Palynological Evidence of Pliocene-Pleistocene Climatic Variations from the Western Niger Delta, Nigeria
Rich and well preserved assemblages of pollen, spores, and organic walled dinoflagellate cysts in 96 and 89 samples of Wells A and B from the Western Niger Delta Nigeria are recorded. The dominance of savanna pollen over wet climate indicators (mangrove, freshwater swamp species, brackish water swamp species and Palmae) and the preponderance of the dinocysts Polysphaeridium zoharyi and Operculodinium centrocarpum, species adapted to very saline and warm waters respectively, with abundant fungal spores dominated by Exesisporites sp., gives credence to a predominantly dry climate and lowered sea level during the Pliocene-Pleistocene (ca. 5.0-1.3 Ma) in the Gulf of Guinea. The most pronounced glacial events were around the 2.0-2.7 Ma, as well as between the 2.7-3.4 Ma.
April 16, 2010; Accepted: July 22, 2010;
Published: September 01, 2010
Palynological studies in Nigeria are largely confined to the Tertiary Niger
Delta, where numerous studies have been carried out following the discovery
of petroleum resources in the Niger Delta in the late 1950s. Most of these reports
exist as confidential reports of the oil and gas prospecting companies. The
few published papers on the Niger Delta geological studies include those of
Allen (1964a, b), Allen
and Wells (1962), Short and Stauble (1967), Weber
(1971), Weber and Daukoru (1975), Evamy
et al. (1978), Doust and Omatsola (1990),
Oboh et al. (1992), Haack
et al. (2000). Furthermore, most palynological studies on the Niger
Delta are concerned with zonation, age determinations and correlation. Only
few had dealt with the vegetation vicissitudes and paleoenvironmental reconstruction
for which palynological research is known for. Unfortunately, none had treated
the global glacial records and the Plio-Pleistocene climatic variations of which
much have been written in other parts of Africa and the world at large (Adam,
1993; Adam et al., 1989, 1990;
Dupont and Welnelt, 1996; Dupont
et al., 1998, 2000; Finch
and Hill, 2008; Hooghiemstra, 1993; Hooghiemstra
et al., 1986, 2006; Jahns
et al., 1998; Lezine and Cazet, 2005; Leroy
and Dupont, 1994, 1997; Weigelt
et al., 2008; Molnar and Cane, 2004).
Since climatic changes do affect the vegetation, the reconstruction of the
past vegetation will help us understand the mechanisms of such changes (Ivanor
et al., 2007). Sowunmi (1987) had discussed
the close relationship between the vegetation, soil and climate of an area.
She opined that the flora of an area offers the clue to the major climatic regimes
in such places. The most popular published works on the Niger Delta palynology
are those of Sowunmi (1981a, b,
1986, 1987, 1999,
2002, 2004), Evamy
et al. (1978), Biffi and Grignani (1983),
Legoux (1978), Clarke and Fredriksen
(1968), Knapp (1971), Jan du
Chene and Salami (1978), Oboh and Salami (1989),
Oboh, 1991a, b, 1993a,
b, 1995), Oboh
et al. (1992), Antolinez et al. (2006),
Demchuk and Morley (2004), Ige (2009)
and Adekanmbi and Ogundipe (2009). The modern Niger
Delta appears, from sedimentological, faunal and floral evidence, to have a
configuration today similar to those of the past. The physiography of the recent
Niger Delta is governed by several factors, which influence transport, ultimate
deposition of the sediment load and shape and growth of the Delta (Short
and Stauble, 1967). Different workers have investigated climatic changes
using palynomorphs in different parts of the world. The reports are scanty for
Africa and the Gulf of Guinea to which the Niger Delta belongs.
Dupont et al. (1998) had used sporomorphs and
dinoflagellate cysts in documenting the vegetation history of the West African
forest during the last 700 ka in relation to changes in salinity and productivity
of the eastern Gulf of Guinea from site GIK16867 in the northern Angola Basin.
They found out that during most of the cool and cold periods, the Afromontane
forest, rather than the open grass-rich dry forest, expanded to lower altitudes
partly replacing the lowland rain forest of the borderlands east of the Gulf
Sowunmi (2004) had studied an 11 m terrestrial core
from Ahanve in Badagry area of the coastal South-Western Nigeria as part of
the Dahomey Gap project. The area was a Typha dominated freshwater swamp
annually flooded by the Badagry Creek. She recognized 141 pollen and spore species
out of which 104 were identified. She reported that the dominant vegetation
communities that prevailed in the coastal South-Western Nigeria was made up
of abundant forests, Rhizophora dominated mangrove forest in the tidal
zone, freshwater swamp forest in the swampy regions under freshwater influence
and drier, semi-deciduous forest on the well drained areas with some woody,
savanna species and sparse grass cover at some intervals from ca. 8576±48
BP to sometime prior to ca. 3109±26 BP. Furthermore, between ca. 5780
and ca. 5682±32 BP, there was a marked decrease in all the three forest
types and a corresponding marked increase in monolete fern spores Lygodium
and Alchornea which she opined indicated a short-lived, drier climate.
