Turbulent bubble plumes and CO 2-driven lake eruptions

Abstract

We examine the motion of turbulent bubble plumes within lakes containing large quantities of dissolved CO 2 such as Lake Nyos, Cameroon. First we describe some new laboratory experiments in which we examine the dynamics of turbulent bubbles plumes produced by electrolysis in an aqueous solution. These experiments were analysed to determine the entrainment rate into the plume. Using the experimental results, we then develop a model of the motion of a bubble plume through a CO 2-rich lake. The model identifies that for a sufficient amount of dissolved CO 2 in the lake or a sufficient flux of CO 2 at the base of the lake, a turbulent bubble plume can rise through the lake and erupt at the surface. The model predicts that owing to the rapid entrainment of ambient fluid, the surface flux of CO 2 may be 10 4–10 5 times greater than that at the base of the lake. We also show that the return flow in the lake acts to decrease the CO 2 content at all depths as the CO 2 is erupted at the surface, and that this eventually leads to termination of the activity. The model provides the first quantitative analysis that CO 2 saturated lakes can overturn convectively and lead to rapid degassing events such as occurred at Lake Nyos in Cameroon in 1986.