CLIMATIC AND HYDROLOGIC CHANGE DURING THE EOCENE-OLIGOCENE TRANSITION IN THE NORTH AMERICAN INTERIOR

Continental records of the Eocene-Oligocene transition (EOT) from North America show limited consensus regarding the magnitude and impact of this climatic shift on terrestrial ecosystems. Stable isotopes of fossil bones and teeth from the White River Group (WRG) in the northern Great Plains have been interpreted to show an 8 °C decrease in mean annual temperatures across the EOT, with no evidence for accompanying changes in aridity. Conversely, other studies have inferred increased aridity in the early Oligocene based on changes in the δ¹⁸O and δ¹³C values of mammalian tooth enamel sampled from the same location. Some of this ambiguity is likely to stem from the difficulty in separating the influence of factors such as temperature, water availability and atmospheric circulation patterns on the δ¹⁸O composition of ungulate tooth enamel.

We present paired leaf wax biomarker and clay hydrogen isotope data from the WRG to produce a new record of hydrological change and/or ecosystem shifts through the EOT. Clay δD data show a negative shift of <15 ‰ from the late Eocene through early Oligocene, while leaf wax n-alkanes record a negative shift in excess of 50 ‰ with a synchronous positive change in average chain length. We infer an increase in aridity from these data, with a potential associated shift from coniferous/evergreen forests to more savannah-style vegetation as the climate became drier throughout the transition into the Oligocene. These data provide new constraints on the impact of the EOT in North America, and highlight the potential role of increasing aridity, in addition to declining temperatures, in shaping terrestrial ecosystems during this interval.