Again, just prior to ca. 3109±26 BP, Rhizophora declined sharply
and later disappeared, it got replaced by freshwater swamp, an expanded freshwater
swamp forest and coastal savannas made up of abundant grasses, Alchornea
and Elaeis guineensis. The drier forest later became more opened leading
to the development of derived savanna. This mosaic of more open forest and savannas
established just prior to ca. 3109±26 BP she said constituted the eastern-most
extension of the Dahomey Gap. She reasoned that from palynological evidence
that from sometime after ca. 3109±26 BP, anthropogenic effects had caused
the phenomenal rise in Elaeis guineensis pollen and concluded that though
climatic and geomorphological factors appeared the main causes of the late Holocene
vegetation changes, information from palynology and archeology indicated outstanding
Leroy and Dupont (1994) studied a 200 m long marine
pollen record from ODP Site 658 (21°N, 19°W) which revealed cyclic fluctuations
in vegetation and continental climate in northwestern Africa from 3.7 to 1.7
Ma. These cycles they found paralleled oxygen isotope stages. Prior to 3.5 Ma,
the distribution of tropical forests and mangrove swamps reached Cape Blanc,
5°N of the present distribution. Between 3.5 and 2.6 Ma, forests occurred
at this latitude during irregular intervals and nearly disappeared afterwards.
Likewise, a Saharan paleoriver flowed continuously until isotope Stage 134 (3.35
Ma). When river discharge ceased, wind transport of pollen grains prevailed
over fluvial transport. They observed that pollen indicators of trade winds
gradually increased between 3.3 and 2.5 Ma. Again, strong aridification of the
climate of northwestern Africa occurred during isotope Stage 130 (3.26 Ma).
Humid conditions were reestablished followed by another aridification around
2.7 Ma. Afterwards repetitive latitudinal shifts of vegetation zones ranging
from wooded savanna to desert flora dominated for the first time between 2.6
and 2.4 Ma as a response to the glacial stages 104, 100 and 98. They reported
that as climatic conditions recorded in the Pliocene were not as dry as those
of the middle and Late Pleistocene, latitudinal vegetation shifts near the end
of the Pliocene resembled those of the interglacial-glacial cycles of the Brunhes
Leroy and Dupont (1997) further carried out a high
resolution pollen analysis on ODP site 658, off Cape Blanc, N-W Africa. They
observed that the periods before 3.5 Ma were marked by warmer and wetter climates
from northwestern Europe as far south as equatorial West Africa. They opined
that the long term variation denoted the onset of a drier climate between 3.5
and 3.2 Ma. At 3.2 Ma, their pollen record revealed the aridification of Northwestern
Africa probably in connection with enhanced Trade winds. After a period of re-established
humidity, a second and stronger step to aridification started at ca. 2.6 Ma.
They documented a reduction of the savannah vegetation from 2.8 Ma which they
suggested could have resulted in the development of a desert in West Africa.
Jahns et al. (1998) had reconstructed the West
African vegetation history for the last 400 ka from pollen analysis of a sediment
core from the Atlantic Ocean off Liberia. They noted that an arid climate was
indicated by the cold oxygen isotope stages 12, 10, 8, 6, 4, 3 and 2, which
resulted in a southward shifting of the southern border of the savanna. They
inferred that the Late Pleistocene glacial stages were more arid than those
of the Middle Pleistocene. They recorded the persistence of the rain forest
even during the glacial stages which they attributed to a glacial refuge of
rain forest situated in the Guinean mountains.
Lezine and Cazet (2005) had carried out a high resolution
pollen study of core KW31 off the mouth of Niger River (3° 31' 1N-05°
34'1E), from the Gulf of Guinea. The core was collected at a water depth of
1181m close to the coast and it revealed vegetation changes between 40,000 and
3500 cal yr B.P. in the West African lowlands. The microflora was highly diverse
from the studied 69 samples with the counts ranging between 124 and 1026, excluding
the mangrove pollen Rhizophora. Fern and herbaceous pollen grains of
Poaceae and Cyperaceae dominated the assemblages. The percentages reaching 46,
76 and 27%, respectively. Though trees, shrubs and lianas never exceeded 33%,
they were represented by 157 pollen taxa (about 62% of the total microflora).
They delineated three main pollen zones based on differences in tree pollen
percentages. Pollen zone 1 from the base (38,500 cal year B.P.)-917 m (22,000
cal year B.P.) was characterized by influx of minimum values of pollen and spores
which they opined resulted from the reduction in the organic material transport
from the continent. However, ferns and Typha were dominant with low arboreal
pollen except Podocarpus. Again pollen types from Sahara, Sahelian desert
and wooded grasslands also occurred in appreciable amounts together with numerous
pollen types from the Guineo-Congolian ecosystems. The results indicated the
permanency of rainforests and secondary forests in the Niger river catchment
possibly as gallery forests along rivers during the last glacial period when
enhanced trade wind conditions led to dry conditions. They pointed out that
the increase in forest diversity and the expansion of rain and secondary forests
in the adjoining continent could have arisen from the post glacial warming coupled
with increase in monsoon fluxes over West Africa. Furthermore, they were able
to note migration rates of tropical forest populations throughout North West
Africa at the beginning of the Holocene through comparison of the KW31 pollen
data and continental pollen data from 5°S to 25°N. They further reported
the vegetation response to the shift toward aridity recorded widely at the end
of the African Humid Period around 4000 cal yr B.P. This wet period had been
proposed by DeMenocal et al. (2000) from the
study of ODP site 658C off Cape Blanc and was corroborated by the fresh water
algal records of Lezine and Cazet (2005).
