Monday, February 11, 2008

Milankovitch Cycles and Stratigraphy

Part of my research deals with the controls on sedimentation, and how stratigraphic packages of rock can be used to reconstruct patterns of erosion, sediment transport, and deposition. This area of sedimentological research has been going on for quite some time, of course, with lots of different workers producing lots of data (and interpretations) that seem to point in many different directions. One of the frameworks employed has been the interpretation of repeatable packages of sedimentary rocks (i.e., stratigraphic cycles) as having been caused by Milankovitch cycles in the climate. Wikipedia has a pretty good summary of Milankovitch cycles at:

Milankovitch cycles attempt to explain how variations in the orientation and orbital pattern of the Earth result in changes in the amount of incoming solar radiation (insolation). The original work (Milankovitch, 1941; translated in 1969) uses this theoretical framework to explain the cyclicity observed in glacial-interglacial periods (the Ice Ages). That’s all well and good, and I think it holds up pretty well when used to explain the past few hundred thousand years of ice dynamics, since the mechanism (insolation changes) can be reasonably hypothesized to strongly effect ice volumes. What I’ve always had a problem with seems to be the desire to extend the Milankovitch cycle into the stratigraphic record where we KNOW that there were no large scale ice sheets (The Eocene, or the Cretaceous, for example).

Finally, what are the actual impacts in a SEDIMENTOLOGICAL sense that these Milankovitch cycles would be expected to have? How would they modulate erosion or transport, and what would their impact be on the rock record? So far, I haven’t found a Milankovitch-supporter who can adequately provide me with an answer.


BrianR said...

I've long pondered whether or not sediment transport/depositional systems record Milankovitch cycles in a recognizable way.

Researchers have intrepreted Milankovitch cycles in relatively quiet and climate-sensitive settings (e.g., varves). But, I think you are wondering more about coarser-grained/higher "energy" systems (e.g., rivers, deltas, alluvial fans, submarine fans, etc.).

Ultimately, I think recognizing true Milankovitch forcings in such systems would be highly interpretive. I think there are just too many aspects of the sediment routing system that include lag times, feedbacks, emergent features, and so on.

I've done some work on a Holocene source-to-sink system (that I will blog about in detail soon) in which we had great temporal control (14C dates) and contextual knowledge. Even was challenging to deconvolve the myriad forcings that all interact together to produce the depositional record.

At a longer time could say the glacial-interglacial cycles are a manifestation of Milankovitch cycles...they have certainly have left a signature on the depositional record (e.g., last glacial max lowstands).

Just found your blog ... the sedimentary geology blogging community keeps getting stronger!

Eric said...

Howdy Brianr; nice to virtually meet-you.

The whole Milankovitch thing is a pretty interesting study of not just geological processes, but also of geological methodologies and philosophies, in my humble opinion.

Of course, ol' Milankovitch really was just talking about the glacial/interglacial cycles of the Pliestocene. Their has also been some recent work (that I think I'll blog about soon) that casts doubt on some of the varve interpretations from older strata, as well.

BrianR said...

check out a very recent paper by Boulila et al. in the Jan 08 edition of Sedimentary Geology about the cyclostratigraphy of Upper Jurassic marl-limestone successions.

A great book about this kind of stuff is one by Weedon called "Time Series Analysis and Cyclostratigraphy" published by Cambridge.

World's Bubbles said...

Im very new in geology and Im really interested to discuss on The role of Milankovick cycle in Petroleum Geology. I wonder if you could give me some ideas.. or where I could find lots of sources on this.

I really appreciate your helps...

Eric said...

Howdy Worlds Bubbles! A good resource wa smentioned by Brianr in the above comments:
Weedon, G., 2005?, Time series analysis and Cyclostratigraphy: Examining Stratigraphic Records of Environmental Cycles. Its a pretty standard resource with a fare amount of background info on the theory and mechanics of cycle analysis.

A very pro-Milankovitch set of papers have been authored by Linda Hinnov, so a google search with her name and "Milankovitch" would produce a number of papers.

Andrew Miall published a book, "The Geology of Stratigraphic Sequences" that discusses some of the pros and cons of cyclostratigraphy.