Pedogenic and sedimentary controls on secondary carbonate accumulation in semi-arid fluvial landscape, Central Botswana

The formation and accumulation of secondary carbonates represent an important pathway for long-term carbon sequestration in global dryland regions. However, accurately assessing their contribution to the regional carbon budget requires distinguishing between pedogenic (in-situ soil formation) and sedimentary processes, which often occur concurrently in dynamic environments. This study investigated the formation and accumulation of carbonate features within four pedosedimentary sections located along ephemeral stream banks in semi-arid central Botswana. The method used involved combining morphological and physicochemical characterisation with geochemical indices in the observed horizons/layers. Geochemical indices include the robust chemical weathering index (RW), the calcification index ((Mg + Ca)/Al), the clayeyness index (Al/Si) and Mg/Ca ratios. Analysis of the sections reveals a cyclical palaeoenvironmental history marked by palaeosols whose development was repeatedly interrupted by successive episodes of fluvial sedimentary deposition, followed by the resumption of modern pedogenesis. Particle size analysis of the deposit layers confirms a pulsed fluvial regime, with sediments ranging from poorly sorted fine sand upstream to extremely poorly sorted coarse silt downstream, indicating hydraulic sorting according controlled by flood energy. Geochemically, the low RW values, ranging overall between 30% and 60%, and the clay index generally below 0.3 indicate a limited degree of chemical weathering and low clay neoformation, reflecting the predominance influence of the arid environment. Calcification was identified as the dominant pedogenic process in all sections (both ancient and modern), characterised by a high CaCO₃ content (14 to 22%), confirmed by the calcification index in the calcic horizons. The low Mg/Ca ratio strongly suggests the predominance of calcite as the main secondary carbonate phase also confirmed by XRD evidence. These findings highlight that the total carbonate accumulation in the study area is a direct result of the complex interplay between repetitive fluvial deposition (pedosedimentary history) and subsequent in-situ pedogenic calcification. The results underscore the critical importance of secondary carbonate accumulation in the soils of Botswana and highlight a complex paleoenvironmental evolution.