Pollen and Spores in Marine Sediments from the East Atlantic
525
Grazzini 1993; Dupont and Agwu 1992;
Hooghiemstra et al. 1992; Dupont and Weinelt
1996; Fredoux 1994; Marret and Turon 1994; Jahns
1996; Jahns et al 1998; Shi and Dupont 1997;
Dupont et al. 1998) have been published.
In this article, I will examine some aspects of
marine palynology focussing on pollen data from
the East Atlantic. I will not discuss the palynology
oflittoral or shelf sediments, because the proximity ofthe continent, the fluctuations in water table,
and the differences in sedimentation regime would
require a very different approach. Neither is an
attempt made to give a review of previous work
since that would surpass the scope of this paper.
In the interpretation of pollen data, several aspects must be taken into account: these include
source and production of pollen grains, transport to
the sea and through the water column, displacement
by ocean currents, sedimentation processes, fossilisation and accumulation in the sediment. Production of pollen grains varies strongly between different species. After pollen and spores reach
the sediments, they still can be displaced by
sedimentological processes such as bioturbation or
sediment movement. Embedded within very slowly
accumulating sediments at great water depths, pollen and spores may decay if bottom waters over
the site are rich in oxygen and therefore corrosive
(Keil et al. 1994), or the pollen and spore content
may become enriched by the dissolution of calcium
out ofthe sediment.
Because pollen grains from marine sediments
are typically transported over long distances, the
evaluation of transport agents is always part of the
interpretation of marine pollen diagrams. The following sections give examples of different transport mechanisms as derived from palynological
evidence of the East Atlantic.
PoUen Transport
Transport of pollen and spores from their source
areas to the marine sediments involves several
steps. After pollen grains are released by the plants
they disperse into the air and are transported by
winds, or the grains fall into a lake or a river and
are then carried by the river into the ocean. Aeolian
transport of pollen and spores predominates in deepsea sediments located far from the coast and along
arid areas with no or small river discharge into the
ocean (Heusser and Morley 1985; Hooghiemstra
et al. 1986). In contrast to desert margins, fluvial
transport of pollen and spores is especially high in
the humid tropical areas. The role of fluvial transport has been shown by an increased pollen concentration in sediments sampled from the vicinity
of river mouths (Muller 1959; Heusser and Balsam
1977; Davey and Rogers 1975; Heusser 1988;
Bengo 1997).
Pollen that has eventually got into the upper layers of the ocean has to be transported down through
the water column to the sea floor. Pollen grains are
too small and too light to have an effective sinking
velocity by themselves. However, pollen is caught
in larger aggregates - in faecal pellets or large
filamental aggregates - that can have sinking velocities of 100 m per day or more (Schrader 1971;
Silver et al. 1978; Honjo 1980; Wefer 1991, 1993;
Wefer and Fischer 1993). The transport through the
water column is more efficient in areas with a high
marine surface productivity where the incorporation of sporomorphs in the food chain is enhanced
(Ratmeyer et al. in press). The increased sinking
velocity reduces drifting by ocean currents. However, along continental slopes, strong boundary currents may displace pollen and spores (Heusser and
Balsam 1985; Dupont and Agwu 1991).
Palynological studies on modern sediments of
continental shelves reflect the influence of both
river input and sorting of particles, whereby the
sedimentation of pollen and spores can be compared
to that of the medium to fine silt fraction, because
of their size and density (Rossignol 1961; Davey
1971).
The relative importance of transport processes
from the continent into the ocean varies from region to region. For the East Atlantic they involve,
among others, trade-wind transport and mid-tropospheric wind transport (Fig. 1). Off the estuaries
of large rivers and in delta areas, river transport
predominates. Depending on the oceanographical
situation, ocean currents may have some influence.
I will give examples of wind transport of pollen from
North and Northwest Africa (by NE-trade winds
and African Easterly Jet) and of river transport of
pollen by the Niger.
525
Grazzini 1993; Dupont and Agwu 1992;
Hooghiemstra et al. 1992; Dupont and Weinelt
1996; Fredoux 1994; Marret and Turon 1994; Jahns
1996; Jahns et al 1998; Shi and Dupont 1997;
Dupont et al. 1998) have been published.
In this article, I will examine some aspects of
marine palynology focussing on pollen data from
the East Atlantic. I will not discuss the palynology
oflittoral or shelf sediments, because the proximity ofthe continent, the fluctuations in water table,
and the differences in sedimentation regime would
require a very different approach. Neither is an
attempt made to give a review of previous work
since that would surpass the scope of this paper.
In the interpretation of pollen data, several aspects must be taken into account: these include
source and production of pollen grains, transport to
the sea and through the water column, displacement
by ocean currents, sedimentation processes, fossilisation and accumulation in the sediment. Production of pollen grains varies strongly between different species. After pollen and spores reach
the sediments, they still can be displaced by
sedimentological processes such as bioturbation or
sediment movement. Embedded within very slowly
accumulating sediments at great water depths, pollen and spores may decay if bottom waters over
the site are rich in oxygen and therefore corrosive
(Keil et al. 1994), or the pollen and spore content
may become enriched by the dissolution of calcium
out ofthe sediment.
Because pollen grains from marine sediments
are typically transported over long distances, the
evaluation of transport agents is always part of the
interpretation of marine pollen diagrams. The following sections give examples of different transport mechanisms as derived from palynological
evidence of the East Atlantic.
PoUen Transport
Transport of pollen and spores from their source
areas to the marine sediments involves several
steps. After pollen grains are released by the plants
they disperse into the air and are transported by
winds, or the grains fall into a lake or a river and
are then carried by the river into the ocean. Aeolian
transport of pollen and spores predominates in deepsea sediments located far from the coast and along
arid areas with no or small river discharge into the
ocean (Heusser and Morley 1985; Hooghiemstra
et al. 1986). In contrast to desert margins, fluvial
transport of pollen and spores is especially high in
the humid tropical areas. The role of fluvial transport has been shown by an increased pollen concentration in sediments sampled from the vicinity
of river mouths (Muller 1959; Heusser and Balsam
1977; Davey and Rogers 1975; Heusser 1988;
Bengo 1997).
Pollen that has eventually got into the upper layers of the ocean has to be transported down through
the water column to the sea floor. Pollen grains are
too small and too light to have an effective sinking
velocity by themselves. However, pollen is caught
in larger aggregates - in faecal pellets or large
filamental aggregates - that can have sinking velocities of 100 m per day or more (Schrader 1971;
Silver et al. 1978; Honjo 1980; Wefer 1991, 1993;
Wefer and Fischer 1993). The transport through the
water column is more efficient in areas with a high
marine surface productivity where the incorporation of sporomorphs in the food chain is enhanced
(Ratmeyer et al. in press). The increased sinking
velocity reduces drifting by ocean currents. However, along continental slopes, strong boundary currents may displace pollen and spores (Heusser and
Balsam 1985; Dupont and Agwu 1991).
Palynological studies on modern sediments of
continental shelves reflect the influence of both
river input and sorting of particles, whereby the
sedimentation of pollen and spores can be compared
to that of the medium to fine silt fraction, because
of their size and density (Rossignol 1961; Davey
1971).
The relative importance of transport processes
from the continent into the ocean varies from region to region. For the East Atlantic they involve,
among others, trade-wind transport and mid-tropospheric wind transport (Fig. 1). Off the estuaries
of large rivers and in delta areas, river transport
predominates. Depending on the oceanographical
situation, ocean currents may have some influence.
I will give examples of wind transport of pollen from
North and Northwest Africa (by NE-trade winds
and African Easterly Jet) and of river transport of
pollen by the Niger.