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Factors driving the late Miocene expansion of C4 grasses remain widely debated. Here, we explored the role of climate and fire in controlling the abundance of C4 vegetation in the Angastaco Basin (Palo Pintado area) and La Viña Basin, NW Argentina, during the late Miocene (ca. 14−5.33 Ma). From paleosol horizons, we reconstructed paleoclimate and paleovegetation conditions using phytolith assemblages, geochemical and isotopic proxies, and polycyclic aromatic hydrocarbons (PAHs) to determine fire input. Our paleoclimate reconstructions suggest a stable mean annual temperature (MAT) of ∼10 °C and a gradual decline in mean annual precipitation (MAP) from 1100 mm yr−1 to 850 mm yr−1. Paleovegetation reconstructions from carbon isotopic composition and phytolith assemblages show a maximum of ∼15% C4 vegetation by 6 Ma. No significant increases in fire occurrence or establishment of fire feedbacks were identified from the PAH data. Though low in abundance (∼3% on average), our data identified the presence of C4 grass by the late Miocene. The lack of significant C4 expansion in this region was likely controlled by the changing hydroclimatic conditions associated with the Andes mountain range—increasing aridity and elevation constraints along with the lack of a fire feedback might have limited the distribution of C4 vegetation. 

The geological record encodes the relationship between climate and atmospheric carbon dioxide (CO₂) over long and short timescales, as well as potential drivers of evolutionary transitions. However, reconstructing CO₂beyond direct measurements requires the use of paleoproxies and herein lies the challenge, as proxies differ in their assumptions, degree of understanding, and even reconstructed values. In this study, we critically evaluated, categorized, and integrated available proxies to create a high-fidelity and transparently constructed atmospheric CO₂record spanning the past 66 million years. This newly constructed record provides clearer evidence for higher Earth system sensitivity in the past and for the role of CO₂thresholds in biological and cryosphere evolution.