Orbital climate forcing has long been accepted as one main factor in modifying physical properties of marine sediments and oxygen isotope profiles in a cyclic pattern (Shackleton, 1987; Jansen et al., 1991; Berger, Kroenke, Mayer, et al., 1993; de Boer and Smith, 1994; Cooper, 1995; Kronen and Wilkens, pers. comm., 2000). The study of cyclic variability in marine sediments has enhanced our knowledge of glacio-eustatic and tectonic processes over time (Diester-Haass et al., 1992; Wright and Miller, 1993; Clark et al., 1999; Krijgsman et al., 1999).
We investigated the nature of Milankovitch cyclicity in the form of density variations in hemipelagic sediments off the southwest African coast to obtain a detailed sedimentation rate (SR) history for our study area. We aim to contribute knowledge about the evolution of the Benguela Current upwelling system (BCUS), a coastal upwelling center that is comparable to other large upwelling systems in the world (Berger et al., 1999). We analyzed wireline and gamma-ray attenuation (GRA) bulk density (Boyce, 1976; Gerland and Villanger, 1995), which, in general, provide high-resolution records over long intervals that are quickly obtained and cost effective. For this study we focused on log and GRA bulk density from Ocean Drilling Program (ODP) Sites 1081, 1082, and 1084 (Fig. F1) drilled during Leg 175. The increased sampling rate of log and GRA bulk density at these sites yielded SR histories of greater detail than those obtained from biostratigraphy.
Hemipelagic clays and oozes at Sites 1081, 1082, and 1084 are composed of a varying abundance of diatoms, foraminifers, radiolarians, and nannofossils (Wefer, Berger, Richter, et al., 1998). Our data analyses on these sediment records revealed a cyclic variability in density, which we attributed to orbital forcing in accord with the Milankovitch periods. Variations in density generally observed in geophysical logs are caused by changes in the microstructure of the sediment, which influence physical properties such as porosity, grain size distribution, and type of grain contacts of the sediment (Kronen and Wilkens, pers. comm., 2000). At our study sites the abundance of large foraminifers relative to fine-grained nannofossil ooze (Fig. F2) is linked to bioproductivity and dissolution (Mayer, 1979; Berger and Mayer, 1987; Meyers, 1992) and to diagenetic processes (Wilkens and Handyside, 1985). We suspect that a variable abundance of calcareous foraminifers at these sites (Fig. F2) strongly influences the local variability in density. Therefore, we suspect that rhythmic variations in our sediment deposits along the BCUS are linked to the presence of these calcareous microfossils, which maintain intratest porosity to great depths.