3. Materials and methods
The three gravity cores (GCs) utilized for this study were collected
from the Niger Delta by Fugro Geotechnical Company for Shell Petroleum
Development Company of Nigeria in 2002 (Adojoh et al., 2020; Figure 1).
The cores were acquired from the seabed within the near shore (shallow
marine) realm at approximately 40 m water depth (Adojoh et al., 2017,
2020). They were well positioned in the eastern (GC1 = Latitude -
4°49″43 N, Longitude - 5°20′20″ E), central (GC2 = Latitude -
4°05′08″ N, Longitude - 6°33′30″ E) and western (GC3 = Latitude -
4°11′59″ N, Longitude - 7°21′29″ E) parts of the delta (Figure 1; Adojoh
et al., 2017, 2020). A detailed lithological description on the scale of
1:10 cm was undertaken to select the intervals for detailed microfossil
and biogeochemical analyses. Each gravity core was sampled at every 2 cm
for detailed stratigraphic, chronostratigraphic and palaeoenvironmental
study, and to further infer the imprints of sea level/climate cycles and
vegetation dynamics over time.
Preparations for palynomorphs, nannoplankton and foraminifera data
followed the standard procedures described in Adojoh et al. (2017,
2020). In addition, the geochemical components of the three GC samples
were analysed at 2 cm intervals for physical and chemical properties
using a Bruker S2 Ranger XRF Spectrophotometer Autosampler. In this
study, 28 samples were selected as required for the maximum batch of 28
clean and dry pots. Nylon film was used to separate the individual pots
while the bases of the pots were kept flat and level with no
protrusions. The samples were compacted gently by using the brass
plunger. All recorded measurements were obtained from the saved drive of
the XRF analyser database and used for graphical plots.
The samples for Ti/Zr and Fe/S ratios were selected in their ranges of
elemental analyses for the XRF Spectrophotometer. High Ti/Zr ratios of
hinterland pollen provide information on the extent of fluvial materials
supplied from terrestrial environments, whereas high Fe/S ratios provide
the extent of inherent marine shale deposited on the continental shelf
(Marius and Lucas, 1991; Zabel et al., 2001; Adegbie et al., 2003;
Doktorgrades, 2004; Mendoza, 2007).
Adojoh et al. (2017, 2020) based the age model for each gravity core
only on biostratigraphy defined by the first occurrences (FO) of
calcareous nannoplankton (NN19 to NN21) and planktonic foraminifera
(Glorobotalia tumida and Glorobotalia truncatulinoides ).
Those studies could not utilize radiometric dating because the impact of
freshwater dilution on the near-shelf margin affected the preservation
and quantitative counts of fossil materials, such as foraminiferal
tests, macrofossil shells (gastropod, bivalves), and wood particles,
that could have been used for dating. However, this present study has
compared the dated multiple proxy records from the GCs with published
data on relative sea level change for the Niger Delta region over the
last 20 ka. In Figures 3-5, we have extrapolated the relative sea level
curve for the Gulf of Guinea region plotted against the radiocarbon date
from a global isostatic model database (Peltier, 1994) and a
representative location (5830V S, 11830V E, T89-16 core) on the Congo
shelf margin (Scourse et al., 2005).