Pollen and Spores in Marine Sediments from the East Atlantic
527
duced in large quantities, in particular from March
to May, but pine pollen release continues at a lower
level until up to July (Hooghiemstra et al. 1986;
Rizzi Longo andCristofolini 1987; Caramielloetal.
1990). Southward winds are dominant in Southwest
Europe during the flowering period. Pine pollen
grains showed up in a sediment trap moored in the
vicinity of the Cape Verde Isles, where a maximum
of pine pollen was found for the period between
21 March to 4 April 1993 and may be attributed to
the spring bloom of pine trees in the western Mediterranean area (Agwu, unpublished results).
Maximum representation of pine pollen in marine surface sediments West of North Africa is
found at some distance from the coast, whereas
percentages decrease coastwards. South of the
Canary Islands, a secondary maximum is present,
apparently representing the pine forests on those
islands. Representation decreases southwards and
isopol lines are roughly parallel to the coast
(Hooghiemstra et al. 1986). The distribution pattern in the modem marine sediments matches the
average wind flow pattern of the NE-trade winds
(Fig. 3).
Ephedra pollen grains are also effectively
downwind tranported by the NE-trade winds. Species of Ephedra occur mainly in the northern and
western Sahara, in the Saharan-Mediterranean
transition zone, and in the drier parts of the Mediterranean (Knapp 1973; White 1983; Agwu and
Beug 1982). Ephedra major grows in the Haifa
steppe along the northern fringe of the Sahara
(Knapp 1973; Queze11965, 1978). The distribution
pattern of Ephedra pollen grains in marine surface
sediments corresponds with the distribution of
Ephedra on the continent. Low percentages in
samples oflower latitudes attest downwind transport by the NE-trade winds as far south as the
equator (Fig. 4; Hooghiemstra et al. 1986).
Compared to the modem situation, the distribution area of Ephedra on the continent strongly
expanded during the Last Glacial Maximum
(Hooghiemstra et al. 1987). The southernmost position of the desert is estimated to have been at 14°N
during glacial maxima (Rossignol-Strick and Duzer
1979; Hooghiemstra 1988b; Hooghiemstra and
Agwu 1988; Dupont 1993). Yet, an increase of
Ephedra pollen in glacial stages is found in marine
sediments at 9°N (Site GlK16415; Dupont and
Agwu 1992). The Ephedra pollen percentage
maxima at 9°N parallel those ofthe cores from Site
GIK12392 at 21 ° (Agwu and Beug 1982) and ODP
Site658at25°N(DupontetaI.1989). The maxima
in the northern cores are interpreted as recording
the enlargement ofthe distribution area ofEphedra on the continent during a glacial stage, whereas
the southern core reflects the effective southward
transport by enhanced NE trade-winds (Fig. 5; see
Fig. 2 for core positions).
Mid-Tropospheric Wind Transport by the
African Easterly Jet
Off West Africa, the lithogenic (siliciclastic)
sediments are dominated by aeolian dust. The main
transport of dust occurs in the Saharan Air Layer
coupled to the zonal mid-tropospheric African
Easterly Jet between 1000 and 5000 m altitude
(Fig. 1). These dust outbreaks follow the temperature gradient linked to the Intertropical Convergence Zone (ITCZ), which today attains its northernmost position near 18°N during northern summer (Siedler 1980; Tetzlaff and Wolter 1980;
Tetzlaff et al. 1989). Additional dust is supplied by
the NE-trade winds below 1000 m altitude. The
aeolian dust contribution to the East Atlantic was
much increased up to 800 km offshore during the
Last Glacial Maximum, but its centre showed almost no latitudinal shift to the south compared to
today (Koopmann 1981; Sarnthein et al. 1981;
Hooghiemstra et al. 1987).
The halophytic shrubs and herbs belonging
to the families of Chenopodiaceae and
Amaranthaceae have a wide distribution in the saline littoral zone of Northwest Africa and the extensive saline evaporation areas of the Sahara.
Their flowers produce great quantities of pollen
which is wind-dispersed. At the northern fringe of
the Sahara the period of main pollen release lasts
from November to April, but at the southern fringe
from June to September. In these periods of the
year, the NE-trade winds and the African Easterly
Jet (AEJ), respectively, are the major wind systems.
