Reconstructing the Late Miocene-Pliocene paleoenvironment and paleoclimate ofsouthern continental South America (SA) in orderto determine the factors driving expansion of C4grasslands: δ13C isotopes, molecular biomarkers,phytoliths, and X-ray fluorescence

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

Congreso; AGU Fall Meeting 2020; 2020

AGU

The application of novel molecular biomarkers paired with traditional bulk δ13C isotopic analysis, phytolithassemblages, and XRF analysis allows for the in-depth reconstruction of various components ofpaleoenvironments including past vegetation, climate, and fire regimes. These methods are used toreconstruct the timing and identify the most influential factors in the expansion of C4grasslands, consideredone of the most important biotic events during the Cenozoic. We hypothesize that increased seasonality inclimate, due to the strengthening of the SA Summer Monsoon, allowed for the spread of C4grasslands inSA. Additionally, this strengthening climatic seasonality may have triggered the development of a positivefeedback loop involving increasing seasonality, increasing fire frequency, and spread in C4grasses, whichhas been observed elsewhere in the world. This study presents results from well-dated paleosol outcropsfrom the Río Iruya site (RI) in NW Argentina. The RI site, located around 22.5°S, is part of the Orán Groupand was deposited as foreland basin sediments related to the eastward migration of the Andean ranges,between ~10 Ma to 1.9 Ma. Isotopic ratios of δ13C obtained from bulk organic matter, and phytolithassemblages present in sampled paleosols are used as fingerprints to reconstruct trends in vegetationalcomposition, in order to identify the timing and extent of vegetational transitions to C4-dominated grasslands.Polycyclic aromatic hydrocarbons (PAH) are used as evidence for fires, with total PAHs normalized to C31concentrations representing fire frequency. Comparison of these data allows for the interpretation of trendsbetween variations in fire frequency and C4grassland expansion. Examination of the alteration of clayspresent in paleosols by XRF analysis are used to reconstruct precipitation seasonality, and leaf waxbiomarkers such as higher order n-alkanes (C20to C36) are used as independent molecular tracers ofvegetational composition and climatic conditions such as aridity and temperature seasonality. We will presenta multiproxy paleoenvironmental reconstruction of RI that will contribute to constraining the causes of C4expansion in SA, including the most important climatic controls and fire regimes, in a region where this eventhas been poorly understood.