Geochemistry of African Soda Lakes

Abstract

Soda lakes are those in which sodium and carbonate species dominate the dissolved ions. They form in hydrologically closed lake basins, where inflow is balanced primarily by evaporation. They are important habitats, closely tied to water resource issues in arid lands, and they support important economic activities. Soda lakes differ from other saline lakes in that their drainage basins typically are dominated by volcanic terrains that produce very alkaline dilute inflow. In East Africa, soda lakes are commonly associated with volcanism and tectonic depressions, mostly along the East African Rift. Upon evaporative concentration, salinity increases, but many solutes are also lost to solid phases through the actions of abiotic mineral precipitation and biological utilization. Thus, soda lake waters tend to be dominated by sodium, bicarbonate and/or carbonate, with potentially significant sulfate and chloride concentrations as well. Alkaline earths (calcium and magnesium) are almost always negligible in concentration in evolved soda brines. Hydrothermal input is an important contributor of silica in soda lake waters and may also play an important role in deep subsurface environments (far below the sediment-water interface) by enriching CO2, enhancing silicate hydrolysis and generating high alkalinity.