Live (Stained) Benthic Foraminifera Off Walvis Bay, Namibia: A Deep-Sea Ecosystem under the Influence of Bottom Nepheloid Layers

Abstract

We present an ecological investigation of live (stained) foraminiferal faunas (>150-μm fraction) from four deep-sea stations sampled in the eastern South Atlantic off Walvis Bay, Namibia. These stations describe a bathymetric transect from ~900 to ~3000-m depth, below the oxygen minimum zone related to the Benguela upweling system. The mineralization of organic detritus, as suggested by oxygen-penetration depths and ΔpH depth profiles, is relatively intense along the overall bathymetrie transect. Organic carbon and amino-acid contents in the surface sediment are high between ~900-2000-m depth (e.g., C org > 1.7%, THAA > 47 nmol/mg), and decrease along the bathymetrie transect. Carbon-14 ages and grain-size distribution determined for the upper sediment suggest a complex balance between erosion, transportation, and deposition at all stations. Bottom nepheloid layers (BNLs) are well-documented in our study area. They are relevant candidates for the dispersion and accumulation of reworked organic compounds along the Namibian slope. Foraminiferal faunas living at the three shallower sites are characterized by species (e.g., Cibicidoides kullembergi, Bulimina inflatalcostata) related to the enhanced productivity. The occurrence of epibiotic species ( Cibicides refulgens, Gavelinopsis translucens, and C kullenbergi ) argues for bottom currents supplying organic compounds in suspension above the sediment-water interface. In those environmental conditions, Reophax bilocularis, R. pilulifer, and Lagenammina difflugiformis contribute significantly to live communities, especially at the lower-slope site. Live foraminiferal faunas are largely restricted to shalow-infaunal oxygenated microhabitats. Finally, we propose that hydro-sedimentary conditions characterized by more or less intermittent near-bottom currents 1) trigger the dominance of arenaceous faunas and 2) preclude the proliferation of a foraminiferal population in deep-infaunal microhabitats.