Continuous monitoring of water chemistry of Lake Nyos using an automated CDT and satellite data

尼奥斯湖和莫农湖（喀麦隆）在20世纪80年代中期爆发，造成约1800人死亡。在这些湖泊中，自上次爆炸以来，底部水的温度、溶解离子总量和二氧化碳浓度一直在稳步上升。因此，除非降低CO_2浓度，否则湖爆可能再次发生。目前正在使用于2001年1月在尼奥斯湖和2003年2月在莫诺恩湖安装的脱气系统进行气体清除。在本次调查中，我们确定了(1)湖泊CO_2的空间分布和CO_2总量随时间的变化；(2)湖泊物理结构的变化，即湖泊的温度、化学成分和密度。我们在尼奥斯湖沿N-S和E-W剖面进行了详细的CTD观测。这使我们第一次能够构建温度和化学的三维分布。结果表明，湖泊具有较强的分层性，人工脱气除了使表层水体中的化学物质富集外，对湖泊的化学和物理结构没有实质性的影响。这是由于在地表加入了脱气的深水。最高（350 mmol/kg）出现在深度190 m以下。深海CO_2补给速率为100 Mmol/yr，至少与前10年一样高。^3He/^4He （8.1 × 10^<-6>）和^<40>Ar/^<36>Ar（600）的比值最高，提示岩浆成因。在单一条件下，65 ~ 95 m深度的co2浓度最高，为160 mmol/kg。如果在任何深度达到CO_2饱和，这些湖泊可能在不久的将来自发爆炸。然而，这些湖泊的气体爆炸随时可能在引入诸如滑坡等外部作用力后发生。

Lakes Nyos and Monoun (Cameroon) exploded in the mid 1980s killing about 1800 people. At these lakes, temperature, total dissolved ions, and CO2 concentration in the bottom water have been steadily increasing since the last explosions. Thus the lake explosion may recur unless CO_2 concentration is lowered. Currently gas removal is going on using a degassing system that was installed in January 2001 at Lake Nyos and February 2003 at Lake Monoun.In the present investigation, we determined (1) the spatial distribution of CO_2 and variation with time in the total amount of CO_2 in the lakes, and (2) the changes in the physical structure of the lakes, I.e., temperature, chemical composition, and density of the lakes. We made a detailed CTD observation at Lake Nyos along N-S and E-W cross sections. This enabled us to construct a three-dimensional distribution of temperature and chemistry for the first time. It was found that the lakes show strong stratification and that the artificial degassing did not give any substantial change in the chemical and physical structures of the lakes except for enrichment of chemical species in surface water. This was caused by addition of degassed deep water at the surface.The highest ( 350 mmol/kg) is observed below 190 m depth a Nyos. The CO_2 recharge rate in the deep waters is 100 Mmol/yr, at least as high as in the previous ten years. The highest ratios of ^3He/^4He (8.1x10^<-6>) and ^<40>Ar/^<36>Ar (600), suggestive of magmatic affiliation. At monoun, the highest CO_2 concentration of 160 mmol/kg is observed between 65-95 m depth. The lakes may spontaneously explode in the near future if CO_2 saturation is reached at any depth. However, gas explosion at these lakes can occur at any time upon introduction of external forcing such as a landslide.