new and simple method of measur- Monoun and Nyos are shown in Fig. 14. Although no – ing the total CO2 (CO2,aq and HCO3 ) has been devel- data were available in 1986 at Lake Monoun, the 1986 oped by Saiki et al. (2016). This method is based on a profile was estimated from later CO2-conductivity re- linear relationship between the total CO2 concentra- lationships (Kusakabe et al., 2008). Figure 14 shows tion and the sound velocity in lake water. that CO2(aq) concentrations in deep lake water were Temporal pre-degassing CO2(aq) variations at Lakes around 130 mmol/kg in layers III and IV, with the CO2

Fig. 15. Evolution of syn-degassing CO2 profiles at (a) Lake Monoun (2003–2014) and (b) Lake Nyos (2001–2014). The saturation of CO2 in water at 25∞C is shown by a dashed line. Recent data were added to figures 5 and 11 of Kusakabe et al.

(2008), and the figures were colored. Note that the CO2 concentrations in 2012, 2013 and 2014 in deep water at Lake Monoun have increased, indicating a re-buildup of gas. CO2 profiles at Lake Nyos have steadily subsided. The highest CO2 concentration at the bottom water in 2014 was reduced to ~150 mmol/kg. See text for details.

shoulder at a depth of ~63 m in 1986. The CO2(aq) pro- change in the bottom water is likely caused by the files evolved with time, especially from 1986 to 1993. gradual addition of recharge fluid having a CO2(aq) The thickness of layers III and IV expanded with time, concentration of ~350 mmol/kg. The CO2(aq) content supporting the hypothesis that CO2-rich recharge fluid of the lake was calculated by integrating CO2(aq) pro- was added from the bottom. In December 2001 and files over the water column below layer II using the January 2003, the CO2 shoulder at a 58 m depth (157 bathymetry in Kling et al. (2005) under the assump- mmol/kg) was very close to the CO2 saturation con- tion that the horizontal distribution of CO2 was uni- centration (Duan and Sun, 2003) at a depth of 50 m. form, as deduced from the conductivity distribution Considering that the rate at which the shoulder was (see figure 8 of Kusakabe et al., 2008) and that CO2 rising was about 1 m/yr, saturation at a 58 m depth loss through the upper chemocline was negligible. could be reached in several years. The formation of Thus, the CO2 accumulation rate can be regarded as CO2 bubbles which could induce a limnic eruption the CO2 recharge rate. Accumulation of CO2 in layer (Kozono et al., 2016) would have occurred soon after II, and in deeper layers, is tabulated in Table 3 for 2003 at Lake Monoun if no degassing operation had Lakes Monoun and Nyos. Considering that the CO2(aq) been undertaken. profile in October 1986 at Lake Monoun was estimated Figure 14b shows the temporal variation of CO2(aq) in an indirect way (Kusakabe et al., 2008), the overall profiles between November 1986 and January 2001 at rate of CO2 accumulation below the upper chemocline Lake Nyos. The general features of Fig. 14b are sum- was calculated using the 1993 to 2003 profiles. The marized as: (i) CO2(aq) concentration was lowest in the change in CO2(aq) content below layer II in the main early days after the explosion; (ii) there was little basin for the pre-degassing period (1993 to 2003) was change with time at mid-depths (~50 m to 150 m); (iii) ~80 (=610–530) Mmol, with a CO2 recharge rate of the greatest change took place at a depth of >170 m, 8.4 Mmol/yr. Almost the same recharge rate of 8.2 where the CO2(aq) concentration at a given depth in- Mmol/yr was reported by Kling et al. (2005) using their creased significantly with time; and (iv) the CO2(aq) own data obtained between 1992 and 2003 for Lake concentration at the bottom-most water was almost Monoun. At Lake Nyos, the CO2(aq) content below layer constant, near 350 mmol/kg since 1999. The constancy II steadily increased by ~1.7 Gmol until January 2001, of the bottom water CO2(aq) concentration was con- when permanent degassing started. The increase in the firmed by later syn-degassing measurements. The CO2(aq) content can be translated into the CO2 recharge

Fig. 16. The reduced height of fountains from degassing pipes at Lakes Nyos and Monoun. Three degassing pipes were in operation at Lake Nyos as of March 2014 (c). Gas self-lift capability was lost in January 2009 at Lake Monoun, leaving a weak bubbly flow from the neck of the pipe (f).

rate, which was 0.12 Gmol CO2/yr between Novem- mmol/kg and ~15 m, respectively, in 2011 (Kusakabe ber 1986 and January 2001 (Table 3). Again, the rate et al., 2011). In 2009, two of three degassing pipes is in good agreement with the value of 0.13 Gmol CO2/ stopped working completely, and the other pipe issued yr given by Kling et al. (2005). only a weak bubbly flow (Fig. 16f). Thus, it can be said that the degassing pipes at Lake Monoun have al-

3-2. Syn-degassing evolution of CO2 content and most lost their gas self-lift capability. Moreover, re- future prospect cent observations (2011–2014) show that CO2 concentrations below 80 m and the layer III thickness are in- The CO2(aq) profiles between 2001 and 2011 at both creasing (Fig. 15a), clearly indicating that natural CO2 lakes during syn-degassing are shown in Fig. 15. Gen- recharge into Lake Monoun still continues. This con- erally speaking, degassing went smoothly, as illustrated firms our prediction that CO2 re-buildup is inevitable by the steady subsidence of the CO2(aq) profiles. This if lake degassing stops. On the basis of a geochemical resulted in a lowering of the fountain height at both study on the generation of CO2 in the Nyos mantle, lakes (Fig. 16). The subsidence has continued up to Aka (2015) has suggested that CO2 will be continu- the present time; however, a buildup of CO2 has re- ously supplied into the lake for a geologically long time sumed recently at Lake Monoun (see below). The over- in the future. This view may also apply to Lake all shape of the profiles did not change with degassing, Monoun. In order to avoid gas re-buildup and to make showing that only bottom water and dissolved CO2(aq) the lake continualy safe, Yoshida et al. (2010) sug- were removed without causing any effect on the strati- gested continuously removing the bottom water that fication of the lake water. At Lake Monoun (Fig. 15a), contains the CO2 at a significantly high concentration. the highest CO2(aq) concentration at the bottom de- The installation of such a bottom water removal sys- creased to 80 mmol/kg, and the thickness of layer III tem was undertaken at Lake Monoun in December 2013 reduced to ~20 m in 2009, and further reduced to ~70 (Yoshida et al., 2016), and details of the system are

Fig. 17. (a) Change with time in the CO2 content at Lakes Monoun (a) and Nyos (b). Modified from figures 6 and 12 of Kusakabe et al. (2008) to which recent data were added. The blue and red circles denote pre-degassing and syn-degassing evolution, respectively. Note that the CO2 content at Lake Monoun started to rise after 2011 at a rate of ~4.8 Mmol/yr, approximately half of the natural CO2 recharge rate of 8.4 Mmol/yr estimated from the pre-degassing data. The degassing rate at Lake Nyos was accelerated after installation of 3 pipes. The CO2 content at Lake Nyos will attain a minimum in several years time.

described in Section 5. tion started. Degassing was effective, reducing the At Lake Nyos (Fig. 15b), CO2(aq) profiles subsided amount of dissolved CO2 at a mean gas removal rate steadily until January 2011, resulting in a very thin of 98 Mmol/yr between January 2003 and December layer III by that time. As two more degassing pipes 2007 (see Table 3). This rate is approximately 12 times with a greater diameter (25.7 cm I.D.) were installed greater than the natural recharge rate as shown by the in December 2011–March 2012, the degassing rate was sharp slope (Fig. 17a). The installation of two addi- greatly enhanced, resulting in a rapid decrease of CO2 tional pipes in April 2006 accelerated the gas removal concentration in deep water in the subsequent years rate. In January 2009, the system had almost lost its (2011–2014). We can expect that most of the CO2-rich gas self-lifting capability, resulting in a reduction of bottom water will disappear from Lake Nyos in sev- gas removal rate to only 5 Mmol/yr in January 2011 eral years from now and that the gas self-lift capabil- (Table 3), although a very weak flow of bubbly water ity will be lost as in Lake Monoun. from one of the three pipes was still visible. At that Using CO2(aq) profiles and lake bathymetry (Kling time, the amount of CO2(aq) dissolved in deep water et al., 2005), the amount of CO2(aq) dissolved below was 55 Mmol, or only 9% of the maximum value ob- the upper chemocline (layers II, III and IV) was calcu- served in 2003 (Kusakabe, 2015). Based on these ob- lated as a function of time since the limnic eruption at servations, it can be said that Lake Monoun has been both lakes (Fig. 17). The amount of dissolved CO2 in made safe. However, the tailing-off of the CO2 con- Lake Monoun (Fig. 17a) increased steadily at a rate of tent after 2009 (Fig. 17a) implied that a buildup of 8.4 Mmol/yr, reaching a maximum value of 610 Mmol CO2 is inevitable at Lake Monoun if the natural re- in January 2003, shortly before the degassing opera- charge of CO2 continues at the previously estimated

Fig. 18. dD-d18O values of monthly collected rain waters (dark asterisks) and ground waters (circles and triangles) sampled in the vicinity of Lake Nyos are shown (Kamtchueng et al., 2015a). Those of Lake Nyos waters (Nagao et al., 2010) are also included. All values are consistent with the global meteoric water line of Rozanski et al. (1993), although the Lake Nyos waters are plotted slightly upper-right of the cluster. It may suggest that the lake water is not recharged by recent groundwater.

rate. Indeed, the re-buildup of CO2 became obvious to 1.2 Gmol/y (Table 3). It is hoped that most of the after March 2012 (Fig. 17a). Using the data between remaining gas will be removed within the next 5 years 2010 and 2014, the rate of gas re-buildup is calculated or so. At the last stage of the degassing operation, the to be ~4.8 Mmol/yr which is about half of the CO2 rate of gas removal will decrease due to a lower CO2 recharge rate of 8.4 Mmol/yr calculated from the pre- concentration at the intake depth. This will lead to gas degassing data (Table 3), although we need to accu- re-buildup, as we have seen at Lake Monoun. Thus, a mulate more recent data for a more reliable determi- system to pump up CO2-rich bottom water needs to be nation of the rate of gas re-buildup. The current amount set up after the current degassing system has lost its of total dissolved CO2 in Lake Monoun is 79 Mmol gas self-lifting capability. (as of April 2014) which is ~13% of the maximum pre- degassing value recorded in 2003. It may take another 3-3. Hydrogen, oxygen, carbon, and noble gas iso- ~100 years to reach the pre-degassing situation if the topic signatures current rate of gas re-buildup remains unchanged. For these reasons, it is essential to continue monitoring the The hydrogen and oxygen isotopic ratios of Lake lake on a regular basis. Nyos waters were first reported by Kling et al. (1987). The evolution of CO2 content over time since 1986 The data for Lakes Nyos, Monoun and Wum (a crater at Lake Nyos is shown in Fig. 17b. The gas removal lake near Lake Nyos) were later added by Kusakabe et rate by a single pipe (0.46 Gmol/yr) is about four times al. (1989) and Nagao et al. (2010). The isotopic deter- greater than the natural recharge rate of 0.12 Gmol/yr. mination was intended to find any input of volcanic At this removal rate, however, it would take another gases into Lake Nyos, for volcanic gases are usually 18 20 years or so to remove all the gas from the lake. For- characterized by high d O signatures, but the data did 18 tunately, using funds from the Government of not indicate any volcanic input. The dD and d O val- Cameroon and UNDP, two additional degassing pipes ues from Lake Nyos plot close to the Global Meteoric were installed in early 2011. Since pipes with a greater Water Line (Craig, 1961; Rozanski et al., 1993) when diameter (25.7 cm) were used and the water intake combined with the data for rain water, groundwater, depth was increased to close to the bottom for the ad- and surface water recently collected in the Lake Nyos ditional pipes, the rate of gas removal greatly increased catchment area as shown in Fig. 18, although the lake

3 4 40 36 3 4 Fig. 19. Relationship between He/ He (in Ratm) and Ar/ Ar ratios of waters in Lakes Nyos and Monoun. Ratm is a He/ He ratio of sample relative to that of air (=1.4 ¥ 10–6). The plots show a mixing of magmatic gases and the atmosphere (green cross). See text for a discussion. Data from Nagao et al. (2010).

