LATE PLEISTOCENE AND HOLOCENE PALAEOENVIRONMENTAL RECORDS OF TÉNÉRÉ, ERG OF TÉNÉRÉ AND ERG OF FACHI BILMA (CENTRAL SAHARA): NEW IMPLICATIONS FROM PALAEOECOLOGICAL DATA

Late Pleistocene and Holocene deposits enable the reconstruction of the palaeoenvironmental development of the vast lowlands of southern Central Sahara. The environmental conditions are deduced from geomorphology, sedimentology, biological remains, palaeolimnological, palaeopedological and prehistoric evidence.

Geomorphology and biological remains (e.g. Lates niloticus, Hippopotamus amphibius, Crocodylus niloticus) suggest from 11,5 ka BP a wide-spread lakeland country with permanent freshwater lakes changing size, level and water balance. A rapid change of the environmental conditions occurs from 6,5 to 6 ka BP: The desiccation to ephemeral ponds with saline/alkaline superficial dilute/stratified waters to swamp environment. Artifacts confirm that the lakes had lost much of their size by the time of the neolithic settlement already. Palaeosoils (60 to 100 cm of preserved thickness) are classified as Chromi-Cambic/Cambic-Arenosols. In the shore area of the palaeolakes the terrestrial palaeosoils grade into hydromorphic Gleyic Arenosols. The former shorelines are marked by fringes of bog-iron accumulations.

The palaeolakes of the lowland areas of Central Sahara have reacted differently to the ground water fed lakes in endorheic depressions in front of cuestas: a more direct influence of the climatic tendencies superimposed on local topographic conditions and changes in geomorphology (e.g. dune activity). The palaeopedological approach shows a close correlation between the degree of rubefication as well as physical, chemical and mineralogical properties of the palaeosoils and the geographical position. The climatically determined gradient of decreasing weathering intensity from sw to ne has been modified only by local differences of the original mineral spectrum and by the influx of aeolian dust. Based on palaeoecological evidence, the annual precipitation rate is estimated at least at 400 mm at the end of Late Pleistocene and Early Holocene, combined with an interaction of monsoonal rains and northern cyclons. In Mid-Holocene this interaction was reduced and the character of precipitation became more frequently violent, indicating a stronger saisonality of climate as well as a reduction of the annual precipitation rate (150-200 mm), with a more and more stronger gradient from sw to ne.