Noble gases in Lakes Nyos and Monoun, Cameroon

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 CO 2 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.3 He/ 4 He 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 4 He 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 deepseated 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, 3 He/ 4 He ratio decreased slightly, suggesting that the fluid from the deep-seated reservoir is supplied to the lake mainly at ca. 190 m. C/ 3 He ratios were 1.5 × 10 10 and 0.6 × 10 10 in deep and shallow layers of Lake Nyos, respectively. The C/ 3 He ratio was 1.7 × 10 10 at Lake Monoun. A gas recharge model to explain the observed layered structure in Lake Nyos is presented.

Lakes Nyos and Monoun Gas Disasters (Cameroon)—Limnic Eruptions Caused by Excessive Accumulation of Magmatic CO₂ in Crater Lakes

“…13). A gas sample collected directly from the dry gas line is perfectly free from air contamination which is essential for noble gas analysis (Nagao et al, 2010). A similar method has been reported by Tassi et al (2009) for gas collection from Lake Kivu.…”

“…18, although the lake 18 O values of monthly collected rain waters (dark asterisks) and ground waters (circles and triangles) sampled in the vicinity of Lake Nyos are shown (Kamtchueng et al, 2015a). Those of Lake Nyos waters (Nagao et al, 2010) are also included. All values are consistent with the global meteoric water line of Rozanski et al (1993), although the Lake Nyos waters are plotted slightly upper-right of the cluster.…”

“…The data for Lakes Nyos, Monoun and Wum (a crater lake near Lake Nyos) were later added by Kusakabe et al (1989) and Nagao et al (2010). The isotopic determination was intended to find any input of volcanic gases into Lake Nyos, for volcanic gases are usually characterized by high d…”

δ18O and δD variations in some volcanic lakes on the Cameroon Volcanic Line (West-Africa): generating isotopic baseline data for volcano monitoring and surveillance in Cameroon

“…Between 1986 and 1999, the surface waters became isotopically lighter before the changes reversed toward a heavier trend from 1999 to 2010. That surface evo- O profiles of lakes Nyos and Monoun (2012, 2010and 1999data from Sasaki et al 2012Nagao et al, 2010 andKusakabe et al, 1989, respectively). Contrary to the TDS that increase from the surface to the bottom, δ 18 O instead shows a negative gradient.…”

“…1). Previous work using the stable isotopes of water encompassed five lakes (Kling et al, 1987;Kusakabe et al, 1989;Tanyileke et al, 1996;Nagao et al, 2010). Only the last two contained isotopic profiles for Lakes Nyos, Monoun and Wum without discussing their variations and controls further.…”