LINKING THE LATE MIOCENE C4 GRASSLAND EXPANSION TO EXPANSION OF THE SOUTH AMERICAN SUMMER MONSOON IN RIO IRUYA CANYON, NORTHWEST ARGENTINA

Though fossil evidence shows that C₄ grasses were present on Earth as far back as the early to mid-Miocene. However, between 8 to 3 million years ago, C₄ grasses expanded so extensively that C₄ grasses now dominate tropical and some temperate grasslands. The driver of this major ecological change is a subject that is still highly debated. The reason for this debate stems from ambiguity over if this expansion was due to changes in global or regional climate, atmospheric composition, or tectonic forces. Our research seeks to determine the underlying cause of the expansion of C₄ grasses in South America by analyzing stable carbon isotope of organic matter contained within paleosols from Rio Iruya Canyon, which comprises late Miocene foreland basin sediments within the Oran Group of northwest Argentina. The stable carbon isotope of these samples reflect the transition of the vegetation regime of the local watershed at the time of sediment deposition. We hypothesize that the onset of C₄ grass expansion correlates with a climatic forcing, namely the strengthening of the South American Summer Monsoon. We will test this hypothesis first by better constraining the timing of the C₄ grass expansion in the region, and then by correlating this expansion with isotope and geochemical proxy precipitation records. Presently, there are many species of both C₃ and C₄ grasses that are staple food crops for people around the world. As such, understanding the cause of the C₄ grassland expansion is relevant towards accurately predicting the impacts of anthropogenic climate change and global food security.