Terrestrial Palaeoecology and Global Change

226 Valentin A. Krassilov. Terrestrial Palaeoecology
Moreover, a lag between the temperature and subsequent CO 2
fluctuations has been
actually recorded in some of the fossil air studies (Raynaud et al., 1993). Incidentally, an
appreciable lowering of CO 2
level has been recorded only 6 k.y. after a cooling event
over the 5.5/5.4 isotopic stage boundary (Fischer et al., 1999), the time lag being perhaps
related to the inertia of such CO 2
-regulating oceanic systems as the vertical displacements
of carbonate lysoclines (Hodell et al., 2001).
The greenhouse records. Modelling of CO 2
changes over geological times has
started with a seminal work by Budyko et al. (1985, 1986). In their model, the atmospheric
CO 2
concentration is determined by the ratio of coeval volcanic (source) and
carbonate (sink) deposits (based on quantitative estimates in Ronov et al., 1980 and
elsewhere). Over times, atmospheric CO 2
allegedly decreases with a fading-out of volcanic
activity. While bringing to view the geospheric factors that potentially may determine
atmospheric comoposition in a very long run, the model fails to account for either
reversibility (e.g., a sink of CO 2
to chemical weathering of volcanic rocks or its release
with thermic decomposition/dissolution of carbonates) or reallocation of CO 2
between
the atmospheric, oceanic and biotic reservoirs that operationally regulate the greenhouse
effect (above).
As for the general trend, the postulated decrease in volcanic activity with dissipation
of radioactive heat is scarcely substantiated by either theoretical considerations or geological
data. The model disregards kinetic heat production by shear at the interior density
boundaries within the lithosphere and over the lithosphere/mantle transition (V.2). Moreover,
the rock volume assessments neglect the most voluminous terrestrial volcanism of
the Permian/Triassic (Siberian traps), Jurassic/Cretaceous (the East Brazilian and Mongolian
basaltic provinces), mid-Cretaceous (the circum-Pacific volcanic belt), and the
Cretaceous/Palaeogene (Deccan traps) events. Neither does it account for the pulses of
oceanic magmatism, as in the Cretaceous. Worse of all, the model is in a poor correspondence
with palaeoclimatic records. A postulated Early Permian peak of atmospheric
pCO 2
falls at the time of glaciation, the next, mid-Triassic, peak is timed to a major
cooling, the Jurassic ascending track passes over the prominent Bathonian/Callovian
cooling, and the Oligocene low indicates a cooler climate than at present.
Contrary to the model by Budyko et al. (1985), the most prominent volcanic events
associate with global cooling rather than warming (Briffa et al., 1998). The greenhouse
effect of volcanic emissions is grossly outweighed by the volcanic ash/aerosol albedo
2-
effects, in particular the backscatter of solar radiation by SO 4
plus a depletion of stratospheric
ozone.
A further elaboration by Berner (1990 and elsewhere) involves a great number of
geological and biotic variables, such as land area, river runoff, biotic production, carbonate
dissolution depth, etc. Atmospheric pCO 2
levels are calculated as a reverse function
of the difference between the CO 2
spent to weathering of silicate rocks (the rates of
which are inferred from the ratios of carbonate to organic deposition) and those emitted
form the contemporaneous volcanic/metamorphic sources. A correlation of the lowest