PALEOCLIMATIC SIGNIFICANCE OF MICROFLORA RETRIEVED FROM WELL ‘Y’, WESTERN NIGER DELTA, NIGERIA

Palynomorph species comprising 53 pollen, 7 spores, 2 algae and 6 dinoflagellate cysts were recovered from a section of well ‘Y’ located in the offshore western Niger Delta and were used for paleoclimatic deductions of the sediments. There was a dominance of the fresh water swamp species over the brackish, savannah and the rain forest types. Stratigraphically, four floral zones corresponding to inferred climatic units were identified. M2 floral zone, at the lower portion of the well, occurred between 8150 ft (2486 m) and 6440 ft (2147 m). It corresponded to wet and humid climate with periodic rainfall. M1 floral zone was between 6440 ft (1964 m) and4820 ft (1470 m) indicating a dry climatic phase in a high sea level stand. The P7 floral zone was also a humid climate in a sea level rising environment while the topmost floral zone between 4370 ft (1333 m) and3125 ft (953 m) was a fluctuating dry and wet climatic phase in a tidally influenced environment. A humid tropical climate characterised by alternation of dry and wet regimes occasioned by sea level changes and cyclic fluctuation in continental climatic condition is inferred from the analysed well that spanned through Late Miocene and Early Pliocene.


INTRODUCTION
Deltaic facies, typified by Niger Delta environment, have abundant palynomorphs whose occurrence does not only reflect variations of sediment provenance but also the environmental conditions that supported the floral growth.The abundance therefore readily becomes a tool for paleoclimatic interpretation as the various morphophyte groups do reflect different vegetational species consequent upon climatic differences (Sowunmi, 1987 andIvanor et al., 2007).In Nigeria, palynological studies had largely been conducted on the Tertiary Niger Delta on account of increasing exploratory activities resulting from the discovery of petroleum resources in the late 1950s.Most of these reports are, however, under the proprietary control of the oil and gas prospecting companies.The few published papers are those of Jan du Chêne et al. (1978), Sowunmi (1981), Odebode and Salami (1984) as well as Lawal and Moullade (1986).The present study presents identified palynomorphs retrieved from well "Y" owned by Chevron Nigeria Limited in the western offshore of Niger Delta (Fig. 1).The palynomorphs enable paleoclimatic deductions for the floral environment.
Ifeoluwadun O. Adelabu, Department of Applied Geology, Federal University of Technology, Akure, Nigeria.The recovered organic matter were uniformly spotted on arranged cover slips of 22/32mm size, and then allowed to dry in sunlight, before being mounted on properly labelled slides.The mounting medium used was the glycerine oil.The cover slips were gently lowered to the slides, so as to prevent air bubbles.
The prepared palynological slides were studied under the transmitted light microscope made by Olympus Corporation; model CHB 213 (R) .Sometimes staining with immersion oil was necessary in order to enhance the visibility of transparent palynomorphs.Palynomorphs were identified, described and counted for as many forms as possible.Relevant literatures were used in their identification like Legoux, 1978;Evamy et al., 1978;Morley, 1995 etc.Some pictures of the palynomorphs were taken using a special camera attached to the microscope, the DP 12 (R) Olympus camera.

RESULTS AND DISCUSSION
A total number of seventy palynomorphs, comprising fifty-three pollen, seven spores, two algae, six dinoflagellates and two fungi sporeswere counted after screening.The zone ranges from 8150 to 6440 ft corresponding to2486 to1964 m.

Polysphaeridium zorhayi
The basal depth (8150ft/2486 m) of the studied interval marks the beginning of zone M2.The grass pollen, Monoporites annulatusis almost nonexistent but there is an observed increase in Zonocostites ramonae up to a depth of 6530ft (1992 m) where a maximum count of 270 is picked.The consistency in the record of this mangrove pollen together with fresh water swamp/rain forest species of Sapotaceae sp., Retibrevitricolporites protudens, Gemmamonoporites spp., Striatricolpites catatumbus and Botryococcus braunii is an indication of preponderance of a wet climate.There is also a rare occurrence of marine elements, dinocysts Polysphaeridium zoharyi and acritarch Leiosphaeridia spp., as well as savannah pollens, Concentricytis spp., Elaeis guineensis, Cyperaceaepollis spp., Pteris spp., Ctenolophonidites costatus.The zone probably experienced a tropical humid climate with periodic rainfall during this time.The presence of the dinocysts species, Polysphaeridium zoharyi, known for its adaptation to very saline waters, and the acritarch Leiospaeridia spp.along with freshwater algae Botryococcus braunii, also suggests sediment deposition in shallow marine environment with frequent freshwater incursions.