water is slightly more enriched in heavy isotopes than et al. (1999), Aka (2000) and Aka et al. (2004) pub- the ground and surface waters (Kamtchueng et al., lished a detailed study of noble gases in basalts and 2015a). xenoliths from CVL volcanic rocks, showing a sym- Noble gas information was used to constrain the ori- metrical distribution of 3He/4He ratios along the CVL 3 4 –6 gin of CO2 dissolved in the lakes, for noble gases do (Fig. 20). The lowest He/ He ratios (4.5 ¥ 10 or ~3 not react with rocks and waters on the way to the Ratm) were found at Etinde, a small volcano next to Earth’s surface. Figure 19 shows the relationship be- Mt. Cameroon, located at the oceanic and continental tween 3He/4He and 40Ar/36Ar ratios for Lakes Nyos and boundary. The 3He/4He ratios become close to the 13 Monoun gases in 1999. Coupled with the d C values MORB values (7~9 Ratm) as we go away from the oce- of –3.3 ~ –3.4‰ (relative to Vienna Pee Dee Belemnite, anic and continental boundary towards both ends (Aka VPDB) for Lake Nyos and –6.8‰ for Lake Monoun, et al., 2004). This symmetric isotopic variation was the helium and argon isotopic ratios suggest a strong explained as reflecting the geochemical characteristics affinity of the dissolved gases with a magmatic source of the mantle, or the continental lithosphere underneath (e.g., Kusakabe and Sano, 1992). Nagao et al. (2010) the boundary which is of a HIMU character (Halliday reported more precise data using air-contamination free et al., 1988). HIMU is geochemical jargon for “high- 3 4 238 204 samples. The He/ He ratios in the gases in the Lake m” with m defined as the ratio of U/ Pb. HIMU Nyos deep waters are ~5.7 Ratm, where Ratm is the at- mantle is characterized by an enrichment in U and Th, –6 3 4 mospheric ratio of 1.4 ¥ 10 . The Lake Nyos He/ He the parent elements of radiogenic Pb and He. Melts ratios are lower than the typical mantle values of 7~9 derived from this HIMU mantle are postulated to have Ratm for depleted mantle producing Mid-Oceanic Ridge been emplaced beneath the oceanic and continental basalts (MORBs) (Graham, 2002). The reasons why boundary. Thus, rocks at the oceanic and continental the 3He/4He ratios of Lake Nyos are lower than the boundary are high in 206Pb/204Pb and low in 3He/4He mantle values are related to the sub-lithospheric struc- ratios. Mineral separates from rocks around Lake Nyos ture beneath the Cameroon Volcanic Line (CVL) as (clinopyroxene and amphibole in xenolith) have 3He/ 4 discussed later (Section 6). Halliday et al. (1988) re- He ratios of 6.7~7.0 Ratm (Aka et al., 2004), slightly ported variations in the radiogenic isotopic ratios (Pb, lower than the typical MORB values, implying a small Nd, and Sr) of volcanic rocks along the CVL, where degree of the HIMU character of the magma source the highest 206Pb/204Pb and 208Pb/204Pb ratios were beneath Lake Nyos. found at the oceanic and continental boundary. Barfod The deep water of the lake has even lower values of

Fig. 20. Symmetrical distribution of 3He/4He and 206Pb/204Pb ratios of rocks along the CVL as a function of distance from Annobon. This distribution suggests a contribution of the HIMU mantle for magma genesis in the oceanic and continental boundary volcanoes (Halliday et al., 1988; Aka et al., 2004). Oceanic sector volcanoes are Annobon (AN), São Tomé (ST) and Principé (PP). Ocean/continent boundary volcanoes are Bioko (BK), Etinde (ETD) and Mount Cameroon (MC). Continental sector volcanoes are Manengouba (MB), Bambouto (BT), Oku (OK), and Ngaoundere (ND).

Fig. 21. Sampling water and gas using the “Flute de Pan” (a). Water and gas gushing out of 11 plastic hoses (O.D. of 15 mm) with different intake depths were collected (b).

~5.7 Ratm, as was stated above. This low ratio may mean CO2-rich waters compared to air so that any samples that He in deep water was originally derived from exposed to the atmosphere during sampling, or stor- magma generated from a slightly HIMU-type mantle, age in an improper way, are not good for analysis. Sam- but acquired radiogenic 4He on the way from the source ples from Lake Nyos (January 2001) were collected magma to the sub-lacustrine fluid reservoir during the using the “Flute de Pan” which had been deployed by passage of the magmatic fluid through granitic base- the French scientific team. This consisted of 11 plastic ment rocks. Nagao et al. (2010) reported the distribu- hoses having an outside diameter of 15 mm with dif- tion of isotopic ratios of not only He but also Ne, Ar, ferent intake depths (83–210 m) (Fig. 21). CO2-rich Kr, Xe and C in Lakes Nyos and Monoun waters col- gas spouting out of a given hose was collected in a lected at closely separated depths. They stressed the glass bottle using an inverted funnel placed in a bucket. importance of samples that were free from air-contami- Although slight contamination of air dissolved in the nation, because noble gas concentrations, especially water was still suspected to some extent, especially for those of Ar and Ne, are so low in gases exsolved from heavy noble gases, this sampling method was found to

Fig. 22. Depth profiles of noble gas concentrations in water (10–6 ccSTP/gwater) measured in 2001 at Lake Monoun (a) and Lake Nyos (b). Noble gas concentrations in air saturated water (ASW) at 30∞C (table 2 in Kipfer et al., 2002) are also shown by arrows for comparison. Modified from figures 1 and 2 of Nagao et al. (2010). be promising. In the December 2001 sampling, a plas- This method was later modified to measure the total tic hose method, essentially the same as the Flute de gas concentration on site (Yoshida et al., 2010). Pan method, was adopted. A single plastic hose (12 Figures 22a, b illustrate the profiles of He, Ne, Ar, mm I.D.) was deployed initially to the bottom, followed Kr and Xe in water, measured in 2001, at Lake Monoun by pulling it upward little by little to a desired depth. and Lake Nyos, respectively. Except for He, they show Exsolved CO2 gas was directly allowed to pass through roughly constant concentrations with respect to depths a sampling bottle made of uranium glass that has a low below 80 m at Lake Nyos, and 50 m at Lake Monoun. He diffusivity. With this method, Nagao et al. (2010) The Ne, Ar, Kr and Xe concentrations are up to sev- were able to collect air-contamination free samples. eral times lower than those in air saturated water

Fig. 23. 3He/4He ratios as a function of depth at Lakes Nyos and Monoun in 1999 and 2001. Modified from figure 4 of Nagao et al. (2010).

(ASW). Depth profiles of the 3He/4He ratio for Lakes tle Ar to the sub-lacustrine Ar may be less than 20% Nyos and Monoun are presented in Fig. 23. The data assuming that the mantle Ar has a 40Ar/36Ar ratio of published by earlier workers are in the same range >1650. This is consistent with the conclusion derived (Kling et al., 1987; Sano et al., 1987, 1990; Kusakabe from the Ne signature (Fig. 24), although the contri- and Sano, 1992). Generally speaking, the 3He/4He ra- bution of mantle Ne to the sub-lacustrine Ne may be tios are almost constant in the depth range of 80–210 ~6%. At Lake Nyos, the highest 40Ar/36Ar ratio, of m at Lake Nyos, and 40–100 m at Lake Monoun. The about 600, was found at the bottom (210 m). The 40Ar/ 3He/4He ratio approaches the atmospheric value in 36Ar profile in January 2001 decreased gradually to- waters shallower than 80 m and 40 m for Lakes Nyos wards the surface approaching the atmospheric ratio, and Monoun, respectively. High 4He/20Ne ratios up to but it showed a zigzag pattern below the lower ~1500, as shown in Fig. 22, support the premise of chemocline at ~180 m with a second maximum value magmatic gas input to the lake as inferred by high 3He/ of 480 at 190 m. The zigzag 40Ar/36Ar profile disap- 4He ratios. peared in December 2001 with consistent ratios around Neon isotopic ratios are presented in Fig. 24. Com- 530 below 190 m. This may have resulted from verti- pared to atmospheric Ne, small excesses of both the cal mixing in this depth range caused by degassing, 20Ne/22Ne and 21Ne/22Ne ratios are observed. Most data because water was pumped out by the degassing pipe points for both lakes lie on the MORB line connecting from an intake depth of 203 m. A tendency for such atmospheric Ne and mantle Ne as reported by homogenization was also observed in the water tem- Staudacher and Allègre (1988). The data clearly indi- perature and electric conductivity at the correspond- cate the presence of mantle Ne in the lakes, and are ing depths (Kusakabe et al., 2008), although they were consistent with the conclusions of Barfod et al. (1999) less clear than the noble gas profiles. At Lake Monoun, and Aka et al. (2004) that the CVL mantle contains the 40Ar/36Ar ratios were in a narrow range of about MORB-like Ne. 470 between 60 m and 100 m (bottom) (Fig. 25). The Argon isotopic ratios are presented in Fig. 25. The ratios are lower than those in the deep waters of Lake 40Ar/36Ar ratios for all samples are higher than the at- Nyos, suggesting that the contribution of atmospheric mospheric value of 296, but much lower than the esti- Ar to the magma-originating gases at Lake Monoun is mated value of >1650 for Ar in the upper mantle be- greater than that at Lake Nyos. neath the CVL (Barfod et al., 1999). This means that The characteristic features of noble gases observed magmatic fluids containing mantle Ar mixed with at- at Lake Nyos can be summarized as follows (Nagao et mospheric Ar on the way to the surface such as in a al., 2010): (i) Helium in the lake water derived origi- 3 4 sub-lacustrine fluid reservoir. The contribution of man- nally from the mantle where He/ He ratios of ~7 Ratm

Fig. 24. 20Ne/22Ne versus 21Ne/22Ne plot for samples collected in 2001 at Lakes Nyos and Monoun. The “mass fractionation line” indicates the isotopic trend of atmospheric Ne due to mass fractionation. The dashed line heading to MORB represents a mixing line between atmospheric Ne and Ne in MORB or the upper mantle (Ballentine et al., 2005). Modified from figure 5 of Nagao et al. (2010).

Fig. 25. Depth profiles of 40Ar/36Ar in Lakes Nyos and Monoun in 2001. Chemoclines were taken from Kusakabe et al. (2008). Modified from figure 6 of Nagao et al. (2010).

are found in mantle xenoliths (Aka et al., 2004), but, 1999) on the way to the lakes. The most likely source on its way to the surface, approximately 20% of radio- of Ar to reduce the mantle 40Ar/36Ar ratio is atmos- genic 4He that accumulated in crustal rocks was ad- pheric Ar-bearing groundwater. (iii) Ne in the lakes mixed to give ratios of ~5.7 Ratm, probably in the sub- may be a mixture of atmospheric Ne and a small amount lacustrine region. (ii) The observed 40Ar/36Ar ratios of of MORB-like Ne from the mantle. The observed He, 450–550 are also explained by the addition of atmos- Ne and Ar isotopic ratios in lake waters can be best pheric Ar (40Ar/36Ar = 296) carried by groundwater to explained by mixing between two noble gas reservoirs, mantle-originating Ar (40Ar/36Ar >1650, Barfod et al., i.e., air dissolved groundwater and the mantle. It is

Fig. 26. (a) “Inflating” CO2 profiles in the deep water of Lake Nyos in the depth range of 160–210 m between 1986 and 2001. (b) 3He profile in the same depth range observed in 2001. Note the sharp maximum of 3He concentration at 188 m where a chemocline (dashed line) existed in 2001. Modified from figure 14 of Volcanic Lakes (Dmitri Rouwet, Bruce Christenson,

