The three LCB sites sampled a complex environment dominated by riverine input, seasonal coastal upwelling, and incursions from the South Equatorial Countercurrent (Fig. 8). Although these three sites represent the same depositional environment, they are located at varying distances from the shelf break, in different water depths, and at different positions with respect to the Congo Canyon and Congo Fan area (Fig. 17).
Site 1075 (APC to 207.2 mbsf) is the deep-water site on the transect (at 3007 m). Sediments consist of greenish gray diatomaceous clay and nannofossil-bearing diatomaceous clay. Calcium carbonate contents generally are <2.5 wt%; thus, paleoclimatic reconstructions will have to rely largely on siliceous microfossils. Nevertheless, it should be possible to generate an oxygen-isotope stratigraphy, in combination with Site 1077. Sedimentation rates are 100 m/m.y. on average. We expected to find methane (CH4) clathrates at this site because of the high-productivity setting and the presence of a bottom-simulating reflector at 500 mbsf, as seen in high-resolution seismic profiles. The stability field for CH4 clathrates, given the temperature gradient of 45°C/km, would suggest a transition from ice to gas near that depth. However, we found no evidence for substantial clathrate abundance at this or any other Leg 175 site. (Such clathrates should have lowered chlorinity in interstitial waters upon melting.) Here and elsewhere in Leg 175 sites, gas was abundant. Consequently, small holes were drilled along the core liners to allow the gas to escape and to prevent the sediment from being pushed out the ends of the liners. About one-half of the gas was CH4; the other half, CO2.
Site 1076 is the shallow-water site of the transect (at 1402 m). The record Congo River sedimentation (Fig. 17) was recovered from this site, and it will tie Congo River activity in with coastal upwelling and eastern tropical ocean dynamics. The changing ratio between direct river input and reworked shelf sediments will be of special interest when reconstructing terrigenous input. Sediments consist of organic carbon-rich olive-gray clay and greenish gray clay. Sedimentation rates vary between 50 and 210 m/m.y. The average concentration of total organic carbon (TOC) is 2.6 wt%, which is rather high for ocean margin areas. There is much evidence of reworked material.
Site 1077 is the intermediate site on the Congo transect (at 2394 m). Sediments are composed of greenish gray diatom-rich, diatom-bearing, nannofossil-bearing, and nannofossil-rich clay. Calcium carbonate values vary between 0.8 and 13.2 wt%. In parts of the section, preservation of calcareous fossils is excellent, as indicated by the presence of pteropods. Hole 1077A was logged (202-74 mbsf) with a limited suite of sensors to test for the presence of gas hydrate, for core-log integration, and to obtain proxy records for paleoclimatic change. The presence of Milankovitch-related cyclicity is apparent from comparison of logging records with standard isotope stratigraphies.
The MAB sites, off the Bight of Angola near 12°S, were drilled to provide information on "most nearly normal" margin sedimentation, being influenced neither by riverine input nor by sustained coastal upwelling activity. Productivity in this region is greatly influenced by variations in the Angola Dome; that is, oceanic upwelling (see Jansen et al., 1996; Fig. 18). Upwelling activity is seasonal, and productivity is not particularly high compared with adjacent regions (Schneider, 1991). This setting allows maximum expression of a pelagic signal in the regional high-productivity record.
Only two sites were occupied at relatively shallow depths (Site 1078 [438 m] and Site 1079 [749 m]) and with penetration limited to 200 and 120 m, respectively. Coring was by APC. Sediments consist of gray silty clay with varying amounts of nannofossils and foraminifers. In parts of the sections, extremely high sedimentation rates (as much as 600 m/m.y.) are present. Much of the material responsible for the high rates may be delivered by coastal erosion. Steep soft-rock cliffs near Lobito, seen during the visit to that port, bear witness to vigorous uplift and erosion along the coast.
