174-7 Triple oxygen isotope compositions of paleosol carbonates spanning the PETM in Wyoming

Session: Environmental Instability During Greenhouse Periods: Impact on Terrestrial and Marine Ecosystems

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Abstract:

The Paleocene–Eocene Thermal Maximum (PETM) was a global event that featured abrupt and extreme warming, with elevated temperatures persisting for ~200,000 years. This hyperthermal interval coincided with significant biotic change including pulses in taxonomic turnover and major biogeographic rearrangement. While climate models suggest major perturbations to the global hydrologic cycle, the regional hydroclimate response on continents remains poorly constrained in the geologic record. One of the most complete terrestrial PETM records is preserved in the Willwood Formation of the Bighorn Basin, Wyoming. Previous studies using bulk geochemistry, paleosol morphology, and fossil plant physiognomy from this section have been interpreted as evidence for transient aridification during the PETM. In contrast, globally amplified soil carbon isotope excursions have been interpreted to reflect increased mid-latitude tropospheric humidity. Triple oxygen isotope geochemistry offers a novel tool for reconstructing evaporation and water balance. We measured the triple oxygen isotope composition of pedogenic carbonates from the Willwood Formation in Wyoming to assess changes in evaporative stress across the PETM interval. Preliminary results do not show a lowering in triple oxygen isotope values that would be expected for local aridification during the PETM.  In contrast, the emerging pattern is an increase in triple oxygen values and increased variance during the PETM compared to pre- and post-event intervals. If confirmed with additional analyses, these preliminary elevated values may reflect increased soil moisture and/or shifts in the timing, intensity, or source of precipitation. Amplified intra-PETM variability in triple oxygen values, if confirmed, would suggest decreased hydroclimatic stability. Alternatively, the triple oxygen values of these soil carbonates may not primarily reflect surface climatic conditions but instead be influenced by the depth at which carbonate formed within the soil profile. Both quantitative models and empirical data from modern soil systems suggest that evaporative effects diminish with depth, particularly in clay-rich, low-permeability soils where groundwater-atmosphere communication is limited. To further evaluate this possibility, our next steps include analyzing triple oxygen and clumped isotope trends across individual paleosol profiles in the Willwood Formation to assess depth-related patterns.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-5961

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