Site 1085, in connection with other sites, especially Sites 1082 and 1084, helps document the path and strength of the Benguela Current system from the Miocene to the Quaternary, as well as the shoreward and seaward migrations of the upwelling center. The upwelling inside the Benguela Coastal Current is fed from the thermocline by South Atlantic Central Water, and its intensity is related to the position and intensity of the Benguela Current system. Filaments of cold, nutrient-rich waters from the coastal upwelling area extend well offshore (as much as ~600 km offshore; Lutjeharms and Stockton, 1987; see Fig. 1, "Site 1084" chapter, this volume). Here, the cold water mixes with low-productivity oceanic water and forms a zone of intermediate productivity.
Coastal upwelling within the Benguela Current system varies with the seasonal extremes of summer and winter (Shannon and Nelson, 1996). The seasonal pattern is used to divide the system into a northern Benguela Region (NBR) and a southern Benguela Region (SBR; Dingle, 1995). This is seen in the modern-day planktonic foraminiferal distributions (Giraudeau, 1993) and has been demonstrated using benthic ostracods (Dingle, 1995) and satellite imaging (Lutjeharms and Meeuwis, 1987). The boundary between the NBR and SBR (Lüderitz Boundary) is the site of maximum upwelling intensity at 26°–27°S, which has the coldest and the most persistent upwelling.
Upwelling in the area north of the Lüderitz Boundary (e.g., at the location of Site 1084) is typified by year-round high productivity and enhanced accumulation of phytoplankton (Brown et al., 1991). Wind speeds are of medium intensity, with a wide oceanic mixing domain characterized by filaments (Lutjeharms and Stockton, 1987). The SBR has a highly seasonal upwelling regime, with its maximum in summer, and a restricted mixing domain (Lutjeharms and Meeuwis, 1987; Giraudeau and Rogers, 1994). Sites 1085, 1086, and 1087 are located within the SBR.
Site 1085 is located off the side of the mouth of the Orange River, which discharges into the South Atlantic throughout the year, delivering additional terrigenous sediments. This effect should be more pronounced during times when the Benguela Current and coastal upwelling were of lower intensity than today, so that sediments from ocean production were less dominant. A close tie-in between pelagic and terrigenous sedimentation is expected to be present within the slope record. Clay mineralogical results from Deep Sea Drilling Project Site 362 give indications for a precursor to the modern Benguela Current in the middle Miocene (14 Ma), which was too weak to produce upwelling but reached the Walvis Ridge during glacial periods and transported montmorillonite northward from the Orange River. In interglacial periods of the middle Miocene, the local source of illite (the Namib Desert) overwhelmed the distant montmorillonite supply (Diester-Haass et al., 1990). The more southern Site 1085 will provide an excellent comparison to Site 362, as it is closer to the clay mineral source area. From these older sediments, we expect a picture of the conditions of circulation before strong coastal upwelling arose.
Compared with Site 1084, which is much closer to the centers of upwelling (especially the one strong cell off Lüderitz), we should find relatively low sedimentation rates. A rate of ~5 cm/k.y. was determined (Schulz et. al., 1992) for an 11-m-long core collected from near the same water depth (Geosciences Bremen [GeoB] 1719-7, 28°55.6'S, 14°10.7'E, water depth 1010m) but ~45 km to the north. Because of the proximity of the Agulhas Retroflection (Lutjeharms, 1996) and the Subtropical Convergence Zone, we expect to find indications of warm-water incursions in the fauna and flora of the plankton embedded into assemblages typical for temperate and cool conditions. Also, for this offshore site, we expect a stronger open-ocean influence on sedimentation compared with the more northern sites, with their strong coastal upwelling imprints.