Hooghiemstra et al. (2006) had studied the distribution
patterns of pollen grains in recent marine sediments off NW Africa. They related
the distribution patterns to modern pollen source areas (vegetation belts) and
the prevailing transport systems (wind belts and ocean currents). Revealing
the positions of these vegetation belts in the adjacent continent were the patterns
of Quercus, Artemisia, Chenopodiaceae-Amaranthaceae, Ephedra,
Gramineae and wet forest trees. The northeast trade winds and the African
Easterly Jet (AEJ) were responsible for wind transport of pollen species. They
further observed that rivers also transported pollen and fern spores in the
rain forest belt. When they compared the recent pollen rain samples from terrestrial
and marine sites between 21 and 12°N, clear latitudinal ranges of the vegetation
belts were evident in the pollen samples of both environments. A migration of
the Southern border of the Sahara was reflected by the changing ratio between
Chenopodiaceae-Amaranthaceae pollen from the desert and Gramineae
pollen from the savannah belt.
Based on the foregoing, this present study was undertaken to investigate the effects of the Plio-Pleistocene climatic changes/sea level changes as contained in Western Niger Delta palynomorphs.
MATERIALS AND METHODS
Description of study area: The Tertiary Niger Delta (Fig.
1) is a large arcuate delta of the destructive, wave dominated type, which
is found in Southern Nigeria. It covers an area of about 7500 km2
and is composed of an overall regressive clastic sequence, which reaches a maximum
thickness of 9,000-12,000 m (30,000-40,000 feet). It covers an area located
between longitude 4°-9° E and latitude 4°-9° N. Its development
has been dependent on the balance between the rate of sedimentation and the
rate of subsidence. The stratigraphic distribution of surface outcrops of Tertiary
rocks in Southern Nigeria has been previously established by Reyment
(1965) and Adegoke (1969). However, the first information
on the subsurface distribution of stratigraphic units in the Delta was provided
by Short and Stauble (1967) and Frankl
and Cordry (1967). According to Short and Stauble (1967),
the recent Niger Delta may be divided into 3 main sedimentary environments:
the continental, transitional and the marine environments. The continental environment
comprises the alluvial environments including the braided stream and meander
belt system of the upper deltaic plain. The sediments deposited in this zone
are predominantly sandy. Feldspar grains are fairly common and sandy grains
commonly are limonite coated. Fine grained sediments (silt and clay) are deposited
in the adjacent fresh water back swamps and ox-bows together with large quantities
of plant remains. The transitional environment comprises the brackish water
lower deltaic plain (mangrove swamp, flood plain, marsh) and the coastal area
with its beaches, barrier bars and lagoons.
The sediment in this environment are distinctly finer-grained than in the continental
environment. Feldspar is scarce and brackish water faunas may occur. The marine
environment includes part of the delta fringe with its fine sand, silt and clay
and the associated marine faunas. The environment grades laterally into the
holomarine environments, which is not affected by deltaic activity (Short
and Stauble, 1967).
Sample collection: Ninty six and eighty nine ditch cutting samples of the wells A and wells B from the offshore Western Niger Delta (Fig. 1), were donated for the investigation by Chevron Nigeria Ltd. in January 2005.
Sample preparation: Thirty grams of each sample was initially treated
with hot Hydrochloric acid to remove carbonates, prior to complete digestion
in (60%) Hydrofluoric Acid (HF) under a fume cupboard. Agitation of the acid/sample
mixture was carried out to aid digestion. The sample was further treated with
hot Hydrochloric Acid (HCL) and wet-sieved over a 5 micro mesh polypropylene
sieve. The 5-micro sieve was used so that small sporomorphs such as Zonocostites
ramonae were not washed off. The Branson Sonifier 250 was further employed
during sieving to facilitate complete removal of silt and clay particles. Each
residue was oxidized using concentrated nitric acid (HNO3). The residues
were prepared for study as strewn mounts using Loctite on 22x32 mm cover slips.
The slides were stained with Safranin O to enhance the study of dinoflagellate
Sample analysis: The prepared samples were analyzed between May to December 2005 in the Paleobotanical Laboratory of the University of Lagos. A Leitz Dialux 20 EB microscope was used for the analysis. All species (pollen, spores, dinoflagellate cysts, fungal remains, algae and foraminiferal linings) present were recorded. This enhanced the palyno-ecological groupings and paleovegetational/paleoenvironmental reconstructions processes.
Coordinates of index markers were aligned with the England Finder coordinates for easier location of the specimens on the slides.
The slides, residues, unprocessed samples, negatives and duplicate prints are
in the palynological collections of the Department of Biological Sciences, Redeemers
University, Mowe, Ogun State, Nigeria.
Species identification: The different palynomorphs (pollen, spores)
were identified by comparison with local palynological catalogues of Shell
(1980), Legoux (1978), Germeraad
et al. (1968), Clarke and Frederiksen (1968),
Evamy et al. (1978), Salard-Cheboldaeff
(1979, 1990). For dinoflagellates and acritarchs,
monographs of Fauconnier et al. (2004), Rochon
et al. (1999), Wrenn et al. (1986),
Lentin and Williams (1973, 1989),
Head and Wrenn (1992), Powell (1992),
Fensome et al., (1993, 1996)
and Sluijs et al. (2003) were used. Generally,
species nomenclature for dinoflagellate cysts followed by Fensome
and Williams (2004).
Photomicrography: The specimens were examined and index species and dinoflagellate cysts photographed at X400 and X1000 using the Motic 2.0 camera at the Paleobotanical Laboratory of the University of Lagos.
