Characteristics and Origins of Laminated Ferromanganese Nodules from Lake Malawi, Central Africa

Fields of laminated ferromanganese nodules up to several decimeters thick are found in waters 80–160 m deep in Lake Malawi, commonly near the boundary of littoral sand and deeper-water mud facies. The nodules (<1–8 mm diameter) have a laminated cortex variably composed of goethite, manganite and other oxides. Compositionally, they range from ferric nodules with <2% Mn to examples in which Mn > Fe. Total Fe+Mn in most nodules exceeds 50 weight percent. A genetic model is proposed whereby Fe²⁺ and Mn²⁺ mobilized from anoxic sediments are oxidized close to the sediment–water interface. The precipitation of Fe and Mn oxides and development of lamination are controlled by many factors, including seasonal and longer term variations in the level of the oxic–anoxic boundary in the lake, and the oxygen status and pH of the lake waters.