12
Air-Vegetation Interface: Pollen
Joël Guiot
From the Production of Pollen to Sediment
In order to persist in difficult conditions, most plants and
terrestrial animals contain hard parts that are preserved in the
sediments after death. For higher-level terrestrial plants,
these parts consist mainly of pollen grains and spores which
provide a widely-used tool in paleoclimatology, thanks to
their abundance in wet sediments. These grains, which are
dispersed over variable distances depending on their shape
and size (from 5 to 100 microns), are an essential factor in
the reproduction of higher-level plants. Their outer envelope
(exine) is composed of sporopollenin, a highly resistant
substance once it is protected from oxidation. Lakes and
bogs are particularly good environments for the conservation
of these plant remains. These grains are scattered by wind,
insects, birds, water. In temperate regions, wind is the predominant vector, and because of its relative inefficiency, lots
of pollen grains and spores are found not far from their
source, in continental sediments and marine sediments near
the coast. In rainforests, animals play a much more important
role, so that many species are underrepresented in sediments.
After choosing a site representative of the surrounding
vegetation, with good conservation of pollen and sufficient
accumulation rates to permit studies over the desired time
scale, cores are extracted, usually at the center of the lake or
bog. These cores are studied stratigraphically and samples
are dated in order to establish an absolute chronology of
climate events. Samples are taken at regular intervals along
the core. They are subjected to physical and chemical
treatments to make the pollen grains clearly visible for
examination under a microscope. The grains are then identified on the basis of their exine which have different
morphologies depending on the plant type. The palynologist
counts each pollen type to work out the relative abundance
of the species of trees or grasses that produced the grains. It
is not always possible to determine each species, and many
plants can only be recognized at the level of the genus or
even family. Because of this heterogeneity in the classification, the term ‘pollen taxon’ is used to characterize the
type of plant that produced it. The total number of grains
counted varies depending on the diversity of the vegetation:
tropical vegetation is more diverse than temperate vegetation
and therefore requires a much higher total number, sometimes more than a thousand grains compared with a few
hundred in a temperate vegetation, and this ensures good
statistical significance of the various fluctuations detected.
The relative abundance of each taxon makes up a pollen
assembly or spectrum. It provides information on the relative
composition of the surrounding vegetation, but this signal is
influenced by the abundance of pollen productivity, its mode
and capacity of dispersion. Statistical methods are needed to
reliably decode the pollen spectrum (Moore et al. 1991).
The Pollen Diagram
The set of pollen spectra along the core is presented
graphically to provide a pollen diagram whose complexity
depends on the number of taxa counted. It can be simplified
either by grouping together similar taxa (i.e. taxa
co-evolving in similar environments), or by representing
only the most important ones. Figure 12.1 shows an example
of a simplified diagram. The interpretation of the diagram is
complex because of the large number of processes occurring
between the pollen production by the vegetation and its
record in the sediment. As in most paleontological disciplines, interpretation is done by comparison with modern
data. This supposes that the principle of uniformity, that is,
where the present is the key to the past, applies. This is
generally the case for data from the Quaternary, and in
J. Guiot (&)
European Centre for Research and Teaching in Environmental
Geosciences CEREGE, Aix-Marseille University, CNRS, IRD,
INRAE, Collège de France, BP 80, 13545, Aix-en-Provence,
Cedex 04, France
e-mail: guiot@cerege.fr
© Springer Nature Switzerland AG 2021
G. Ramstein et al. (eds.), Paleoclimatology, Frontiers in Earth Sciences,
https://doi.org/10.1007/978-3-030-24982-3_12
151
Air-Vegetation Interface: Pollen
Joël Guiot
From the Production of Pollen to Sediment
In order to persist in difficult conditions, most plants and
terrestrial animals contain hard parts that are preserved in the
sediments after death. For higher-level terrestrial plants,
these parts consist mainly of pollen grains and spores which
provide a widely-used tool in paleoclimatology, thanks to
their abundance in wet sediments. These grains, which are
dispersed over variable distances depending on their shape
and size (from 5 to 100 microns), are an essential factor in
the reproduction of higher-level plants. Their outer envelope
(exine) is composed of sporopollenin, a highly resistant
substance once it is protected from oxidation. Lakes and
bogs are particularly good environments for the conservation
of these plant remains. These grains are scattered by wind,
insects, birds, water. In temperate regions, wind is the predominant vector, and because of its relative inefficiency, lots
of pollen grains and spores are found not far from their
source, in continental sediments and marine sediments near
the coast. In rainforests, animals play a much more important
role, so that many species are underrepresented in sediments.
After choosing a site representative of the surrounding
vegetation, with good conservation of pollen and sufficient
accumulation rates to permit studies over the desired time
scale, cores are extracted, usually at the center of the lake or
bog. These cores are studied stratigraphically and samples
are dated in order to establish an absolute chronology of
climate events. Samples are taken at regular intervals along
the core. They are subjected to physical and chemical
treatments to make the pollen grains clearly visible for
examination under a microscope. The grains are then identified on the basis of their exine which have different
morphologies depending on the plant type. The palynologist
counts each pollen type to work out the relative abundance
of the species of trees or grasses that produced the grains. It
is not always possible to determine each species, and many
plants can only be recognized at the level of the genus or
even family. Because of this heterogeneity in the classification, the term ‘pollen taxon’ is used to characterize the
type of plant that produced it. The total number of grains
counted varies depending on the diversity of the vegetation:
tropical vegetation is more diverse than temperate vegetation
and therefore requires a much higher total number, sometimes more than a thousand grains compared with a few
hundred in a temperate vegetation, and this ensures good
statistical significance of the various fluctuations detected.
The relative abundance of each taxon makes up a pollen
assembly or spectrum. It provides information on the relative
composition of the surrounding vegetation, but this signal is
influenced by the abundance of pollen productivity, its mode
and capacity of dispersion. Statistical methods are needed to
reliably decode the pollen spectrum (Moore et al. 1991).
The Pollen Diagram
The set of pollen spectra along the core is presented
graphically to provide a pollen diagram whose complexity
depends on the number of taxa counted. It can be simplified
either by grouping together similar taxa (i.e. taxa
co-evolving in similar environments), or by representing
only the most important ones. Figure 12.1 shows an example
of a simplified diagram. The interpretation of the diagram is
complex because of the large number of processes occurring
between the pollen production by the vegetation and its
record in the sediment. As in most paleontological disciplines, interpretation is done by comparison with modern
data. This supposes that the principle of uniformity, that is,
where the present is the key to the past, applies. This is
generally the case for data from the Quaternary, and in
J. Guiot (&)
European Centre for Research and Teaching in Environmental
Geosciences CEREGE, Aix-Marseille University, CNRS, IRD,
INRAE, Collège de France, BP 80, 13545, Aix-en-Provence,
Cedex 04, France
e-mail: guiot@cerege.fr
© Springer Nature Switzerland AG 2021
G. Ramstein et al. (eds.), Paleoclimatology, Frontiers in Earth Sciences,
https://doi.org/10.1007/978-3-030-24982-3_12
151