DID INCREASING SEASONALITY AND FIRE FREQUENCY CAUSE THE C4GRASSLAND TRANSITION IN SOUTH AMERICA (SA)? INVESTIGATIONSFROM TWO PALEOSOL SITES IN NW ARGENTINA USING δ13C ISOTOPES,MOLECULAR BIOMARKERS, PHYTOLITHS, AND X-RAY FLUORESCENCE (XRF)

ADIT GHOSH; COTTON JENNIFER; ETHAN HYLAND; HAUSWIRTH SCOTT; M. SOL RAIGEMBORN; DAVID TINEO; INSEL NADJA

Congreso; GSA 2020 Connects Online; 2020

GSA

Recent advances in molecular biomarkers along with traditional bulk δ13C isotopic analysis, phytolith assemblages, and XRF analysisallow for the detailed reconstruction of continental paleoecological and paleoclimatic conditions. We use these methods to reconstructconditions during the expansion of C4grasses, considered one of the most important biological events during the Cenozoic. Wehypothesize that in SA, the spread of C4grasses was driven by the intensification of the SA Summer Monsoon and that this increasein C4vegetation caused an intensification of fire regimes, due to both seasonality-fire and fire-C4expansion feedbacks, that has longbeen suspected but poorly documented and never studied in this context in SA. This study presents results from well-dated paleosolsfrom two sites in N.W. Argentina: La Viña (LV) and Palo Pintado (PP) in the Salta Province. The LV site includes the Jesus MariaFormations, Guanaco and Piquete (~15 - 4 Ma), while the PP site includes the Palo Pintado Formation (~7.5 - 6 Ma). Previous studieshave interpreted these sites as foreland basin sediments deposited in a meandering stream to lacustrine environment. Shifts in theδ13C obtained from bulk organic matter and phytolith assemblages in these paleosols are used to identify vegetation transitions fromC3-dominated grasslands and forests, to C4-dominated grasslands. Leaf wax biomarkers include higher order n-alkanes (C20to C36),and are used to independently determine vegetation composition and changes to aridity and seasonality. XRF analysis is carried out todetermine the chemical index of alteration in paleosols, which is a comparable proxy for precipitation seasonality. Polycyclic aromatichydrocarbons (PAH) are used to determine fire regimes, in order to verify whether C4expansion promoted fires. δ13C analysis of LVreveals a steady change from a C3-dominated ecosystem at 15 Ma to a C3-C4intermediary ecosystem by 7.5 Ma. The PP site doesnot show any directional change in vegetation type during the investigated period between 7.2 Ma to 6 Ma, though δ13C valuesindicate a mixed C3-C4ecosystem throughout this period. However, results from the PP site show that increasing C4cover,determined from bulk organic matter δ13C, is positively correlated with an increased fire frequency, determined from increasing PAHconcentrations. Ultimately, these data contribute to constraining the causes of C4expansion in NW Argentina, including the mostimportant climatic controls and feedbacks from fire regimes, in continental SA.