Stable-Isotope and trace-element geochemistry of Molluscan fossils from the Cretaceous Bearpaw Marine Cyclotherm of the Western Interior Basin of North american
Date
1996-06-01
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ORCID
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Masters
Abstract
The Late Cretaceous Bearpaw Formation and its equivalents (products of deposition during the last major transgressive-regressive marine cycle to affect the Western
Interior Basin) contain a molluscan fauna dominated by ammonites and bivalves. Other
than molluscs, representatives of the invertebrate megafauna! groups that were prevalent
in the open seas and oceans are relatively rare or absent. Usually, this is attributed to
brackish-water conditions or oxygen-deficient environments in the Bearpaw epicontinental
sea. Investigation of stable-isotope and major- and trace-element compositions of well-preserved
carbonate shells of the ammonites and bivalves from the Bearpaw cyclothem
provides valuable information about environmental conditions within the Bearpaw sea.
A clear relationship exists between the δ¹⁸O values of Bearpaw zonal baculitids and
their biostratigraphic sequence. The baculitids from the zones denoting peak transgression
have the lowest average δ¹⁸O values (-2.3 to- 0.7‰), whereas those from the underlying
and overlying zones have higher δ¹⁸O values (-0.8 to 0.22‰). This pattern of the δ¹⁸O values can only be explained by fluctuations in temperature, rather than variations in freshwater
influx, which may have been reduced due to lower precipitation and run-off under
drier, warmer, climatic conditions. Inoceramids usually have the highest δ¹³C values (0.6
to 5.0 ‰) and lowest δ¹⁸O values (-4.0 to -2.8 ‰), whereas coeval baculitids have the
lowest δ¹³C values (-4.8 to -0.3‰) and the highest δ¹⁸O values (-2.3 to 0.0 ‰). The large
differences in stable-isotope values imply that these animals must have inhabited
isotopically distinct reservoirs within the Bearpaw sea. Thus, the baculitids probably
hovered and swam mostly in the upper part of the water column in contrast to the
inoceramids which are known to have been benthonic. Didymoceras, with isotopic
compositions similar to those of benthonic inoceramids, also probably lived very close to
the bottom. The heterogeneity of the stable-isotope compositions of these coeval
benthonic and nektonic molluscs points to isotopically distinct, stratified, reservoirs within
the seaway, much like those inferred for the older Claggett and Greenhorn seas. Causes
of this stratification may have involved modification of bottom water through isotopic
exchange reactions between the water and the sediments near or at the bottom of the sea.
Relatively constant element/Ca ratios of nektonic baculitids indicate a stable
chemical composition for the upper water column of the seaway. This suggests a low
fresh-water influx, as indicated also by the stable-isotope compositions. Benthonic fossils
have more variable compositions due partly to modification by pore waters derived from
the sediments. Both nektonic and benthonic molluscs exhibit relatively high Ce/Ce* values
( -0. 16 to 0. 02 ), similar to the seawater in anoxic and dysoxic environments of modern
oceans and seas. Therefore, the seaway was probably dysoxic in both its upper and bottom
waters through most of the Bearpaw cycle. Low oxygen-level conditions are reflected by
characteristics of the Bearpaw sedimentary rocks and fauna in general. With the seeming
absence of brackish waters, dysoxia may account for the rarity of other normal, open
ocean, invertebrate organisms.
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Degree
Master of Science (M.Sc.)
Department
Geological Sciences
Program
Geological Sciences