At Site 1078, dolomite concretions were first encountered during Leg 175. They are between 3 and 7 cm thick and are present at various depths. Laminated intervals are present in parts of the sections (in one case cemented by dolomite) and point to sporadic expansion of anoxic conditions. A shallow sulfate reduction zone (complete reduction by 30 mbsf) and high TOC content (2.5 wt%) indicate high primary productivity. Together with Site 1079, this site will provide information on the changing position of the Angola-Benguela Front and on the upwelling activity at the Angola Dome.
Site 1079, outside the Bight of Angola, has 60% lower sedimentation rates than Site 1078. Sediments are uniform olive-gray silty clays with varying amounts of nannofossils and foraminifers. TOC content averages 3 wt%, which is somewhat higher than at Site 1078 (less dilution by terrigenous material). Sulfate reduction goes to completion at 50 m.
Site 1080, off the Kunene River, is positioned to sample the northern end of the Angola-Namibia upwelling region and for tying shifts in the Benguela Current system to changes in climate in the African hinterland, as reflected in wind-borne dust. Coring at this site proceeded only to 52 mbsf (Hole 1080A) and 38 mbsf (Hole 1080B). Hard dolomite layers resisted drilling at these depths. Because of this, the considerable disturbance of the section, and poor preservation of calcareous fossils, shown by preliminary stratigraphy, drilling was terminated to save time for alternate sites. Sediments from Site 1080 are composed of greenish gray diatom-bearing, diatom-rich silty clays with varying abundances of nannofossils and foraminifers.
The Walvis group consists of the three Leg 175 sites on WR and in WB, as well as DSDP Sites 532 and 362 (Legs 75 and 40), which were drilled at a 1300-m water depth. The DSDP sites are seaward of the upwelling center but contain an upwelling signal, which has been transported (as eddies and filaments) by the Benguela Current. Glacial/interglacial cycles are represented as carbonate dissolution cycles, productivity cycles, and continental sedimentation cycles. The transect, located above the regional calcite compensation depth in a passive margin area with high sedimentation rates, can provide high-resolution records for the reconstruction of climatic processes and sea-level change. The phase relationships between carbonate and opal cycles were previously used to propose reconstructions of the path of the Benguela Current (Fig. 19).
Site 1081 (at 760 m) is the shallow-water site of the group. Sediments consist of gray clays, which contain varying amounts of diatoms, nannofossils, foraminifers, and radiolarians. Authigenic minerals, such as glauconite, framboidal pyrite, and dolomite, are present. Sedimentation rates are high, varying between 70 and 150 m/m.y.
Site 1082 (at 1290 m) is intermediate in depth and directly comparable to the DSDP sites farther offshore. Cyclic sedimentation is well developed and spans the last 5.8 m.y. Sediments are composed of green clays containing varying abundances of diatoms, nannofossils, foraminifers, and radiolarians.
Site 1083 (at 2190 m) is the deep-water site in the Walvis group. It has a hemipelagic section going back to 2.6 Ma. Sediments consist of clayey nannofossil ooze. Sedimentation rates vary between 60 and 140 m/m.y. Productivity changes are reflected in dark-light color cycles throughout the drilled sequence.
Site 1084, off Lüderitz Bay, will help document the stepwise and fluctuating northward migration of the Benguela Current system from the Miocene to the Quaternary periods, as well as the fluctuations in intensity of coastal upwelling in the nearby upwelling center. Such fluctuations are expected to coincide with the movement of the Angola-Benguela Front through coupling via the strength of the trade-wind system. A close tie-in between pelagic and terrigenous sedimentation within the slope record should allow oceanic changes to be matched with continental climate changes. Previous work in this area has documented anaerobic, in part varved, sedimentation in the upper margin region (for a summary, see Dingle et al., 1987; Summerhayes et al., 1995).
