Elemental Analysis by SEM of Late Ordovician Brachiopod and Bryozoan Fossils Wonnell, Chloe and Neff, David. Department of Geology, College of Science, Marshall University

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Elemental Analysis by SEM of Late Ordovician
Brachiopod and Bryozoan FossilsWonnell, Chloe
and Neff, David. Department of Geology, College
of Science, Marshall University
Introduction
Results and Discussion
    All fossils are preserved in sedimentary
environments. Geologists use fossils to
interpret depositional environments and to
produce a biostratigraphic correlation within
different rock units. Fossils are divided into
body fossils and non-body fossils, also known as
trace fossils (e.g. footprints). The unaltered
remains of soft body parts are preserved by means
of refrigeration, burial in bogs, or amber
secretion. Hard body parts are also fossilized
as unaltered remains, and are found as original
bones, teeth, or shells of the organism. The
altered remains of body fossils occur or form by
replacement, recrystallization, compression,
carbonization, and as molds and casts (Ref. 1).
In this study, the specimen used is a body
fossil our interest is to gain insight into
chemistry of its preservation through elemental
analysis. The specimen is an encrusting bryozoan
on a brachiopod shell. Geologic Strata of
Interest The specimen was collected from the
Dillsboro Formation of the Upper Ordovician
strata in Southeastern Indiana off of U.S. Route
421, north of Madison, IN (figure 1A) from
Indiana Geological Survey). The Dillsboro
Formation is approximately 300 feet thick, and it
overlies the Kope Formation and underlies the
Saluda Formation. The formation consists of a
richly fossiliferous series of alternating gray
shales and thin slabs of coquina limestone, which
are well-known to fossil collectors (Ref. 2).
The fossils found in the Dillsboro Formation
include brachiopods, bryozoans, molluscs,
arthropods, and echinoderms. Fauna of the
Ordovician Epeiric Sea By the Upper Ordovician
(figure 1A), the continent was covered by a large
shallow sea or epeiric sea, which deposited shale
and limestone over most of the continental
platform. The shallow, warm, calcium-carbonate
rich environment was ideal for shell-secreting
organisms. The most abundant organisms found in
the Upper Ordovician, specifically the Dillsboro
Formation, are bryozoans and brachiopods.
Bryozoans found in the Upper Ordovician are
marine animals. They are colonial organisms
whose encrusting masses often contributed to the
formation of limestones. An overview of the
diversity of Ordovician Bryozoans can be seen in
figure 2 (Ref. 4). In this study, the notable
aspects of Bryozoan anatomy are their calcareous
protective tube and the soft body it surrounds.
Brachiopods are generally larger and more complex
animals, their shells can be confused with those
of bivalve molluscs. Brachiopod remains also
contribute to the formation of limestone (Ref.
5). Elemental Analysis by Scanning Electron
Microscopy (SEM) The electron beam formed by the
SEM is composed of high energy electrons. When
the electrons hit the sample, some are scattered
back off the sample and allow us to form
backscattered electron images (as seen in figure
3A-E). Beam electrons also cause specific energy
transitions to occur in the specimen these
create X-rays whose energies are characteristic
of specific elements. In order to get
uncomplicated X-ray spectra, we imaged our
samples without any conductive coating. This can
lead to an unstable imaging condition called
charging, which we eliminate by using the low
vacuum mode of our SEM. This allows for a small
amount of atmospheric moisture (water molecules)
to enter the chamber and dissipate the electron
charge. We collected X-rays generated by the
sample with an energy dispersive spectrometer
(EDS). This device is capable of measuring and
counting X-ray photons these counts are then
plotted on an energy spectrum (figure 3F-I) or
mapped to spatial coordinates (figure 4). All
research was carried out using the environmental
JEOL 5130 SEM with an Oxford Instruments Pentafet
thin window EDS. This detector has a beryllium
(Be) window which can detect elements with atomic
numbers gt 10.  The microscope has a tungsten
filament, we used the solid state backscattered
electron detector.
F.
A.
B.
G.
C.
D.
Y-axis units are X-ray photon counts
H.
Figure 3 Backscattered electron (BSE) images
taken in low vacuum mode to prevent specimen
charging (A-E) and X-ray spectra (F-I).
Brachiopod (A, B, and F), bryozoan (C, D, G,
and H), and in E, a second smaller brachiopod
shell (arrow) overlying encrusting bryozoan. 3D
is an image of intrashell area of the Bryozoan,
the cube shaped crystal in D. has higher relative
Mg than the sur-rounding material (data for
crystal not shown). Spectrum I. is from the rock
as seen in figure 2B, left. All X-ray spectra
are qualitative, scale is normalized to the
largest calcium Ka peak.
