540
Dupont
Podocarpus pollen percentages show another
maximum in the early to middle Holocene (Shi and
Dupont 1997) but decline afterwards.
From 70 to 10 ka BP, high pollen percentages
of Poaceae and increasing ones of tubuliflorous
composites show a northward shift of the Namib
desert. However, the maximum percentages of
Poaceae never reach values as high as those found
directly west of the Namib desert (Van Zinderen
Bakker 1984). So, the northernmost position ofthe
Namib probably did not cross the latitude ofthe site
(l2°S), although it came close at the end of Stage
2.
Concluding Remarks
Palynological studies of marine sediments allow
estimates as to the spatial changes in major vegetation zones in Africa over the last 150 ka.
Information of the regional extension of
phytogeographical units and the latitudinal position
of vegetation zones during climatic cycles provides
important clues about the climatic regime of the
continent.
Fig. 10 provides a schematic representation of
vegetation and pollen distributions of the Last
Glacial Maximum (LGM) compared to those of
modern interglacial conditions. The LGM can be
regarded as the most extreme time-slice in terms
of reduction of rain forest and mangrove swamp
areas. Other time-slices show distinctive characteristics. For example, Stage 5 shows large variation in the extent of Po do carpus-rich Afromontane
forest areas.
The natural Mediterranean vegetation consists
mainly of Mediterranean oak forests (both
sclerophylous and deciduous) and conifer forests
(dominated by pine and ceder). These communities strongly declined or disappeared completely
from western Morocco during the LGM and were
replaced by an open steppe-like vegetation with
Artemisia, Ephedra, and Chenopodiaceae. The
southward transport of pollen from the Mediterranean area and the northern Sahara strongly increases during the LGM (Fig. lO, signature #1).
Pollen from the Sahara and the Sahel are effectively transported by the African Easterly Jet
(Fig. 10, signature #2). Although vegetation zones
shift in latitude through the course of a climatic
cycle, the latitudinal position of the African Easterly Jet is stable. The pollen load ofthis wind system depends on the vegetation growing between
ca. 15 and J 7°N. A northern position of the Saharan-Sahel ian boundary results in increased abundance of grass pollen in marine sediments around
20oN, transported by the northern branch of the
African Easterly Jet.
The representation ofthe modern savanna vegetation zones in the marine sediments is not differentiated as the high grass pollen content dominates
over pollen from more specific elements. High
grass pollen values (30-50% or higher; Fig. 10, signature #3) include the area west of the Sahelian
and Sudanian savanna zone (between line A and
B) and the area west of the southern Zambezian
woodland and the northern Kalahari shrub land (between line C and D). During the LGM, grass pollen percentages are high (>30%) in marine
sediments between lines A and D, with exception
of the northernmost Angola basin. They indicate
the expansion of open vegetation types displacing
denser forested ones.
Marine areas adjacent to tropical rain forests
have a rich flora including pollen of many different
trees from the tropical rain forest and the dry deciduous forest (Fig. 10, signature #4). Because
most of those trees produce little pollen, they are
represented only in small numbers. An exception
is Alchornea, which is a pioneer tree and, therefore, not growing in the denser parts of the rain
forest. Nevertheless, its comparably large pollen
production strongly improves the representation of
the rain forest in marine sediments (between lines
B and C). Pollen grains from Rhizophora are numerous (> J 0%) along the coastal areas with mangrove swamps (Fig. J 0, signature #5). During the
LGM, between lines A and D, the representation
of the rain forest and the mangrove swamp dropped
dramatically, although they did not completely disappear. Biogeographical evidence indicates rain
forest refuge areas southwest of the Guinean
mountains, along the coast in easternmost Ivory
Coast, and west of the mountains in Cameroon,
Gabon, and Congo (Sosef 1996).
Dupont
Podocarpus pollen percentages show another
maximum in the early to middle Holocene (Shi and
Dupont 1997) but decline afterwards.
From 70 to 10 ka BP, high pollen percentages
of Poaceae and increasing ones of tubuliflorous
composites show a northward shift of the Namib
desert. However, the maximum percentages of
Poaceae never reach values as high as those found
directly west of the Namib desert (Van Zinderen
Bakker 1984). So, the northernmost position ofthe
Namib probably did not cross the latitude ofthe site
(l2°S), although it came close at the end of Stage
2.
Concluding Remarks
Palynological studies of marine sediments allow
estimates as to the spatial changes in major vegetation zones in Africa over the last 150 ka.
Information of the regional extension of
phytogeographical units and the latitudinal position
of vegetation zones during climatic cycles provides
important clues about the climatic regime of the
continent.
Fig. 10 provides a schematic representation of
vegetation and pollen distributions of the Last
Glacial Maximum (LGM) compared to those of
modern interglacial conditions. The LGM can be
regarded as the most extreme time-slice in terms
of reduction of rain forest and mangrove swamp
areas. Other time-slices show distinctive characteristics. For example, Stage 5 shows large variation in the extent of Po do carpus-rich Afromontane
forest areas.
The natural Mediterranean vegetation consists
mainly of Mediterranean oak forests (both
sclerophylous and deciduous) and conifer forests
(dominated by pine and ceder). These communities strongly declined or disappeared completely
from western Morocco during the LGM and were
replaced by an open steppe-like vegetation with
Artemisia, Ephedra, and Chenopodiaceae. The
southward transport of pollen from the Mediterranean area and the northern Sahara strongly increases during the LGM (Fig. lO, signature #1).
Pollen from the Sahara and the Sahel are effectively transported by the African Easterly Jet
(Fig. 10, signature #2). Although vegetation zones
shift in latitude through the course of a climatic
cycle, the latitudinal position of the African Easterly Jet is stable. The pollen load ofthis wind system depends on the vegetation growing between
ca. 15 and J 7°N. A northern position of the Saharan-Sahel ian boundary results in increased abundance of grass pollen in marine sediments around
20oN, transported by the northern branch of the
African Easterly Jet.
The representation ofthe modern savanna vegetation zones in the marine sediments is not differentiated as the high grass pollen content dominates
over pollen from more specific elements. High
grass pollen values (30-50% or higher; Fig. 10, signature #3) include the area west of the Sahelian
and Sudanian savanna zone (between line A and
B) and the area west of the southern Zambezian
woodland and the northern Kalahari shrub land (between line C and D). During the LGM, grass pollen percentages are high (>30%) in marine
sediments between lines A and D, with exception
of the northernmost Angola basin. They indicate
the expansion of open vegetation types displacing
denser forested ones.
Marine areas adjacent to tropical rain forests
have a rich flora including pollen of many different
trees from the tropical rain forest and the dry deciduous forest (Fig. 10, signature #4). Because
most of those trees produce little pollen, they are
represented only in small numbers. An exception
is Alchornea, which is a pioneer tree and, therefore, not growing in the denser parts of the rain
forest. Nevertheless, its comparably large pollen
production strongly improves the representation of
the rain forest in marine sediments (between lines
B and C). Pollen grains from Rhizophora are numerous (> J 0%) along the coastal areas with mangrove swamps (Fig. J 0, signature #5). During the
LGM, between lines A and D, the representation
of the rain forest and the mangrove swamp dropped
dramatically, although they did not completely disappear. Biogeographical evidence indicates rain
forest refuge areas southwest of the Guinean
mountains, along the coast in easternmost Ivory
Coast, and west of the mountains in Cameroon,
Gabon, and Congo (Sosef 1996).