The southern Benguela region, which extends along the western flank of southern Africa from the Orange River mouth to the Cape of Good Hope, is one of the most complex hydrological systems of the South Atlantic. There, the nature of surface and intermediate waters is under the influence of three distinct processes (e.g., Shannon, 1985): (1) upwelling of cold, nutrient-rich South Atlantic central water from the combination of the equatorward Benguela drift and the prevailing southerly winds, (2) input of warm, salty Indian thermocline waters and rings, which are shed from the Agulhas Current retroflection area, and (3) variable admixture of Southern Ocean waters induced by instabilities in the region of the Subtropical Convergence (Fig. F1). These last two phenomena may allow the transfer of allochthonous planktonic fauna from Indian subtropical and Atlantic subantarctic water masses to a region otherwise characterized by populations adapted to the coastal upwelling regime.
Exploring the Neogene and Quaternary history of these Indian and subantarctic surface-water advections and, ultimately, assessing the implications for the glacial-interglacial heat balance between the South and the North Atlantic were the primary objectives for drilling at Site 1087. This site, located in the southernmost area of the Cape Basin in 1371-m-deep water (31°28´, 15°19´E), is presently affected by wind-induced upwelling that carries a clear seasonal pattern, with the maximum upwelling occurring during spring and summer (Lutjeharms and Meeuwis, 1987). This seasonal pattern is determined by the combined seasonal latitudinal shifts of the South Atlantic high-pressure system and east-moving cyclones in the south. The seasonal nature of upwelling in the southern Benguela region most probably accounts for the overall lower phytoplankton biomass in surface waters (Brown et al., 1991) compared to the situation in the north, where upwelling is perennial. Sediments over the continental margin off the Olifants River, where Site 1087 was drilled, are therefore carbonate rich and organic carbon and opal poor (Rogers and Bremner, 1991), accumulating at an average rate of 2 to 7 cm/k.y., compared to 10 to 20 cm/k.y. off Namibia (Giraudeau et al., 1998). Site 1087 recovered a relatively continuous pelagic section to 430 meters below seafloor (mbsf), spanning the last 9 m.y.
The generally high abundance and good preservation of calcareous microfossils contained in sediments at Site 1087, coupled with recent studies that document the reliability of planktonic foraminifers as tracers of present surface-water masses in the southeast Atlantic (Giraudeau, 1993; Niebler and Gersonde, 1998), indicate that the downcore distribution of species assemblages may allow for precise reconstruction of surface circulation changes around the southern tip of Africa.
Here we report on census counts of planktonic foraminifers at Site 1087 that are representative of the last four glacial-interglacial cycles. High-resolution sampling coupled with benthic stable isotope stratigraphy provide the necessary data to reconstruct the evolution of the southern Benguela region and adjacent regions throughout the last 460 k.y.