Ecological grouping: The definition of the ecological groups were based
on the study of Sowunmi (1981a, b,
1986), Salard-Cheboldaeff (1981),
Rao (2001), Frederiksen (1985),
Germeraad et al. (1968) and Rull
(2003). Only Zonocostites ramonae and Avicennia sp. are taken
as mangrove following (Rao, 2001; Richards,
2001; Germeraad et al., 1968; Frederiksen,
1985), while Monoporites annulatus is listed under savanna according
to Germeraad et al. (1968) (Table
1 and 2; Fig. 2-5).
Chronostratigraphy: The chronostratigraphic ages were provided by diagnostic
foraminifera using planktonic foraminiferal Zones of Blow
(1969, 1979) and Hardenbol et
al. (1998). The MFSs were used as the zonal boundaries for convenience.
|| Well A ecological groups abundance/glacial record indicative
dinoflagellate cysts data
|| Well B Ecological groups abundance/glacial record indicative
dinoflagellate cysts data
||Photomicrographs of some recovered Savanna pollen species
from well A and B with corresponding well, sample and England finder location.
(a) Cyperaceaepollis sp. Well B (8640-8730) G33/2, (b) Echitricolporites
spinosus well A (7350-7440) G30/0, (c) Fenestrites spinosus well
B (4590 - 4680) V39/ 1, (d) Retistephanocolpites gracilis well B
(7740-7830) T19/2, (e) Monoporites annulatus well A (1980-2040) M24/3,
(f) Multiareolites formosus well A (5010-5100) G24/1, (g) Gardenia
imperialis well A (5010-5100) P35/2, (h) Polygonum sp. Well
A (6720- 6810) N42/2, (i) Peregrinipollis nigericus well B (3690-3780)
T52, (j) Numulipollis neogenicus well B (5170-5260) O34/1, (k) Retistephanocolpites
gracilis well B (5170-5260) K24/3, (l) Echistephanoporites echinatus
well B (2790-2880) T33/4, (m) Tubifloridites antipodica Cookson,
1947; well A (7530 -7620) M41 (n) Margocolporites vanwijhei well
B (7650-7740) S39/1, (o) Chenopodipollis sp. (Chenopodiaceae) well
B (7740-7830) T18/1 and (p) Tubifloridites antipodica well B (5400-5490)
Presentation of results: The analysis sheets (data sheets) were inputed
on STARTABUGS, from which distribution charts showing the palynomorph species
arranged by stratigraphic appearances and disappearances against increasing
depth in the wells were generated, with the log and systems tracts by the side.
The marker species were plotted separate from other species. This facilitated
the picking of the palynological zonal boundaries easily.
Statistical analysis: Mean, standard deviation, student T-test, one
way Analysis of Variance (ANOVA) and correlation analysis were performed on
the different ecological groups using the Prism version 5.00 computer software
programme (GraphPad Software, San Diego, CA).
||Photomicrographs of some Freshwater swamp species recovered
from wells A and B with corresponding well, sample and England Finder location.
(a) Nympheaepollis clarus well B (2790 -2880) M22/2, (b) Retitricolporites
irregularis well A (2130 -2220) Q48/4, (c) Psilatricolporites operculatus
well A(2310 -2400) V36/3, (d) Momipites sp. Well A (7260-7350) G35/1,
(e) Striatricolpites catatumbus well B(5400-5490)W26/1, (f) Laevigatosporites
sp. Well A (1980-2040) L28/2, (g) Lycopodium sp. Well A (1980-2040)
K42/4, (h) Crassoretitriletes vanraadshooveni well B (3330-3420)
D27/2, (i) Nympheaepollis clarus well B (4590-4680) R38/2, (j) Verrucatosporites
sp. Well A (4590-4680) R38/2, (k) Gemmamonoporites sp. well B
(8370-8460) R22/2, (l) Retibrevitricolporites obodoensis well B (5400-5490)
U19/3, (m) Magnastriatites howardi well A (2490-2580) S42/1, (n)
Anthocleista vogelii well B (3780-3870) S38/4, (o) Anacolosidites
cf. luteoides well A (5400-5490)P28/2 and (p) Rauwolfia vomitora
well A (5400-5490) S35/3
Significant limits were set at the 95% confidence interval.
Pollen, spores and algae: Ninety six and eighty nine samples of wells A
and well B between (1980-10570 feet) and (1920-9860 feet) were analyzed. The
palynomorphs recovered were abundant and diverse in most of the horizons studied.
The assemblages were dominated by land derived sporomorphs particularly Monoporites
annulatus, Zonocostites ramonae, Podocarpus milanjianus, Stereisporites sp.,
Cyperaceaepollis sp., Sapotaceoidaepollenites sp., Retistephanocolpites
gracilis (aff. Borreria verticillata), Podocarpidites sp.,
Verrucatosporites sp., Laevigatosporites sp., Polypodiaceoisporites
sp. and Acrostichum aureum.
||Photomicrographs of some Lowland Rainforest species recovered
in well A and B, with the corresponding well, sample and England Finder
liocation. (a) Stereisporites sp. (Smooth) well B (2880-2970)H33/4,
(b) Sapotaceoidaepollenites sp. Well B (5400-5490) S32/1, (c) Canthium
sp. well B (5760-5850) X17/3, (d) Corsinipollenites jussiaensis well
A (10400-10490) J41/1, (e) Spirosyncolpites bruni well B (6750-6840)
S45/2, (f) Sterisporites sp. (Rough) well B (2700 -2790) X48/2, (g)
Bombacidites bellus well A (7800 -7890) N38/4, (h) Polypodiaceoisporites
sp. well B (2700 -2790) M43/3, (i) Psilatricolporites crassus well
A (5640 -5730) T34/3, (j) Pachydermites diederixi well A(4470 -4560)
N36/3, (k) Acrostichum aureum well A (9590 -9680) N36/3, (l) Echitriporites
trianguliformis well B (3420 -3510) H27/3, (m) Marginipollis concinnus
well B (2700 -2790) X48/2, (n) Podocarpus milanjianus well A
(6000-6090)T35/3, (o) Alnipollenites verus well A (6000-6090)T35/3
and (p) Podocarpidites sp. well A (2220-2310) J30/1
Two hundred and thirty palynomorph species were recorded in well A, while well
B yielded one hundred and seventy-three species.