Franco Tassi, Jean Vandemeulebrouck, eds.), Evolution of CO2 content in Lakes Nyos and Monoun, and sub-lacustrine CO2- 3 recharge system at Lake Nyos as envisaged from CO2/ He ratios and noble gas signatures, 2015, pp. 427–450, Kusakabe, M., „ Springer-Verlag Berlin Heidelberg 2015 with permission of Springer. conceivable that the mantle-derived gases with the those of 3He in the same depth range, because of the addition of radiogenic 4He from crustal rocks and at- almost constant 3He/4He ratios (Fig. 23), although the mospheric Ar and Ne carried by groundwater are fi- 4He concentrations are not graphically shown. Between nally homogenized in the sub-lacustrine reservoir. Note 190 and 210 m, the 3He concentrations are nearly con- –10 3 that the contribution of atmospheric He to deep lake stant at ~5 ¥ 10 ccSTP/g-water. The He concentra- water, if any, is difficult to find, since the He concen- tion gradually decreases as the depth decreases (layer tration in deep lake water is more than 3 orders of II). magnitude higher than that in ASW, whereas the con- The C/3He ratios of volcanic fluids have been widely tribution of the other noble gases is more easily dis- used to constrain magma sources. The C/3He ratios of cernible because of their similar concentrations in deep MORB glasses are shown to be fairly constant at 0.20 10 lake water and ASW (see Fig. 22). (±0.05) ¥ 10 , suggesting that the source region of As stated previously, the greatest chemical change MORB in the upper mantle has little variation in the in Lake Nyos took place at depths greater than 180 m. C/3He ratio (Marty and Jambon, 1987). The ratios for The CO2 profiles (1986–2001) in the depth range 160– volcanic gases from subduction volcanism, however, 210 m are enlarged in Fig. 26a. This shows that the have been found to be significantly greater than the 10 10 increase of CO2 concentration in the deep water of Lake MORB values, i.e., 0.7 ¥ 10 ~ 3 ¥ 10 . These high 13 Nyos after the 1986 limnic eruption resulted from wid- ratios, coupled with d C values, indicate the existence ening of CO2-rich water leading to the formation of a of recycled carbon (marine carbonates, slab carbon- clear lower chemocline at the top of the CO2-rich wa- ates and/or organic materials) in subduction zone mag- ter. The 3He concentration observed in 2001 in the same mas (Sano and Williams, 1996, and references therein). 3 depth range was compared with the CO2 profiles (Fig. Figure 27 shows the C/ He ratios in the depth range 26). The 3He profile was obtained from the 4He pro- 160–210 m in Lake Nyos. The C/3He ratios range from 3 4 10 file (Fig. 22) and the He/ He profile (Fig. 23). It 0.5~1.7 (¥ 10 ). These values are higher than the man- 3 10 should be noted that the He concentration below 160 tle values of ~0.2 ¥ 10 . It is interesting to note that m in January 2001 and December 2001 shows a sharp the C/3He ratios in waters below the lower chemocline 10 maximum at around 188 m with a concentration up to are significantly high at around 1.6 ¥ 10 , and sharply –10 10 9.1 ¥ 10 ccSTP/g-water (December 2001) (Fig. 26b). decrease to 0.5 ¥ 10 above the lower chemocline. 4 3 The He concentrations have a pattern very similar to Thus, the behavior of CO2 and He are decoupled be-

Fig. 27. C/3He atomic ratios observed in the depth range of 160–210 m at Lake Nyos in 2001. The C/3He ratios were calculated 3 from the CO2 and He profiles shown in Fig. 26. A clear difference is seen across the chemocline (dashed line). Modified from figure 15 of Volcanic Lakes (Dmitri Rouwet, Bruce Christenson, Franco Tassi, Jean Vandemeulebrouck, eds.), Evolution of 3 CO2 content in Lakes Nyos and Monoun, and sub-lacustrine CO2-recharge system at Lake Nyos as envisaged from CO2/ He ratios and noble gas signatures, 2015, pp. 427–450, Kusakabe, M., „ Springer-Verlag Berlin Heidelberg 2015 with permission of Springer.

3 Fig. 28. (a) Change with time of the CO2/ He ratio in fumarolic gases from Mammoth Mountain in the Long Valley caldera, California (1988–1998). (b) Change with time of the He concentration in the same gases. Data of Sorey et al. (1998) were used.

Fig. 29. Schematic presentation of the sub-lacustrine fluid reservoir which is encircled by a green circle. The geological cross- section of Lake Nyos was taken from Lockwood and Rubin (1989). Blue arrows indicate the possible flow of groundwater, and red arrows indicate a magmatic fluid coming from the magma underneath. Noble gas and carbon isotopic ratios of respec- tive reservoirs are shown. Modified from figure 16 of Volcanic Lakes (Dmitri Rouwet, Bruce Christenson, Franco Tassi, Jean

Vandemeulebrouck, eds.), Evolution of CO2 content in Lakes Nyos and Monoun, and sub-lacustrine CO2-recharge system at 3 Lake Nyos as envisaged from CO2/ He ratios and noble gas signatures, 2015, pp. 427–450, Kusakabe, M., „ Springer-Verlag Berlin Heidelberg 2015 with permission of Springer.

3 low and above the chemocline. The cause(s) of the with time in the CO2/ He ratio has been observed in decoupling may be explained by the underplating of fumarolic gases from Mammoth Mountain in the Long the recharge fluid from the bottom that is character- Valley caldera, California, where “tree-kill” took place 3 ized by different C/ He ratios. By “underplating”, I due to an anomalous discharge of magmatic CO2 into meant that the recharge fluid is added to the bottom- soils (Farrar et al., 1995; Sorey et al., 1998). This most water from beneath. It is possible that the ratio anomalous CO2 discharge was induced by an episode was low before the limnic eruption and high after the of shallow dyke intrusion beneath Mammoth Moun- 3 limnic eruption. At the time of the limnic eruption, the tain in 1989–1990. The CO2/ He ratios of the fumarolic 10 10 lake was not completely mixed, suggesting that deep gases there changed from ~0.3 ¥ 10 to 1.6 ¥ 10 in water still contained a large fraction of “pre-eruption” about 10 years (Fig. 28). The change was caused by a water (Giggenbach, 1990; Tietze, 1992; Evans et al., trend of decreasing He concentration and little change 1994) which may have been proportionally higher in in the concentration of CO2, which is a major compo- 3 3 4 He and lower in CO2 concentrations with the CO2/ He nent of the gases. The He/ He ratios stayed at around 10 ratio of ~0.5 ¥ 10 . Recharge fluids entering the lake 5.5 Ratm with a few exceptions. These observations 3 after the eruption may have a CO2/ He ratio of ~1.6 ¥ indicate that the above geochemical parameters (CO2/ 10 3 3 10 . This interpretation implies that the CO2/ He ra- He ratio and He concentration) that carry information tio in the recharge fluids may vary with time and has about magmatic fluids can change within a geologi- changed from low to high values with time. A change cally very short period of time, i.e., in the order of 10

doi:10.5047/gems.2017.00101.0001 © 2017 TERRAPUB, Tokyo. All rights reserved. 28 M. Kusakabe / GEochem. Monogr. Ser. 1: 1–50, 2017 years, at a single volcanic system. Thus, it is conceiv- between tritium and TDS was interpreted to reflect the 3 able that the decoupling of CO2 and He observed at destruction of the pre-existing gradient at mid-depth Lake Nyos after the limnic eruption was caused by the during the eruption, suggesting that CO2 exsolved from addition of “recent” recharge fluids that were charac- deep water. In their model, the upper chemocline was terized by relatively low 3He concentrations. From the placed at ~50 m depth, similar to the chemocline depth foregoing discussions based on noble gas signatures observed in January 2001, 14 years after the limnic and C/3He ratios, we can envisage the sub-lacustrine eruption, and just prior to the initiation of artificial CO2-recharge system at Lake Nyos to be as shown in degassing (see Fig. 9). Some triggers, such as a com- Fig. 29. bination of seasonal decline in the water column stability, landslide and/or seiche, pushed water upward at 4. Limnic eruption, models, triggers and cyclicity a layer around the chemocline to the CO2 saturation depth. Bubble formation then followed and relatively 4-1. Models quiet degassing continued. A local reduction in the hydrostatic pressure beneath the release area created a Many hypotheses have been put forward to explain rising column of shallow, slightly gassy water. This why the limnic eruption occurred. Sigurdsson et al. was followed by mixing with pre-release surface wa- (1987) proposed that a landslide slumped into deep ter (low TDS) to form the surface water that was ob- water, pushed CO2-rich water up and induced the 1984 served soon after the limnic eruption. The base of the limnic eruption at Lake Monoun. The same idea was column became slowly deeper, bringing CO2-rich, more also suggested for the 1986 Lake Nyos event (Kling et saline deep water upward. When the base of the col- al., 1987). Tietze (1987) suggested supersaturation of umn reached the deeper chemocline, below which CO2 dissolved CO2 just below the shallowest chemocline and TDS concentrations were much higher, gas release (~8 m depth in 1986) to be the main cause of the erup- became more violent and created wave damage along tion. The strong density stratification of this layer the lake shore such as the flattening of vegetation and worked as a lid for rising gases, inhibiting them from the passing of water over an 80-m-high promontory in penetrating this density divide. The supersaturation that the southern part of the lake. The duration of this vio- followed led to the exsolution of gases to form a foun- lent fountaining was short (<1 min), and the amount tain. This process was self-intensified and deeper wa- of CO2 released was estimated to be 6.3 Gmoles. This ter was steadily degassed in turn. Since the water from scenario is consistent with the testimonies of survivors. the fountain was cooler than the surface water, it sank Giggenbach (1990) proposed that the gas release at around the fountain, forming a cylindrical “density Lake Nyos was triggered by a climatic factor. The de- wall”. This wall limited lake-wide exsolution of gases, scent of a parcel of unusually cold rain water (18.5∞C) leaving CO2 dissolved in deep water (>150 m?) intact pushed initially CO2-rich shallow water upward. The during the eruption. Assuming that Lake Nyos was iso- uplift of the CO2-rich water above the saturation depth thermal and fully saturated with CO2, Kanari (1989) induced bubble formation which accelerated upward presented a fluid-dynamics model to explain how the movement by a reduction of density, leading to the for- limnic eruption proceeded. In his model, degassing mation of a convecting water flow that entrained started from the bottom but was confined to a limited deeper, more CO2-enriched water, and, finally, to the area at the surface. Circulation of water was confined limnic eruption. Less-dense degassed waters accumu- in small cells that stacked at various depths. Accord- lated at the surface, making it difficult for deeper CO2- ing to this model, stratification within the lake was rich water (>100 m) to reach the surface, thus termi- hardly affected. Kanari estimated that (i) the released nating the eruption. Deep water CO2 was therefore left 3 gas volume (0.68 km ) was the difference between the almost intact. The amount of CO2 released was esti- saturation and the CO2 profile observed in 1986 by mated at 5.4 Gmoles. Kusakabe et al. (1989), (ii) the maximum height of the In contrast to the previous models for the cause of gas cloud was 110 m, and (iii) the speed of the gas the limnic eruption, spontaneous exsolution of dis- cloud running down the valley was 19 m/s. However, solved gases has been suggested by Kusakabe et al. later observations indicated that full CO2 saturation (2008) and Kusakabe (2015). In this scenario, atten- over the entire lake was unlikely (Kusakabe et al., tion was paid to the pre-degassing evolution of dis- 2008). solved CO2 at Lake Monoun (see Fig. 14a) which in- Obviously, any degassing model depends on a knowl- dicated that CO2(aq) profiles evolved with time and that edge of the pre-eruption distribution of CO2 in the lake. CO2-rich layers below the lower chemocline (layers Evans et al. (1994) proposed a model based on a linear III and IV) widened due to the continuing recharge of pre-eruption relationship between CO2 and TDS (total CO2-charged fluid from beneath. Note that the CO2(aq) dissolved solids) at Lake Nyos using water chemistry, concentration in water below layer III was constant at CTD measurements, gas analyses and tritium profiles ~150 mmol/kg. In January 2003, just prior to the ini- obtained between 1987 and 1992. A linear relationship tiation of the degassing operation, the CO2(aq) concen-

doi:10.5047/gems.2017.00101.0001 © 2017 TERRAPUB, Tokyo. All rights reserved. M. Kusakabe / GEochem. Monogr. Ser. 1: 1–50, 2017 29 tration immediately below the chemocline at the boundary between layers II and III was very close to saturation. If no degassing operation were undertaken, the saturation of CO2(aq) would have been attained at that depth in a short time (within several years), and bubble formation would have followed by additional input of the recharge fluid. Thus, at Lake Monoun another limnic eruption could have occurred spontaneously within several years after 2001, if any external trigger had been introduced to this critical situation.

