Ocean Drilling Program (ODP) Leg 175, the "Benguela Leg," set out to drill as many as four or five transects off the western coast of Africa, beginning off the Congo River, just south of the equator, and ending in the Southern Cape Basin, just north of the Cape of Good Hope. Eight sites were planned for the leg; 13 were in fact drilled (Fig. 1), and 8003 m of core were recovered. Our goal is to reconstruct the late Neogene history of the Benguela Current and the associated upwelling regimes. These regimes constitute one of the few great upwelling regions of the world. Like other eastern boundary upwelling systems studied during recent ODP legs (Peru and California), the Angola-Benguela regimes are characterized by organic-rich sediments that contain an excellent record of productivity history, which can be read on a very fine scale. In addition, this environment provides a prime setting for natural experiments in diagenesis.
The individual transects reflect a compromise between geographic coverage, accessibility, and time constraints. Off the Congo River and off Angola (where active exploration for offshore hydrocarbons is taking place), drilling was limited to maximally 200 m below sea floor (mbsf) for safety reasons (six sites). On the Walvis Ridge and off Namibia and South Africa, we drilled to advanced hydraulic piston (APC) refusal (two sites) or down to between 400 and 600 mbsf (five sites). This overall drilling strategy resulted in the recovery of Quaternary records north of the Walvis Ridge and upper Neogene records on and south of the ridge. In the south, a number of sites include upper Miocene sediments; Site 1087 (the last drilled) includes upper Eocene sediments, but the record is not continuous in the lower section.
The hemipelagic sediments off the Congo (Sites 1075, 1076, and 1077) are dominated by diatomaceous, partially carbonate-bearing clays. Two shallow-water sites off Lobito (Angola) have extremely high sedimentation rates and contain silty clays (Sites 1078 and 1079). Site 1080, drilled off the Kunene River (Angola), contains diatomaceous clays (the section is disturbed). The sites on the Walvis Ridge and in the Walvis Basin (Sites 1081, 1082, and 1083) contain calcareous clays, which are diatom rich in the upper Pliocene and lower Quaternary sections. Site 1084, off Lüderitz, contains sections with extremely high organic carbon content. The southern sites (1085, 1086, and 1087) near the Cape of Good Hope have pelagic calcareous oozes.
The major aim of the expedition was to provide the basis for reconstruction of productivity variations in the South Atlantic, off western Africa, as a means to obtain clues for the history of the Benguela Current, which is the eastern boundary current for the Subtropical Gyre of the South Atlantic (Fig. 2). The Benguela Current originates near the Cape of Good Hope, fed by the South Atlantic Current and, to some degree, by the Agulhas Current. It moves parallel to the coast off southwest Africa and Namibia, but turns westward at the latitude of the Walvis Ridge and merges with the South Equatorial Current. A front develops over the coastal portion of the Walvis Ridge (the Angola-Benguela Front, see Fig. 2) and moves north and south with the seasons (through ~5°). To the north, sluggish cyclonic circulation marks the Angola Basin, which results in offshore upwelling (the Angola Dome). This region has high nutrient contents in subsurface waters and a strong oxygen minimum.
Productivity is low within the Subtropical Gyre (Fig. 3). It is high along the entire coastal region of southwestern Africa as a result of upwelling and vertical mixing. Areas of especially high productivity within this belt occur both north and south of the Walvis Ridge. To the north, the outflow of the Congo River provides offshore estuarine-type circulation, whereby the dispersing freshwater layer entrains nutrient-rich waters from below the mixed layer. Off Angola, waters rising within the Angola Dome provide nutrients to surface waters, raising the general level of productivity. South of the Walvis Ridge, vigorous wind-driven upwelling interacting with the northward flow of the Benguela Current dominate production dynamics.
The Benguela Current plays a key role in the heat transfer from the South Atlantic to the North Atlantic, which, in turn, dominates climate developments in the Northern Hemisphere in the late Neogene (see articles in Wefer et al., 1996a). The principle of heat transfer is simple: warm surface waters and relatively warm subsurface waters move across the equator, from south to north, to balance the deep return flow of cold waters within the North Atlantic Deep Water (NADW). This results in major heat export from the South Atlantic to the North Atlantic (Woods, 1981; Gordon, 1986; Berger et al., 1987; Broecker and Denton, 1989). Of special interest in this context is the import of warm subtropical waters from the Indian Ocean, around the Cape of Good Hope (Gordon, 1985). The sediments recovered from the Southern Cape Basin will be crucial in reconstructing the history of this process.
Both ocean currents and winds transfer heat from the South Atlantic to the North Atlantic. Warm, moist, tropical air moves across the equator toward the northerly Intertropical Convergence Zone. As a result of this energy transfer, evaporation is greatly increased in the North Atlantic, which produces high-salinity surface waters. Upon cooling in northern latitudes, these waters sink to great depth and return to the south within the NADW. The process started roughly 10 m.y. ago, as seen in a drop in the carbonate compensation depth in the North Atlantic at that time and in other indicators (opal deposition and stable isotopes; summarized in Berger and Wefer, 1996).
The results of Leg 175 thus far provide much encouragement regarding the opportunities for detailed reconstructions of productivity histories all along the southern rim of western Africa. High sedimentation rates in most of the sites will facilitate the task. Initial results highlighted here show that productivity involves a different mix of processes in each major region. Nevertheless, long-term changes in these various regions show some striking coherencies. On orbital scales, evidence for cyclic sedimentation similarly suggests large-scale forcing acting on linked subsystems. The Congo sites, recording tropical wind and runoff, provide a window into the interplay of monsoon and trade winds and the response of the African interior. The Walvis and Lüderitz sites monitor the Benguela system proper. The Cape of Good Hope sites help keep track of the warm-water influx from the Indian Ocean through the late Neogene. Establishing the linkage between these major parts of the Angola-Benguela system is the main task yet ahead.