Sites 1078 and 1079, within the Bight of Angola, were chosen as part of a transect planned to provide information on "background" sedimentation that is unaffected by coastal upwelling for the latest Neogene, between the high-productivity regions to both the north and south. The region is characterized by salt tectonics and is being explored for hydrocarbon potential, especially to the north. Permission to drill other sites along the planned transect was thus denied because of safety concerns. The two remaining shallow sites, being very close to shore, are not typical for "background sedimentation."
Earlier studies (e.g., Wefer et al., 1988) established that compared with the adjacent upwelling areas, sediments from this region indicate low primary productivity in overlying waters. Unlike off northern Angola, where the estuarine dynamics of the Congo are dominant, and off Namibia, where upwelling is strong year-round, upwelling of subsurface waters is seasonal here and relatively weak. Thus, the influence of the open ocean is more pronounced at Sites 1078 and 1079 and provides a tie-in of coastal-ocean history to the record of the pelagic environment.
The region is part of a large complex of fronts, gyres, and thermal domes, which reflect the interactions of the trade-wind-driven South Equatorial Current and the eastern boundary currents along southwest Africa: the Angola and Benguela Currents. In analogy to the eastern tropical Pacific, the interactions between these currents result in open-ocean and coastal upwellings over large areas within a highly productive region (van Bennekom and Berger, 1984; Jansen and van Iperen, 1991). Within that complex, the sites selected for drilling are situated in an island of relatively low productivity (Fig. 1).
The general level of primary productivity in the Eastern Angola Basin is between 125 and 180 g C/m2/yr, with the higher values occurring close to shore (Berger and Wefer, 1991). Silicate values are low in the subsurface Benguela Current, which flows northward and feeds the oceanic upwelling area off Angola. The low silicate supply from subsurface waters results in relatively low opal production rates, despite the high primary production, with high organic carbon export. This export relies mainly on the contributions from cocco-lithophores and dinoflagellates, which characterize the phytoplankton community in the oceanic upwelling area off Angola (Hentschel, 1933; Shannon and Pillar, 1986).
A core taken near the drill sites (Geosciences Bremen [GeoB] 1016-3; water depth 3441 m; Wefer et al., 1988) shows paleoproductivity changes related to precessional cycles. These cycles influence upwelling via trade-wind-forced shallowing of the thermocline and nutricline and by changing advection of cold Benguela Current waters from the south (Schneider et al., 1997). Lack of silica, as at present, has prevented significant accumulation of opal in these sediments for at least the last 300,000 yr. As a result, fluctuations (such as might be expected from glacial/interglacial cycles) are greatly subdued, despite considerable variation in the abundance of total organic carbon (Fig. 2). Schneider et al. (1997) conclude from this evidence that during this time period, the Namibian coastal upwelling cell has never made its influence known this far north.
We expect a close tie-in of climatic records of coastal and open ocean in this area, allowing us to study the cross-correlations of climate-driven ocean dynamics across these two regimes. Previous work on Pleistocene sediments in the region (Schneider et al., 1997) has shown that an excellent record with good time resolution can be expected for the purpose of relating pelagic stratigraphy to the record of the coastal ocean. One of the intriguing aspects of this record is the low opal content associated with high organic matter accumulation. This paradox indicates a strong influence of the quality of subsurface waters, which is set elsewhere in the system, possibly in the Subtropical Convergence Zone. Other topics of importance are the control of variation by precession, reflecting the changing dominance of trade-wind and monsoonal effects.