lakes nyos and monoun gas disasters (cameroon)—limnic ...phreato-magmatic eruption (lockwood and...

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

Received on December 5, 2015Accepted on May 11, 2016Online published on

April 7, 2017

Keywords• Cameroon• Lakes Nyos and Monoun• gas disaster• crater lakes• magmatic CO2• limnic eruption• disaster mitigation• degassing• Cameroon Volcanic Line• SATREPS

© 2017 TERRAPUB, Tokyo. All rights reserved.doi:10.5047/gems.2017.00101.0001 ISSN: 2432-8804

degassing of CO2 is the quiet discharge of gas oftenderived from a magmatic source, with varying degreesof contamination by crustal or biological CO2. Craterlakes usually sit on top of volcanic conduits and act ascondensers or traps for magmatic volatiles. The LakeNyos gas disaster in 1986, and a similar event in 1984at Lake Monoun, both in Cameroon, Central Africa,resulted from an excessive accumulation of magmaticCO2 in the bottom layers of the lakes. These volcanic

AbstractThis is a review paper on the Lakes Nyos and Monoun gas disasters that took place in themid-1980s in Cameroon, and on their related geochemistry. The paper describes: (i) thegas disasters (the event and testimonies); (ii) the unusual geochemical characters of thelakes, i.e., strong stratification with high concentrations of dissolved CO2; (iii) the evolu-tion of the CO2 content in the lakes during pre- and syn-degassing; (iv) the noble gassignatures and their implications; (v) a review of models of a limnic eruption; (vi) arevision of a spontaneous eruption hypothesis that explains the cyclic nature of a limniceruption (Kusakabe 2015); (vii) a brief review of the origin of the Cameroon VolcanicLine (CVL) and the geochemistry of CVL magmas; (viii) a brief review of other CO2-rich lakes in the world; and (ix) concluding remarks.

Degassing of the two lakes has been successful. Most of the dissolved CO2 has beenremoved from Lake Monoun, resulting in the stoppage of the degassing system. How-ever, the CO2 content in the lake started to increase in recent years due to the continuingsupply of gas from the underlying magma, indicating the necessity of the continuousremoval of gas from the lake. Lake Nyos will attain the same situation in several yearswhen degassing will stop. Thus, a continuation of scientific monitoring of the lakes isessential. Since the transfer to Cameroonian scientists of monitoring techniques, includ-ing analytical equipment necessary for the monitoring, has been effected through theSATREPS 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 CameroonianGovernment and local scientists.

Lakes Nyos and Monoun Gas Disasters(Cameroon)—Limnic Eruptions Caused byExcessive Accumulation of Magmatic CO2in Crater Lakes

Minoru Kusakabe

Department of Environmental Biology and ChemistryUniversity of Toyama3190 Gofuku, Toyama 930-8555, Japane-mail: [email protected]

Citation: Kusakabe, M. (2017) Lakes Nyos and Monoun gas disasters (Cameroon)—Limnic erup-tions caused by excessive accumulation of magmatic CO

2 in crater lakes. GEochem. Monogr. Ser.

1, 1–50, doi:10.5047/gems.2017.00101.0001.

1. Introduction

Volatiles in the deep interior of the Earth are broughtto the surface mainly by volcanic activity. In terms ofthe present-day global carbon cycle, the CO2 dischargefrom subaerial volcanism including the passive dis-charge from the craters or flanks of volcanoes, is themajor non-anthropogenic contributor to atmosphericCO2 (e.g., Kerrick, 2001; Gerlach, 2011). The passive

2 M. Kusakabe / GEochem. Monogr. Ser. 1: 1–50, 2017

doi:10.5047/gems.2017.00101.0001 © 2017 TERRAPUB, Tokyo. All rights reserved.

crater lakes are considered to be the sites of passivedegassing of CO2. On 26th August, 1986, a largeamount of CO2 was suddenly released from Lake Nyosthat asphyxiated 1746 people, and an unaccountablenumber of cattle, living near the lake (Sigvaldason,1989). A very similar gas event took place in August1984 at Lake Monoun, with 37 casualties (Sigurdssonet al., 1987). Lake Monoun is located only 100 kmsouth-east of Lake Nyos (Fig. 1). A term “limnic erup-tion” was coined by J.-C. Sabroux to describe a gasoutburst from a lake (Halbwachs et al., 2004), and willbe used in this review. Given that this type of gas dis-aster had not been previously recorded (Sigurdsson,1987a), the Lakes Monoun and Nyos events attracteda great deal of attention, not only from the media butalso from a disaster science perspective. At that time,nobody imagined that the lakes had accumulated somuch lethal gas and that the gas was released into theatmosphere without any precursor. Subsequent

geochemical investigations revealed that the gas wasCO2 that originated from magma and had accumulatedpassively in the deep part of these lakes. The physico-chemical characteristics of the lakes are unique andhave evolved with time, even after the gas release, dueto the continuing supply of magmatic CO2.

In the present paper, issues related to these gas dis-charges are reviewed in the following sections; (III)what happened at the time of the Lakes Nyos andMonoun gas disasters?; (IV) pre- and syn-degassingchemical evolution of the lakes; (V) possible causesof the disasters, the models and the repetitive natureof a limnic eruption. In relation to the recurrence pre-vention of a limnic eruption, a bilateral scientificproject between Japan and Cameroon called SATREPS-NyMo was carried out during 2011 and 2016, and isoutlined in Section 5.

The upper 40 m of Lake Nyos is bounded on the northby a narrow dam of poorly consolidated pyroclastic

Fig. 1. Location of Lakes Nyos and Monoun (red circles) and volcanoes along the Cameroon Volcanic Line (solid black) inCameroon, Central Africa. Modified from figure 1 of Environmental Monitoring and Assessment Journal, Hydrogeochemistryof surface- and groundwater in the vicinity of Lake Monoun, West Cameroon: Approach from multivariate statistical analysisand 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 Switzerland2015 with permission of Springer.

M. Kusakabe / GEochem. Monogr. Ser. 1: 1–50, 2017 3


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. Awarning was given that the collapse of the dam couldcause a flood that would affect inhabited areas over a220 km distance (Lockwood et al., 1988). An accurateestimation of the rate of back erosion of the dam iscritical for the safety of people living downstream.Thus, the age of the dam formation (or Nyos maar for-mation) has been hotly debated using different agedetermination techniques. Recent progress on the ageof the dam is briefly reviewed in Section 6.

Thirty nine crater lakes including Lakes Nyos andMonoun and numerous soda springs are located alongthe Cameroon Volcanic Line (CVL). An understand-ing of the origin and the geochemistry of CVL mag-mas is essential. These subjects are reviewed in Sec-tion 6, which constitutes the basis on which CO2 accu-mulation in these lakes is scientifically interpreted. Wealso need to understand why CO2 becomes enriched inmagmatic volatiles as they leave the magma. The LakesNyos and Monoun events have stimulated geochemicalinterest in other CO2-rich volcanic lakes in the worldfor their gas hazard potential. This is reviewed in Sec-tion 7.

2. Gas disasters at lakes Nyos and Monoun,Cameroon

2-1. Cameroon: Location and physiography

Cameroon is a country in Central Africa located be-tween 2–13∞N latitude, and 8–16∞E longitude (Fig. 1).It is bounded by 6 countries: Chad to the northeast,Nigeria to the west, Central African Republic to theeast, Equatorial Guinea, Gabon and Congo to the south.Cameroon can be divided into ten major ecologicalregions. These regions are classified under four re-

gional units which are differentiated by their geogra-phy, climate and vegetation characteristics as follows:(1) The Sudano-Sahelian zone in the North is composedof the Mandara mountains, Diamaré plains and theBenue Valley. (2) The savanna zone is composed ofthe Adamawa highlands, the Tikar plain, the low landsavanna of the Center and East regions, and the high-land of the West and Northwest regions. (3) The tropi-cal forest zone is composed of the degraded forests ofthe Central and Littoral regions, and the tropical rain-forests of the Southwest and East regions. (4) Thecoastal and marine zone spreads along the Gulf ofGuinea. The country’s economy is driven by agro-in-dustry in the coastal, central and southern zones (Moluaand Lambi, 2006). Because of the above geographiccharacteristics, its wide range of climatic types, andits cultural diversity, Cameroon is often nicknamed“Africa in miniature”. The population of Cameroon isestimated to be ~23 million as of January 2015 (http://countrymeters.info/en/Cameroon). According to theDemographics of Cameroon (http://en.wikipedia.org/wiki/Demographics_of_Cameroon), the country com-prises an estimated 250 distinct ethnic groups, whichmay be classified into five large regional-cultural di-visions: (1) the western highlanders (Semi-Bantu orgrassfielders), including the Bamileke, Bamoun, andmany smaller Tikar groups in the Northwest (~38% ofthe total population); (2) the coastal tropical forestpeoples, including the Bassa, Duala (or Douala), andmany smaller groups in the Southwest (12%); (3) thesouthern tropical forest peoples, including the Beti-Pahuin with subgroups called Bulu, Fang, Maka, Njem,and Bakapygmies (18%); (4) the predominantly Islamicpeoples of the northern semi-arid regions (the Sahel)and central highlands, including the Fulani (or Fulbe)(14%); and (5) the “Kirdi”, non-Islamic, or recently



Islamic, peoples of the northern desert and central high-lands (18%). Since people of different ethnic groupsspeak different languages, French and English, inher-ited from colonialism, are used for mutual communi-cation, although they retain their original languages.

2-2. Cameroon lakes

There are at least 39 lakes of volcanic origin that aredistributed along the CVL (Kling, 1988; Issa et al.,2014a). Lake Nyos in the Northwest Region ofCameroon (06∞26¢ N and 10∞17¢ E) is a meromicticvolcanic crater lake with a N-S length of ~2.0 km, E-W length of ~1.2 km, surface area of 1.58 km2, and amaximum depth of 209.5 m. It lies in the Oku volcanicfield along the CVL, and was formed by a basalticphreato-magmatic eruption (Lockwood and Rubin,1989). The age of the lake, which has been a topic ofcontroversy, will be described later (Subsection 6-1).Lake Monoun in the West region of Cameroon lies at05∞35¢ N and 10∞35¢ E, and is also a meromictic vol-canic crater lake with a NEE-SWW length of ~1.6 km,a maximum NW-SE width of ~0.7 km, a surface areaof 0.31 km2, and a maximum depth of 99 m. It belongsto the Oku volcanic center along the CVL. The age ofthe lake is unknown.

2-3. Lake Nyos disaster: The event and testimo-nies

Unusual news raced around the world in late August1986. The first news that reached Japan reported that40 local people had been killed by a “poisonous” gasreleased from a volcanic lake (Lake Nyos) inCameroon. The number of casualties increased to~1200 in a later report. In response to a request for

international support by the Government of the Repub-lic of Cameroon, the Japan International CooperationAgency (JICA) under the Ministry of Foreign Affairsof Japan sent a Japan Disaster Relief Team (JDR) tothe site. I was asked to join the team as a volcanic gasexpert. It was my first visit to Cameroon. The JDR teamarrived at Douala on 28 August, 1986. A few days laterthe team was taken to Lake Nyos by helicopter becauseof poor road conditions and heavy rains in the Nyosarea. We were shocked by the terrible scenes we wit-nessed (Fig. 2), and the reddened surface water of thelake (Fig. 3), which increased our anxiety concerningthe cause(s) of the disaster. Since the main purpose ofJDR was to provide relief supplies and medical care tothe refugees, we made an initial cursory scientific sur-vey during this first visit. There was no indication ofthe direct involvement of volcanic gases as initiallysuggested, for we did not find any trace of acid gaseslike SO2, H2S and HCl, which are major componentsof high-temperature volcanic gases. Later reports in-dicated that the cause of the deaths was CO2 gas re-leased from Lake Nyos on the evening of 21 August,1986, and that the gas killed 1746 people and ~8000livestock by asphyxiation (Kling et al., 1987; Kusakabeet al., 1989). Exactly 2 years prior to the Lake Nyosdisaster, there had occurred a very similar gas releasefrom Lake Monoun on 15 August, 1984, that killed 37people also by asphyxiation by CO2 gas released fromthe lake. These extremely unusual disasters had neverbeen recorded before, and therefore constituted a newtype of natural disaster (Sigurdsson, 1987a).

August 21, 1986, was a Thursday, and a market dayat the Nyos village. Local people were selling and ex-changing their agricultural products and articles fordaily use. At the time when the catastrophe occurred(8~9 p.m.), people must have been relaxed and chat-

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



ting at home, or drinking beer outside. Some peoplemay have already been in bed. Such a peaceful situa-tion was disrupted by a sudden release of lethal gasfrom Lake Nyos. It was indeed a nightmare. It is obvi-ous that the local people did not understand what hap-pened. From the testimonies collected later from sur-vivors by journalists and researchers, the event mayhave proceeded as follows. Some people heard faintrumbles and noises like a car coming from a distance.They went out of the house to look around, and thenfelt a tepid breeze with a smell of rotten eggs or gunpowder. Most people fell down, lost consciousness anddied (Fig. 2). At Nyos village, where 1200 people livedat that time, only a few people (4~6) survived. Thesurvivors were stunned to find that they had lost mostof their family members, relatives and friends.Sigurdsson (1987b) noted that “some survivors of thedisaster attributed it to the wrath of their dead tribalchief, who, on his deathbed in 1983, ordered that hisbest cattle be driven off the sheer cliffs above LakeNyos 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 anexpression of the chief’s posthumous displeasure”.Sigurdsson (1987b) also described that, four days be-fore the lethal event, local herdsmen noticed unusualbubbling on the lake’s surface, which prompted twentyfive of them to move to a distant village. There werealso unconfirmed reports claiming the emission offoaming water and vapor from the lake two to threeweeks earlier. At about 4:00 p.m. on August 21, nearbyherdsmen heard strange bubbling sounds and observeda slight emission of vapor from the lake. At about 8:00p.m., villagers in Cha, a village about 7 km northwestof the lake, heard two loud noises, followed by threerumbles at about 9:00 p.m., when activity built up tothe climactic disaster.

According to Aramaki et al. (1987) who interviewedMallam Jae, a local farmer who lived at a place 120 mhigher than the lake surface, the gas explosion tookplace at about 8:30 p.m. on August 21 and continueduntil 1 a.m. the next day. This account of the time atwhich the events took place may be reliable, sinceMallam Jae was wearing a nice wrist watch. Initially,Mallam Jae heard sounds like a murmur followed bydetonations. He also felt tremors and a smell of gunpowder. The next morning he found the lake quite unu-sual. Le Guern et al. (1992) published details of inter-views with some local people who spoke in pidginEnglish (which was translated into English) about whatthey saw. Observations by local people included: (1)Minor upwelling of hydrothermal waters from the bot-tom of Lake Nyos on August 20, one day before theevent. (2) A small explosion that took place at 4:00p.m. followed by a major explosion between 8:00 and8:30 p.m. on August 21. (3) A water jet accompaniedby white illuminations. (4) A detonation was heard in

the village at 11:00 p.m. (5) Minor events with anupwelling of hydrothermal water and gas occurred inLakes Nyos and Njupi, a small and shallow lake 2 kmeast of Lake Nyos (Chevrier, 1990). (6) White cloudwas seen during the catastrophe on August 21. Basedon these testimonies and observations, Le Guern et al.(1992) preferred the interpretation that the Lake Nyoscatastrophe was caused by the input of hot hydrother-mal fluids containing CO2 into the lake and surround-ing area. Their interpretation seemed to have been in-fluenced by their experience at Dieng volcano (Indo-nesia) where CO2 gas, originating from a phreatic erup-tion of the volcano, killed 142 local people who werefleeing from the site (Le Guern et al., 1982). How-ever, there is a view that the anecdotal evidence (suchas “the smell of rotten eggs and gun powder, rumblingnoise heard at distance”, etc.), collected soon after thedisaster through interviews with local survivors byjournalists and scientists, should be interpreted withcare because stories told by local people may have beentailored to give answers to please the visiting inter-viewers (Freeth, 1990). The author adopts this viewand believes that the phreatic hypothesis did not havea firm scientific basis, because this interpretation ofthe events was largely based on the testimonies andanecdotal evidence.

In March 1987, a Cameroon Government and

Fig. 4. Photograph showing a white cloud still remainingalong the valley downstream of Lake Nyos. Nyos village isseen at the bottom. The photo was taken 2 days after theeruption by a helicopter pilot carrying a Catholic mission(supplied by G. Tanyileke).



