Abstract

A large amount of CO₂ is stored in the deep waters of Lake Nyos, a volcanic crater lake in Cameroon. The lake is meromictic and thus anoxic in the deeper areas, where dissolved iron exists as Fe²⁺. Since 2001, a controlled degassing of the lake has been underway. The degassing brings deep water containing Fe²⁺ to the lake surface as a fountain. This resulted in the formation of Fe(OH)₃ precipitates and turned the lake surface red-brown. This coloration was accelerated after the addition of two further degassing pipes in 2011. The Fe(OH)₃ precipitates sink to deeper parts of the lake, re-dissolve and are reduced back to Fe²⁺, which is thought to be precipitating as siderite.

The rates of dissolution and precipitation of siderite in Lake Nyos were examined. Fragments of siderite crystals were covered with gel (Epikote) and placed in lake water for 54 h at several depths characterized by chemical features. The change in the thickness of the crystal surface before and after the reaction was analysed by an interferometer in the laboratory. The siderite dissolves at a rate of −0.09 µm a⁻¹ for samples placed at a depth of 50 m, whereas precipitation took place in deeper waters. The precipitation rate of siderite increased by 0.29 µm a⁻¹ with an increase in depth from 100 to 200 m, and decreased to 0.25 and 0.09 µm a⁻¹ at depths of 208 and 210 m, respectively. However, the calculated saturation indices of siderite in the lake waters increased with depth. The objective of this study is to examine the observed rates of dissolution and precipitation of siderite and to compare them with the saturation index of siderite calculated from the chemistry of the lake water. We also discuss the change in Fe species from the oxidation of Fe²⁺ to Fe³⁺ at the surface and reduction to Fe²⁺ and consequent precipitation as siderite in deeper parts of the lake by degassing of CO₂ from bottom water in Lake Nyos.