Coupled primary production, benthic foraminiferal assemblage, and sulfur diagenesis in organic-rich sediments of the Benguela upwelling system

Episodically deposited, dark, organic-rich Pleistocene and Late Pliocene se diments from the lower continental slope off southwest Africa reveal comple x interactions between changes in primary production, benthic foraminiferal assemblage, and anaerobic microbial processes. The organic-rich layers con tain diatom assemblages characteristic of intense seasonal coastal upwellin g whereas stratigraphically adjacent sediments reflect pelagic primary prod uction. Coastal upwelling-dominated depositional intervals coincide with pe riods of enhanced carbon flux to the seafloor. Enhanced organic carbon expo rt during dark layer deposition was accompanied by decreases in the diversi ty of benthic foraminifera to few opportunistic species adapted to high phy todetritus accumulation rates and low O-2 conditions. In all sediments the sulfur isotopic composition of pyrite indicates redox cycling of sulfide cl ose to the sediment/water interface. The sulfur isotopic evidence and the p ermanent presence of abundant low O-2-adapted benthic foraminifera througho ut the organic-rich layers suggest an oxygenated benthic environment. Effic ient oxidation of sulfide and removal of sulfide by sulfidization of organi c matter inhibited buildup of toxic hydrogen sulfide from bacterial sulfate reduction at the sediment/water interface. These data imply that in contin ental slope sediments underneath productive surface waters benthic dysoxic conditions are maintained by the lateral advection of dissolved oxygen to s upport a small, but well-adapted benthic community. (C) 2000 Elsevier Scien ce B.V. All rights reserved.