Pollen and Spores in Marine Sediments from the East Atlantic
531
borne pollen and spores (Bengo 1997). The composition of river-borne pollen spectra reflects the
source area of the drainage basin of the river. Along
the northern coast of the Gulf of Guinea, source
areas of river-borne pollen from the numerous rivers partly coincide with those of pollen transported
by the NE-trade winds blowing offthe continent
during boreal winter, when the ITZC is at its
southernmost position. The SW-monsoon is not
expected to bring much pollen from southern
Africa because of its long trajectory over sea. Another possibility is transport by mid-tropospheric
southeasterly and easterly winds that potentially
could bring pollen from Central Africa (Wyputta
pers comm). The distribution of pollen in the marine surface sediments reflects rather accurately
the distribution of the forest on the adjacent continent indicating that the majority of pollen comes
from the regions north and northeast of the Gulf of
Guinea (Dupont and Agwu 1991).
Around the Niger delta, however, source areas
of wind and water transported pollen have no complete overlap. The Niger river flows in a northeasterly direction from the Guinean mountains to Timbuktu, reaching 17°N before flowing southward in
the direction of the Gulf of Guinea, thereby crossing several vegetation zones. Just south of8°N, the
Niger is joined by the Benue draining the Jos Plateau. The drainage area of the Niger thus includes
the Sudanian, Sahelian, (and southern Saharan)
vegetation zones as well as the Guineo-Congolian
rain forests and the mangrove swamps of its delta.
Members of the grass family grow in various
vegetation types all over the world. The abundance
and diversity in the African savannas, however,
makes this region a main source area for grass
pollen. Grasses are most abundant between 12 and
19°N. This latitudinal zone includes the semi-desert
grasslands, the Sahel Acacia wooded grasslands
and the Sudanian wooded grasslands. Grasses also
occur in the Sahara itself, but this source area is
probably oflesser importance.
The distribution pattern ofPoaceae (grass) pollen in the marine sediments west of West Africa
(Fig. 7) reflects the latitudes ofthe savanna to the
open dry forest (Sahelian and Sudanian vegetation
zones). Highest relative amounts of grass pollen are
found west of the savanna area between 10 and
200N with pollen percentages exceeding 30%
(Dupont and Agwu 1991). The distinct change in
percentages of Chenopodiaceae-Amaranthaceae
and Poaceae pollen at 19°N clearly reflects the
transition from Saharan desert to Sahelian
grasslands (Lezine and Hooghiemstra 1990).
In a zone ranging from 8-12°N in Guinea to 48°N in Cameroon, the natural vegetation gradually
changes from Sudanian wooded savanna to tropical rain forest. A savanna corridor, also called Togo
or Dahomey Gap, reaches the coast in Togo and
Benin and divides the Guineo-Congolian rain forest into a western and a eastern part. As a consequence, grass pollen percentages are much lower
off the northern coast of the Gulf of Guinea. However, the areas south of the Niger delta and the
Volta mouth form exceptions. Although the coastal
mangrove swamps provide large amounts of
Rhizophora pollen, grass pollen percentages rise
up to 20% of the total. The high grass pollen percentages in this area are attributed, on the one hand,
to represent the savanna and dry forest corridor
reaching down to the coast in Togo and Benin, and
on the other hand, to river transport. Surface samples from the central eaquatorial Atlantic are
also comparatively rich in grass pollen. Those
sediments, however, were sampled too far from
the coast to include river-borne material
(Westerhausen et al. 1993), but lay under the winter dust plume, which may bring grass pollen from
the Sudanian savanna (see also Wagner and
Dupont this volume).
Confirmation of the importance of river transport in the case of grass pollen is found in the
sediments of Site GIK16856. The 17 m long piston core from Site GIKI6856 mostly consists of
silty clay and clay. The stable isotope record of the
planktonic foraminifer Globigerinoides ruber
(white variety) covers the period back to late Stage
6. Sediments dated before 145 kaand between 122
and 125 ka do not contain foraminifera as the marine portion is strongly diluted by clastic sediments.