The upwelling areas of the Benguela Coastal Current are fed from the thermocline by South Atlantic Central Water, which originates at the Subtropical Convergence Zone by mixing and sinking of subtropical and subantarctic surface waters (Lutjeharms and Valentine, 1987). 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. 20). Here, remnants of the filaments mix with low-productivity oceanic water, forming a zone of intermediate productivity. Coastal upwelling is strongly seasonal (Shannon and Nelson, 1996), but response to the seasons differs in the northern Benguela Region (NBR) from that in the southern Benguela Region (SBR; Dingle, 1995). Differences between these regimes are seen in the modern-day planktonic foraminiferal distributions (Giraudeau, 1993; Fig. 21) and have been demonstrated using satellite imaging (Lutjeharms and Meeuwis, 1987). The boundary separating the two regions (Lüderitz Boundary) is the site of maximum upwelling intensity at 26°-27°, with unusually low temperatures persisting throughout the year.
Upwelling in the area north of the Lüderitz Boundary, at the location of Site 1084, shows persistent high productivity and high rates of accumulation of phytoplankton (Brown et al., 1991). Wind speeds are of medium intensity with a wide oceanic and filamentous mixing domain (Lutjeharms and Stockton, 1987). Surface sediments are rich in organics, with a maximum in the inner-shelf belt of diatomaceous ooze off Walvis Bay (Bremner, 1983; Rogers and Bremner, 1991).
Site 1084 has sedimentation rates between 100 and 270 m/m.y., with the highest values within the last 1 m.y. A diatom-rich interval in the uppermost Pliocene sequence also is marked by elevated sedimentation rates. Site 1084's close proximity to the Lüderitz upwelling cell results in well-expressed organic carbon, diatom, and coccolith cycles, via cyclic productivity intensity, as described from the late Quaternary record (Little et al., 1997). Sediments are composed of clay-rich nannofossil diatom ooze, diatomaceous nannofossil ooze, and clay-rich nannofossil ooze. Conspicuous decimeter-thick intervals of dark, organic-rich clay layers are present between 120 and 410 mbsf and are characterized by lower carbonate contents. The biogenic component of the dark layers is commonly dominated by diatom resting spores.
Site 1084 showed intense sulfate reduction in the uppermost few meters and had the second highest ammonia values ever measured for an ODP site. The sediment is unusually gas rich, and the offensive smell produced by outgassing during laboratory studies proved difficult to cope with.
The last three sites occupied during Leg 175 are located in the southern part of the Cape Basin (Fig. 1). They are rather close to the continent and should be most useful in detecting upwelling signals and clues to changes in continental climate (pollen, clay minerals, and terrigenous silt) and in sea level. Sites 1085, 1086, and 1087 are located within the SBR (Dingle, 1995; see Fig. 21). This region has a highly seasonal upwelling regime, with a maximum in summer, and a restricted mixing domain (Lutjeharms and Meeuwis, 1987; Giraudeau and Rogers, 1994).
Site 1085 is located offshore and to the side of the mouth of the Orange River, which flows year-round and could deliver terrigenous materials to the site. Such an effect should show more clearly during times when the Benguela Current and coastal upwelling activity were less vigorous than they are today. It will be most interesting to compare the results from Site 1085 with those from the more northern Site 362 (Diester-Haass et al., 1990), which is farther from a source of terrigenous input. Together with other sites, especially 1082 and 1084, Site 1085 will help document the path and strength of the Benguela Current from the Miocene to the present, as well as the shoreward and seaward migrations of the upwelling centers along the coast. Because sedimentation rates are more moderate at Site 1085, drilling penetrated into sediments of middle Miocene age so that a complete record of the development of the Benguela Current system should be available.
Site 1086 (at 793 m) is the shallow-water site of the SCB transect. Because of its proximity to 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. At present, the addition of warm water from the Indian Ocean into the region where the Benguela Current originates (Fig. 22) is an important element of the heat budget of the South Atlantic. Much or all of the Quaternary record is missing at this site, apparently because of winnowing at these shallow depths.
Site 1087 (at 1383 m) is the deep-water site of the transect. For this offshore site, we expect a strong open-ocean influence on sedimentation compared with the more northern sites in the Cape Basin, with their strong imprint of coastal upwelling. This site is located at a crossroads for west-wind drift, the Benguela Current, and the Agulhas Retroflection and should sensitively record the evolution of this complicated system at the point of origin of the Benguela Current.