35 Years of Diversity– Stable Isotope Geochemistry, Volcano-Fluid Interaction, andGeochemistry Linked to Mitigation of Natural Disasters –Minoru Kusakabe
Institute for Study of the Earth’s Interior, Okayama University
Misasa, Tottori-ken 682-0193, Japan
E-mail: Kusakabe@misasa.okayama-u.ac.jp
I started my career as a geochemist 35 years ago, initially studying chemical behavior ofvolcanic gases from some Japanese volcanoes, followed by experimental work on isotope
fractionation of some light elements such as oxygen and sulfur (e.g., Kusakabe and Robinson,
1977). The latter was later applied to the studies of volcano-fluid interaction. The Lake Nyos
gas disaster in 1986 made a turning point in my career. At that time, the view that a volcanic
eruption caused the gas disaster was widely circulated in the mass media. The Japanese
Government asked me, as a volcanic gas geochemist and as a member of the Japanese disaster
relief team, to investigate what happened at the lake. In the field I had an intuition that thedisaster was caused by a process related to the lake. Since then, I continued a geochemical
survey together with my Japanese colleagues and Cameroonian and American scientists. We
found that Lakes Nyos and Monoun (where a similar but smaller scale gas disaster had takenplace in 1984) were charged by a natural input of magmatic CO2 from the bottom of the lakes.
Recharge of CO2 resulted in an increase of the total amount of dissolved CO2 at an alarming rate
after the gas burst at L. Nyos (Fig. 1). A similar observation has been obtained for L. Monoun.

Fig. 1. Pre-degassing evolution of CO2 profiles (left) and the amount of dissolved CO2 below 170 m(right) as a function of time at Lake Nyos. Recharge rate is ~130 mega-mole CO2/year.

Funded mainly by OFDA (USAID), one degassing pipe each was installed at Lake Nyos
in 2001 and at Lake Monoun in 2003, to remove the dissolved gases in a controlled manner(Halbwachs, 2004). So far degassing has gone on without inducing any change in the stability ofthe lakes as opposed to initial concerns that it was going to induce lake instability. The total
amount of dissolved gases in these lakes is now steadily decreasing, indicating that animpending threat of gas explosion, especially at L. Monoun, has decreased. Based on simple
mass balance calculations on the CO2 inventory of each lake, a future prospect on the total
amount of dissolved CO2 has been made as shown in Fig. 2 (Kling et al., 2005).

Fig. 2. Model calculations of the change with time in the amount of dissolved CO2 in Lakes
Nyos (left) and Monoun (right) (Kling et al., 2005). Solid circles indicate observed values inJanuary 2005 (unpublished data).

It will take more than 30 years to degas Lake Nyos almost completely with a single pipe, soadditional pipe installation is required to make the lake safe within several years (Fig. 2a). The
amount of CO2 in Lake Monoun will become steady very shortly because of the high recharge
rate of CO2 relative to the removal rate through a single pipe (Fig. 2b). Presently two more pipes
are being installed at L. Monoun, which will certainly accelerate the gas removal in a reasonable
period of time.
After 35 years of diversity, I will come back to classic stable isotope geochemistry,
especially to the 16O-17O-18O systematics of extraterrestrial materials as initiated by Prof.
Clayton some 35 years ago.
References
Halbwachs et al., EOS 85, 281-288 (2004).
Kling et al., Proc. Nat. Acad. Sci., USA, 102, 14185-14190 (2005).

Kusakabe & Robinson, Geochim. Cosmochim. Acta, 41, 1033-1040 (1977).