Quantifying Climate Change Over the Early Cretaceous Ruby Ranch Member of the Cedar Mountain Formation, East-Central Utah

Date
2018
Authors
Knight, John A., II
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Abstract

The age of the Ruby Ranch Member (RRM) of the Cedar Mountain Formation in East-Central Utah was recently constrained using carbon isotope chemostratigraphy to span known excursions associated with the late Aptian. The RRM is characterized by calcrete horizons that are thought to occur across the C10 carbon isotope excursion. Along with carbonate stable isotope analyses and the region’s paleo-position in a depositional basin on the leeward rain shadow of the Sevier Orogenic belt, this interval is hypothesized to coincide with an aridification event. Our research objective is to quantify the extent of this aridity using clumped isotope paleothermometry (n = 7) and paleoprecipitation proxies (n = 51) for samples collected across the C10 chemostratigraphic interval. Two weathering indices, CIA-K and CALMAG, were applied to data obtained using X-ray fluorescence spectrometry. Using these proxies, we determined mean annual precipitation across the RRM at its type section. Precipitation values (n = 27) obtained through CIA-K for identified paleosol horizons ranged between 795 and 1275 mm/year, and through CALMAG ranged between 735 and 1042 mm/year. Precipitation values decreased through the C10 interval which may indicate increased aridity. Clumped isotopes provided Δ47 values ranging from 0.647 to 0.693‰. Paleotemperature measurements (n = 4) from accepted carbonate samples were between 27.9 and 46.3 °C. Isotopic compositions of water calculated from carbonates ranged between -4.4‰ and -1.9‰ VSMOW. Precipitation values and temperatures were not lowest during the C10 interval. Temperatures peaked at the end of the C10 interval and decreased afterward, indicating a potential for cooler, more arid conditions. These results suggest that carbon cycle changes during the mid-Cretaceous may have influenced paleoclimate conditions experienced in terrestrial settings.

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Keywords
Albian, Aptian, clumped isotope, Cretaceous, paleoclimate, Ruby Ranch
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Department
Geosciences