カメルーン・ニオス湖ガス災害(1986年8月)の原因 : 火口湖からの炭酸ガスの突出

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Geochemical and limnological survey was carried out at lake Nyos, Cameroon, where the tragic gas disaster took place on 21 August 1986, killing more than 1700 people and an uncomtable number of animals. The water temperature and concentrations of dissolved chemical species increase with depth. The chemistry was overwhe1mingly dominated by C0_<2(sq)> and HCO_3- with a substantial concentration of ferrous iron below 40m. Very low concentrations of Cl- and S0_4^<2-> argue against the volcanic gas injection hypothesis as a cause of the disaster. The gases discharged from the lake consisted mainly of C0_2 but sulfurous gases such as S0_2 and H_2S were hardly present. Stable isotopic ratios of carbon and helium strongly indicated a mantle origin of the gases. The present level of the sissolved C0_2 is about 20% of the saturation. Assuming the pre-event saturation of the lake with respect to C0_2, the maximum amount of C0_2 discharged is calculated to be O.63km^3, which is consistent with the estimates based on the field observation. Density of the present lake water calculated from the profiles of temperature and chemical compositions increases with depth, indicating the stable stratification of the lake, The C0_2 gas burst was caused by upward displacement of deep water creating oversaturation of C0_2 with formation of C0_2 bubbles which move upwards dragging a part of ambient water. Since this is a self-sustaining process, most of the dissolved C0_2 would have been released from the lake by the above process once triggered by the increased input rate of warmer, C0_2- saturated groundwater into the lake bottom during the rainy season. Since exsolution of C0_2 from water and adiabatic expansion of C0_2 bubbles would have lowered the water temperature, the circulation of water may have been restricted in relatively small cells, thus preventing the initial chemical stratification from a drastic change. The lake Nyos tragedy and the Monoun event in 1984 preesnt a new type of natural disaster. It is essential to accumulate the geochemical and limnological baseline-data as presented here in order to understand the causes of the disaster in more detail and to predict possible future gas bursts from crater lakes of the world under similar geological situations.
特定非営利活動法人日本火山学会の論文
1987-04-30

カメルーン・ニオス湖ガス災害(1986年8月)の原因 : 火口湖からの炭酸ガスの突出

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