Samples taken off Jamestown, Saint Helena Island (South Atlantic Ocean)

Three dredgings samples taken off the coast of Saint Helena, reveal two micropalaeontological associations. One is an infralittoral association, characterized by Miliolina (essentially Quinqueloculina), and by Rotaliina with various species of Amphistegina. The other is a circalittoral association, in which the following changes in family distribution may be observed: a decrease in Miliolina and others littoral species, and an increase in Discorbidae, Eponididae, Cibicididae, and Anomalinidae which indicate an increase in sea depth. The presence of relict Amphisteginidae might point to a Holocene stillstand of the transgressive sea.


INTRODUCTION
Saint Helena is situated in the South Atlantic 800km east of the mid-ocean ridge and 2,600km from the West African coast (Fig. 1). It consists of Miocene to recent basalt volcanics (Abdel-Monem & Gast, 1987;Baker, 1969Baker, , 1973, forming an island with an area of 120 square kilometres, rising to a height of 2,850 feet above sea level. Our knowledge of hydrological conditions in the tropical south central Atlantic Ocean is very limited; this study is based on the works of Gallardo (1966), Neumann (1965), and Piton et al. (1977). Surface temperatures of the sea range between 20°C and 22°C in the vicinity of St. Helena, with salinity between 35%0 and 36%0.
The three samples studied were collected by the o c e a n o g r a p h i c v e s s e l ' L A . N i z e r y " of t h e O.R.S.T.O.M. in Pointe Noire, Congo Peoples Republic, by means of a Rallier du Bathy dredge in Jamestown Harbour (15 j j ' S -5 43'W) (Fig. 1). Dredge sample DSHl contained two separate sediments, referred to as DSHla and DSHlb.
The sediments are poorly sorted, and can be classified as muddy sand (DSH1 and DSH3) and sand (DSH2). The carbonate, with a higher percentage in the sand, consists of reworked calcareous algae and madrepores. Molluscan shells are common and the two samples from the shallowest water (DSH2 and DSH3) have abundant amphisteginids (Fig. 2). X-ray diffraction shows the clays to consist of kaolinite, or more rarely smectite interlayering with illite. Table 1 gives details for the samples.

MICROPALAEONTOLOGY Ostracoda
In the region studied the fauna is very limited. Only five species of ostracods have been found, three of which remain in open nomenclature: Ruggeria lekkii Omatsola 1972, R. martinsoni Omatsola 1972, Ruggeria sp., Bairdia sp., Loxoconcha sp.. These species are not unknown in this region; Ruggeria abounds in the Gulf of Guinea, in shallow, coastal waters with a normal salinity (Jouval, 1978;Keen 1972Keen , 1975Babinot & Kouyoumontzakis, 1985).

Planktonic Foraminifera
Planktonic and benthic foraminifera are considered separately. Given the sampling method, the samples may be considered as average representations of the sediment. Since the "birth" of the Saint Helena volcano dates from the end of the Miocene, a mixture of middle Neogene populations and recent ones is quite possible. Boltovskoy (1968) recorded planktonic species typically found in tropical waters to be represented in these regions by the following percentages: Globigerinoides ruber 36.5%, G. sacculifer 1.5%, G. trilobus 45%, Globoquadrina dutertrei 6.5%, Globorotafia menardii 6.5% and accessory species 4%, the number of which varies according to the season. These percentages were calculated from samples taken by trawling with a plankton net in the open ocean, whereas the data used in this study was gathered from coastal dredging. The planktonic foraminifera encountered here cannot therefore be easily compared with Boltovskoy's study.
The most important species are Globigerinoides trilobus which represents between 23% and 45% of the planktonic species and Globigerinoides ruber, which accounts for between 10% arid 36% of the population. Globorotalia crassaformis gp. form between 14% and 35% of the planktonic fauna, being more abundant in the samples from deeper water. The quantity of the planktonic species as a percentage of the total foraminifera microfauna increases with depth: 7.5% of the microfauna at 50m, compared with 32% at 96m (Table 2).

Benthic Foraminifera
There is very little difference between the microfauna encountered in this study, and that found on the African coasts (Kouyoumontzakis, 1982). Sixty species are found of which seven are in open nomenclature. Textulariina are almost totally absent from the studied sediments. Two thirds of the species found are often represented by a single specimen, and have therefore only a limited value (Table 3).
The absence of Textulariina could be attributed to hydrological factors, or to the fact that agglutinated foraminifera are unable to find particles that are fine enough for them to form their test. The quantity of Miliolina is more or less constant at around 13%, mainly represented by five species: Quinqueloculina pulchella, Q. venusta, Q. kerimbatica, Q. pseudoreticulata and Q. aff. crenulata.
The number of Rotaliina varies between 45% (DSH1) and 79% (DSH3) depending on the presence or absence of Amphisteginidae. This family, the percentage of which are very high in the two most littoral sediments, DSH3 (50m) 77% and DSH2 (75 m) 72%, is renowned for its reef way of life, and its presence in Gulf of Guinea is well known (Kouyoumontzakis, 1982;Lagaaij, 1973). The state of wear of these Amphisteginidae makes it possible to compare them with those found on the Congolese continental shelf, and could correspond to a stillstand of the sea during the Holocene transgression; this thanatocoenosis dates from 12,000 years B.P. off the coast of the Congo (Delibrias et al., 1973). Radiocarbon dating was not carried out since too few Amphisteginidae tests were picked out. In the deepest sediments (DSH l b and DSH la), littoral species are replaced by Rotaliina whose habitat is circalittoral. Amphisteginidae account for only 25% of the benthic microfauna in these areas; the species which take their place are Lenticulina suborbicularis, Eponides repandus, Eponides repandus var . concomeratus, Cerato b ulimina pacifica.

CONCLUSIONS
The study of just three dredging samples cannot serve as a basis for generalizations concerning associations of Foraminifera around Saint Helena island, but the study does demonstrate that the microfauna found here is quite similar to that encountered on the coasts of the Gulf of Guinea, and this is true for both Ostracoda and Foraminifera.
Schematically, two associations of Foraminifera can be defined (Fig. 2): The first association from depths of up to 75m includes calcareous algae, and could correspond to an infralittoral fauna characterized by Spiroloculina concava, Quinqueloculina kerimbatica, Q . aff. crenulata, Q . pseudoreticulata, Q . venusta,  Amphistegina bilobata, A . gibbosa, A . lessonii, A . radiata. The sediments are characterized by quite high levels of CaC03; 58.8% and 47.2%. These calcium carbonate level are due to the presence of numerous calcareous algal arbusculs and large foraminifera. The Amphisteginidae could be the type of foraminifera that produces the most calcium carbonate. Experiments conducted in the Pacific ocean, show that Amphistegina madagascariensis produces 500g of Ca C 0 3 by square meter every year (Muller, 1974). Table 2. Numerical repartition of planktonic species.
The second association for which only one sample was found, can be termed circalittoral, and is characterized by an increase in the quantity of benthic Rotaliina: Lenticulina suborbicularis, Cancris auriculatus, Eponides repandus, Eponides repandus var. concomeratus, Cibicididae, Anomalinidae and Ceratobulirnina pacifica. It is also characterized by a decrease in the number of Amphisteginidae. A striking fact is the absence of the Textulariina in these littoral environments, which are poor in fine particles.