Gas buildup in Lake Nyos, Cameroon: The recharge process and its consequences

• Published in: Applied geochemistry 1993, Vol.8 (3), p.207-221
• Main Authors: Evans, W.C., Kling, G.W., Tuttle, M.L., Tanyileke, G., White, L.D.
• Format: Article
• Language: English
• Subjects: Crystalline rocks; Earth sciences; Earth, ocean, space; Engineering and environment geology. Geothermics; Exact sciences and technology; Freshwater; Geochemistry; Igneous and metamorphic rocks petrology, volcanic processes, magmas; Mineralogy; Natural hazards: prediction, damages, etc; Silicates; Water geochemistry
• ISSN: 0883-2927; 1872-9134
• DOI: 10.1016/0883-2927(93)90036-G

The gases dissolved in Lake Nyos, Cameroon, were quantified recently (December 1989 and September 1990) by two independent techniques: in-situ measurements using a newly designed probe and laboratory analyses of samples collected in pre-evacuated stainless steel cylinders. The highest concentrations of CO 2 and CH 4 were 0.30 mol/kg and 1.7 mmol/kg, respectively, measured in cylinders collected 1 m above lake bottom. Probe measurements of in-situ gas pressure at three different stations showed that horizontal variations in total dissolved gas were negligible. 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). Comparing the CO 2 profile constructed from the 1990 data to one obtained in May 1987 shows that CO 2 concentrations have increased at depths to below 150 m. Based on these profiles, the average rate of CO 2 input to bottom waters was 2.6 × 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 CO 2, heat, and major ions into the hypolimnion suggest that a low-temperature reservoir of free CO 2 exists a short distance below lake bottom and that convective cycling of lake water through the sediments is involved in transporting the CO 2 into the lake from the underlying diatreme. Increased CH 4 concentrations at all depths below the oxycline and a high 14C content (41% modern) in the CH 4 4 m above lake bottom show that much of the CH 4 is biologically produced within the lake. The CH 4 production rate may vary with time, but if the CO 2 recharge rate remains constant, CO 2 saturation of the entire hypolimnion below 50 m depth would require ∼140a, given present-day concentrations.