LATE MIOCENE CLIMATE VARIABILITY AND C4 GRASS DYNAMICS IN SOUTH AMERICA: INSIGHTS FROM CLIMATE SIMULATIONS AND PALEOCLIMATE RECONSTRUCTIONS

Understanding past regional climate and ecosystem changes is crucial for predicting future responses to climate change. The Late Miocene witnessed substantial global climate and environmental changes, impacting ecosystems worldwide. Central South America saw the development of more open, arid environments favoring the more efficient photosynthetic pathways of C₄ plants over C₃ species, especially under low atmospheric CO₂ concentrations and higher temperatures. However, the specific drivers of this shift remain debated. This study employs a high-resolution global general-circulation model (CESM) to investigate changes in temperature and precipitation patterns from 10 to 3 million years ago (Ma). Our analyses are complemented with multi-proxy paleovegetation and paleoclimate reconstructions from field sites in Argentina, aiming to enhance our understanding of environmental and climatic influences on C₃/C₄ grassland ecosystems.

Our findings suggest an increase in annual precipitation and seasonal intensification during the Late Miocene. Simulations show higher temperatures and intensified summer monsoons at 3 Ma compared to 10 Ma, especially east of the Andes. Monthly precipitation rose along the Andean slopes between 8 and 3 Ma, with the highest monthly increases observed in November and December. Paleoprecipitation proxies from the Angastaco and La Viña areas suggest locally increased aridity despite regional increases in precipitation. Seasonal precipitation variability strengthened at 3 Ma compared to 8 Ma, particularly in March and April, with greater variability in Angastaco during the late Miocene and in La Viña during the Pliocene. This notable shift in increased variability correlates with an overall spatial and temporal extension of the monsoon seasons. Field studies indicate a moderate (5-20%) increase in C₄ vegetation, consistent with the emergence of enhanced seasonal climates featuring distinct hot-wet and dry-cool periods, alongside increased warm-season precipitation. Our results suggest that in a future warming climate, C₄ grasses may expand their dominance and geographical range, particularly in regions experiencing warmer temperatures and seasonal precipitation patterns.