Soil cover and landscape evolution in the Senegal floodplain: a review and synthesis of processes and interactions during the late Holocene

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The aim of this paper is to summarize the successive biological, pedological, hydrodynamic, geomorphological and geochemical processes that have occurred in the Senegal valley, and to describe how their interactions during the late Holocene conditioned soil cover formation and landscape evolution. Potential acidity accumulated as pyrite in the floodplain sediment during the last marine transgressions, and was expressed during the following regressions because of oxidation. Soil acidification was mitigated by the soil buffer capacity and by the interaction with the slightly alkaline continental freshwater of the river. Two pedogenetic transformation processes that resulted from the succession of acidic and neutral conditions, transformed unripe muds with pyrite (potential Acid Sulphate soil) to actual Acid Sulphate soils, and then to Vertisols. Geochemical modelling with PHREEQC quantitatively confirmed the feasibility of the processes involved. These two pedogenetic processes also controlled two independent salt accumulation processes: (i) the transformation of shell accumulation beds into gypsum layers and (ii) aeolian deflation and formation of clay dunes. The study shows that pedogenetic effects on alluvial material can lead to contrasting horizons that cannot be explained stratigraphically. It also shows that the presence of saline areas in the Senegal middle valley results from much more complex processes than a simple salt deposition during transgressions.