INTRODUCTION

The main objectives of Ocean Drilling Program (ODP) Leg 175, in which the participants drilled along the west coast of southern Africa, were to reconstruct the past history of the Benguela Current, which is presently associated with a system of very active upwelling cells located off shore of Angola, Namibia, and south Africa. High productivity levels (>180 g C/m2/yr) characterize the present system, which is considered to be a major zone for carbon export and thus probably exerts a strong control on the global carbon budget.

Previous studies in the region have shown that the relative positions of the oceanic fronts (the Angola-Benguela Front, located presently at ~17S; the Subtropical Convergence [STC], located presently at ~38S; and the Polar Front Zone located presently ~50S) moved farther north by a few degrees of latitude during glacial periods (Morley and Hays, 1979; Jansen et al., 1986). The northward shift of the STC was also responsible for the transfer of thermocline waters from the southwest Indian Ocean to the southeast Atlantic Ocean (Winter and Martin, 1990; Wefer et al., 1996). During glacial periods, surface productivities were much higher than during interglacials because of the strengthened trade-wind stress; at depth, the general consensus is that the glacial North Atlantic Deep Water (NADW) flux was weakened and brought nutrient-rich 13C-depleted waters, whereas the Antarctic Intermediate Water (AAIW) flux was relatively strong and brought nutrient-poor 13C-rich waters, compared to the interglacial situations (Sarnthein and Tiedemann, 1990; Howard and Prell, 1994).

The recovery of continuous Neogene sedimentary sequences during Leg 175 makes it possible for the first time to obtain the complete paleoceanographic record of the Angola-Benguela Current System for the last 15 m.y. The present study examines the stable isotope record of planktonic and benthic foraminifers of the last 500 k.y. at Site 1087, which is in the Southern Cape Basin. Our main objectives are to

  1. Establish the precise chronological framework with the oxygen isotope stratigraphy,
  2. Estimate the fluctuations of paleoproductivity from the carbon isotopic compositions of planktonic foraminifers and from the surface-to-bottom 13C gradient, and
  3. Measure the isotopic gradients between different planktonic species in order to deduce temperature and productivity gradients in surface waters.

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