Micromorphological approach to polycyclic pedogenesis on the Messak Settafet plateau (central Sahara): Formative processes and palaeoenvironmental significance.

The Messak Settafet is a wide plateau located in the Libyan central Sahara (SW Fezzan); it is cut into the Nubian sandstone and interpreted as relic of a Tertiary peneplain, heavily affected by aeolian deflation. Notwithstanding degradation phenomena, discontinuous, thin soil bodies are locally preserved below the hamada. Several profiles, located in correspondence of different geomorphological units (the hamada, the endorheic depressions, and the southern escarpment), were described and sampled for micropedological analyses. Micromorphology outlined that the main characteristics of the residual B horizons from each geomorphological units are (i) high concentration of ferruginous nodules, (ii) multiple generations of clay illuviation, (iii) fragmentation and deformation of textural pedofeatures, and (iv) calcite mobilization and redeposition. According to horizon macro- and micromorphological characteristics, these pedologic bodies do not seem to be in equilibrium with the present-day climatic conditions and should be regarded as paleosols generated by processes regulated by large water availability and warm temperature, such as rubification, clay translocation, and calcite redistribution. The development of horizons acted mainly since the Early/Middle Pleistocene during subsequent pluvial periods and was interrupted by several erosional and aeolian input phases (arid periods). Pedological processes ended at the onset of the severe dry environmental conditions at the Mid-Late Holocene transition. The development of this kind of paleosols in the central Sahara is also discussed in comparison with pedogenetic processes occurring in temperate regions during interglacial and interstadial intervals.