Terrestrial Palaeoecology and Global Change

234 Valentin A. Krassilov. Terrestrial Palaeoecology
the low-latitude volcanic eruptions (El Chichón, Mount Pinatoba) enter stratosphere at
the tropical tropopause, spreading to the poles (Boering et al., 1996; Volk et al., 1996).
The intensity of tropospheric impact on the stratospheric ozone layer thus depends on
convectivity of tropospheric air that is stirred by the greenhouse warming. Redistribution of
heat by the tropospheric concentration of greenhouse gases results in a cooler stratosphere,
hence an acceleration of ozone-destroying reactions on stratospheric ice clouds
(Wang et al., 1991; Shindell et al., 1998). However, the effects are non-linear owing to
superposition of other atmospheric phenomena, such as a contraction of polar vortices that
isolate polar air from ozone-destroying substances borne by the mid-latitude air masses
(Stine, 1994; Robinson, 1998). A combination of positive and negative effects gives the 2 –
4 year ozone cycles that correlate with oceanic – tropospheric events, such as El Niño.
On account of the volcanic, as well as the tropospheric, greenhouse effects, fluctuations
of the ozone layer are bound to geological and climatic processes as an integral part
of global change.
A depletion of stratospheric ozone is potentially damaging to the biosphere because of
an increased influx of UV radiation. This effect is presently mitigated by a concomitant
increase in tropospheric ozone excessively produced by photochemical decomposition of
technogenic nitrogen dioxide (Penkert, 1989). The photochemical fog of modern cities,
though damaging to respiratory systems, shields the urban life, also the adjacent rural life,
from UV radiation. In pre-industrial time, a reduction of stratospheric ozone by natural
agents, such as volcanic aerosols, might have had even more dramatic consequences.
The biotic turnovers of the fossil record correlate with huge magmatic events, such
as the Siberian and Deccan traps at, respectively, the Permian/Triassic and Cretaceus/
Tertiary boundaries. A massive discharge of volcanic aerosols to stratosphere inevitably
resulted in at least temporary reduction of the ozone layer. At such critical moments, the
mutagenic effect of UV radiation might have contributed both to the mass extinctions
and innovations.
Evidence of UV damage comes from plant cuticular studies. Thus, an exceptionally
high frequency of epidermal anomalies, such as an irregular differentiation of epidermal
zones, interrupted stomatal rows, underdeveloped and/or contiguous stomata, disorganized
stomatal morphologies, etc. (Fig. 96), are recorded in peltasperms and conifers
from the Permian/Triassic boundary clay of Nedubrovo, central European Russia, a
smectitic horizon correlated with the eruptive phase of Siberian and Petchorian traps
(Krassilov et al., 1999a, 2000).
VII.2.6.Vegetation effects
Terrestrial vegetation bears on the albedo of the earth’s surface (about 35% over deserts,
9 -12% over forested areas: Sellers et al., 1997), air temperature (decreasing by 3-5 K in
forested areas), evaporation, wind velocity, air convection, meridional heat transport (Hadley