CONSTRAINING C4 EXPANSION IN CONTINENTAL SOUTH AMERICA DURING THE LATE MIOCENE-PLIOCENE USING D13C ISOTOPIC PROXIES AND COMPLEX ORGANIC CARBON MOLECULES

The spread of C4 grasslands from 8-3 million years ago is one of the most significant, yet least understood continental scale biological transitions of the Cenozoic. Understanding the climatic drivers that caused this global vegetation change is consequential in predicting the future of modern grassland biodiversity in an age of anthropogenic climate change. We hypothesize that in South America (SA), the spread of C4 grasses was driven by the intensification of the SA Summer Monsoon (SASM). We test this hypothesis by analyzing carbon isotopes in organic matter from well dated paleosols, which indicates the exact timing of vegetational change. Oxygen isotopes from carbonates and soil clays constrain climatic factors during this period. Linking the two determines the most important climatic factors responsible for the vegetation transition. We also measure the abundance of complex organic carbons such as polycyclic aromatic hydrocarbons (PAHs). Analyzing PAH levels will enable us to determine fire intensity and see if the expansion of C4 grasses is correlated to increased fire regimes. We perform preliminary analysis of vegetation data for two sites, La Viña (LV) and Palo Pintado (PP), in Northwest Argentina. The LV site includes the Guanaco and San Felipe Formation. The PP site includes the Palo Pintado Formation. Previous studies interpreted these sites as foreland basin sediments deposited in a meandering stream to lacustrine environment. δ13C analysis of LV reveals a steady change from 16% to 41% C4 vegetation. δ13C analysis of PP does not show any discernible change in vegetation type and δ13C values indicate a C3 dominated ecosystem. We aim to constrain the exact timing of global vegetational change in these sites from a C3 dominated ecosystem to a C4 dominated ecosystem, information for which is sparse in continental SA. Ultimately, we will constrain the causes of C4 expansion, including the most important climatic controls, in continental SA where this event is poorly understood. If C4 expansion coincides with strengthening seasonality over a large spatio-temporal scale, increasing seasonality could play a vital role in determining future ecosystems. Anthropogenic climate change resulting in stronger seasonality will cause an increase in C4 vegetation with severe implications for modern biodiversity.