Terrestrial Palaeoecology and Global Change

178 Valentin A. Krassilov. Terrestrial Palaeoecology
cratic epochs. Relative extents of marine deposition seem to record a decrease of epeiric
seas over geological history. However, on evidence of Cretaceous transgressins, this
tendency was not sustained over the entire Phanerozoic.
It should be noted that the fossil record actually documents shoreline displacements,
which are not exactly the same as sea-level fluctuations relative to the geoid mid-level.
Shore lines apparently tend to approach the boundary of continental and oceanic crust
domains, but are displaced by either eustatic or tectonic events, or both. The following
causative types of sea-level changes are distinguished:
(1) Eustatic, related to changes in water volume, including the glacioeustatic fluctuations
of 10 2 m scale recorded in the Pleistocene and extrapolated back in time. The
commonly recognized glacioeustatic fluctuations are phased by the obliquity cycles,
with a period of 41 k.y. However a similar cyclicity is recorded for non-glacial epochs as
well.
Eustatic events caused by thermal expansion of seawater correlate with low latitude
insolation maxima of the orbital eccentricity cycles. Such eccentricity-driven sea-level
fluctuations are globally synchronous, of 100 k.y. and 400 k.y. periodicities. The Tethyan
transgression cycles of respective periodicities are recorded in the Cretaceous (Stroll &
Schrag, 1996). The 400 k.y. is of a biospheric significance, as indicated by a chronology
of speciation events and the average species longevity in such rapidly evolving groups as
the Permian conodonts or Cretaceous foraminifers and ammonoids.
(2) Geodetic, pertaining to deformations – redistributions of elevations and depressions
– of the geoid (V.3). A reversal of the present-day geoid highs (+ 100 m near New
Guinea) and lows (- 80 m near the Maldives) would generate a sea-level wave of amplitude
about 180 m.
With deceleration of the earth’s rotation rates, sea level would have been elevated at
the high latitudes and depressed at the lower latitudes (Mörner, 1976; 1980), and the
opposite with acceleration. Expansion of the astenospheric equatorial bulge would coincide
with depression of polar areas. The Boreal to Tethyan transgressive/regressive
events of the geodetic sea-level cycles would then be at counter-phase, and the sealevel
fluctuations would correlate with magmatic pulses.
The chronology of large-scale sea-level events seems consistent with this model. At
least, a major mid-Cretaceous Tethyan transgression was accompanied by an emergence
of vast land areas in the northern North Atlantic (the Thulean Land), Beringia and elsewhere
in the Arctic Basin, as well as in the Tasmantis area of southern Pacific (Krassilov,
1985 and similar examples in Lapkin & Katz, 1990). Conversely, the maximal Late Cretaceous
advance of high-latitude epeiric seas over northern European, West Siberian and the
interior North America (Fig. 77) coincides with the peaks of low-latitude magmatism over
the Tethyan ophiolite belts and oceanic fracture zones (V.6.5, V.7). There are, however,
incidents of globally synchronous transgressions and regressions (e.g., in the terminal Permian
or terminal Cretaceous), as well as magmatic pulses, that are not explained by the
geodetic model, hence of a different origin (e.g., isostatic, below).