'Floral' zone M1
The zone ranges from 6440 to 4820 ft corresponding to 1964 to 1470 m.
There is a sharp quantitative (numerical) drop of mangrove pollen, Zonocostites ramonae from 270counts at 6530ft (1992 m) to 33counts at 6350ft (1937 m) and 26counts at 5900ft (1800 m), but with a gentle downhole increase in Monoporites annulatus within this zone.The Dinocysts diversity also peaked within this zone (at 6170ft/1882 m), with no record of fresh water species Botryococcus braunii.Generally the zone seems to have records of species adapted to very saline and warm water forms like Polysphaeridium zoharyi and Operculodinium centrocarpum respectively.Marine species Lingulodinium machaerophorum is also restricted to this zone.There are also noticeable fluctuations in the occurrence of ferns, Laevigatosporites spp., Verrucatosporites spp., and Stereiosporites spp.The absence of Botryocoocus braunii at 6170ft (1882 m) strongly suggests a 100% marine environment.Nevertheless, important few savannah and rain forest/fresh water swamp species also occur within the zone.The savannah species include Concentricytis spp., Proteacidites spp., Cyperaceaepollis spp., Corylus spp., Pteris spp., Chenophodipollis spp while the rainforest/fresh water swamp species include Canthium spp., Brevitricolporites guinetii, Psilatricolporites operculatus.This zone is probably a marine prone area having a dry climatic phase with warm temperatures at times of lowered sea levels and low fluvial activity.

Floral' zone P7
The zone ranges from 4820 to 4370 ft corresponding to 1470 to1333 m.
Within this depth slice, a relatively low percentage composition of Monoporites annulatus predominates with the ratio of Monoporites annulatus to Zonocostites ramonae ranging from 0.40 to 3.40.Conversely, abundant quantities of mangrove pollen, Zonocostites ramonae, are recorded with common to abundant fresh water algae, Botryococcus braunii.Also noticeable is an increase in brackish water swamp species, Pachydermites diederixi and Psilatricolporites crassus.Large representation of fern spores was also a major feature in this zone.The only record of Palmae pollen, Retitricolpites spp. is at 4370 ft (1333) m.Some of the fresh water swamp species that characterize the zone includes Sapotaceae sp, Retitricolporites irregularis, Gemmamonoporites spp., Retibrevitricolporites protudens and Acrostichum aureum.The pollen record indicates a rise in sea level and an extension of mangrove swamps.

'Floral' zone P6
The zone ranges from 4370 to 3125 ft corresponding to 1333 to 953 m.
In this zone, the pollen record indicates that mangrove swamp forest vegetation is now well established, with the quantitative occurrence of Zonocostites ramonae reaching its highest numerical value.So also is the high occurrence of fresh water swamp species, Sapotaceae, Retitricolporites irregularis and Retibrevitricolporites protudens.There is, however, a rare occurrences of other fresh water sporomorphs of Verrutricolporites rotundiporus, Gemmamonoporites spp., Rauwolfia vomitoria, Verrutricolporites microporus, Marginipollis concinnus including fresh water algae, Botryococcus braunii.There is also a rare to common occurrences of the brackish water swamp species of Pachydermites diederixi and Psilatricolporites crassus.Within the zonal interval it is worthy to note the increase in abundance of pteridophytes (fern spores), Laevigatosporites spp., Verrucatosporites spp.and Stereiosporites spp.coupled with an almost consistent occurrence of Acrostichum aureum but rare occurrences of lowland rainforest species of Nympheaepollis clarus, Echistephanoporites echinatus, Racemonocolpites hians, Bombacacidites bellus, Canthium spp.There is also an overall increase in the recorded quantities of grass pollen, Monoporites annulatus, comparable numerically to mangrove pollen.This is in tandem with a visible common to abundant occurrences of Corylus spp., Cyperaceaepollis spp.and Pteris spp., and rare to common occurrences of Ctenolophondites costatus, Chenophondipollis spp., Numulipollis neogenicus, Peregrinipollis nigericus, including charred gramineae cuticle, which are all savannah species and are indicative of a dry climate.An initial rise in sea level with increasing expanse of mangrove vegetation is interpreted followed by a probable alternation of dry and wet climates in this zone.

