Terrestrial Palaeoecology and Global Change

Chapter 7. Climate change
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with reversals coming up later on. What we actually know of past climate changes is a
restructuring of zonal patterns, from which global climatic variables can be inferred with
a varied degree of confidence.
Major climate changes are indicated by alternation of the following zonal patterns:
Humid/dry equatorial zone. The Late Cenozoic and Early Permian are the times
of a predominantly humid equatorial zone supporting rainforests of macrophyllous angiosperms
and gigantopterids respectively. The mid-Mesozoic is the time of a predominantly
arid or semiarid equatorial zone supporting a xeromorphic vegetation of microphyllous
(brachyphyllous) gymnosperms (IV.4). Dry equatorial zone signifies a pattern
of global atmospheric (and, by inference, oceanic) circulation considerably different from
the present-day norm, but corresponding to the present-day short-lived anomalous situations,
such as desiccation of equatorial rainforests caused by El Niño (VII.2.3). Over the
Cenozoic, as well as over the Permian, equatorial humidity is apparently correlated with
the advances of continental glaciers.
Evergreen/deciduous temperate zones. The ratios of evergreen to deciduous
vegetation types vary with seasonality, extreme temperatures, precipitation, atmospheric/soil
pore CO 2
levels and the length of growing season. In areas of ample precipitation,
deciduousness is due to a prolonged period of soil freezing inflicted by a spread of cold
air generated by polar ice caps. Cold winters make temperate zones unfavourable for
broadleaved evergreens, but, in combination with heavy snowfalls, they promote the
needle-leaved evergreens. Dwarf-shrub evergreens are adapted to a short growing season
(that need not be wasted on unfolding the leaves). Temperate deciduousness appears
with polar ice caps and spreads to lower mid-latitudes with their expansion, replacing
both the temperate and xerothermic evergreens. In the fossil record, deciduousness
is recognized on the basis of leaf morphology, taphonomy and associated lithologies,
among which the intracontinental coal measures furnish evidence of summer-wet climate.
Ice-capped/ice-free poles. Glaciogenic deposits are direct evidence of past glaciations,
but continental tillites are scarcely distinguishable from glaciomarine deposits and
allotillites (Chumakov, 1990), let alone a possible confusion with taphrogenic mixtites.
More reliable evidence comes from palaeophytogeography. Vast continental ice sheets
generate a periglacial dry zone that reaches to the mid-latitudes. The well-defined temperate
deciduous zones correlate with moderate polar ice caps, as at present. On evidence
of mid-latitude temperization, polar ice caps might have existed over most of the
mid-Palaeozoic to Cenozoic times.
The alternative situation of ill-defined temperate zones, with a significant proportion
of arborial evergreens spreading over the polar circles, is recorded over relatively short,
about 6-10 m.y., intervals in the Carboniferous (Visean), Early Triassic (Olenekian),
Late Cretaceous (Campanian) and Early-Middle Eocene (Sparnacian to Lutetian).
It is widely held that glaciations are exceptional, occurring over no more than 10% of
the Phanerozoic time, while the normal situation is ice-free (Lloyd, 1983). Hence recent