Calcareous nannoplankton assemblages in sediments and their distribution within the KwaZulu-Natal Bight. M. N. Ovechkina1,2, R. Uken1, M. B. Mostovski2,3 1School of Geological Sciences, University of KwaZulu-Natal, P. Bag X54001, Durban, 4000 South Africa 2A.A. Borissiak Paleontological Institute, RAS, 123 Profsoyuznaya Str., Moscow, 117997 Russia 3KwaZulu-Natal Museum, P. Bag 9070, Pietermaritzburg, 3200 South Africa and School of Biological & Conservation Sciences, University of KwaZulu-Natal, P. Bag X01, Scottsville, 3209 South Africa; mmostovski@nmsa.org.za Introduction Coccolithophores are marine unicellular golden-brown algae, which produce and bear minute calcium carbonate plates called coccoliths. Coccoliths make an important contribution to translocation of the inorganic carbon produced in pelagic areas to the ocean floor and thus to the sedimentary archive. Coccolithophores form the base of the oceanic food web, and coccoliths are extremely valuable for stratigraphic and paleoceanographic purposes, especially in reconstructing the past productivity in the ocean. Due to their capacity to control ocean-atmosphere CO2 exchange, coccoliths participate actively in the formation of the climate system (Westbroek et al. 1993; Rost & Riebesell 2004). Coccolith assemblages in sediments serve as excellent proxy indicators of temperature, salinity and limiting nutrients of surface-waters in which they once lived (Roth 1994). They also monitor carbonate dissolution on the sea-floor and in the sediment and, finally, coccoliths are sensitive indicators of diagenesis in ancient rocks. Understanding the processes of sedimentation and preservation of coccoliths in Recent sediments is crucial for understanding the ancient rock record where coccoliths serve as one of the major environmental guides for the last 200 million years of ocean history. While composition and distribution of the coccoliths assemblages have been studied along the southwest African continental margin (Giraudeau 1992), little is known from the eastern continental shelf of South Africa. Material and methods From February 2010, seafloor sediment samples were collected aboard a Marine and Coastal Management marine research vessel, the F.R.S. Algoa. Twenty samples were identified for calcareous nannoplankton analysis. Samples were washed through a 63 micron sieve over a ten-litre bucket in order to concentrate the nannoplankton. Such concentration was essential, because most samples were sandy and contain very little nannoplankton. The bucket was set aside for about a week to settle nannoplankton. Water was cautiously drained out of each bucket and the residue was poured into a jar for further settling. After another week the excess water was drained again, and the nannoplankton-enriched sediment used for the preparation of smear-slides which were prepared from each sample using standard techniques (e.g. Green et al. 2008), and examined using a cross-polarized light microscope at 1250× magnification. For palaeoenvironmental reconstructions and estimation of abundance, and hence a relative productivity, over 500 Recent coccoliths, including Emiliania huxleyi, were counted for each sample, recording the number of fields of view. The number of all coccoliths calculated per one field of view was used as a proxy to estimate a relative productivity for each sample.

