A model of overturn of CO2 laden lakes triggered by bottom mixing

We develop a model for the ascent of a discrete cloud of mixed water from the base of a lake which is saturated in CO2 at the lake bed, and which becomes progressively under-saturated with height above the lake bed. We show that, for a given CO2 profile, there is a critical size of cloud which can ascend to the lake surface, and that smaller clouds only rise to a finite height within the lake and then intrude at that level. The model provides a simplified picture of how disturbances which mix the water in the lower few meters of the lake, may lead to release of CO2 at the lake surface, through the buoyant ascent of discrete parcels of mixed fluid. This provides a possible rationalisation of the potential for CO2 driven lake overturn to be triggered by a landslide or the lake bottom turbulence associated with a wind-driven seiche. We consider the relevance of the model to historic eruptions of Lake Nyos 1986 and Lake Monoun, 1984 in Cameroon.