TERRESTRIAL EVIDENCE FOR TWO INTENSE GREENHOUSE EVENTS NEAR THE CRETACEOUS-TERTIARY BOUNDARY

Environmental interpretations near the K/T boundary are controversial and require additional empirical data to enhance our understanding of this important period of Earth’s history. As a consequence, we present a continuous record of stable carbon and oxygen isotopes from paleosol carbonate dating to between 69 and 63 Ma for climate interpretations. The study area is located within the Tornillo Basin of Big Bend National Park, southwest Texas. The Tornillo Basin filled with a fluvial sedimentary succession adjacent to the Western Interior Seaway during the K/T transition (30 °N paleolatitude). The absolute age of the succession is provided by a synthesis of previous vertebrate biostratigraphic and magnetostratigraphic studies. Thirty-five paleosols with Bk horizons containing pedogenic carbonate nodules were sampled. The micritic carbonate phase was collected from each nodule with a micro-drill and analyzed for carbon and oxygen isotopic compositions. d¹³C values were converted to pCO₂ using the soil diffusion model and d¹⁸O values to temperature by the simultaneous solution of two equations describing the relationship between ambient temperatures and oxygen isotopic composition of meteoric waters. Isotopic ratios point to covarying and elevated atmospheric pCO₂ and terrestrial temperatures between 69.0 and 67.5 and 65.5 and 65.0 Ma. These two intense greenhouse episodes were governed by pCO₂ levels between 1100 and 1300 ppmV and regional mean annual temperatures near 22 °C. Our data suggests that a doubling of pCO2 induces a 1.8 °C increase in temperature, which is slightly less than forecasted for future greenhouse climates. When comparing temperature proxy from west Texas and North Dakota, a temperature gradient of 0.30 to 0.35 °C per degree latitude for North America is established for the K-T transition. Data presented here demonstrate strong coupling between terrestrial climates and ocean temperatures that were possibly forced by Deccan trap volcanic degassing leading to dramatic global climate changes prior to, and during, the end-Cretaceous mass extinction.