Paleosols on Pleistocene dunes as indicators of paleo-monsoon events in the Sahara of East Niger

In a SW–NE traverse across the Ténéré desert and the southern Tchigai mountainous region, only one generation of ancient dunes was found, overlain by recent, active eolian sand sheets and dunes. Paleosols on these dunes display red-brown to yellow-brown Bw horizons up to 100 cm thick and are classified as Chromi-Cambic Arenosols and Cambic Arenosols, respectively. The structure of the soil horizons is stabilized by pedogenic cementation and often shows effects of bioturbation. Near the shores of previously more extensive paleolakes, these paleosols change into Gleyic Arenosols. The former shore lines are frequently marked by seams of goethite rhizoconcretions (“bog iron ores”). Within the paleolake depressions, ancient dune sediments bleached by gleying are covered by silt-rich lacustrine sediments. Neolithic artifacts on the lacustrine sediments indicate that during an arid climatic period, the paleolakes contracted in size and did not reach their fullest original extent during the late Neolithic humid period. This suggests the existence of earlier periods with enhanced humidity. Because of a decrease in humidity from SW to NE, the degree of rubefication and several physical, chemical and mineralogical properties of the paleosols are related to their position along the traverse. A gradient of decreasing weathering intensity from SW to NE is paralleled by local variations in mineralogical properties of the parent materials and in rates of dust deposition. Consequently, the two effects on soil properties are difficult to separate.