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Carbonate Carbon Isotope Chemostratigraphy from the Ruby Ranch Member of the Cedar Mountain Formation in the Western San Rafael Swell

Gottberg, Amy
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Abstract
The Ruby Ranch Member of the Cedar Mountain Formation in East-Central Utah represents terrestrial deposits in the early Cretaceous Period. This member has been studied at multiple field locations and has been determined to span the Aptian-Albian Boundary based on chemostratigraphic and detrital zircon studies. The study location presented here, Moore CutoffRoad, is further west than previous studies, and thought to be a more expanded section that may contribute more detail to the chemostratigraphic and paleoclimatic record. The Ruby Ranch Member in this area consists mainly of reddish grey and greenish gray silty-sandy mudstones with abundant carbonate nodules suggestive of paleosol development. For this study samples were collected approximately every 25 cm and were used to construct a C-isotope curve using carbonate carbon. These were used to compare to existing bulk sedimentary organic carbon isotope curves. A total of 48 samples were analyzed and values from -7.45 ‰ vs. VPDB to -3.52 ‰ vs. VPDB were observed. The C-isotope data follows the trend of existing organic carbon isotope curves which exhibits the C10 positive isotope excursion near the Aptian-Albian boundary. A set of samples have been analyzed for pCO2 values. These values were calculated using both a high and low value for both S(z) and temperature. pCO2 values range from 795 ppmV to 2850 ppmV over the 4 scenarios presented. A subset of samples have been analyzed for clumped isotope paleothermometry to test a hypothesis that this positive isotope excursion corresponds with a late Aptian “cold snap” in an otherwise greenhouse climate. This data produced values that were not reliable estimates of temperature. Additional studies are needed to confirm a temperature change over the C10 positive isotope excursion and refine the paleoclimate record at this location.
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Date
2022-05-31
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University of Kansas
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Keywords
Geochemistry, Geochemistry, Paleoclimate
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