REGIONAL SETTING

Hydrography

Detailed studies of modern hydrography in the eastern Angola Basin and the Congo Fan area are given by Eisma and van Bennekom (1978), van Bennekom and Berger (1984), and Schneider et al. (1995). The complex hydrography of the area involves different water masses with characteristic physical and biological properties.

In the eastern Angola Basin (0-20S), the surface and shallow subsurface circulation is dominated by the Angola Current (AC) and the Benguela Coastal Current (BCC). The AC flows southward along the African coast and is fed by the eastward-flowing, shallow-subsurface warm South Equatorial Countercurrent (SECC). The BCC transports cold, nutrient-rich waters northward across Walvis Ridge. The two currents converge between 14S and 16S, depending on the season, and form the Angola Benguela Front (ABF) (Fig. F1). In a zone between 10S and 16S, the interaction between the SECC, AC, and BCC creates a complicated pattern of fronts, gyres, and thermal domes (e.g., Angola Dome), bringing nutrient-rich shallow subsurface waters into the euphotic zone (oceanic upwelling). North of the ABF, the BCC can be traced as a shallow subsurface current to 5S (van Bennekom and Berger, 1984). The ABF delineates the northern boundary of the zonally directed tradewind field. North of the ABF, winds weaken and change to a meridional direction (summary in Schneider et al., 1995). Coastal upwelling is restricted to two narrow areas north and south of the Congo estuary and is considered to be the result of upwelling of colder waters of the Equatorial Undercurrent.

Superimposed on the system described above is the influence of the Congo River and fan area, which supplies freshwater, nutrients (including large amounts of dissolved SiO2), and sediments to the ocean. The Congo River is the second largest river in the world. It has a peculiar estuarine hydrography caused by the small river mouth, which includes the canyon head (Jansen, 1984). This forces a rapid outflow of river water toward the ocean in a sharply bounded turbid surface layer 5-15 m thick, which also entrains subsurface oceanic waters rich in phosphate and nitrate (Eisma and van Bennekom, 1978). The plume of Congo water, characterized by reduced surface-water salinity, can be detected as far as 800 km offshore during austral summer, when monsoonal circulation and precipitation reach their maximum seasonal intensity (Eisma and van Bennekom, 1978; van Bennekom and Berger, 1984). Salinity is <30 in the inner plume area. Farther offshore (150-200 km from the river mouth), the plume broadens, salinity rises to ~30, and maxima in primary production (river-induced upwelling) (van Bennekom and Berger, 1984), diatom cell numbers (Cade, 1978, 1984), and diatom accumulation rates in the sediments (van Iperen et al., 1987) are found.

As a result of coastal, oceanic, and river-induced upwelling, modern primary productivity is very high in the surface waters off the Congo; Berger et al. (1989) gives values of 90-125 gC/m2/yr. Data on biogenic silica production indicate high diatom productivity in the surface waters surrounding the central Congo plume, accounting for 40%-60% of the total carbon productivity (van Bennekom and Berger, 1984).

Sediments

Sedimentation within the LCB is dominated by rainout of suspended clay derived from the Congo River and by pelagic settling of biogenic debris. Since the Congo River drops most of its coarse load before reaching the ocean (unlike other river-influenced hemipelagic systems), LCB sediments lack a significant river-borne sand and silt fraction (Jansen et al., 1984). Wind-derived silt is minimal in relation to the amount of river-deposited clay (Jansen et al., 1984; van der Gaast and Jansen, 1984). Kaolinite and smectite are the most important clay minerals in the surface sediments of the Congo Fan (van der Gaast and Jansen, 1984; Gingele et al., 1998). Other important terrigenous components include fresh- and brackish-water diatoms, plant remains, and phytoliths. The marine biogenic components include marine diatoms (dominating), coccoliths, planktonic and benthic foraminifers, silicoflagellates, radiolarians, dinoflagellate cysts, ebridians, and sponge spicules (e.g., Jansen et al., 1984; Mikkelsen, 1984; van Iperen et al., 1987; Jansen and van Iperen, 1991).

Site 1077 (510S, 1026E) is the intermediate-water drill site on a depth transect in the LCB, located in 2382 m water depth at the northern edge of the Congo River plume (Fig. F1). Three holes (Holes 1077A, 1077B, and 1077C) were cored with the advanced hydraulic piston corer to a maximum depth of 205.1 meters below seafloor (mbsf), which recovered a continuous hemipelagic sedimentary section spanning the entire Pleistocene. Sediments are dominated by diatomaceous, partially carbonate-bearing clays (Shipboard Scientific Party, 1998c). Sedimentological evidence suggests that Site 1077 is not affected by turbidity currents (Pufahl et al., 1998). The sediment composition of ODP Site 1075 does not differ significantly from Site 1077 and consists entirely of greenish gray diatomaceous clay and nannofossil-bearing diatomaceous clay (Shipboard Scientific Party, 1998b.

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