without vesicles or sacci (see Figures 1f–i and 2g–i).
Conifer pollen grains are often characterized by a pair of
sacci and are designated as bisaccate. The size of gymnosperm pollen is variable, ranging from about 10 mm in simple forms to about 200 mm for large bisaccate taxa.
The pollen grains of angiosperms have a great morphological diversity (see Figures 1j–r and 2j–s). They are
identified on the basis of their geometry (spherical, oblate,
prolate), the type of aperture (circular (pore) or elongate
(colpus)), the number and distribution of apertures, the
ornamentation (smooth, spiny, reticulate, striated), and
structure of the exine or pollen grain outer membrane.
Their size is usually comprised between 5 and 50 mm.
In contrary to many other microfossil groups, pollen
grains and spores are characterized by homogenous morphology within their taxonomical categories, at family,
genus, or species level. Hence, keys based on the
abovementioned characters have been developed for the
identification of pollen and spores (e.g., Faegri and
Iversen, 1989; Moore et al., 1991; Reille, 1992;
Kapp et al., 2000).
One particularity of pollen and spores is the chemical
compound of the sporopollenin that forms the exine and
sporoderm, respectively, and which consists in extremely
resistant biological polymers (e.g., Traverse, 2007). The
chemical composition of sporopollenin, however, is very
complex and differs according to the taxonomical affinities of pollen and spores (e.g., Hemsley et al., 1993;
Hemsley and Poole, 2004).
Evolution of land plants and pollen and spore
stratigraphy
The evolution of land plants through the geological history has been mostly documented from microfossils,
including pollen and spores. The land plants or embryophytes developed from green algae ancestors in the
Paleozoic. Their first occurrence might be inferred from
spores recovered in mid-Ordovician deposits dated of
ca. 470–460 Ma (e.g., Steemans et al., 2009), and the
diversification of vascular plants occurred in the Silurian
about 420 Ma ago (e.g., Kenrick and Crane, 1997;
Kenrick et al., 2012). Seedless plants remained dominant
until about 320–300 Ma ago when the first spermatophytes developed. Seed plants were first represented by
the gymnosperms, which diversified during the Carboniferous and Permian with the successive development of
cycadophytes,
ginkgophytes,
gnetophytes,
and
pinophytes (e.g., Magallón et al., 2013). The occurrence
of angiosperms or flowering plants, which are characterized not only by a reproduction from the sporophyte but
also by flowers and protected seeds, took place much later
but with a spectacular diversification of taxa during the
Jurassic and early Cretaceous, from about 200 to 130 Ma
(e.g., Magallón et al., 2013).
From a stratigraphical point of view, spores date from
the Ordovician, whereas inaperturate and saccate pollen
grains date from the Carboniferous. Pollen grains of
angiosperms bearing pores and colpi range mostly from
the Cretaceous.
Because the sporopollenin is highly resistant to diagenetic alteration, except under highly oxidizing conditions,
all sedimentary rocks dating from the Ordovician to recent
potentially contain fossil pollen and spores. Hence,
reworked Paleozoic or Mesozoic pollen and spores
are frequently recovered in late Cenozoic sediments.
The distinction of “fresh” versus old reworked pollen
and spores can be made from taxa identification and from
the state of preservation as aged pollen grains and spores
have lost the ultrastructure and fluorescent properties of
their exine or sporoderm through diagenetic processes.
Pollen and spore transport into marine sediments
Pollen and spores are routinely recovered in marine sediments from coastal areas and continental margins. They
are necessarily allochthonous, and their occurrence
depends upon production in the source area in addition
to transport mechanisms through air or water. Pollen and
spore deposition decreases rapidly with the distance from
source areas. Pollen grains produced at high elevation
above ground, from the tree canopy, for example, are more
prone to be carried by wind than pollen produced from the
herb strata closer to the ground. Moreover, pollen grains
from anemophilous plants have attributes fostering transport by wind. in particular, the pinophytes characterized
by saccate morphology. In addition to atmospheric transport, hydrodynamic transport through runoff, rivers, and
marine currents has to be taken into account.
The long-distance atmospheric transport of pollen
results in a nearly asymptotic decrease of pollen and spore
concentration with the distance of the source area (e.g.,
Rochon and de Vernal, 1994). It also results in important
modifications of taxa percentages in assemblages, because
of their selective transport by wind. For example, as
shown from the distribution of pollen in surface sediment
samples, there is an offshore overrepresentation of some
tree taxa and especially of Pinus pollen grains that have
particularly efficient aerial dispersion properties (e.g.,
Heusser and Balsam, 1977; Mudie, 1982; Sun et al.,
1999).
The transport of pollen grains and spores with water is
less selective than that in the atmosphere. In estuarine
environment, deltas, and epicontinental seas, pollen grains
and spores can be very abundant reaching concentration of
10
5 g
À1
, which are comparable to those of lakes (e.g., de
Vernal and Giroux, 1991; Marret et al., 2001; Traverse,
2007). Detailed studies from neritic settings along southwest European margins, where relatively humid climate
prevails, have shown that most pollen and spores recovered in marine sediments are intimately related to fluvial
inputs from adjacent lands, allowing direct land-sea correlations (e.g., Sánchez Goñi et al., 1999; see Figure 3a). In a
different context, the distribution pattern of pollen grains
off northwest Africa permitted to identify aeolian transport through trade winds, in addition to input by rivers,
656
PALYNOLOGY (POLLEN, SPORES, ETC.)
