The distribution of Recent Radiolaria in surficial sediments of the continental margin off northern Namibia

47 Species of radiolaria have been identified from 30 surface sediment samples collected along transects across the continental margin of northern Namibia between the Kunene River and Walvis Bay. From the distribution patterns of the 24 most abundant species, it was possible to identify a warm water, high salinity population and a cold water, low salinity population. The distribution patterns of each population shows a close correspondence with the known positions of the Angola Current (warm, high salinity water) and the Benguela Current (cold, low salinity water) respectively. Two other trends are apparent from the overall radiolaria distribution; dilution of the nearshore samples by terrigeneous input and a strong preference for open ocean conditions. There is no apparent correlation with upwelling.


REGIONAL SETTING
The continental margin of Namibia is a region of strong oceanic upwelling (Hart & Currie, 1960; and others) and it is situated off an extremely arid coastline from which there is low terrigerieous input (Bremner, 1976). T h e northern margin of Namibia has been subdivided by  into the Kunene Margin in the north and the Walvis Margin in the south separated by a transitional zone, the Walvis Ridge Abutment which lies between 18" 40' S and 19' 30' s.
The surface water masses over the continental margin essentially comprise two contrasting ocean current systems. The northward flowing Benguela Current (an eastern boundary current) has been described by Stander (1 964), hloroshkin et ul., (1970), Hart & Currie (1960 and others, as a slow moving (ma>.. velocity 25 cmisec.:), cool (1 2-14"C), low salinity (34.1>-35,10/oo) and relatively well oxygenated and nutrient rich water mass. The main body of the Benguela Entering the margin from the north is the warm water ( 16-1 8" C), high salinity (35.3O,100), oxygen poor ( 3 m l i l ) Angola Current. This current reaches a maximum velocity of more than 70cms~sec. off Angola although on the Kunene Margin its flow rate is reduced to 5-8 cmsisec. T h e maximum southward penetration of the Angola Current is to 23" S (Moroshkin er ul., 1970;Bremner pers. comm.).

METHODS
The samples were collected during joint University of Cape Town/Ceological Survey Marine Geoscience Unit cruises on the R.V. Thomas H. Davie. A modified van Veen grab was used in shallow waters and a short gravity corer in deeper water.
T h e sample sites were chosen to cover the width of the continental margin from close inshore to the upper slope. Six transects of five sites each lie between the mouth of the Kunene river in the north (17" 20' S) and a position (22" 15' S) midway between Cape Cross and Walvis Bay in the south. 'The position and depth of the sample stations are listed in Table 1 and are shown in Fig. 1.
The samples were prepared and the radiolaria counted using the method developed by Go11 & Bjorklund ( 1974). Species identification was carried out by scanning electron microscope and the taxonomy used was that of Nigrini & Moore (1979) in the first instance and Haeckel ( 1887) where the species is not listed by Nigrini & Moore.
Current moves offshore at the latitude of Walvis'Bay (23" S) and it is actually three offshoots of the current RESULTS that traverse the northern continental margin of Where radiolaria were present in the samples they Namibia, (Moroshkin et d., 1970).
were well preserved with little sign of dissolution.

