An early Paleogene terrestrial paleoclimate record from Big Bend National Park, Texas: Insights from carbonate clumped isotope thermometry

Understanding the profound climatic transitions between the extreme greenhouse conditions of the Cretaceous and early Eocene and the intermediate cooler Paleocene epoch represents a critical component in deciphering the past behavior of the global climate system. However, the Paleocene is relatively understudied and the spatial distribution of climate data that do exist for this time period is quite limited, especially in the terrestrial realm. A more thorough understanding of subtropical, terrestrial greenhouse climate conditions is essential in order to document climate change through this period and to test the accuracy of climate models for terrestrial regions. This study presents a record of summer-season temperatures derived from carbonate clumped isotope thermometry of paleosol carbonates from early Paleogene floodplain deposits in Big Bend National Park, Texas. The carbonate clumped isotope thermometry method is less widely understood than many other methods of paleoclimate reconstruction and it is technically challenging. However, it provides unique information that other methods cannot, most importantly, temperatures independent of the compositions of coexisting waters. The average summer-season temperature estimate for Big Bend during the time period studied is ~34 °C, with a range of 29 to 37 °C. The coolest temperatures occur during the mid-to late Paleocene while the warmest temperatures occur during the early Paleocene and early Eocene. The pattern of temperature change correlates well with the early Paleogene temperature pattern from marine records. The calculated soil water isotopic compositions average -0.5‰, with a range of -1.6‰ to 0.6‰, and suggest humid conditions through the early Paleocene and a transition to drier conditions through the middle Paleocene to early Eocene. A latitudinal temperature gradient generated using Big Bend and northern Wyoming temperature data indicates the summer-season temperature gradient was near zero for the late Paleocene to early Eocene, suggesting hot summer temperatures throughout the continental interior of North America.