Late Miocene Hydroclimate Variability and its Impact on Vegetation in NW and Central Argentina- a Multiproxy Investigation

IFFAT AZMI; ETHAN HYLAND; JENNIFER M. COTTON; SHELBY LITTLETON; MARIA SOL RAIGEMBORN; D.E. TINEO; INSEL, NADJA

San Francisco

Congreso; AGU23 Annual Meeting; 2023

AGU

The Late Miocene witnessed a rapid global increase in C4 vegetation, yet its timing and cause remain unclear. In South America, the Late Miocene Andean orogeny was accompanied by changes in regional atmospheric circulation patterns, such as the development of the South American Monsoon System. We investigate the role of climatic variability on vegetation changes in Argentina by reconstructing paleoclimate and paleovegetation using a multiproxy approach. We explored the Huayquerian Cerro Azul and Epecuén formations (9–6.8 Ma) in central Argentina (La Pampa and Buenos Aires provinces), along with coeval deposits (9.8–7.4 Ma) from the Cacheuta Basin (Mendoza province) in northwest Argentina. We used the geochemical composition of paleosols to estimate mean annual precipitation (MAP) and mean annual temperature (MAT). We used carbon isotope signatures of paleosol organic matter (δ13C) and phytolith assemblages for paleovegetation reconstruction. Paleoclimate reconstructions from the Cacheuta Basin show an average MAP of 663±182 mm/yr with a stable MAT of 10±5°C. δ13C values vary between -25.2 ‰ and -23.4 ‰, while phytolith assemblages suggest a mixed forest-grassland ecosystem with minimal C4 presence (~1.5%). The Cerro Azul samples show an average MAP of 370±182 mm/yr and MAT of 8±5°C. Here, δ13C values range between -25.7‰ and -21.3‰, while phytolith assemblages suggest a mostly grassland-dominated ecosystem with a maximum of 15.7% C4 vegetation. Notably, the highest C4 percentages were observed in NW sites, which recorded higher precipitation. Our findings suggest a correlation between MAP and vegetation, suggesting a relationship between hydroclimate conditions and the expansion of C4 vegetation during the Late Miocene (Huayquerian). However, the higher average MAP and the lack of C4 vegetation in the Cacheuta Basin compared to Cerro Azul sites indicate regional variability in controls on C4 grass distributions across the region. Topographic differences between the sites are likely to influence both hydroclimate patterns and vegetation dynamics, leading to different ecological responses to evolving climate in these areas. This study provides valuable insights into Late Miocene paleoclimatic conditions and their implications for understanding vegetation responses in future climate scenarios.