Reconstructing precipitation changes in northeastern Africa during the Quaternary by clay mineralogical and geochemical investigations of Nile deep-sea fan sediments.
Résumé
Clay mineralogy combined with high-resolution element geochemistry of core MD90-9064, located in the distal part of the Nile deep-sea fan (Levantine Basin), have been investigated to reconstruct rainfall changes in northeastern Africa during the Quaternary and to determine possible climatic controls. Clay minerals of core MD90-964 are derived mainly from three sedimentary sources (the Sahara, Nile River and Egyptian wadis) and are characterized by contrasted mineralogical composition. Variations in illite content and logarithm ratios of Si/Al and K/Al permit the tracking of eolian input from Sahara to the Mediterranean Sea. It is suggested that precipitation changes in the Sahara are mainly dominated by glacial-interglacial cycles. Such variations are owing to a shift of climatic conditions in the North Atlantic from a NAO-positive-like condition in glacial times to a NAO-negative-like condition in interglacial times. Fe content in the Levantine sediments is mostly derived from Fe-bearing heavy minerals brought by the Nile River. Therefore, variations of Fe/Al ratios can be used to establish precipitation changes in the Nile River basin. Long-term variation in precipitation in the Nile River basin is governed by precessional and eccentricity signals, implying that the African monsoon is the most significant controlling factor for precipitation changes in this region. Precipitation changes in the northeastern coasts of Africa are reconstructed using kaolinite contents provided by the Egyptian wadis. It is reported that precipitation in coastal northeastern Africa is mainly of the Mediterranean-climate type. Long-term variations in rainfall in this region are also affected by the NAO-like climatic variability and thus dominated by the glacial-interglacial cycles.