Whether a global greenhouse interval is a distinct or distant future, it is important
to understand the dynamics of a greenhouse system. During such intervals the oceans, in
the absence of sizeable polar ice caps, flood the continental shelf. The stratification and
circulation of these epicontinental seas are open to debate, because there are no Recent
analogs. The carbon and oxygen stable isotope record of fossil molluscan shell from
epicontinental seas has the potential to reveal their stratification and seasonal cycles.
Whether a global greenhouse interval is a distinct or distant future, it is important
to understand the dynamics of a greenhouse system. During such intervals the oceans, in
the absence of sizeable polar ice caps, flood the continental shelf. The stratification and
circulation of these epicontinental seas are open to debate, because there are no Recent
analogs. The carbon and oxygen stable isotope record of fossil molluscan shell from
epicontinental seas has the potential to reveal their stratification and seasonal cycles.
As a study sample, mollusks from the
Baculites compressus and Baculites
cuneatus
biozones of the Western Interior Seaway of North America were collected from
three locations: Kremmling, Colorado; Trask Ranch, South Dakota; Game Ranch, South
Dakota. These fossils date to the Campanian (Late Cretaceous). Taxa include
ammonites, bivalves, gastropods, and nautiloids.
The first part of this investigation, described in Chapter 2, investigates the degree
of alteration in these specimens. Elevated concentrations of minor elements such as
magnesium and strontium reveal alteration from the original aragonite and/or calcite
skeletons. Concentrations of these elements obtained by ICP-OES analysis are compared
within several suites of specimens: mode of preservation, shell testing location, shell
color, cementation, appearance under light microscope, and appearance under scanning
electron microscope. Each of these suites tests a hypothesis about optimal shell
preservation. Shell was found to be preserved best in shale rather than concretions,
ammonite phragmacone rather than septa, opalescent specimens rather that nonopalescent
ones, and uncemented shells rather than cemented shells, especially those with
second-order versus first-order cement. Salinity and temperature values were derived for
the organisms in the Western Interior Seaway: while bivalves produced unusually low
temperatures, the others were reasonable for an inland sea.
The second part of this study, described in Chapter 3, examines the isotopic
record within exemplary mollusk shells, taken perpendicular to growth lines. The data
for this investigation in sclerochronology documents the dominant isotopically enigmatic
bottom-water habitat of the
Inoceramus, the geochemical signature of the overlying water
mass inhabited by
Baculites, and short-term migrations between the two water masses in
the nautiloid
Eutrephoceras.
| Identifer | oai:union.ndltd.org:USF/oai:scholarcommons.usf.edu:etd-5117 |
| Date | 11 April 2006 |
| Creators | da Silva, Ashley |
| Publisher | Scholar Commons |
| Source Sets | University of South Flordia |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Graduate Theses and Dissertations |
| Rights | default |
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