microfossil preparation

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I got the following question by private e-mail and want to post the answer
to the net because it may promote the discussion about microfossil
preparation techniques.

"..., does it really work for getting out forams? My experience is
that when forams are really indurated and diagenetically altered they
disintegrate in the process of getting them out of the rocks. What is your
experience?"

Yes, this is a real problem. Diagenetically altered forams may become
fragile but I am not quite sure if it is always the preparation that
destroys them. I did my PhD thesis work on foram stratigraphy and basin
dynamics in Cretaceous pelagic (hard) limestones in the Alps. I had to
study them in polished slabs and thin sections because the foraminifera
were broken already in many samples. In fact it was the rock matrix that
kept them together. Any isolation (e.g. washing with any technique) would
have resulted in a tremendous loss of specimens. I can really recommend to
look at thin sections of such problematic rocks before decisions are made
on preparation techniques. Much of what is considered as alteration of a
fauna by the preparation technique could be "in the rock" already:
Fracturing by mechanical forces (tectonic or diagenetic), recrystallisation
and subsequent deformation (which can be extreme), dissolution of the
shell, and much more. It is often better to determine planktic foraminifera
(at least the Cretaceous forms that I know) in large polished surfaces or
thin sections rather than to try to wash the sample. The problem is the
sample statistics. Small sections may not yield enough specimens for a
reliable record of all species. The size of the polished surfaces to look
at can be computed from the abundance of forams in the rock. In polished
slabs I appreciated to see the sedimentology of the foram samples:
winnowing, sorting of forams by species through hydrodynamics, forams as
building materials of burrows, indications of all sorts of reworking, and
many more processes that can alter the information content of a microfossil
sample. When I look at a washed sample I feel somewhat "blind" after this
experience.

Yours Heinz


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Heinz Hilbrecht
Geological Institute, ETH Zentrum, Sonneggstr. 5, CH-8092 Zuerich, Switzerland
Tel.: ++41-1-63 23676, Fax: ++41-1-63 21080, Internet: Hilbrecht@erdw.ethz.ch
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