Lake Nyos in dual agony-An assessment of the limnic eruption and dam collapse-

点击翻译按钮获取中文摘要

The following results have been obtained through this geochemical studies on the limnic eruption at Lake Nyos and the age of Lake Nyos maar.(1)Buildup_of_magmatic_CO_2_at_lakes_Nyos_and_Monoun_and_its_artificial_degassing : Lakes Nyos and Monoun have been known as notorious crater lakes, because they suddenly exploded in mid-80s, claiming about 1800 lives. Continuous observations since 1986 by the head investigator and his colleagues have demonstrated that the lakes are continuously charged by magmatic CO_2 of mantle origin, resulting in a steady increase in the amount of dissolved CO_2, and that recurrence of the limnic eruption followed by a gas disaster is highly likely at both lakes. Artificial degassing apparatus was installed at each lake in 2001-2003. However the degassing rate is not high enough to reduce the gas content to a safe level in several years. Thus danger of the recurrence of limnic eruption still remains.In this study, we estimated the current rates of gas recharge … More and removal. Based on the estimate we have modeled the change with time in the CO_2 content of the lakes. It will take more than 30 years to make Lake Nyos safe. At Lake Monoun the gas content would become steady within several years under the current CO_2 recharge-removal rates. Fortunately additional degassing pipes were installed at Lake Monoun in 2006. If degassing pipes keep working, Lake Monoun would become safe in the near future. Lake Nyos still remains unsafe.(2)Chronology of Lake Nyos dam : Lake Nyos is bound by a 45 m high natural dam at the north end. The dam consists of poorly consolidated pyroclastic materials that deposited at the time of the maar formation. It has been suggested that the dam is fragile and has been eroded away in a high rate since the formation of the lake. It may collapse in the near future. The dam collapse would result in a large scale flooding downstream which reaches Nigeria. In order to estimate the erosion rate, it is essential to obtain a reliable age of the maar formation. We have determined the age of the dam to be ca.5000 years B.P.using the U-Th-Ra dating method. This age is much older than 400 years obtained by the ^<14>C method and much younger than the K-Ar dates of ca.400 ka. Our new age would give an important constraint on the erosion rate of the dam. Less