Dinoflagellates/marine elements: Abundant records of dinoflagellate
cysts which peaked at some horizons characterized the well A. The dominant species
were Polysphaeridium zoharyi, Operculodinium centrocarpum, Spiniferites
ramosus, Achomosphaera ramulifera, Tuberculodinium vancampoae,
Hystrichokolpoma rigaudiae, Spiniferites sp. (S. membranaceus,
S. mirabilis, S. delicatus, S. bulloideus, S. elongatus,
S. bentorii and S. hyperacanthus), Selenopemphix nephroides, Achomosphaera
andalousiensis, Achomosphaera sp., Selenopemphix sp. and the acritarch
||Photomicrographs of some dinoflagellate cycts and accessories
recovered from wells A and B, with the corresponding well, sample and England
Finder location. (a) Polysphaeridium zoharyi well A (2880-2970) H33/4,
(b) Operculodinium centrocarpum well B (3690- 3780) O35/1, (c) Botryococcus
braunii well B (4860-4950) K38/4, (d) Fungal spore. well B (5760-5850)
O38/2, (e) Charred Graminae Cuticle well A (2130-2220) X36/1, (f)
Pediastrum sp. Well B (8370-8460) W38/1, (g) Diatom Frustule well
B (4680-4770) K20/2, (h) Polysphaeridium zoharyi well A (2880-2970)
H33/4, (i) Concentricytes circulus well B (4680-4770) K20/2, ( j)
Fungal spore well A (10400-10490) V18/2, (k) Botryococcus braunii
well B (3690-3780)K24/2, (l) Zonocostites ramonae well B (3690-3780)
L24/3, (m) Elaeis guineensis well A (7260- 7350) T12/1, (n) Fungal
spore well A (3510-3600) M37/4, (o) Zonocostites ramonae well
B (3780-3870) K24/1 and (p) Pediastrum sp. well B (5760-5850) K38/4
The recoveries were moderate in well B, with Polysphaeridium zoharyi,
Spiniferites ramosus, Operculodinium centrocarpum, Protoperidinium
sp. (Pteridinium sp. of Wall et al., 1977),
Selenopemphix nephroides, Hystrichokolpoma rigaudiae, Spiniferites sp.,
Achomosphaera sp., Selenopemphix sp., Brigantedinium sp.,
Tectatodinium sp. and the acritarch Leiosphaeridia sp. as
the commonest species. Deepening at some intervals was indicated by the records
of Impagidinium sp., Nematosphaeropsis labyrinthus and N. lemniscata.
The spot records of Dinogymnium euclaense, Muderongia sp. and
Odontochitina sp., which are Maastrichtian and older dinocysts at 6000 and
8160 feet, in the well A, together with such older palynomorphs as Monocolpopollenites
sphaeroidites, Ephedripites sp., Gnetaceaepollenites sp.,
Foveotriletes margaritae, Proteacidites longipinosus, Psilamonocolpites
marginatus, Cinctiperiporites mulleri, Corsinipollenites jussiaeensis,
Retimonocolpites obaensis all suggest the reworking of older sediments onto
The preponderance of the paleoenvironmental indicators dinoflagellate cysts, microforaminiferal wall linings and freshwater algae Botryococcus braunii and Pediastrum sp., in the studied sections of the wells suggests sediment deposition in a shallow marine environment with frequent freshwater incursions.
Age: The wells A and B sequences fell within the broad pan-tropical
Echitricolporites spinosus zone of Germeraad et
al. (1968) and the P800-P900 zones of Evamy et
al. (1978). The sequence also correlated with the J2-J3 Zones of Legoux
(1978). The ages of the studied sections of the wells ranged from Late Miocene
(P860) at the base to Early Pleistocene (P900) subzone at the top. This is attributed
to the Quantitative Base and Base Occurrences of the Niger Delta palynological
Miocene/Pliocene boundary marker species Retistephanocolpites gracilis (aff.
Borreria verticillata) at 8520 and 9030 feet respectively in wells A and
B (Evamy et al., 1978). Again the 5.0 Ma MFSs
were delineated at 8545 feet and 8915 feet in wells A and B, defined by the
FDOs of Cyclammina minima, Eggerella scabra and LDO of Globorotalia
tumida as well as the marked influx of Arenaceous foraminifera. These events
further indicated the penetration of Late Miocene below these depths. The uppermost
part of the wells are dated Lower Pleistocene based on the first peak abundance
of Podocarpus milanjianus at 2040 feet and the presence of Spiniferites
elongatus, Spiniferites bulloideus and Spiniferites hyperacanthus (Powell,
1992; Rochon et al., 1999), coupled with the
FDO: Globorotalia miocenica dated 2.0 Ma at 4680 and 4650 feet, respectively
for wells A and B.
