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Isotopic Variation in Aquatic Gastropods From the Continental Late Cretaceous Kaiparowits Formation, Southern Utah, U.S.A.
Department: Geology
ResourceLengthWidthThickness
Paper000
Specimen Elements
Pocatello
Unknown to Unknown
Amy Hudson
Idaho State University
Thesis
Yes
9/13/2016
digital
City: Pocatello
Master
ABSTRACT Base level exerts a fundamental control on continental sedimentary architecture, affecting the vertical and lateral deposition of sediments along the coastal plain. In the Late Cretaceous Kaiparowits Formation of southern Utah, coastal and alluvial plain deposition is recorded in a continuous ~2.1 myr section, with abundant and well-preserved continental fossils, of which mollusks are the dominant fauna. This study examines δ13C and δ18O stable isotopes in primary aragonite shell material from aquatic gastropods in pond and river deposits to evaluate the impact of changes in fluvial architecture, such as changes in pond stability and increased overbank flooding. Analysis of 67 gastropod shells from three morphotaxa display similar carbon and oxygen values, and show no significant variance attributed to vital effect differences among species. Primary aragonite shell material is distinguished from diagenetically altered calcite and dolomite bearing samples by X-ray diffraction methods. Samples composed of more than 75% aragonite are inferred to record close to primary δ13C and δ18O isotope signatures, while those containing less than 50% aragonite reflect a significant increase in δ13C and δ18O shell isotopic values. Mixed carbonate shell δ18O values are corrected to original aragonite isotopic composition using isotopic mixing equations. Pond gastropods record a distinct shift in the mean δ18O values between each unit. The lower unit records a mean δ18O value of -12.7±4.3‰, while the marine influenced portion of the middle unit has a heavier mean δ18O value of -9.8±4.2‰. The post marine influenced middle unit records a similar mean δ18O value to the marine influenced portion, with a value of -10.9±1.8‰, while the upper unit returns to δ18O values more xi similar to the lower unit with a mean of -12.3±4.4‰. This increase in δ18O values and return to lighter δ18O values correspond to changes in the fluvial geometry between meandering and anastomosing style rivers, and an incursion of marine waters up the river channels. Average values for pond unionoids (-9.5±1.6‰ δ18O) and pond gastropods (-10.3±3.8‰ δ18O) are statistically similar. In most units, the range of values recorded by the pond gastropods overlaps the range recorded by the pond unionoids. Occasionally, the range of pond gastropods overlaps with the fluvial unionoids as well. The consistent small offset of heavier δ18O and lighter δ13C values between gastropods and unionoids suggests these two mollusks record a similar environmental signal, but may record slightly different positions within the Late Cretaceous microenvironments, such as shallow vegetated areas versus deeper non-vegetated areas. These relationships suggest that aquatic gastropods promise to be an additional archive for geochemists, sensitive to both local and regional environmental changes, with the potential to expand our understanding of basin scale processes on the flora and fauna living in the Western Interior Basin.

Isotopic Variation in Aquatic Gastropods From the Continental Late Cretaceous Kaiparowits Formation, Southern Utah, U.S.A.

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