Planktonic Foraminiferal Assemblage in Surface Sediments from the Thukela Shelf, South Africa

ABSTRACT Twenty-three species of planktonic Foraminifera have been identified from surface sediments on the Thukela Shelf (KwaZulu-Natal Bight, South Africa). The assemblage is dominated by Orbulina universa, Globigerinoides ruber and Neogloboquadrina dutertrei, and is referred to the tropical-subtropical zone. Surface-water species (Globigerinoides ruber, G. trilobus, G. sacculifer, and G. conglobatus) dominate over deep-water species (Globorotalia inflata, G. tumida, G. menardii, and Sphaeroidinella dehiscens). Identification keys to species of Globorotalia, Globigerina, and Globigerinoides from the KwaZulu-Natal Bight are provided.


INTRODUCTION
Planktonic Foraminifera are an important source of palaeoceanographic information (Schiebel & Hemleben 2005). Their long palaeontological record, good preservation in sediments and easy collection make them perfect indicators for studies on changes in sea-surface temperature, salinity, and nutrient content (e.g., Spezzaferri & Spiegler 2005;Fraile et al. 2009).
South African foraminifers have been studied for more than 150 years, but most studies, especially recent ones, have been of a geological nature, without any investigation of their distribution pattern (e.g., McMillan 2003;Toefy et al. 2005; and references therein). The diversity and distribution pattern of recent foraminifers, and their application to environmental analyses, have so far been largely ignored. Noteworthy studies on southern African planktonic foraminifers include those of Giraudeau (1993) on the south-western African continental margin and Rau et al. (2002) on the southern margin. Bé and Hutson (1977) provided a regional distribution of planktonic foraminifers in the Indian Ocean, but no systematic research has been undertaken on the eastern continental shelf of South Africa.
Recent interest was initiated by the proposed development of a dam on the Thukela River and the associated impacts on a unique marine ecosystem, the Thukela Shelf, comprising an uncharacteristically wide muddy continental shelf and an associated economically important prawn industry. This system depends on the interaction of fresh water and sediment from the Thukela River, the strong Agulhas Current, and a semi-permanent anti-cyclonic spinning gyre within the KwaZulu-Natal Bight (Bosman et al. 2007).
In this paper we provide results of the taxonomic study of planktonic foraminiferal assemblages from sediments across the KwaZulu-Natal Bight.   TABLE 1  General information from samples collected at the Thukela River and Thukela Shelf (TB).

MATERIAL AND METHODS
The study area, approx. 18×25 km in extent, is located immediately offshore of the Thukela River on the Thukela Shelf and forms part of the KwaZulu-Natal Bight (Fig. 1). Surface sediment samples were collected by Shipek grab sampler in 2006 aboard the Marine and Coastal Management research vessel the FRS Algoa (Hunter 2007), and during the Oceanographic Research Institute (ORI) survey in 2008.
In total, 21 samples were analyzed for the foraminiferal fauna (Fig. 2). Samples were washed through 500, 125 and 63 μm sieves, were dried and the two fractions (>500, 500-125 μm) were analyzed. Due to the relative proximity of the sampling sites to the shore, foraminifers were found to be scarce and the required number of 300 specimens per sample could not be obtained for most samples. Thus this study presents results of a semi-quantitative approach.
The relative abundance of individual foraminiferal taxa is expressed as follows: R -rare (1-5 specimens/sample), C -common (6-10 specimens/sample), F -frequent (11-25 specimens/sample), A -abundant (>25 specimens/sample). The sample numbers, depth, location of each sample and sediment character are given in Table 1. Photographs were taken using a Nikon AZ-100 stereomicroscope and 3-D Nikon Tappan (1987Tappan ( , 1994, Saito et al. (1981) and The Taxonomicon & Systema Naturae (2000). All examined material has been deposited in the palaeontological collection of the Department of Natural Sciences, Natal Museum, Pietermaritzburg, South Africa. TAXONOMY Order Globigerinida Lankester, 1885 Superfamily Globorotaliacea Cushman, 1927 Family Globorotaliidae Cushman, 1927Genus Globorotalia Cushman, 1927Globorotalia cultrata (d'Orbigny, 1839 Figs 3A, 3D differs from other Globorotalia species encountered in having subglobular chambers and in lacking a keel. Distribution: This species characterizes cool-temperate waters. It is uncommon in the tropical region (Bylinskaya et al. 2002) and very rare in our material.
Globorotalia tumida (Brady,  Medium to large; biconvex trochospiral test with 5-6 wedge-shaped chambers in tures. Peripheral keel distinct, thick. Aperture interiomarginal, extraumbilical, a low arch with a large, plate-like lip. Remarks: G. tumida has a more elongate and thick test than G. menardii, with a less Distribution: An equatorial/tropical species, appearing in warm-water regions only. A single specimen was found in our material. Bermúdez, 1961 Figs 3C, 3F, 3I

