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.