Intensity of chemical weathering and its implications for soil erosion across lithological and climatic gradients in the Chemoga watershed, Upper Blue Nile Basin

Abstract

The geochemistry of rocks, regolith/soil, surficial and core sediments, and clay mineral assemblages from these sediments was analyzed to elucidate the transition from fresh to chemically altered materials. These data were used to evaluate the intensity of chemical weathering and its implications for erosion across contrasting lithological and climatic gradients in the Chemoga watershed of the Upper Blue Nile Basin, Ethiopia. Our findings show depletion of Na+, Ca2+, Mg2+, and K+, and enrichment of Al3+, Fe3+, Ti4+, and Mn2+ in the regolith/soil, upper part of surficial sediment, and upper layer of the core sediment (10 m). Chemical Index of Alteration (CIA) values are rocks 35–47, regolith/soil 75–96, surficial river sediments 45–96, and core sediments 50–90. River sediments in the highland area are dominated by kaolinite (39%), illite (34%), and chlorite (27%), while the lower study area has illite (76%), chlorite (11%), and kaolinite (12%). Geochemistry, ternary plots, and clay mineralogy show that the highland region underwent intensive chemical weathering, while the midland experienced intermediate to high weathering, and the lowland had weak weathering. The highlands experience severe weathering due to a humid, bimodal precipitation climate; despite having younger, weathering-resistant rocks (basalt, andesite, trachyte), the lower regions consist of older, more mafic rocks (basalt and trachybasalt). Like the surficial river sediment, core sediment showed intense chemical weathering and kaolinite dominance in the upper layers (0–10 m depth, CIA > 80). Weathering intensity decreased with depth, suggesting changes in weathering and in clay mineral formation that reflect past climate. The high weathering gradient from highland to lowland correlates with erosion, indicating that weathering strongly influences soil erosion variability in the watershed. This work provides a reasonable basis for improving our understanding of the degree of weathering across contrasting environments and its implications for soil erosion by connecting the relationship between parent materials and chemically altered samples with the degree of soil erosion.