Environmental Engineering Reference
In-Depth Information
observed in a section and the total size available for investigation. For instance,
it makes a difference whether a core slab restricted by a size of 100
10 cm
comprises tiny burrows such as Chondrites or Trichichnus with a burrow dia-
meter in the millimeter range or whether it contains large burrows like Thalas-
sinoides or Phoebichnus in the range of centimeters. The same number of
burrows in each example would result in completely different amounts of bio-
turbation (e.g., the area they occupy within the section), low versus high,
respectively. Therefore, it is essential to select a reasonably sized section for
investigation with a representative statistical distribution.
Another issue is related to the seven categories (0-6) with a disproportionate
weight. While the current scheme works fine in semiquantitative analysis, it
influences a statistical analysis of the amount of bioturbation, a method which
is relevant for the comparison of depositional settings or stratigraphical changes
(e.g., Knaust, 2010a; Martin, 1993; Pemberton and Frey, 1984 ). Moreover, ich-
nofabric constituent diagrams are typically constructed on the basis of percent-
age of bioturbation ( Taylor and Goldring, 1993 ). For such analysis, categories
with the same proportions (increments) and a linear increase would enhance
their comparability in a statistical way ( Martin, 1993 ). In contrast to the semi-
quantification approach with varying proportions (percentage) of the catego-
ries, statistical analysis should be based on categories with constant
proportions between the two extremes 0% and 100%, such as 0%, 20%,
40%, 60%, 80%, and 100% of bioturbation as maxima ( Fig. 2 ). The content
of each category can be selected on the basis of the quality and quantity of
the available material and the purpose of the analysis. This procedure has been
used occasionally (e.g., Rebata-H et al., 2006 ) but so far is not in common use.
From a technical aspect, point counting can be employed for collecting data
on bedding planes and vertical (e.g., core) sections (for details, see Fl¨gel, 2004;
Marenco and Bottjer, 2010 ). Image analysis techniques for quantifying biotur-
bation were applied by Francus (2001) , L¨wemark (2003) , and Honeycutt and
Plotnick (2008) . Challenges regarding the logging of the BI in core, particularly
in carbonates, are addressed by Knaust (2009b) .
3. LABORATORY TECHNIQUES
3.1 Sectioning and Enhancement of Visibility
An important method to enhance the visibility of biogenic structures is the sec-
tioning of rocks and staining the smooth surfaces with oil or other chemicals.
The employed method is dependent on the kind of lithology, the degree of con-
solidation, and the composition of the rock. Cutting and polishing of rock faces
may already give some reasonable results ( Miller, 1988 ) but can still be
improved by photography under alcohol ( Bockelie, 1973 ), by applying polar-
izing filters to camera and light sources ( Bengtson, 2000 ), or subsequent stain-
ing techniques ( Farrow, 1975 ). Less-consolidated siliciclastic rocks with relicts
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