UNESCO-sponsored international conference on theLake Nyos Disaster was held in Yaoundé, the capitalof Cameroon. More than 200 scientists participated andpresented the results of their initial studies on the geo-logical, geochemical, physical, medical and socio-an-thropological aspects of the disaster (Sigvaldason,1989). Regarding the cause of the gas burst, there wasa sharp confrontation between a group of scientists whobelieved that the lake played a key role in the accumu-lation of the CO2 which was subsequently released (thisinterpretation was later named “limnic eruption hypoth-esis”) and another group of scientists who believed thatthe cause of the Nyos catastrophe was due to a vol-canic (phreatic) eruption from the bottom of the lake(volcanological or phreatic hypothesis) (Tazieff, 1989;Barberi et al., 1989; Le Guern et al., 1992). Disagree-ment between the two scientific views resulted in acompromise of the resolutions of the Yaoundé Confer-ence (Sigvaldason, 1989), and encouraged the need forfollow-up investigations, which clearly indicated asteady supply of magmatic CO2 from the lake bottomand its accumulation in the lake. This gave strong sup-port to the limnic eruption hypothesis (Kling et al.,2005; Kusakabe et al., 2000, 2008). This hypothesiswill be described in detail in Sections 3 and 4.

The gas released from Lake Nyos was almost pureCO2 (Kling et al., 1987; Kusakabe et al., 1989). SinceCO2 gas is 1.5 times denser than air at room tempera-ture, and since it may have been cooled due to adi-abatic expansion when released from the pressurizeddeep part of the lake, the density of the gas was likelyto have been significantly greater than that of the am-bient atmosphere. This would have facilitated its flow

along low-lying areas, such as valleys, before mixingwith air. Costa and Chiodini (2015) simulated the gasflow, using a computer code TWODEE-2, for 4 differ-ent scenarios that considered different gas masses andfluxes from Lake Nyos in 1986. The simulations, indi-cating how far and fast the cloud dispersed after thelimnic eruption, are useful for making up a hazard mapof the area. Figure 4 is a photograph taken two daysafter the eruption by a Helimission helicopter pilot (G.Tanyileke, pers. commun.) and shows that the whitecloud was still present along a valley downstream ofthe lake. Figure 5 (modified from Sigurdsson et al.,1987a) shows the gas flow path estimated from thedistribution of victims around Lake Nyos. The gascloud traveled more than 20 km, asphyxiating peopleon its way before dissipating into air. The number ofvictims was 1200 at Nyos village, 300 at Cha villageand 52 at Subum village. More than 8000 cattle werealso killed. Survivors were evacuated to 7 resettlementcamps, namely, Kimbi, Buabua, Kumfoutu, Yemge,Ipalim, Esu and Upkwa (around Lake Wum). As of July2015, these people were still cut off from the generalpopulation, as neither the national radio, electricitygrid, nor television signals reached them.

Since the victims were asphyxiated almost instantly,the oxygen concentration in the gas must have beenextremely low compared to normal the atmosphere, orthe CO2 concentration was very high. Table 1 showsthe effect of some gases on human health (Kusakabeet al., 1989). Mammals, including human beings, liveon a normal atmosphere that contains 21 vol % of O2.If air is breathed containing less than half of this nor-mal air concentration of O2, a coma, fainting, cyano-sis, syncope, respiratory arrest, and ultimately, cardiacarrest can result. If we breathe air containing high con-centrations of CO2 (e.g., >10 vol %), a coma, and even-tually, death can result. Some survivors of the LakeNyos disaster were found to suffer from pulmonaryedema, respiratory distress, conjunctivitis, and skinlesions or “burns” (Baxter et al., 1989). The skin burnswere taken by the phreatic hypothesis supporters asevidence that the gas was at a high temperature andcontained some acidic, corrosive components, such asSO2 (which turned to sulfuric acid later) and HCl thatare commonly contained in high-temperature volcanicgases. This interpretation was highly unlikely, sincevegetation, and clothes on the victims stayed intact andno appreciable level of sulfur and chlorine componentswere found in the lake water (Kusakabe et al., 1989).The medical interpretation of the skin damage or blis-ters was that the body’s metabolic rate was drasticallyreduced in a state of deep coma, inducing a severelyrestricted circulation of blood. As a consequence, thecapillary vessels of the skin lacked circulation, result-ing in necrosis and the development of skin lesions onapproximately 5% of the patients (Baxter et al., 1989).

Fig. 5. Distribution of localities where victims were foundaround Lake Nyos (red circles), and estimated flow paths ofthe gas (arrows). Modified from Sigurdsson (1987a).



2-4. Lake Monoun disaster

Lake Monoun experienced a gas burst on 15 August,1984, that killed 37 people by asphyxiation. A recon-struction of the event (Sigurdsson et al., 1987) showedthat at almost midnight of that day, people in Njindoun,a village about 5 km north of the lake, heard a loudnoise in the vicinity of the lake. They informed thenearby police early next morning. A policeman togetherwith a medical doctor went to the site where they sawa whitish, smoke-like cloud that covered the ground toa height of ~3 m. Since they became nauseous anddizzy, they left the site and moved to Njindoun vil-lage. After the cloud dissipated, they came back to thesite and found dead people lying on the road. The vic-tims had skin lesions or blisters. Clothes were not af-fected. Domestic and wild animals were also founddead. Altogether 37 people were killed by this event.A survivor described the smell of the gas cloud as“sulfurous like a car battery”. It was found in a latersurvey that vegetation at the east end of the lake wasflattened, indicating that the water wave locally reachedup to 5 m high, and that the color of the lake waterchanged to a reddish brown. From the above state-ments, the Lake Monoun event was very similar to theLake Nyos event. For this reason, it seems appropriateto describe and compare, at the same time, the resultsof the geochemical surveys made after the gas burstsat both lakes.

2-5. Unusual geochemical characters of LakesNyos and Monoun

A scientific survey of Lake Monoun was undertakenby the Office of Foreign Disaster Assistance (USAID)

several months after the gas burst, upon a request bythe Cameroonian Government (Sigurdsson et al.,1987). They found unusual chemical characteristics.Waters below 50 m were anoxic, dominated by highFe2+ (~200 mg/l) and HCO3

– (~1000 mg/1), and su-persaturated with siderite, a major component of thecrater floor sediments. The unusually high Fe2+ levelswere attributed to the reduction of laterite-derived fer-ric iron that was gradually brought into the lake as loessand in river input. Sulfur compounds were below de-tection limits in both water and gas. Table 2 shows thechemical composition of Lake Monoun water samplescollected between 27 February and 16 March, 1985(Sigurdsson et al., 1987). It includes data for samplescollected in 1986 (Kusakabe et al., 1989) and 1993(Kusakabe et al., 2008). Analysis of the 1985 and 1986samples showed lower gas and ionic contents than theoriginal solution. This was interpreted to be due to (i)loss of CO2 from the waters during retrieval of theNiskin sampler from the lake, (ii) loss of CO2 fromwaters collected in the sample containers, and/or (iii)oxidation and precipitation of iron prior to the analy-sis. During the early stages of investigation of the LakesMonoun and Nyos disasters, these problems high-lighted the difficulty of sampling and the analysis ofCO2-rich waters. The data for the 1993 samples weremore reliable (Kusakabe et al., 2008). Based on thedata obtained in 1985, however, Sigurdsson et al.(1987) had come to the important conclusion that ac-cumulation of CO2(aq) in the lake was attributed to thelong-term emission of magmatic CO2(aq) from ventswithin the crater, which led to a gradual build-up ofCO2(aq) and HCO3

– in the lake, i.e., an essential con-cept of the “limnic eruption hypothesis”. It is notedthat the manuscript of the paper by Sigurdsson et al.

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

Concentration in atmosphere Stage Response and symptoms

O2 (%)21 1 Normal16-12 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 Normal1.5 2 Changes in physiological ranges (techypnea etc.)5 Shortness of breath, headache, coma

10 4 Coma in 10-15 min. exposure>40 5 Sudden death

3

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



(1987) had been prepared prior to the 1987 Interna-tional Conference on the Lake Nyos disaster inYaoundé, so the American team who started an initialscientific survey at Lake Nyos must have had a gen-eral idea of the cause of the disaster.

Lake Nyos also has unusual chemical and physicalcharacteristics similar to Lake Monoun. Dissolved spe-cies are overwhelmingly dominated by CO2(aq) fol-lowed by HCO3

–, Fe2+, Mg2+, Ca2+, SiO2, NH4+, Na+,

K+ in decreasing order. The concentration of the dis-solved species in the water column increases with depthwith maximum values reached at the bottom (210 m).Follow-up studies of Lakes Nyos and Monoun clearlyindicated that the CO2 content in the lakes was increas-ing at an unusually high rate for a geological phenom-

enon (Evans et al., 1993; Kusakabe et al., 2000). Thissituation led scientists working on Lakes Nyos andMonoun to warn of the possible recurrence of a limniceruption in the near future and to recommend the arti-ficial removal of dissolved gases from the lakes (Freethet al., 1990; Tietze, 1992; Kling et al., 1994; Kusakabeet al., 2000). To achieve this goal, the Nyos-MonounDegassing Program (NMDP) was set up by scientistswho were deeply involved in the disaster mitigationissues of the limnic eruption. After experimentaldegassing at Lake Monoun (Halbwachs et al., 1993)and Lake Nyos (Halbwachs and Sabroux, 2001), a per-manent degassing apparatus was installed at Lake Nyosin 2001 and at Lake Monoun in 2003 under the NMDP,funded by the U.S. Office of Foreign Disaster Assist-

Depth pH Na+ K+ NH4+ Mg2+ Ca2+ Fe2+ SO4

2- 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 2615 5.8 17 4.7 6.2 22 20 0.03 <1 1.8 40 265 95261 6.3 25 5.6 15 29 41 200 <1 3.2 35 1050 108690 6.4 24 5.7 13 30 42 220 <1 3.4 45 1050 89090 6.1 24 5.7 15 30 41 170 <1 3.3 50 775 131190 6.3 25 5.8 18 30 43 260 <1 3.2 48 805 85190 6.4 24 5.9 13 30 42 190 <1 3 41 805 68090 6.0 26 5.5 15 29 42 290 <1 3.4 50 1000 211290 5.9 26 5.5 12 30 41 190 <1 3.2 51 885 2357

October 1986*2

0 æ 11 3 æ 4.2 4 1.7 0.1 0.4 17 57 16

15 æ 14 4.2 6 13 10 110 0.3 1.0 40 421 202

25 æ 17 5.3 11 17 12 190 0.2 1.4 37 657 542

50 æ 17 5.1 17 13 10 340 0.4 2.5 38 1087 2859

75 æ 21 7.2 26 22 18 540 0.4 2.5 42 1520 2385

95 æ 22 7.2 26 23 19 590 0.2 2.6 44 1660 2922

January 1993*3

10 6.58 13 3.5 10 8 13 100 0.1 1.1 34 82 420 6.46 15 4.3 12 15 21 259 0.1 1.5 58 105 1830 6.00 18 4.6 18 24 30 464 0.1 1.5 86 1352 180940 5.91 19 5.0 19 24 33 505 0.1 1.6 91 1448 227150 5.80 21 5.3 22 23 38 533 0.1 1.8 95 1546 321755 5.60 23 5.5 28 27 46 641 0.1 2.3 110 1823 676075 5.58 24 5.8 28 27 46 646 0.1 2.4 114 1862 684890 5.60 24 5.4 30 28 48 682 0.1 2.4 114 1961 681395 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

-

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

*1Sigurdsson et al. (1987).*2Reproduced from table 2 of J. Volcanol. Geotherm. Res. 39, Kusakabe, M., Ohsumi, T. and Aramaki, S., The Lake Nyos gasdisaster: chemical and isotopic evidence in waters and dissolved gases from three Cameroonian crater lakes, Nyos, Monounand 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.



ance (USAID) and the Cameroonian Government. Con-trolled degassing is continuing successfully at bothlakes. Figure 6 shows the amazingly beautifulfountains in the initial phase of degassing at LakesNyos and Monoun, a 45 m high fountain at Lake Nyos(Feb. 2001) and a 8 m high fountain at Lake Monoun(Jan. 2004). The degassing system and the construc-tion of the degassing pipes have been described inHalbwachs et al. (1993, 2004). There was concern thatartificial degassing might trigger another limnic erup-tion, since degassing could bring deep water to thesurface, which will become cooler due to adiabaticcooling, and therefore may sink and destabilize the lake(e.g., Freeth, 1994). However, numerical modeling ofthe evolution of CO2 in the lake under different inputconditions (McCord and Schladow, 1998; Kusakabe etal., 2000; Schmid et al., 2003, 2006) suggested thatdestabilization of the water column due to controlleddegassing could not pose an immediate threat from a“man-made” limnic eruption. However, the possibil-ity of thermal instability of the water column between50–70 m, which could become a trigger for a limniceruption, was suggested by Schmid et al. (2004), forthey found double-diffusive convection at that depthrange. In agreement with the results of the numericalmodeling, the observed chemical structure of the lakesafter the initiation of the controlled degassing opera-tion indicated that a stable stratification was estab-lished, which remained basically the same as the pre-degassing situations at both lakes (Kling et al., 2005;Kusakabe et al., 2008).

As stated above, the chemical and physical structureof Lakes Nyos and Monoun evolved steadily with time

until the early 2000s when degassing started. After theinitiation of gas removal, the lake structure was obvi-ously affected by degassing. For this reason, the evo-lution is better described separately as “pre-degassing”and “syn-degassing”.

3. Pre- and syn-degassing evolution of Lakes Nyosand Monoun

3-1. Pre-degassing evolution

The first scientific reports on the 1986 Lake Nyosgas disaster were published by Freeth and Kay (1987),Kling et al. (1987) and Tietze (1987). Of these, Klinget al. (1987), which is easily accessible, gave the mostcomprehensive results of the initial survey of the dis-aster, which included the geology of the region, thegeochemistry of water and gas from the lake, and thepathology of hospitalized people and victims. Theyconcluded that (i) the gas released was CO2 that hadbeen stored in the lake’s hypolimnion (bottom layer),(ii) the victims died of CO2 asphyxiation, (iii) CO2 wasderived from magmatic sources, and (iv) there was nodirect volcanic activity involved. Kusakabe et al.(1989) reached similar conclusions on the basis ofwater chemistry and carbon and noble gas isotopiccompositions of the gases dissolved in Lakes Nyos andMonoun. They noted that the H2S concentration in thereleased gas must have been far below a lethal level, apoint that precluded the phreatic eruption hypothesis(see above). The same authors also reported the firstpetrochemical data of ejecta around Lakes Nyos andMonoun, which indicated that the lavas were transi-

Fig. 6. Fountains from the degassing pipes. The fountain heights were 45 m at Lake Nyos, February 2001 (a) and 8 m at LakeMonoun, January 2004 (b). The tapping depth of the pipes was 203 m and 73 m, respectively.



tional to slightly alkaline in composition (see below).A detailed temporal variation of the chemical struc-

ture of Lakes Nyos and Monoun since the limnic erup-tions at both lakes was reported by Kusakabe et al.(2008). This paper presented the most comprehensivedata set of chemical composition, conductivity, tem-perature, pH and CO2 profiles obtained from measure-ments taken almost every 2–3 years from 1986 to 2006,which enabled an evaluation of the evolution of CO2in the lakes over a period of about 20 years and whichencompassed pre- and syn-controlled degassing peri-ods. The chemical structure of the lakes is best repre-sented by a conductivity profile (Fig. 7). Both lakeshave a similar chemical structure which is character-

ized by four distinct layers. At Lake Monoun, layer Iis the shallowest, is well-mixed, and contains low con-ductivity water. A sharp chemocline separates layers Iand II at 23 m in January 2003. Layer II extends downto a 51 m depth, where a second chemocline develops.A well-mixed layer III continues down to ca. 85 m.Below this depth, conductivity increases steadily to-ward the bottom (layer IV). Lake Nyos has basicallythe same structure as Lake Monoun in January 2001:layer I is the shallowest, is well mixed, and containslow conductivity water. A sharp chemocline at about a50 m depth separates layers I and II, the latter extend-ing down to about a 180 m depth. A lower chemoclinedevelops around this depth, below which a well-mixed

Fig. 7. Profiles of electric conductivity normalized at 25∞C (abbreviated as C25) at Lake Monoun, January 2003 (left) andLake 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.



layer III continues down to ~203 m. Below this depth,conductivity sharply increases towards the bottom(layer IV).

Pre-degassing temperature variations at LakesMonoun (October 1986 to January 2003) and Nyos(November 1986 to January 2001) are shown in Figs.8a,b, respectively, (reproduced from Kusakabe et al.,2008). Temperature profiles at Lake Monoun show aminimum at the 5–21 m range (layer I); followed by(i) an increasing temperature to about 23∞C down tothe lower chemocline at depths of 50–63 m (layer II),(ii) constant values down to around 90 m (layer III),and (iii) a second increase to >23∞C towards the bot-tom (layer IV). It is worth noting that the temperatureof the layer III water increased significantly between1986 and 1999, and that, at the same time, layer III(thermally homogeneous zone) widened, forming a“shoulder” at a depth of 51 m. This widening suggeststhat warmer water was added to layer IV, and the pro-files were pushed upward. A simple heat balance indi-cates that the heat accumulated in layers III and IVduring 15 years (October 1986 to January 2003) is 7.8¥ 1012 J, supplying heat at an average rate of 5.1 ¥1011 J/yr (~0.02 MW). The incremental upward move-ment of the lower thermocline (Fig. 8a) indicates theaddition of water to layer IV, most likely from the bot-tom. If 4.1 ¥ 108 tons of water having a temperature of27∞C were added, it would account for the heat accu-mulation during that period. Diffusive and conductiveheat loss to layer II and above was not taken into con-sideration in this simple heat balance calculation, thusgiving a minimum heat supply. Note that the rate ofheat and water supply to layers III and IV initially ap-

pears high judging from the change in the temperatureprofiles (Fig. 8a).

