The expansion of C₄ grasses and global change in the late Miocene: Stable isotope evidence from the Americas

δ¹³C values in paleosols and fossil teeth have been used to document the expansion of C₄ plants in South Asia, Africa, and North America during the late Miocene. However, the exact timing and rate of expansion of C₄ vegetation is unclear outside the Old World because of a lack of high-resolution records. We present a high-resolution record from northwest Argentina in which the δ¹³C values of soil carbonate rise above a threshold of -8‰, suggesting the presence of C₄ plants, starting at 7.3-6.7 Ma. δ¹³C values of fossil teeth from well dated sections in South and North America display a concomitant increase of C₄ plants in the diets of large herbivores. These results show that the late Miocene expansion of C₄ plants was global, but occurred at different rates in each region. While it is has been suggested that declining pCO₂ levels during the late Neogene caused C₄ plant expansion, climate change, such as an increase in summer-dominated rainfall regimes globally, is an alternative explanation. The δ¹⁸O soil carbonate records from South Asia, East Africa and now also northwest Argentina all show an increase of at least 3-4‰ in the late Neogene, either the result of climate change or of greater evaporation in average grassland soils.