Controls on Palaeovegetation and Delta Evolution: Implications from the Coastal Margin, Niger Delta, Gulf of Guinea

Delta systems are not only the interface between the land and the ocean, but also sites of long-term sediment accumulation, pollen and organic matter storage that can be used for environmental investigations. Deltas are particularly sensitive to climate and sea level change, which can be expressed with change in the shelf morphology and the associated vegetation. The Niger Delta, ideally located in a region of great sensitivity to climate change and sea level fluctuations, offers the possibility to provide an insight on the “late” Quaternary history of environmental change in West Equatorial Africa. Up to now, this deltaic system has been sparsely studied, compared with other major tropical deltas and therefore, its response to past climate change and sea level fluctuation is poorly understood. This study aims to fill this scientific gap through a detailed analysis of a multi-proxy dataset collected from three gravity cores obtained from the shallow margin of the Niger Delta. The dataset included sedimentology, grain size, geochemistry, calcareous nannoplankton, foraminifera and palynomorphs. Using these data, a robust biostratigraphic framework was established for the littoral shallow offshore deltaic sequence. In addition, a refined biostratigraphic technique was developed based on foraminiferal and nannofossil (NN) indicators linked to the Marine Isotopes Stages, which helped to define the age model of the sequences (NN19= MIS2, NN20 & NN21=MIS1). Within this new biostratigraphic framework the spatial and temporal evolution of the coastal environment of the Niger Delta were evaluated. This study hypothesised that the dynamics of the Intertropical Convergence Zone (ITCZ), the West African Monsoon (WAM) and sea level fluctuation played a major role in shaping the landscape in the Niger Basin and adjacent coastal regions. The palynological sequences defined in the three gravity cores show very similar fluctuations, with a dominance of the Afromontane Forest (Podocarpaceae), Freshwater Swamp (Cyperaceae), Savannah (Poaceae) and Lowland Rainforest (Polypodiaceae) during the late glacial and deglaciation period, followed by the iv development of the Mangrove (Rhizophoraceae) during the Early to mid-Holocene. In addition, high values of charred grass cuticles and Ti/Zr are observed during the late glacial, whereas high values of Fe/S ratio and planktonic foraminiferal are concomitant to the expansion of mangrove vegetation during the interglacial period. These records suggest dry conditions, with lower sea level during the late glacial and deglaciation periods, and warm conditions leading to the rise of the sea level during the interglacial in the Early to mid-Holocene periods respectively. To summarise, the outcome of this study indicates that the records link well in time to the known hypotheses of the land-ocean interactions providing the main drivers for study of the climate and sea level change in relation to the sedimentary and vegetation evolution of the Niger Delta compared with previous studies from West Equatorial Africa. The results permit a re-evaluation of the controls of climate, sea level and sediment supply contributing to the understanding of the two stages in palaeovegetation and the littoral/coastal evolution of the Niger Delta (progradation and retrogradation) for the future exploration, exploitation and sustainability of the region.