The individual transects selected for drilling within the ABC-system reflect a compromise among geographic coverage, accessibility, quality of sedimentary record, and time constraints. Variations in productivity are generated in different ways, within different geographic settings (off the Congo, near the Angola Dome, at Walvis Ridge, in the upwelling cells south of the ridge). One of the major goals is to document fluctuations in productivity in these different settings in relationship to large-scale climate change within the late Neogene, including the onset of glacial cycles in the Northern Hemisphere. Another major goal is to tie the fluctuations in oceanic conditions to the corresponding changes in climate on the adjacent continent.
Most of the sites drilled have high sedimentation rates (ca. 100 m/m.y.), which offers an opportunity to develop detailed paleoceanographic records, with a resolution near 1000 yr. Sediments are largely diatomaceous and carbonate-rich clays with variable (and occasionally very high) organic carbon contents. Analysis of these sediments will greatly extend and refine results concerning paleoceanography and paleoclimate of the late Neogene that were provided by Deep Sea Drilling Project Sites 362 and 532.
The northernmost sites (Sites 1075, 1076, and 1077) contain the record of sediment supply by the Congo River, intercalated with the oceanic record. Pollen, freshwater diatoms, phytoliths, and clay minerals will provide clues to climatic change in the drainage basin of the Congo. Fluctuations in the accumulation of pelagic diatoms and of marine organic matter track the changes in productivity in this peri-estuarine environment.
Sedimentation patterns at Sites 1078 and 1079 are greatly influenced by changes in intensity of upwelling around the Angola Dome. The two sites show extremely high rates of accumulation, presumably due to the supply of silt from vigorous coastal erosion (as seen in the morphology of the coast around Lobito). Sediments of Site 1081 contain a record of variation in seasonal coastal upwelling near the northern boundary of the string of coastal upwelling cells off Namibia and South Africa ("southwest Africa upwelling cells"). This record is closely related to the southeasterly winds driving the Benguela Current, which is documented (in part) in the supply of dust from the Namib Desert. Sites 1082, 1083, and especially 1084, lie close to the major upwelling centers along southwest Africa, with year-round upwelling activity. Thus, these sites directly record the variability in the intensity of coastal upwelling, mainly through the eddies and filaments generated at the centers and passing overhead and generating high export production. The three sites in the southern part of the Cape Basin (1085, 1086, and 1087) document the history of the Benguela Current near its point of origin and contain a record of the influence of warm water from the Indian Ocean, brought by the Agulhas Current. Also, these sites contain the evidence for incursions of cold antarctic waters, which apparently reached a maximum near the Pliocene/Pleistocene boundary.
Preliminary results focus on the interplay between high- and low-latitude Milankovitch forcing (obliquity vs. precession), the role of the 100-k.y. oscillation, the effects of the mid-Pleistocene Climate Step (near 920 ka) on upwelling and African climate, the nature of the late Pliocene-early Pleistocene productivity maximum, the onset of enhanced upwelling at the beginning of the late Pliocene (near 3 Ma), and the implications of changes in productivity and sediment supply for diagenesis, which affects interpretation of seismic profiles. Concerning this latter item, dolomite layers were found to be abundant at certain sites, whereas evidence for clathrates was lacking at all sites.
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