4-2. Spontaneous eruption hypothesis

The evolution of pre-degassing CO2 profiles at Lake Monoun gives a clue to estimate a pre-eruptive CO2 profile at Lake Nyos. It is conceivable that it was similar in shape to the 2001–2003 profiles at Lake Monoun (see Fig. 14a). It is interesting to note that the CO2 profiles at the deep layer of Lake Nyos (>180 m in Fig. 30. A model of the spontaneous limnic eruption at Lake 1999 and 2001; Fig. 14b) was developing in a way Nyos. An assumed pre-eruption CO2 profile is shown by red similar to that observed at Lake Monoun. The thick- small open circles as “Before 1986”. After the eruption, the ness of CO -rich water close to the bottom kept in- 2 CO2 profile turned to the post-eruption profile shown as creasing after the 1986 eruption till 2001 due to the “Nov. 1986” (process 1). It evolved to the January 2001 pro- addition of the recharge fluid from beneath. The CO2(aq) file (blue) in 15 years (process 2). If the natural recharge concentration below 195 m reached 350 mmol/kg in continues, the January 2001 profile may “recover” the pre- 1999. This concentration remained unchanged until eruption situation (process 3). Modified from figure 9 of January 2011 down to the bottom (Kusakabe et al., Volcanic Lakes (Dmitri Rouwet, Bruce Christenson, Franco Tassi, Jean Vandemeulebrouck, eds.), Evolution of CO con- 2008; Kusakabe, unpublished data). This observation 2 tent in Lakes Nyos and Monoun, and sub-lacustrine CO2- suggests that the CO2(aq) concentration of the recharge 3 recharge system at Lake Nyos as envisaged from CO2/ He fluid is constant at ~350 mmol/kg. If no degassing was ratios and noble gas signatures, 2015, pp. 427–450, undertaken, and if the natural recharge of CO contin- 2 Kusakabe, M., „ Springer-Verlag Berlin Heidelberg 2015 ued as before, the thickness of the bottom CO2-rich with permission of Springer. water would have continued to increase, and the top level of the CO2-rich layer could have eventually reached saturation at some shallower depth. This speculation is schematically presented in Fig. 30. In this generated from a growing CO2-saturated surface (the model, the pre-eruption profile, shown as “Before top of the “Before 1986” curve in Fig. 30) can reach 1986”, has a shoulder that touches the saturation curve the lake surface with a high flux of CO2, i.e., limnic at a depth of ~110 m. A limnic eruption would take eruption, if any external forcing triggers bubble for- place spontaneously, releasing the dissolved gases to mation at the growing CO2-saturated surface. The trig- the atmosphere, resulting in a CO2 profile shown as ger may be an instability caused by double diffusive “November 1986” in Fig. 30 (process 1). The observed convection (Schmid et al., 2004), or a seiche near the evolution of the CO2 concentration between Novem- CO2-saturated surface where the density gradient is ber 1986 and January 2001 is shown as “process 2” in strong. Fig. 30. If no degassing took place, and if the natural If our model is correct, the difference between the recharge of CO2 continued as before, the CO2(aq) pro- pre- and post-eruption profiles integrated over the lake file would have shifted upward following “process 3”, volume gives the amount of gas released at the time of and would eventually have touched the saturation the eruption, which was calculated to be ~14 Gmol or curve. Saturation is a necessary condition, but may not 0.31 km3 (at STP). This value is greater than the esti- be a sufficient condition, for a limnic eruption to take mate of 0.14 km3 by Evans et al. (1994) by a factor of place. Rising bubbles may re-dissolve in under-satu- ~2, but significantly smaller than earlier estimates 3 rated water during ascent. However, if sufficient CO2 (0.7~1 km ) by Faivre Pierret et al. (1992), and Kanari flux is given, the bubbles can reach the surface, possi- (1989). The estimated amount of CO2 released obvi- bly leading to a limnic eruption. Based on the above ously depends on the assumptions involved. As long model, a numerical approach to the recurrence of a as the lake receives a continual natural recharge of CO2, future limnic eruption was made by Kozono et al. limnic eruptions can occur repetitively (Tietze, 1992), (2016). They demonstrated that a plume of bubbles but may not be regular as described in the model of

Chau et al. (1996), which considered a possible varia- formed their Fon, who proclaimed a day when they tion in the rate of the natural recharge of CO2. How- would all go into the lake to catch the fish. The day ever, if the conceptual model shown in Fig. 30 is cor- came and the Kom people watched the people of rect, it would take ~100 years to attain the pre-erup- Bamessi assemble at the lakeside; then the Bamessi tion CO2 level shown by the “Before 1986” curve start- went into the lake to catch fish for their Fon. Then the ing from the curve “November 1986” assuming a con- Bamessi went back to catch fish for themselves. At that stant CO2 recharge rate of 0.12 Gmol/year (process 3). point, Kom people say, the lake “exploded”, then sank and disappeared, taking with it most of the Bamessi 4-3. Repetitive nature of a limnic eruption population. Thus was the Kom Fon’s curse fulfilled; the people of Bamessi were destroyed, leaving the en- The above model implies that the time of repetition emy Fon with only a few retainers as he had left the of a limnic eruption is ~100 years. A possibility of cy- Kom Fon when the two houses were burned. As they clic gas bursts from lakes which are charged by a gas watched from the hills, the python trail appeared to influx from the lake bottoms was also pointed out by the Kom people and they turned away to begin the long Tietze (1992). He argued that dissolution of CO2(aq) journey west, to the area they now occupy. will inevitably create a stratification of the lake be- The Oku Story cause the density of CO2-containing water is higher At Oku there is a good-sized crater lake and Oku than that of pure water, due to a small partial volume people say that at one time two groups were settled of dissolved CO2(aq) in water (Ohsumi et al., 1992), beside the lake. On the western slope were the Babanki and that the stratification will limit upward gas trans- or Kijem people and on the eastern slope were the Oku port, leading to an accumulation of the gas below the people. Each had their own Fon. There were many dis- stratified layers. If limnic eruptions take place repeti- putes between them, one being a disagreement as to tively on a timescale of ~100 years, evidence of past which group owned Lake Oku. One day a stranger came eruptions might be found in geological records and and asked the Fon of Kijem for land on which to build local documents. Unfortunately, no geological evidence a compound. The Kijem Fon was a disagreeable fel- has been recorded, and no such written documents are low and he refused to give land. The stranger then went known to exist in the Nyos-Monoun areas. However, to the Oku Fon, who gave him a building plot. But the Shanklin (1989, 1992, 2007) published interesting stranger did not like the land that was given, so he folklores that are common in the grassfields of west- went back to the Oku Fon and asked for a different ern Cameroon; the Kom story and the Oku story. The plot. The Fon allocated him another, but again the folklores are suggestive of limnic eruptions that took stranger was not happy, so he returned to the Fon, ask- place in the past. The following paragraphs (shown in ing for a different place. Once again, he was given a italic) are reproduced from Shanklin (1992). plot, and once again, he returned to complain about The Kom Story it. Finally the Oku Fon, seeing that the man would not Kom people were living at Bamessi (near Lake Nyos) be satisfied, told him to choose his own land. The man as guests of the Fon (a ruler is called Fon in the settled down beside Lake Oku, and, as it is said in Grassfields), but the Bamessi Fon was afraid the Kom Pidgin English, no one ever knew what he did there. were becoming too powerful and he devised a trick to (The implication is that the man had no visitors be- rid himself of them: he suggested to the Kom Fon that cause he was a witch.) When the stranger died, the since their young men were showing signs of their Kijem and Oku people went to celebrate his death, the reigns, they each should build a house and entice the Kijem people on their side of the lake and the Oku peo- young men inside, then bar the doors and set the houses ple on theirs. Both Fons were called to come into the afire. But the wily Bamessi Fon built his house with lake (presumably by the now-dead stranger) and they two doors and so all the Bamessi men escaped, while did so, each entering from his side. They were then all the Kom men died. Soon the Kom Fon discovered taken to the lake bottom and, soon after they disap- the trick and vowed revenge. First, he called his sister peared, streaks of red (blood) began to appear on the to him and told her of his plans. He would hang him- Oku side. As they watched the red streaks come up, the self and Kom people were not to cut his body down, Oku people thought their Fon was dead and they be- nor even go near it; instead, they were to watch and gan to mourn for him. At the same time, there appeared wait for the appearance of a python track that would in the distance a Fon dressed in fine new clothes, and lead them to their new home. Led by the Fon’s sister, the Kijem people began to cheer, believing their Fon the Kom people followed their Fon’s instructions pre- was being returned to them, having been honored by cisely. After he hanged himself, his body fluids dripped the host with precious garments. But, in fact, it was down and formed a lake; the Kom people watched. the Oku Fon who was dressed in fine clothes and the Maggots from the Fon’s body fell into the lake and Kijem Fon who had been slaughtered. The two groups became fish; the Kom people watched. The people of returned to their homes, wondering what would come Bamessi were delighted with the new lake and they in- next. Soon after, the waters of Lake Oku left the lake

doi:10.5047/gems.2017.00101.0001 © 2017 TERRAPUB, Tokyo. All rights reserved. M. Kusakabe / GEochem. Monogr. Ser. 1: 1–50, 2017 31 bed and destroyed most of the homes and people on the Kijem side; the remnant moved away from the lake, further west into the nearby Belo Valley. Oku people still elaborate annual sacrifices to the lake, which showed by its actions that it wished to belong to the Oku people. From that day to this, no red streaks have appeared in the lake. From the above stories we can find one common theme that “maleficent” water misbehaves in a spec- tacular way and sets in motion the migration of ethnic groups. I believe that this point indicates the occur- rence of a limnic eruption of Lake Nyos in the past, although we cannot specify the date(s) and lake(s) of the past eruptions.

5. SATREPS-NyMo: A project to reduce the risk of another limnic eruption

The Science and Technology Research Partnership for Sustainable Development (abbreviated as SATREPS) is a program for joint research cooperation between Japan and developing countries for resolving Fig. 31. Schematic presentation of the deep water removal global issues, e.g., environment, energy, natural disas- system. ter prevention and infectious diseases control. SATREPS is sponsored by the Japan International Co- operation Agency (JICA) and the Japan Science and Technology Agency (JST). It was launched in 2008. monitoring of Lakes Nyos and Monoun. During the The JICA and JST are the organizations under the Min- project, scientific cooperation between the two coun- istry of Foreign Affairs of Japan, and the Ministry of tries was encouraged through the exchange of scien- Education, Culture, Sports, Science and Technology tists. Capacity building included scholarships to train of Japan, respectively. Cameroonian students and technicians in Japan, and Under the umbrella of the SATREPS, we were able the donation of scientific instruments to IRGM. The to obtain funds for a project entitled “Magmatic fluid progress of the SATREPS-NyMo can be seen in the supply into Lakes Nyos and Monoun, and the mitiga- website “http://www.satreps.u-tokai.ac.jp”. The project tion of natural disasters through capacity building in went well in terms of scientific achievement. Many Cameroon”. This started in 2011. The project was nick- scientific papers were published, e.g., Issa et al. (2013, named “SATREPS-NyMo”. It was a 5-year project and 2014a, 2014b), Asaah et al. (2014, 2015), Chako continued until March 2016. The project was headed Tchamabé et al. (2013), Fouépé et al. (2013), Fantong by Professors Takeshi Ohba (Tokai University, Japan) et al. (2013, 2015), Tiodjio et al. (2014, 2015, 2016), and Minoru Kusakabe (co-leader, University of Kamtchueng et al. (2014, 2015a, 2015b), Yoshida et Toyama, Japan). The counterpart organization in al. (2016), Ohba et al. (2016), Kozono et al. (2016), Cameroon was the Institute for Geological and Min- and Saiki et al. (2016). ing Research (IRGM) headed by Dr. Joseph V. Hell, As described in Section 3, a bottom water removal under the Ministry of Scientific Research and Innova- system was installed at Lake Monoun in December tion (MINRESI). The goal of the project was to miti- 2013 to stop re-buildup of CO2 (Yoshida et al., 2016). gate natural disasters in Cameroon through capacity The system is shown in Figs. 31, 32. As the degassing building, specifically for issues related to the Lakes pipes in Lake Monoun had lost their gas self-lift capa- Nyos and Monoun gas disasters. To accomplish the bility, one of the pipes was utilized to set up a solar goal, we planned the following sub-projects: (1) a CO2 power driven rotary pump, in order to reduce the total discharge system beneath Lakes Nyos and Monoun; cost of the installation. The intake depth of the pipe is (2) the hydrological regime around the lakes; (3) the ~99 m, very close to the bottom (100 m). A small ro- eruptive history of volcanoes along the Cameroon Vol- tary water pump with an outer diameter of 74 mm was canic Line (CVL); (4) the CO2 distribution in Lakes placed inside the pipe which had an internal diameter Nyos, Monoun and other lakes along the CVL; (5) the of 100 mm. Four small solar modules with a total out- setup of an experimental system for removing CO2- put of 320 W were used as a power source. Although rich deep water to prevent gas re-buildup in Lake the system shown in Fig. 32 works only during the Monoun; and (6) the continuation of geochemical daytime, it is capable of pumping bottom water at an