Similar to Lake Monoun, the temperature of the LakeNyos bottom water increased continuously after thelimnic eruption in 1986 (Fig. 8b), indicating a heat in-put into the lake. The heat input to layers III and IVwas reported to decrease from an initially high valueof 0.93 MW (August 1986 to May 1987) to 0.43 MW(November 1986 to December 1988, Nojiri et al., 1993)to 0.32 MW (May 1987 to September 1990, Evans etal., 1993).

Figure 9 shows the temporal change in the pre-degassing conductivity profiles at both lakes. As pre-viously stated, Lake Monoun profiles have a “shoul-der” between layers II and III. The shoulder becameshallower and sharper, and layer III widened with timeand its conductivity increased (Fig. 9a). Vertical con-ductivity profiles in layer III suggest that the layer iswell mixed. The rise of the shoulder indicates an addi-tion of recharge water from the bottom, pushing bot-tom water upward. This is consistent with the changesin the temperature (Fig. 8a). By combining the con-ductivity profiles from October 1986 to January 2003(15 years) with the bathymetry used in Kling et al.(2005), we calculated an overall increase of 2.7 ¥ 103

tons of Total Dissolved Solids (TDS) in layers III andIV. This translates into an average annual TDS inputrate of 1.7 ¥ 102 tons/yr. The sharp conductivity risetoward the bottom in layer IV may be related to disso-lution and reduction of particles containing ferric com-pounds to release Fe2+ and HCO3

– in the sediments thatare rich in organic material. The concentration of Fe2+,HCO3

– and NH4+ increased significantly with depth

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.



only in layer IV whereas the other ions such as Na+,K+, Mg2+, Ca2+ showed a steady increase with depth(Kusakabe et al., 2008).

At Lake Nyos, shallow water in November 1986 hada higher conductivity, even at about 7 m (Fig. 9b), thanthat in later years, indicating that deep, TDS-rich wa-ter was brought to the surface during the limnic erup-tion, the effect still remaining 3 months after the limniceruption. This upper chemocline in November 1986deepened gradually with time down to 30 m in 1988,47 m in 1993 and 50 m in 2001. Conductivity profilesat mid-depths (70–160 m) stayed almost unchanged for15 years after the eruption, suggesting that transportof dissolved chemical species through layer II was lim-ited. The conductivity in layers III and IV (170–210m) increased notably with time (Fig. 9b). In January2001, the conductivity profile between 185 m and 202m became steep, with an associated slight reduction of

earlier high conductivity in layer IV, indicating the ini-tiation of mixing in the deepest zone. This tendencyhad started in 1998, although the 1998 profile is par-tially obscured behind the 2001 profile in Fig. 9b. Fromthe depth of 205 m to the bottom, the conductivity in-creased sharply. This trend is the same as observed atthe bottom water of Lake Monoun. The pre-degassingincrease of the conductivity in layers III and IV fromNovember 1986 to January 2001 (14 years) correspondsto an increase of 7700 tons of TDS, with the averageannual input of 540 ton/yr. Initially, the input rate wasrelatively high, but later decreased by at least a factorof two as shown by the close spacing of the conductiv-ity profiles (Fig. 9b). This temporal trend was similarto that of the water temperature.

Before describing the temporal variation of CO2 pro-files in the lakes, it may be worthwhile mentioning theanalytical methods used to determine the dissolvedCO2. As stated before, during the early days of ourobservations, we encountered difficulties in measur-ing CO2 from deep water. The partial pressure of dis-solved gases in Lake Nyos was so high (~1.1 MPa in1990, Evans et al., 1993) that we could not use a Niskinwater sampler to collect water and gases, since the lidof the sampler was forced open before retrieval due to

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,S. G., Recent pH and CO

2 profiles at Lakes Nyos and

Monoun, Cameroon: implications for the degassing strategyand its numerical simulation, 241–260, Copyright 2000, withpermission from Elsevier.

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 opensquares, April 1992, Evans et al., 1994). Dotted curves alongthe pH-based CO

2 profile indicate possible errors due to an

uncertainty in the pH measurement of ±0.05. Modified fromfigure 2 of J. Volcanol. Geotherm. Res. 97, Kusakabe, M.,Tanyileke, G., McCord, S. A. and Schladow, S. G., RecentpH and CO

2 profiles at Lakes Nyos and Monoun, Cameroon:

implications for the degassing strategy and its numericalsimulation, 241–260, Copyright 2000, with permission fromElsevier.



the exsolution of high-pressure dissolved gases. Thisdifficulty was partially solved by attaching a gas bagto the Niskin sampler, which enabled us to collect wa-ter samples (Kusakabe et al., 1989), but it was stilldifficult to measure the dissolved CO2 accurately. Toovercome this difficulty we developed a new methodcalled the “the MK or syringe method” (Kusakabe etal., 2000). The sampler in the MK method is schemat-ically shown in Fig. 10. In this technique, the total dis-solved carbonate species (H2CO3 + HCO3

– + CO32–) at

a given depth is fixed in situ in a 50-ml plastic syringecontaining concentrated (5 M) KOH solution. The to-tal carbonate concentration in the alkaline solution isdetermined later in the laboratory by a classical mi-cro-diffusion method (Conway, 1958). Subtraction ofthe HCO3

– concentration and the blank carbonate inthe KOH solution from the total carbonate concentra-tion gives the H2CO3 (or CO2,aq) concentration.

Since the total carbonate species dissolved in LakesNyos and Monoun are essentially controlled by car-

bonate equilibria, it is possible to determine the H2CO3(or CO2,aq) concentration from pH values (measuredby CTD) if the HCO3

– concentration at a correspond-ing depth is known (Kusakabe et al., 2000). The HCO3

–

concentration is closely related to the electric conduc-tivity, so we can calculate the concentration of H2CO3(or CO2,aq) of a water column using the CTD data. It isa big advantage that we can obtain a continuous CO2profile, although very careful calibration of the pHsensor is an important prerequisite. This method iscalled “the pH method”. Figure 11 compares CO2 pro-files at Lake Nyos in 1995 obtained by the pH and sy-ringe methods. In the figure, the results obtained bythe “cylinder method” are included. In the cylindermethod (Fig. 12a), deep water was sucked into a re-motely-operated evacuated stainless steel cylinder andthe exsolved total CO2 was later analyzed in the labo-ratory (Evans et al., 1993). They also introduced aninteresting device called a “gas-probe” (Fig. 12b) withwhich the total gas pressure at a given depth of a water

Fig. 12. (a) Cylinder sampler used by Evans et al. (1993). A pre-evacuated cylinder is deployed to a desired depth, and a checkvalve is opened to sample water. (b) Gas pressure probe used by Evans et al. (1993). Dissolved gas molecules except waterdiffuse through the membrane unit consisting of multiple silicone rubber tubing. The total gas pressure inside the collectionchamber 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.



column was measured. Figure 11 shows the satisfac-tory agreement between the CO2 concentration ob-tained by the different methods.

Recently, “the plastic hose method” (Yoshida et al.,2010) for CO2 determination has also been used. Thisis based on a self-gas-lifting principle in a plastic hosethat is deployed into the deep water of the lake. A mix-ture of gas and water spouting from the hose is sepa-rated into liquid and gas phases by using a plastic sepa-

rator. The liquid phase accumulates in the separatorand is collected as the water sample, while the dry gasflows through a volumetric gas meter to measure thegas volume at the sampling time (Fig. 13). A gas sam-ple collected directly from the dry gas line is perfectlyfree from air contamination which is essential for no-ble gas analysis (Nagao et al., 2010). A similar methodhas been reported by Tassi et al. (2009) for gas collec-tion from Lake Kivu.

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 flowmeter. The amount of water accumulated in the separator and the volume of gas that goes through the flow meter in a giventime are measured, from which the CO

2 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.



More recently, a new and simple method of measur-ing the total CO2 (CO2,aq and HCO3

–) has been devel-oped by Saiki et al. (2016). This method is based on alinear relationship between the total CO2 concentra-tion and the sound velocity in lake water.

Temporal pre-degassing CO2(aq) variations at Lakes

Monoun and Nyos are shown in Fig. 14. Although nodata were available in 1986 at Lake Monoun, the 1986profile was estimated from later CO2-conductivity re-lationships (Kusakabe et al., 2008). Figure 14 showsthat CO2(aq) concentrations in deep lake water werearound 130 mmol/kg in layers III and IV, with the CO2

Date Time aftereruption Total CO2 CO2 below layer II CO2 accumulation rate CO2 removal rateyear giga mol giga mol giga-mol/yr giga-mol/yr

Lake Monoun: Pre-degassingOctober 1986 2.17 0.38 0.38 æ æNovember 1993 9.25 0.53 0.53 æ æApril 1996 11.67 0.59 0.59 æ æNovember 1999 15.25 0.60 0.60 æ æDecember 2001 17.33 0.61 0.61 æ æJanuary 2003 18.42 0.61 0.61 0.0084 (1993-2003) æ

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

Lake Nyos: Pre-degassingNovember 1986 0.17 13.1 12.9 æ æDecember 1988 2.33 13.3 13.3 æ æNovember 1993 7.25 13.6 13.6 æ æApril 1998 11.67 14.1 14.0 æ æNovember 1999 13.25 14.4 14.0 æ æJanuary 2001 14.42 14.8 14.6 0.12 (1986-2001) æ

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

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

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.



shoulder at a depth of ~63 m in 1986. The CO2(aq) pro-files evolved with time, especially from 1986 to 1993.The thickness of layers III and IV expanded with time,supporting the hypothesis that CO2-rich recharge fluidwas added from the bottom. In December 2001 andJanuary 2003, the CO2 shoulder at a 58 m depth (157mmol/kg) was very close to the CO2 saturation con-centration (Duan and Sun, 2003) at a depth of 50 m.Considering that the rate at which the shoulder wasrising was about 1 m/yr, saturation at a 58 m depthcould be reached in several years. The formation ofCO2 bubbles which could induce a limnic eruption(Kozono et al., 2016) would have occurred soon after2003 at Lake Monoun if no degassing operation hadbeen undertaken.

Figure 14b shows the temporal variation of CO2(aq)profiles between November 1986 and January 2001 atLake Nyos. The general features of Fig. 14b are sum-marized as: (i) CO2(aq) concentration was lowest in theearly days after the explosion; (ii) there was littlechange with time at mid-depths (~50 m to 150 m); (iii)the greatest change took place at a depth of >170 m,where the CO2(aq) concentration at a given depth in-creased significantly with time; and (iv) the CO2(aq)concentration at the bottom-most water was almostconstant, near 350 mmol/kg since 1999. The constancyof the bottom water CO2(aq) concentration was con-firmed by later syn-degassing measurements. The

change in the bottom water is likely caused by thegradual addition of recharge fluid having a CO2(aq)concentration of ~350 mmol/kg. The CO2(aq) contentof the lake was calculated by integrating CO2(aq) pro-files over the water column below layer II using thebathymetry in Kling et al. (2005) under the assump-tion that the horizontal distribution of CO2 was uni-form, as deduced from the conductivity distribution(see figure 8 of Kusakabe et al., 2008) and that CO2loss through the upper chemocline was negligible.Thus, the CO2 accumulation rate can be regarded asthe CO2 recharge rate. Accumulation of CO2 in layerII, and in deeper layers, is tabulated in Table 3 forLakes Monoun and Nyos. Considering that the CO2(aq)profile in October 1986 at Lake Monoun was estimatedin an indirect way (Kusakabe et al., 2008), the overallrate of CO2 accumulation below the upper chemoclinewas calculated using the 1993 to 2003 profiles. Thechange in CO2(aq) content below layer II in the mainbasin for the pre-degassing period (1993 to 2003) was~80 (=610–530) Mmol, with a CO2 recharge rate of8.4 Mmol/yr. Almost the same recharge rate of 8.2Mmol/yr was reported by Kling et al. (2005) using theirown data obtained between 1992 and 2003 for LakeMonoun. At Lake Nyos, the CO2(aq) content below layerII steadily increased by ~1.7 Gmol until January 2001,when permanent degassing started. The increase in theCO2(aq) content can be translated into the CO2 recharge

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 CO

2 concentra-

tion at the bottom water in 2014 was reduced to ~150 mmol/kg. See text for details.



rate, which was 0.12 Gmol CO2/yr between Novem-ber 1986 and January 2001 (Table 3). Again, the rateis in good agreement with the value of 0.13 Gmol CO2/yr given by Kling et al. (2005).

3-2. Syn-degassing evolution of CO2 content and

future prospect

The CO2(aq) profiles between 2001 and 2011 at bothlakes during syn-degassing are shown in Fig. 15. Gen-erally speaking, degassing went smoothly, as illustratedby the steady subsidence of the CO2(aq) profiles. Thisresulted in a lowering of the fountain height at bothlakes (Fig. 16). The subsidence has continued up tothe present time; however, a buildup of CO2 has re-sumed recently at Lake Monoun (see below). The over-all shape of the profiles did not change with degassing,showing that only bottom water and dissolved CO2(aq)were removed without causing any effect on the strati-fication of the lake water. At Lake Monoun (Fig. 15a),the highest CO2(aq) concentration at the bottom de-creased to 80 mmol/kg, and the thickness of layer IIIreduced to ~20 m in 2009, and further reduced to ~70

mmol/kg and ~15 m, respectively, in 2011 (Kusakabeet al., 2011). In 2009, two of three degassing pipesstopped working completely, and the other pipe issuedonly a weak bubbly flow (Fig. 16f). Thus, it can besaid that the degassing pipes at Lake Monoun have al-most lost their gas self-lift capability. Moreover, re-cent observations (2011–2014) show that CO2 concen-trations below 80 m and the layer III thickness are in-creasing (Fig. 15a), clearly indicating that natural CO2recharge into Lake Monoun still continues. This con-firms our prediction that CO2 re-buildup is inevitableif lake degassing stops. On the basis of a geochemicalstudy on the generation of CO2 in the Nyos mantle,Aka (2015) has suggested that CO2 will be continu-ously supplied into the lake for a geologically long timein the future. This view may also apply to LakeMonoun. In order to avoid gas re-buildup and to makethe lake continualy safe, Yoshida et al. (2010) sug-gested continuously removing the bottom water thatcontains the CO2 at a significantly high concentration.The installation of such a bottom water removal sys-tem was undertaken at Lake Monoun in December 2013(Yoshida et al., 2016), and details of the system are

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



described in Section 5.At Lake Nyos (Fig. 15b), CO2(aq) profiles subsided

steadily until January 2011, resulting in a very thinlayer III by that time. As two more degassing pipeswith a greater diameter (25.7 cm I.D.) were installedin December 2011–March 2012, the degassing rate wasgreatly enhanced, resulting in a rapid decrease of CO2concentration in deep water in the subsequent years(2011–2014). We can expect that most of the CO2-richbottom water will disappear from Lake Nyos in sev-eral years from now and that the gas self-lift capabil-ity will be lost as in Lake Monoun.

Using CO2(aq) profiles and lake bathymetry (Klinget al., 2005), the amount of CO2(aq) dissolved belowthe upper chemocline (layers II, III and IV) was calcu-lated as a function of time since the limnic eruption atboth lakes (Fig. 17). The amount of dissolved CO2 inLake Monoun (Fig. 17a) increased steadily at a rate of8.4 Mmol/yr, reaching a maximum value of 610 Mmolin January 2003, shortly before the degassing opera-

tion started. Degassing was effective, reducing theamount of dissolved CO2 at a mean gas removal rateof 98 Mmol/yr between January 2003 and December2007 (see Table 3). This rate is approximately 12 timesgreater than the natural recharge rate as shown by thesharp slope (Fig. 17a). The installation of two addi-tional pipes in April 2006 accelerated the gas removalrate. In January 2009, the system had almost lost itsgas self-lifting capability, resulting in a reduction ofgas removal rate to only 5 Mmol/yr in January 2011(Table 3), although a very weak flow of bubbly waterfrom one of the three pipes was still visible. At thattime, the amount of CO2(aq) dissolved in deep waterwas 55 Mmol, or only 9% of the maximum value ob-served in 2003 (Kusakabe, 2015). Based on these ob-servations, it can be said that Lake Monoun has beenmade safe. However, the tailing-off of the CO2 con-tent after 2009 (Fig. 17a) implied that a buildup ofCO2 is inevitable at Lake Monoun if the natural re-charge of CO2 continues at the previously estimated

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-degassingevolution, respectively. Note that the CO

2 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.



rate. Indeed, the re-buildup of CO2 became obviousafter March 2012 (Fig. 17a). Using the data between2010 and 2014, the rate of gas re-buildup is calculatedto be ~4.8 Mmol/yr which is about half of the CO2recharge rate of 8.4 Mmol/yr calculated from the pre-degassing data (Table 3), although we need to accu-mulate more recent data for a more reliable determi-nation of the rate of gas re-buildup. The current amountof total dissolved CO2 in Lake Monoun is 79 Mmol(as of April 2014) which is ~13% of the maximum pre-degassing value recorded in 2003. It may take another~100 years to reach the pre-degassing situation if thecurrent rate of gas re-buildup remains unchanged. Forthese reasons, it is essential to continue monitoring thelake on a regular basis.

