Sediment generation under volcanic environment and arid tropical climate: a perspective based on geochemical maps of top soils and stream sediments of Santiago Island (Cape Verde)

The present research tests the application of geochemical atlas of soils and stream sediments in the investigation of weathering and erosion processes on volcanic islands. The composition of surface soils collected in six... [ view full abstract ]

The present research tests the application of geochemical atlas of soils and stream sediments in the investigation of weathering and erosion processes on volcanic islands. The composition of surface soils collected in six catchment areas from Santiago Island (Cape Verde) are compared with bedload stream deposits sourced by these catchments in order to evaluate the spatial variability of weathering and erosion processes. The geochemistry of bedload stream deposits is between that of the fresh rocks and the topsoils of their source areas and, depending on the intensity of weathering and erosion processes, may be either closer to non-weathered rocks or to the surface soils. When the variability of topsoil weathering intensity in the catchment areas is high and the composition of bedload deposits is substantially different from the average soil composition, bedload deposits should incorporate significant amounts of non-weathered rocks and erosion protect sectors occur within the drainage basin. Ratios of non-mobile elements allow the identification of highly vulnerable and erosion-protected sectors within the catchments. A comparable composition of surface soils and bedload sediments suggests widespread and homogeneously distributed erosion. It is proposed that the catchments of the rivers in the SW flanking side of Santiago Island include sectors where lava mantles formed during the shield building and post-erosional eruptive phases are capable of an effective protection to erosion. Conversely, in the NE-facing area the denudation is presently more advanced and the erosion-protection by these mantles is more limited. Simple compositional parameters derived from the databases of geochemical maps of soil and stream sediments provide important information for the analyses of weathering, erosion and denudation processes at the catchment scale.