In this work, we favoured grouping taxa in order to offer an immediate and synthetic ecological framework. Taxa were combined, grouping species with distinctive morphologies and biogeographies, which dominate coccolithophores assemblages in particular environments (Incarbona et al. 2008). Emiliania huxley (Plate 1) which is the most ubiquitous species obscuring the variations of minor taxa, has been excluded from the quantitative analysis at this stage. After Young (1994), we recognise four groups of the calcareous nannoplankton. The Placolith group includes species of small Gephyrocapsa, G. muellerae, G. omega, G. carribeanica, G. ericsonii, and the genus Reticulofenestra. Emiliania huxleyi, which also belongs to this group, was counted separately. Gephyrocapsa oceanica, in spite of its peculiar ecological preferences, is not included in the placolith group and was also counted separately. The Umbelliform group which includes Discosphaera tubifera, Umbellosphaera spp., Calciosolenia spp., holococcoliths, Rhabdosphaera spp., Syracosphaera pulchra, and Thoracosphaera heimii are thought to have a similar affinity for warm and oligotrophic surface waters (Incarbona et al. 2008). The Floriform group includes lower photic Florisphaera profunda and very rare specimens of Gladiolithus flabellatus. Algirosphaera sp. is absent from surface sediment samples of KwaZulu-Natal Bight. The Miscellaneous group includes all the other taxa. Results In general, the taxonomic composition of the calcareous nannoplankton assemblage is richer than the assemblage recorded by Giraudeau (1992) along the southwest African continental margin, and comprises 27 Recent species and taxonomic groups, although some species occur as isolated specimens. Across the KwaZulu-Natal, the relative overall productivity increases seaward and southward, with the highest number of coccoliths per one field of view recorded at the Thukela Mouth (77 at site 73) and at the Durban Eddy (71 & 72 sites 48 & 28 respectively) focus areas. Emiliania huxleyi is the most abundant species and constitutes 30.9–49.4% of the assemblage. E. huxleyi is a cosmopolitan and highly eurytophic species, and does not convey much palaeoenviromental information. Florisphaera profunda is the second most abundant and ecologically important species, constituting 13.9–63.8% of the assemblage (after the exclusion of E. huxleyi). Another lower photic species, Gladiolithus flabellatus, is very rare. F. profunda normally dwells in the lower photic zone and plays a key role as an indicator of past productivity and upwelling (Ahagon et al. 1993; Guptha et al. 2005; Mergulhao et al. 2006). According to Okada (1983, 1989) the relative abundance of F. profunda is usually just a few percent in shelf sediments but may reach 70% on continental slopes and in deep basins. The relative abundance of this species in the south-eastern Indian Ocean is very high, reaching about 50–70% (Takahashi, Okada 2000, 2001). A relatively low abundance of F. profunda is characteristic of the Thukela Mouth focus area and near-shore sites of the Richard Bay South and Richard Bay North focus areas. Offshore sites of the Richard Bay South and Richard Bay North focus areas are characterised by medium abundances of this species, while most the northern parts of the Richard Bay North and Durban Eddy areas show the highest percentages of F. profunda. The umbelliform taxa demonstrate a more patchy distribution. On average, they are more abundant seaward, except for the offshore part of the Richard Bay South area (sites 102 & 109) and central part of the Richard Bay North area (site 150), and less abundant at near-

shore sites, with the exception of sites 81, 120 & 165. Such distribution may reflect a complex combination of temperature and trophic conditions across the study area. There are also a number of reworked species. They are either broadly Upper Cretaceous Chiastozygus litterarius, Eiffellithus turriseiffelii, Cribrosphaerella ehrenbergii, Zeugrhabdotus sp., Zeugrhabdotus spiralis, Helicolithus trabeculatus, Watznaueria barnesae, Prediscosphaera cretacea, Pr. sp., Calculites ovalis, Microrhabdulus decoratus, Gartnerago obliquum, Staurolithites imbricatus, or confined to narrow stratigraphic intervals (Santonian–Maastrichtian Arkhangelskiella specillata, A. cymbiformis, A. confusa and Upper Maastrichtian Lithraphidites quadratus). There are also some Cenozoic taxa, viz. Eocene–Miocene Sphenolithus moriformis and Neogene Discoaster sp. The reworked taxa probably originate from sediments outcropping closer to the shore. ACKNOWLEDGEMENTS The staff of the Microscopy Unit of the University of KwaZulu-Natal is acknowledged for facilitating access to their equipment and generous help. MNO and MBM’s research has been funded by the NRF (via The African Coelacanth Ecosystem Project and Incentive Funding for Rated Researchers). REFERENCES AHAGON, N., TANAKA , Y. & UJIIE´, H. 1993. Florisphaera profunda, a possible

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Plate 1. SEM images of calcareous nannofossils from the surface samples of the KwaZulu-Natal Bight. 1. Emiliania huxleyi, distal side; sample 87. 2. small Gephyrocapsa, distal side; sample 87. 3. small Gephyrocapsa, distal side; sample 48. 4. Gephyrocapsa oceanica, distal side; sample 87. 5. Florisphaera profunda, general appearance; sample 87. 6. Anoplosolenia brasiliensis, general appearance; sample 48. 7. Umbilicosphaera sibogae, distal side; sample 87. 8. Helicosphaera carteri, distal side; sample 48. 9. Umbellosphaera irregularis, distal side; sample 48. 10. Syracosphaera sp., proximal side; sample 87. 11. Rhabdosphaera sp., general appearance; sample 87.