Chemical Denudation Rates in the Humid Tropics of East Africa and Comparison with 10Be-Derived Erosion Rates

Based on 2 years of hydrochemical monitoring of 15 rivers in the western East African Rift, together with the geochemistry of soils, river sediments, and bedrocks, we calculated chemical denudation rates and tracked shifts of major cation ratios via chemical weathering. In contrast to previous studies emphasizing low chemical denudation rates in tropical cratonic areas, we also found high rates due to more easily soluble Quaternary volcanic tephras, and to a minor degree due to amphibolites. Quartz-derived 10Be erosion rates fail to predict these high rates because major sources of solutes are provided by tephras and mafic minerals. Although cosmogenic denudation rates significantly increase with increasing topography this is not reflected in chemical rates. Hence, the strong tectonic rejuvenation in this rift setting does not significantly accelerate chemical denudation. Cation ratios from rocks over soils, sediments to waters show enrichment in K and Ca with respect to Na and Mg for all rock types.