Ecological group assessment: The ecological group trends in the two
wells appeared similar with the savanna dominating over mangrove and the other
ecological groups especially in well A (Fig. 6 and 7;
Table 1 and 2). The least were beach, other
pollen, other spores and Palmae. The abundance of freshwater algae especially
Botryococcus braunii is in agreement with Morley (1995),
who recognized its abundance in the Pliocene-Pleistocene of the Niger Delta.
There were clear crests and troughs in the microfloral abundance, suggesting
alternations of dry and wet climates.
The statistical tests revealed significant relationships between all the ecological
groups (at 95%) confidence interval except the marine elements in the well A.
A comparison of the wells A and B showed that the means of the ecological groups
were significantly different and the Bartlett's test for equal variances depicted
significant differences. An entirely different scenario was observed in the
well B, there were significant relationships between the mangrove, savanna,
montane and marine elements while none existed for the freshwater, brackishwater,
lowland rainforest, palmae, beach, algae and other spores.
|| Well A ecological groups abundance plot
|| Well B ecological groups abundance plot
This shows that though the two wells were from the same field, their miospore
associations are slightly different.
Dinoflagellate stratigraphy: The dominant dinoflagellate cysts in well
A were Operculodinium centrocarpum and Polysphaeridium zoharyi
accounting for 11.15 and 10.14% respectively (Fig. 5). The
dominance of these two species which were concentrated at some points could
have resulted from lowered sea levels following glacial maxima (Mozardec-Kerfourn,
1988, 1992a, b). A critical
evaluation of well B, revealed the dominant dinoflagellate cysts to be Operculodinium
centrocarpum, Polysphaeridium zoharyi, Protoperidinium sp. (Peridinium
sp. of Wall et al., 1977), Spiniferites
ramosus, Invertocysta sp. and the acritarch species Leiosphaeridia sp.
Evidence of climatic variations, sea level changes and contributions to
the global glacial history: As highlighted earlier the chronostratigraphic
ages for the sediments were provided by diagnostic foraminifera which enabled
correlation to worldwide chronostratigraphy. The MFSs were used as the
zonal boundaries for convenience. The recognized SBs were dated by comparison
with the Global Sequence Cycle Chart of Haq et al.
(1988) as revised by Berggren et al. (1995)
and updated by Grandstein et al. (2004).
Early pleistocene (c.a 1.3-2.0 Ma): Top defined by the FDO of Globorotalia
truncatulinoides and Hyalinea balthica. This horizon corresponds
to the 1.3 Ma MFS. The zonal base is defined by the 2.0 Ma MFS marked by the
FDO of Globorotalia miocenica and the LDOs of Pulleniatina obliquiloculata
and Pulleniatina finalis. Well A: (2070-4680 feet) and Well B: (2040-3750
feet) Early Pleistocene.
In the well A, except at few points where the counts of Zonocostites ramonae exceeded Monoporites annulatus, (Table 1) the microfloral assemblage is dominated by Monoporites annulatus and other savanna species. This trend occurred with common records of the dinocysts Polysphaeridium zoharyi and Operculodinium centrocarpum between 1980-2670 feet suggesting a marked dry climate possibly associated with lowered sea level (glacial maximum) around this period in the Niger Delta. This event is demonstrated again around the 2.0Ma MFS at 4680, where Polysphaeridium zoharyi, occurred regularly from 4560-5190 feet in association with abundant savanna species. In the well B (Table 2), around the 2.0Ma MFS at 3750 feet are clusters of the dinocysts Polysphaeridium zoharyi and Operculodinium centrocarpum between 3510-3870 feet. The few mangrove peaks at (2070 and 3780 feet) show possible wet periods sandwiched in-between an extremely dry climate.
Early-late pliocene (c.a 5.0-2.0 Ma): The top of this Zone is defined
by the FDO of Globorotalia miocenica and the LDOs of Pulleniatina
obliquiloculata and Pulleniatina finalis which coincided with the
2.0 Ma MFS. The base is defined by the FDOs of Cyclammina minima, Eggerella
scabra and LDO of Globorotalia tumida. The horizon coincides with
the 5.0 Ma MFS. It also lies close to the Base Occurrence/Quantitative Occurrence
of the savanna pollen specie Retistephanocolpites gracilis (aff. Borreria
sp.), which is the index Pliocene /Miocene boundary marker in the Niger
Delta (Evamy et al., 1978).
0-2.7 Ma: The top of this Zone is defined by the FDO of Globorotalia miocenica and the LDOs of Pulleniatina obliquiloculata and Pulleniatina finalis which coincide with the 2.0 Ma MFS. The base is defined by the FDOs of Bulimina marginata and Cyclammina cancellata as well as the LDO of Sphaeroidenella dehiscens, which is dated 2.7 Ma.
2.0-2.7 Ma Well A: (4680-5580 feet); Well B: (3750-5490 feet): Savanna species dominated the well A microfloral assemblage within this time (2.0-2.7 Ma) except at 5190-5370 feet where mangrove peaked, showing a possible wet period in-between an extremely dry climate. The wet period is supported by common freshwater swamp species. The common records of Polysphaeridium zoharyi from 5550-6180 feet, revealed a well defined dry climate around the 2.7 Ma.
Within this time lapse in well B, there were peaks of mangrove at (4500 and 4860 feet) and a longer wet period between 5080-5850 feet. A marked drier period supported by common records of Polysphaeridium zoharyi occurred between 4860-5080 feet.