Robson
Barren samples contained no rccognisable radiolaria fragments suggesting that selective post mortem dissolution had not occurred. The radiolaria distribution pattern on the northern continental margin of Namibia reveals a distinctly higher abundance where open ocean conditions prevail. Sea-ward of the shelf break the mean concentration of radiolaria is 40,339 specimensigm of dry sediment (calcium carbonate free) with a range from 4,572 to 153,670; samples from the shelf break to the coast have a mean radiolaria concentration of 607 with a range from 0 to 2435.  The taxonomy of the radiolaria identified in the samples is given in Table 2, the percentage of the total faun,a (all 47 taxa observed) represented b'y a species and the number of samples in which it is found are shown in parenthesis after each entry. Where the percentage presence of the species is less than 1% it is indicated by an asterisk (*). The species frequency in individual samples is listed in Table 3.
Species diversity patterns (using all 47 species identified) show high values where intermixing of the two current systems takes place indicating that contrasting radiolaria populations are being brought together. These area$, of intermixing and the consequent high species diversity (more than 11 speciesi100 specimens) are found in two regions. I n the south centred around sample 3768 and in the two northernmost transects where samples 3666, 3657, 3901, 3902 and 3908 have species diversity values in excess of 20 species1100 specimens Fice radiolaria species common on the Namibian continental margin have been recorded as abundant in the Peru and Chile current systems by Molina-Cruz ( 1977). The summed distribution patterns of these five species ( T . octacantha, A . stauraxonium, L . maritalis, S. validispiizu and C. davisiana) show some correlation with the position of the offshore components of the Benguela Currtmt, underlining the similarities of temperature and salinity of the Benguela, Peru and Chile Currents.
The zones of upwelling described by Hart & Currie ( 1960) show n o correlation with radiolaria abundance, similarly thiere is no correlation with the position of the diatomaceous mud belts or the distribution of organic matter in the surficial sediments described by

CONCLUSIONS
The low radiolaria abundance o n the shelf is not due to dilution in high porosity sand dominated sediments (there is n o correlation between radiolaria lows and high concentrations of sand sized sediments). Neither is there evidence to show that it is due to the mechanical destruction of the radiolaria skeletons by their interaction with medium to coarse grained sediments. Polychete worms are abundant o n the shelf but an examination of their feacal pellets revealed no recognisable traces of radiolaria skeletons eliminating their activities as a possible cause for the low abundances on the shelf.
Fine grain sediments are brought onto the shelf by the Kunene River in the north and by windblown material from the arid coastline. It is thought that the low concentration of radiolaria in the shelf samples is due partly to their dilution by these terrigeneous sediments especially on the Kunene Margin, whilst on the Walvis Margin the dilution is mostly caused by the high concentrations of diatoms and carbonate microfossils. The most important factor in radiolaria distribution in this area seems to be their predilection for open ocean conditions.
Comparisons between the plotted distributions of the 24 most numerous species and the current systems active o n the continental margin of Namibia frequently show excellent correlations with their known temperature and salinity preference (taken from the literature). In this way it is possible to characterise the Angola and Benguela Currents by the species composition of their radiolaria assemblages as found in the surficial sediments (Fig. 2). Samples dominated by warm water species lay beneath the Angola Current and cold water species dominated the sediments beneath the Benguela Current.

Robson
Eight radiolaria species predominate in the sediments hexugonutum, Cenosphaeru spp. and Phormospyris beneath the Angola Current and can therefore be stabilis scaphipes. This population is best typified by the associated with warm, high salinity, oxygen poor water. distribution pattern of Ommatartus tetrathalamus and is These are Ommatartus tetrathalamus, Spongocore given that title in Fig. 2. These species are illustrated on puella, Hexacontium enthacanthum, Curpocanarium Plate 1. papillosum, Axoprunum stauraxonium, Eucyrtidium   Five radiolaria species preferentially occupy the ACKNOWLEDGEMENTS Benguela Current and are therefore associated with cool, Advice was received from Prof. R.V. Dingle, Dr G. F. low salinity, oxygen rich water. These five species are Birch, Dr Kjell Bjorklund of Bergen University and Spongotrochus venustum, Stylodictya validispinu, from Dr Mike Bremner whose published work on the Cornutella profunda, Tetrapyle octacantha and Lithosediments of the Namibian continental margin provided melissa setosa. Spongotrochus venustum best defines this nearly all the background information for the present population and it is so identified in Fig. 2. The thirteen study. species characterising these two populations are illustrated in Plate 1.   Table 3. Percentage ot cach species per sarnple. ( R = rare 1.e. less than 2 % )