LATE MIOCENE-PLIOCENE VEGETATION, FIRE AND HYDROCLIMATE DYNAMICS IN THE RÍO IRUYA BASIN, NORTHWEST ARGENTINA

Drivers for the spread of C₄ grasses through the Late Miocene-Pliocene in South America have remained a mystery due to sparse terrestrial archives and a lack of corresponding vegetation and fire proxies. We hypothesize that the increased seasonality of precipitation in NW Argentina associated with a strengthening summer monsoon since the Late Miocene enabled the spread of C₄ grasses in the Río Iruya basin, and this increase in C₄ vegetation resulted in a positive feedback with fire frequency. To test this hypothesis, the carbon isotopic ratio of bulk organic carbon (δ¹³C_BOM) and specific Polycyclic Aromatic Hydrocarbon (PAH) biomarkers were used as proxies for vegetation type, concentrations of PAHs were used to reconstruct fire regimes and bulk geochemistry of paleosol B-horizon were used to reconstruct climatic changes. We observe the appearance of ~50% C₄ cover in the basin by 5.13 Ma from δ¹³C_BOM values. PAH vegetation provenance from the retene, DMP-x, and DMP-y proxies indicate a mixed gymnosperm forest with a grassy understory across the studied period. Thus, we conclude that C₄ grasses were replacing C₃ grasses in a gymnosperm dominated woodland in Río Iruya. Fire frequency reconstructed from total PAH concentrations normalized to the concentration of C₃₁ alkane appears relatively stable throughout the investigated period. We observe an increase in mean annual precipitation of <1000 mm/yr to >1200 mm/yr derived from the CIA-K proxy between 6.6 to 5.4 Ma. We conclude that the increase in C₄ cover did not drive an increase in fire frequency at Río Iruya which may be due to the presence of fire resistant C₃ taxa, to the lack of threshold C₄ grass cover, or increased precipitation. Thus, we conclude that significant changes in vegetation, but not fire regimes, took place in Río Iruya during the Late Miocene to Pliocene, and precipitation and seasonality estimates suggest that these vegetation changes may be linked to the strengthening of the South American summer monsoon.