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During the eruption of Nyiragongo Volcano in January 2002 about 10 6 m 3 of lava entered Lake Kivu. The high concentrations of CO 2 and CH 4 dissolved in the deep waters of Lake Kivu raised serious concerns about a potential gas outburst with catastrophic consequences for the population in the Kivu-Tanganyika region. Therefore, 3 weeks after the volcanic eruption, we performed an ad hoc lake survey of the stability of the water column stratification. Vertical profiles of temperature and turbidity revealed signatures of the lava, which had penetrated to 100 m depth; however, there was no substantial warming or destratification of the gas-containing deep layers below. The deep double-diffusive structures also remained unaltered. Based on these observations, we conclude that a thermally driven gas outburst in Lake Kivu is not to be expected from future eruptions of comparable dimensions. In addition, the recent measurements allowed for an update and gave new insight into the stratification and double-diffusive mixing phenomena in Lake Kivu. A comparison with former measurements revealed a warming of the upper part of the lake of up to 0.5ЊC within the last 30 yr, which could be attributed to climate variability.

A hstractAn outburst of lethal gas from Lake Nyos, Cameroon, killed more than 1,700 people on 2 1 August 1986. The surveys carried out so far indicate that a considerable portion of the CO, dissolved in the lake was released to the atmosphere and asphyxiated the people. We revisited the lake in December 1988. The conductivity-temperature-depth profiler (CTD) measurements and chemical analysis of the lake water revealed that temperature, total dissolved solids (TDS), and CO, content of the bottom water increased in parallel fashion, especially in the bottom layer, during the preceding 25 months. The result supports the view thdt CO1 is being supplied to the lake bottom in the form of warm, CO,-charged, mineralized water. From the increments of temperature and CO, during the period, fluxes of heat and CO, were estimated to be 0.43 MW and 1 .O Mmol yr I. The CO1 flux is large enough to saturate the lake's hypolimnion within -30 yr. In the 1988 survey, the very bottom layer of the lake was estimated to be close to saturation with CO,. Using the CO,-TDS-Si relationship and temperature dependence of the solubility of amorphous silica, we estimated the chemical composition of the warm, mineralized water; these estimates suggest the existence of a CO,-saturated fluid below the sedimentary cover.

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