Globorotalia menardii ungulata
Small to medium biconvex trochospiral test with about 5 wedge-shaped chambers in keel thin. Aperture interiomarginal, a very low slit-like opening at the base of umbilical shoulder with a large plate-like umbilical lip.
Remarks: This species can be easily recognized by its high umbilical face and its thin, G. ungulata was occasionally considered to be related to G. tumida; however, we believe it to be closer to the G. menardii-G. cultrata complex.   Test variable in size, usually medium, a medium-height trochospire with 4-6 subsphesize as added, sutures depressed. Aperture a wide and deep opening directly into umbilicus.
Remarks: A few individuals with a lower trochospire and more umbilical aperture were N. eggeri (Rhumbler, 1901). Saito et al. (1981) separate these species on the basis of a higher spire in N. dutertrei, but Kennett and Srinivasan (1983) suppose that N. eggeri is a junior synonym of N. dutertrei. We tend to agree with the latter opinion at this stage.
N. dutertrei differs from N. incompta in having a bigger and less compact test, and Distribution: Equatorial and tropical waters. A common species in our material.
Neogloboquadrina incompta (Cifelli, 1961) Figs 4D, 4G slowly increasing in size. Chambers spherical or slightly ovoid. Aperture umbilical, interiomarginal, a high arch. Wall smoothly perforated. Remarks: The species differs from G. falconensis in having a high aperture without a lip. G. bulloides has a more hispid and thicker wall than does Globigerinella calida. Distribution: A cosmopolitan species, equatorial to subpolar waters. Most abundant in the temperate Atlantic. It is also known for its association with upwelling zones (Bylinskaya et al. 2002). This species is very rare in our material.
Globigerina falconensis Blow, 1959 Figs 6A-C  (Bé & Hutson 1977;Bylinskaya et al. 2002). A single specimen was found in our material. Hofker, 1956 Figs 6D-F Remarks: This species occasionally has a distinctive pink to red pigmentation in the Late Pleistocene and younger sediments of the Atlantic (Saito et al. 1981;Bylinskaya et al. 2002). Red-coloured forms have not yet been found in surface sediments of the Indian Ocean. The species differs from Globigerinoides tenellus and Globigerinoides ruber in having a more lobulate periphery and in the absence of supplementary apertures; and from other Globigerina species in having a coarsely perforate wall. Distribution: Temperate to equatorial waters. Very rare in our material.     (Parker, 1962), sample 4.

Globigerinella calida (Parker): Saito et al.
Size variable. Low trochospiral test with about 4 initially spherical to slightly radially detached from the previous whorl and the aperture is visible from the lateral and even spiral side (appears as slit); sutures deep. The aperture is developed as a large umbilical arch. Remarks: Differs from Globigerina bulloides in loose embracing, radial elongation of less hispid wall. It also differs from juvenile trochoid Globigerinella siphonifera in having less involute chambers and a less hispid surface of the wall. Distribution: G. calida appears in most of the climatic areas from equatorial to temperate, but is never abundant in assemblages (Bylinskaya et al. 2002). This species is rare in our material. This species is very variable, with the test varying from very low trochospiral to planispiral. The test is medium to large. Chambers are spherical to ovate, rapidly enlarging in the adult whorl. Sutures distinct and depressed. The aperture appears as a low, wide symmetrical equatorial arch in the interiomarginal position. Remarks: G. siphonifera is more hispid than Hastigerina pelagica, and more tightly coiled and planispiral than Globigerinella calida. Distribution: Tropical -subtropical waters. It appears in practically all our samples, in low abundance.
Genus Globigerinoides Cushman, 1927Globigerinoides bulloideus Crescenti, 1966 Figs 7C, 7F Globigerinoides bulloideus slowly increasing in size. Primary aperture umbilical, interiomarginal, a wide arch with very thin lip; only one small supplementary aperture. Remarks: The test of this species is very similar to that of Globigerina bulloides, and it is normally characterized by a wide primary aperture (Bolli & Saunders 1985). Our specimen has an atypically small primary aperture with a thin lip, similar to that of Globigerina falconensis. However, it differs from both G. bulloides and G. falconensis in having a supplementary aperture. G. bulloideus differs from Globigerinoides tenellus in being larger and in having slowly increasing non-embracing chambers.
Distribution: Known from the Middle Miocene of Italy (Bolli & Saunders 1985). Temtwo specimens in our material.   (Thompson et al. could be considered pink varieties have an early whorl or two of light pink chambers and gradually become white in later chambers (Saito et al. 1981).
In our material we also found seven individuals that show aberrant features (Figs 9D, 9E; Table 2). Their tests are medium to large, medium-high trochospiral, very loosely last chamber is completely detached from the previous whorl and is connected with the penultimate chamber only. The test wall is spinose, strongly perforated. The primary aperture is umbilical, in the form of a broad and deep arch. Secondary apertures are smaller, semicircular or drop-shaped. This form was mentioned as G. ruber forma helicina (Saito et al. looser than illustrated in the aforementioned work. Distribution: Equatorial to temperate waters, most abundant in the tropical and subtropical areas; it is indicative of warm-water conditions. The species is abundant in our material. Globigerinoides sacculifer (Brady, 1877) Figs 8G-I Primary aperture interiomarginal, umbilical, a low but quite wide symmetrical arch; supplementary apertures elliptical or subtriangular. Remarks: The species differs from G. trilobus in the characteristic sac-like shape of the last chamber and in being generally larger. Distribution: A tropical species, it is an indicator of warm-water conditions. It appears in practically all our samples, in small numbers. Parker, 1958 Figs 7G-I