Fig. 32. Photograph showing the solar power-driven deep water removal system installed at Lake Monoun. Fig. 33. Photograph showing the 45-m-wide natural dam at the northern edge of Lake Nyos. The area surrounded by a green curve is the head of the valley where pyroclastic materials are said to have been eroded away. estimated rate of ~100 m3/day. Based on this rate and the current CO2 concentration of deep water (~90 mmol/kg as of March 2014), the annual removal rate of CO2 is calculated to be 3.3 Mmol/year. Since this an interpretation that the trees grew in magmatic CO2- removal rate is less than half the natural recharge rate rich atmosphere at the center of the present maar where (8.2~8.4 Mmol/year, Kling et al., 2005; Kusakabe et the eruption took place. Magmatic CO2 is character- al., 2008), it would be advisable to install 2 additional ized by “dead carbon” (no or very little 14C), and its systems at Lake Monoun to equal the natural CO2 re- incorporation in trees resulted in older ages. The charge, in order to reduce the risk of a limnic eruption pyroclastic rocks that form the dam once extended in the future. The system is robust and can work for a much farther to the northwest (~600 m), but the lake long time without complicated maintenance and trans- water overflowing the spillway has back-eroded these portation of fuel, which is an important factor that any rocks along the stream bed, leaving only the 45-m-wide system should have in a remote area like Lakes Nyos dam at the present time (Fig. 33). An average erosion and Monoun in Cameroon. rate calculated from these data is 1.5 m/year. At this rate, the 45-m-wide dam will be eroded away in 30 6. The Cameroon Volcanic Line years, if the age of the dam is correct and the erosion proceeds at the mean constant rate. It is, however, more 6-1. Eruption age of the Nyos maar and potential realistic to imagine that the dam collapse will take place collapse of the natural dam in an irregular and catastrophic way. Figure 34 shows many joints at the surface of the moderately consoli- Lakes Nyos and Monoun are maar crater lakes situ- dated upper unit and the seepage of lake water through ated along the Cameroon Volcanic Line (CVL) (Fig. the poorly consolidated lower unit. The seepage of 1). The northern edge of Lake Nyos consists of a 45- CO2-containing lake water may have chemically eroded m-wide natural dam (Fig. 33) that holds surface lake the lower unit in the past, resulting in fall-out of the water down to a depth of 40 m. The dam (Fig. 33) is lower unit, as suggested by the existence of caves. made of pyroclastic materials deposited at the time of There may be an associated breakage of the jointed the volcanic eruption that formed the maar. The upper upper unit. Thus, the erosion rate may vary irregularly unit is moderately consolidated with visible cracks at with time, but it is still alarmingly high. On this basis, the surface, whereas the lower unit is poorly consoli- Lockwood et al. (1988) warned that the dam may even- dated and looks readily eroded as indicated by a con- tually collapse releasing >50 million tons of water and cave structure beneath the upper unit (Lockwood et inducing a catastrophic flood on downstream areas in- al., 1988). Erosion of the lower unit may be facilitated cluding part of Nigeria. This warning was seriously by seeping water. Lockwood and Rubin (1989) deter- taken up by the Cameroonian authorities. They asked mined 14C ages of 2 pieces of charcoal found at the support from the United Nations Office for the Coor- base of the lower unit to be ~400 and ~5100 years BP dination of Humanitarian Affairs (OCHA) and the (before present). They took the age of 400 years to in- United Nations Environmental Program (UNEP) for a dicate the age of trees that were growing at the time of detailed survey of the dam. A team of experts from maar formation. The older age was discarded based on OCHA and UNEP concluded that a failure of the dam

Fig. 34. Cross-section of the natural dam at Lake Nyos. Some explanatory words were added on the cross-section originally drawn by Lockwood et al. (1988). Figure 3 of Bull. Volcanol., The potential for catastrophic dam failure at Lake Nyos maar, Cameroon, 50, 1988, 340–349, Lockwood, J. P., Costa, J. E., Tuttle, M. L., Nni, J. and Tebor, S. G., „ Springer-Verlag 1988 with permission of Springer.

is highly likely “to occur within the next 5 years” based et al. (2008) analyzed 12 samples collected from the on their geotechnical survey (Joint UNEP/OCHA En- Lake Nyos area, including 5 samples of the dam-form- vironment Unit, 2005). They recommended reinforce- ing surge deposit and 5 nearby lava flows. They used ment of the dam by cementing fractures and the XRF and ICP-MS for the analysis of major and trace unconsolidated part of the dam. As a result, the dam element compositions including (238U/232Th), (230Th/ has been reinforced by engineering methods. 232Th), (226Ra/230Th) and (238U/230Th) ratios. The re- The warning by Lockwood et al. (1988) also drove sults of the Th-Ra disequilibria are reproduced in Fig. geochronological studies of the dam, since the age is 35. The (230Th/232Th) ratios of 10 alkaline rock sam- directly related to the erosion rate and thus to the safety ples vary from 0.886 to 1.024, and the (238U/232Th) of the dam. The age of the dam has long been debated. ratios vary from 0.716 to 0.880. Data for 26 samples The debate on the age is concisely summarized in Aka from the Mt. Cameroon volcano, which has erupted and Yokoyama (2013). Freeth and Rex (2000) proposed during the last 100 years, are also included (Yokoyama that the age of eruption of the Nyos maar was in ex- et al., 2007). The Lake Nyos and Mt. Cameroon sam- cess of 100,000 years, based on K-Ar dates (Fig. 34) ples lie closely on a line marked as 238U/230Th = 0.82 and evidence from aerial photographs taken in 1963– with a few exceptions, significantly above the equi- 1964 that showed no change in the width of the dam line which is 238U/230Th = 1.00. This feature indicates since that time. They concluded that the dam materials the presence of a 15 to 28% enrichment of 230Th over were eroding at a “geologically realistic rate” and that 238U, suggesting strongly that the Lake Nyos maar for- “there is no reason to suspect that the rate at which it mation is younger than ~375 ka which is 5 times the is currently eroding away is, in itself, sufficient to pose half-life of 230Th. If the time which has elapsed since an immediate threat”. However, the application of the the volcanic eruption is greater than 375 ka, then (230Th/ 238 K-Ar dating method to the basaltic rocks from the Nyos U)A (activity ratio) becomes unity, or a secular equi- dam area was criticized by Lockwood and Rubin librium is established, and no dating can be made (equi- (1989), because the Nyos basalts contain fine shards line in Fig. 35a). Tholeiitic samples, D26 and D27 in of K-feldspars which were derived from basement Fig. 35a plot on the line 238U/230Th = 1.00, an indica- monzonite (Fig. 34). The K-Ar dating of the rocks con- tion that they are in the 238U-230Th radioactive equilib- taining the shards gives much older ages than their true rium, giving their formation age older than 375 ka, with age due to the inclusion of K-feldspars with a high ra- no more information about the age. It is important to 230 232 238 diogenic Ar concentration. note that the variation in the ( Th/ Th)A and ( U/ 232 Aka et al. (2008) applied a U-series dating method Th)A ratios of the Mt. Cameroon samples (~0.99 and to Lake Nyos maar basalts. The basic principles, as- ~0.82, respectively), and the corresponding excess sumptions and applications of the U-Th dating method 230Th over 238U (18–24%) were all within the range are summarized in Chabaux and Allègre (1994). Aka for Lake Nyos samples (Fig. 35a). Figure 35b is a

Fig. 35. (a) (230Th/232Th)-(238U/232Th) activity ratio diagram for Lake Nyos and Mt. Cameroon samples. Some Lake Nyos samples are enriched in 230Th compared to 238U by 15–28%. (b) (226Ra/230Th)-(238U/230Th) activity ratio diagram for the samples showing 2–19% excess 226Ra over 230Th, suggesting a Th-Ra fractionation of <10 ka BP. Reproduced from figure 4 of J. Volcanol. Geotherm. Res. 176, Aka, F. T., Yokoyama, T., Kusakabe, M., Nakamura, E., Tanyileke, G., Ateba, B., Ngako, V., Nnange, J. and Hell, J., U-series dating of Lake Nyos maar basalts, Cameroon (West Africa): Implications for potential hazards on the Lake Nyos dam, 212–224, Copyright 2008, with permission from Elsevier.

226 230 238 230 plot of ( Ra/ Th)A vs. ( U/ Th)A for the studied ratio has to be known to calculate the age of the dam samples. They were compared to published data for using the excess 226Ra. Since there are no eruptions of 226 230 MORB and OIB (inset). The ( Ra/ Th)A ratios for a known age which have occurred in the Lake Nyos the alkaline rock samples range from 1.017 to 1.040 area, the assumption was made that the initial ratio was with a mean value of 1.028 ± 0.008. These Nyos data the same (1.15 ± 0.02) as that measured in Mt. 226 230 plot above the ( Ra/ Th)A =1 line (equilibrium), in- Cameroon lavas that are erupting today (Yokoyama et dicating an enrichment of 226Ra compared to 230Th that al., 2007). Using this assumption, the 226Ra-230Th age was acquired during partial melting of the mantle of Lake Nyos was calculated to be 8.75 ± 0.49 ka (Aka source, as is generally observed in oceanic basalts and Yokoyama, 2013) after a careful examination of (Thomas et al., 1999). Similar to 238U-230Th systemat- the samples. Based on this age, they consider that a ics, the tholeiitic samples are in 226Ra-230Th equilib- collapse of the Nyos dam from erosion alone is not as rium. It is highly contrasting that the Mt. Cameroon imminent and alarming as has been suggested. How- 226 230 data have higher ( Ra/ Th)A ratios (1.09–1.21) than ever, making the dam more stable is necessary to com- the Lake Nyos samples (1.01~1.04), although the two pletely eliminate the potential flood hazard. volcanoes are similar in their degree of 238U-230Th dis- Stabilization by grouting of the dam has been under- equilibria (Aka et al., 2008). The initial 226Ra/230Th taken.

Fig. 36. Chemical composition of rocks from CVL volcanoes. (a) K2O+Na2O versus SiO2, and (b) Mg number versus SiO2 plots. Reproduced from figure 2 of Asaah et al. (2014) which should be referred to for the abbreviations.

6-2. Origin of the Cameroon Volcanic Line Bambouto, Oku, Ngaoundéré Plateau, Mandara Moun- tains and Biu Plateau) to the northeast. The volcanic The Cameroon Volcanic Line (CVL) is an alignment islands in the oceanic sector are made up of rocks rang- of Cenozoic volcanoes stretching for 1600 km from ing from nephelinite, basanite and basalt to trachyte Annobon in the Gulf of Guinea to Biu Plateau in the and phonolite (Halliday et al., 1988; Deruelle et al., continental part of central Africa (Fig. 1). It straddles 1991). The volcanoes in the ocean-continent bound- both oceanic and continental lithosphere. The CVL can ary are located SW of Mt. Cameroon, and are made of be grouped into 3 sectors, i.e., the oceanic sector to mostly nephelinitic lavas for Etindé (Nkoumbou et al., the southwest (Annobon, Saõ Tomé, and Principe), the 1995) and basalts and basanites for Bioko and Mt. ocean-continent boundary (Bioko, Etinde and Mt. Cameroon (Yokoyama et al., 2007; Asaah et al., 2014). Cameroon), and the continental sector (Manengouba, Mt. Cameroon is the only active volcano in the CVL

Fig. 37. Trace element patterns for mafic rocks from the oceanic and continental sectors of the CVL. Reproduced from figure 4 of Asaah et al. (2014). The patterns are generally similar to each other and akin to OIB, suggesting an origin from a similar source. Reproduced from figure 5 of Asaah et al. (2014).

