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 CO
2(sq) and HCO
3- with a substantial concentration of ferrous iron below 40m. Very low concentrations of Cl- and SO
42- argue against the volcanic gas injection hypothesis as a cause of the disaster. The gases discharged from the lake consisted mainly of CO
2 but sulfurous gases such as SO
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 CO
2 is about 20% of the saturation. Assuming the pre-event saturation of the lake with respect to CO
2, the maximum amount of CO
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 CO
2 gas burst was caused by upward displacement of deep water creating oversaturation of CO
2 with formation of CO
2 bubbles which move upwards dragging a part of ambient water. Since this is a self-sustaining process, most of the dissolved CO
2 would have been released from the lake by the above process once triggered by the increased input rate of warmer, CO
2- saturated groundwater into the lake bottom during the rainy season. Since exsolution of CO
2 from water and adiabatic expansion of CO
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.
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