Globigerinoides tenella: Globigerinoides tenellus et al. Globoturborotalita tenella
Small trochospiral test with strongly lobate equatorial outline, with 4 globular chamrapidly enlarging. Primary aperture umbilical, an open and high arch (almost circular in outline) with a thin lip, secondary aperture(s) small and narrow, but visible on even the smallest specimens. Remarks: The species differs from G. rubescens in having supplementary aperture(s) and a smoother wall, and from G. ruber by its more lobulate periphery and low trochospire. Distribution: Equatorial to temperate waters. The species is never numerous in assemblages, and very rare in our material.
Globigerinoides trilobus (Reuss, 1850) Figs 8E, 8F  Remarks: Differs from G. sacculifer by its globular last chamber, and from G. ruber in the general chamber arrangement and the form of the primary aperture. Distribution: A tropical species, indicative of warm-water conditions. It appears in practically all our samples, in low abundance.
Key to the species of Globigerinoides from the Thukela Shelf Remarks: This species demonstrates a very wide variation in the size and shape of the test, and in the development of apertures. Distribution: Equatorial to tropical waters, warm-water species. We found three specimens in our material.

Globigerina bilobata
Test large, spherical. Wall very thin, coarsely perforated with pores of two distinct sizes.
Remarks: This species is very distinctive in having two life stages, viz. a multichambered trochospiral stage followed by a single-chambered spherical stage (Bé et al. 1973). However, the trochospiral form was not found in our material. Distribution: Equatorial to temperate waters. Exceptionally abundant in our material.  (Bé & Hutson 1977) (Table 2), the assemblage of planktonic foraminifers in surface sediments on the Thukela Shelf is as rich as assemblages from the south-western continental margin of the whole of southern Africa. Giraudeau (1993) reported 23 species off South Africa and Namibia, a region which includes two different biogeographic zones, i.e. transitional and subtropical (Bé & Hutson 1977).
Although all our samples represent shallow-water sediments, species with very different depth preferences have been found. The studied assemblage is characterized by a high proportion of Globigerinoides ruber (>20 %), with G. trilobus, G. sacculifer and G. conglobatus being less abundant. These species prefer the upper part of the euphotic zone (Venéc-Peyré et al. 1995;Schiebel et al. 2004). Although Globigerina quinqueloba and G. rubescens are known as surface species as well, they are very rare in our material. Deep-water species (Globorotalia menardii, G. tumida, , and Sphaeroidinella dehiscens) are generally exceptionally scarce, with the exception of Neogloboquadrina dutertrei that reaches over 11 % in the assemblage. Orbulina universa, Globigerinella siphonifera, Globigerina bulloides, Globigerinita glutinata, and Pulleniatina obliquiloculata do not show a strong preference for particular depths within 300 m of water column (Bé & Tolderlund 1971).
The taxonomic diversity of the planktonic foraminifers generally increases toward the outer shelf across the Thukela Shelf. This is hardly surprising, as these unicellular organisms prefer normal marine conditions.
In general, the planktonic foraminiferal assemblage of the Thukela Shelf agrees with the distributional pattern of species drawn by Bé and Hutson (1977), with several Globigerinella siphonifera is approximately the same as expected, whereas the proportions of Globigerinita glutinata, Globigerinoides sacculifer and are slightly lower than those shown by Bé and Hutson (1977), although these authors analysed a different size fraction. These variations are most probably of little value, since the distribution of planktonic forams often demonstrates patchiness (Boltovskoy 1971).
The most striking oddity is a proportion of O. universa, which constitutes over 32 % of the total Thukela Shelf assemblage. Earlier researchers (Bé et al. 1973;Bé & Hutson 1977) reported much lower occurrences of this species east of South Africa, with its share not exceeding 5 % in sediment ("fossil") assemblages. O. universa is considerably more abundant (up to 10 %) in live plankton assemblages east of Kenya and south of Madagascar, and is believed to be associated with water temperatures between 18 and concentration (ca 0.3 μg/l) (Bé et al. 1973;Bé & Hutson 1977).
O. universa to the KwaZulu-Natal waters. On a smaller scale, O. universa is substantially  The Agulhas Current may also be responsible for the appearance of Globorotalia menardii (Figs 3B,3E) in sediments on the Thukela Shelf, although as isolated specimens. This species occurs predominantly in the equatorial region, the Arabian Sea, the Gulf of Aden, and the Bay of Bengal (Bé & Hutson 1977), but may be transported by currents over large distances (Schiebel & Hemleben 2005).