with seven eruptions recorded in the last 100 years, the CVL has long been a subject of controversy, and i.e., 1909, 1922, 1954, 1959, 1982, 1999, and 2000 (Suh various hypotheses have been proposed. They are sum- et al., 2003). It is a composite volcano made of alka- marized by Aka et al. (2004) and more recently by line basanitic and basaltic flows interbedded with small Asaah et al. (2014), as follows: (1) Reactivation of pre- amounts of pyroclastic materials and numerous cinder existing tectonic structures in the Cenozoic associated cones (Suh et al., 2003; Yokoyama et al., 2007). The with crustal melting (Gorini and Bryan, 1976; Moreau continental sector of the CVL includes Mount et al., 1987; Fairhead, 1988). (2) Membrane stresses Bambouto and Mount Oku. They are Oligocene to generated by the movement of the African plate away Quaternary strato volcanoes with lava successions com- from the equator (Freeth, 1978). (3) Displacement of prising a strongly bimodal basalt-trachyte-rhyolite suite the African plate (Fitton, 1980). (4) Hotspot trail (Marzoli et al., 2000, 2015; Kamgang et al., 2013). (Morgan, 1983). (5) Hotline hypotheses (Meyers et al., Mt. Manengouba is also in the continental sector and 1998). (6) A plate-wide shallow mantle convection is a well-preserved stratovolcano whose summit hosts model (Burke, 2001). (7) Edge convection and two concentric calderas with lakes. Lavas range from lithospheric instability (Reusch et al., 2010). Of the basalts to trachytes, quartz trachytes, and rare rhyolites above, Fitton’s classic hypothesis is still attractive in (Pouclet et al., 2014). The Ngaounderé Plateau in the that the Benue Trough and the CVL are related to a northeastern continental part of the CVL consists of common “Y”-shaped hot zone in the asthenosphere alkaline basalts and basanites capped by trachytes and over which the African plate moved during the period phonolitic flows. The Biu Plateau, which is located in of 110 to 70 Ma (Fitton, 1980). The “Y”-shaped hot the northern part of the Ngaounderé Plateau, consists zone was a rift zone that extended from a triple junc- of basaltic flows with a maximum thickness of 250 m. tion originally located at the Gulf of Guinea that was This plateau is composed of basanite to transitional underlain by the St. Helena hotspot at the time of the basalts (Rankenburg et al., 2005). opening of the south Atlantic. The CVL developed over Since Lakes Nyos and Monoun are situated on the this rift zone. In this sense, the magmatism in the CVL CVL, it may be informative to give a brief summary may have been similar to that in the currently active of the origin of the CVL to understand the characteris- East African Rift Zone. Asaah et al. (2014) went for tics of the Nyos and Monoun volcanoes. The origin of the “hotline” model which invokes multiple plumes

Fig. 38. (a) 207Pb/204Pb-206Pb/204Pb diagram, and (b) 143Nd/144Nd-87Sr/86Sr diagram for the CVL lavas. (c) Positive and negative trends were seen in the 143Nd/144Nd-206Pb/204Pb relationship for the CVL lavas. Reproduced from figures 6, 7 and 9 of Asaah et al. (2014). EM1 is for Enriched Mantle type 1, EM2 for Enriched Mantle type 2, NHRL for Northern Hemisphere Reference Line, HIMU for high-m (=238U/204Pb ratio), FOZO for Focal Zone, MORB for Mid-Ocean Ridge Basalt, OIB for Ocean Island Basalt, and DMM for Depleted MORB Mantle.

originating from the same source in the upper mantle, volcanic activity. each of which produced volcanoes independently, as Magmatism of the CVL is characterized by melting the model appears to explain the diverse features of in the garnet lherzolite stability fields (Marzoli et al., the CVL, i.e., geophysical, structural and geochemical 2000; Yokoyama et al., 2007; Kamgang et al., 2013), evidence, including the absence of time-dependent although melting in the spinel lherzolite stability field

doi:10.5047/gems.2017.00101.0001 © 2017 TERRAPUB, Tokyo. All rights reserved. 38 M. Kusakabe / GEochem. Monogr. Ser. 1: 1–50, 2017 has been reported in the Ngaoundere Plateau (e.g., Lee from source materials that are different from the other et al., 1996; Nkouandou and Temdjim, 2011). In addi- CVL lavas, as suggested by the Mg# versus SiO2 trend tion, mixing of both garnet and spinel melting fields (Fig. 36b) and a strong high m (HIMU) character there. has been reported in Mt. Cameroon (Tsafack et al., The radiogenic isotope (Sr-Nd-Pb) geochemistry of 2009). Both mafic and felsic rocks show chemical fea- the CVL rocks is also summarized in Fig. 38, adapted tures consistent with a plume activity outlined by their from Asaah et al. (2014). The Sr-Nd-Pb isotopic com- ocean island basalt (OIB) characters, and their isotopic positions of the CVL basalts overlap those of OIB. In ratios (Mbassa et al., 2012). the 207Pb/204Pb vs. 206Pb/204Pb diagram (Fig. 38a), the data plot parallel to the Northern Hemisphere Refer- 6-3. Geochemistry of CVL magmas ence Line (NHRL) of Hart (1984), and to the right of the 4.53 Ga geochron. However, some data from the Asaah et al. (2014) made a comprehensive review Oku Volcanic Group (OVG) with low 206Pb/204Pb and of the geochemistry of CVL rocks. They compiled the 207Pb/204Pb ratios overlap with the MORB end mem- existing geochemical data of the CVL rocks (580 sam- bers (Atlantic, Indian, and Pacific MORBs). The 143Nd/ ples) consisting of major and trace element composi- 144Nd ratios and 87Sr/86Sr ratios of the mafic rocks show tions and radiogenic (Sr-Nd-Pb) isotope compositions. a limited range of variation (Fig. 38b). The 87Sr/86Sr Figure 36 shows some chemical characteristics of the ratios range from 0.70286 in a sample from the Biu CVL rocks in terms of (a) K2O+Na2O versus SiO2, and Plateau to 0.70515 in a sample from Mt. Bambouto. 143 144 (b) Mg number versus SiO2 plots. The SiO2 contents The Nd/ Nd ratios vary from 0.51302 in a sample show a wide range of variation from 38% (oceanic from the Biu Plateau to 0.52771 in a sample from Mt. CVL) to 79% (continental CVL) reflecting the diverse Cameroon. Some lavas from the Biu Plateau and the rock types. The Mg number (Mg#), defined as Mg# = oceanic CVL show relatively low 87Sr/86Sr and high 143 144 MgO/(MgO+FeO)*100, is often used as an index of Nd/ Nd ratios, implying that they are more primi- the level of evolution of volcanic rocks. It shows dif- tive than other continental volcanic rocks (Mt. ferent trends from one volcanic center to another. The Bambouto, Mt. Manengouba, and the OVG). Isotope CVL rocks from the oceanic and continental sectors data for the OVG show a wider spread than those of are dominantly alkali basalts and basanites. The Mg# the other CVL volcanoes. This difference is conspicu- of mafic samples ranges from 60~69 (least evolved ous in the Pb isotopes. In Fig. 38b, a negative correla- basalts) to 40~49 (evolved rocks), indicating various tion is observed between 143Nd/144Nd and 87Sr/86Sr ra- fractional crystallization paths (Fig. 36b). Refer to tios and the correlation slope matches the mantle array Asaah et al. (2014) for further discussion. of MORB-OIB samples. From these figures it is sug- Abundance patterns of trace elements are often used gested that the CVL lavas formed by a dominant con- to discuss magma genesis, since they provide tribution of EM2 to the Depleted MORB Mantle geochemical and geological information through their (DMM). The 143Nd/144Nd versus 206Pb/204Pb plots (Fig. unique chemical properties and sensitivity to processes 38c) show positive and negative correlations with dif- to which major elements are insensitive. Primitive- ferent slopes, where the role of EM2 becomes domi- mantle normalized trace element patterns (Palme and nant over EM1. Mixing with various end members in O’Neill, 2003) for mafic rocks from the oceanic and different proportions may account for the complex iso- continental sectors of the CVL are presented in Fig. topic characteristics of the CVL lavas. 37. The patterns are generally similar to each other Based on trace element and isotope geochemistry, it (except for the Mt. Etindé samples) and akin to OIB, has been suggested that these magmas derived from suggesting an origin from a similar source. They show the sub-lithosphere without interaction with the over- a marked enrichment of light rare earth elements lying lithosphere (Fitton and Dunlop, 1985). A differ- (LREEs) and a strong fractionation of heavy rare earth ent view, however, was given by Halliday et al. (1990) elements (HREEs) relative to LREEs. The most strik- that the continent/ocean boundary magmas (Bioko, ing features of Fig. 37 are: (1) the Mt. Etindé samples Etindé and Mt. Cameroon) are characterized by 206Pb/ have high trace elemental abundances compared to the 204Pb ratios that are higher (more radiogenic) than those other CVL alkaline basalts; (2) relatively high posi- of typical continental and oceanic sector magmas. This tive anomalies of Nb, La and Nd; and (3) the occur- radiogenic feature has also been confirmed by the dis- rence of a K-trough. Nearly constant elemental ratios tribution of 3He/4He ratios of lavas and mineral sepa- of incompatible elements in CVL rocks suggest that rates from the CVL rocks showing a clear 3He/4He magma processes, such as zone refining melting, valley as already illustrated in Fig. 20 (Aka et al., magma mixing, and extensive fractionation and replen- 2004). Together with Sr, Nd and O isotopic variations, ishment, were not dominant processes during the gen- Halliday et al. (1988, 1990) suggested that the radio- eration of the CVL mafic lavas, because the above proc- genic nature of the 206Pb/204Pb ratios of rocks from the esses can efficiently fractionate incompatible elements. ocean-continent boundary reflects melt migration from The peculiar features of Mt. Etindé may have resulted the St. Helena fossil plume head that took place at 125

Fig. 39. Schematic presentation of a model for source enrichment in high m elements following plume emplacement at 125 Ma beneath the ocean/continent boundary of CVL (St. Helena). Reprinted by permission from Macmillan Publishers Ltd: Nature 347, 523–528, Halliday, A. N., Davidson, J. P., Holden, P., DeWolf, C., Lee, D.-C. and Fitton, J. G., Trace-element fractionation in plumes and the origin of HIMU mantle beneath the Cameroon line, Copyright 1990.

Ma, and that some of the CVL magmas derive from aphyric (Aka et al., 2008) and difficult to use for the the upper metasomatized part of the fossil plume in analysis of pre-eruptive volatile contents by the afore- the lithospheric mantle (Fig. 39). The degree of trace- mentioned techniques. Instead, based on major and element enrichment (U/Pb, or m in this case) varies as trace elements systematics, Aka (2015) proposed that a function of the vertical thickness of the plume head the Nyos basalts formed by a small degree (1~2%) of through which the melts migrated. At the margins partial melting of the primitive mantle to which where the flattened plume head is thinnest, the source amphibole and phlogopite had been added by regions are dominated by more depleted mantle carbonatitic fluids, and that decarbonation reactions of (Halliday et al., 1990). The radiogenic nature of 206Pb/ the carbonatitic metasomatism are responsible for pro- 204 3 4 Pb ratios and the He/ He valley observed at the ducing the magmatic CO2. However, based on the ocean-continent boundary region can be explained by geochemical data of the Nyos volcanic rocks, Asaah et the magma genesis affected by the fossil plume head. al. (2015) suggest that CVL magmatism is predominantly of an asthenospheric source with little contri- 6-4. Volatiles in magma bution from the subcontinental lithospheric mantle (SCLM). The lavas show evidence of enrichment by A fundamental question arises as to whether Lake metasomatic fluids probably in the Mesozoic (e.g., Nyos magmas are enriched in CO2. Volatile contents Halliday et al., 1990; Aka, 2015; Asaah et al., 2015). in pre-eruptive magmas have been estimated by vari- The metasomatism affected the SCLM, inducing hy- ous techniques. One of the techniques is to analyze melt drous minerals like amphibole and phlogopite that are inclusions in phenocrysts, since melt is trapped in not stable in the asthenosphere. Asaah et al. (2015) growing phenocrysts as melt inclusions in magma and suggest that the metasomatic fluids crystallized as small is quenched to glass at the time of eruption. The pockets or veins in the SCLM. An ultimate source of volatiles, mainly H2O and CO2, in the glass inclusions CO2 in the Nyos magma may derive from the are determined by microanalytical techniques such as decarbonation of such crystallized metasomatic fluids. Fourier transform infra-red spectroscopy (FTIR), la- It is unlikely that the CVL magmas, including the Nyos ser-Raman spectrometry, and secondary ion mass magma, have abnormal CO2 in their mantle source. spectrometry (SIMS), etc. (Ihinger et al., 1994). An- Lake Nyos and Lake Monoun volcanoes are located other approach is experimental petrology where min- in the Oku and Bambouto volcanic centers, respec- eral stabilities and assemblages are calibrated under tively, in the middle of CVL (Fig. 1). Lake Nyos is a different (but controlled) conditions, such as tempera- maar lake created by a phreato-magmatic eruption. ture, pressure, and water fugacity. Comparison of ex- There are some other maar lakes near Lake Nyos, i.e., perimental products with natural phenocrystic assem- Oku, Elum, Nyi, Wum and Enep, but only Lake Nyos blages allows us to constrain the pre-eruptive volatile contains a large amount of dissolved CO2. According contents (Johnson et al., 1994). Unfortunately for us, to Lockwood and Rubin (1989) who described the ge- however, lavas from the Nyos volcano are mostly ology of the Nyos volcano, eruption sequences are sum-

Table 4. Concentration of volatiles in magma and expected gas composition from the magma.

