The ocean's role in climatic change through heat transport and control of carbon dioxide is increasingly being recognized. This new awareness and the urgency that must be accorded to the attempt to understand the mechanisms of climatic change have led to the initiation of large integrated efforts in physical and chemical oceanography. Likewise, the potential of using the oceanic record to understand climatic change has received increased attention in recent years (CLIMAP, 1976; COSOD II, 1987). The Angola-Benguela Currents (ABC-system) with its associated upwelling regimes need to be studied because of their importance in the global ocean-carbon cycle and their capacity to provide for comparison with the systems off Peru and California.
By comparing these systems with one another, we shall learn which elements of a system are peculiar and which have general validity through time and on a global scale. To further these goals, the JOIDES Resolution occupied 13 sites off the southwestern coast of Africa (Fig. 1) for drilling with the advanced hydraulic piston corer (APC) and the extended core barrel (XCB). The sediments recovered during Leg 175 contain the record of climatic change and productivity variation of the ABC-system, with emphasis on the late Neogene.
Eastern boundary upwelling is strongly involved in the marine carbon cycle. This process helps set the partial pressure of carbon dioxide (pCO2) both by "biological pumping" (removal of carbon from surface waters to deep waters) and by "biological dumping" (removal of organic carbon to sediments; e.g., Berger and Keir, 1984; Sund-quist and Broecker, 1985; Boyle, 1988; Sarnthein et al., 1988; Berger et al., 1989). It is now generally thought that the efficiency of biological pumping (Broecker, 1982) is a crucial factor for the explanation of short-term fluctuations in atmospheric CO2, as seen in ice cores. However, the effect is probably insufficient in magnitude to serve as the sole or main cause. Biological dumping also has to be considered, as well as the accumulation and redissolution of carbonate. That evidence from sediments is relevant to the reconstruction of atmospheric CO2 is shown in the good correlation between productivity indices in a core from the eastern equatorial Pacific and the ice-core record of pCO2 (Fig. 2). Likewise, there is good correlation between the ice-core record and estimates of CO2 pressure in surface water from a core taken off Angola (Fig. 3).
With respect to Neogene climate steps, Vincent and Berger (1985) have postulated that carbon dumping by coastal upwelling is responsible for rapid changes in the general level of atmospheric pCO2. They propose climatic preconditioning by upwelling-induced carbon extraction from the ocean-atmosphere system for the beginning of the modern ice-cap-dominated world. Their argument is based on the observation that carbon isotopes in deep-sea benthic foraminifers become enriched in 13C just when organic-rich phosphatic sediments begin to accumulate around the Pacific margins (Fig. 4). In this view, eastern boundary upwelling, and therefore upwelling off Angola, Namibia, and South Africa, has global implications for the long-term history of the carbon cycle and climate and for the evolution of life and biogeography on land and in the sea.
If we are to assess the effects of changes in productivity on the CO2 content of the atmosphere, the interrelationships among ocean circulation, nutrient transport, and the sedimentation of organic compounds and carbonate must be established for each of the important productivity regions. Before Leg 175, little information was available on upwelling fluctuations off Angola and Namibia—except for the late Quaternary period.
The most important period for understanding the workings of the present system is the time since the late Miocene. Within this period, we see the evolution of the present planetary orography, the buildup of ice caps on both poles, the development of modern wind and upwelling regimes, and the stepwise increase in North Atlantic Deep Water (NADW) production, which dominates the style of deep circulation in the ocean. The present system is characterized by a strong 100-k.y. climatic cycle, beginning at ~700 ka (Berger et al., 1996). High-amplitude fluctuations associated with buildup and decay of northern ice sheets began at ~2.8 Ma (Shackleton et al., 1984; Hodell and Venz, 1992; see Fig. 5).