Conifer pollen grains are often characterized by a pair of
sacci and are designated as bisaccate. The size of gymnosperm pollen is variable, ranging from about 10 mm in simple forms to about 200 mm for large bisaccate taxa.
The pollen grains of angiosperms have a great morphological diversity (see Figures 1j–r and 2j–s). They are
identified on the basis of their geometry (spherical, oblate,
prolate), the type of aperture (circular (pore) or elongate
(colpus)), the number and distribution of apertures, the
ornamentation (smooth, spiny, reticulate, striated), and
structure of the exine or pollen grain outer membrane.
Their size is usually comprised between 5 and 50 mm.
In contrary to many other microfossil groups, pollen
grains and spores are characterized by homogenous morphology within their taxonomical categories, at family,
genus, or species level. Hence, keys based on the
abovementioned characters have been developed for the
identification of pollen and spores (e.g., Faegri and
Iversen, 1989; Moore et al., 1991; Reille, 1992;
Kapp et al., 2000).
One particularity of pollen and spores is the chemical
compound of the sporopollenin that forms the exine and
sporoderm, respectively, and which consists in extremely
resistant biological polymers (e.g., Traverse, 2007). The
chemical composition of sporopollenin, however, is very
complex and differs according to the taxonomical affinities of pollen and spores (e.g., Hemsley et al., 1993;
Hemsley and Poole, 2004).
Evolution of land plants and pollen and spore
stratigraphy
The evolution of land plants through the geological history has been mostly documented from microfossils,
including pollen and spores. The land plants or embryophytes developed from green algae ancestors in the
Paleozoic. Their first occurrence might be inferred from
spores recovered in mid-Ordovician deposits dated of
ca. 470–460 Ma (e.g., Steemans et al., 2009), and the
diversification of vascular plants occurred in the Silurian
about 420 Ma ago (e.g., Kenrick and Crane, 1997;
Kenrick et al., 2012). Seedless plants remained dominant
until about 320–300 Ma ago when the first spermatophytes developed. Seed plants were first represented by
the gymnosperms, which diversified during the Carboniferous and Permian with the successive development of
cycadophytes,
ginkgophytes,
gnetophytes,
and
pinophytes (e.g., Magallón et al., 2013). The occurrence
of angiosperms or flowering plants, which are characterized not only by a reproduction from the sporophyte but
also by flowers and protected seeds, took place much later
but with a spectacular diversification of taxa during the
Jurassic and early Cretaceous, from about 200 to 130 Ma
(e.g., Magallón et al., 2013).
From a stratigraphical point of view, spores date from
the Ordovician, whereas inaperturate and saccate pollen
grains date from the Carboniferous. Pollen grains of
angiosperms bearing pores and colpi range mostly from
the Cretaceous.
Because the sporopollenin is highly resistant to diagenetic alteration, except under highly oxidizing conditions,
all sedimentary rocks dating from the Ordovician to recent
potentially contain fossil pollen and spores. Hence,
reworked Paleozoic or Mesozoic pollen and spores
are frequently recovered in late Cenozoic sediments.
The distinction of “fresh” versus old reworked pollen
and spores can be made from taxa identification and from
the state of preservation as aged pollen grains and spores
have lost the ultrastructure and fluorescent properties of
their exine or sporoderm through diagenetic processes.
Pollen and spore transport into marine sediments
Pollen and spores are routinely recovered in marine sediments from coastal areas and continental margins. They
are necessarily allochthonous, and their occurrence
depends upon production in the source area in addition
to transport mechanisms through air or water. Pollen and
spore deposition decreases rapidly with the distance from
source areas. Pollen grains produced at high elevation
above ground, from the tree canopy, for example, are more
prone to be carried by wind than pollen produced from the
herb strata closer to the ground. Moreover, pollen grains
from anemophilous plants have attributes fostering transport by wind. in particular, the pinophytes characterized
by saccate morphology. In addition to atmospheric transport, hydrodynamic transport through runoff, rivers, and
marine currents has to be taken into account.
The long-distance atmospheric transport of pollen
results in a nearly asymptotic decrease of pollen and spore
concentration with the distance of the source area (e.g.,
Rochon and de Vernal, 1994). It also results in important
modifications of taxa percentages in assemblages, because
of their selective transport by wind. For example, as
shown from the distribution of pollen in surface sediment
samples, there is an offshore overrepresentation of some
tree taxa and especially of Pinus pollen grains that have
particularly efficient aerial dispersion properties (e.g.,
Heusser and Balsam, 1977; Mudie, 1982; Sun et al.,
1999).
The transport of pollen grains and spores with water is
less selective than that in the atmosphere. In estuarine
environment, deltas, and epicontinental seas, pollen grains
and spores can be very abundant reaching concentration of
10
5 g
À1
, which are comparable to those of lakes (e.g., de
Vernal and Giroux, 1991; Marret et al., 2001; Traverse,
2007). Detailed studies from neritic settings along southwest European margins, where relatively humid climate
prevails, have shown that most pollen and spores recovered in marine sediments are intimately related to fluvial
inputs from adjacent lands, allowing direct land-sea correlations (e.g., Sánchez Goñi et al., 1999; see Figure 3a). In a
different context, the distribution pattern of pollen grains
off northwest Africa permitted to identify aeolian transport through trade winds, in addition to input by rivers,
656
PALYNOLOGY (POLLEN, SPORES, ETC.)