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

In a SW-NE traverse across the Tenere desert and the southern Tchigai mount ainous region, only one generation of ancient dunes was found, overlain by recent, active eolian sand sheets and dunes. Paleosols on these dunes displ ay red-brown to yellow-brown Bw horizons up to 100 cm thick and an classifi ed as Chromi-Cambic Arenosols and Cambic Arenosols, respectively. The struc ture 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 ir on ores"). Within the paleolake depressions, ancient dune sediments bleache d by gleying are covered by silt-rich lacustrine sediments. Neolithic artif acts on the lacustrine sediments indicate that during an arid climatic peri od, the paleolakes contracted in size and did not reach their fullest origi nal extent during the late Neolithic humid period. This suggests the existe nce of earlier periods with enhanced humidity. Because of a decrease in hum idity from SW to NEI the degree of rubefication and several physical, chemi cal and mineralogical properties of the paleosols an related to their posit ion along the traverse. A gradient of decreasing weathering intensity from SW to NE is paralleled by local variations in mineralogical properties of t he parent materials and in rates of dust deposition. Consequently, the two effects on soil properties are difficult to separate. (C) 2000 Elsevier Sci ence B.V. All rights reserved.