Concentration in magma Expected gas compostion

H2OCO2 SClH2OCO2 SHCl wt% ppm ppm ppm mmol/mol mmol/mol mmol/mol mmol/mol Glassy margin of MORB Eastern Pacific Ocean 0.12 1630 690 50 526 292 170 11 Atlantic Ocean 0.20 1320 1170 60 621 167 203 9

Melt inclusions from hotspot basalts Kilauea, ocean floor 0.46 3100 1050 æ 712 196 91 æ Kilauea, summit 0.23 800 1300 80 677 96 215 12

Melt inclusions from subduction zone volcanoes Basalt 1.0 >1000 1000 1000 871 36 49 44 Andesite 3.0 >1000 400 3000 933 13 7 47 Rhyolite 5.0 >1000 100 2000 971 8 1 20

Shinohara (2003) and Giggenbach (1996).

marized as shown below. The formation of the Nyos the decarbonation of metasomatized mantle facilitated maar is directly related to the ascent of alkali basalt partial melting (Aka, 2015). Once a melt is formed, it magma. The first lava reached the surface in a rela- rises through the mantle due to its lower density (higher tively gentle, fire-fountaining fashion, depositing buoyancy) and approaches the surface of the Earth to scoria and fluid bombs over a wide area around the form magma. The melt may remain as a magma reser- present north end of Lake Nyos. This phase was fol- voir in the shallow part of the crust. When the magma lowed by an explosive and violent eruption due to vola- further ascends, crystallization in the magma begins tile expansion. The violence of the activity increased because of the reduction of temperature and pressure. rapidly, however, and basalt is only included as shat- Magma contains various volatile materials, such as tered fragments in upper parts of the pyroclastic sec- H2O, CO2, S, Cl, etc. Since the volatile materials will tion. Neither the basalt flow nor the associated scoria not all be incorporated into crystals (or minerals), they at the base of the pyroclastic section were found to tend to be concentrated as fluids in the magma as it contain ultramafic xenoliths, suggesting that mantle rises and cools. A volcanic eruption is often facilitated rocks were only transported to the surface during the by magma ascent driven by a lowered density due to later more explosive phases of the eruption. The depth the accumulation and expansion of bubbles of the of the explosive activity may have gradually increased volatiles in the magma. during the eruption, and the initial explosion crater The chemical composition and concentration of mag- gradually widened, which resulted in the formation of matic volatiles have been estimated through the analy- the maar crater, or the present Lake Nyos. It can be sis of high temperature volcanic gases, chilled glassy imagined that the magma subsided after the eruption, margins of lava that has extruded onto the bottom of but the release of CO2-rich volatiles from the magma the deep ocean, and glass inclusions in phenocrysts of continues until today. volcanic rocks. Volatiles in magma are almost com- Magma is generally generated by a partial melting pletely discharged into the atmosphere at the time of of rocks in the lower crust or upper mantle. Mantle volcanic eruption. For this reason, the chemical analy- rocks, mainly comprised of peridotite, exist as a solid, sis of high-temperature volcanic gases, if collected and because the geothermal gradient within the Earth is analyzed properly, can give the volatile composition generally below the solidus of mantle rocks. It has been (not concentration) in magma. Table 4 shows the con- hypothesized that part of solid mantle, if heated lo- centration of H2O, CO2, S and Cl in some types of cally, can ascend as a diapir and cross the solidus where magma, and the composition of volcanic gases that is partial melting starts to take place. Volatile materials expected from the degassing of each type of magma such as H2O and CO2, if they coexist with the rocks, (Shinohara, 2003). The concentration of the magmatic reduce the solidus temperature and facilitate a partial volatiles in magma is highly variable depending on its melting of the rocks, or magma genesis. Thus, the co- type. Water concentration in Mid-Oceanic Ridge Ba- existence of volatiles is important for partial melting. salt (MORB) is low (0.1~0.5 wt%), whereas that of Metasomatic fluids may have affected the primitive subduction zone magma is more than an order of mag- mantle beneath Lake Nyos and the fluids produced by nitude higher (1~5 wt%). The concentration of CO2 in

Fig. 40. Solubilities of H2O and CO2 in silicate melts (Holloway and Blank, 1994; Shinohara, 2003).

MORB magma is 1000~3000 ppm, which is generally higher than that of subduction zone magma. The calculated composition of gases exsolved from magma is also shown in Table 4. Such calculated gas compositions are in general agreement with the observed gas compositions (not shown, but can be found in Allard, 1983; Gerlach, 1983; Shinohara, 2003). In gases from hotspot basaltic volcanoes such as Kilauea (Hawaii), the water content is lower than that from subduction volcanoes, whereas the CO2 content is significantly higher in hotspot and mid-oceanic volcanoes, reflecting its higher concentration and low solubility in basaltic melts. Solubilities of H2O and CO2 in silicate melts have been experimentally determined as shown in Fig. 40 (Holloway and Blank, 1994, and references therein). The solubility depends on the temperature, pressure and the chemistry of melts. It increases as the partial pressure of the volatile species in question increases, Fig. 41. Solubilities of CO and H O in basaltic melts at and decreases as the temperature of the melt increases. 2 2 1200 C as a function of the total pressure of the volatiles Generally speaking, water is approximately an order ∞ (Holloway and Blank, 1994; Shinohara, 2003). of magnitude more soluble than CO2. Water dissolves slightly more in silicic melts than in basaltic melts, whereas CO2 dissolves more in basaltic than in silicic melts (Fig. 40). Figure 41 illustrates the solubility of position in the melt will follow the thin dotted line depending on the co-existing H O concentration as CO2 and H2O in basaltic melts at 1200∞C as a function 2 of the total pressure of the volatiles. The non-linear shown in Fig. 41. If degassing takes place in an open relationship of this binary system in the melts comes system, the melt composition may follow a different from the non-ideal mixing properties of these species path, as indicated by the thick long dashed line, since (Holloway and Blank, 1994). Using Fig. 41, it can be the CO2-rich fluid leaves the magma when the system envisaged how the volatile composition in the melt becomes open due to the low CO2 solubility in the changes as the decompression proceeds. For example, melts, making the remaining magma progressively CO -poor, while the H O concentration decreases only at point A of Fig. 41, where CO2 = 540 ppm and H2O = 2 2 1.6 wt%, the melt is saturated with the coexisting fluid a little. As long as the magma keeps open-system degassing, CO -rich fluid is continuously released from of which the mole fraction of H2O equals 0.2 and that 2 the magma. This solubility-controlled behavior of CO of CO2 is 0.8. This implies that the fluid coexisting 2 in basaltic magma may explain a CO -rich nature of with the basaltic melt is extremely rich in CO2. As the 2 magma ascends, or the confining pressure is reduced, fluids separated from the magma. The ultimate source the fluid exsolves, or degassing takes place. If of CO2 in the Nyos magma may derive from the degassing proceeds in a closed system, the fluid com- decarbonation of crystallized metasomatic fluids in the

doi:10.5047/gems.2017.00101.0001 © 2017 TERRAPUB, Tokyo. All rights reserved. 42 M. Kusakabe / GEochem. Monogr. Ser. 1: 1–50, 2017 subcontinental lithosphere (Aka, 2015; Asaah et al., centration at Kabuno Bay (a small basin on the north- 2015). The permanent supply of such CO2 is likely to western end of Lake Kivu) is relatively close to satu- be responsible for the high concentration of CO2 gas ration. Since Kabuno Bay is shallower than the main in the fluids feeding into Lakes Nyos and Monoun. basin and is characterized by the highest input of CO2- rich magmatic fluid, the bay is considered to be poten-

7. Other CO2-rich volcanic lakes in the world tially most hazardous in terms of the possibility of limnic eruption. Continuous monitoring is recom- Of 714 volcanoes in the world, 86 volcanoes host mended (Tassi et al., 2009). The concentration of dis- lakes (Pasternak and Varekamp, 1997). Information on solved CH4 is highest (~17 mmol/L), approx. 12% of the volcanic lakes is now available from the VOLADA dissolved CO2 at the bottom of the main basin. The (2013) database (https://vhub.org/resources/2822). gas is produced by the bacterial reduction of CO2 and Although Lakes Monoun and Nyos in Cameroon be- acetate fermentation (Schoell et al., 1988). It is im- came notoriously famous because of the gas disasters portant to note that microbial activity contributes to in the mid-1980s, other CO2-rich volcanic lakes exist the gas chemistry in deep, stratified and anaerobic in the world, e.g., Lake Kivu (Democratic Republic of lakes, as recently found also at Lakes Nyos and the Congo and Rwanda, see below for references), Monoun (Tiodjio et al., 2014, 2016). Carbon isotopic 13 Laacher See (Germany) (Aeschbach-Hertig et al., ratios (d C) of CO2 dissolved in the main basin of the 1996), Lake Van (Anatolia in eastern Turkey) (Kipfer Lake Kivu range from –7 to –6‰ (relative to VPDB), et al., 1994), Lago Albano and the two Monticchio suggesting also a large contribution from mantle-origi- lakes (Italy) (Anzidei et al., 2008; Caracausi et al., nating CO2. Those at Kabuno Bay (–11 ~ –13‰), how- 2009), Laguna Hule and Rio Cuarto (Costa Rica) ever, are significantly lower than the values for the (Alvarado et al., 2011), and Lac Pavin (France) main basin, probably reflecting the interaction of mag- (Aeschbach-Hertig et al., 1999). matic fluids with organic-rich sedimentary materials Of these lakes, Lake Kivu has been known to con- that underlie volcanic rocks derived from nearby tain a high concentration of CO2 and CH4 in its deep Nyamulagira and Nyiragongo volcanoes (Tassi et al., water since well before the Lake Nyos event (e.g., 2009). The 3He/4He ratio of Kabuno Bay water is 5.5 Deuser et al., 1973; Tietze et al., 1980). Carbon diox- Ratm, indicating a large contribution of a magmatic 13 ide dissolved in the lake is basically of magmatic ori- component regardless of the low d C values, whereas gin, a situation similar to that in Lakes Nyos and the ratio ranges from 2.1–2.6 Ratm in the main Kivu Monoun in Cameroon, although the magmatic CO2 is basin water. Fumarolic gases collected at the summit mixed with a variable proportion of biogenic CO2. The crater of the Nyiragongo volcano may best represent lake is located along the East African Rift on the bor- the 13C/12C and 3He/4He ratios of magmatic end-mem- der between the Democratic Republic of the Congo bers in the fluids that are supplied to Lake Kivu and (DRC) and Rwanda. The lake area is tectonically and its surroundings (Tedesco et al., 2010). These authors 13 volcanically active as part of the East African Rift Sys- observed typical mantle values of d C = –3.5 ~ –4‰ 3 4 tem. Because of the high gas concentrations in the lake and He/ He ratios up to 8.7 Ratm for the fumarolic and the large population around it, Lake Kivu has a gases. The influence of this magmatic signature be- potential risk of a gas disaster caused by a limnic erup- comes smaller, and the crustal components increase, tion which may be triggered by a possible volcanic as we move southward (toward Lake Kivu). The C/ 3 10 eruption at the lake bottom (Schmid et al., 2005) or a He ratio of ~30 ¥ 10 was observed for summit plunge of lava flows from Nyiragongo, the nearest ac- fumarolic gases. This high value probably reflects the tive volcano (only 20 km NE to the lake). Indeed, the high CO2 solubility in the Nyiragongo magma which 2002 eruption of the volcano generated lava from flank is foiditic (alkaline), different from typical MORB 3 10 fissures flowed into the city of Goma, the provincial magmas. High C/ He ratios up to 36 ¥ 10 were mea- capital, resulting in destruction of local structures and sured for the main basin water of Lake Kivu. Although the evacuation of local people, and these lava flows these ratios are close to the Nyiragongo magmatic eventually ran into the lake. Fortunately, no limnic value, it is more likely that the addition of CO2 in lo- eruption was induced at that time (Tedesco et al., 2007). cal groundwater that has interacted with organic mate- Detailed gas and water chemistry of Lake Kivu and rials enhanced the lake’s C/3He ratio, as suggested by 13 the surrounding region has been published by several d C values. These observations show that magmatic authors (Tietze et al., 1980; Tassi et al., 2009; Schmid fluids interact with surrounding materials in varying et al., 2005). The lake has 5 basins, each of which is degrees, and that the gas geochemistry of this area is characterized by a different chemistry, CO2 profile, and controlled by the local tectonic-geologic settings biology. The main basin (>250 m) contains the highest (Tedesco et al., 2010). CO2 concentration with a horizontal heterogeneity. Lake Mashu is a small, dimictic (mixing twice a year) Although the highest CO2 concentration in the main caldera lake in Hokkaido, Japan, with a surface area 2 basin is far from saturation at any depth, the CO2 con- of 19 km and a maximum depth of 211 m. A hot spring

