On August 15, 1984, a cloud of gas burst out of Lake Monoun in Cameroon, killing 37 people by asphyxiation. A study of the lake revealed that it occupies a 96-m-deep crater and that very high levels of dissolved carbon dioxide prevail in the anoxic deep waters [Sigurdsson et al., 1987]. The carbon isotope characteristics of Lake Monoun indicate derivation of the CO2 from mantle source (high 14C age of lake; ∂13C of −7) , but the chemistry of the lake showed no indication of input of hot magmatic gases, such as elevated levels of sulfur or evidence of halogen compounds or gases in the water. We therefore proposed that the Lake Monoun event was not related to a volcanic explosion but was rather the consequence of long-term buildup of mantlederived CO2, which was dissolved in the anoxic lake waters and was derived from soda springs or other carbon dioxide-rich groundwater flow into the submerged crater. Sudden degassing of the CO2-rich waters was attributed to an upset of the density stratification of the lake by a landslide from the crater's eastern rim.
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