Distribution of foraminifera and calcareous nannoplankton in quaternary sediments of the Eastern Angola Basin in response to climatic and oceanic fluctuations

Abstract

The impact of the Zaire River on the oceanic environment is clearly illustrated in the surface sediments by anomalously high carbonate dissolution rates over a large area off the river mouth. This anomaly results from the high supply of terrestrial organic matter brought into the Angola Basin by the outflowing river water. Carbonate dissolution indices demonstrate that downcore fluctuations in carbonate content are strongly controlled by carbonate dissolution processes. In the deep water cores variations in the rate of dissolution follow from changes in the position of the CCD. Variations in carbonate dissolution in the shallow cores are of local origin, and are linked to differences in the flux of land-derived organic matter. Associations of planktonic Foraminifera in the cored sediments of the eastern Angola Basin consist of tropical-subtropical and transitional-subantarctic species. The latter are considered as “southern intruders” brought northward by the Benguela Current and are potentially useful for deciphering changes in surface-water circulation during the Quaternary. Unfortunately, in almost all cores the planktonic foraminiferal signal in the “cold” periods has been obliterated by dissolution. The compositional pattern (in core T78-46) indicates that the Benguela Current had a stronger influence during the last glacial than it has today. High joint percentages of Epistominella exigua and Osangularia? umbonifera (in core T78-45) during the Holocene and Emian I document periods in which bottom waters were made up of an admixture of North Atlantic Deep Water and Antarctic Bottom Water just as they are today. Recurrent peak frequencies of Uvigerina peregrina may reflect intermittent stages of oxygen-poor bottom water. A distinct drop in radiolarian numbers across the W-X and Y-Z zonal boundaries is attributed to a periodically increased discharge of river water, which lowered primary productivity due to the short residence time and high turbidity of the surface water. The resultant diminished supply of biogenic opal to the bottom prevents the opaline skeletons from being preserved in the sediments.