Microbioerosion - Trace Fossils as Indicators of Sedimentary Environments - page 209

methodological tools at hand, and a review and outlook of their applicability for
paleoenvironmental studies with respect to relative bathymetry, temperature,
salinity, and the trophic regime. For all of these applications, the current state
of knowledge on relevant ichnotaxa, their known or inferred producers, and their
geological record is compiled in tables that can serve as a guideline during
paleoenvironmental assessments.
For further information on the various aspects of microbioerosion, the reader
is referred to the review papers on bioerosion in general ( Warme, 1975 ), micro-
bioerosion in general ( Golubic et al., 1975, 1981; Tribollet et al., 2010 ), cyano-
bacterial microbioerosion ( Schneider and Le Campion-Alsumard, 1999 ), fungal
microbioerosion ( Golubic et al., 2005 ), high-latitude bioerosion ( Wisshak, 2006 ),
tropical reef microbioerosion ( Tribollet, 2008 ), the fossil record ( Glaub and
Vogel, 2004 ), and geological implications ( Glaub et al., 2007 ). An extensive bib-
liographic source is the continuously growing online bibliography maintained by
Wilson (2008, 2012) .

2. THE METHODOLOGICAL TOOLKIT
At present, three methods are at our disposal for visualizing trace fossils left by
microbioerosion agents in lithic substrates. These are microscopy of petrographic
thin sections, scanning electron microscopy (SEM) of casts obtained by the epoxy
cast-embedding technique, and sophisticatedmicrocomputed tomography (micro-
CT) that is about to conquer a resolution in the nanometer scale, enabling non-
destructive investigations of even minute bioerosion features.

2.1 Thin Sectioning
Being a standard tool in microfacies analyses, transmission microscopy of pet-
rographic thin sections may also be applied for microbioerosion analyses, but
bears the limitations adherent to any two-dimensional (2D) visualization of
three-dimensional (3D) objects ( Fig. 1 ). On the other hand, this method does
comprise a number of advantages such as that (1) microbioerosion patterns
can be visualized together with the surrounding matrix, (2) the true penetration
depth of microborings can easily be determined since they are not prone to col-
lapse such as in the cast-embedding technique (see below), and (3) even those
fossil microborings can be studied that are filled with spar cement or other sec-
ondary mineralizations prohibiting the standard cast-embedding technique.
When preparing thin sections of semiconsolidated and/or relatively porous
and permeable samples for microbioerosion analyses, an impregnation with a
low-viscosity epoxy resin such as Ciba-Geigy Araldite BY158

þ

Aradur 21 is
advised. It is advantageous to stain the resin with 2% (by weight) of Keystone
Oil Blue powder for an easy visualization of the pore space within the micro-
bioerosion cavities. The degree of impregnation can be enhanced when placing