The evolution of CO2 content over time since 1986at Lake Nyos is shown in Fig. 17b. The gas removalrate by a single pipe (0.46 Gmol/yr) is about four timesgreater than the natural recharge rate of 0.12 Gmol/yr.At this removal rate, however, it would take another20 years or so to remove all the gas from the lake. For-tunately, using funds from the Government ofCameroon and UNDP, two additional degassing pipeswere installed in early 2011. Since pipes with a greaterdiameter (25.7 cm) were used and the water intakedepth was increased to close to the bottom for the ad-ditional pipes, the rate of gas removal greatly increased

to 1.2 Gmol/y (Table 3). It is hoped that most of theremaining gas will be removed within the next 5 yearsor so. At the last stage of the degassing operation, therate of gas removal will decrease due to a lower CO2concentration at the intake depth. This will lead to gasre-buildup, as we have seen at Lake Monoun. Thus, asystem to pump up CO2-rich bottom water needs to beset up after the current degassing system has lost itsgas self-lifting capability.

3-3. Hydrogen, oxygen, carbon, and noble gas iso-topic signatures

The hydrogen and oxygen isotopic ratios of LakeNyos waters were first reported by Kling et al. (1987).The data for Lakes Nyos, Monoun and Wum (a craterlake near Lake Nyos) were later added by Kusakabe etal. (1989) and Nagao et al. (2010). The isotopic deter-mination was intended to find any input of volcanicgases into Lake Nyos, for volcanic gases are usuallycharacterized by high d18O signatures, but the data didnot indicate any volcanic input. The dD and d18O val-ues from Lake Nyos plot close to the Global MeteoricWater Line (Craig, 1961; Rozanski et al., 1993) whencombined with the data for rain water, groundwater,and surface water recently collected in the Lake Nyoscatchment area as shown in Fig. 18, although the lake

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



water is slightly more enriched in heavy isotopes thanthe ground and surface waters (Kamtchueng et al.,2015a).

Noble gas information was used to constrain the ori-gin of CO2 dissolved in the lakes, for noble gases donot react with rocks and waters on the way to theEarth’s surface. Figure 19 shows the relationship be-tween 3He/4He and 40Ar/36Ar ratios for Lakes Nyos andMonoun gases in 1999. Coupled with the d13C valuesof –3.3 ~ –3.4‰ (relative to Vienna Pee Dee Belemnite,VPDB) for Lake Nyos and –6.8‰ for Lake Monoun,the helium and argon isotopic ratios suggest a strongaffinity of the dissolved gases with a magmatic source(e.g., Kusakabe and Sano, 1992). Nagao et al. (2010)reported more precise data using air-contamination freesamples. The 3He/4He ratios in the gases in the LakeNyos deep waters are ~5.7 Ratm, where Ratm is the at-mospheric ratio of 1.4 ¥ 10–6. The Lake Nyos 3He/4Heratios are lower than the typical mantle values of 7~9Ratm for depleted mantle producing Mid-Oceanic Ridgebasalts (MORBs) (Graham, 2002). The reasons whythe 3He/4He ratios of Lake Nyos are lower than themantle values are related to the sub-lithospheric struc-ture beneath the Cameroon Volcanic Line (CVL) asdiscussed later (Section 6). Halliday et al. (1988) re-ported variations in the radiogenic isotopic ratios (Pb,Nd, and Sr) of volcanic rocks along the CVL, wherethe highest 206Pb/204Pb and 208Pb/204Pb ratios werefound at the oceanic and continental boundary. Barfod

et al. (1999), Aka (2000) and Aka et al. (2004) pub-lished a detailed study of noble gases in basalts andxenoliths from CVL volcanic rocks, showing a sym-metrical distribution of 3He/4He ratios along the CVL(Fig. 20). The lowest 3He/4He ratios (4.5 ¥ 10–6 or ~3Ratm) were found at Etinde, a small volcano next toMt. Cameroon, located at the oceanic and continentalboundary. The 3He/4He ratios become close to theMORB values (7~9 Ratm) as we go away from the oce-anic and continental boundary towards both ends (Akaet al., 2004). This symmetric isotopic variation wasexplained as reflecting the geochemical characteristicsof the mantle, or the continental lithosphere underneaththe boundary which is of a HIMU character (Hallidayet al., 1988). HIMU is geochemical jargon for “high-m” with m defined as the ratio of 238U/204Pb. HIMUmantle is characterized by an enrichment in U and Th,the parent elements of radiogenic Pb and He. Meltsderived from this HIMU mantle are postulated to havebeen emplaced beneath the oceanic and continentalboundary. Thus, rocks at the oceanic and continentalboundary are high in 206Pb/204Pb and low in 3He/4Heratios. Mineral separates from rocks around Lake Nyos(clinopyroxene and amphibole in xenolith) have 3He/4He ratios of 6.7~7.0 Ratm (Aka et al., 2004), slightlylower than the typical MORB values, implying a smalldegree of the HIMU character of the magma sourcebeneath Lake Nyos.

The deep water of the lake has even lower values of

Fig. 19. Relationship between 3He/4He (in Ratm

) and 40Ar/36Ar ratios of waters in Lakes Nyos and Monoun. Ratm

is a 3He/4Heratio of sample relative to that of air (=1.4 ¥ 10–6). The plots show a mixing of magmatic gases and the atmosphere (greencross). See text for a discussion. Data from Nagao et al. (2010).



~5.7 Ratm, as was stated above. This low ratio may meanthat He in deep water was originally derived frommagma generated from a slightly HIMU-type mantle,but acquired radiogenic 4He on the way from the sourcemagma to the sub-lacustrine fluid reservoir during thepassage of the magmatic fluid through granitic base-ment rocks. Nagao et al. (2010) reported the distribu-tion of isotopic ratios of not only He but also Ne, Ar,Kr, Xe and C in Lakes Nyos and Monoun waters col-lected at closely separated depths. They stressed theimportance of samples that were free from air-contami-nation, because noble gas concentrations, especiallythose of Ar and Ne, are so low in gases exsolved from

CO2-rich waters compared to air so that any samplesexposed to the atmosphere during sampling, or stor-age in an improper way, are not good for analysis. Sam-ples from Lake Nyos (January 2001) were collectedusing the “Flute de Pan” which had been deployed bythe French scientific team. This consisted of 11 plastichoses having an outside diameter of 15 mm with dif-ferent intake depths (83–210 m) (Fig. 21). CO2-richgas spouting out of a given hose was collected in aglass bottle using an inverted funnel placed in a bucket.Although slight contamination of air dissolved in thewater was still suspected to some extent, especially forheavy noble gases, this sampling method was found to

Fig. 20. Symmetrical distribution of 3He/4He and 206Pb/204Pb ratios of rocks along the CVL as a function of distance fromAnnobon. This distribution suggests a contribution of the HIMU mantle for magma genesis in the oceanic and continentalboundary volcanoes (Halliday et al., 1988; Aka et al., 2004). Oceanic sector volcanoes are Annobon (AN), São Tomé (ST) andPrincipé (PP). Ocean/continent boundary volcanoes are Bioko (BK), Etinde (ETD) and Mount Cameroon (MC). Continentalsector 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).



be promising. In the December 2001 sampling, a plas-tic hose method, essentially the same as the Flute dePan method, was adopted. A single plastic hose (12mm I.D.) was deployed initially to the bottom, followedby pulling it upward little by little to a desired depth.Exsolved CO2 gas was directly allowed to pass througha sampling bottle made of uranium glass that has a lowHe diffusivity. With this method, Nagao et al. (2010)were able to collect air-contamination free samples.

This method was later modified to measure the totalgas concentration on site (Yoshida et al., 2010).

Figures 22a, b illustrate the profiles of He, Ne, Ar,Kr and Xe in water, measured in 2001, at Lake Monounand Lake Nyos, respectively. Except for He, they showroughly constant concentrations with respect to depthsbelow 80 m at Lake Nyos, and 50 m at Lake Monoun.The Ne, Ar, Kr and Xe concentrations are up to sev-eral times lower than those in air saturated water

Fig. 22. Depth profiles of noble gas concentrations in water (10–6 ccSTP/gwater) measured in 2001 at Lake Monoun (a) andLake Nyos (b). Noble gas concentrations in air saturated water (ASW) at 30∞C (table 2 in Kipfer et al., 2002) are also shownby arrows for comparison. Modified from figures 1 and 2 of Nagao et al. (2010).



(ASW). Depth profiles of the 3He/4He ratio for LakesNyos and Monoun are presented in Fig. 23. The datapublished by earlier workers are in the same range(Kling et al., 1987; Sano et al., 1987, 1990; Kusakabeand Sano, 1992). Generally speaking, the 3He/4He ra-tios are almost constant in the depth range of 80–210m at Lake Nyos, and 40–100 m at Lake Monoun. The3He/4He ratio approaches the atmospheric value inwaters shallower than 80 m and 40 m for Lakes Nyosand Monoun, respectively. High 4He/20Ne ratios up to~1500, as shown in Fig. 22, support the premise ofmagmatic gas input to the lake as inferred by high 3He/4He ratios.

Neon isotopic ratios are presented in Fig. 24. Com-pared to atmospheric Ne, small excesses of both the20Ne/22Ne and 21Ne/22Ne ratios are observed. Most datapoints for both lakes lie on the MORB line connectingatmospheric Ne and mantle Ne as reported byStaudacher and Allègre (1988). The data clearly indi-cate the presence of mantle Ne in the lakes, and areconsistent with the conclusions of Barfod et al. (1999)and Aka et al. (2004) that the CVL mantle containsMORB-like Ne.

Argon isotopic ratios are presented in Fig. 25. The40Ar/36Ar ratios for all samples are higher than the at-mospheric value of 296, but much lower than the esti-mated value of >1650 for Ar in the upper mantle be-neath the CVL (Barfod et al., 1999). This means thatmagmatic fluids containing mantle Ar mixed with at-mospheric Ar on the way to the surface such as in asub-lacustrine fluid reservoir. The contribution of man-

tle Ar to the sub-lacustrine Ar may be less than 20%assuming that the mantle Ar has a 40Ar/36Ar ratio of>1650. This is consistent with the conclusion derivedfrom the Ne signature (Fig. 24), although the contri-bution of mantle Ne to the sub-lacustrine Ne may be~6%. At Lake Nyos, the highest 40Ar/36Ar ratio, ofabout 600, was found at the bottom (210 m). The 40Ar/36Ar profile in January 2001 decreased gradually to-wards the surface approaching the atmospheric ratio,but it showed a zigzag pattern below the lowerchemocline at ~180 m with a second maximum valueof 480 at 190 m. The zigzag 40Ar/36Ar profile disap-peared in December 2001 with consistent ratios around530 below 190 m. This may have resulted from verti-cal mixing in this depth range caused by degassing,because water was pumped out by the degassing pipefrom an intake depth of 203 m. A tendency for suchhomogenization was also observed in the water tem-perature and electric conductivity at the correspond-ing depths (Kusakabe et al., 2008), although they wereless clear than the noble gas profiles. At Lake Monoun,the 40Ar/36Ar ratios were in a narrow range of about470 between 60 m and 100 m (bottom) (Fig. 25). Theratios are lower than those in the deep waters of LakeNyos, suggesting that the contribution of atmosphericAr to the magma-originating gases at Lake Monoun isgreater than that at Lake Nyos.

The characteristic features of noble gases observedat Lake Nyos can be summarized as follows (Nagao etal., 2010): (i) Helium in the lake water derived origi-nally from the mantle where 3He/4He ratios of ~7 Ratm

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

Fig. 24. 20Ne/22Ne versus 21Ne/22Ne plot for samples collected in 2001 at Lakes Nyos and Monoun. The “mass fractionationline” indicates the isotopic trend of atmospheric Ne due to mass fractionation. The dashed line heading to MORB representsa mixing line between atmospheric Ne and Ne in MORB or the upper mantle (Ballentine et al., 2005). Modified from figure5 of Nagao et al. (2010).

are found in mantle xenoliths (Aka et al., 2004), but,on its way to the surface, approximately 20% of radio-genic 4He that accumulated in crustal rocks was ad-mixed to give ratios of ~5.7 Ratm, probably in the sub-lacustrine region. (ii) The observed 40Ar/36Ar ratios of450–550 are also explained by the addition of atmos-pheric Ar (40Ar/36Ar = 296) carried by groundwater tomantle-originating Ar (40Ar/36Ar >1650, Barfod et al.,

1999) on the way to the lakes. The most likely sourceof Ar to reduce the mantle 40Ar/36Ar ratio is atmos-pheric Ar-bearing groundwater. (iii) Ne in the lakesmay be a mixture of atmospheric Ne and a small amountof MORB-like Ne from the mantle. The observed He,Ne and Ar isotopic ratios in lake waters can be bestexplained by mixing between two noble gas reservoirs,i.e., air dissolved groundwater and the mantle. It is



conceivable that the mantle-derived gases with theaddition of radiogenic 4He from crustal rocks and at-mospheric Ar and Ne carried by groundwater are fi-nally homogenized in the sub-lacustrine reservoir. Notethat the contribution of atmospheric He to deep lakewater, if any, is difficult to find, since the He concen-tration in deep lake water is more than 3 orders ofmagnitude higher than that in ASW, whereas the con-tribution of the other noble gases is more easily dis-cernible because of their similar concentrations in deeplake water and ASW (see Fig. 22).

As stated previously, the greatest chemical changein Lake Nyos took place at depths greater than 180 m.The CO2 profiles (1986–2001) in the depth range 160–210 m are enlarged in Fig. 26a. This shows that theincrease of CO2 concentration in the deep water of LakeNyos after the 1986 limnic eruption resulted from wid-ening of CO2-rich water leading to the formation of aclear lower chemocline at the top of the CO2-rich wa-ter. The 3He concentration observed in 2001 in the samedepth range was compared with the CO2 profiles (Fig.26). The 3He profile was obtained from the 4He pro-file (Fig. 22) and the 3He/4He profile (Fig. 23). Itshould be noted that the 3He concentration below 160m in January 2001 and December 2001 shows a sharpmaximum at around 188 m with a concentration up to9.1 ¥ 10–10 ccSTP/g-water (December 2001) (Fig. 26b).The 4He concentrations have a pattern very similar to

those of 3He in the same depth range, because of thealmost constant 3He/4He ratios (Fig. 23), although the4He concentrations are not graphically shown. Between190 and 210 m, the 3He concentrations are nearly con-stant at ~5 ¥ 10–10 ccSTP/g-water. The 3He concentra-tion gradually decreases as the depth decreases (layerII).

The C/3He ratios of volcanic fluids have been widelyused to constrain magma sources. The C/3He ratios ofMORB glasses are shown to be fairly constant at 0.20(±0.05) ¥ 1010, suggesting that the source region ofMORB in the upper mantle has little variation in theC/3He ratio (Marty and Jambon, 1987). The ratios forvolcanic gases from subduction volcanism, however,have been found to be significantly greater than theMORB values, i.e., 0.7 ¥ 1010 ~ 3 ¥ 1010. These highratios, coupled with d13C values, indicate the existenceof recycled carbon (marine carbonates, slab carbon-ates and/or organic materials) in subduction zone mag-mas (Sano and Williams, 1996, and references therein).Figure 27 shows the C/3He ratios in the depth range160–210 m in Lake Nyos. The C/3He ratios range from0.5~1.7 (¥ 1010). These values are higher than the man-tle values of ~0.2 ¥ 1010. It is interesting to note thatthe C/3He ratios in waters below the lower chemoclineare significantly high at around 1.6 ¥ 1010, and sharplydecrease to 0.5 ¥ 1010 above the lower chemocline.Thus, the behavior of CO2 and 3He are decoupled be-

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 achemocline (dashed line) existed in 2001. Modified from figure 14 of Volcanic Lakes (Dmitri Rouwet, Bruce Christenson,Franco Tassi, Jean Vandemeulebrouck, eds.), Evolution of CO

2 content in Lakes Nyos and Monoun, and sub-lacustrine CO

2-

recharge system at Lake Nyos as envisaged from CO2/3He ratios and noble gas signatures, 2015, pp. 427–450, Kusakabe, M.,

„ Springer-Verlag Berlin Heidelberg 2015 with permission of Springer.



