Geochemical and limnological survey on crater lakes in Cameroon

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Outburst of a large amount of CO_2 dissolved in Lake Nyos, Cameroon, killed more than 1700 people on 21 August 1986. The surveys made soon after the gas disaster indicate that the CO_2 is of magmatic origin and the lake is chemically stratified. We revisited the lake in December 1988. Our bottom sounding study revised the bathymetry by Hassert (1912). The conductivity-temperature-depth profiller (CTD) measurements and chemical analysis of the lake water revealed that temperature, total dissolved solids (TDS) and CO_2 content of water below 160 m increased considerably over the last 25 months. This supports the view that CO_2 is being supplied to the lake bottom in the form of warm, CO_2-charged, mineralized water. Fluxes of heat and CO_2 were estimated to be 0.36 MW and 515-560x10^6 mol/y, respectively. The CO_2 flux is consistent with that estimated from the He-T relationship. The CO_2 flux is large enough to saturate the lake's hypolimnion within 50 y. Using the CO_2-TDS-Si relationship and temperature dependence of amorphous silica solubility, chemical composition of the warm mineralized water was estimated. Existence of a CO_2-saturated fluid below the sedimentary cover is suggested. These are the important findings for disaster prevention.