OBSERVED SHIFT IN WESTERN INTERIOR BASIN CLIMATES ACROSS THE PEAK CRETACEOUS THERMAL MAXIMUM (OAE2)

Accurately assessing correlations between chemostratigraphic and paleoclimate data is important in interpreting past ecosystems and how those ecosystems changed throughout time. The early Late Cretaceous Naturita, Tununk, and Straight Cliffs Formations of the Western Interior Basin (WIB) of western North America (Utah) experienced dynamic geological and environmental changes in part due to the evolution of the Western Interior Seaway. The transition between the Naturita and the Tununk/Straight Cliffs Formations was previously shown to capture the Cenomanian-Turonian ocean anoxic event (OAE2) that happened ~94 Ma, and therefore should also record the peak of the Cretaceous Thermal Maximum (KTM).

In order to more thoroughly describe this event in the WIB, we compared stratigraphy between two localities of the Naturita-Tununk/Straight Cliffs transition, and examined bulk sediment samples and paleosols. We analyzed bulk sediment samples on a dual inlet isotope ratio mass spectrometer (DI-IRMS) to produce δ¹³C_org values. We found that the δ¹³C_org values ranged from -26% to -9% in the Naturita and Tununk/Straight Cliffs Formations at the OAE2 boundary in both localities. With the enrichment of δ¹³C_org, this indicates a strong correlation between the positive carbon isotope excursion (CIE) and the OAE2 during the Cenomanian-Turonian ~94 Mya, and provides a direct stratigraphic linkage between localities in the WIB. We used portable and benchtop XRFs to quantify bulk elemental compositions of identified paleosols (specifically B-horizons), and used major elements and corresponding climofunctions to estimate mean annual precipitation (MAP), mean annual temperature (MAT), and Köppen aridity index (AI_Koppen). We found that during the peak of the KTM, MAT increased from approximately 13°C to 22℃ and MAP increased from approximately 900 to 1600 mm/yr. These results also provided an aridity index (AI_Koppen) range of 17 to 39, which places the pre-event Naturita Formation as humid subtropical while the Straight Cliffs Formation is classified more as a temperate rainforest. Overall this suggests that the KTM (and specifically peak OAE2) had a significant effect on climate regimes and resultant ecosystems in the WIB.