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 calculatedfrom the CO

2 and 3He 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 ofCO


2-recharge system at Lake Nyos as envisaged from CO

2/3He

ratios and noble gas signatures, 2015, pp. 427–450, Kusakabe, M., „ Springer-Verlag Berlin Heidelberg 2015 with permis-sion of Springer.

Fig. 28. (a) Change with time of the CO2/3He 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) wereused.



low and above the chemocline. The cause(s) of thedecoupling may be explained by the underplating ofthe recharge fluid from the bottom that is character-ized by different C/3He ratios. By “underplating”, Imeant that the recharge fluid is added to the bottom-most water from beneath. It is possible that the ratiowas low before the limnic eruption and high after thelimnic eruption. At the time of the limnic eruption, thelake was not completely mixed, suggesting that deepwater still contained a large fraction of “pre-eruption”water (Giggenbach, 1990; Tietze, 1992; Evans et al.,1994) which may have been proportionally higher inHe and lower in CO2 concentrations with the CO2/3Heratio of ~0.5 ¥ 1010. Recharge fluids entering the lakeafter the eruption may have a CO2/3He ratio of ~1.6 ¥1010. This interpretation implies that the CO2/3He ra-tio in the recharge fluids may vary with time and haschanged from low to high values with time. A change

with time in the CO2/3He ratio has been observed infumarolic gases from Mammoth Mountain in the LongValley caldera, California, where “tree-kill” took placedue to an anomalous discharge of magmatic CO2 intosoils (Farrar et al., 1995; Sorey et al., 1998). Thisanomalous CO2 discharge was induced by an episodeof shallow dyke intrusion beneath Mammoth Moun-tain in 1989–1990. The CO2/3He ratios of the fumarolicgases there changed from ~0.3 ¥ 1010 to 1.6 ¥ 1010 inabout 10 years (Fig. 28). The change was caused by atrend of decreasing He concentration and little changein the concentration of CO2, which is a major compo-nent of the gases. The 3He/4He ratios stayed at around5.5 Ratm with a few exceptions. These observationsindicate that the above geochemical parameters (CO2/3He ratio and He concentration) that carry informationabout magmatic fluids can change within a geologi-cally very short period of time, i.e., in the order of 10

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, JeanVandemeulebrouck, eds.), Evolution of CO


2-recharge system at

Lake Nyos as envisaged from CO2/3He ratios and noble gas signatures, 2015, pp. 427–450, Kusakabe, M., „ Springer-Verlag

Berlin Heidelberg 2015 with permission of Springer.



years, at a single volcanic system. Thus, it is conceiv-able that the decoupling of CO2 and 3He observed atLake Nyos after the limnic eruption was caused by theaddition of “recent” recharge fluids that were charac-terized by relatively low 3He concentrations. From theforegoing discussions based on noble gas signaturesand C/3He ratios, we can envisage the sub-lacustrineCO2-recharge system at Lake Nyos to be as shown inFig. 29.

4. Limnic eruption, models, triggers and cyclicity

4-1. Models

Many hypotheses have been put forward to explainwhy the limnic eruption occurred. Sigurdsson et al.(1987) proposed that a landslide slumped into deepwater, pushed CO2-rich water up and induced the 1984limnic eruption at Lake Monoun. The same idea wasalso suggested for the 1986 Lake Nyos event (Kling etal., 1987). Tietze (1987) suggested supersaturation ofdissolved CO2 just below the shallowest chemocline(~8 m depth in 1986) to be the main cause of the erup-tion. The strong density stratification of this layerworked as a lid for rising gases, inhibiting them frompenetrating this density divide. The supersaturation thatfollowed led to the exsolution of gases to form a foun-tain. This process was self-intensified and deeper wa-ter was steadily degassed in turn. Since the water fromthe fountain was cooler than the surface water, it sankaround the fountain, forming a cylindrical “densitywall”. This wall limited lake-wide exsolution of gases,leaving CO2 dissolved in deep water (>150 m?) intactduring the eruption. Assuming that Lake Nyos was iso-thermal and fully saturated with CO2, Kanari (1989)presented a fluid-dynamics model to explain how thelimnic eruption proceeded. In his model, degassingstarted from the bottom but was confined to a limitedarea at the surface. Circulation of water was confinedin small cells that stacked at various depths. Accord-ing to this model, stratification within the lake washardly affected. Kanari estimated that (i) the releasedgas volume (0.68 km3) was the difference between thesaturation and the CO2 profile observed in 1986 byKusakabe et al. (1989), (ii) the maximum height of thegas cloud was 110 m, and (iii) the speed of the gascloud running down the valley was 19 m/s. However,later observations indicated that full CO2 saturationover the entire lake was unlikely (Kusakabe et al.,2008).

Obviously, any degassing model depends on a knowl-edge of the pre-eruption distribution of CO2 in the lake.Evans et al. (1994) proposed a model based on a linearpre-eruption relationship between CO2 and TDS (totaldissolved solids) at Lake Nyos using water chemistry,CTD measurements, gas analyses and tritium profilesobtained between 1987 and 1992. A linear relationship

between tritium and TDS was interpreted to reflect thedestruction of the pre-existing gradient at mid-depthduring the eruption, suggesting that CO2 exsolved fromdeep water. In their model, the upper chemocline wasplaced at ~50 m depth, similar to the chemocline depthobserved in January 2001, 14 years after the limniceruption, and just prior to the initiation of artificialdegassing (see Fig. 9). Some triggers, such as a com-bination of seasonal decline in the water column sta-bility, landslide and/or seiche, pushed water upward ata layer around the chemocline to the CO2 saturationdepth. Bubble formation then followed and relativelyquiet degassing continued. A local reduction in thehydrostatic pressure beneath the release area created arising column of shallow, slightly gassy water. Thiswas followed by mixing with pre-release surface wa-ter (low TDS) to form the surface water that was ob-served soon after the limnic eruption. The base of thecolumn became slowly deeper, bringing CO2-rich, moresaline deep water upward. When the base of the col-umn reached the deeper chemocline, below which CO2and TDS concentrations were much higher, gas releasebecame more violent and created wave damage alongthe lake shore such as the flattening of vegetation andthe passing of water over an 80-m-high promontory inthe southern part of the lake. The duration of this vio-lent fountaining was short (<1 min), and the amountof CO2 released was estimated to be 6.3 Gmoles. Thisscenario is consistent with the testimonies of survivors.

Giggenbach (1990) proposed that the gas release atLake Nyos was triggered by a climatic factor. The de-scent of a parcel of unusually cold rain water (18.5∞C)pushed initially CO2-rich shallow water upward. Theuplift of the CO2-rich water above the saturation depthinduced bubble formation which accelerated upwardmovement by a reduction of density, leading to the for-mation of a convecting water flow that entraineddeeper, more CO2-enriched water, and, finally, to thelimnic eruption. Less-dense degassed waters accumu-lated at the surface, making it difficult for deeper CO2-rich water (>100 m) to reach the surface, thus termi-nating the eruption. Deep water CO2 was therefore leftalmost intact. The amount of CO2 released was esti-mated at 5.4 Gmoles.

In contrast to the previous models for the cause ofthe limnic eruption, spontaneous exsolution of dis-solved gases has been suggested by Kusakabe et al.(2008) and Kusakabe (2015). In this scenario, atten-tion was paid to the pre-degassing evolution of dis-solved CO2 at Lake Monoun (see Fig. 14a) which in-dicated that CO2(aq) profiles evolved with time and thatCO2-rich layers below the lower chemocline (layersIII and IV) widened due to the continuing recharge ofCO2-charged fluid from beneath. Note that the CO2(aq)concentration in water below layer III was constant at~150 mmol/kg. In January 2003, just prior to the ini-tiation of the degassing operation, the CO2(aq) concen-



tration immediately below the chemocline at the bound-ary between layers II and III was very close to satura-tion. If no degassing operation were undertaken, thesaturation of CO2(aq) would have been attained at thatdepth in a short time (within several years), and bub-ble formation would have followed by additional in-put of the recharge fluid. Thus, at Lake Monoun an-other limnic eruption could have occurred spontane-ously within several years after 2001, if any externaltrigger had been introduced to this critical situation.

4-2. Spontaneous eruption hypothesis

The evolution of pre-degassing CO2 profiles at LakeMonoun gives a clue to estimate a pre-eruptive CO2profile at Lake Nyos. It is conceivable that it was simi-lar in shape to the 2001–2003 profiles at Lake Monoun(see Fig. 14a). It is interesting to note that the CO2profiles at the deep layer of Lake Nyos (>180 m in1999 and 2001; Fig. 14b) was developing in a waysimilar to that observed at Lake Monoun. The thick-ness of CO2-rich water close to the bottom kept in-creasing after the 1986 eruption till 2001 due to theaddition of the recharge fluid from beneath. The CO2(aq)concentration below 195 m reached 350 mmol/kg in1999. This concentration remained unchanged untilJanuary 2011 down to the bottom (Kusakabe et al.,2008; Kusakabe, unpublished data). This observationsuggests that the CO2(aq) concentration of the rechargefluid is constant at ~350 mmol/kg. If no degassing wasundertaken, and if the natural recharge of CO2 contin-ued as before, the thickness of the bottom CO2-richwater would have continued to increase, and the toplevel of the CO2-rich layer could have eventuallyreached saturation at some shallower depth. This specu-lation is schematically presented in Fig. 30. In thismodel, the pre-eruption profile, shown as “Before1986”, has a shoulder that touches the saturation curveat a depth of ~110 m. A limnic eruption would takeplace spontaneously, releasing the dissolved gases tothe atmosphere, resulting in a CO2 profile shown as“November 1986” in Fig. 30 (process 1). The observedevolution of the CO2 concentration between Novem-ber 1986 and January 2001 is shown as “process 2” inFig. 30. If no degassing took place, and if the naturalrecharge of CO2 continued as before, the CO2(aq) pro-file would have shifted upward following “process 3”,and would eventually have touched the saturationcurve. Saturation is a necessary condition, but may notbe a sufficient condition, for a limnic eruption to takeplace. Rising bubbles may re-dissolve in under-satu-rated water during ascent. However, if sufficient CO2flux is given, the bubbles can reach the surface, possi-bly leading to a limnic eruption. Based on the abovemodel, a numerical approach to the recurrence of afuture limnic eruption was made by Kozono et al.(2016). They demonstrated that a plume of bubbles

generated from a growing CO2-saturated surface (thetop of the “Before 1986” curve in Fig. 30) can reachthe lake surface with a high flux of CO2, i.e., limniceruption, if any external forcing triggers bubble for-mation at the growing CO2-saturated surface. The trig-ger may be an instability caused by double diffusiveconvection (Schmid et al., 2004), or a seiche near theCO2-saturated surface where the density gradient isstrong.

If our model is correct, the difference between thepre- and post-eruption profiles integrated over the lakevolume gives the amount of gas released at the time ofthe eruption, which was calculated to be ~14 Gmol or0.31 km3 (at STP). This value is greater than the esti-mate of 0.14 km3 by Evans et al. (1994) by a factor of~2, but significantly smaller than earlier estimates(0.7~1 km3) by Faivre Pierret et al. (1992), and Kanari(1989). The estimated amount of CO2 released obvi-ously depends on the assumptions involved. As longas the lake receives a continual natural recharge of CO2,limnic eruptions can occur repetitively (Tietze, 1992),but may not be regular as described in the model of

Fig. 30. A model of the spontaneous limnic eruption at LakeNyos. An assumed pre-eruption CO

2 profile is shown by red

small open circles as “Before 1986”. After the eruption, theCO

2 profile turned to the post-eruption profile shown as

“Nov. 1986” (process 1). It evolved to the January 2001 pro-file (blue) in 15 years (process 2). If the natural rechargecontinues, the January 2001 profile may “recover” the pre-eruption situation (process 3). Modified from figure 9 ofVolcanic Lakes (Dmitri Rouwet, Bruce Christenson, FrancoTassi, Jean Vandemeulebrouck, eds.), Evolution of CO

2 con-

tent in Lakes Nyos and Monoun, and sub-lacustrine CO2-

recharge system at Lake Nyos as envisaged from CO2/3He

ratios and noble gas signatures, 2015, pp. 427–450,Kusakabe, M., „ Springer-Verlag Berlin Heidelberg 2015with permission of Springer.



Chau et al. (1996), which considered a possible varia-tion in the rate of the natural recharge of CO2. How-ever, if the conceptual model shown in Fig. 30 is cor-rect, it would take ~100 years to attain the pre-erup-tion CO2 level shown by the “Before 1986” curve start-ing from the curve “November 1986” assuming a con-stant CO2 recharge rate of 0.12 Gmol/year (process 3).

4-3. Repetitive nature of a limnic eruption

The above model implies that the time of repetitionof a limnic eruption is ~100 years. A possibility of cy-clic gas bursts from lakes which are charged by a gasinflux from the lake bottoms was also pointed out byTietze (1992). He argued that dissolution of CO2(aq)will inevitably create a stratification of the lake be-cause the density of CO2-containing water is higherthan that of pure water, due to a small partial volumeof dissolved CO2(aq) in water (Ohsumi et al., 1992),and that the stratification will limit upward gas trans-port, leading to an accumulation of the gas below thestratified layers. If limnic eruptions take place repeti-tively on a timescale of ~100 years, evidence of pasteruptions might be found in geological records andlocal documents. Unfortunately, no geological evidencehas been recorded, and no such written documents areknown to exist in the Nyos-Monoun areas. However,Shanklin (1989, 1992, 2007) published interestingfolklores that are common in the grassfields of west-ern Cameroon; the Kom story and the Oku story. Thefolklores are suggestive of limnic eruptions that tookplace in the past. The following paragraphs (shown initalic) are reproduced from Shanklin (1992).The Kom Story

Kom people were living at Bamessi (near Lake Nyos)as guests of the Fon (a ruler is called Fon in theGrassfields), but the Bamessi Fon was afraid the Komwere becoming too powerful and he devised a trick torid himself of them: he suggested to the Kom Fon thatsince their young men were showing signs of theirreigns, they each should build a house and entice theyoung men inside, then bar the doors and set the housesafire. But the wily Bamessi Fon built his house withtwo doors and so all the Bamessi men escaped, whileall the Kom men died. Soon the Kom Fon discoveredthe trick and vowed revenge. First, he called his sisterto him and told her of his plans. He would hang him-self and Kom people were not to cut his body down,nor even go near it; instead, they were to watch andwait for the appearance of a python track that wouldlead them to their new home. Led by the Fon’s sister,the Kom people followed their Fon’s instructions pre-cisely. After he hanged himself, his body fluids drippeddown and formed a lake; the Kom people watched.Maggots from the Fon’s body fell into the lake andbecame fish; the Kom people watched. The people ofBamessi were delighted with the new lake and they in-

formed their Fon, who proclaimed a day when theywould all go into the lake to catch the fish. The daycame and the Kom people watched the people ofBamessi assemble at the lakeside; then the Bamessiwent into the lake to catch fish for their Fon. Then theBamessi went back to catch fish for themselves. At thatpoint, Kom people say, the lake “exploded”, then sankand disappeared, taking with it most of the Bamessipopulation. Thus was the Kom Fon’s curse fulfilled;the people of Bamessi were destroyed, leaving the en-emy Fon with only a few retainers as he had left theKom Fon when the two houses were burned. As theywatched from the hills, the python trail appeared tothe Kom people and they turned away to begin the longjourney west, to the area they now occupy.The Oku Story

At Oku there is a good-sized crater lake and Okupeople say that at one time two groups were settledbeside the lake. On the western slope were the Babankior Kijem people and on the eastern slope were the Okupeople. Each had their own Fon. There were many dis-putes between them, one being a disagreement as towhich group owned Lake Oku. One day a stranger cameand asked the Fon of Kijem for land on which to builda compound. The Kijem Fon was a disagreeable fel-low and he refused to give land. The stranger then wentto the Oku Fon, who gave him a building plot. But thestranger did not like the land that was given, so hewent back to the Oku Fon and asked for a differentplot. The Fon allocated him another, but again thestranger was not happy, so he returned to the Fon, ask-ing for a different place. Once again, he was given aplot, and once again, he returned to complain aboutit. Finally the Oku Fon, seeing that the man would notbe satisfied, told him to choose his own land. The mansettled down beside Lake Oku, and, as it is said inPidgin English, no one ever knew what he did there.(The implication is that the man had no visitors be-cause he was a witch.) When the stranger died, theKijem and Oku people went to celebrate his death, theKijem people on their side of the lake and the Oku peo-ple on theirs. Both Fons were called to come into thelake (presumably by the now-dead stranger) and theydid so, each entering from his side. They were thentaken to the lake bottom and, soon after they disap-peared, streaks of red (blood) began to appear on theOku side. As they watched the red streaks come up, theOku people thought their Fon was dead and they be-gan to mourn for him. At the same time, there appearedin the distance a Fon dressed in fine new clothes, andthe Kijem people began to cheer, believing their Fonwas being returned to them, having been honored bythe host with precious garments. But, in fact, it wasthe Oku Fon who was dressed in fine clothes and theKijem Fon who had been slaughtered. The two groupsreturned to their homes, wondering what would comenext. Soon after, the waters of Lake Oku left the lake



bed and destroyed most of the homes and people onthe Kijem side; the remnant moved away from the lake,further west into the nearby Belo Valley. Oku peoplestill elaborate annual sacrifices to the lake, whichshowed by its actions that it wished to belong to theOku people. From that day to this, no red streaks haveappeared in the lake.

