ABSTRACT

He, Ne and Ar isotopic ratios and concentrations of noble gases in different depths of Lakes Nyos and Monoun in Cameroon, West Africa, were investigated. Samples were collected in November 1999, January 2001 and December 2001. Improved sampling method applied in 2001 enabled us to detect detailed isotopic distribution in the lakes. Concentrations of Ne and Xe in the lake waters were low by factors of ∼10 and ∼3, respectively, compared with those of air saturated water (ASW), whereas in CO₂ gas they were depleted by 2 orders of magnitude compared with those in air. In contrast, He was strongly enriched in lake water by more than three orders of magnitude compared to ASW. ³He/⁴He ratios of 8 × 10^-6 (Lake Nyos) and 5 × 10^-6 (Lake Monoun) indicated recharge of magmatic gases of mantle origin to the lakes, though the ratios were lowered by the addition of radiogenic ⁴He from crustal rocks. A small contribution (<2%) of MORB-like Ne was detected in both lake waters. A simple mixing between two end members, i.e., atmosphere and gases from a deep-seated reservoir, can explain the noble gas signatures. Depth profile of He concentration in Lake Nyos showed a clear layered structure with a maximum at 190 m, 20 m above the lake bottom. Below this depth, ³He/⁴He ratio decreased slightly, suggesting that the fluid from the deep-seated reservoir is supplied to the lake mainly at ca. 190 m. C/³He ratios were 1.5 × 10¹⁰ and 0.6 × 10¹⁰ in deep and shallow layers of Lake Nyos, respectively. The C/³He ratio was 1.7 × 10¹⁰ at Lake Monoun. A gas recharge model to explain the observed layered structure in Lake Nyos is presented.