BENTHIC FORAMINIFERA FROM THE DIATOMACEOUS MUD BELT OFF NAMIBIA: CHARACTERISTIC SPECIES FOR SEVERE ANOXIA

Carola Leiter and Alexander V. Altenbach

ABSTRACT

Surface sediments were recovered at 35 stations from the oxygen depleted diatomaceous mud belt off Namibia, at water depths from 28 to 152 m. The oxygen content of the bottom near water masses was dysoxic (0.2 to 0.8 ml l^-1) at 16 stations, microxic (0.1 ml l^-1) at nine stations, and free of dissolved O₂ at 10 stations. Four species of benthic foraminifera were repeatedly found living (stained) in anoxic and sulfidic environments of the central part of the diatomaceous mud belt. Virgulinella fragilis is restricted to sulfidic, anoxic, and weakly dysoxic environments, with a distinct upper threshold of dissolved oxygen in the bottom near water masses at 0.3 ml O₂ l^-1. Nonionella stella, Fursenkoina fusiformis, and Bolivina pacifica were found stained under such oxygen depleted conditions, but also in better oxygenated environments. These four species were recovered stained also from deep infaunal habitats where sulfate reduction persists. They also represent the main contributors to the thanatocoenosis. Lateral advection of unstained test and migrational tries of some other species is favored by prevailing landward bottom currents. Among all species observed, V. fragilis is considered a characteristic element of sulfidic to dysoxic environments. Even the earliest records of the genus are linked to carbonate aggressive, sulfidic, or oxygen depleted palaeoenvironments.

Carola Leiter. Department of Earth and Environmental Science, Ludwig-Maximilians-University, Richard-Wagner-Str. 10, 80333 Munich, Germany
Alexander V. Altenbach. GeoBio-Center^LMU and Department of Earth and Environmental Science, Ludwig-Maximilians-University, Richard-Wagner-Str. 10, 80333 Munich, Germany.

KEY WORDS: anoxia; sulfidic; inner shelf; Virgulinella fragilis; Benguela-current