INTRODUCTION

The Congo River Basin is one of the largest drainage basins in the world. The materials deposited by its main rivers provide the opportunity to study the products of denudation of a large fraction of the upper continental crust of the African continent (Fig. F1). This report characterizes different phases carried by the Congo rivers and deposited into the Lower Congo Basin by using magnetic properties and high-resolution X-ray fluorescence (XRF) core-scanner measurements as proxies for the changing sediment input. The depositional system at Site 1075 in the Lower Congo Basin includes the sediment-producing drainage basin of the Congo River with processes that include river transport, redistribution (including possible intermediate storage on the shelf), and marine processes (Jansen et al., 1984; Olausson, 1984; Jansen and van Iperen, 1991; Wefer et al., 1996; Schneider et al., 1994, 1996, 1997; Berger et al., 1998a, 1998b). Marine factors include primarily productivity patterns of both the open ocean and the coastal ocean as well as the changing carbonate chemistry of deep waters. The production of sediment in the Congo Basin is largely a function of the wetness of the climate on the African continent (Jansen, 1990). Ocean productivity is a function of winds, which in the tropical Atlantic depend on monsoonal amplitudes and the nutrient supply within the thermocline waters.

Site 1075 was drilled during Ocean Drilling Program (ODP) Leg 175 (Wefer, Berger, Richter, et al., 1998) in the equatorial Atlantic Ocean (447.12S, 104.50E). The site is located in 2995-m-deep water in an environment dominated by sediment input from the Congo River, seasonal coastal upwelling and associated filaments and eddies moving offshore, and incursions of open-ocean waters from the South Equatorial Countercurrent.

Three holes were cored with the advanced hydraulic piston corer to a maximum depth of 207.2 meters below seafloor (mbsf), which recovered a continuous Pleistocene to late Pliocene-age interval (Wefer, Berger, Richter, et al., 1998). A spliced record using sediment cores from all three holes was constructed and yielded a complete stratigraphic sequence with no significant coring gaps. The recovered materials are cyclic deep-sea sediments, which consist of greenish gray diatomaceous clay and nannofossil-bearing diatomaceous clay. Sedimentation rates for the recovered sequence are high and average 100 m/m.y., which permits the determination of records with excellent temporal resolution.

High-resolution shipboard measurements of physical properties such as magnetic susceptibility (Fig. F2), color reflectance, and gamma-ray attenuation from Site 1075 material vary on orbital time scales and show an excellent correlation to the global oxygen isotope curve (Berger et al., 1998a). Magnetic susceptibility has been frequently compared to commonly known paleoclimate indicators such as calcium carbonate percentages and oxygen isotopes in deep-sea sediments (e.g., Bloemendal et al., 1988; Bloemendal and deMenocal, 1989; Kukla et al., 1988).

We performed detailed rock magnetic measurements on 218 discrete samples from the composite section of two depth intervals at Site 1075 (Fig. F2). The younger interval was sampled between 14 and 32 meters composite depth (mcd) (0.09-0.21 Ma) and the older between 141 and 163 mcd (1.31-1.54 Ma) at a resolution of 20 cm, which represents a temporal resolution of 2.0 and 1.3 ka, respectively. These intervals were also measured with an XRF core scanner (Jansen et al., 1998) at 5-cm resolution to obtain the geochemical composition of the sediment.

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