Thirteen sites were occupied during Ocean Drilling Program Leg 175 along southwestern Africa, starting with the Congo Fan in the north and ending off the Cape of Good Hope. North of the Walvis Ridge, penetration was limited because of safety concerns. Sediments from the Congo Fan are rich in opal. Maximum rates of sedimentation were found at holes drilled off Lobito (Angola), with rates as high as 60 cm/k.y. because of high terrigenous influx from a rising coast. On the Walvis Ridge and in the Walvis Bay, organic matter–rich calcareous clays were recovered. The highest organic content, the most vigorous development of methane and carbon dioxide, and abundant noxious gases were found at Site 1084, located off Lüderitz, Namibia. The sites occupied off the Cape of Good Hope are characterized by calcareous oozes.
Organic matter abundance, diatom accumulation, and composition of benthic faunas, among other indicators, are the most important tools for the reconstruction of the productivity history of the Angola-Benguela upwelling system. Problems arising include (1) the origin of the Matuyama Opal Maximum, centered near 2.2 Ma, (2) the question of phase relationships between diatom productivity and organic matter supply, and (3) the role of precessional effects in the modulation of productivity as a function of latitude. Regarding the first problem, it emerged that the maximum supply of diatoms is linked to incursions of both warm pelagic water and Antarctic water during the early Matuyama, presumably resulting in the formation of chaotic frontal zones. Coastal upwelling dominates after the maximum. Thus, opal deposition goes through an optimum situation within the latest Pliocene. We hypothesize that increased silicate concentrations in subsurface waters made this optimum possible. As the Benguela Current became stronger in the Quaternary, a more vigorous flow of underlying Antarctic Intermediate Water increased ventilation of the thermocline off southwestern Africa, thereby helping to decrease the silicate content of subsurface waters. Observations during Leg 75 regarding glacial/interglacial cycles on the Walvis Ridge (that warming is accompanied by increased opal deposition) support our hypothesis. Precessional effects are of great importance in the productivity history recorded on the Congo Fan (as seen in color cycles). The sensitivity of the depositional system off the Congo to precessional forcing apparently is especially high within the Milankovitch Chron (last third of the Quaternary).
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