SCIENTIFIC OBJECTIVES

The results from DSDP Sites 362 and 532 suggest that there has been a general northward migration of the Benguela Current upwelling system during the last 14 m.y. Because the shape of the South Atlantic Ocean has not changed appreciably during this time, the changes in the upwelling system must reflect large-scale, perhaps global, changes in ocean circulation. Leg 175 focused primarily on the paleoceanographic and paleoclimatic aspects of the area. How-ever, there is interest in investigating samples from the upwelling area off Angola and Namibia with regard to early diagenetic processes taking place in this unique environment. Possible work includes study of the formation of dolomite (Baker and Kastner, 1981; Kulm et al., 1984; Kelts and McKenzie, 1982, and other articles in Garrison et al., 1984) and phosphorite (Calvert and Price, 1983). We also hope to examine the organic matter type and distribution as a function of time and climatic cycles.

Important goals of Leg 175 include the following:

  1. Determine the history of the Benguela Current for the late Neogene. Of special interest is the changing response to orbital forcing, as seen in spectral amplitudes and phase relationships of various proxies (e.g., McIntyre et al., 1989; Schneider, 1991; Berger and Wefer, 1996a, 1996b; Jansen et al., 1996; Schneider et al., 1996; Wefer et al., 1996).
  2. Study the history of upwelling productivity off Angola and Namibia and the influence of the Congo River, thereby extending available information about the late Quaternary (Bremner, 1983; Jansen et al., 1996) to earlier periods. The history of opal deposition off the Congo River is of interest (Schneider, 1991), as well as the origin of cycles of carbonate, organic matter deposition, and diatoms in each region.
  3. Determine what kind of oceanographic changes (regarding the Agulhas Current, polar front position, Equatorial Current, and Brazil-Argentine Currents) occur simultaneously with the shifting of the Benguela Current. Quaternary studies, in conjunction with results from physical oceanography (Wefer et al., 1996), can help refine the nature of such shifts. Results from Ocean Drilling Program (ODP) Legs 108, 114, and 177 are useful in defining the past equatorial and polar boundaries of the Benguela Current. The final aim is to reconstruct the late Neogene paleocirculation pattern of the South Atlantic Ocean to evaluate implications for the glacial/interglacial heat balance through time between the South and North Atlantic Oceans. Of special interest is the identification of changes in modes of circulation, as seen in changes in correlations between proxy variables, as a function of time.
  4. Assess the relationships between changes in the ABC-system and changes in climates of western South Africa. For example, how is the origin of the Namib Desert related to the initiation and intensification of upwelling off southwest Africa? Sites close to the continent probably contain enough information (clay minerals, grain size of terrigenous material, pollen, phytoliths, and freshwater diatoms) to allow reconstruction of continental climatic changes and to determine whether these changes are synchronous with oceanographic changes.
  5. Examine the effect, if any, of sea-level changes on sedimentation below the Benguela Current. Published eustatic sea-level curves (Haq et al., 1987) will be useful for this purpose.
  6. Study early diagenetic processes in environments with very high organic carbon and opal contents for comparison with studies undertaken during Leg 46 in the Gulf of California (Curray, Moore, et al., 1982) and Leg 112 off Peru (Suess, von Huene, et al., 1990). The sediments deposited below the upwelling areas off the Peru margin are deposited in forearc basins in a disturbed tectonic setting, whereas sedimentation off Angola and Namibia occurs on a steadily sinking passive margin with quite stable conditions.

These goals guided the planning of Leg 175. On 12 August 1997, the JOIDES Resolution set out from Las Palmas (Canary Islands) with a course for the first site (1075) off the Congo River. During the 57-day expedition, we occupied 13 sites (Fig. 1) and drilled 40 holes. Overall penetration totaled 8210.5 m, with 8003.2 m of recovery (Table 1). On 9 October 1997, the vessel entered the port of Cape Town, South Africa, ending the leg.

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