Geoscience Reference
In-Depth Information
Table 16.1.
Processes involved in the
production of fossil pollen assemblages from
parent plants and subsequent analysis and
interpretation. (After Williams et al.,
1998
,
fig. 10.1.)
Pollen production from plants
Dispersal
Deposition
Fossilisation
Fossil pollen assemblage
Sample collection
Pollen extraction
Identification and counting
Analysis and interpretation
Vegetation history and biostratigraphy
Environmental (and climatic) reconstruction
Pollen analysis involves a series of steps (
Table 16.1
), each of which can add errors
to the final interpretation of the vegetation history (Williams et al.,
1998
, pp. 185-
199). Plants first produce pollen, some in great abundance and others less so. The
pollen is then dispersed and deposited. Once deposited, some grains will be des-
troyed by the processes of weathering and erosion, while the remaining grains will
become fossilised. These fossil grains comprise the fossil pollen assemblage. The
next step is to extract a sample of the pollen-bearing sediment using a variety of
coring methods and taking great care to avoid contamination. Then begins the long
process of identifying and counting individual pollen grains. The pollen counts are
then grouped into pollen zones that are defined according to the relative abund-
ance of different plant species or genera, expressed as a percentage of the total
pollen count. Changes in plant assemblages inferred from the pollen are then used
to reconstruct the vegetation history. Ideally, the interpretation is calibrated using
samples of the modern pollen rain, provided the former plants have living counter-
parts. The entire process of output-transport-storage-retrieval-sediment preparation-
analysis-portrayal-interpretation requires considerable skill and a thorough know-
ledge of plant ecology. Another possible stumbling block arises when the fossil
plant assemblages have no modern counterparts, as Margaret Bryan Davis (
1976
)so
brilliantly demonstrated for the postglacial deciduous forests that colonised North
America after the retreat of the great ice sheets some 12,000 years ago.
Pollen analysis enabled each of the European and North American interglacials to
be in part defined according to certain diagnostic ferns and other plants. In addition,
