Cenozoic tectonic–climate–fire evolution of the northern North American Cordillera

The Cenozoic uplift of the northern Rocky Mountains profoundly reshaped regional hydroclimate and ecosystems, yet linkages among mountain-building, moisture deficiency, and wildfire remain unconstrained. Here, we integrate multiple geochemical proxies — including polycyclic aromatic hydrocarbons (PAHs) as fire indicators, and hydrogen isotopes of leaf-wax n-alkanes (δ²Hn-C29) and volcanic glass (δ²Hvg) as paleohydrological proxies — to reconstruct fire and moisture shifts in southwestern Montana through the Cenozoic. Before ~50 million years ago (Ma), elevated concentrations of (5+6)-ring PAHs and retene (a conifer-derived compound) indicate high-intensity burning in gymnosperm-dominated landscapes. By ~40 Ma, fire regimes shifted to lower-temperature, regional fires, reflected by increased (3+4)-ring PAHs. Concurrently, rising Oleanane Index (an angiosperm-derived biomarker) values indicate expansion of angiosperm vegetation, consistent with phytolith evidence for low-stature angiosperms indicative of intermittently open canopies. These ecological and fire regime transitions coincide with depleted δ²Hn-C29 values after ~50 Ma, suggesting the establishment of high topography in the northern Rockies by this time. Volcanic glass δ²Hvg values (−130‰ to −95‰) from leaf-wax-bearing sediments further record regional evaporative enrichment in closed basins. We infer that tectonically-driven drying in the early Cenozoic and climate-driven aridification in the late Cenozoic enhanced wildfire activity, contributing to the decline of conifer inputs and the spread of angiosperm vegetation. This feedback among uplift, climate drying, and fire likely accelerated late Cenozoic grassland expansion, highlighting the integral role of fire in coupling tectonic and ecological evolution in mountain landscapes.