Paleoclimatic Implication
A wet condition is suggested for the studied section of the well between 8150 ft (2486m) and 6440 ft (195 m) on account of the regular occurrence of mangrove pollen, abundant fresh water algae and brackish water swamp species.However remarkable increase in the Gramineae pollen and a rare record of humid climatic indicator, Cyperraceaepollis spp., allow the interpretation of a tropical humid climate with irregular rainfall.Mozardec-Kerfourn (1992) while working on the deep sea sediments of the West African margin attributed the dominance of Operculodinium centrocarpum and Polysphaeridium zoharyi to lowered sea levels following glacial maxima.In another consideration, Udeze and Oboh-Ikuenobe (2005) linked Operculodinium centrocarpum observed among palynological data from Cape Basin of South Africa to warm climate.With the flourishing of mangrove and fresh water forms in this unit, it is therefore inferred that sediment deposition is in a shallow marine environment with frequent freshwater incursions.
The unit between depth slice 6440 ft (1964 m) and 4820 ft (1470 m) is a dry climatic phase with warm temperatures as indicated with the increase in the quantities of Monoporites annulatus.Vermoere et al. (1999)   Occurrence of small quantities of mangrove pollen in this zone is probably due to minor local short-lived transgressions of the sea.
The pollen record within depth slice 4820 ft (250 m) and4370 ft (1333 m) indicates a rise in sea level and an extension of mangrove swamps.It has been reported that high values of Zonocostites ramonae characterize periods with a high sea level (Lezine and Vergnaud-Grazzini, 1993;Crowley and Gagan, 1995).The preponderance of ferns and occurrence of Cyperaceae in this zone coupled with the presence of fresh water swamp forest species indicate a dominance of wet and humid conditions that supported ample vegetation.Nevertheless, a substantial occurrence of Gramineae pollen, Monoporites annulatus and a few savannah species like Corylus spp.indicates fluctuation in environmental conditions as the sea level rose.
The topmost unit of the investigated section between depth 4370ft (1333 m) and 3125 ft (953m) give an indication of increasing mangrove vegetation as the sea level continued to rise.However tidal influence is inferred on account of the fluctuations in percentage occurrence of Zonocostites ramonae.The rise and fall of tides are capable of bringing about drier conditions resulting in a reduction of forest vegetation and subsequently promoting extension of the savannah.This agrees with the reports of Leroy and Dupont (1994) establishing cyclic fluctuations in the vegetation and continental climatic conditions of NW Africa in the Pliocene.

CONCLUSION
Palynomorphs recovered from well 'Y'located in the western offshore of Niger Delta provide large quantities of pollen and spores as well as some quantities of algae and dinocysts.The forms are 84 SAKA ADELAYO OPELOYE AND IFEOLUWADUN O. ADELABU classified into ecologic groups of fresh water swamp, brackish, savannah and rain water species.Stratigraphically, four floral zones on the basis of phytoecological assemblage are recognised from the investigated section of the well.The four floral zones correspond to the M2, M1, P7 and P6 of the Microfloral zones of Morley (1995) Morley (1995).The unit is characterised by tidally influenced regime which promoted dry and wet climatic phases (Fig. 4).
A humid tropical climate characterised by alternation of and wet regimes occasioned by sea level changes and cyclic fluctuation in continental climatic condition is inferred from the analysed well.River Niger catchment being along the corridors of mangrove and forest region might be responsible for the persistence in the occurrence of mangrove and rain forest pollen Zonocostites ramonae and Retitricolporites irregularis, Canthium spp.throughout the stratigraphic column of the well.

Fig. 1 :
Fig.1: Location of Well 'Y' in the offshore of western Niger Delta Oil Province (Modified after Whiteman 1982)

Fig. 4 :
Fig. 4: Recognised Microfloral zones in well 'Y' with palynological and inferred paleoclimatic events with paleoclimatic significance.M2 zone is characterised by sudden increase in Zonocostites ramonae and decrease in Monoporites annulatus.It is between 8150 ft (2486 m) and 6440 ft (1964 m) of the section of the investigated well.Wet humid climatic condition at the fall of sea level is interpreted.M1 zone is marked by increase in Monoporites annalatus and some marine related species.It is within 6440 (1964 m) and 4820 ft (1470 m) of the well.It represents a transgressive deposit within a dry climatic phase.P7 is between 4820 ft (1470 m) and 4370 ft (1333 m) of the well.It represents a humid climate marked by fluctuating atmospheric condition and transgressive deposits that supports extension of mangrove swamp.The upper portion of the well, between 4370 ft (1333 m) and 3125 ft (953 m), falls within the P6 unit of in a study in SW Turkey reported that high percentages of Gramineae pollen types in sediment points to drier local conditions.There was a noticeable presence of savannah species Echitricolporites spinosus, which further confirms the prevalence of a dry climate.Other important savanna species occurring within this zone include Concentricytis spp., Proteacidites spp., Cyperaceaepollis spp., Corylus spp., Pteris spp., Chenophodipollis spp., and more.However, a few rainforest/fresh water swamp species