Terrestrial Palaeoecology and Global Change

Chapter 7. Climate change
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- The development, since the Late Carboniferous, of seasonal deciduousness that
increased the carbon sink with a shed leaf mass,
- An increase of trophic cascades with elaboration of detritvorous and herbivorous
adaptations the diversity of which seems to have been saturated by the Permian.
Thus, a major increase of terrestrial biomass occurred in the Palaeozoic, with subsequent
developments switching to a fluctuation mode (one can argue that biotic capacity
have increased with the relatively recent appearance of such overproductive vegetation
types as the tropical rainforests; yet, as discussed above, the rainforests allocate more
carbon to their standing crop biomass, but less to their dead mass production). Fluctuations
potentially affecting the stabilizing role of terrestrial vegetation in respect to atmospheric
composition and climate are induced by sea level and climate through their effect
on the area of plant cover, as well as the productivity of terrestrial biomes. Over the
post-Palaeozoic history, these variables fluctuate with:
- An extent of vegetated land changing with sea-level fluctuations (the consequences
of Cretaceous transgressions are discussed in VII.2), desertification and continental
glaciations,
- The ratios of arboreal to non-arboreal vegetation changing with the periodic
afforestation/deforestation waves, with afforestation maxima in the mid-Jurassic, early
Palaeocene and Oligocene (VII.5), affecting the net carbon uptake by the biota,
- An extent of peatlands, maximal in the Carboniferous, mid-Jurassic, mid-Cretaceous
and mid-Eocene affecting both CO 2
and CH 4
levels,
- The ratios of evergreen to deciduous vegetation, with a spread of deciduousness
in the mid-Jurassic, Early Palaeocene and Oligocene (IV.2.2), affecting the dead-mass
accumulation/decomposition rates,
- The ratios of C 3
to CAM and C 4
photosynthetic types, typically correlated with
deforestation (Moore, 1982; Ehleringer et al., 1991; Ku et al., 1996).
Land plant photosynthesis depletes the atmospheric carbon reservoir of the lighter
isotope that is then released with forest fires to contribute to the Suess effect, a decrease
in carbonate δ 13 C that is in equilibrium with the atmospheric 12 C / 13 C ratio. The δ 13 C
fluctuations are partly ascribed to the changing ratios of C 3
to C 4
photosynthetic types
(Morgan et al., 1994; Cerling et al., 1997), the latter presumably appearing about 15-20
Ma, with peaks at 7.3 Ma and a few subsequent levels over the Neogene (VII.2.4).
However, the anatomical syndrome of C 4
photosynthesis (the Kranz anatomy) has been
recorded in the Mesozoic Pseudofrenelopsis already (Bocherens et al., 1993) suggesting
a much longer evolutionary history.
VII.3. Climate trends
This chapter summarises the discussion of climate change in VII.1-2. A popular
climatological theory holds that, with dissipation of the earth’s internal heat, the global