Great Geologists | 95
Britain, have long been debated. The application of time-series
analysis to a range of properties of the sedimentary succession
(for example, percentage carbonate or organic carbon) has
demonstrated that they can be explained in terms of the various
Milankovitch cycles.
Moreover, if Milankovitch cyclicity can be recognised in the rock
record, it can be used to “orbitally tune or “astronomically tune”
the geological timescale. The geological timescale classically
relies on radiometric dating. Unfortunately, reliable radiometric
ages are not available for every stage or period. Therefore,
recognition of orbitally-forced (Milankovitch) cyclicity can be
used to estimate the duration of a period of geological time
between two established datums. This technique is now a major
contributor to updating the geological timescale.
Why Milankovitch cyclicity should be present in the rock record
during periods considered to have minimal polar glaciation
(parts of the Mesozoic, for example) is something of a mystery.
One possibility is that glaciation is more frequent in the
geological past than often considered. Alternatively, it may
be that Milankovitch cyclicity affects the rock record by, for
example, increasing and decreasing the amount and intensity of
precipitation over a particular landmass. This in turn influences
run-off which changes clay content of marine rocks and drives
productivity changes and the deposition of organic matter,
leading to a cyclic depositional pattern.
A further uncertainty is how far back in geological time the
celestial mechanics that drive Milankovic cyclicity can be
deemed valid. Milankovitch cyclicity has readily been recorded
from Carboniferous cyclothems and has even been described
from banded ironstone formations formed in the Precambrian.
What cannot be debated is that Milankovitch’s work continues
to resonate through geology, both providing answers and asking
important questions.
REFERENCES
This essay has drawn upon information from the following
sources:
Frakes, L.A., Francis, J.E. & Syktus, J.I. 1992. Climate Modes
of the Phanerozoic. Cambridge University Press, 274pp.
Grotzinger, J.P. & Jordan, T.H. 2014. Understanding Earth
(Seventh Edition). W.H. Freeman & Company, 672pp.
Hinnov, L.V. & Hilgen, F.J. 2012. Cyclostratigraphy and
Astrochronology. In: Gradstein, F.M., Ogg J.G., Schmitz, M.D.
& Ogg, G.M. (eds.). The Geologic Time Scale 2012. Cambridge
University Press, 63-83.
Imbrie, J. & Imbrie, K.P. 1979. Ice Ages: Solving the Mystery.
Macmillan, 224pp.
Nield, T. 2007. Supercontinent: Ten Billion Years in the Life of
our Planet. Granta Books, 288pp.
Weedon, G.P. 2003. Time-Series Analysis and
Cyclostratigraphy. Cambridge University Press, 259pp.
http://www.teslasociety.com/milankovic.htm
https://www.youtube.com/watch?v=vCzDAm1nYlI
Rhythmic bedding (light grey limestone, dark grey marls) in the Blue Lias (Early Jurassic) at Lyme Regis in
southern England. Such bedding patterns can be interpreted as being the product of Milankovitch cyclicity.