Reconstructing the Late Miocene-Pliocene paleoenvironment and paleoclimate of southern continental South America (SA) in order to determine the factors driving expansion of C4 grasslands: δ13C isotopes, molecular biomarkers, phytoliths, and X-ray fluorescence (XRF) from Río Iruya, NW Argentina.

Abstract:

The application of novel molecular biomarkers paired with traditional bulk δ13C isotopic analysis, phytolith assemblages, and XRF analysis allows for the in-depth reconstruction of various components of paleoenvironments including past vegetation, climate, and fire regimes. These methods are used to reconstruct the timing and identify the most influential factors in the expansion of C4 grasslands, considered one of the most important biotic events during the Cenozoic. We hypothesize that increased seasonality in climate, due to the strengthening of the SA Summer Monsoon, allowed for the spread of C4 grasslands in SA. Additionally, this strengthening climatic seasonality may have triggered the development of a positive feedback loop involving increasing seasonality, increasing fire frequency, and spread in C4 grasses, which has been observed elsewhere in the world. This study presents results from well-dated paleosol outcrops from the Río Iruya site (RI) in NW Argentina. The RI site, located around 22.5°S, is part of the Orán Group and 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 phytolith assemblages present in sampled paleosols are used as fingerprints to reconstruct trends in vegetational composition, 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 C31 concentrations representing fire frequency. Comparison of these data allows for the interpretation of trends between variations in fire frequency and C4 grassland expansion. Examination of the alteration of clays present in paleosols by XRF analysis are used to reconstruct precipitation seasonality, and leaf wax biomarkers such as higher order n-alkanes (C20 to C36) are used as independent molecular tracers of vegetational composition and climatic conditions such as aridity and temperature seasonality. We will present a multiproxy paleoenvironmental reconstruction of RI that will contribute to constraining the causes of C4 expansion in SA, including the most important climatic controls and fire regimes, in a region where this event has been poorly understood.