From the above stories we can find one commontheme that “maleficent” water misbehaves in a spec-tacular way and sets in motion the migration of ethnicgroups. 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) ofthe past eruptions.

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

The Science and Technology Research Partnershipfor Sustainable Development (abbreviated asSATREPS) is a program for joint research cooperationbetween Japan and developing countries for resolvingglobal issues, e.g., environment, energy, natural disas-ter prevention and infectious diseases control.SATREPS is sponsored by the Japan International Co-operation Agency (JICA) and the Japan Science andTechnology Agency (JST). It was launched in 2008.The JICA and JST are the organizations under the Min-istry of Foreign Affairs of Japan, and the Ministry ofEducation, Culture, Sports, Science and Technologyof Japan, respectively.

Under the umbrella of the SATREPS, we were ableto obtain funds for a project entitled “Magmatic fluidsupply into Lakes Nyos and Monoun, and the mitiga-tion of natural disasters through capacity building inCameroon”. This started in 2011. The project was nick-named “SATREPS-NyMo”. It was a 5-year project andcontinued until March 2016. The project was headedby Professors Takeshi Ohba (Tokai University, Japan)and Minoru Kusakabe (co-leader, University ofToyama, Japan). The counterpart organization inCameroon was the Institute for Geological and Min-ing Research (IRGM) headed by Dr. Joseph V. Hell,under the Ministry of Scientific Research and Innova-tion (MINRESI). The goal of the project was to miti-gate natural disasters in Cameroon through capacitybuilding, specifically for issues related to the LakesNyos and Monoun gas disasters. To accomplish thegoal, we planned the following sub-projects: (1) a CO2discharge system beneath Lakes Nyos and Monoun;(2) the hydrological regime around the lakes; (3) theeruptive history of volcanoes along the Cameroon Vol-canic Line (CVL); (4) the CO2 distribution in LakesNyos, Monoun and other lakes along the CVL; (5) thesetup of an experimental system for removing CO2-rich deep water to prevent gas re-buildup in LakeMonoun; and (6) the continuation of geochemical

monitoring of Lakes Nyos and Monoun. During theproject, scientific cooperation between the two coun-tries was encouraged through the exchange of scien-tists. Capacity building included scholarships to trainCameroonian students and technicians in Japan, andthe donation of scientific instruments to IRGM. Theprogress of the SATREPS-NyMo can be seen in thewebsite “http://www.satreps.u-tokai.ac.jp”. The projectwent well in terms of scientific achievement. Manyscientific papers were published, e.g., Issa et al. (2013,2014a, 2014b), Asaah et al. (2014, 2015), ChakoTchamabé et al. (2013), Fouépé et al. (2013), Fantonget al. (2013, 2015), Tiodjio et al. (2014, 2015, 2016),Kamtchueng et al. (2014, 2015a, 2015b), Yoshida etal. (2016), Ohba et al. (2016), Kozono et al. (2016),and Saiki et al. (2016).

As described in Section 3, a bottom water removalsystem was installed at Lake Monoun in December2013 to stop re-buildup of CO2 (Yoshida et al., 2016).The system is shown in Figs. 31, 32. As the degassingpipes in Lake Monoun had lost their gas self-lift capa-bility, one of the pipes was utilized to set up a solarpower driven rotary pump, in order to reduce the totalcost of the installation. The intake depth of the pipe is~99 m, very close to the bottom (100 m). A small ro-tary water pump with an outer diameter of 74 mm wasplaced inside the pipe which had an internal diameterof 100 mm. Four small solar modules with a total out-put of 320 W were used as a power source. Althoughthe system shown in Fig. 32 works only during thedaytime, it is capable of pumping bottom water at an

Fig. 31. Schematic presentation of the deep water removalsystem.



estimated rate of ~100 m3/day. Based on this rate andthe current CO2 concentration of deep water (~90mmol/kg as of March 2014), the annual removal rateof CO2 is calculated to be 3.3 Mmol/year. Since thisremoval rate is less than half the natural recharge rate(8.2~8.4 Mmol/year, Kling et al., 2005; Kusakabe etal., 2008), it would be advisable to install 2 additionalsystems at Lake Monoun to equal the natural CO2 re-charge, in order to reduce the risk of a limnic eruptionin the future. The system is robust and can work for along time without complicated maintenance and trans-portation of fuel, which is an important factor that anysystem should have in a remote area like Lakes Nyosand Monoun in Cameroon.

6. The Cameroon Volcanic Line

6-1. Eruption age of the Nyos maar and potentialcollapse of the natural dam

Lakes Nyos and Monoun are maar crater lakes situ-ated along the Cameroon Volcanic Line (CVL) (Fig.1). The northern edge of Lake Nyos consists of a 45-m-wide natural dam (Fig. 33) that holds surface lakewater down to a depth of 40 m. The dam (Fig. 33) ismade of pyroclastic materials deposited at the time ofthe volcanic eruption that formed the maar. The upperunit is moderately consolidated with visible cracks atthe surface, whereas the lower unit is poorly consoli-dated and looks readily eroded as indicated by a con-cave structure beneath the upper unit (Lockwood etal., 1988). Erosion of the lower unit may be facilitatedby seeping water. Lockwood and Rubin (1989) deter-mined 14C ages of 2 pieces of charcoal found at thebase of the lower unit to be ~400 and ~5100 years BP(before present). They took the age of 400 years to in-dicate the age of trees that were growing at the time ofmaar formation. The older age was discarded based on

an interpretation that the trees grew in magmatic CO2-rich atmosphere at the center of the present maar wherethe eruption took place. Magmatic CO2 is character-ized by “dead carbon” (no or very little 14C), and itsincorporation in trees resulted in older ages. Thepyroclastic rocks that form the dam once extendedmuch farther to the northwest (~600 m), but the lakewater overflowing the spillway has back-eroded theserocks along the stream bed, leaving only the 45-m-widedam at the present time (Fig. 33). An average erosionrate calculated from these data is 1.5 m/year. At thisrate, the 45-m-wide dam will be eroded away in 30years, if the age of the dam is correct and the erosionproceeds at the mean constant rate. It is, however, morerealistic to imagine that the dam collapse will take placein an irregular and catastrophic way. Figure 34 showsmany joints at the surface of the moderately consoli-dated upper unit and the seepage of lake water throughthe poorly consolidated lower unit. The seepage ofCO2-containing lake water may have chemically erodedthe lower unit in the past, resulting in fall-out of thelower unit, as suggested by the existence of caves.There may be an associated breakage of the jointedupper unit. Thus, the erosion rate may vary irregularlywith time, but it is still alarmingly high. On this basis,Lockwood et al. (1988) warned that the dam may even-tually collapse releasing >50 million tons of water andinducing a catastrophic flood on downstream areas in-cluding part of Nigeria. This warning was seriouslytaken up by the Cameroonian authorities. They askedsupport from the United Nations Office for the Coor-dination of Humanitarian Affairs (OCHA) and theUnited Nations Environmental Program (UNEP) for adetailed survey of the dam. A team of experts fromOCHA and UNEP concluded that a failure of the dam

Fig. 32. Photograph showing the solar power-driven deepwater removal system installed at Lake Monoun.

Fig. 33. Photograph showing the 45-m-wide natural dam atthe northern edge of Lake Nyos. The area surrounded by agreen curve is the head of the valley where pyroclastic ma-terials are said to have been eroded away.



is highly likely “to occur within the next 5 years” basedon their geotechnical survey (Joint UNEP/OCHA En-vironment Unit, 2005). They recommended reinforce-ment of the dam by cementing fractures and theunconsolidated part of the dam. As a result, the damhas been reinforced by engineering methods.

The warning by Lockwood et al. (1988) also drovegeochronological studies of the dam, since the age isdirectly related to the erosion rate and thus to the safetyof the dam. The age of the dam has long been debated.The debate on the age is concisely summarized in Akaand Yokoyama (2013). Freeth and Rex (2000) proposedthat the age of eruption of the Nyos maar was in ex-cess of 100,000 years, based on K-Ar dates (Fig. 34)and evidence from aerial photographs taken in 1963–1964 that showed no change in the width of the damsince that time. They concluded that the dam materialswere eroding at a “geologically realistic rate” and that“there is no reason to suspect that the rate at which itis currently eroding away is, in itself, sufficient to posean immediate threat”. However, the application of theK-Ar dating method to the basaltic rocks from the Nyosdam area was criticized by Lockwood and Rubin(1989), because the Nyos basalts contain fine shardsof K-feldspars which were derived from basementmonzonite (Fig. 34). The K-Ar dating of the rocks con-taining the shards gives much older ages than their trueage due to the inclusion of K-feldspars with a high ra-diogenic Ar concentration.

Aka et al. (2008) applied a U-series dating methodto Lake Nyos maar basalts. The basic principles, as-sumptions and applications of the U-Th dating methodare summarized in Chabaux and Allègre (1994). Aka

et al. (2008) analyzed 12 samples collected from theLake Nyos area, including 5 samples of the dam-form-ing surge deposit and 5 nearby lava flows. They usedXRF and ICP-MS for the analysis of major and traceelement compositions including (238U/232Th), (230Th/232Th), (226Ra/230Th) and (238U/230Th) ratios. The re-sults of the Th-Ra disequilibria are reproduced in Fig.35. The (230Th/232Th) ratios of 10 alkaline rock sam-ples vary from 0.886 to 1.024, and the (238U/232Th)ratios vary from 0.716 to 0.880. Data for 26 samplesfrom the Mt. Cameroon volcano, which has eruptedduring the last 100 years, are also included (Yokoyamaet al., 2007). The Lake Nyos and Mt. Cameroon sam-ples lie closely on a line marked as 238U/230Th = 0.82with a few exceptions, significantly above the equi-line which is 238U/230Th = 1.00. This feature indicatesthe presence of a 15 to 28% enrichment of 230Th over238U, suggesting strongly that the Lake Nyos maar for-mation is younger than ~375 ka which is 5 times thehalf-life of 230Th. If the time which has elapsed sincethe volcanic eruption is greater than 375 ka, then (230Th/238U)A (activity ratio) becomes unity, or a secular equi-librium is established, and no dating can be made (equi-line in Fig. 35a). Tholeiitic samples, D26 and D27 inFig. 35a plot on the line 238U/230Th = 1.00, an indica-tion that they are in the 238U-230Th radioactive equilib-rium, giving their formation age older than 375 ka, withno more information about the age. It is important tonote that the variation in the (230Th/232Th)A and (238U/232Th)A ratios of the Mt. Cameroon samples (~0.99 and~0.82, respectively), and the corresponding excess230Th over 238U (18–24%) were all within the rangefor Lake Nyos samples (Fig. 35a). Figure 35b is a

Fig. 34. Cross-section of the natural dam at Lake Nyos. Some explanatory words were added on the cross-section originallydrawn 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 1988with permission of Springer.



plot of (226Ra/230Th)A vs. (238U/230Th)A for the studiedsamples. They were compared to published data forMORB and OIB (inset). The (226Ra/230Th)A ratios forthe alkaline rock samples range from 1.017 to 1.040with a mean value of 1.028 ± 0.008. These Nyos dataplot above the (226Ra/230Th)A =1 line (equilibrium), in-dicating an enrichment of 226Ra compared to 230Th thatwas acquired during partial melting of the mantlesource, as is generally observed in oceanic basalts(Thomas et al., 1999). Similar to 238U-230Th systemat-ics, the tholeiitic samples are in 226Ra-230Th equilib-rium. It is highly contrasting that the Mt. Cameroondata have higher (226Ra/230Th)A ratios (1.09–1.21) thanthe Lake Nyos samples (1.01~1.04), although the twovolcanoes are similar in their degree of 238U-230Th dis-equilibria (Aka et al., 2008). The initial 226Ra/230Th

ratio has to be known to calculate the age of the damusing the excess 226Ra. Since there are no eruptions ofa known age which have occurred in the Lake Nyosarea, the assumption was made that the initial ratio wasthe same (1.15 ± 0.02) as that measured in Mt.Cameroon lavas that are erupting today (Yokoyama etal., 2007). Using this assumption, the 226Ra-230Th ageof Lake Nyos was calculated to be 8.75 ± 0.49 ka (Akaand Yokoyama, 2013) after a careful examination ofthe samples. Based on this age, they consider that acollapse of the Nyos dam from erosion alone is not asimminent and alarming as has been suggested. How-ever, making the dam more stable is necessary to com-pletely eliminate the potential flood hazard.Stabilization by grouting of the dam has been under-taken.

Fig. 35. (a) (230Th/232Th)-(238U/232Th) activity ratio diagram for Lake Nyos and Mt. Cameroon samples. Some Lake Nyossamples are enriched in 230Th compared to 238U by 15–28%. (b) (226Ra/230Th)-(238U/230Th) activity ratio diagram for the samplesshowing 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 potentialhazards on the Lake Nyos dam, 212–224, Copyright 2008, with permission from Elsevier.



Fig. 36. Chemical composition of rocks from CVL volcanoes. (a) K2O+Na

2O versus SiO

2, and (b) Mg number versus SiO

2

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

The Cameroon Volcanic Line (CVL) is an alignmentof Cenozoic volcanoes stretching for 1600 km fromAnnobon in the Gulf of Guinea to Biu Plateau in thecontinental part of central Africa (Fig. 1). It straddlesboth oceanic and continental lithosphere. The CVL canbe grouped into 3 sectors, i.e., the oceanic sector tothe southwest (Annobon, Saõ Tomé, and Principe), theocean-continent boundary (Bioko, Etinde and Mt.Cameroon), and the continental sector (Manengouba,

Bambouto, Oku, Ngaoundéré Plateau, Mandara Moun-tains and Biu Plateau) to the northeast. The volcanicislands in the oceanic sector are made up of rocks rang-ing from nephelinite, basanite and basalt to trachyteand phonolite (Halliday et al., 1988; Deruelle et al.,1991). The volcanoes in the ocean-continent bound-ary are located SW of Mt. Cameroon, and are made ofmostly nephelinitic lavas for Etindé (Nkoumbou et al.,1995) and basalts and basanites for Bioko and Mt.Cameroon (Yokoyama et al., 2007; Asaah et al., 2014).Mt. Cameroon is the only active volcano in the CVL



with seven eruptions recorded in the last 100 years,i.e., 1909, 1922, 1954, 1959, 1982, 1999, and 2000 (Suhet al., 2003). It is a composite volcano made of alka-line basanitic and basaltic flows interbedded with smallamounts of pyroclastic materials and numerous cindercones (Suh et al., 2003; Yokoyama et al., 2007). Thecontinental sector of the CVL includes MountBambouto and Mount Oku. They are Oligocene toQuaternary strato volcanoes with lava successions com-prising a strongly bimodal basalt-trachyte-rhyolite suite(Marzoli et al., 2000, 2015; Kamgang et al., 2013).Mt. Manengouba is also in the continental sector andis a well-preserved stratovolcano whose summit hoststwo concentric calderas with lakes. Lavas range frombasalts to trachytes, quartz trachytes, and rare rhyolites(Pouclet et al., 2014). The Ngaounderé Plateau in thenortheastern continental part of the CVL consists ofalkaline basalts and basanites capped by trachytes andphonolitic flows. The Biu Plateau, which is located inthe northern part of the Ngaounderé Plateau, consistsof basaltic flows with a maximum thickness of 250 m.This plateau is composed of basanite to transitionalbasalts (Rankenburg et al., 2005).

