Wednesday, August 8, 2007

Cyclothems: Layers Of Rock Record Past Climate Change

In discussing the effects of variations in solar energy received by the Earth, and its role in global warming, or climate change, I recalled the geologic term "cyclothem", or repeated layers of different sedimentary rock types. The MSNBC discussion board where this is appearing is here: (you will recognize me as GeoPete, or geopeter

A quick google search of cyclothems turned up the following interesting information. As I have said before, these alternating layers of rock represent cyclical climate change. The only thing affecting the Earth's climate that is cyclical is the changes in amount of solar energy received. Carbon dioxide is not causing these climate changes. Carbon dioxide doesn't cause climate change, it responds to it. It is just along for the ride on Earth, along with everything else.

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In geology, cyclothems are alternating stratigraphic sequences of marine and non-marine sediments, interbedded with coal seams. Unique to the Carboniferous period, they apparently formed as a result of marine transgressions and regressions related to decay and growth of ice sheets, respectively, as the Carboniferous was a time of widespread glaciation. Cyclothems were possible because of the extremely low topography of the interior lowlands the seas covered and uncovered, which is why they have been absent in the current ice age.

and this:

Cyclothems as solar-system pulse recorders
Geologists can help astronomers look back in time. The sunspot cycle can be seen in variations of varves; i.e., annual layers of sediment; and the growth rings of shells have been used to estimate the number of days in the lunar month when the solar system was younger. Cyclothems may also be useful. Cyclothems are groups or bundles of strata that repeat themselves in stratigraphic columns. A generalized cyclothem from Illinois is shown in the illustration.

In the U.S. western interior, rhythmic sedimentation appears in the Fort Hays Limestone Member of the Niobrara Formation. These cyclothems can be correlated over distances exceeding 800 kilometers and are believed to be the consequence of climatic changes associated with the earth's precession and orbital eccentriciy. These rhythms have been captured in bundles of shale-limestone couplets. A bundle of five coup lets, for example, is thought to express 21,000- and 100,000-year Milankovitchtype climatic cycles, as impressed by variations in the earth's orbital precession and eccentricity.

Analysis of the Fort Hays Limestone Member, however, reveals that while bundles of five couplets do occur, the number may vary from 1 to 12. Clearly, things are not clear-cut.
(Laferriere, Alan P., et al; "Effects of Climate, Tectonics, and Sea-Level Changes on Rhythmic Bedding Patterns in the Niobrara Formation (Upper Cretaceous), U.S. Western Interior," Geology, 15:233, 1987.)


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