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

1986年尼奥斯湖爆发的毒气和1984年在西非喀麦隆莫农湖发生的类似事件，造成约1800人死亡，数千只动物死亡。死亡是由于两个火山口湖底部溶解的岩浆CO_2出溶所致。由于这些湖泊所在的火山既没有喷发记录，也没有地热表现，因此，这些瓦斯灾害可以视为一种“冷”火山灾害和一种新型自然灾害。对这些湖泊在戏剧性事件发生后7年的连续观测表明：(1)瓦斯灾害是由湖泊中溶解的二氧化碳大量释放所致；而不是火山潜水喷发，(2)二氧化碳以温暖的充气矿化泉水的形式提供给湖底，(3)气体的聚集和随后的释放受到湖泊密度结构的高度制约，(4)尼奥斯湖底层的CO_2聚集正在以惊人的速度增加，(5)湖泊再次发生类似的气体爆发的可能性很大；但可以有效地预防。根据上述发现，应尽快采取以下补救行动，以减轻未来瓦斯爆炸的危险。(1)对湖泊进行脱气处理。应鼓励启动但由于资金限制而停止的受控脱气。(2)具有相似补给系统的热带深水湖泊具有积累CO_2和其他气体的潜力。因此，强烈建议对热带火山湖进行定期的地球化学和湖沼学调查，以此作为应对尼奥斯型灾害的一种缓解措施。

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.