Enrichment of ferrous iron in the bottom water of Lake Nyos

Abstract Lake Nyos is a meromictic lake with strong stratification separated into 4 distinct layers; shallow, intermediate, transitional, and bottom layers. This article discusses enrichment of Fe 2+ in the bottom water in January 2003 when the effect of artificial degassing that started in January 2001 on the chemical structure of the lake was still minimal. In the bottom layer (205.5–209.5 m), Fe 2+ and HCO 3 − are the major dissolved species and their concentrations increase sharply toward the bottom. Enrichment of Fe 2+ and HCO 3 − is most likely caused by anaerobic microbial reduction of Fe(OH) 3 precipitates at the water-sediment interface. The Fe(OH) 3 precipitates sank to the bottom after atmospheric oxidation of Fe 2+ dispersed by the degassing pipe. Lateritic hematite may also participate in the reduction reaction. Production of Fe 2+ and HCO 3 − significantly increases the alkalinity and electric conductivity of the bottom-most water and is responsible for a small rise of pH between 205 m and 209.5 m, the bottom. The other dissolved species, e.g., Na + , K + , Ca 2+ , Mg 2+ , SiO 2(aq) etc., result from the CO 2 -water-basalt interaction at low temperatures (25°∼30 °C) in the sub-lacustrine fluid reservoir that is believed to exist in the diatreme beneath the lake bottom, and they seep into the bottom layer.