2.7-3.4 Ma: The top is defined by the FDOs of Bulimina marginata and Cyclammina cancellata as well as the LDO of Sphaeroidenella dehiscens, which are dated 2.7 Ma. The base of the Zone is defined by the FDOs of Rectuvigerina multicostata, Amphistegina lessonii, Globigerinoides bollii, Globorotalia margaritae and Globorotalia pseudomiocenica, which is correlated with the 3.4 Ma MFS.
Well A: (5580-6900 feet) and well B: (5490-6390 feet): In well A, savanna species dominated the microfloral assemblages, the common freshwater swamp species suggests possible climatic fluctuations. Polysphaeridium zoharyi became common from 6900-7170 feet, Operculodinium centrocarpum was common between 5820-6180 and 6990-7260 feet. This indicates a possible glacial maxima and lowered sea level around 2.7 Ma. On the other hand in well B, there were peaks of mangrove between 5490-5850 feet and 6210-6390), in-between savanna dominated microflora. The dinocysts Polysphaeridium zoharyi and Operculodinium centrocarpum occurred in spots around 5310-6030 feet.
3.4-4.0 Ma: The top of this Zone is defined by the FDOs of Rectuvigerina multicostata, Amphistegina lessonii, Globigerinoides bollii, Globorotalia margaritae and Globorotalia pseudomiocenica, which is correlated with the 3.4 Ma MFS. Its base coinciding with the 4.0 Ma MFS is defined by the FDOs of Marginulina costata, Haplophragmoides compressa and Globigerina nepenthes.
3.4-4.0 Ma-well A: (6900-7980 feet) and well B: (6390-7170 feet): Abundant savanna species dominated the microfloral assemblage. Polysphaeridium zoharyi and Operculodinium centrocarpum were common between 6900-7170 and 6990-7350 feet, respectively. The assemblagein well A indicates an extremely dry climate. While in well B, the microfloral assemblage was dominated by savanna species. The dinocyst Polysphaeridium zoharyi was absent, while Operculodinium centrocarpum occurred in spots around 6660-6930 feet. This suggests an extremely dry climate and agrees with the results from well A.
4.0-5.0 Ma: The top of this Zone is defined by the FDOs of Marginulina costata and Globigerina nepenthes. It coincides with the 4.0 Ma MFS. The base is defined by the FDOs of Cyclammina minima, Eggerella scabra and LDO of Globorotalia tumida.
Well A: (7980-8570 feet) and well B: (7170-8880 feet): This interval is characterized by extremely high records of savanna pollen and common Retistephanocolpites gracilis (aff. Borreria sp.). The records of the dinocysts Polysphaeridium zoharyi and Operculodinium centrocarpum were sparse in well A, while mangrove pollen, in association with common freshwater swamp, brackish water swamp species dominated the microfloral assemblage in well B. The dinocyst Polysphaeridium zoharyi was common between 7200-7470 feet and later occurred in spots within this section. Operculodinium centrocarpum occurred in spots all through. This suggests a period of climatic fluctuations with brief dry periods and fllowered sea levels.
Late miocene: 5.0-5.8 Ma: The top is defined by the FDOs of Cyclammina minima, Eggerella scabra and LDO of Globorotalia tumida. The base of this Zone dated 5.8 Ma MFS is defined by the FDOs of Haplophragmoides narivaensis and Globoquadrina dehiscens.
Well A: 5.0-5.8 Ma(8570-9380 feet) and well B: (8880-9720 feet): This section is of Late Miocene age, savanna species still dominated the assemblage in well A. This occurred with common freshwater swamp species, mangrove peaked between 9770-10040 feet, indicating a possible wet inter-phase between a dominantly dry climate. This section is characterized by abundant records of savanna species, mangrove just peaked between 8940-9120 feet in well B. The dinocysts Polysphaeridium zoharyi and Operculodinium centrocarpum were sparse in occurrence.
>5.8 Ma well A: (9380-10570 feet) and well B: (9720-9860 feet): Savanna species still dominated the microfloral assemblage within this thin section older than 5.8 Ma. The dinocysts Polysphaeridium zoharyi and Operculodinium centrocarpum were sparse to absent.
Paleoclimatic deductions: The dominance of Polysphaeridium zoharyi
and Operculodinium centrocarpum which were concentrated at some points
in the two wells could have resulted from lowered sea levels following glacial
maxima (Mozardec-Kerfourn, 1988, 1992a,
b). The 1.3, 2.0, 2.7, 3.4, 4.0, 5.0 and 5.8 Ma Maximum
Flooding Surfaces were respectively dated using diagnostic foraminifera. Close
to these MFSs were clusters of Polysphaeridium zoharyi and Operculodinium
centrocarpum, especially around the 2.0-2.7 Ma MFSs in well A (4650-5640
feet) and (3780-5850 feet) in well B (Table 1 and 2).
The signature being more pronounced in well A. Furthermore, this trend was slightly
repeated between the 2.7-3.4 Ma MFSs (5640-6900 feet) and (5850-6390 feet)
in wells A and B, respectively. Further downhole, between the 3.4-4.0 Ma MFSs
well A (6900-7900 feet) and well B (6350-7270 feet), were spot records of the
two species. The recoveries between the 4.0-5.0 Ma MFSs were similar to
the former, while below the 5.0 Ma MFS (Late Miocene) in well B were spot records
of these species contrary to well A, in which a remarkable increase in the abundance
of the two species especially below the 5.8 Ma MFS down to the last sample analyzed
10570 feet were observed.
