LATE MIOCENE EXPANSION OF GRASSLANDS IN NW ARGENTINA LINKED TO SHIFTING HYDROCLIMATE

The Late Miocene marks the global expansion of C₄ grasslands, with several factors being suggested as potential drivers in different regions. In South America, C₄ expansion has been linked to the rise of the Andes and the development of the Summer Monsoon (SASM); however, spatio-temporal variability of C₄ abundance in different regions indicates the complexity of the event. This study aims to identify the effects of SASM on the distribution and abundance of C₄ grasses in NW Argentina through proxy-based reconstruction of Late Miocene paleoclimate and paleovegetation. It presents a multi-proxy paleoclimate and paleovegetation reconstruction through bulk paleosol geochemistry, organic carbon isotopes of soil organic matter, and phytoliths preserved in paleosols at two different sites in southern Salta Province. The first site contains the Palo Pintado Formation (7.24–5.96 Ma) deposited in an intermontane setting, and the second site (La Viña) contains the Jesus Maria, Guanaco, and Piquete Formations (17.01–4.08 Ma). Bulk geochemical analyses of paleosol B-horizons resulted in average mean annual precipitation (MAP) and mean annual temperature (MAT) values from Palo Pintado of ~900 mm/yr and 10⁰ C, with a slight decrease in precipitation after 6.3 Ma to ~800 mm/yr. From La Viña, average MAP and MAT were estimated to be ~800 mm/yr and 10⁰ C with a gradual decrease in precipitation to a minimum of ~300 mm/yr by 8.5 Ma. This study also uses both a δ¹³C_org mixing model and phytolith assemblages for paleovegetation reconstructions. We find minimal C₄ vegetation in the Palo Pintado Formation while a substantial increase of C₄ grasses is detected in La Viña beginning around 8 Ma. We identified 26 different phytolith morphotypes and a >5‰ carbon isotope excursion within the studied intervals indicating a progressive increase in C₄ vegetation. Paleoclimatic conditions (including preliminary estimates of growing season precipitation) coupled with this paleovegetation reconstruction suggests a significant relationship between the increase in C₄ grasslands and changing/variable hydroclimate in the region. The onset of this Late Miocene aridification and shifts coincide with modeled uplift in the region and illustrates the complicated interplay between tectonics, climate, and ecosystems in South America.