Gas bursts from tropical crater lakes constitute a hitherto unrecognized natural hazard, which claimed 37 lives around Lake Monoun in 1984 and 1,746 lives in 1986 around Lake Nyos in Cameroon, west Africa. Studies of these events indicate that the lethal gas clouds were dominantly CO(2) which exsolved catastrophically from deep waters of the lakes, producing in the case of Lake Nyos a gas cloud of 1.94 times 10(6) tons CO(2) . Carbon-isotope data indicate a magmatic source of the carbon dioxide, but the geochemistry of deep water and gases does not support a sudden injection of volcanic gas from a deep source into the lakes. Rather, it is proposed that the gas bursts were preceded by gradual build-up of dissolved bicarbonate in deep waters, where anoxic conditions in enclosed and stagnant basins led to low pH and pCO(2) close to saturation. Steady input from the Earth's mantle to submerged mofettes or CO(2) -rich soda springs within the lakes is most likely the primary source of carbon dioxide. Lethal effects of the gas bursts are almost entirely due to CO(2) -induced asphyxia. A small percentage of victims awoke from coma one or two days after the event, but most died. Unusual skin lesions on about 5% of victims arose from the comatose state. It is shown that the mass of gas required to account for the lethal effects and observed gas clouds is an order of magnitude less than the potential gas yield from the lakes. In view of the lethal gas bursts from the small Cameroon lakes, the potential hazard of future gas bursts from other much larger density-stratified equatorial lakes must be seriously considered, particularly in Lake Kivu in east Africa, where methane and carbon dioxide gas content is higher by two to four orders of magnitude than that of the Cameroon lakes. A gas burst from Lake Kivu would form a carbon dioxide cloud up to 340 km(3) in volume and expansion of the exsolving gas from deep water to atmospheric pressure would correspond to an energy release equivalent to 8 megatons of explosive.

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