Tectonics & Paleo (4):The world of CPOs and ODFs

Materials in geoscience and biology are often not isotropic - their properties, such as conductivity, soundwave velocity, and shear strength, vary with direction.

The reason can be that they are crystalline - if so, the orientation of the crystal lettice of (one, a few, or) many individual crystal grains has an influence on the properties of the compound material. The individual crystal lettice orientations can be at random or their can be a preferred orientation.

A whole branch of mineralogy and material science deals with the analysis of materials which show crystal preferred orientations (CPOs).

From Tectonics...

Structural geologists, who are dealing with microscopic phenomena of tectonic deformation, use mineral textures of rocks, i.e. crystal preferred orientations of the rockforming minerals, as indicators of tectonic movement and deformational regimes (temperature, pressure conditions; deformation rate).

The so-called Orientation Distribution Density Function (ODF) describes how the crystal axes of mineral grains in a sample are oriented relative to an outer coordinate system (e.g. geographic XYZ coordinates). From the ODF, usely depicted as a couple of stereographic density plots (e.g. plots for the crystal a-axis, b-axis and c-axis orientations) the direction and sense of shearing of a rock can be inferred.

...to Paleontology

Not only tectonic forces are governing the crystalline properties of natural materials. In a similiar way the (often monomineralic) mineralized tissues of organisms are underlying specific biological formation conditions - the different layers of a skeleton can show rather perfect CPOs.

Crystal orientations in mineralized tissue are, of course, not defined relative to a geographic coordinate system but to the anatomic directions (or the axis of accretionary growth) of the animal as a reference system.

Coming from the tectonical side I was participating in a workshop on "Textures & Microstructures in Geosciences" in 2005: I was really surprised that there is an application of texture analysis in palaeontology and that some people are really doing it.

Methodology

You can do either single grain measurements or methods integrating all orientations of crystal grains in a certain volume of the sample.

U-stage and EBSD are involving single grain measurements of a thin section: For each grain in a certain raster the individual crystal orientation is determined - either by its optical properties using the universal stage (this old-fashioned manual method is rather time-consuming) or by the way how electrons from an electron microbeam are backscattered on a detection screen (Electron Backscatter Diffraction, EBSD). The detected Kikuchi line patterns are indicative for how an individual crystal is oriented (they can be interpreted automatically). What you get in both approaches is not only an ODF but also a crystal orientation map of your thin section.

XRD, Neutron Diffraction. In these methods an X-ray or neutron beam is used to measure a larger volume of a sample comprising several crystals, thereby neutron radiation can penetrate even larger samples completely while the X-ray has a relatively low depth of penetration. From the detected line sprectra pole figures, representing the distributions of certain crystal lettice plane orientations can be derived (from which in turn the ODF of all the crystals in the measured sample volume can be deduced).

Examples from Paleontology

Chateigner, D., Hedegaard, C. & Wenk, H.-R. (2000): Mollusc shell microstructures and crystallographic textures. - Journal of Structural Geology 22: 1723-1735.

The authors employ X-ray diffraction measurements and demonstrate thate the microstructures and crystallographic textures of aragonite layers of species from different mollusc taxa including bivalves, cephalopods, gastropods and monoplacophorans are highly specific and contain a phylogenetic signal: closer relatives are more similar in their shell's crystal orientations.

Pyzalla, A.R., Sander, P.M., Hansen, A., Ferreyo, R., Yi, S.-B., Stempniewicz, M. & Brokmeier, H.-G. (2006): Texture analysis of Sauropod bones from Tendaguru. - Material Science and Engineering A 437: 2-9.

This neutron diffraction approach addresses the question whether the apatite crystallite textures in adolescent and adult Brachiosaurus long bones show some signal indicative for specialized crystal orientations which can be attributed to the giant growth of sauropod dinosaurs. However, comparing their results to the measurements of turkey and other dinosaur long bones they found no significant difference in texture strength or in the predominant direction of fibres.

Prospects

Given the elaborateness of most approaches, measurements of crystallographic textures are rarely used in paleontology - I suppose this will change if it turns out that the analysis of the crystal orientations can provide substantial information which is not obtained from the usual analysis of skeletal histology and microstructures.

Tectonics & Paleo (III):The shearing of fossils and how to reverse it

Let's start in 2D: You have a flat fossil and regard only the deformation in the two dimensions of the fossil plane:


Easy, you may say. Fossils specimens like that are good strain indicators and it's not difficult to deduce the amounts of simple shear and flattening/lengthening necessary to transform the undeformed into the deformed specimens or vice versa:


There was a story, my tectonics prof told me from his study time when he was working for a famous German paleontologist: He was doing the retrodeformation of fossils using some kind of algorithm/ computing procedure - but only, until his sponsor found that he could reach the same effect by holding the fossil oblique over a photocopying machine (you can imagine what a disillusionment that was...).

In some cases, however, the problem is not as simple as in the example displayed above. Insect wings from Madygen and other localities often display a considerable amount of deformation but occur isolated and as palaeontological samples they have not been taken oriented (i.e. referenced to a system of external coordinate axes).



What was the original shape?

