Terrestrial Palaeoecology and Global Change

194 Valentin A. Krassilov. Terrestrial Palaeoecology
The episodes of massive black shale deposition (the “oceanic anoxic events”) in the
Toarcian, Aptian, Albian/Cenomanian and Cenomanian/Turonian are marked by the positive
δ 13 C spikes, which are considered to be a response (with an appreciable time lag)
of inorganic carbon reservoirs to a high-rate burial of 13 C-depleted organic carbon (Arthur
et al., 1988; Prokoph et al., 2001; Wilson & Norris, 2001). These isotopic events typically
correlate with transgressions, global warming and extinction of benthic organisms
intolerant of low oxygen levels. Inflow of warm saline waters from epeiric seas enhances
thermohaline stratification in oceanic depressions and the resulting spread of anoxic
conditions (Thierstein & Berger, 1978).
The major biotic turnovers, such as at the Permian/Triassic, Cretaceous/Tertiary and
the Palaeocene/Eocene boundaries (IX.1-2), are marked by short-term negative δ 13 C
excursions probably reflecting a drop of biotic productivity with a disruption of the oceanic
– epeiric circulation systems. Later on, with a spread of highly productive pioneer
vegetation over the emerging land and an increase of organic matter in the terrestrial
runoff, the anoxic facies reappear accompanied by a rise of δ 13 C.
VII.1.3. Fossil plants
Fossil plant record as a source of palaeoclimatic inference has much the same limitations
as the sedimentological and isotope records. Plant life is controlled not by independently
quantifiable climatic variables but by their concerted impacts. The temperature
tolerances depend on precipitations, and vice versa.
Plants are thought to be the most climatically sensitive components of biotic communities,
but this may not necessarily be so, e.g., for tree lines primarily controlled by the
mycorrhizal tolerances. Plants are linked to climate not only through their tolerances per
se (of which we scarcely have sufficient evidence even for extant plants), but also, and
perhaps to a larger extent, through their symbiotic organisms, such as mycorrhizal fungi,
insect pollinators, vertebrate dispersers, etc. (Bonan & Shugart, 1989; Van der Heiden et
al., 1998; Christian, 2001, examples in VIII.1). In other words, what we actually see in
the fossil plant record is a vegetation component of co-adapted response to climate of a
biotic community as a whole.
Plant characters habitually considered as climate correlates might have initially developed
in respect to a different kind of causation. Since plants were impacted by animals
from the very beginning of terrestrial plant life (VIII.3.1), most plant adaptations
are plant-animal co-adaptations. As discussed above (III.1.3), the leaf margin morphologies
are related to plant – insect interactions and only secondarily to climate. Flowers
and fruits are more conspicuous on leafless twigs (Fig. 84). A shut-down of photosynthesis
in respect to pollination and dispersal occurs over a wide range of climates suggesting
this to be the primary function of deciduousness, overshadowed by a derived
adaptive response to seasonality of water supply.