Geoscience Reference
In-Depth Information
fully modern climates occurred by 4 ka. Van Devender (
1990a
, p. 127) also observed
that 'community composition continued to vary subsequently during lesser climatic
fluctuations, suggesting that differential responses of plant species to climate changes
and continuous variation in climate on several scales have resulted in dynamic plant
communities that rarely if ever reach equilibrium (Davis,
1986
)'. A corollary to this is
that different plants will likely have different lags in responding to any future climatic
changes. This short-term Holocene instability is in contrast to the apparently stable
early to middle Wisconsin woodland assemblages in certain parts of the Chihuahuan
Desert, which persisted without noticeable change for 15,000-20,000 years.
16.6 Plant microfossils
16.6.1 Pollen and spores
Palynology is the study of fossil pollen and spores, and pollen analysis is the study
of fossil pollen grains. Pollen analysis was placed on its modern footing by Von Post
(1884-1951) in his oral account of Swedish peat bogs in 1916, published two years
later (Von Post,
1918
), and for many decades it has been the method most widely used
to reconstruct terrestrial environments in humid areas during the Quaternary (West,
1977
; Faegri and Iversen,
1989
;LoweandWalker,
1997
; Williams et al.,
1998
). It
relies on the generally good preservation of fern spores and pollen grains in lake,
swamp and bog sediments. Pollen grains have a resistant outer layer composed of a
substance called sporopollenin, which enables pollen grains to remain well-preserved
in waterlogged deposits. Pollen grains are best preserved in wet, slightly acidic sed-
iments but soon become oxidised and degraded in dry, alkaline environments, which
is why they are scarce in desert sediments. Only in the last few decades has pollen
analysis been applied with some limited success in arid areas, although problems of
low pollen counts and partial preservation remain significant obstacles (Bonnefille,
1972
; Bonnefille,
1976
; Bonnefille,
1980
;Maley,
1980
;Maley,
1981
; Bonnefille,
1983
;Lezine et al.,
1990
; Bonnefille et al.,
2004
;Vincensetal.,
2007
;Lezine et al.,
2011
). Ritchie et al. (
1985
) analysed the pollen from a desert lake in northern Sudan
and concluded that the savanna vegetation zone extended around 400 km further north
during the early Holocene, which is consistent with other evidence from that region
(Ritchie and Haynes,
1987
).
Pollen grains are produced in the male flowers of gymnosperms (conifers) and
angiosperms (flowering plants) and are dispersed by wind, water, insects, birds and
other animals. One immediate problem involves distinguishing between near and far
sources of pollen. Pollen from aquatic plants is usually deposited quite locally, within
the lake, swamp or peat bog in which the plants are growing. However, pollen from
plants that rely primarily on wind to disperse their pollen grains can travel many
hundreds of kilometres and even thousands of kilometres, as in the case of pollen
carried out to sea during dust storms.