This question is crucial if you want to define and distinguish taxa (how many unnecessary species have been erected because the similarity of fossil specimens got lost in deformation?) but also searching for intraspecific variation, e.g. branching points that are highly variable in individuals of the same species.

My "bureau-mate" Olivier Bethoux, paleoentomologist, is currently doing his postdoc research working on that problem. I won't say much about his solution which involves morphometrics/ landmark analysis but keep you informed about results when they are published.

Tectonics & Paleontology (II): Sclerochronology

Some five years ago I had one of my first presentation-preparing seminars and the list of available topics included 'sclerochronology', supervised by the tectonophysics prof. Searching the literature I found that the term referred to the study of the accretionary growth of mineralized organismic hard parts - a bit like the tree-ring chronology transferred to animal skeletons ('interesting', so I thought and chose 'sclerochronology' for my seminar talk).

There were many papers on mollusc life strategies and environmental change during the younger Cenozoic, mostly analyzing some long-living clams. Not many studies involved 'sclerochronology' as an actual dating method, often researchers were looking for either ontogenetic signals or climatic signals, often involving distinct taxa, localities, and stratigraphic levels for comparison.

Among the animal groups considered were brachiopods, bivalves, corals, belemnites, fish (otoliths) but also higher vertebrates: Enamel and accretionary growing bone can yield sclerochronological data - the method is also called 'skeletochronology' when applied on vertebrate hard parts.

And the link to tectonics? If you consider the cross section of a shell as representing a time series of fast and slow growth phases and phases of arrested growth, how exactly can you expect a tectonic signal to show up? I asked my tectonophysics prof what story he wanted me to tell and it was this:

A reference book on Quaternary dating methods (Lettis et al. 2000) also includes a chapter on sclerochronology in recent to subrecent corals inhabiting a shallow tropical tectonically active shelf. If the shelf area is part of a block which is elevated over another block by thrust faulting (as under compression along a convergent tectonic plate margin), this tectonic movement - which is not continuous but (mostly) discrete with larger earthquakes releasing most of the stress - can have consequences for the coral growth:

During an earthquake (along a thrust fault) the uppermost part of a colony is lifted over the water level, dies, and stops growing, while deeper-lying sections stay intact and continue their growth. If you count the annual growth bands and locate the points of growth arrestment after an earthqake you can derive the timing of earthquakes and also the amount of vertical displacement for each event. These data are sufficient for deriving the earthquake characteristic of the responsible fault - a classical aim of paleoseismology.

You could argue that all that has nothing to do with deep time processes and you are right: While the paleobiological and paleoclimatological approaches employing sclerochronology are not strictly limited in time, sclerochronological dating is restricted to the youngest few thousand years of the Holocene.

Some refs: Sclerochronology

... & Tectonics:
Buddemeier, R.W. & F.W. Taylor (2000): Sclerochronology. In: Lettis, W.R., J.S. Noller & J.M. Sowers (eds): Quarternary Geochronology: Methods and Applications. - Washington, AGU, pages 25- 40.

... in vertebrates
MacFadden, B.J. (2004)(ed): Incremental Growth in Vertebrate Skeletal Tissues: Paleobiological and Paleoenvironmental Implications. In: Palaeogeography, Palaeoclimatology, Palaeoecology 206(3-4).

... in marine animals
Schöne, B.R. & D. Surge (2005)(eds): Looking back over Skeletal Diaries - High-resolution Environmental Reconstructions from Accretionary Hardparts of Aquatic Organisms. In: Palaeogeography, Palaeoclimatology, Palaeoecology 228(1-2).

Tectonics and Paleontology (I): Series Intro

In Germany paleontologists often have graduated in geoscience study programs. At some universities, such as my Freibergian alma mater, paleontology is mostly taught as a branch of geology dealing with fossils for the purpose of solving geoscientific problems: Fossils provide information about the age of sedimentary rocks (biostratigraphy) or their maturity (see for example: conodont alteration index) or formation conditions (biofacial analysis) or are relevant for paleogeographic reconstructions (paleobiogeography) or for paleoclimatic inference. As a matter of fact fossils are useful and paleontology is not the end in itself, no art pour l'art...

...and so forth. Perhaps some of you heard a similar story.

There are some advantages, though, when you are coming from the geological side: You know your rocks and minerals alright. You have learned how to draw maps and what geoinformation is and all those analytical methods for rock samples and how to get a picture of an ancient biotope from sedimentological criteria and how to find the most fossiliferous places and strata.

In my M.Sc. studies of geology/paleontology I was supposed to chose 3 out of 13 electives, including petrology, tectonics/geodynamics, geology of mineral deposits, geochemistry, sedimentology, pedology, hydrogeology, geotechnics, paleontology, mathematical geology/ geoinformatics and mineralogy. I did a bit of everything with the exception of hydrogeology and focussed on all that non-applied basic research stuff, including paleontology and tectonics.

And when the project in Kyrgyzstan started somewhat later than expected I did - not only for reasons of timing - my master thesis on fissures and normal faults in the Ethiopian rift (see here). Over the years I found more and more links between tectonics and paleontology including rather subtle ones. Some posts will help me to keep them in mind.