During Leg 175, 13 sites were occupied off the western coast of Africa (Congo, Angola, Namibia, and South Africa) and 40 holes were drilled using advanced hydraulic piston coring and the extended core barrel method. The goal is to reconstruct the late Neogene history of the Benguela Current and the associated upwelling regimes between 5° and 32°S. The area investigated contains one of the great upwelling regions of the world, intermediate in intensity between the systems off Peru and California. The Angola-Benguela Current system (ABC-system) with its associated upwelling regions is characterized by organic-rich sediments that contain an outstanding record of productivity history, which can be read on a very fine scale. In addition, this environment provides an excellent setting for natural experiments in diagenesis.
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 the Walvis Ridge, and 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 climatic change within the late Neogene, including the onset of glacial cycles in the Northern Hemisphere. Another major goal is to tie fluctuations in oceanic conditions with the corresponding changes in climate on the adjacent continent.
Most of the drilled sites have high sedimentation rates (~100 m/m.y.), which offers an opportunity to develop detailed paleoceanographic records with a resolution close to 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 the results concerning paleoceanography and paleoclimate of the late Neogene that were provided by Deep Sea Drilling Project Sites 362 and 532.
The northernmost 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 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 the upwelling around the Angola Dome. The two sites show extremely high rates of accumulation, which presumably are caused by the supply of silt from vigorous coastal erosion (as seen in the morphology of the coast around Lobito).
Sediments from Site 1081 contain a record of variation in the 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 that form at the centers, pass over the sites, and generate high export production here.
The three sites in the southern part of the Cape Basin (Sites 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 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-latitude 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 the interpretation of seismic profiles. Concerning the last item, dolomite layers were found to be abundant at certain sites, whereas evidence for clathrates was lacking at all sites.
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