Rock magnetic and geochemical signals of late Quaternary climate variability over northwest Africa

This thesis investigates the paleo- climatic and environmental changes over northwest Africa by focusing on the Senegal region and the Gulf of Guinea. Emphasis was placed on the timing of past climate changes, the frequency of occurrence, and regional impact. Possible shifts in the position of the intertropical conversion zone (ITCZ) were also investigated. A multi proxy approach integrating rock and environmental magnetic parameters diffuse reflectance spectrophotometry, scanning electron microscopy, color reflectance and climate modeling experiments was employed. The results show that a climatic boundary which certainly reflects the southward shift of the northern limit of summer position of the ITCZ existed between 13 and 12à �à °N during the last glacial. The West African monsoon varied at D O times-scales at 12à �à °N, demonstrating that drier conditions occurred further south of the Sahel than previously reported. A fresh water hosing experiment using coupled climate model experiment suggested that the North Atlantic sea surface temperature and West African climate are linked by shifts in the position of the monsoon trough and the mid tropospheric African Easterly jet. The cooling of the North Atlantic sea surface is directly linked to the weakening of the Atlantic Meridional Overturing Circulation. Further more, climate change regulated the influx of magnetic minerals and post depositional alteration in the Senegalese sediments. Interglacial showed lower concentrations of magnetic particles, with high concentration of euhedral and framboidal pyrite. Wind derived sediments dominated the glacial with hematite and goethite prominent at these intervals as a result of increase input of Saharan dust. This studies shows that climate over this region varies at different times scales and is interconnected with global climatic signals.