A predominantly dry climate is inferred for these sediments due to the abundance
of Monoporites annulatus (Graminae), Cyperaceaepollis sp., Chenopodipollis
sp. and other savanna species. This agrees with the reports of Morley
(1995) who inferred from late Miocene data from the Niger Delta which spanned
a period of about 1.7 Ma, that the abundance of Gramineae pollen and common
mangrove pollen within lowstand fan deposits was consistent with a dry climate
and widespread representation of mangroves on the lower delta plain. Possibly
this trend continued into the Pliocene and younger ages. Again, Hooghiemstra
et al. (1986) had also associated the abundance of Gramineae pollen
and Chenopodiaceae-Amaranthaceae with the dry conditions of desert and savanna.
Moreover, Pocknall et al. (2001) in carrying
out an integrated paleontological study of Pliocene to Pleistocene sediments
of the Orinoco Delta in Eastern Venezuela and Trinidad, had attributed the increase
in the abundance of Cyperaceae in the Pleistocene to the same climatic (cooling)
event that led to the introduction of Alnus into the Andes which they
observed coincided with the appearance of Avicennia sp. In these studied
Niger Delta wells apart from the abundant records of Cyperaceae pollen, the
moderate records of Podocarpus milanjianus could have arisen from the
same cooling event reported by Pocknall and co-workers.
An assessment of the dinoflagellate cysts records further gives credence to
a dominantly dry climate coupled with warm temperatures in the Pliocene-Pleistocene
of the Niger Delta. Mozardec-Kerfourn (1988, 1992a,
b) had associated Polysphaeridium zoharyi with
dry climate and lowered sea levels, while Udeze and Oboh-Ikuenobe
(2005) linked Operculodinium centrocarpum with warm climate. These
trends were clearly revealed in the well A, around the 2.0 Ma MFS at 4680 feet,
where Polysphaeridium zoharyi, occurred regularly from 4560-5190 feet
in association with abundant savanna species. In the well B, around the 2.0
Ma MFS at 3750 feet were clusters of the dinoflagellate cysts Polysphaeridium
zoharyi and Operculodinium centrocarpum between 3510-3870 feet. The
most pronounced period of glacial maxima and lowered sea level as revealed by
the results from the wells A and B are the periods close to and around the 2.0-2.7
Ma MFS. This particular event has been reported by Molnar
and Cane (2004) who opined that early to Middle Pliocene global climate
was clearly different from those since 2.7 Ma. They further reported that, around
2.7 Ma, the warm equable climates of early and middle Pliocene time (5-2.7 Ma)
were replaced by recurring ice ages.
The inferred warm climate and lowered sea levels around the 3.4-2.7 Ma from
the studied samples concurs with the earlier reports from Colombia by Hooghiemstra
(1993) who suggested warm climatic conditions around the 3.2-2.7 Ma.
Leroy and Dupont (1994) had recognized this event from
their study of a 200 m marine pollen record from northwestern Africa. Their
study revealed cyclic vegetational and continental climatic fluctuations in
this area from 3.7-1.7 Ma. The cycles paralleled oxygen isotope stages. Tropical
forests and mangrove swamps got to Cape Blanc about 5°N of the present distribution
before 3.5 Ma. Between 3.5 and 2.6 Ma, forests occurred and later disappeared
after which a Saharan paleo-river flowed until isotope stage 134 (3.35 Ma).
After this period wind transport of pollen grains prevailed over fluvial, as
pollen indicators of trade winds increased between 3.3 and 2.5 Ma. They suggested
a strong aridification of the Northwestern African climate during isotope stage
130 (3.26 Ma) after which humid conditions got re-established followed by another
aridification around 2.7 Ma.
Pocknall et al. (2001) had related the dominance
and subsequent decline of Grimsdalea magnaclavata to a possible climatic
warming and cooling which coincided with the major regional lowstand displayed
on Haq et al. (1987) Global sea level curve at
around 2.1 to 2.0 Ma. This same event is clearly revealed in the well A, around
the 2.0 Ma MFS at 4680 feet, where Polysphaeridium zoharyi, occurred
regularly from 4560-5190 feet in association with abundant savanna species.
In the well B, around the 2.0 Ma MFS at 3750 feet were clusters of the dinoflagellate
cysts Polysphaeridium zoharyi and Operculodinium centrocarpum
between 3510-3870 feet. Furthermore, the cooling events must have encouraged
the preponderance of Podocarpus milanjianus between 4650-4920 feet in
well A and 3510-4230 feet in well B respectively. The few mangrove peaks at
(2070 and 3780 feet) in well B indicate possible wet period in-between an extremely
The 2.0 and 2.7Ma glacial maxima events which had been reported in different parts of the world, has been indicated in Western Niger Delta sediments. The evidence is clearly demonstrated by both marine and terrestrial palynomorphs.
The author is grateful to the Management of Chevron Nigeria limited for donating the ditch cutting samples and providing other data for this research work. I also appreciate Prof. O.K Ulu of the Earth Sciences and Reservoir Services Division for his guidance and assessment of the manuscript.
To Prof. Niels Poulson of the Geological Survey of Denmark and Greenland (GEUS) for the identification and confirmation of the dinoflagellate cysts.
To Prof. O. S. Adegoke and the Management of Mosunmolu limited for allowing me use some of their facilities.
To Dr. Francisca Oboh-Ikuenobe of the Geology Dept. University of Missouri, Rolla for her wise counsel and for providing relevant journals.
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