REGIONAL VARIABILITY IN TERRESTRIAL PALEOCLIMATE RECORDS DURING THE EOCENE OLIGOCENE TRANSITION

During the Eocene-Oligocene transition (EOT, ca. 35-30 Ma), Earth's climate returned to a persistent icehouse state for the first time since the Paleozoic and has remained in this state since. Cores in the Southern, tropical Atlantic, and Pacific Oceans indicate that the EOT is characterized globally by a +1‰ shift in the oxygen isotope composition (δ¹⁸O) of benthic forams associated with rapid growth of the Antarctic ice sheet (Oi-1) and a coincident +0.7‰ carbon isotope excursion, suggesting organic carbon burial and greenhouse forcing of Oi-1. The EOT is represented in the Great Plains by the White River Group and overlying Sharps Formation, both of which contain numerous superposed calcareous paleosols. At Toadstool Park, NE, paleosol carbonate δ¹⁸O values are variable in the latest Eocene Chadron Fm (20.6±1.62‰), do not change through the Oi-1 interval in the Orella Mbr, Brule Fm (20.3±0.93‰), then increase sharply in the Whitney Mbr, Brule Fm (23.8±1.40‰). The Chadron Fm has not been sampled in the Big Badlands, SD, but δ¹⁸O values in the Scenic and Poleslide Mbrs of the Brule Fm there (23.4±1.53 and 25.8±0.72‰, respectively) are higher than in correlative sections at Toadstool. Based on δ¹⁸O values of vein calcite at Toadstool (14-17‰), the regional offset cannot be explained easily by diagenesis, suggesting differences in soil hydrology or local climate as the cause. Comparison to other proxy records from the region (paleosol maturity, fluvial morphology, floral and faunal composition) suggests moderate cooling and increased aridity explain the early Oligocene increase in δ¹⁸O values. The Great Plains paleosol record contrasts with contemporaneous continental paleoclimatic records from other regions. Taxonomic diversity of thermophilic plants and marine molluscs suggests protracted cooling during the EOT in western Oregon. In contrast, δ¹⁸O values of rodent teeth, gastropods, charophytes, and fish otoliths from the U.K. and of mammal teeth from Argentina suggest little or no cooling during the EOT. Unsurprisingly, terrestrial climate varied regionally during the transition into the icehouse state and studies of the terrestrial biotic response to climate change during this interval cannot rely on the global signal of the marine record but must use suitable regional climatic records.