GSA Connects 2021 in Portland, Oregon

Paper No. 166-5
Presentation Time: 2:35 PM


GHOSH, Adit1, COTTON, Jennifer M.2, HAUSWIRTH, Scott3, HYLAND, Ethan4, AZMI, Iffat5, RAIGEMBORN, Maria Sol6, TINEO, David6, HAYDUK, Tyler7 and INSEL, Nadja8, (1)Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089, (2)Geological Sciences, California State University, Northridge, Northridge, CA 91330, (3)Geological Sciences, California State University, Northridge, 18111 Nordhoff St, Northridge, CA 91330, (4)Dept. of Marine, Earth & Atmospheric Sciences, North Carolina State University, 2800 Faucette Dr., Raleigh, NC 27695, (5)Dept. of Marine, Earth and Atmospheric Sciences, North Carolina State University, 2800 Faucette Dr., Raleigh, NC 27695, (6)Centro de Investigaciones Geologicas, Universidad Nacional de La Plata, La Plata, C1925, Argentina, (7)Canoga Park, 22035 Valerio Street, Canoga Park, CA 91303, (8)Marine, Earth, and Atmospheric Sciences, NC State University, Raleigh, NC 27695

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 (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 B-horizon 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 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.