GEOGRAPHIC SETTING

Site 1079 was drilled outside the Angola Bight on the upper continental slope during Leg 175 (Wefer, Berger, Richter, et al., 1998); it is located in the eastern Mid-Angola Basin (1155.78S, 1318.54E; water depth = 749 m) (Fig. F1). Upwelling here is seasonal and strongly related to the Angola Thermal Dome (AD), which pumps cold nutrient-rich waters upward (denoted by cyclonic surface circulation off Angola in Fig. F1). The dome forms as a result of the interaction between the Benguela ocean current (BOC) (Fig. F1) and the South Equatorial Counter Current (SECC). A river plume (and associated estuarine pumping) in combination with eastern equatorial upwelling generates high productivity off the Congo, and the three fields (dome, river plume, and upwelling) are thought to merge in the sedimentary record. Productivity in the surface waters off the Angola Bight is somewhat lower than in the adjacent upwelling areas to the north and south, according to satellite information and based on previous geologic studies (Berger and Wefer, 1996; Shipboard Scientific Party, 1998a), which would put both core GeoB1016 and Site 1079 into a region of reduced production. The site was drilled to obtain information on Angola margin sedimentation where it is neither influenced by riverine input nor sustained coastal upwelling activity. Penetration was limited for safety reasons; recovery of sediments was through advanced piston coring (maximum penetration = 128.3 mbsf).

At present, the Angola-Benguela Front (ABF) (Fig. F1) is located south of Site 1079. For most of the past 200 k.y., this region of increased production may have been much closer to Site 1079 or may have even moved across its latitude toward a more northerly position (Jansen et al., 1996). These apparent movements of the ABF display a periodicity somewhere near 30 k.y. (fig. 4 of Jansen et al., 1996). Jansen et al. (1996) used spectral analysis to identify a strong 15-k.y. signal, which they relate to the sum frequency of the 23- and 41-k.y. cycles. One task in reconstructing the productivity of Site 1079 is to check the possibility that migrations of the front across the site resulted in cyclic productivity compatible with the periods identified by Jansen et al. (1996).

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