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Abstract

Episodically deposited, dark, organic-rich Pleistocene and Late Pliocene sediments from the lower continental slope off southwest Africa reveal complex interactions between changes in primary production, benthic foraminiferal assemblage, and anaerobic microbial processes. The organic-rich layers contain diatom assemblages characteristic of intense seasonal coastal upwelling whereas stratigraphically adjacent sediments reflect pelagic primary production. Coastal upwelling-dominated depositional intervals coincide with periods of enhanced carbon flux to the seafloor. Enhanced organic carbon export during dark layer deposition was accompanied by decreases in the diversity of benthic foraminifera to few opportunistic species adapted to high phytodetritus accumulation rates and low O2 conditions. In all sediments the sulfur isotopic composition of pyrite indicates redox cycling of sulfide close to the sediment/water interface. The sulfur isotopic evidence and the permanent presence of abundant low O2-adapted benthic foraminifera throughout the organic-rich layers suggest an oxygenated benthic environment. Efficient oxidation of sulfide and removal of sulfide by sulfidization of organic matter inhibited buildup of toxic hydrogen sulfide from bacterial sulfate reduction at the sediment/water interface. These data imply that in continental slope sediments underneath productive surface waters benthic dysoxic conditions are maintained by the lateral advection of dissolved oxygen to support a small, but well-adapted benthic community.