doi:10.5047/gems.2017.00101.0001 © 2017 TERRAPUB, Tokyo. All rights reserved. M. Kusakabe / GEochem. Monogr. Ser. 1: 1–50, 2017 43 has been identified at the bottom of the lake. The chemi- outlet blocked by lava flows from the nearby Nemrut cal characteristics of the lake were given by Nojiri et volcano. Lake Nemrut is one of the caldera lakes near al. (1990). Based on noble gas data of the lake water Lake Van. The injection of He, derived from depleted 3 4 collected at various time, points and depths, Igarashi mantle with He/ He ratio of 7.4 Ratm, into Lakes 3 4 et al. (1992) estimated a He/ He ratio of 6.7 Ratm for Nemrut and Van was documented by Kipfer et al. helium supplied from the lake bottom through the hot (1994). It is likely that CO2 is also supplied to the lakes, spring, suggesting the addition of mantle helium to the but unfortunately there is no mention of CO2 in the lake from the underlying magma. The accumulation of lakes in Kipfer et al. (1994). mantle helium between two overturns (spring and au- Alban Hills in the volcanic area near Rome, Italy, 7 –2 –1 tumn) was estimated to be 9.2 ¥ 10 atoms cm s has been characterized by high emissions of CO2 from 4 2 –2 –1 3 ( He) and 8.7 ¥ 10 atoms cm s ( He) using the a pressurized CO2-rich aquifer, and small-scale gas helium profiles and a one-dimensional diffusion model. outbursts from the aquifer have been recorded Since the CO2 supply rate was estimated to be 5.3 ¥ (Carapezza and Tarchini, 2007). Carbon isotopic ra- 108 mol/y (Nojiri et al., 1993), a high C/3He ratio of tios were reported to be in a limited range around 10 18 ¥ 10 can be calculated for the supply fluid. The +1.3‰ (relative to VPDB), which suggests the contri- C/3He ratio, similar to that estimated for the Alban Hills bution of decomposed marine carbonates as the source 3 4 volcanic district (see below), is two orders of magni- of CO2. The He/ He ratio of He in the associated gas tude greater than the MORB value. Igarashi et al. was 1.9 Ratm, very low compared to MORB and sub- (1992) attributed this high value to the enrichment of duction volcanic gas values, but still suggestive of a 3 CO2 in the source magma beneath Lake Mashu. Al- magmatic affiliation. The C/ He ratio of gases collected 8 11 though the CO2 supply rate of 5.3 ¥ 10 mol/y is greater from a nearby well is 2.3 ¥ 10 , 2 orders of magnitude 8 than that for Lake Nyos (1.2 ¥ 10 mol/y, Kusakabe et greater than typical MORB values. This value is con- al., 2008) by a factor of ca. 4, the dimictic nature of sistent with a high contribution of CO2 that was most the lake does not allow an excessive accumulation of likely derived from the thermal decarbonation of lime- CO2 in it, which is fortunate from the limnic eruption stone involved in magma genesis at the Alban Hills perspective. volcanic district. Historical evidence has shown that Laacher See is also a 53-m-deep holomictic (com- Lake Albano, a 160-m-deep crater lake located in the plete vertical mixing once a year) maar lake in the East center of the district, experienced lahars associated with Eifel volcanic district in Germany, where the discharge water overflow (Carapezza and Tarchini, 2007). The of CO2 gas from the lake has been observed for years. present water and gas chemistry of the lake indicates Helium and neon isotopes dissolved in the lake were that dissolved CO2 concentration increases with depth measured twice (spring and early autumn) in 1991 by in anoxic hypolimnion (>80 m). However, the total gas Aeschbach-Hertig et al. (1996) with the aim of esti- pressure calculated from the CO2 concentration is far mating the helium flux from the lake bottom, since below the hydrostatic pressure at all depths, suggest- gases supplied from the bottom were considered to ing that a gas hazard at the lake is unlikely, unless CO2 accumulate in the lake during summer stratification. from the pressurized aquifer is suddenly injected into Both the He concentration and 3He/4He ratios increased the lake (Carapezza et al., 2008). The Monticchio cra- with depth, and the rate of increase was more clearly ter lakes in Southern Italy are also receiving passive 3 4 observed in early autumn. The He/ He ratio of the magmatic CO2, and the potential risk of a Nyos-type incoming He was estimated to be 5.4 Ratm, suggesting gas hazard has been described (Caracausi et al., 2009). a large contribution of magmatic He with a minor crustral contribution. Using the amount of He stored 8. Concluding remarks during summer stratification and a one-dimensional vertical mixing model, the 4He flux into the lake was This review mainly summarizes the author’s achieve- 8 2 –1 3 4 estimated to be 10 ¥ 10 atoms/cm s with a He/ He ments in work and related matters on the Lakes Nyos ratio of 5.3 Ratm. Since gas samples from the lake were and Monoun gas disasters that took place in the mid- 3 9 >99% CO2, a C/ He ratio of 8.6 ¥ 10 was calculated. 1980s in Cameroon. At that time, nobody knew that 3 3 –2 –1 Combining the He flux of 7.4 ¥ 10 atoms cm s , a lakes could accumulate so much CO2 gas and then sud- CO2 flux into Laacher See was estimated to be 3.3 denly release it to induce such disasters. The Lake Nyos mmol cm–2 y–1 (Aeschbach-Hertig et al., 1996). This and Monoun events had a strong impact on scientists 8 is equivalent to an annual release of 1.1 ¥ 10 mol CO2 working on gas emissions from the interior of the Earth. to the atmosphere. Even if this value represents the This impact especially boosted volcanic lake studies. annual recharge of CO2 to the lake, the holomictic na- Soon after the 1986 gas burst at Lake Nyos, scientists ture does not allow the accumulation of CO2 as was working on the initial phase of their research created a the case in Lakes Nyos and Monoun. small informal group “The International Working Lake Van in Anatolia, eastern Turkey, was formed Group on Crater Lakes (IWGCL)” to exchange scien- during the Pleistocene in a tectonic depression with its tific information about the Lake Nyos gas disaster, to

doi:10.5047/gems.2017.00101.0001 © 2017 TERRAPUB, Tokyo. All rights reserved. 44 M. Kusakabe / GEochem. Monogr. Ser. 1: 1–50, 2017 coordinate follow-up field trips planned by those who the gas content at Lake Monoun has recently been were interested in the subject, and to organize scien- found to be increasing due to the continuing gas sup- tific meetings as a forum for further discussions. The ply from the underlying magma, the duration of which scope of IWGCL was later expanded to include not is much longer than the span of human life. It is al- only studies of gassy lakes in Cameroon but also those most certain that the same situation will occur at Lake of other volcanic lakes in general. The new objectives Nyos within several years when the gas self-lift capa- were to obtain information on the activity and bility is lost. Now is the turn for Cameroonian scien- degassing state of shallow magmatic bodies so that tists and technicians to work toward defusing the new forecasting volcanic eruptions and the mitigation of risks of the increasing gas content in the lakes, for they volcanic lake-related hazards could be achieved. Ex- have acquired the needed knowledge and techniques. I pansion of the scope of IWGCL naturally meant a hope the safety of the lakes is secured and that the sur- greater number of scientists, and resulted in acquiring rounding populations can return to their ancestral roots a formal IAVCEI status as the Commission on Volcanic and go about their daily lives without the fear of fur- Lakes (CVL) in 1993. I was satisfied by these organi- ther gas disasters. zational developments as the leader of IWGCL and CVL in those early days. The CVL has organized sci- Acknowledgments entific meetings every 2–3 years, reports of which can This article is a review of scientific achievements con- be found in the website “http://www.ulb.ac.be/sciences/ cerning the Lakes and Monoun Nyos gas disasters and re- cvl/”. On a personal note, and as a scientist who has lated subjects. The review could not have been made with- worked on Lakes Nyos- and Monoun-related disaster out the cooperation of many colleagues and friends who worked together with me in the field. I express my sincere reduction issues for close to 30 years, I was particu- thanks to: Y. Yoshida, T. Ohba, K. Nagao, G. Tanyileke, F. larly happy to know that the CVL-9 meeting took place T. Aka, Issa, Y. W. Fantong, J. V. Hell, G. W. Kling, W. C. in Yaoundé, Cameroon, in March 2016, to commemo- Evans, D. Rouwet, and many others, who worked together rate the 30th anniversary of the Lake Nyos gas disas- in the field and have provided me with important scientific ter. information. Special thanks go to T. Ohba who kindly sup-

During my career, I have acquired experiences of plied recent data (unpublished) on the CO2 concentrations working in national and international scientific com- in the lakes used in Fig. 15. Fieldwork since in the period munities, and, consequently, have made friendships 1986–2006 was mostly supported by the Grant-in-Aid for with many wonderful scientists worldwide. Such ex- Scientific Research from JSPS (Japan Society of Promotion periences have led me to obtain research funding that of Science). Recent fieldwork (2011–2015) has been sup- has made it possible for me to continue to work in ported by the SATREPS-NyMo project. Logistic support from IRGM and its technicians is appreciated. The Embassy Cameroon for ~30 years. As described in Section 5 of of Japan and the JICA office in Yaoundé are acknowledged this article, a typical example was the success in get- for their help while I was in Cameroon. ting support from JICA and JST for the SATREPS- K. Oshida of TerraPub is acknowledged for giving me the NyMo project. The resolution of solving various prob- chance to write this review. I also thank Y. Matsuhisa who lems associated with the Lakes Nyos and Monoun gas made constructive comments on an early version of the disasters, such as the continuation of scientific moni- manuscript. W. C. Evans, D. Rouwet and F. Aka are also toring of the lakes, the monitoring of the reinforced thanked for their comments that helped improve the manu- natural dam at Lake Nyos, the rehabilitation and set- script. The English of the final version was improved by D. ting up of an infrastructure for the displaced people, Larner who kindly checked the manuscript in a very careful etc., are obviously domestic issues for which the manner and suggested the corrections. Cameroonian Government and scientists should, in principle, take responsibility. But the reality is differ- References Aeschbach-Hertig, W., Kipfer, R., Hofer, M., Imboden, D. ent; the economic insufficiency of Cameroon has hin- M., Wieler, R. and Signer, P. (1996) Quantification of gas dered the principle. The main goal of the SATREPS- fluxes from the subcontinental mantle: The example of NyMo project is to mitigate natural disasters in Laacher See, a maar lake in Germany. Geochim. Cameroon through capacity building, specifically for Cosmochim. Acta 60, 31–41. issues related to the Lakes Nyos and Monoun gas dis- Aeschbach-Hertig, W., Hofer, M., Kipfer, R., Imboden, D. asters. The risks of the recurrence of limnic eruptions M. and Wieler, R. (1999) Accumulation of mantle gases can be defused if proper and timely actions are taken. in a permanently stratified volcanic lake (Lac Pavin, The SATREPS-NyMo capacity building included the France). Geochim. Cosmochim. Acta 63, 3357–3372. donation of some analytical instruments necessary to Aka, F. T. (2000) Noble gas systematics and K-Ar chronol- help Cameroonian scientists achieve the project’s goals. ogy: Implications for the Cameroon Volcanic Line, West Also included was the training of young Cameroonian Africa. Ph.D. thesis, Okayama University, Japan. Aka, F. T. (2015) Depth of melt segregation below the Nyos scientists and technicians in Japan, so that, after they maar-diatreme volcano (Cameroon, West Africa): Major- get back home, they can play an important role in the trace element evidence and their bearing on the origin of field of mitigation of natural disasters. Unfortunately, CO2 in Lake Nyos. Volcanic Lakes (Rouwet, D.,

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