Past Environmental Changes Recorded By Deep-sea Sediment Cores From Eastern Equatorial Atlantic

The last decade has seen major advances in both the quality and diversity of the study of African palaeoclimate records from marine sediments,adding to a nearly 40 years history of research that started with marine sedimentation work off NW Africa.Ongoing research on sediment fluxes,composition,grain size,and organic biomolecular vegetation proxies preserved in deep-sea sediments has greatly clarified to our current understanding of how and why African climate has changed in the past.However,fewer researches relating to marine sedimentation have been done off West Africa,eastern Equatorial Atlantic(EEA)compared with off NW Africa and South Atlantic.This study aims at reconstructing the environmental changes and sedimentation history of EEA using two sediment cores,DY26?-Nig-S60-GC2 and DY26?-Nig-S71-GC8 collected from the slope and abyssal basin in EEA respectively.Variations in Quaternary terrigenous grain-size distributions of NW African continental margin sediments have been linked to changes in fluvial and aeolian inputs,which are in turn associated with fluctuations of continental humidity.In this work,a marine sediment core DY26?-Nig-S60-GC2 was analyzed for C-,O-isotopes,grain size,clay minerals content,geochemical composition and magnetic susceptibility.During glacial stages,the increase in grain-size,K/Al,Ti/Al and magnetic susceptibility,and decrease in Fe/K ratio depicts a southward shift in the mean latitude position of the ITCZ and by implication increased terrigenous input delivery to the basin.Interglacial periods are characterized by decrease in grain-size,K/Al,Ti/Al ratios and magnetic susceptibility,and increase in Fe/K ratio,which depicts a northward shift in the mean latitude position of the ITCZ and by implication a reduced terrigenous input delivery to the basin.Fluctuations in Fe/K,K/Al and Ti/Al reflect changes in the low latitude insolation cycle(23 kyr).The terrigenous components show strong dominance of the highly coherent 23 kyr precessional forcing of insolation at low latitude due to monsoonal variations and intensities.In addition,less Kaolinite content during the glacial periods depicts low or moderate chemical weathering in the sediment source area.On the contrary,the more kaolinite content during the interglacial periods depicts increased action of chemical weathering in the sediment source area.The high-frequency variations in terrigenous influx during the last glacial in core DY26?-Nig-S60-GC2 from the tropical water off Nigeria support the idea that occurrences of rapid climate change existed during the last glacial in both high and low latitudes.On the continental slope,turbidity currents are one of the most important mechanisms for the rapid sediments transportation from continental margins to the deep-sea.In the 170 cm long core DY26?-Nig-S71-GC8 retrieved from the abyssal basin of the eastern Equatorial Atlantic(EEA)revealed turbidite layer at a depth of 90 cmbsf.The turbidite deposit consists of feldspar and quartz with fining upward sequence.They are characterized by poor sorting,rounded to sub-rounded coarse grain sizes.The source area of the turbidite is inferred to be located on the shelf of the Nigerian continental margin.The turbidite occurred during the last glacial stage is due to the sea-level lowstand and rivers draining,also the slope canyons on Nigerian continental margin carrying material downslope effectively.They are likely formed by the sediment gravity flow that might have been assisted due to 80-90 cm drop in sea level during MIS 3 period that contained a wide range of sediment type which ultimately made their way down into the canyons and adjacent plain.Marine sediments accumulating along the margins and deep-sea off the African continent serve as repositories of Africa's paleoclimate history.As part of this work,the sediments obtained in sediment cores off the coast of West-Africa,northwest Africa and south West Africa were complied and compared to provide insight into the sources and transport pathways of terrigenous sediments that are deposited into the ocean from the west of African continent.Four distinctly types of terrigenous sediments were identified in both sediment cores DY26?-Nig-S60-GC2 and DY26?-Nig-S71-GC8 based on grain-size distribution patterns using end-member modeling algorithm.Type 1(distal-aeolian)is dominated by fine-silt which is likely picked from the Bodele depressions by the cold and dry wind system from the north latitude to coastal West-Africa,sometimes reaching far to the equator.Type 2(proximal-aeolian)is composed of coarse and fine silt,derived from the expanding source area caused by locally widespread aridification.Type 3(Hemipelagic),this is made up of clay and very fine-silt,associated with continental runoff.Type 4(mass-movement deposit)consist of medium and coarse sand which mainly contains quartz and feldspar.The Type 4 sediment layer in DY26?-Nig-S60-GC2 was deposited from ca.11 to 49 ka,but at ca.35 and 40.9 ka in the Core DY26?-Nig-S71-GC8.Variations in the proportion of the aeolian end-members reflects variations in aridity which shows relatively arid climate conditions during interglacial stages and relatively humid conditions during glacial stages in African continent.