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 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 will focus primarily on the paleoceanographic and paleoclimatic aspects of the area. However, 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), phosphorite (Calvert and Price, 1983), and chert (see articles in Garrison et al., 1984). We will also be able to examine the organic matter type and distribution as a function of time and climatic cycles. Important questions that are being addressed by Leg 175 include the following:

• 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 (e.g., McIntyre et al., 1989; Schneider, 1991; Berger and Wefer, 1996; Jansen et al., 1996; Schneider et al., 1996; Wefer et al., 1996).

• Study the history of productivity of the upwelling region off Angola and Namibia and the influence of the Zaire River, extending available information about the late Quaternary (Bremner, 1983; Jansen et al., 1996) to earlier periods. The history of opal deposition off the Zaire River is of interest (Schneider, 1991), as well as the origin of cycles of carbonate, organic matter deposition, and diatoms in each region.

• Determine what kind of oceanographic changes occur simultaneously in the Atlantic Ocean (Agulhas Current, polar front position, Equatorial Current, Argentine Current) with the shifting of the Benguela Current. Results from Ocean Drilling Program (ODP) Legs 108 and 114 can help define 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. 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.

• Determine if changes in the surface-current and upwelling patterns of the Benguela Current cause, or are related to, changes in climates of western South Africa. For example, is the origin of the Namib Desert related to the initiation of upwelling activity 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 (i.e., the establishment of upwelling regions off southwest Africa).

• 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.

• Study early diagenetic processes in environments with very high organic carbon and opal contents, which will offer an interesting contrast to the studies undertaken during Leg 112, off Peru (Suess, von Huene, et al., 1990). The upwelling sediments off the Peruvian active margin are deposited in forearc basins in a disturbed tectonic setting, whereas off Angola and Namibia, sedimentation occurs on a steadily sinking passive margin with quite stable conditions. Therefore, we expect a more continuous and longer record in comparison to the sites drilled off Peru, although the sedimentation rate might not be quite as high.

To 175 Results Site 1075

To 175 Table of Contents

ODP Publications

ODP Homepage