Foraminifera, Radiolaria, and Calcareous Nannofossil Biofacies Succession in a Core at the Base-Of-Slope of South Brazilian Continental Margin

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FORAMINIFERA, RADIOLARIA, AND CALCAREOUS NANNOFOSSIL BIOFACIES SUCCESSION IN A CORE AT THE BASE-OF-SLOPE OF SOUTH BRAZILIAN CONTINENTAL MARGIN. Barbosa, C. F. 1,a, Eilert, V.P. 2,b, Rios-Netto, A.M. 2,b, Wanderley, M.D.2,b, Fridlund, F.E.1,c, and McManus J.F.3 1 Departamento de Geologia – Lagemar/UFF. Av. Litorânea s/n, Boa Viagem, Rio de Janeiro CEP 24.310- 240; [email protected]; 2 LabMicro/Depto de Geologia/IGEO/UFRJ; Av. Brigadeiro Trompowsky, sl. J2018, CEP:21.949-900, Rio de Janeiro, RJ 3 Woods Hole Oceanographic Institution, 121 Clark MS #23, Woods Hole, MA 02543, USA a FAPERJ Proc. no: E-26/151.104/97 b Fundação Universitária José Bonifácio (FUJB) c Coordenação de Aperfeiçoamento de Pessoal de Ensino Superior (CAPES)

Benthic and planktonic foraminifera, radiolaria, and calcareous nannofossil biofacies succession have been analyzed from samples collected at the base-of-slope in the South Brazilian Continental Margin. The studied core is located in a relatively small sedimentary sub-basin limited by the continental slope (West), the São Paulo Plateau (North), the Rio Grande Rise (East) and by a basement structural high (South), which also separates Santos and Pelotas basins (Gomes et al. 1993). These features correspond to morphological obstacles that influence both the oceanic circulation and the sedimentation in this sub-basin. The main thermohaline currents acting in Southwestern Atlantic, after Hogg et al. (1996), are: the Antarctic Intermediate Water (AAIW, 700-1100m depth), the North Atlantic Deep Water (NADW, 2000-3000m depth), and the Antarctic Bottom Water (AABW, 3500-4300m depth). In this area the superficial water masses corresponds to the Subtropical-Subantarctic Convergence Zone (Boltovskoy, 1981). One core have been described and studied in order to interpret the paleoecology of the sediment trough the analysis of foraminifera, radiolaria and nannofossil biofacies. The biofacies succession of these microfossils can help to understand the upper Holocene deposition of the base-of-slope system which, as Galloway (1998) states, is a major component of most marine basin fill and constitute primary targets for hydrocarbon exploration. Radiocarbon dating of the deposit is going to be obtained by AMS, to the bottom, middle, and upper part of the core, providing the sedimentation rate to the analyzed interval.

OBJECTIVES 1) to determine the taxonomic composition and vertical distribution of foraminifera, radiolaria, and nannofossil assemblages, for a better interpretation of the fossil record representative of this base-of- slope deposit; 2) the know the microfaunistic response, through time, to the variability in ecological parameters in this area; 3) to contribute to the understanding of the sedimentology, and circulation patterns dominant in the area during the time span represented by the studied core.

METHODS In October 1998 multicores were collected from sediment of the continental slope and base-of-slope of the South Atlantic Ocean, by the research team of the Woods Hole Oceanographic Institution (WHOI), in cooperation with LAGEMAR-UFF, on the board of the R.V. Knorr, during the cruise 159-5. The samples were collected using a multicore sampler at the station MC 58 (Fig.1). Further, in order to integrate the benthic foraminiferal research with other microorganism groups, the UFRJ research team joins in this project contributing with the knowledge of planktonic forams, radiolaria, and nannofossil groups. This integrated study that is being performed for the first time in this area, uses the material from the deepest multicore recovered during this cruise, from a depth of 4.015m. It was observed differences in compaction of the mud material, which increased from the middle to the bottom. A total of 13 samples were collected through the core at vertical intervals of 2cm, since 11cm

CUSHMAN FOUNDATION RESEARCH SYMPOSIUM - Quaternary Micropaleontology: Ecological Studies and Paleoenvironmental Applications VII Brazilian Association for Quaternary Studies (ABEQUA), - Porto Seguro, Brazil, October 03-09 1999 to 50cm. The bedding at the top was tilted due to moving when the core was removed. Therefore, the core top was not sampled because of the unconsolidated, mixed character of the sediment (Fig. 2). A set of 13 smear slides was prepared in order to observe the total calcareous and siliceous biogenic content in this sediment. The observation of benthic and planktonic foraminifera, radiolaria, calcareous nannofosils, few silicoflagellates, sponge spicules, and diatoms frustules, lead to this integrated approach. Two sub-sampling were performed, one for foraminiferal analyses and the other one for radiolaria and calcareous nannofossil preparations. The volume of each sub-sample was standardized in 10 cm3. The foraminifera sub-samples were washed trough the 63-µm mesh sieve, while for the radiolaria the preparation used the 42-µm mesh sieve.

PRELIMINARY RESULTS Quantitative data on foraminiferal fauna reveals an increasing in the relative abundance and diversity upward. Different species from Textulariina suborder occur preferably toward the top of the core. The dominance of very small planktonic specimens is remarkably noted throughout the core. The radiolarian fauna increases in diversity from the bottom to the top, though the relative abundance remains low. The species that predominate in these sediments indicate deposition in temperate to warm water conditions. The typical warm-water assemblage is composed by the surface water living colonial forms and the spongy forms. A few cold water species were also found in some intervals. The calcareous nannofossils are abundant and well preserved. The identified species are common in Quaternary sediment.

REFERENCES

BOLTOVSKOY, E. 1981. Masas de Água en El Atlántico Sudoccidental. In: Boltovskoy, D. (ED.). Atlas del Zooplancton del Atlantico Sudoccidental y Métodos de Trabajo con el Zooplancton Marino. Inidep, Mar del Plata, Argentina, p.227-237.

GALLOWAY, W. E., 1998. Siliciclastic slope and base-of-slope depositional systems: Component facies, stratigraphic architecture, and classification: AAPG Bulletin, v.82, no. 4, p. 569-595.

GOMES, P.O.; SEVERINO, M.C.G.; GOMES, B.S. 1993. Projeto LEPLAC: Interpretação integrada dos dados geofísicos do Prospecto LEPLAC-IV – Margem Continental Sul Brasileira. In: Cong. Int. SBGf, Anais do 3°…, Rio de Janeiro, v.2, p.; 1275-1280.

HOGG, N.G.; OWENS, W.B.; SIEDLER, G.; ZENK, W. 1996. Circulation in the Deep Brazil Basin. In: Wefer, G.; Berger, W.H.; Siedler, G.; Webb, D.J. (eds.). The south Atlantic: present and past

circulation. Springer-Verlag, Berlin Heidelberg, p 249-260.

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