Furthermore, river systems are hardly present or
totally absent in these areas.
527
duced in large quantities, in particular from March
to May, but pine pollen release continues at a lower
level until up to July (Hooghiemstra et al. 1986;
Rizzi Longo andCristofolini 1987; Caramielloetal.
1990). Southward winds are dominant in Southwest
Europe during the flowering period. Pine pollen
grains showed up in a sediment trap moored in the
vicinity of the Cape Verde Isles, where a maximum
of pine pollen was found for the period between
21 March to 4 April 1993 and may be attributed to
the spring bloom of pine trees in the western Mediterranean area (Agwu, unpublished results).
Maximum representation of pine pollen in marine surface sediments West of North Africa is
found at some distance from the coast, whereas
percentages decrease coastwards. South of the
Canary Islands, a secondary maximum is present,
apparently representing the pine forests on those
islands. Representation decreases southwards and
isopol lines are roughly parallel to the coast
(Hooghiemstra et al. 1986). The distribution pattern in the modem marine sediments matches the
average wind flow pattern of the NE-trade winds
(Fig. 3).
Ephedra pollen grains are also effectively
downwind tranported by the NE-trade winds. Species of Ephedra occur mainly in the northern and
western Sahara, in the Saharan-Mediterranean
transition zone, and in the drier parts of the Mediterranean (Knapp 1973; White 1983; Agwu and
Beug 1982). Ephedra major grows in the Haifa
steppe along the northern fringe of the Sahara
(Knapp 1973; Queze11965, 1978). The distribution
pattern of Ephedra pollen grains in marine surface
sediments corresponds with the distribution of
Ephedra on the continent. Low percentages in
samples oflower latitudes attest downwind transport by the NE-trade winds as far south as the
equator (Fig. 4; Hooghiemstra et al. 1986).
Compared to the modem situation, the distribution area of Ephedra on the continent strongly
expanded during the Last Glacial Maximum
(Hooghiemstra et al. 1987). The southernmost position of the desert is estimated to have been at 14°N
during glacial maxima (Rossignol-Strick and Duzer
1979; Hooghiemstra 1988b; Hooghiemstra and
Agwu 1988; Dupont 1993). Yet, an increase of
Ephedra pollen in glacial stages is found in marine
sediments at 9°N (Site GlK16415; Dupont and
Agwu 1992). The Ephedra pollen percentage
maxima at 9°N parallel those ofthe cores from Site
GIK12392 at 21 ° (Agwu and Beug 1982) and ODP
Site658at25°N(DupontetaI.1989). The maxima
in the northern cores are interpreted as recording
the enlargement ofthe distribution area ofEphedra on the continent during a glacial stage, whereas
the southern core reflects the effective southward
transport by enhanced NE trade-winds (Fig. 5; see
Fig. 2 for core positions).
Mid-Tropospheric Wind Transport by the
African Easterly Jet
Off West Africa, the lithogenic (siliciclastic)
sediments are dominated by aeolian dust. The main
transport of dust occurs in the Saharan Air Layer
coupled to the zonal mid-tropospheric African
Easterly Jet between 1000 and 5000 m altitude
(Fig. 1). These dust outbreaks follow the temperature gradient linked to the Intertropical Convergence Zone (ITCZ), which today attains its northernmost position near 18°N during northern summer (Siedler 1980; Tetzlaff and Wolter 1980;
Tetzlaff et al. 1989). Additional dust is supplied by
the NE-trade winds below 1000 m altitude. The
aeolian dust contribution to the East Atlantic was
much increased up to 800 km offshore during the
Last Glacial Maximum, but its centre showed almost no latitudinal shift to the south compared to
today (Koopmann 1981; Sarnthein et al. 1981;
Hooghiemstra et al. 1987).
The halophytic shrubs and herbs belonging
to the families of Chenopodiaceae and
Amaranthaceae have a wide distribution in the saline littoral zone of Northwest Africa and the extensive saline evaporation areas of the Sahara.
Their flowers produce great quantities of pollen
which is wind-dispersed. At the northern fringe of
the Sahara the period of main pollen release lasts
from November to April, but at the southern fringe
from June to September. In these periods of the
year, the NE-trade winds and the African Easterly
Jet (AEJ), respectively, are the major wind systems.
Furthermore, river systems are hardly present or
totally absent in these areas.