Late Miocene-Pliocene vegetation, fire and hydroclimate dynamics in the Río Iruya basin, Northwest Argentina

Abstract

The spread of C4 grasses and fire history through the Late Miocene-Pliocene in South America has remained a mystery due to sparse terrestrial archives and lack of corresponding geochemical vegetation and fire proxies. We hypothesize that the increased seasonality of precipitation associated with a strengthening summer monsoon 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 (δ13CBOM) 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 clay minerals were used to reconstruct climatic changes. We observe the appearance of ~50% C4 cover in the basin by 5.13 Ma from δ13CBOM values. PAH vegetation provenance from the retene, DMP-x and DMP-y proxy indicated 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 forest 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 derived from the CIA-K proxy from 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. This lack of increase in fire frequency is likely 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 investigated period, and precipitation estimates suggest that these vegetation changes may be linked to the strengthening of the South American summer monsoon.