65 Hydroclimate Variability Across Timescales in Western North America: Causes and Consequences (Posters)

This session aims to bring together researchers studying past climate change over a range of timescales using innovative data and modeling approaches and using these data to inform future changes in climate and their impacts.

Sunday, 19 October 2025
8:00 AM - 5:30 PM
HBGCC, Hall 1

Climate and Vegetation Controls on Hydroclimate in Western North American during the Eocene-Oligocene Transition

Paper number: 65-0
Program: Technical Sessions
Time: 8:00 AM – 8:15 AM

Authors

Abstract

The gradual cooling of the late Eocene culminated in the Eocene-Oligocene Transition (EOT) ~33.9 million years ago, marking the transition to the modern icehouse climate. Across the EOT, a notable increase in benthic foraminifera δ18O accords with global cooling and the onset of permanent Antarctic glaciation primarily driven by a decrease in pCO2, with the opening of Southern Ocean gateways and ice sheet growth as secondary forcings. Alongside these events, drastic hydroclimate shifts epitomized by the emergence of loess deposits are documented in the mid-latitude continental interiors of Asia and Western North America (WNA). To identify causes of these shifts, we are conducting a series of simulations using the water isotope enabled Community Earth System Model (iCESM 1.3) at a 1-degree grid resolution. Our experiments include 1) a late Eocene baseline simulation featuring a paleogeographic reconstruction at 38 Ma, and late Eocene Antarctic ice sheet, vegetation and 3 times (840 ppm) preindustrial level CO2, and 2) two sensitivity experiments separately featuring CO2 decline to 2 times (560 ppm) preindustrial level and a shift from forests to early grasslands across the EOT. Based on our CO2 sensitivity experiments, decreasing CO2 causes worldwide cooling of 1-6°C and increases the meridional temperature gradient, with 2-3°C of cooling in WNA representing nearly half of reconstructed cooling. Poleward moisture transport also increases, though a muted precipitation response is observed in WNA. Simulated paleo-precipitation δ18O and δD values for 3x and 2x CO2 scenarios were additionally converted and compared to pre- and post-EOT isotope values from paleosol carbonate and hydrated volcanic glass proxy data in WNA. Using an OLS regression, we found at 95% confidence that we cannot reject that δ18O values for the 3x and 2x CO2 scenarios may correspond 1:1 with proxy values before and after the EOT, suggesting a potential role of CO2 decline on controlling carbonate δ18O at this time. While Eocene δD modelled and proxy values may also describe a 1:1 relationship, Oligocene results are rejected at 95% confidence, and proxy δD variability overall is underestimated by model values due to potentially missing high topography. Ongoing vegetation sensitivity experiments, involving a comparison between late Eocene forestland and modern grassland vegetation in WNA, will help quantify how grassland expansion drove WNA hydroclimate change during this time.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-412760

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