Reconstructing Geochemical and Mineralogical Signatures of Weathering and Sedimentation: A L ate P leistocene– H

ABSTRACT

This study aimed to reconstruct weathering intensity, sedimentation rates and to elucidate associated environmental changes recorded in floodplain deposits of the Chemoga watershed of the Upper Blue Nile Basin (UBNB) in Ethiopia. A 20‐m sediment core, CGC2023, was analysed for major oxide geochemistry ( n  = 23), clay mineralogy ( n  = 23) and carbon dating ( n  = 18) to characterise temporal variations in weathering and sedimentation processes. Radiogenic dating reveals that the core stratigraphy experienced multiple hiatuses, sediment discontinuities and an age reversal, which collectively indicate substantial paleoenvironmental changes in the region. The major oxide concentrations indicate a depth‐dependent concentration change, reflecting temporal shifts in weathering intensity. The upper section (10 m) of the core shows intense chemical alteration, with enrichment of Al ₂ O ₃ , Fe ₂ O ₃ and MnO, and depletion of SiO ₂ , Na ₂ O, MgO and K ₂ O. In addition to the geochemical data, the clay mineralogy result illustrated the clay mineralogy assemblage is depth dependent change, kaolinite (up to 67%) in the upper stratigraphy, absent when the depth increases, while illite dominates (up to 100%) on depth increase, chlorite (up to 56%), and smectite (up to 18%) occur in the core profile with distribution varying with depth and age. The sediments were produced through four distinct stages of chemical weathering and erosion (~42.5–1.3 cal kyr BP). Stage I (undated) marks the beginning, with a basal layer showing minor chemical alteration. The transition to Stage II (42.58–39.99 cal kyr BP) involves an increase in chemical weathering intensity (CIA, 75.2–79.9) and a sedimentation rate (0.42–0.44 mm year ⁻¹ ). Stage III (37.19–18.58 cal kyr BP) follows, characterised by declining chemical weathering (CIA, 51.6–74.4) and rising sedimentation rate (0.06–1.105 mm year ⁻¹ ). Transitioning to Stage IV (9.012–1.686 cal kyr BP), advanced chemical weathering (CIA, 85–89) and a high depositional rate (0.27–4.17 mm year ⁻¹ ) emerge, likely reflecting the intensified Holocene climate variability and anthropogenic impact.