We have investigated the development of iron crust lateritic systems and rain forest soils in tropical environments using in situ-produced 10Be 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 autochthonous (in situ formation) processes and hence for determining whether colluvial transport or in situ chemical weathering dominates soil development in these environments. In the stable West African Craton in Southwest Burkina Faso at Larafella, the exponential decrease in 10Be 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 development. Models of the 10Be depth profile in this unit suggest that this surface has been subject to erosion at a mean rate of ~2 m.My-1 and that ~1.5% of the surface 10Be production rate is associated with muon-induced reactions. In a contrasting environment, tropical rain forest (Malemba, Congo), 10Be concentrations have 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 few meters from the quartz veins by downslope lateral creeping, whereas the round cobbles have an allochthonous origin. Models of 10Be distributions as this site yield an erosion rate of ~12 m.My-1 and a rate of lateral creeping on the order of 60 m.My-1.
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We thank J. Lestringuez and D. Deboffle for their continuing expertise in AMS measurements. Fieldwork was highly facilitated by the logistic support of the Brazzaville and Ouagadougou ORSTOM Centers. This work was supported by INSU-CNRS through the DBT Program Thème 1: Fleuves et érosion. Tandétron operation is supported by the CNRS, CEA, and IN 2 P 3 .