Since Lakes Nyos and Monoun are situated on theCVL, it may be informative to give a brief summaryof the origin of the CVL to understand the characteris-tics of the Nyos and Monoun volcanoes. The origin of

the CVL has long been a subject of controversy, andvarious hypotheses have been proposed. They are sum-marized by Aka et al. (2004) and more recently byAsaah et al. (2014), as follows: (1) Reactivation of pre-existing tectonic structures in the Cenozoic associatedwith crustal melting (Gorini and Bryan, 1976; Moreauet al., 1987; Fairhead, 1988). (2) Membrane stressesgenerated by the movement of the African plate awayfrom the equator (Freeth, 1978). (3) Displacement ofthe African plate (Fitton, 1980). (4) Hotspot trail(Morgan, 1983). (5) Hotline hypotheses (Meyers et al.,1998). (6) A plate-wide shallow mantle convectionmodel (Burke, 2001). (7) Edge convection andlithospheric instability (Reusch et al., 2010). Of theabove, Fitton’s classic hypothesis is still attractive inthat the Benue Trough and the CVL are related to acommon “Y”-shaped hot zone in the asthenosphereover which the African plate moved during the periodof 110 to 70 Ma (Fitton, 1980). The “Y”-shaped hotzone was a rift zone that extended from a triple junc-tion originally located at the Gulf of Guinea that wasunderlain by the St. Helena hotspot at the time of theopening of the south Atlantic. The CVL developed overthis rift zone. In this sense, the magmatism in the CVLmay have been similar to that in the currently activeEast African Rift Zone. Asaah et al. (2014) went forthe “hotline” model which invokes multiple plumes

Fig. 37. Trace element patterns for mafic rocks from the oceanic and continental sectors of the CVL. Reproduced from figure4 of Asaah et al. (2014). The patterns are generally similar to each other and akin to OIB, suggesting an origin from a similarsource. Reproduced from figure 5 of Asaah et al. (2014).



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

originating from the same source in the upper mantle,each of which produced volcanoes independently, asthe model appears to explain the diverse features ofthe CVL, i.e., geophysical, structural and geochemicalevidence, including the absence of time-dependent

volcanic activity.Magmatism of the CVL is characterized by melting

in the garnet lherzolite stability fields (Marzoli et al.,2000; Yokoyama et al., 2007; Kamgang et al., 2013),although melting in the spinel lherzolite stability field



has been reported in the Ngaoundere Plateau (e.g., Leeet al., 1996; Nkouandou and Temdjim, 2011). In addi-tion, mixing of both garnet and spinel melting fieldshas been reported in Mt. Cameroon (Tsafack et al.,2009). Both mafic and felsic rocks show chemical fea-tures consistent with a plume activity outlined by theirocean island basalt (OIB) characters, and their isotopicratios (Mbassa et al., 2012).

6-3. Geochemistry of CVL magmas

Asaah et al. (2014) made a comprehensive reviewof the geochemistry of CVL rocks. They compiled theexisting geochemical data of the CVL rocks (580 sam-ples) consisting of major and trace element composi-tions and radiogenic (Sr-Nd-Pb) isotope compositions.Figure 36 shows some chemical characteristics of theCVL rocks in terms of (a) K2O+Na2O versus SiO2, and(b) Mg number versus SiO2 plots. The SiO2 contentsshow a wide range of variation from 38% (oceanicCVL) to 79% (continental CVL) reflecting the diverserock types. The Mg number (Mg#), defined as Mg# =MgO/(MgO+FeO)*100, is often used as an index ofthe level of evolution of volcanic rocks. It shows dif-ferent trends from one volcanic center to another. TheCVL rocks from the oceanic and continental sectorsare dominantly alkali basalts and basanites. The Mg#of mafic samples ranges from 60~69 (least evolvedbasalts) to 40~49 (evolved rocks), indicating variousfractional crystallization paths (Fig. 36b). Refer toAsaah et al. (2014) for further discussion.

Abundance patterns of trace elements are often usedto discuss magma genesis, since they providegeochemical and geological information through theirunique chemical properties and sensitivity to processesto which major elements are insensitive. Primitive-mantle normalized trace element patterns (Palme andO’Neill, 2003) for mafic rocks from the oceanic andcontinental sectors of the CVL are presented in Fig.37. The patterns are generally similar to each other(except for the Mt. Etindé samples) and akin to OIB,suggesting an origin from a similar source. They showa marked enrichment of light rare earth elements(LREEs) and a strong fractionation of heavy rare earthelements (HREEs) relative to LREEs. The most strik-ing features of Fig. 37 are: (1) the Mt. Etindé sampleshave high trace elemental abundances compared to theother CVL alkaline basalts; (2) relatively high posi-tive anomalies of Nb, La and Nd; and (3) the occur-rence of a K-trough. Nearly constant elemental ratiosof incompatible elements in CVL rocks suggest thatmagma processes, such as zone refining melting,magma mixing, and extensive fractionation and replen-ishment, were not dominant processes during the gen-eration of the CVL mafic lavas, because the above proc-esses can efficiently fractionate incompatible elements.The peculiar features of Mt. Etindé may have resulted

from source materials that are different from the otherCVL lavas, as suggested by the Mg# versus SiO2 trend(Fig. 36b) and a strong high m (HIMU) character there.

The radiogenic isotope (Sr-Nd-Pb) geochemistry ofthe CVL rocks is also summarized in Fig. 38, adaptedfrom Asaah et al. (2014). The Sr-Nd-Pb isotopic com-positions of the CVL basalts overlap those of OIB. Inthe 207Pb/204Pb vs. 206Pb/204Pb diagram (Fig. 38a), thedata plot parallel to the Northern Hemisphere Refer-ence Line (NHRL) of Hart (1984), and to the right ofthe 4.53 Ga geochron. However, some data from theOku Volcanic Group (OVG) with low 206Pb/204Pb and207Pb/204Pb ratios overlap with the MORB end mem-bers (Atlantic, Indian, and Pacific MORBs). The 143Nd/144Nd ratios and 87Sr/86Sr ratios of the mafic rocks showa limited range of variation (Fig. 38b). The 87Sr/86Srratios range from 0.70286 in a sample from the BiuPlateau to 0.70515 in a sample from Mt. Bambouto.The 143Nd/144Nd ratios vary from 0.51302 in a samplefrom the Biu Plateau to 0.52771 in a sample from Mt.Cameroon. Some lavas from the Biu Plateau and theoceanic CVL show relatively low 87Sr/86Sr and high143Nd/144Nd ratios, implying that they are more primi-tive than other continental volcanic rocks (Mt.Bambouto, Mt. Manengouba, and the OVG). Isotopedata for the OVG show a wider spread than those ofthe other CVL volcanoes. This difference is conspicu-ous in the Pb isotopes. In Fig. 38b, a negative correla-tion is observed between 143Nd/144Nd and 87Sr/86Sr ra-tios and the correlation slope matches the mantle arrayof MORB-OIB samples. From these figures it is sug-gested that the CVL lavas formed by a dominant con-tribution of EM2 to the Depleted MORB Mantle(DMM). The 143Nd/144Nd versus 206Pb/204Pb plots (Fig.38c) show positive and negative correlations with dif-ferent slopes, where the role of EM2 becomes domi-nant over EM1. Mixing with various end members indifferent proportions may account for the complex iso-topic characteristics of the CVL lavas.

Based on trace element and isotope geochemistry, ithas been suggested that these magmas derived fromthe sub-lithosphere without interaction with the over-lying lithosphere (Fitton and Dunlop, 1985). A differ-ent view, however, was given by Halliday et al. (1990)that the continent/ocean boundary magmas (Bioko,Etindé and Mt. Cameroon) are characterized by 206Pb/204Pb ratios that are higher (more radiogenic) than thoseof typical continental and oceanic sector magmas. Thisradiogenic feature has also been confirmed by the dis-tribution of 3He/4He ratios of lavas and mineral sepa-rates from the CVL rocks showing a clear 3He/4Hevalley as already illustrated in Fig. 20 (Aka et al.,2004). Together with Sr, Nd and O isotopic variations,Halliday et al. (1988, 1990) suggested that the radio-genic nature of the 206Pb/204Pb ratios of rocks from theocean-continent boundary reflects melt migration fromthe St. Helena fossil plume head that took place at 125



Ma, and that some of the CVL magmas derive fromthe upper metasomatized part of the fossil plume inthe lithospheric mantle (Fig. 39). The degree of trace-element enrichment (U/Pb, or m in this case) varies asa function of the vertical thickness of the plume headthrough which the melts migrated. At the marginswhere the flattened plume head is thinnest, the sourceregions are dominated by more depleted mantle(Halliday et al., 1990). The radiogenic nature of 206Pb/204Pb ratios and the 3He/4He valley observed at theocean-continent boundary region can be explained bythe magma genesis affected by the fossil plume head.

6-4. Volatiles in magma

A fundamental question arises as to whether LakeNyos magmas are enriched in CO2. Volatile contentsin pre-eruptive magmas have been estimated by vari-ous techniques. One of the techniques is to analyze meltinclusions in phenocrysts, since melt is trapped ingrowing phenocrysts as melt inclusions in magma andis quenched to glass at the time of eruption. Thevolatiles, mainly H2O and CO2, in the glass inclusionsare determined by microanalytical techniques such asFourier transform infra-red spectroscopy (FTIR), la-ser-Raman spectrometry, and secondary ion massspectrometry (SIMS), etc. (Ihinger et al., 1994). An-other approach is experimental petrology where min-eral stabilities and assemblages are calibrated underdifferent (but controlled) conditions, such as tempera-ture, pressure, and water fugacity. Comparison of ex-perimental products with natural phenocrystic assem-blages allows us to constrain the pre-eruptive volatilecontents (Johnson et al., 1994). Unfortunately for us,however, lavas from the Nyos volcano are mostly

aphyric (Aka et al., 2008) and difficult to use for theanalysis of pre-eruptive volatile contents by the afore-mentioned techniques. Instead, based on major andtrace elements systematics, Aka (2015) proposed thatthe Nyos basalts formed by a small degree (1~2%) ofpartial melting of the primitive mantle to whichamphibole and phlogopite had been added bycarbonatitic fluids, and that decarbonation reactions ofthe carbonatitic metasomatism are responsible for pro-ducing the magmatic CO2. However, based on thegeochemical data of the Nyos volcanic rocks, Asaah etal. (2015) suggest that CVL magmatism is predomi-nantly of an asthenospheric source with little contri-bution from the subcontinental lithospheric mantle(SCLM). The lavas show evidence of enrichment bymetasomatic fluids probably in the Mesozoic (e.g.,Halliday et al., 1990; Aka, 2015; Asaah et al., 2015).The metasomatism affected the SCLM, inducing hy-drous minerals like amphibole and phlogopite that arenot stable in the asthenosphere. Asaah et al. (2015)suggest that the metasomatic fluids crystallized as smallpockets or veins in the SCLM. An ultimate source ofCO2 in the Nyos magma may derive from thedecarbonation of such crystallized metasomatic fluids.It is unlikely that the CVL magmas, including the Nyosmagma, have abnormal CO2 in their mantle source.

Lake Nyos and Lake Monoun volcanoes are locatedin the Oku and Bambouto volcanic centers, respec-tively, in the middle of CVL (Fig. 1). Lake Nyos is amaar lake created by a phreato-magmatic eruption.There are some other maar lakes near Lake Nyos, i.e.,Oku, Elum, Nyi, Wum and Enep, but only Lake Nyoscontains a large amount of dissolved CO2. Accordingto Lockwood and Rubin (1989) who described the ge-ology of the Nyos volcano, eruption sequences are sum-

Fig. 39. Schematic presentation of a model for source enrichment in high m elements following plume emplacement at 125 Mabeneath the ocean/continent boundary of CVL (St. Helena). Reprinted by permission from Macmillan Publishers Ltd: Nature347, 523–528, Halliday, A. N., Davidson, J. P., Holden, P., DeWolf, C., Lee, D.-C. and Fitton, J. G., Trace-element fractionationin plumes and the origin of HIMU mantle beneath the Cameroon line, Copyright 1990.



marized as shown below. The formation of the Nyosmaar is directly related to the ascent of alkali basaltmagma. The first lava reached the surface in a rela-tively gentle, fire-fountaining fashion, depositingscoria and fluid bombs over a wide area around thepresent north end of Lake Nyos. This phase was fol-lowed by an explosive and violent eruption due to vola-tile expansion. The violence of the activity increasedrapidly, however, and basalt is only included as shat-tered fragments in upper parts of the pyroclastic sec-tion. Neither the basalt flow nor the associated scoriaat the base of the pyroclastic section were found tocontain ultramafic xenoliths, suggesting that mantlerocks were only transported to the surface during thelater more explosive phases of the eruption. The depthof the explosive activity may have gradually increasedduring the eruption, and the initial explosion cratergradually widened, which resulted in the formation ofthe maar crater, or the present Lake Nyos. It can beimagined that the magma subsided after the eruption,but the release of CO2-rich volatiles from the magmacontinues until today.

Magma is generally generated by a partial meltingof rocks in the lower crust or upper mantle. Mantlerocks, mainly comprised of peridotite, exist as a solid,because the geothermal gradient within the Earth isgenerally below the solidus of mantle rocks. It has beenhypothesized that part of solid mantle, if heated lo-cally, can ascend as a diapir and cross the solidus wherepartial melting starts to take place. Volatile materialssuch as H2O and CO2, if they coexist with the rocks,reduce the solidus temperature and facilitate a partialmelting of the rocks, or magma genesis. Thus, the co-existence of volatiles is important for partial melting.Metasomatic fluids may have affected the primitivemantle beneath Lake Nyos and the fluids produced by

the decarbonation of metasomatized mantle facilitatedpartial melting (Aka, 2015). Once a melt is formed, itrises through the mantle due to its lower density (higherbuoyancy) and approaches the surface of the Earth toform magma. The melt may remain as a magma reser-voir in the shallow part of the crust. When the magmafurther ascends, crystallization in the magma beginsbecause of the reduction of temperature and pressure.Magma contains various volatile materials, such asH2O, CO2, S, Cl, etc. Since the volatile materials willnot all be incorporated into crystals (or minerals), theytend to be concentrated as fluids in the magma as itrises and cools. A volcanic eruption is often facilitatedby magma ascent driven by a lowered density due tothe accumulation and expansion of bubbles of thevolatiles in the magma.

The chemical composition and concentration of mag-matic volatiles have been estimated through the analy-sis of high temperature volcanic gases, chilled glassymargins of lava that has extruded onto the bottom ofthe deep ocean, and glass inclusions in phenocrysts ofvolcanic rocks. Volatiles in magma are almost com-pletely discharged into the atmosphere at the time ofvolcanic eruption. For this reason, the chemical analy-sis of high-temperature volcanic gases, if collected andanalyzed properly, can give the volatile composition(not concentration) in magma. Table 4 shows the con-centration of H2O, CO2, S and Cl in some types ofmagma, and the composition of volcanic gases that isexpected from the degassing of each type of magma(Shinohara, 2003). The concentration of the magmaticvolatiles in magma is highly variable depending on itstype. Water concentration in Mid-Oceanic Ridge Ba-salt (MORB) is low (0.1~0.5 wt%), whereas that ofsubduction zone magma is more than an order of mag-nitude higher (1~5 wt%). The concentration of CO2 in

Shinohara (2003) and Giggenbach (1996).

Concentration in magma Expected gas compostion

H2O CO2 S Cl H2O CO2 S HClwt% ppm ppm ppm mmol/mol mmol/mol mmol/mol mmol/mol

Glassy margin of MORBEastern Pacific Ocean 0.12 1630 690 50 526 292 170 11Atlantic Ocean 0.20 1320 1170 60 621 167 203 9

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

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

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



MORB magma is 1000~3000 ppm, which is generallyhigher than that of subduction zone magma. The cal-culated composition of gases exsolved from magma isalso shown in Table 4. Such calculated gas composi-tions are in general agreement with the observed gascompositions (not shown, but can be found in Allard,1983; Gerlach, 1983; Shinohara, 2003). In gases fromhotspot basaltic volcanoes such as Kilauea (Hawaii),the water content is lower than that from subductionvolcanoes, whereas the CO2 content is significantlyhigher in hotspot and mid-oceanic volcanoes, reflect-ing its higher concentration and low solubility in ba-saltic melts.

Solubilities of H2O and CO2 in silicate melts havebeen experimentally determined as shown in Fig. 40(Holloway and Blank, 1994, and references therein).The solubility depends on the temperature, pressureand the chemistry of melts. It increases as the partialpressure of the volatile species in question increases,and decreases as the temperature of the melt increases.Generally speaking, water is approximately an orderof magnitude more soluble than CO2. Water dissolvesslightly more in silicic melts than in basaltic melts,whereas CO2 dissolves more in basaltic than in silicicmelts (Fig. 40). Figure 41 illustrates the solubility ofCO2 and H2O in basaltic melts at 1200∞C as a functionof the total pressure of the volatiles. The non-linearrelationship of this binary system in the melts comesfrom the non-ideal mixing properties of these species(Holloway and Blank, 1994). Using Fig. 41, it can beenvisaged how the volatile composition in the meltchanges as the decompression proceeds. For example,at point A of Fig. 41, where CO2 = 540 ppm and H2O =1.6 wt%, the melt is saturated with the coexisting fluidof which the mole fraction of H2O equals 0.2 and thatof CO2 is 0.8. This implies that the fluid coexistingwith the basaltic melt is extremely rich in CO2. As themagma ascends, or the confining pressure is reduced,the fluid exsolves, or degassing takes place. Ifdegassing proceeds in a closed system, the fluid com-

position in the melt will follow the thin dotted linedepending on the co-existing H2O concentration asshown in Fig. 41. If degassing takes place in an opensystem, the melt composition may follow a differentpath, as indicated by the thick long dashed line, sincethe CO2-rich fluid leaves the magma when the systembecomes open due to the low CO2 solubility in themelts, making the remaining magma progressivelyCO2-poor, while the H2O concentration decreases onlya little. As long as the magma keeps open-systemdegassing, CO2-rich fluid is continuously released fromthe magma. This solubility-controlled behavior of CO2in basaltic magma may explain a CO2-rich nature offluids separated from the magma. The ultimate sourceof CO2 in the Nyos magma may derive from thedecarbonation of crystallized metasomatic fluids in the

Fig. 41. Solubilities of CO2 and H

2O in basaltic melts at

1200∞C as a function of the total pressure of the volatiles(Holloway and Blank, 1994; Shinohara, 2003).

