The cyclic nature of Quaternary sedimentation in the deep ocean has been well established since the Swedish Deep-Sea Expedition. It is evident in the carbonate patterns of the tropical Pacific (Arrhenius, 1952) and in the deposition of terrigenous sediments in the North Atlantic (Ericson et al., 1961). The color of sediments changes in response to the cyclic changes in composition, in some places markedly (as in the contrast between white oozes and brown clay), in others more subtly (in hues of green or brown). Such changes in color have recently become important in cyclostratigraphic studies and as a tool for correlation of cores from adjacent holes at Ocean Drilling Program (ODP) sites (Busch, 1991; Mix et al., 1992, 1995; Hagelberg et al., 1992; Schneider et al., 1995). Other physical properties, such as bulk sediment density, magnetic susceptibility, and velocity, also are useful in this context (e.g., Mayer, 1991; Herbert and Mayer, 1991). In many cases, color changes are a result of changes in the abundance of organic carbon, which, in turn, are tied to glacial/interglacial productivity cycles (Müller et al., 1983). These are the type of color cycles we analyze in this study.
Here we present a preliminary statistical analysis of color and magnetic susceptibility cycles in ODP Site 1075, which was drilled during Leg 175 in the Lower Congo Basin, along the northern rim of the Congo Fan, off the Congo River, the second largest river in the world (Peters, 1978). The recovered sediment consists entirely of greenish gray to olive-gray clay with varying amounts of nannofossils and diatoms. The ~200-m-long record spans more than the last 2 m.y., with the overall sedimentation rate just less than 100 m/m.y.
We aim to show that the physical properties of these sediments can be used to refine the age scale, and that such refinement can bring out cyclic properties in color and reflectance that are useful in developing conceptual models about the dynamics of the regional sedimentary province. These concepts can guide further research regarding the origin of the cycles and the climate-related messages they contain. Results of our analysis suggest that such messages are complicated by changes in the processes dominating the expression of physical properties through time. Nevertheless, clues to changes in the relative influence of high- and low-latitude forcing on the dynamics of regional sedimentation (characterized by different cycles) can still be recognized. In particular, we identify differences in the influence of cryocyclic variations, with periods of 100 k.y. and 41 k.y., and monsoonal variations, with periods near 21 k.y., for different properties and over different time periods.