Abstract
This paper examines the impact of climate-sea level controls on the vegetation and evolution of the Niger Delta during the Late Quaternary. The extraneous controls on the environment outlined in this context confirm a direct link between vegetation dynamics (pollen data), sediment supply, and the landscape evolution of the Niger Delta between 20 ka and 6.5 ka. Two phases of sedimentation are recognised based on multiple proxies analysed in three gravity cores obtained from the shallow offshore at ~40 m water depth. Phase I records abundant occurrences of Poaceae, Cyperaceae, and Podocarpuspollen from a dry hinterland, charred grass cuticles, nonmarine algaPediastrum , high Ti/Zr ratio, and lower sedimentation from 20-11.7 ka. Phase II records an expansion of mangrove vegetation, high Fe/S ratio, and increase in planktonic foraminifera between 11.7 ka and 6.5 ka. This second phase is attributed to sea level rise and higher sedimentation during the development of delta plain and mangrove vegetation on the gently sloping shelf. These sequential records provide a new clue about the link between the evolutionary stages of the Niger Delta landscape and vegetation dynamics during two distinct time bound intervals, which potentially delineate the boundary between two Marine Isotope Stages: MIS2 (late glacial period) and MIS1 (interglacial period).
Keyword : Mangrove pollen, biogeochemistry, Niger Delta, landscape evolution, sea level- climate change, Late Quaternary
Introduction
The reconstruction of the vegetation dynamics and deltaic shift of the Niger Delta during the late to post-glacial period (Marine Isotope Stages MIS1 and MIS2) over the last 20 ka has been the subject of debate (2009; Adojoh et al., 2017; Adeonipekun and Sowunmi, 2019). This study presents a model that explains the interplay between the driving mechanisms and palaeoenvironmental settings in the basin based on biotic and abiotic evidence (Zong et al., 2009; Adojoh et al., 2017; Boyden et al., 2021). Previous studies of the Late Quaternary sea level in the shallow offshore of the Niger Delta and other West African regions indicate post-glacial sea level rise (SLR) and a middle Holocene SLR, followed by a subsequent fall to the present time (e.g., Lézine, 1997; Lézine et al., 2005; Scourse et al., 2005; Miller and Gosling, 2013; Bouimetarhan et al., 2015; Joo-Chang et al., 2015; Chadwick et al., 2020). Climate-driven sea level change and local fluvial sediment discharge are among the factors controlling evolution of the Niger Delta during the Quaternary (Lézine, 1997; Adojoh et al., 2017; Adeonipekun and Sowunmi, 2019). Several shallow offshore palaeoenvironments exist in this setting, such as the littoral realm (mangrove swamps, barrier islands, lagoons, deltas), and inner neritic realm. These palaeoenvironments may be composed of an integrated depositional system, and thus cannot be examined in sequestration (Woodroffe, 2002, Cohen et al. 2014). The major depositional systems under rising sea level on a gently sloping sandy coast are barrier islands or littoral system, while strand-plains (sand belt) with beach-ridges are essentially absent (Cohen et al., 2014, Chadwick et al., 2020).
The response of the shallow offshore paleoenvironments of the Niger Delta to sea level changes is influenced by tidal impact, and climate driven-nearshore wave and fluvial discharge relative to the nature of the sedimentary budget (Lézine et al., 2005; Adojoh et al., 2017). All littoral settings assumed their present structure during the sea level transgression that occurred during the Last Glacial Maximum (LGM), about 20-25 ka (Poumot 1989; Lézine, 1997; Riboulot et al., 2012; Cohen et al., 2014 Adojoh et al., 2017; Chadwick et al., 2020). Nevertheless, a sea level fall promotes extremely adverse conditions for the origination and preservation of the littoral and neritic realms, especially in wave-controlled settings of the delta. Thus, if rapid fluvial/sediment supply occurs during shoreline progradation, it may generate a delta (Cohen et al., 2014, Chadwick et al., 2020). Under this circumstance and depositional complexities, lagoons, shoreface and the neritic realm evolve, and wave-dominated deposits may rapidly prograde, causing regressive sand strands (Martin and Suguio, 1992; Chadwick et al., 2020).
Some major questions associated with this complexity remain unanswered in relation to the link between the nature of the vegetation dynamics and timing of sediment supply to the onshore and shallow offshore Niger Delta areas, as well as the impact of the Niger and Benue Rivers and their tributaries (Adojoh et al., 2017) (Figure 1). The significance of the key vegetation distribution over the post glacial evolution of the Niger Delta is still not well known, in particular how the mangrove ecosystem has responded to sea level changes and the impact of Holocene warm and wet conditions on the coastal vegetation. This study attempts to answer some of these questions through the evaluation of palaeovegetation dynamics and sediment supply using three gravity cores (GCs) (Figure 1). When present, mangrove vegetation and marine sediments along the littoral realm can be used as markers of coastal and delta dynamics, since their locations within the intertidal zone are strongly influenced by SLR (Woodroffe,1989, 1995, 2002; Lézine, 1997; Scourse et al., 2005; Adojoh et al., 2017; Adeonipekun and and Sowunmi, 2019). The mangroves and littoral fringe of most tropical settings have kept up with sedimentation and can accommodate eustatic SLR rates of ~ 3.8 mm/year. However, when eustatic rates surpass 5.2 mm/year, then the mangroves would not be preserved (Mckee et al., 2007; Cohen et al., 2014; Chadwick et al., 2020).
The main aim of this study is to establish the relationship between the evolution of the Niger Delta landscape and vegetation dynamics during the Late Quaternary. In addition, it explores the significance of detailed and integrated multiproxy data (key pollen indicators, biogeochemistry, planktonic foraminifera and sedimentology) to identify the main controls on the distinct stages of the Niger Delta evolution and its reorganisation during the Late Quaternary (MIS1 and MIS2). The methodological approaches will contribute to the body of knowledge on the factors affecting the depositional succession (stages of evolution) of the West African region through the impact of climate-driven sea level fluctuations on the coastal/littoral vegetation and delta dynamics.