During Ocean Drilling Program (ODP) Leg 175, the Lower Congo Basin (LCB) off west Africa was the target for three drilling sites (1075, 1076, and 1077) along a transect on the northern rim of the Congo Fan (Shipboard Scientific Party, 1998a). Here, one of the largest rivers in terms of freshwater discharge joins an oceanic high-fertility area. The regional environment is dominated by seasonal coastal upwelling and associated filaments and eddies moving offshore, by riverine input from the Congo River, and by incursions of open-ocean waters, especially from the South Equatorial Countercurrent (Fig. F1). Thus, the combination of pelagic and terrigenous information contained in these fan-margin deposits provides an excellent opportunity for studying simultaneous climatic changes on land and sea.
One of the goals of Leg 175 included the reconstruction of the history of productivity off Angola and Namibia and the influence of the Congo River, thereby extending available information about the late Quaternary (e.g., Schneider et al., 1994, 1996, 1997; Jansen et al., 1996, and references therein) to earlier periods (Shipboard Scientific Party, 1998a). According to Jansen (1985), river-induced phytoplankton activity extends ~160 km beyond the shelf edge and would affect all three sites drilled. However, it is also evident from these earlier studies that major productivity changes off the Congo are determined by wind forcing and oceanic subsurface nutrient supply rather than merely reflecting fertility changes induced by river discharge of nutrients. In an elegant work using oxygen and carbon isotope data of two planktonic foraminifers, Schneider et al. (1994) showed that off the Congo, ocean dynamics have overwhelmed the influence of one of the world's largest rivers on marine coastal productivity over the past 190 k.y. This is contrary to other areas off major rivers where a strong freshwater signal was described (Pastouret et al., 1978; Showers and Bevis, 1988).
High opal content characterizes the Congo Fan sediments (Müller and Schneider, 1993) where diatoms dominate. Schneider et al. (1997) suggested that enhanced opal production in this region was the result of additional fluvial supply of dissolved silica during humid climates characterized by more intense chemical weathering on the continent, whereas total paleoproductivity created by oceanic upwelling was high in periods of increased zonal trade wind intensity at precessional insolation minima and during cold, more arid glacial climate conditions.
Here, we present a record of siliceous microfossil paleoproductivity spanning the last 460 k.y. from upper Pleistocene sediments of ODP Site 1077. By using accumulation rates and the relative contribution of siliceous components of both continental origin (freshwater diatoms, phytoliths, and chrysophycean cysts) and marine origin (marine diatoms, silicoflagellates, and radiolarians) in combination with organic matter and opal data from nearby sites (ODP Site 1075 and GeoB1008), we aim to reconstruct temporal fluctuations in the Congo River freshwater outflow, coastal upwelling, and open-ocean contributions to the dynamics of the region. Our findings are integrated with and compared to those of previous studies from nearby sites in the Congo Fan and off Angola (e.g., Mikkelsen, 1984; Jansen et al., 1984; van Iperen et al., 1987; Jansen and van Iperen, 1991; Schneider et al., 1994, 1995, 1997; Gingele et al., 1998).