E.
I.
B.
A.
From the context of the collection site, we know
the age of the source strata to be Upper
Ordovician (see introduction). The animals
whose remains are shown here are known to have
lived in a shallow epeiric sea that once covered
much of the continent. Consistent with that
habitat are the calcareous remains of the hard
parts of the animals. This type of anatomical
adaptation, calcium rich shell, is very common in
former and present shallow sea dwellers. Both
animals seen in this study were dominant in the
shallow seas of the Paleozoic, but have now been
supplanted by other calcium fixing organisms such
as mollusks (Ref. 6). The X-ray spectra in
figure 3 F, G, and H and the X-ray maps in
figure 4 show that shells (also known as
carbonate skeleton) of both brachiopod and
bryozoan are rich in calcium, presumably some
form of calcite. Whether the ratios of elements
seen in the spectra represent the original
composition of the shells or replacement minerals
cannot be determined from this data. In
contrast, the pore filling intrashell material of
the bryozoan is identified by the silicon,
aluminum, and magnesium peaks (figure 3H), and in
the corresponding X-ray map (figure 4A and C).
The finding of these elements in the intrashell
regions of the bryozoan is of interest, because
it implies replacement by elements other than
calcium, which is found predominately in the
outer tubing of the bryozoan and brachiopod shell
(figure 4 A and C). It was suggested (by
personal correspondence, Dr. Martino) that these
elements indicate filling by a mud matrix.
This finding is consistent with the Dillsboro
Formation (see introduction) that has a lithology
of mostly coquina limestone and gray shales (Ref.
7). The rock spectrum (figure 3 I) was taken
from the limestone matrix that surrounded the
fossil as it was in situ. We think silicate
minerals related to the gray shale may have
filled the intrashell areas (formerly occupied by
the animals soft body) of the bryozoan. The
dominance of calcium and the absence of magnesium
in the brachiopod shell (figure 3A, B, F) helped
us reason that the brachiopod shell was likely
replaced by low-Mg calcite during diagenesis. In
studies of calcium carbonate minerals,
specifically of low-Mg calcite and high-Mg
calcite, it is known that carbonate sediments are
often converted to low-Mg calcite, and that
high-Mg calcite at times will lose its Mg (Ref.
6). Based on this knowledge, we hypothesize
that our fossilized brachiopod may have undergone
diagenesis from high-Mg calcite to
low-Mg-calcite. Future work may involve this
hypothesis by analyzing cross sections.
A.
X-axis units are X-ray photon energy
A.
B.
C.
Figure 4 X-ray maps of the fossil show spatial
distribution of detected elements. A is
bryozoan, B is brachiopod. C is both with
bryozoan at left side of each pane and brachiopod
at lower right of each pane. Elements mapped in
each pane are identified at top of pane, the
5310lv pane is a BSE image.
Figure 1 Above left (A) is a general
stratigraphic column of Paleozoic rocks in
Indiana, the red box indicates the formation of
origin for the fossils in this study. At right
(B) we see where the Ordovician period falls in
the overall geologic time scale.
Figure 2A The SEM images above, from reference
4 show the diversity of Ordovician bryozoans.
These specimens were collected from an other
Ordovician epeiric sea in present day
India. Scale bars represent 0.5 mm in 12, 48,
10, 12, 15, 17 1 mm in 3, 9, 11, 1314, 16 2 mm
in 18. Figure 2B A photograph of the back of
specimen showing the rock matrix from which the
fossil was collected (left). The photograph at
right shows the brachiopod shell (approximately
1.5 inches across). The white arrow points to
the encrusting bryozoan.
References
B.
1. Pullen, Stephanie. What is a fossil? UCMP.
April 2004. 20 March 2009. lthttp//www.ucmp.berkel
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G.P., 1992, Middle and Upper Ordovician
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and Bellerophontina) of the Cincinnati arch
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Geologic Time Scale. Geology.com . 2005. 12
April 2009. lthttp//geology.com/time/geologic-time
-scale-550.gifgt.4. Suttner, Thomas J., and
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Acknowledgements
Thanks to Dr. Martino and Dr. El-Shazly for their
advice. And thanks to Marshall Universitys
MBIC for maintenance of SEM facilities.
http//www.marshall.edu/mbic/