Evolution of CO₂ Content in Lakes Nyos and Monoun, and Sub-lacustrine CO₂-Recharge System at Lake Nyos as Envisaged from CO₂/³He Ratios and Noble Gas Signatures

Abstract

Lakes Nyos and Monoun in Cameroon are known as “killer lakes” because they killed ~1,800 people in the mid-80s after a gas explosion or limnic eruption, a sudden release of carbon dioxide accumulated in deep waters. This event attracted the interest of the international scientific community, not only for the sake of disaster mitigation but also for its volcanological significance. Follow-up studies indicated that the CO₂ concentration in deep water increased at an alarming rate at both lakes, and that a future gas explosion was likely to occur. To avoid recurrence of the explosion, artificial degassing of the lakes was set up using pipes based on the gas self-lift principle. This chapter compiles the results of the geochemical monitoring of the two lakes for the last 25 years. Pre- and syn-degassing evolution of CO₂ profiles was translated into the temporal changes of CO₂ content in the lakes, and thus estimate CO₂ recharge and removal rates. Degassing went on smoothly, and more than 90 % of the maximum amount of CO₂ was removed from Lake Monoun by 2011, reaching safe levels. Since the lake has lost its gas self-lift capability, degassing will no longer continue. At Lake Nyos, degassing is on-going and 33 % of the maximum amount of CO₂ has now been removed. A model of spontaneous gas exsolution is proposed. This idea is based on the temporal variation of CO₂ profiles at Lake Monoun between 1986 and 2003. During this period the profiles evolved rather steadily, i.e., the deep CO₂-rich layer thickened while its CO₂ concentration remained constant. This suggests that the recharge fluid coming from depth had a CO₂ concentration similar to that of the bottom water, and that it pushed the deepest water upward, resulting in CO₂ saturation at the top of the CO₂-rich layer, which led to the 1984 limnic eruption. A similar situation was observed at Lake Nyos just before the degassing operation started in 2001, i.e., the deep, CO₂-rich layer thickened while its CO₂ concentration remained unchanged at about 350 mmol/kg, which is still far below CO₂ saturation. In this lake, pre-degassing deep water had low CO₂/³He ratios (~0.5 × 10¹⁰), whereas the recharge fluid was characterized by relatively high CO₂/³He ratios (~1.7 × 10¹⁰). This suggests that the behavior of CO₂ and He in the lake was decoupled. The recharge fluid is likely produced by mixing of magmatic fluids, which are characterized by relatively high ³He/⁴He, ⁴⁰Ar/³⁶Ar, ²⁰Ne/²²Ne and ²¹Ne/²²Ne ratios, with groundwater having air isotopic signatures.