June 2, 2009 | David F. Coppedge

Milankovitch Cycles Indistinguishable from Randomness

A claim has often been made by geologists that the rock sediments record cyclical changes in Earth’s orbit.  Milankovitch cycles, named for the man who analyzed them, are a set of regular periodic changes to the orbital eccentricity, obliquity, and axial precession of the Earth over tens and hundreds of thousands of years.  These subtle changes, it is alleged, produce climate change and sea level fluctuations.  The climate forcing, in turn, produces periodic differences in the thickness of sedimentary layers.  The search for Milankovitch signatures in rock records has been used as a method of dating sediments.
    Geologists at Virginia State and Virginia Polytechnic tested this hypothesis with computer models.  They specifically encoded Milankovitch-like cycles in the production of sediments.  The layering produced was indistinguishable from randomness, according to their report in the Journal of Geology.1  Here was their conclusion:

The simulations used a cyclic Milankovitch driver to produce cyclic stratigraphy, but the lithofacies thickness frequencies and autocorrelation methods used to analyze the resultant rock successions found that these records often appeared independent of periodic orbital forcing.  This indicates that the factors involved in depositing cyclic sedimentary layers, as simulated in the model, tend to mask the original periodic signal (such as Milankovitch orbital forcing) and produce the appearance of independence or stochasticity.  The hypothesis is that the rocks are independent of extrabasinal forcing, and these simulations indicate how difficult it is to disprove such independence.  Real rock successions are very likely to have been historically more complex than our simulations governed by merely a few basic parameters.  This poses a challenge to even most cleverly designed quantitative methods used to test for stratigraphic patterns, with their statistical outcomes being inherently ambiguous: does a given outcome indicate that the record was not formed in a cyclic fashion, or does it merely reflect the fact that an original cyclic driver has been masked by the complexity of depositional processes?  It is important, therefore, to have controls by which these methods can be tested.  The use of simulations can provide such controls by producing synthetic data with known Milankovitch cyclic drivers and thus providing an independent assessment of statistical methods applied to test real empirical records.

They said the results they got with known cyclic drivers was “extremely noisy.”  Obtaining a significant signature required extreme climate differences, like between greenhouse and icehouse conditions for 100,000 years.  Even then, the results were ambiguous: “even with high-magnitude sea level fluctuations, a periodic driver of sediment deposition can be concealed.”.  And that’s not the only factor: “The incompleteness of the carbonate stratigraphic record may act to conceal cyclic driving forces,” they said, “in turn making it difficult to assess the quality of methods developed to measure cyclicity.”  Their computer simulations, they felt, provided a missing control on the theory:

The methods for testing for the presence or absence of a Milankovitch driver in ancient successions must demonstrate patterns that are distinct from what would be expected if the rocks were deposited independent of orbital forcing.  One of the problems with many of the methods for detecting cyclicity is that they test a single series (e.g., a stratigraphic column).  This tends to miss lateral substitution of facies that occurs at similar water depths in real settings.  The benefit of using simulations is their ability to capture information such as periods of no deposition or gaps in deposition from erosion that would otherwise be difficult to quantify in real successions.


1.  Dexter, Kowalewski and Read, “Distinguishing Milankovitch-Driven Processes in the Rock Record from Stochasticity Using Computer-Simulated Stratigraphy,” The Journal of Geology,2009, volume 117, p. 349�361, DOI: 10.1086/599021.

Another dating method is shown to be a bruised reed.  Unfortunately, some well-meaning books like The Privileged Planet have leaned on this reed: “Finally, there are the Milankovitch cycles, probably the single most useful type of clock for layered deposits” (p. 30).  If this is the best, what about the others?  They tried to defend it with mathematical talk about Fourier analysis and power spectra (p. 370 n25), assuming that sophisticated math can discern a reliable signal in noise.  They did not consider the possibility of getting false signals in actual noise.  Then they used it and other methods to portray an old earth embedded with log records of its history over vast ages.
    Although that section did not harm the basic thesis of the book (that our planet appears designed for scientific discovery), it exposes a weakness of some well-meaning attempts to ground design inferences in shaky foundations.  Layers of rocks record something, obviously, but the time scale and explanations become increasingly tenuous when eyewitnesses are unavailable and multiple causes are involved.  We should be wary of taking published scientific claims uncritically and placing too much authority in the ability of secular scientists to discern unobservable history through their worldview-tainted glasses.
    Be wary especially of the divination methods of pagans (examples: 11/06/2008, 07/26/2008, 06/12/2008, 01/25/2008).  Would Daniel have referred to the scholarship of the Babylonian hepatoscopists as a reliable source?

(Visited 104 times, 1 visits today)
Categories: Solar System

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.