Terrestrial Palaeoecology and Global Change

Chapter 7. Climate change
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Since tree line is controlled by soil freezing, its low position indicates a downslope
displacement of respective snow line. In the Sikhote Alin Ranges, Russian Far East,
the birch woodland belt, now next to the tree line, descended to river valleys, now
more than 600 m lower, before the climate-driven vegetation turnover in the lowlands
at the Cretaceous/Tertiary boundary (VII.6, IX.9). The Middle Eocene (45 Ma) taphoflora
of Idaho Plateau represents a sparse conifer woodland, with the tree line about
550 m lower than at present corresponding to a cooling of about -3°C (Axelrod, 1990,
1992) when the lowland vegetation still retained a paratropical aspect. Altitudinal shifts
of tree lines in the Andes over the Pleistocene glacial/interglacial cycles indicate an
amplitude of temperature fluctuations of about 8°C (Hooghiemstra & Ran, 1994 and a
similar information for other low-latitude regions in Lin & Colinvaux, 1985; Newsome
& Flenley, 1988) against the lowland estimates not exceeding 3°C. A downslope expansion
of the upland conifer belts during the Pleistocene glacial maxima is palynologically
recorded both in subtropical and tropical zones (Heusse & Sirocko, 1997; Taylor
et al., 2001), suggesting an increase in the altitudinal temperature gradient disproportionate
to the lowland cooling there.
The extensive glaciations in the Vendian, Ordovician, Late Carboniferous/Early Permian
and the Neogene closely followed the main orogenic pulses of the Transvaalian,
Taconian, Hercynian and Alpine tectonic epochs, respectively. The turning point of Cenozoic
evolution is marked by a major mid-Eocene orogenic event in the Tethys belt, with
multistage overthrusts and high-rate elevation over the bulging area of Tibet–Himalayas
(Burg et al., 1984; Hirn et al., 1984), and the concomitant rise of the central African
plateaux. Climatic consequences were immediately felt over the vast areas of northern
Asia, up to the South Urals and Kazakhstan, where the laurophyllous Eocene floras
were rapidly replaced, first by the serratophyllous broadleaved and then by the mixed
conifer-broadleaved Arcto-Tertiary assemblages. Such a large-scale temperization signalled
an expansion of the North Pole ice-cap.
Cyclic fluctuations over the temperization trend, with the successively deeper pessima
in the terminal Oligocene to basal Miocene, late Middle Miocene and the Pleistocene,
were correlated with orogenic pulses causing a stepwise shrinking of residual
Tethys seaways.
The Cenozoic glaciation was thus linked to the low-latitude orogeny. The low-latitude
mountain ranges, elevated above the potential snow line, were covered with montane
glaciers generating cool air masses that spread to the poles. The atmospheric
heat export from the overheated tropical zone was barred by orogenic barrier enhancing
the extratropical cooling trend. With its southern margin uplifted by the orogeny,
Eurasia has acquired a northward tilt that brought most of its runoff to the northern
North Atlantic and Arctic basins, diluting surface waters and promoting water-borne
glaciers. With their growth, surges of marine ice stirred cyclonic activity bringing heavy
snowfalls to northern lands, feeding the continental ice sheets. This scenario is further
considered below.