The regional environment of Site 1075 (as well as companion Sites 1076 and 1077) is dominated by three major influences: (1) the freshwater input from the Congo River, (2) seasonal coastal upwelling and associated filaments and eddies moving offshore, and (3) incursions of open-ocean waters, especially from the South Equatorial Countercurrent (see "Background and Objectives" section, "Site 1075" chapter, this volume). According to Jansen (1985), river-induced phytoplankton activity extends ~160 km beyond the shelf edge, thus affecting Site 1075. However, much or most of the regionally enhanced productivity is not river related. Divergence and doming, as well as cyclic interactions between the South Equatorial Countercurrent and the Benguela Current, may be the dominant factors.
The high (near 100 m/m.y.) and comparatively steady sedimentation rate is a result of a sustained high supply of fine-grained suspended material from the river and the high productivity of overlying waters. Calcareous microfossils are only a minor constituent; on the whole, concentrations rise toward the present, with maxima in mid-Brunhes time. The biosiliceous fraction has a concentration similar to terrigenous clay. The sediments are rich in organic matter (typically 1–4 wt%), with sulfate reduction leading to complete removal of sulfate in the upper 30 m. Correspondingly, pyrite is ubiquitous at this site (see "Lithostratigraphy" section, "Site 1075" chapter, this volume).
The bulk of the sediment delivered by the Congo River bypasses the area of Site 175, moving down-canyon to feed the channel-and-levee systems of the Congo Cone. The canyon is unusual in that it cuts deeply across the shelf, with water depths of several hundred meters, extending far into the lower stretches of the river (Peters, 1978; Eisma and van Bennekom, 1978).
Whatever the mix of factors dominating the sediment patterns, they are expected to change through time and in a cyclic fashion (Jansen et al., 1984; Olausson, 1984; Zachariasse et al., 1984; Jansen, 1985; Schneider et al., 1997). Our analysis will show that the nature of these cycles is complex.