Terrestrial Palaeoecology and Global Change

Chapter 6. See-level fluctuations
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Fig. 77. Epirift subsidence in the Cretaceous, with epeiric seaways extending over the meridional Triassic and
older rift zones: (NA) North American Interior Basin, (NE) North Sea – Northern European Basin, (WS)
West Siberian and Yenisey-Khatanga basins.
The pole-wander model ascribes sea-level fluctuations to migration of equatorial tidal
bulge with displacement of the axis of rotation (Mörner, 1980; Steinberger & O’Connell,
1997; Mound & Mitrovica, 1998). Sea level falls in front of the migrating pole and rises
in the rear (Mound & Mitrovica, 1998). According to this model, a sea-level wave would
diachronously spread over the globe transverse to the polar path. The effect remains
hypothetical however either for the observed short-term Chandler wobble or the alleged
large-scale pole wander of geological history.
(3) Isostatic, pertaining to relative hypsometric levels of oceanic and continental lithosphere
defined by their isostatic compensation at given earth’s rotation rates. A rotation forcing
would displace mean hypsometric levels for the isostatically compensated oceanic and
continental lithosphere because of their density contrasts resulting in a differential centrifugal
acceleration (V.2). Since oceanic lithosphere is denser than continental lithosphere, a change
in the earth’s rotation rates would either depress or raise sea floor relative to the continents.
Mean hypsometric levels of land and sea floor would converge with deceleration and diverge
with acceleration of the earth’s rotation rates. Convergence results in shallow oceans – low
continents, hence a global transgression, and the opposite under divergence.
This model (Fig. 78) explains alternation of thalassocratic/geocratic epochs, with the
lower order sea-level fluctuations imposed by the associated isostatic effects of surface
loads (e.g., the glacioisostatic effects of continental ice load and rebound of deglaciated
areas appreciable over the periods of vast continental glaciations: Peltier, 1996). In addition,
the tidal deceleration of the earth’s rotation rates may appreciably increase with the tidal
flat areas during thalassocratic epochs, hence a positive feedback enhancing the trend.
In respect to the geochronology, the model predicts coincidence of global transgressions
with sea floor magmatism related to an expansion–elevation of oceanic lithosphere.
Such a correlation is actually observed during the Cretaceous, a period of vast
epicontinental seas and the most voluminous sea-floor basalts (details in V.5.2, V.7, VI.1