Terrestrial Palaeoecology and Global Change

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Chapter 7. Climate change 241 gins, temperate zones of mixed evergreen-deciduous vegetation and the forested polar areas of high biological productivity (IV.3.5). The widespread mm-scale cyclicity of lacustrine and marine black shale sequences indicates a high susceptibility of mid-latitude Cretaceous climates to the rotationally induced (Milankovitch) precipitation cycles. Climatic fluctuations over the Cretaceous, with the temperature maxima in the Berriasian, Aptian, mid-Cenomanian – Turonian and Campanian, and the minima in the Hauterivian, Albian/Cenomanian, Coniacian and over the Maastrichtian/Palaeocene boundary (palaeobotanical evidence in Krassilov, 1973b, 1975a, 1984), correlate with sea-level fluctuations and the related orogenic, volcanic and biotic events. The mid-Cretaceous and Campanian maxima coincide with the larger transgressions, whereas the minima correspond to the orogenic–volcanic pulses of, successively, the circum-Pacific, Austro- Alpine, Mediterranean and Laramid tectonism (Krassilov, 1985). Palaeobotanical and sedimentological evidence of polar glaciations, more definitive in the southern high latitudes, appear no earlier than in the late Maastrichtian. The climatic asymmetry of the hemispheres indicates a leading role of geographic factors, such as land/sea distribution, in which they primarily differ, dominating over the atmospheric heat budget (greenhouse) factors. Contrary to the greenhouse theory of global climate change, the black-shale events, a massive carbon sink potentially decreasing the atmospheric CO 2 level, overlap the warming phases while the major volcanic events, a source of atmospheric CO 2 , coincide with the cooling episodes. In the preceding chapters, the concerted geographic changes have been related to the rotation forcing that inflicts a divergence (with acceleration of rotation rates) or convergence (with deceleration) of hypsometric levels for the isostatically compensated oceanic and continental domains. Elevation of the continents is accompanied by marginal faulting and cratonic volcanism, while the rise of sea level is correlated with extensive marginal shear and sea-floor basalts. The greenhouse component of climate change is bound to sea level (VII.2.4) that concomitantly affects the albedo of the earth’s surface (increases with land area), as well as the oceanic heat transfer (interrupted by land barriers) contributing to the net effect. This explains the recorded transgression/warming – regression/cooling correlations. The following climatic scenarios (summarizing VII.2.1-6) are here suggested for each of the trends (Fig. 97). (1) The convergence trend: - The continental crust is inundates by the rising sea, - The thermohaline stratification in the shallow oceans is sustained by the water exchange with epeiric seas, - The deep-water circulation is further restricted by the emerging oceanic rises, with anoxic conditions spreading over oceanic depressions, - The epeiric seas decrease the net earth’s surface albedo, - The epeiric seaways connecting the Tethys with Arctic basins enhance the meridional heat transfer,
242 Valentin A. Krassilov. Terrestrial Palaeoecology Fig. 97. A schematic interpretation of climate change through the rotation-driven epeiric, orogenic and biotic effects. - A broad anticyclonic gear generated by the Tethys seas sustains a summer-dry climate over the tropical – lower middle latitudes, - The tropical monsoons weaken due to a high STT and a low ocean to continent temperature gradient, - The atmospheric CO 2 level is elevated as the net effect of a decreased land area, reduced terrestrial biomass/pedogenic weathering and the reduced deep-water circulation,
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Terrestrial Palaeoecology and Global Change

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