Geochemical study of CO_2 build-up in Lake Nyos, Cameroon

1986年尼奥斯湖的瓦斯爆发和1984年莫农湖的类似事件，都发生在西非喀麦隆，造成大约1800人和数千只动物死亡。这两个火山口湖的底部层中溶解的岩浆CO_2的出溶导致了死亡。由于湖泊所在的火山既无喷发记录，也无地热表现，因此，这些气灾可视为“冷”火山灾害，是一种新型的自然灾害，对这些湖泊的连续观测表明：（1）这些气灾是由于溶解在湖泊中的二氧化碳大量释放而引起的;而不是由火山潜水喷发引起的;（2）CO_2是以温暖的充CO_2矿化泉的形式提供给湖底的;（3）气体的积累和随后的释放受湖泊密度结构的高度制约，（4）尼奥斯湖底层的CO_2积累正以惊人的速度增加，（5）因此，尼奥斯湖再次发生类似的气体爆发的可能性很大;但却可以有效地预防。根据上述发现，当局应尽快采取下列补救措施，以减低日后发生瓦斯爆炸的危险。(1)这些湖应该脱气。应鼓励已开始但因资金限制而停止的有控制的脱气。(2)具有类似补给系统的热带深湖具有积累CO_2和其他气体的潜力。因此，强烈建议定期对热带火山湖进行地球化学和湖沼学调查，作为防范Nyos型灾害的一项缓解措施。

The gas outburst from Lake Nyos in 1986 and a similar event at Lake Monoun in 1984, both in Cameroon, West Africa, killed approximately 1800 people and several thousand animals. Death resulted from the exsolution of magmatic CO_2 dissolved in the bottom layers of these two crater lakes. Since the volcanoes which host the lakes have neither the records of eruption nor geothermal manifestation, these gas disasters can be regarded as "cold" volcanic hazards and a new type natural disaster.Seven years of continuous observation of these lakes following the dramatic events have revealed that (1) the gas disasters were caused by a massive release of carbon dioxide dissolved in the lakes ; and not by a volcanic phreatic eruption, (2) the carbon dioxide is supplied to the lake bottoms in the form of warm CO_2-charged mineralized springs, (3) the gas build-up and subsequent release was highly conditioned by the density structure of the lakes, (4) the CO_2 build-up within the bottom layers of Lake Nyos is increasing at an alarming rate, and (5) the recurrence of similar gas bursts from the lakes is, therefore, highly probable ; but can effectively be prevented.Based on the above findings, the following remedial actions should be undertaken as soon as possible so as to alleviate the dangers from future gas bursts. (1) The lakes should be degassed. Controlled degassing, initiated but halted due to funding constraints, should be encouraged. (2) Deep tropical lakes with a similar recharge system have the potential to accumulate CO_2 and other gasses. Regular geochemical and limnological surveys of tropical volcanic lakes are, therefore, highly recommended as a mitigative measure against the Nyos-type disasters.