GAS BUILDUP IN LAKE NYOS, CAMEROON - THE RECHARGE PROCESS AND ITS CONSEQUENCES

The gases dissolved in Lake Nyos, Cameroon, were quantified recently ( December 1989 and September 1990) by two independent techniques: in-si tu measurements using a newly designed probe and laboratory analyses o f samples collected in pre-evacuated stainless steel cylinders. The hi ghest concentrations of CO2 and CH4 were 0.30 mol/kg and 1.7 mmol/kg, respectively, measured in cylinders collected 1 m above lake bottom. P robe measurements of in-situ gas pressure at three different stations showed that horizontal variations in total dissolved gas were negligib le. Total dissolved-gas pressure near the lake bottom is 1.06 MPa (10. 5 atm), 50% as high as the hydrostatic pressure of 2.1 MPa (21 atm). C omparing the CO2 profile constructed from the 1990 data to one obtaine d in May 1987 shows that CO2 concentrations have increased at depths b elow 150 m. Based on these profiles, the average rate of CO2 input to bottom waters was 2.6 x 10(8) mol/a. Increased deep-water temperatures require an average heat flow of 0.32 MW into the hypolimnion over the same time period. The transport rates of C02, heat, and major ions in to the hypolimnion suggest that a low-temperature reservoir of free CO 2 exists a short distance below lake bottom and that convective cyclin g of lake water through the sediments is involved in transporting the C02 into the lake from the underlying diatreme. Increased CH4 concentr ations at all depths below the oxycline and a high 14C content (41 % m odern) in the CH4 4 m above lake bottom show that much of the CH4 is b iologically produced within the lake. The CH4 production rate may vary with time, but if the CO2 recharge rate remains constant, CO2 saturat ion of the entire hypolimnion below 50 m depth would require approxima tely 140 a, given present-day concentrations.