Palaeoclimatology and biostratigraphic significance of late Neogene/Quaternary vegetational changes recorded in the offshore western Niger Delta

Abstract

Attempts at reconstructing palaeoclimatic changes over time using palynomorphs of three oil wells drilled in the shallow offshore Niger Delta led to cluster analysis-aided recognition of seven pollen zones. These pollen zones are equivalent to ten palaeoclimatic zones with alternating dry and wet conditions. The palynomorphs were classified into phytoecological groups and changes in their relative abundances were employed to interpret the palaeoclimatic conditions of their source areas. Contrasting fluctuations in the proportions of the Spore (Pteridophytes) phytoecological group and those of the Poaceae were the main basis for palaeoclimatic inferences. Trends of the occurrence of other phytoecological groups were used to substantiate our palaeoclimatic inferences. In the latest Miocene (5.8-5.5 Ma), climatic conditions were mainly wet until between 5.5 and 5.0 Ma, when extreme dry conditions prevailed. The early Pliocene part (5.0->3.4 Ma) was generally wet, while the late Pliocene part (<3.0-2.7 Ma) was extremely dry, with wet conditions re-occurring at the latest Pliocene/earliest Pleistocene boundary at a lower magnitude than those of the early Pliocene. This is inferred from the bloom of open vegetation Acanthaceae undif., Polygala sp. and Asystacia gangetica, along with montane Podocarpus milanjianus from 2.4 Ma through 2.0 Ma and younger. The Acanthaceae bloom recorded the evolution of A. gangetica in the latest Pliocene/earliest Pleistocene at around 2.0 Ma in the Niger Delta. The upper Early Pliocene regional wet event is associated with distinct peaks of riverine forest, freshwater swamp and mangrove pollen. Our results further support earlier findings from other parts of West Africa with respect to palaeoclimatic changes in the late Neogene/earliest Quaternary. Equivalent qualitative palynostratigraphic events were recognized within the pollen zones which are useful for age determination, and the significance of biostratigraphic correlation of the zones is stressed.