Water and gas chemistry of Lake Nyos and its bearing on the eruptive process

Abstract

The isotopic and chemical composition of water samples collected from Lake Nyos some two and eight weeks after the eruption of August 21, 1986 point to the existence of three distinct mixing regimes involving three water components. An essentially homogeneous, unmixed body of water at depths > 100 m, overlain by water increasingly affected by surface evaporation and a 5–10-m layer containing recent, but pre-eruption rain water. The cationic constituents (Na, K, Mg, Ca, Mn, Fe) of the lake water correspond to the dissolution of around 0.1 g of local rock, the waters are close to saturation with respect to siderite. The composition of the gas dissolved in the deep lake waters (0.65% b.w. of CO2, PCO2 = 4.4b) corresponds (in mmol mol−1) to 996 CO2, 2.0 CH4, 2.0 N2, 0.05 Ar, 0.004 He, 0.0002 H2, 0.0001 Ne, < 0.01 O2, < 0.004 H2S, < 0.001 CO. The isotopic compositions of CO2 (δ13C = −3.4‰) and of He (Rair = 5.5) suggest deep magmatic origins, the 13C and 2H content of CH4 organic sedimentary origin, the presence of aromatic hydrocarbons indicates very high temperatures of hydrocarbon formation. Exsolution of gas will first lead to the precipitation of siderite then iron hydroxide. The chemistry of the lake waters points to a loss of some 240,000 t of CO2 from the upper 100 m of the lake, their isotopic composition is consistent with the assumption that the eruption was triggered by the accumulation of cold rain waters at the lake surface prior to the eruption inducing partial convective overturn. There is no need to invoke addition of chemical or isotopic constituents from deeper levels during the eruption.