Terrestrial Palaeoecology and Global Change

Chapter 5. Tectonic factors of global changes
129
observations, but might have been different in the recent past owing to geoidal effects of
glaciations and tectonic napping.
Palaeomagnetic data suggest a far greater polar wander over geological periods than
inflicted by the observable wobble of rotation axis. Frictional deceleration in the Earth –
Moon system theoretically amounts to about 2 ms per 100 years, with the monthly/
seasonal fluctuations related, causally with feedbacks, to atmospheric, glacial and tectonic
(earthquakes) processes (Pariyskiy, 1984). Fragmentary evidence of tidelites and
growth increments on skeletal structures either agree with theoretical expectations (Sonett
et al., 1966) or indicates a long-term variation (Rosenberg, 1982), yet quality of the data
is not always adequate.
Increments on shells of marine invertebrates reflect growth periodicities in phase
with tidal variations over diurnal, weekly, fortnight and monthly cycles, recording rotation
rates as the length of day. An assessment of growth increments on epitheca of fossil
corals (Wells, 1963) has been taken as evidence of constant deceleration rates from the
Devonian (396 days per year) to present (Runcorn, 1967,1979; however compare the
figures for days per lunar month in Wells, 1963, Scrutton, 1964 and Aveni, 1966). Actually
Wells (1963) gives a mid-Devonian range of 385 to 410 days per year. In his material,
studied under low magnification, ranking of growth increments is scarcely discernible,
which makes recognition of diurnal increments problematic.
My SEM studies of growth lines on Cretaceous corals suggest that increments of
several ranks are preserved, marking not only diurnal, but also weekly/monthly periodicities
(Krassilov, 1985). Day lengths can only be deduced from the lower rank increments
in the hierarchy of growth lines (Fig. 58). The coarser ridges/furrows are seasonal increments
(specimens 4 cm and 2.5 cm long have 9 and 5 such ridges respectively). The
second rank growth lines, still discernible at low magnification, correspond to monthly
increments (perhaps corresponding to “daily increments” shown in Wells, 1963). They
are grouped into zones of narrower (5-6 months) and wider (6-7 months) bands between
the annular ridges. The weekly and daily lines are discernible with SEM over the betterpreserved
areas of the epitheca. They give about 24 days per lunar month, nearly in
agreement with the Late Cretaceous estimates by Khan & Pompea (1978). Despite
many complications, such data suggest a fluctuant, rather than linear, change in rotation
rates over geological time.
V.5. Planetary fault system
Geometry of the earth’s rotational stress field is defined by (1) the polar – equatorial
compression – extension gradient, (2) the inertial (Coriolis/Eötvös) forcing, and (3) a
disparity of angular momentum over the density heterogeneities of lithospheric structures.
In the outer shell, the rotational stresses generate a pattern of faults that divide the
lithosphere into a great number of larger and smaller blocks.