Fig. 40. Solubilities of H2O and CO

2 in silicate melts (Holloway and Blank, 1994; Shinohara, 2003).



subcontinental lithosphere (Aka, 2015; Asaah et al.,2015). The permanent supply of such CO2 is likely tobe responsible for the high concentration of CO2 gasin the fluids feeding into Lakes Nyos and Monoun.

7. Other CO2-rich volcanic lakes in the world

Of 714 volcanoes in the world, 86 volcanoes hostlakes (Pasternak and Varekamp, 1997). Information onthe volcanic lakes is now available from the VOLADA(2013) database (https://vhub.org/resources/2822).Although Lakes Monoun and Nyos in Cameroon be-came notoriously famous because of the gas disastersin the mid-1980s, other CO2-rich volcanic lakes existin the world, e.g., Lake Kivu (Democratic Republic ofthe Congo and Rwanda, see below for references),Laacher See (Germany) (Aeschbach-Hertig et al.,1996), Lake Van (Anatolia in eastern Turkey) (Kipferet al., 1994), Lago Albano and the two Monticchiolakes (Italy) (Anzidei et al., 2008; Caracausi et al.,2009), Laguna Hule and Rio Cuarto (Costa Rica)(Alvarado et al., 2011), and Lac Pavin (France)(Aeschbach-Hertig et al., 1999).

Of these lakes, Lake Kivu has been known to con-tain a high concentration of CO2 and CH4 in its deepwater since well before the Lake Nyos event (e.g.,Deuser et al., 1973; Tietze et al., 1980). Carbon diox-ide dissolved in the lake is basically of magmatic ori-gin, a situation similar to that in Lakes Nyos andMonoun in Cameroon, although the magmatic CO2 ismixed with a variable proportion of biogenic CO2. Thelake is located along the East African Rift on the bor-der between the Democratic Republic of the Congo(DRC) and Rwanda. The lake area is tectonically andvolcanically active as part of the East African Rift Sys-tem. Because of the high gas concentrations in the lakeand the large population around it, Lake Kivu has apotential risk of a gas disaster caused by a limnic erup-tion which may be triggered by a possible volcaniceruption at the lake bottom (Schmid et al., 2005) or aplunge of lava flows from Nyiragongo, the nearest ac-tive volcano (only 20 km NE to the lake). Indeed, the2002 eruption of the volcano generated lava from flankfissures flowed into the city of Goma, the provincialcapital, resulting in destruction of local structures andthe evacuation of local people, and these lava flowseventually ran into the lake. Fortunately, no limniceruption was induced at that time (Tedesco et al., 2007).Detailed gas and water chemistry of Lake Kivu andthe surrounding region has been published by severalauthors (Tietze et al., 1980; Tassi et al., 2009; Schmidet al., 2005). The lake has 5 basins, each of which ischaracterized by a different chemistry, CO2 profile, andbiology. The main basin (>250 m) contains the highestCO2 concentration with a horizontal heterogeneity.Although the highest CO2 concentration in the mainbasin is far from saturation at any depth, the CO2 con-

centration at Kabuno Bay (a small basin on the north-western end of Lake Kivu) is relatively close to satu-ration. Since Kabuno Bay is shallower than the mainbasin and is characterized by the highest input of CO2-rich magmatic fluid, the bay is considered to be poten-tially most hazardous in terms of the possibility oflimnic eruption. Continuous monitoring is recom-mended (Tassi et al., 2009). The concentration of dis-solved CH4 is highest (~17 mmol/L), approx. 12% ofdissolved CO2 at the bottom of the main basin. Thegas is produced by the bacterial reduction of CO2 andacetate fermentation (Schoell et al., 1988). It is im-portant to note that microbial activity contributes tothe gas chemistry in deep, stratified and anaerobiclakes, as recently found also at Lakes Nyos andMonoun (Tiodjio et al., 2014, 2016). Carbon isotopicratios (d13C) of CO2 dissolved in the main basin of theLake Kivu range from –7 to –6‰ (relative to VPDB),suggesting also a large contribution from mantle-origi-nating CO2. Those at Kabuno Bay (–11 ~ –13‰), how-ever, are significantly lower than the values for themain basin, probably reflecting the interaction of mag-matic fluids with organic-rich sedimentary materialsthat underlie volcanic rocks derived from nearbyNyamulagira and Nyiragongo volcanoes (Tassi et al.,2009). The 3He/4He ratio of Kabuno Bay water is 5.5Ratm, indicating a large contribution of a magmaticcomponent regardless of the low d13C values, whereasthe ratio ranges from 2.1–2.6 Ratm in the main Kivubasin water. Fumarolic gases collected at the summitcrater of the Nyiragongo volcano may best representthe 13C/12C and 3He/4He ratios of magmatic end-mem-bers in the fluids that are supplied to Lake Kivu andits surroundings (Tedesco et al., 2010). These authorsobserved typical mantle values of d13C = –3.5 ~ –4‰and 3He/4He ratios up to 8.7 Ratm for the fumarolicgases. The influence of this magmatic signature be-comes smaller, and the crustal components increase,as we move southward (toward Lake Kivu). The C/3He ratio of ~30 ¥ 1010 was observed for summitfumarolic gases. This high value probably reflects thehigh CO2 solubility in the Nyiragongo magma whichis foiditic (alkaline), different from typical MORBmagmas. High C/3He ratios up to 36 ¥ 1010 were mea-sured for the main basin water of Lake Kivu. Althoughthese ratios are close to the Nyiragongo magmaticvalue, it is more likely that the addition of CO2 in lo-cal groundwater that has interacted with organic mate-rials enhanced the lake’s C/3He ratio, as suggested byd13C values. These observations show that magmaticfluids interact with surrounding materials in varyingdegrees, and that the gas geochemistry of this area iscontrolled by the local tectonic-geologic settings(Tedesco et al., 2010).

Lake Mashu is a small, dimictic (mixing twice a year)caldera lake in Hokkaido, Japan, with a surface areaof 19 km2 and a maximum depth of 211 m. A hot spring



has been identified at the bottom of the lake. The chemi-cal characteristics of the lake were given by Nojiri etal. (1990). Based on noble gas data of the lake watercollected at various time, points and depths, Igarashiet al. (1992) estimated a 3He/4He ratio of 6.7 Ratm forhelium supplied from the lake bottom through the hotspring, suggesting the addition of mantle helium to thelake from the underlying magma. The accumulation ofmantle helium between two overturns (spring and au-tumn) was estimated to be 9.2 ¥ 107 atoms cm–2 s–1

(4He) and 8.7 ¥ 102 atoms cm–2 s–1 (3He) using thehelium profiles and a one-dimensional diffusion model.Since the CO2 supply rate was estimated to be 5.3 ¥108 mol/y (Nojiri et al., 1993), a high C/3He ratio of18 ¥ 1010 can be calculated for the supply fluid. TheC/3He ratio, similar to that estimated for the Alban Hillsvolcanic district (see below), is two orders of magni-tude greater than the MORB value. Igarashi et al.(1992) attributed this high value to the enrichment ofCO2 in the source magma beneath Lake Mashu. Al-though the CO2 supply rate of 5.3 ¥ 108 mol/y is greaterthan that for Lake Nyos (1.2 ¥ 108 mol/y, Kusakabe etal., 2008) by a factor of ca. 4, the dimictic nature ofthe lake does not allow an excessive accumulation ofCO2 in it, which is fortunate from the limnic eruptionperspective.

Laacher See is also a 53-m-deep holomictic (com-plete vertical mixing once a year) maar lake in the EastEifel volcanic district in Germany, where the dischargeof CO2 gas from the lake has been observed for years.Helium and neon isotopes dissolved in the lake weremeasured twice (spring and early autumn) in 1991 byAeschbach-Hertig et al. (1996) with the aim of esti-mating the helium flux from the lake bottom, sincegases supplied from the bottom were considered toaccumulate in the lake during summer stratification.Both the He concentration and 3He/4He ratios increasedwith depth, and the rate of increase was more clearlyobserved in early autumn. The 3He/4He ratio of theincoming He was estimated to be 5.4 Ratm, suggestinga large contribution of magmatic He with a minorcrustral contribution. Using the amount of He storedduring summer stratification and a one-dimensionalvertical mixing model, the 4He flux into the lake wasestimated to be 10 ¥ 108 atoms/cm2 s–1 with a 3He/4Heratio of 5.3 Ratm. Since gas samples from the lake were>99% CO2, a C/3He ratio of 8.6 ¥ 109 was calculated.Combining the 3He flux of 7.4 ¥ 103 atoms cm–2 s–1, aCO2 flux into Laacher See was estimated to be 3.3mmol cm–2 y–1 (Aeschbach-Hertig et al., 1996). Thisis equivalent to an annual release of 1.1 ¥ 108 mol CO2to the atmosphere. Even if this value represents theannual recharge of CO2 to the lake, the holomictic na-ture does not allow the accumulation of CO2 as wasthe case in Lakes Nyos and Monoun.

Lake Van in Anatolia, eastern Turkey, was formedduring the Pleistocene in a tectonic depression with its

outlet blocked by lava flows from the nearby Nemrutvolcano. Lake Nemrut is one of the caldera lakes nearLake Van. The injection of He, derived from depletedmantle with 3He/4He ratio of 7.4 Ratm, into LakesNemrut and Van was documented by Kipfer et al.(1994). It is likely that CO2 is also supplied to the lakes,but unfortunately there is no mention of CO2 in thelakes in Kipfer et al. (1994).

Alban Hills in the volcanic area near Rome, Italy,has been characterized by high emissions of CO2 froma pressurized CO2-rich aquifer, and small-scale gasoutbursts from the aquifer have been recorded(Carapezza and Tarchini, 2007). Carbon isotopic ra-tios were reported to be in a limited range around+1.3‰ (relative to VPDB), which suggests the contri-bution of decomposed marine carbonates as the sourceof CO2. The 3He/4He ratio of He in the associated gaswas 1.9 Ratm, very low compared to MORB and sub-duction volcanic gas values, but still suggestive of amagmatic affiliation. The C/3He ratio of gases collectedfrom a nearby well is 2.3 ¥ 1011, 2 orders of magnitudegreater than typical MORB values. This value is con-sistent with a high contribution of CO2 that was mostlikely derived from the thermal decarbonation of lime-stone involved in magma genesis at the Alban Hillsvolcanic district. Historical evidence has shown thatLake Albano, a 160-m-deep crater lake located in thecenter of the district, experienced lahars associated withwater overflow (Carapezza and Tarchini, 2007). Thepresent water and gas chemistry of the lake indicatesthat dissolved CO2 concentration increases with depthin anoxic hypolimnion (>80 m). However, the total gaspressure calculated from the CO2 concentration is farbelow the hydrostatic pressure at all depths, suggest-ing that a gas hazard at the lake is unlikely, unless CO2from the pressurized aquifer is suddenly injected intothe lake (Carapezza et al., 2008). The Monticchio cra-ter lakes in Southern Italy are also receiving passivemagmatic CO2, and the potential risk of a Nyos-typegas hazard has been described (Caracausi et al., 2009).

8. Concluding remarks

This review mainly summarizes the author’s achieve-ments in work and related matters on the Lakes Nyosand Monoun gas disasters that took place in the mid-1980s in Cameroon. At that time, nobody knew thatlakes could accumulate so much CO2 gas and then sud-denly release it to induce such disasters. The Lake Nyosand Monoun events had a strong impact on scientistsworking on gas emissions from the interior of the Earth.This impact especially boosted volcanic lake studies.Soon after the 1986 gas burst at Lake Nyos, scientistsworking on the initial phase of their research created asmall informal group “The International WorkingGroup on Crater Lakes (IWGCL)” to exchange scien-tific information about the Lake Nyos gas disaster, to



coordinate follow-up field trips planned by those whowere interested in the subject, and to organize scien-tific meetings as a forum for further discussions. Thescope of IWGCL was later expanded to include notonly studies of gassy lakes in Cameroon but also thoseof other volcanic lakes in general. The new objectiveswere to obtain information on the activity anddegassing state of shallow magmatic bodies so thatforecasting volcanic eruptions and the mitigation ofvolcanic lake-related hazards could be achieved. Ex-pansion of the scope of IWGCL naturally meant agreater number of scientists, and resulted in acquiringa formal IAVCEI status as the Commission on VolcanicLakes (CVL) in 1993. I was satisfied by these organi-zational developments as the leader of IWGCL andCVL in those early days. The CVL has organized sci-entific meetings every 2–3 years, reports of which canbe found in the website “http://www.ulb.ac.be/sciences/cvl/”. On a personal note, and as a scientist who hasworked on Lakes Nyos- and Monoun-related disasterreduction issues for close to 30 years, I was particu-larly happy to know that the CVL-9 meeting took placein Yaoundé, Cameroon, in March 2016, to commemo-rate the 30th anniversary of the Lake Nyos gas disas-ter.

During my career, I have acquired experiences ofworking in national and international scientific com-munities, and, consequently, have made friendshipswith many wonderful scientists worldwide. Such ex-periences have led me to obtain research funding thathas made it possible for me to continue to work inCameroon for ~30 years. As described in Section 5 ofthis article, a typical example was the success in get-ting support from JICA and JST for the SATREPS-NyMo project. The resolution of solving various prob-lems associated with the Lakes Nyos and Monoun gasdisasters, such as the continuation of scientific moni-toring of the lakes, the monitoring of the reinforcednatural dam at Lake Nyos, the rehabilitation and set-ting up of an infrastructure for the displaced people,etc., are obviously domestic issues for which theCameroonian Government and scientists should, inprinciple, take responsibility. But the reality is differ-ent; the economic insufficiency of Cameroon has hin-dered the principle. The main goal of the SATREPS-NyMo project is to mitigate natural disasters inCameroon through capacity building, specifically forissues related to the Lakes Nyos and Monoun gas dis-asters. The risks of the recurrence of limnic eruptionscan be defused if proper and timely actions are taken.The SATREPS-NyMo capacity building included thedonation of some analytical instruments necessary tohelp Cameroonian scientists achieve the project’s goals.Also included was the training of young Cameroonianscientists and technicians in Japan, so that, after theyget back home, they can play an important role in thefield of mitigation of natural disasters. Unfortunately,

the gas content at Lake Monoun has recently beenfound to be increasing due to the continuing gas sup-ply from the underlying magma, the duration of whichis much longer than the span of human life. It is al-most certain that the same situation will occur at LakeNyos within several years when the gas self-lift capa-bility is lost. Now is the turn for Cameroonian scien-tists and technicians to work toward defusing the newrisks of the increasing gas content in the lakes, for theyhave acquired the needed knowledge and techniques. Ihope the safety of the lakes is secured and that the sur-rounding populations can return to their ancestral rootsand go about their daily lives without the fear of fur-ther gas disasters.

AcknowledgmentsThis article is a review of scientific achievements con-

cerning the Lakes and Monoun Nyos gas disasters and re-lated subjects. The review could not have been made with-out the cooperation of many colleagues and friends whoworked together with me in the field. I express my sincerethanks to: Y. Yoshida, T. Ohba, K. Nagao, G. Tanyileke, F.T. Aka, Issa, Y. W. Fantong, J. V. Hell, G. W. Kling, W. C.Evans, D. Rouwet, and many others, who worked togetherin the field and have provided me with important scientificinformation. Special thanks go to T. Ohba who kindly sup-plied recent data (unpublished) on the CO2 concentrationsin the lakes used in Fig. 15. Fieldwork since in the period1986–2006 was mostly supported by the Grant-in-Aid forScientific Research from JSPS (Japan Society of Promotionof Science). Recent fieldwork (2011–2015) has been sup-ported by the SATREPS-NyMo project. Logistic supportfrom IRGM and its technicians is appreciated. The Embassyof Japan and the JICA office in Yaoundé are acknowledgedfor their help while I was in Cameroon.

K. Oshida of TerraPub is acknowledged for giving me thechance to write this review. I also thank Y. Matsuhisa whomade constructive comments on an early version of themanuscript. W. C. Evans, D. Rouwet and F. Aka are alsothanked for their comments that helped improve the manu-script. The English of the final version was improved by D.Larner who kindly checked the manuscript in a very carefulmanner and suggested the corrections.

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