PHYSICAL AND CHEMICAL WEATHERING RATES AND CO2 CONSUMPTION IN A TROPICAL LATERITIC ENVIRONMENT: THE UPPER NIGER BASIN

Abstract

The chemical composition of Niger river water measured bimonthly at Bamako (Mali) during the period 1990-1992 provides an estimate of present weathering rates in the upper Niger basin. The dominant weathering process is kaolinite formation (`monosiallitization'). However, seasonal variations promote gibbsite formation in the rainy season (September) and smectite development in the dry season (May). The results show that lateritic profiles continue to develop even during very dry episodes. The rate of profile development, calculated as the difference between the chemical weathering rate at the base of the soil profile and mechanical erosion rate at the soil surface, is about 1.3 to 3.7 m/Myr. A comparison between 43 river basins of the world shows that, for similar runoff, the CO2 flux consumed by silicate weathering is about two times lower in lateritic areas than in nonlateritic zones.