Terrestrial Palaeoecology and Global Change

26 Valentin A. Krassilov. Terrestrial Palaeoecology
eutrophic (rapid dead mass disintegration at low pH values) and oligotrophic (rapid dead
mass accumulation at high pH values) regimes.
The Milankovitch-type cyclicity might have been involved in each of these groups of
factors.
II.7.2. Variegate cyclothems
In the alternations of light/dark, green/brown, bluish/purple deposits, the colour is
given mainly by the iron and manganese compounds, silica and dispersed organic matter.
It can be primary, related to the chemistry of sediment and the pore water, or secondary,
related to ground water chemistry and fluctuations of water table. The brown/green
alternation is due to fluctuation of the Fe (III) – Fe (II) redox boundary (Lyle, 1983).
Both Fe and Mn redox boundaries shift with influx of organic matter. Mobilization,
accretion and scavenging of metals across redox boundary has been studied in detail in
relation to Mn nodules (Stumeyer & Marchig, 2001). The cyclicity of nodule accretion
and metallic concentrations is related to the surface water productivity. In particular, the
nodule growth rates reflect short-term – of El Niño time scale – vertical oscillations of
redox zonation (König et al., 1999). The redox potential is also affected by a rapid
deposition of turbidite or volcanic ash. Oxygen diffusion beneath tephra is affected by
manganese concentrations and, on evidence from the Mount Pinatubo fallout, is completely
prevented by a tephra layer more than 3 cm thick (Haeckel et al., 2001).
In mixed water bodies, organic deposition is low while iron is precipitated as ferric
compounds (brown to purple bands). In contrast, the ogranic-rich sediments of stratified
water bodies are grey to black, with different shades of green owing to accumulation of
ferrous compounds. Fe ++ concentrations are high in the hypolimnion at low pH and redox
potential, but rapidly decrease with water mixing and oxygenation. A flow of phosphrus
over the sediment/water interface is closely linked to Fe ++ concentrations. Ferruginous
deposits adsorb phosphates that are released with anoxy and circulate in the hypolimnion
or, with episodic mixing, fertilize the epilimnion.
Thus, colour banding is related to fluctuations of aquatic ecosystems, for which the
terrestrial runoff, the depth of thermocline, and the biotic production to dead mass deposition
ratios are the critical variables. Tectonic settings and volcanism control the input of
iron and other metallic ions in sedimentary environments. Both bedrocks and volcanic
fallouts affect pH of surface waters and soil, while climatic effects prevail in postsedimentary
ferrous/ferric replacements and the origins of massive redbeds (usually
containing residual greenish inclusions).
A vast realm of Permian–Triassic variegate (red) deposits extends from the Urals
over the Russian Plains to central Europe. In the European Rotliegenden, their repeated
sequences commonly include eolian sands, playa lake mudstones and calcareous fossil
soil, with a Milankovitch-scale cyclicity (Bailey, 2001). The transboundary Permian–