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

Cenozoic climate, tectonic and wildfire evolution of the Northern Cordillera of North America

Paper number: 65-1
Program: Technical Sessions
Time: 8:15 AM – 8:30 AM

Authors

Abstract

The Cenozoic uplift of the northern Rocky Mountains dramatically reshaped regional hydroclimate and ecosystems, yet linkages among mountain-building, hydroclimate, and wildfire remain unresolved. Here, we integrate multiple geochemical proxies, including polycyclic aromatic hydrocarbons (PAHs) as fire proxies, and hydrogen isotopes of leaf wax n-alkanes (δ²HnC29), and volcanic glass (δ²Hvg) as paleohydrological proxies, to reconstruct uplift- and climate-driven aridification and fire regime shifts in southwestern Montana through the Cenozoic. Before 50 million years (Ma), high concentrations of (5+6)-ring PAHs and retene (conifer-derived compound) indicate high-intensity fires in conifer-dominated forests. Due to a larger number of rings and lower volatility, (5+6)-ring PAHs indicate localized, high-temperature combustion.  By ~40 Ma, fire regimes shifted to lower-temperature, more regional fires, as reflected by increased (3+4)-ring PAHs. Simultaneously, the Oleanane (angiosperm-derived biomarker) Index records a shift to angiosperm-dominated ecosystems, that also coincides with phytolith evidence for low-stature, high-light angiosperms (gingers) consistent with intermittently open canopies. These ecological and fire regime transitions coincide with depleted δ²Hn-C29 values post-50 Ma, reflecting orographic rainout and aridification during mountain uplift. Volcanic glass δ²Hvg values (−130‰ to −95‰) further support regional evaporative enrichment in closed basins. We propose that tectonically-driven aridification intensified wildfire activity, likely contributing to the decline of fire-sensitive conifers and promoting fire-adapted angiosperms. This feedback between uplift, climate drying, and fire activity may have accelerated the late Cenozoic grassland expansion, highlighting fire as an integral role in tectonic-climate-ecosystem feedbacks.

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

doi: 10.1130/abs/2025AM-413994

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