Terrestrial Palaeoecology and Global Change

Chapter 5. Tectonic factors of global changes
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vegetation. Notably, the Early-Middle Eocene thermal event correlates with the emergence
of the North Atlantic land bridge (McKenna, 1975; Tiffney, 1985a, 1985b) that not
only provided for floristic exchanges between Eurasia and North America, but also interrupted
the psychrospheric circulation in the Atlantic. A closure of gateways for cold
water masses resulted in a warmer North Atlantic, allowing the paratropical coastal
vegetation to spread beyond 60°N (VII.5). A similar, though shorter-term, vegetation
event in the Pacific sector (palms in northern Kamchatka and Alaska) attests to an
emergence of the Bering Shelf.
This prominent phytogeographic anomaly is thus related to a general upheaval of
Arctic land as an epeirogenic consequence of decelerated earth’s spin (VII.1). A subsequent
temperization of climate and biota in the Late Eocene, continuing into the Oligocene,
correlates with separation of the North Atlantic landmasses due to the opposite
tendency of tectonic evolution – a contraction/depression of the polar areas, with consequences
for oceanic circulation and climate.
V.9.3.Palaeoecological constrains of continental rearrangements
Continental reassemblies are palaeogegraphic inferences based on sea-floor spreading
geometry and palaeomagnetology. Since both these sources are hypothetical, the
reassemblies have to be warranted by palaeogeography in the broadest sense, including
palaeoclimatology and palaeoecology. If an apparently anomalous palaeogeographic feature
can be explained otherwise than by a continental rearrangement it need not be used
as an incentive for the latter. If, on the other hand, the neutral data are used as a confirmation
or the contradictory data are neglected, then we have conspiracy rather than
science.
For example, the floristic differences of Laurasia and Gondwanaland are commonly
used as evidence of a wide continental separation neglecting mixed floras of the Middle
East, South China, New Guinea, etc. (IV.3). Floristic uniformity is a venerable argument
for assembling the Gondwanaland supercontinent, although a floristic differentiation of
the western/eastern Gondwanas became evident far before its alleged break-up in the
Jurassic. At the same time, floristic similarities, as between the northern and southern
Chinese blocks or over the Palaeozoic Uralian seaway, negate tectonic arguments for
vast oceanic barriers.
A critical test for continental reassemblies is climatic/vegetational zonation. On the
global scale it reflects solar flux, hence latitudinal. In comparison, the cross-latitude vegetation
boundaries are owing to such factors as rain shadows, monsoons, substrates
(permafrost in the case of the mid-Siberian differentiation), etc., hence regional. In this
respect the Palaeozoic and Mesozoic phytogeographies (IV.3) are more consistent with
the present-day coordinates in which the planetary boundaries are roughly latitudinal,
than with continental reassemblies in which they are differently positioned.