Graduation Year

2006

Document Type

Thesis

Degree

M.S.

Degree Granting Department

Geology

Major Professor

Peter J. Harries, Ph.D.

Co-Major Professor

Gregory S. Herbert, Ph.D.

Committee Member

Eric A. Oches, Ph.D.

Keywords

Cretaceous, Campanian, epicontinental sea, paleoclimatology, paleooceanography, fossil preservation, mollusks, oxygen, carbon, minor elements

Abstract

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.

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