Terrestrial Palaeoecology and Global Change

28 Valentin A. Krassilov. Terrestrial Palaeoecology
is dryland forests in the Devonian: Driese et al., 1997). Yet wetland trees, such as
swamp cypress, also produce taproots, and the Nysso-Taxodietum assemblage is a constant
associate of redbed cyclothems. In ombrotrophic environments, penetrating roots
are functional before and in the process of decoupling from ground water feeding. Alternatively,
in rheotrophic environments, the ferruginous sheaths on vertical roots indicate
penetration beneath anoxic layer to avoid a microbial damage of root tips. Alternation of
these trophic types might have been responsible for certain types of variegate cyclicity.
Pedogenesis itself shows a regular periodicity of soil development/truncation cycles
related to the vegetation cover (sparse at the time of truncation), precipitation, upland
runoff and fluctuations of the ground water table (Retallack, 1985b). In the Palaeogene
of Bighorn Basin, Wyoming such cycles might have been related to avulsion with a
periodicity of about 2.000 years (Wing et al., 1991), that is, of the time-scale of a seral
vegetation/soil development.
As in the case of coal-bearing cyclothems (above), current interpretations of Palaeozoic
variegate cyclothems with evaporites and pedogenic horizons are strongly influenced
by the idea of Gondwanic glaciations. In particular, glacioeustatic fluctuations are
inferred as a cause of Pennsylvanian evaporites (regression, dry equatorial climate)
alternating with silicoclastics, dolomites and black shales (transgression, humid climate).
Such alternations might have been related to precession-driven climatic fluctuations even
in the absence of considerable continental ice sheets.
The opposite correlation of climatic phases with glacioeustatic events has been postulated
by Miller et al. (1996) for the Permian (Wolfcampian) carbonate/variegate mudstone
cyclothems, with carbonate deposition under an arid climate at the sea-level rise –
high stand phase (deglaciation) switching to silicoclastic deposition with pedogenic horizons
under a monsoonal dry/wet climate at the sea-level fall – low stand phase (glaciation).
However, no evidence of glaciation is known for the Wolfcampian and chronostratigraphic
correlates. The lithofacies alternation can be fully explained by periodic fluctuations
of river runoff plus the filtering effect of coastal vegetation. Since Potonieisporites
is a common palynomorph of evaporitic sequences while Lycospora occurs in
silicoclastic deposits (Rueger, 1996), a replacement of pteridosperm–coniferoid mangroves
by freshwater lepidophytic marshes with an increase of river runoff may seem a
parsimonious interpretation of their succession.
Similarly, a reduction of coastal vegetation (mangroves) has been invoked as a factor
of continental runoff and a consequential increase in palynomorph diversity over the
Milankovitch-scale sedimentological–palynological cyclicity in the Palaeogene (Rull, 2000).
II.7.3. Lacustrine cyclothems
Biochemical cyclicity is well studied for lacustrine ecosystems (Henderson-Sellers &
Markland, 1987). Depending on the type of water body and climate, the cycles are