The upwelling system associated with the Benguela Current, the eastern boundary current offshore southwest Africa, is one of the most productive areas of the modern ocean and is consequently a major locus of carbon burial. Sediments from Deep Sea Drilling Project (DSDP) Sites 362 and 532, which are at virtually the same location on Walvis Ridge, have provided a preliminary record of the evolution of upwelling and enhanced biological productivity of this system. Siesser (1980) concluded from concentrations of organic carbon and diatom assemblages in rotary-drilled cores from Site 362 that upwelling-enhanced productivity has gradually increased since its onset ~10 Ma. Hydraulic piston coring (HPC) at Site 532 yielded a less disturbed and more complete record of the upwelling history (Hay et al., 1982). Maxima in concentrations of organic carbon and diatom microfossils indicate that productivity peaked at this location in the late Pliocene to early Pleistocene (Meyers et al., 1983; Dean et al., 1984). The HPC record also revealed light-dark alternations that correspond to cycles in concentrations of organic carbon, calcium carbonate, and clay minerals. Diester-Haass et al. (1986, 1990, 1992) explain the color cycles as resulting from a combination of glacial-interglacial shifts in the axis of the Benguela Current, sea-level changes that modified the delivery of organic matter, and climate changes that changed the delivery of clastic sediment components. Moreover, these authors conclude that carbonate dissolution fueled by the oxidation of organic matter is a major factor in the cause of the cycles.

One of the goals of Leg 175 was to improve the record of the evolution of the Benguela Current by obtaining sediment from multiple locations along the margin of southwestern Africa. An important part of this record is to describe the history of burial of inorganic and organic carbon at these different locations. This report provides a summary of measurements of the carbonate and organic carbon contents of sediment from five Leg 175 sites and from Site 532. These six sites comprise a transect along the axis of the Benguela Current. Organic geochemical parameters are used to evaluate the sources of the total organic matter and the degree of its preservation in sediment from the six sites.