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.