Terrestrial Palaeoecology and Global Change

230 Valentin A. Krassilov. Terrestrial Palaeoecology
confirmed by the parallel variation of the cycadophyte index, a proxy of temperature
changes over the Mesozoic (Krassilov, 1973b; Fig. 85).
Dispersion of stomatal measurements has been calculated upon the assumption that
a low level of atmospheric CO 2
would have caused a wider variation. The maximal
dispersion was obtained for the Karatau assemblage but the next low in the mid-Neocomian
coincides with a relatively narrow dispersion. Perhaps a more meaningful correlation
will emerge with accumulation of data.
A qualitative phytogeographic approach (Krassilov, 1997c) is based on a correlation
of atmospheric/soil pCO 2
with plant tolerances in respect to other limiting factors. In
particular, a water uptake by photosynthesis typically decreases with elevation of atmospheric
pCO 2.
In effect, the greenhouse conditions would have rendered plant species
more tolerant of water deficit. Their geographic ranges would have been less dependent
on precipitation.
Since phytogeographic zonation is controlled both by temperature and precipitations,
a reduced water uptake would make zonal boundaries more transparent, resulting in a
mixing of zonal elements. An azonal plant distribution is thus a phytogeographic testimony
of greenhouse climate. In the fossil plant record, the distinctness of the boundary
between the mesotemperate deciduous and xerothermic evergreen zones varies with
climate fluctuations. Smearing of the boundary is recorded for the relatively short “azonal”
intervals in the Early Carboniferous (Visean), Early Triassic (Olenekian) and the
Early-Middle Eocene, the supposed greenhouse episodes (VII.4).
CO 2
trends and feedbacks. As discussed above, the atmospheric CO 2
levels depend
not so much on the total amount of carbon released from the earth’ interior, as on its
reallocation between the atmospheric, lithospheric, oceanic and biotic reservoirs. A directional
change of atmospheric CO 2
concentration over times would have occurred
with directional evolution of these reservoirs or any of them. Net CO 2
fluctuations result
from a disbalance of multiple sinks and sources, their interaction being affected, among
other variables, by the air temperatures and the SST. The time-average atmospheric
CO 2
levels are sustained, with short-term fluctuations, by the negative feedbacks upon
the temperature → CO 2
→ temperature scheme.
There is little doubt about the role of outgassing of the earth’s interior in the origin and
regulation of atmosphere before the biospheric regulating mechanisms came to action and
took over, of which the Proterozoic banded ore formations might have been an early evidence.
Sea-level fluctuations, with an uncertain directional component, but probably of a
deminishing amplitude since stabilization of continental crust over the Precambrian fold
belts, administer a control over the sink rates of atmospheric CO 2
to deep-water circulation,
terrestrial biomass and carbonate deposition. During the global regressions, both terrestrial
biomass and chemical weathering increased with land area, consuming more CO 2
while,
simultaneously, deep-water circulation was enhanced by shutting-down an influx of warm
saline waters from epeiric seas. Transgressions inflicted the opposite trends (Fig. 94).