Color-related variables in the Quaternary sediments recovered from Site 1075 in the northern Congo Fan region show distinctive fluctuations, which reflect regional and global orbital forcing and local response of the depositional system. The color pattern (red/blue) seems less complex than the reflectance pattern (albedo). Color variation is dominated by precessional cycles, whereas reflectance has power in a larger number of competing cycles (Fig. 11). Also, total reflectance has a much stronger long-term trend than the red/blue ratio (compare Fig. 1 and Fig. 2).
The depositional system includes the sediment-producing drainage basin of the Congo River; the river transport; the redistribution processes, including possible intermediate storage on the shelf; and marine processes. The marine factors include productivity patterns of both the open ocean and the coastal ocean, as well as changing carbonate chemistry of deep waters. The production of sediment in the Congo Basin is largely a function of the wetness of the climate (Jansen, 1990). This factor should be strongly related to precession, which controls the intensity of monsoons (Rossignol-Strick, 1985; Schneider et al., 1997). Redistribution of sediment involves the position of sea level and is therefore co-determined by global ice volume. Ice volume varies with obliquity and eccentricity and also has its own oscillatory behavior, which gradually changes from the early to the late Quaternary (Berger et al., 1995).
Ocean productivity is a function of winds, which in the tropical Atlantic, depend on monsoonal amplitudes (McIntyre et al., 1989; Schneider et al., 1994, 1996; Wefer et al., 1996) and on the nutrient supply within the thermocline waters (Molfino and McIntyre, 1990; Hay and Brock, 1992; Herguera and Berger, 1994; Berger and Lange, 1997). Carbonate saturation of deep waters is a function of water age: the older the deep water, the less saturated it is. Whenever the influence of North Atlantic Deep Water (NADW) is strong (despite the limited access to the Angola Basin), carbonate saturation is elevated and carbonate preservation is enhanced. The production of NADW is a result of both evaporation and cooling of North Atlantic surface waters. Thus, NADW depends both on subtropical and high-latitude processes, and it should therefore carry both low-latitude (precessional) and high-latitude (obliquity- and ice-mass-related) cyclicities.
Against this background, it is clear that we will not be able to separate global from regional, high-latitude from low-latitude, and continental from oceanic influences in the color cycles based on spectral analysis alone. However, when this analysis is combined with calibration against the main components, much insight will be gained. The available data suggest that productivity variations are responsible mainly for the fluctuations in the red/blue ratio through the changing abundance of organic matter (Fig. 3). The strong negative correlation with sea level in the 100-k.y. band (Fig. 9A) suggests a greatly enhanced supply of organic matter during glacial periods, with a distinct shift toward red in the red/blue ratio. The same relationship is seen in the (assumed) phase with obliquity (Fig. 9B). There is a hint that the same phase holds with respect to precession (Fig. 10), with higher productivity when perihel occurs in northern winter.
The most important result is that the sensitivity of the depositional system, as reflected in color variation, changes through time. Indications are that the largest amplitudes in red/blue ratio occurred in the Milankovitch Chron (last 625 k.y.) in both precessional and obliquity-related bands. This interval is characterized by a dominant 100-k.y. oscillation in global ice mass. Apparently, the presence of extreme global conditions provide the overall framework for increased variability in sediment supply, both from land and the ocean. It is as though more power is available (e.g., from an increased planetary temperature gradient) to drive the subsystems to greater extremes as they obey the forcing from changing insolation patterns.