PALEOCLIMATE AND CHEMOSTRATIGRAPHY OF THE HENSEL FORMATION, KIMBLE COUNTY, TEXAS

The Hensel Formation is one of the few dominantly terrestrial Cretaceous units in Central Texas and provides the unique opportunity to investigate paleoclimate archives preserved in paleosols in this area prior to the Cretaceous thermal maximum. Bulk sedimentary organic carbon isotopes, carbonate isotopes (δ¹³C, δ¹⁸O, and Δ₄₇ ) of paleosol carbonates, and major element geochemistry were utilized to estimate paleotemperature, paleoprecipitation, pCO₂, and to correlate the δ¹³C values in the Hensel Formation to existing carbon isotope chemostratigraphic records. Organic C isotope values range from -28.15‰ to -19.62‰ vs. VPDB. Carbonate oxygen isotope values range from about -4.97‰ to +0.23‰ vs. VPDB and carbonate carbon isotope values range from -8.29‰ to -5.04‰ vs. VPDB. As expected, samples are on average primarily dominated by SiO₂, with other major element oxides in decreasing abundance including CaO, Al₂O₃, and MgO. Paleotemperature estimates were determined using the clumped isotope (Δ₄₇) paleothermometer and paleoprecipitation estmates were determined using the chemical index of alteration and its relationship to mean annual precipitation. Preliminary paleotemperature and paleoprecipitation data (ranging between 35 and 42°C and ~314mm/year on average) suggests a subtropical desert biome existed in Central Texas during the Albian Stage. The temperature estimates are also consistent with and overlap temperatures from coeval temperature estimates farther north in the Cedar Mountain Formation and Newark Canyon Formation (Suarez et al. 2020, GSL spec. pub. 507; Fetrow et al., 2020, AGU, EP026-03) consistent with a shallow latitudinal temperature gradient. The soil carbonate paleobarometer indicates a range of CO₂ concentration estimates from just under pre-industrial CO₂ concentrations to as much as ~5 times pre-industrial CO₂ concentrations. Refinement of paleosol paleobarometer variables will likely provide more constrained CO₂ estimates. This will contribute to ongoing community efforts to improve Phanerozoic CO₂ records.