Helium and carbon fluxes in Lake Nyos, Cameroon: constraint on next gas burst

On 21 August, 1986, a lethal gas burst issued from Lake Nyos in Cameroon, western Africa, killing at least 1700 people. Although the consensus is that the victims died of asphyxiation by CO2 of magmatic origin, the frequency of such catastrophic degassing events is still unknown. The CO2 flux at the bottom of Lake Nyos is estimated based on the measurement of 3He 4He and 4He 20Ne ratios and the chemical composition of gases exsolved from the lake water. The calculated CO2 flux at the bottom of Lake Nyos is (4.4 ± 2.3) × 1013 cm3 STP/year. Combined with the estimated release in the August 1986 event of (8 ± 2) × 1014 cm3 STP, the CO2 flux suggests that the gas burst may happen about once in 18 ± 10 years, although the significant uncertainty should be taken into account for the frequency resulting from assumptions such as steady-state fluxes in the lake and small fractionization of the C/He ratio during the degassing event. Several yearly measurements of hypolimnetic fluxes and seasonal measurements of epilimnetic fluxes are needed to constrain better the recurrence interval. In addition, the results of a regional 3He 4He survey of carbonated mineral springs in northwestern Cameroon are discussed in the context of the regional geotectonics. © 1990.