Sea-floor distribution and Late Quaternary faunal patterns of planktonic and benthic foraminifers in the Angola Basin

Abstract

The distribution of planktonic and benthic foraminifers was examined in some hundred core-tops from the Angola Basin, mainly from depths below 2000 metres. A number of biofacies have been discriminated in the planktonic faunas on the sea-floor and these show a clear relation with the hydrography of the (near-) surface waters. Species distribution can generally be described satisfactorily in terms of surface-water temperature and fertility. However, if the thermocline reaches into the photic zone, faunas different from those of the surface-mixed layer are found at the top of the thermocline. The faunal differences between the top of the thermocline and the surface-water can not simply be attributed to differences in temperature and fertility. We suppose that some species are specifically linked up with a steep thermal gradient, whereas others can flourish in thermally homogeneous water only. The deep-sea benthic foraminiferal faunas vary primarily with depth. In addition there appear to be differences between the area along the African continent and areas far away from the continent. Faunal contrasts are found also within these areas. The lateral differences must be due to variations in sediment-related parameters and it is suggested that the amount of organic matter at and in the bottom plays a crucial role. Depth-related changes in the faunas are thought to be controlled by vertical gradients in bottom-water temperature and in the amount of organic matter arriving at the bottom. It should be mentioned that our results do not support the widely accepted idea that there is a relation between Nuttallides umboniferus and Antarctic Bottom Water. In a more general sense, we contend that fauna/water-mass relations are actually to be reduced to relations between fauna and bottomwater temperature. Late Quaternary faunal change was studied in five piston-cores from depths between 2000 and 4000 metres in the marginal area of the north-eastern Angola Basin. A climate stratigraphy was established for the last 150,000 years on the basis of regional changes in the planktonic foraminiferal faunas. The regional changes in the planktonic faunas are thought to be essentially due to variations in cold-water advection by the Benguela Current and in intensity of equatorial divergence processes. These two factors do not only influence the temperature of the surface-water, but also control the degree to which the photic layer is thermally stratified. It is concluded that the primary production of the photic layer was much higher during cold than during warm climatic stages. The influence of the Benguela Current has been stronger during cold than during warm climatic phases and reached a maximum in isotope stage 4. Equatorial divergence was intense during isotope stages 2 and 3 and weak during stage 4, substage 5e and periods of deglaciation. It is suggested that intensification of the Benguela Current and intensification of equatorial divergence have generally been out of phase. Changes in wind direction could offer an explanation for this inverse relation. Consistent differences between warm and cold climatic stages show up also in the benthic faunas. In most cores, distinction must be made between an autochthonous and an allochthonous faunal signal. At depths beteen 2700 and 3500 metres in the area north of the Zaire deep-sea fan, the contribution of allochthonous elements reaches maxima in cold climatic stages. This suggests increased down-slope transport during periods of low sea-level stands. Climate related changes in the frequencies of autochthonous species are recognized at all sites and are to be ascribed primarily to variation in the amount of organic matter at and in the bottom. The amount of organic matter must have been larger in cold than in warm climatic intervals. This is to be attributed mainly to increased primary production of the surface-waters during cold climatic periods. It is further suggested that bottom-water temperatures have varied in the Angola Basin over the past 150,000 years. Variation in bottom-water temperature seems only partially linked up with climatic change. At great depth, bottom waters were colder than today during isotope stages 2 and 4 and the middle of stage 5. Interpretation of glacial/interglacial differences in benthic deep-sea faunas reported in the literature for the NW Atlantic Ocean and Southern Ocean, indicates that these differences may also be due to variation in the organic matter content of the sediments. Again, increased primary production during cold climatic periods seems likely.