Those sediments are also characterised by high
sedimentation rates and the occurrence of colonies
of the fresh-water aglae Pediastrum and
Botryococcus. During the periods before 142 ka,
from 121 to 125 ka, and to a lesser extent from 100
to 105 ka, the deposition of fresh-water algae
531
borne pollen and spores (Bengo 1997). The composition of river-borne pollen spectra reflects the
source area of the drainage basin of the river. Along
the northern coast of the Gulf of Guinea, source
areas of river-borne pollen from the numerous rivers partly coincide with those of pollen transported
by the NE-trade winds blowing offthe continent
during boreal winter, when the ITZC is at its
southernmost position. The SW-monsoon is not
expected to bring much pollen from southern
Africa because of its long trajectory over sea. Another possibility is transport by mid-tropospheric
southeasterly and easterly winds that potentially
could bring pollen from Central Africa (Wyputta
pers comm). The distribution of pollen in the marine surface sediments reflects rather accurately
the distribution of the forest on the adjacent continent indicating that the majority of pollen comes
from the regions north and northeast of the Gulf of
Guinea (Dupont and Agwu 1991).
Around the Niger delta, however, source areas
of wind and water transported pollen have no complete overlap. The Niger river flows in a northeasterly direction from the Guinean mountains to Timbuktu, reaching 17°N before flowing southward in
the direction of the Gulf of Guinea, thereby crossing several vegetation zones. Just south of8°N, the
Niger is joined by the Benue draining the Jos Plateau. The drainage area of the Niger thus includes
the Sudanian, Sahelian, (and southern Saharan)
vegetation zones as well as the Guineo-Congolian
rain forests and the mangrove swamps of its delta.
Members of the grass family grow in various
vegetation types all over the world. The abundance
and diversity in the African savannas, however,
makes this region a main source area for grass
pollen. Grasses are most abundant between 12 and
19°N. This latitudinal zone includes the semi-desert
grasslands, the Sahel Acacia wooded grasslands
and the Sudanian wooded grasslands. Grasses also
occur in the Sahara itself, but this source area is
probably oflesser importance.
The distribution pattern ofPoaceae (grass) pollen in the marine sediments west of West Africa
(Fig. 7) reflects the latitudes ofthe savanna to the
open dry forest (Sahelian and Sudanian vegetation
zones). Highest relative amounts of grass pollen are
found west of the savanna area between 10 and
200N with pollen percentages exceeding 30%
(Dupont and Agwu 1991). The distinct change in
percentages of Chenopodiaceae-Amaranthaceae
and Poaceae pollen at 19°N clearly reflects the
transition from Saharan desert to Sahelian
grasslands (Lezine and Hooghiemstra 1990).
In a zone ranging from 8-12°N in Guinea to 48°N in Cameroon, the natural vegetation gradually
changes from Sudanian wooded savanna to tropical rain forest. A savanna corridor, also called Togo
or Dahomey Gap, reaches the coast in Togo and
Benin and divides the Guineo-Congolian rain forest into a western and a eastern part. As a consequence, grass pollen percentages are much lower
off the northern coast of the Gulf of Guinea. However, the areas south of the Niger delta and the
Volta mouth form exceptions. Although the coastal
mangrove swamps provide large amounts of
Rhizophora pollen, grass pollen percentages rise
up to 20% of the total. The high grass pollen percentages in this area are attributed, on the one hand,
to represent the savanna and dry forest corridor
reaching down to the coast in Togo and Benin, and
on the other hand, to river transport. Surface samples from the central eaquatorial Atlantic are
also comparatively rich in grass pollen. Those
sediments, however, were sampled too far from
the coast to include river-borne material
(Westerhausen et al. 1993), but lay under the winter dust plume, which may bring grass pollen from
the Sudanian savanna (see also Wagner and
Dupont this volume).
Confirmation of the importance of river transport in the case of grass pollen is found in the
sediments of Site GIK16856. The 17 m long piston core from Site GIKI6856 mostly consists of
silty clay and clay. The stable isotope record of the
planktonic foraminifer Globigerinoides ruber
(white variety) covers the period back to late Stage
6. Sediments dated before 145 kaand between 122
and 125 ka do not contain foraminifera as the marine portion is strongly diluted by clastic sediments.
Those sediments are also characterised by high
sedimentation rates and the occurrence of colonies
of the fresh-water aglae Pediastrum and
Botryococcus. During the periods before 142 ka,
from 121 to 125 ka, and to a lesser extent from 100
to 105 ka, the deposition of fresh-water algae