Terrestrial cooling and changes in hydroclimate in the continental interior of the United States across the Eocene-Oligocene boundary

One of the most pronounced climate transitions in Earth's history occurred at the Eocene-Oligocene transition, approximately 34.0-33.6 m.y. ago. Marine sedimentary records indicate a dramatic decline in pCO (sub 2) coeval with global cooling during the transition. However, terrestrial records are relatively sparse, with conflicting interpretations of hydroclimate in continental interiors. Here, we provide quantitative constraints on the response of the continental hydroclimate in the western United States across the Eocene-Oligocene boundary by studying clumped isotope temperatures [T(Delta 47)], delta (super 13) C and delta (super 18) O values of vadose carbonates, and delta (super 13) C values of bulk organic matter (delta (super 13) C (sub org) ) in eastern Wyoming. Our results show that T(Delta 47) dropped from approximately 28 degrees C to approximately 21 degrees C, indicating approximately 7 degrees C cooling in air temperature, which occurred parallel to the decrease in atmospheric pCO (sub 2) during the latest Eocene-early Oligocene. We find that aridity and the biome were stable, and ice-volume-corrected precipitation delta (super 18) O decreased approximately 1.6 ppm across the Eocene-Oligocene boundary, attributable to reduced vapor condensation temperatures. These new quantitative data add to the growing body of evidence suggesting a marked terrestrial response in temperature and hydroclimate across the Eocene-Oligocene transition. Our findings indicate a pattern of greenhouse-gas-induced global temperature change in the continental interior of the United States that was roughly 1.5-2X the magnitude of cooling in the global ocean.