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
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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.
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