INTRODUCTION AND OVERVIEW

The goal of Leg 175 of the Ocean Drilling Program (ODP) was to provide for the reconstruction of the late Neogene history of the great eastern boundary current off Namibia and of the upwelling regimes north and south of the Walvis Ridge. Upwelling north of the Walvis Ridge is dominated by the Congo River outflow and the influence of a large cyclonic gyre, the Angola Dome. Upwelling on the landward side of Walvis Ridge and off southwestern Africa in general is intimately tied to the dynamics of the Benguela Current. The front between the two systems runs southeast to northwest just north of the Walvis Ridge (Fig. F1). On 12 August 1997, the JOIDES Resolution left Las Palmas (Canary Islands) on a course for the first site (1075) on the Congo Fan. During the 57-day expedition, we occupied 13 sites (Fig. F2) and drilled 40 holes. Overall penetration totaled 8210.5 m, with 8003.2 m of recovery (see the "Appendix" for a summary of drilling results). On 9 October 1997, the vessel entered the port of Cape Town, South Africa, ending the leg.

Like other eastern boundary upwelling systems studied during recent ODP legs (Peru and California), the Angola-Namibia regimes are characterized by organic-rich sediments that contain an excellent record of productivity history that can be read on a very fine scale, thanks to high sedimentation rates. In addition, this environment is a prime setting for natural experiments in diagenesis, driven largely by bacterial action, including the formation of dolomite and the production of methane and carbon dioxide. The drilling transects realized represent a compromise between geographic coverage, likelihood of continuous stratigraphy, hydrocarbon hazards, and time constraints. Off the Congo and off Angola, where active exploration for offshore hydrocarbons is in progress, drilling was limited to a maximum of 200 meters below seafloor (mbsf) (six sites), and some of the sites initially proposed were rejected altogether for safety reasons. On the Walvis Ridge and off Namibia and South Africa, we drilled to refusal of advanced hydraulic piston coring (APC) at two sites and down to between 400 and 600 mbsf at five more sites.

This overall drilling strategy resulted in the recovery of Quaternary sediments at the sites north of the Walvis Ridge and of upper Neogene sediments on the Walvis Ridge and south of it. At some of the southern sites, the drill reached Miocene deposits. Site 1087 (the last drilled) includes upper Eocene sediments, but the record in the lower section has large gaps. The chief types of facies encountered are diatomaceous clays (Congo sites) and diatomaceous calcareous clays (Walvis Ridge and Walvis Bay sites). The southernmost three sites in the Cape Basin (Cape Basin group sites) had calcareous ooze with very little opal.

The main scientific objectives of Leg 175 were concerned with paleoceanographic reconstruction, but also included diagenetic processes within upwelling sediments. Extensive preparations for the drilling expedition, in addition, yielded much information on seismic stratigraphy and Quaternary sediment patterns. The major questions driving Leg 175 research were formulated based on this preparatory work (carried out by Bremen scientists) (Bleil et al., 1995, 1996) and on previous studies by many others (see Summerhayes et al., 1992, 1995; Wefer et al., 1996) and by the scientists of cruises of the Deep Sea Drilling Project (see Bolli, Ryan, et al., 1978; Moore, Rabinowitz, et al., 1984; Hay, Sibuet, et al., 1984; Hsü and Weissert, 1985). They may be summarized as follows (cf. Shipboard Scientific Party, 1998a):

1. Regarding Quaternary upwelling cycles, what is their precise nature, what is their relationship to Milankovitch forcing, and what are the implications for changing heat transport from south to north across the equator?
2. What can the sediments deposited off the mouth of the Congo River tell us about the evolution of climate in central Africa in the late Neogene?
3. In what ways does the Benguela Current change its course and strength through geologic time, and how are such changes related to the general circulation in the South Atlantic?
4. Are there fundamental differences in the dynamics of upwelling through time off the coast of Namibia?
5. How do the Namibia upwelling system and the Benguela Current respond to known climate shifts on various geologic timescales?
6. What kinds of diagenetic processes help determine the nature of the deposits off southwestern Africa, and how fast does diagenesis proceed?

Many new insights have been gained regarding these and related questions in the wake of Leg 175. The principal results may be summarized as follows:

1. The Congo Fan sediments contain clues to wet-dry cycles in the interior of Africa; a major change in climatic conditions was initiated somewhat earlier than 1 m.y. ago (Uliana et al., Chap. 11, this volume).
2. Monsoon winds forced by North African summer insolation modify the southeast trade winds and affect upwelling intensity well south of the equator.
3. Increased upwelling does not necessarily imply increased deposition of diatom debris. Over much of the time span studied, the reverse is the case, for reasons yet unknown (presumably low silicate content in the thermocline).
4. The great cooling steps between 3 and 2 Ma and the climate shift near 1 Ma (toward large-amplitude glacial-interglacial cycles) affected the upwelling history in fundamental ways and resulted in marked changes in the pelagic and benthic environments.
5. Chemical activity within the sediments is driven by the availability of reactive organic matter; where it is abundant, there is intense production of methane and carbon dioxide and formation of new minerals, especially dolomite.

Many or most of the sections drilled showed continuous sequences with high rates of sedimentation. This was a result not of a lack of disturbance along the margin of southwestern Africa (slumping is common, in fact) but of good control from seismic stratigraphy, gained through the efforts of Volkhard Spiess and his collaborators. An outstanding difference between the sites north and south of the Walvis Ridge is that sites to the north show the effects of salt tectonics and those to the south do not (Fig. F3). The salt tectonic structure is a reminder that back in the Aptian, when the Walvis Ridge and the Rio Grande Rise formed a continuous barrier across the opening South Atlantic, there was a restricted evaporite basin north of this barrier.

Continuity of recovery was affected, in many cases, by the strong development of gas pressure from abundant methane and carbon dioxide, which produced sediment expansion and created voids and cracks. This process also impacted the measurement of physical properties. Although the high gas content had its drawbacks (besides posing a hazard in cases), it also retarded compacting of sediments, permitting fast drilling. Because of this, 13 sites were ultimately occupied, rather than 8 as planned.

The detailed guidance from seismic profiles and the strict application of safety principles resulted in the avoidance of clathrates. We found no evidence for the decrease in salinity (or chlorinity) corresponding to an addition of freshwater from melting of clathrates in sediments recovered from areas where clathrates might be expected (Murray et al., 1998). Presumably, the highly reflective "cloud" structures seen in the seismic profiles over anticlinal features in the Congo Fan region (Shipboard Scientific Party, 1998b, p. 56) indicate the presence of clathrates. Also, narrow disturbed zones where sound is dispersed (leaving a blank record) are seen, suggesting venting of gas from some depth. Clearly, the formation of gas within margin deposits and the return of gas to the atmosphere are problems of prime importance, but owing to the limitations of the JOIDES Resolution, we have but little to contribute in this regard.

Our focus in this overview is entirely on paleoceanographic problems, from the nature of Quaternary sediment cycles to the response of the upwelling systems to global cooling in the late Neogene. The outstanding issue in this context is the "Walvis Paradox," that is, the observation that increased upwelling during glacial stages results in decreased deposition of opal. The paradox manifests itself on two levels: within the scale of glacial-interglacial fluctuations and within the scale of the last 4 m.y., where the onset of strong glaciations within the Quaternary results in reduced deposition of diatoms during the last ~2 m.y. off Namibia. The complex evidence surrounding the Walvis Paradox makes interpretation difficult.