The Lower Congo Basin (set in the northeast corner of the Angola Basin) was the target for three drilling sites (1075, 1076, and 1077), along an east-west transect on the northern rim of the Congo Fan, off the Congo River. The regional environment is dominated by three major influences: (1) the freshwater input from the Congo River (the second largest river in the world), (2) seasonal coastal upwelling and associated filaments and eddies moving offshore, and (3) incursions of open-ocean waters, especially from the South Equatorial Countercurrent (Fig. 1). According to Jansen (1985), river-induced phyto-plankton activity extends ~160 km beyond the shelf edge, which would affect all three sites drilled, where this influence should be seen in high opal content and low-salinity diatoms (among other indicators). River-induced productivity, however, may not be the most important influence. Instead, divergence and doming, as well as cyclic interactions between the South Equatorial Countercurrent and the Benguela Current, may be the dominant factors. In the site closest to shore, the effects of seasonal coastal upwelling should be superimposed on the riverine effects.
We expect a close tie-in of climatic records from the continent and the ocean in this area. In these fan-margin deposits, the intercalation of pelagic and terrigenous information provides an excellent opportunity for studying simultaneous climatic changes on land and at sea. Previous work on Pleistocene sediments in the region (e.g., Jansen et al., 1984; Olausson, 1984; van der Gaast and Jansen, 1984; Zachariasse et al., 1984; Jansen, 1985, 1990; Jansen, et al., 1986, 1989; Jansen and van Iperen, 1991; Schneider et al., 1994, 1996, 1997) have shown that excellent records with high time resolution can be expected.
These previous investigations have clearly demonstrated that sediment records from this region reveal changes not only of the general climatic and oceanographic processes but also reflect variations based on local features, such as coastal or oceanic upwelling, river discharge, surface waters from equatorial or Southern Ocean source regions, as well as prevailing monsoonal or trade-wind influence. For example, export flux or preservation of organic carbon was enhanced during glacials in all areas along the continental margin, reflecting increased surface-water productivity under cold climate conditions off southwest Africa. In contrast, productivity records from the Angola Basin north of the Walvis Ridge reveal a strong 23-k.y. periodicity that is coherent with precessional orbital forcing, which diminishes at the Walvis Ridge (see TOC peaks at 85 and 110 k.y. in Fig. 2). This apparent decline in the 23-k.y. periodicity reflects the southward-decreasing influence of the West African Monsoon system, which modulates trade-wind zonality in step with boreal summer insolation changes and, consequently, the wind-forced coastal upwelling and open-ocean shallowing of the thermocline.
It is also evident that major productivity changes off the Congo follow the pattern determined by wind-forcing and oceanic subsurface nutrient supply rather than merely reflecting fertility changes induced by river discharge of nutrients (Schneider et al., 1994). The Congo Fan sediments are characterized by exceptionally high opal content (Müller and Schneider, 1993), about 10 times higher than that in the slope sediments recovered from elsewhere on the southwest African margin. Only off the Kunene River and on the shelf beneath the coastal upwelling area of Walvis Bay (23°S) does the opal content reach values as high as those in the Congo Fan sediments. Although these high opal values in the Congo Fan presumably are related to riverine input of silicate and to river-induced estuarine upwelling (in addition to a possible role of aluminum in enhancing preservation; van Bennekom, 1996), the opal fluctuations appear controlled by large-scale climatic patterns, judging from the presence of Milankovitch cycles (Fig. 3).
The three sites drilled in the transect allow us to reconstruct the changing influence of Congo River, coastal upwelling, and open-ocean contributions to the dynamics of the region. The influences of these competing subsystems will respond differently to forcing by the dominant climatic cycles (100, 41, 23, and 19 k.y.). The relative strength of the main climate controls (e.g., sea level and general planetary temperature gradient compared with cold water advection and monsoonal variations) should be revealed by spectral analysis. It will be interesting to see how these various controls modify continental climate, particularly within the Congo drainage basin.