A Depression Containing CO2-Enriched Water at the Bottom of Lake Monoun, Cameroon, and Implications for the 1984 Limnic Eruption

In 1984, a limnic eruption occurred in Lake Monoun, Cameroon, and the CO 2 gas released from the lake surface resulted in casualties in the neighboring communities. Subsequent scientific research revealed that the CO 2 gas released from the lake surface was CO 2 of magmatic origin dissolved in the lake water; however, the mechanism of that limnic eruption remains unclear. In this study, we analyzed in detail the lake-bottom bathymetry of the eastern basin, i.e., one of the three basins in Lake Monoun, to understand the mechanism of the 1984 limnic eruption. We discovered two significant depressions at the lake bottom near the scarp and obtained vertical profiles of several parameters of the lake water at the depression locations. The northeastern depression (D1) was ∼ 1.2 m deeper than the lake bottom and contained water with higher temperature and electrical conductivity and lower pH relative to the lake water. Conversely, the southern depression (D2) was ∼ 2.2 m deeper than the lake bottom, and there were no anomalies regarding its water parameters. Although the warm water discharged from the bottom of D1 was not saturated with dissolved CO 2 , bubbles likely existed at the bottom of D1, influenced by the partial pressure of dissolved CH 4 in the lake water. Our results suggest that just before the 1984 limnic eruption, water containing high concentrations of dissolved CO 2 was discharged from D1; this water would have reached the lake surface with bubbles. According to earlier numerical simulations of the limnic eruption, rising bubbles could have induced the limnic eruption. The rising bubbles entrained the surrounding lake water containing high concentrations of dissolved CO 2 , which amplified the flow rate of CO 2 degassing from the lake water and resulted in a limnic eruption. The limnic eruption that occurred just above D1 displaced lake water on the eastern shore. It is estimated that the impact of the displaced water eroded the scarp and deposited sediment as a mound near D1. A similar mound also exists near D2, suggesting that D2 is a trace of another limnic eruption that occurred earlier than 1984. Of the three basins that make up Lake Monoun, the two smaller basins to the west have high concentrations of dissolved CO 2 in their deep waters. This dissolved CO 2 was not supplied from the bottom of the basins but is likely a remnant of the dissolved CO 2 that existed in 2003 before the start of artificial CO 2 degassing. Our results suggest that