AFRICAN LATERITE DYNAMICS USING IN SITU-PRODUCED BE-10

We have investigated the development of iron crust lateritic systems a nd rain forest soils in tropical environments using in situ-produced B e-10 in quartz veins and cobbles. The variability of its concentration as function of subsurface depth in soil profile provides criteria for distinguishing between allochthonous (elsewhere formation) and autoch thonous (in situ formation) processes and hence for determining whethe r colluvial transport or in situ chemical weathering dominates soil de velopment in these environments. In the stable West African Craton in Southwest Burkina Faso at Larafella, the exponential decrease in Be-10 concentration observed along a quartz vein from the lowland lateritic system corresponds to the decrease in its production rate associated with attenuation of cosmic rays and thus indicates an autochthonous de velopment. Models of the Be-10 depth profile in this unit suggest that this surface has been subject to erosion at a mean rate of similar to 2 m.My(-1) and that similar to 1.5% of the surface Be-10 production r ate is associated with muon-induced reactions. In a contrasting enviro nment, tropical rain forest (Malemba, Congo), Be-10 concentrations hav e been measured in quartz vein and in round and angular quartz cobbles incorporated in a downslope ''stone-line.'' Comparison with data from Larafella suggests that the angular cobbles have been transported a f ew meters from the quartz veins by downslope lateral creeping, whereas the round cobbles have an allochthonous origin. Models of Be-10 distr ibutions as this site yield an erosion rate of similar to 12 m.My(-1) and a rate of lateral creeping on the order of 60 m.My(-1). Copyright (C) 1998 Elsevier Science Ltd.