Stable carbon isotopes in paleosol carbonate show that C4 tropical grasses expanded rapidly onto the floodplains of the Indo-Gangetic Plain starting about 7.3 Ma. At about the same time, C4 grasses began their expansion in East Africa. Carbon isotope evidence from fossil teeth from the midwestern U.S. suggests a latest Miocene (8 to 6 Ma) date for the appearance of C4 grasses there as well. Our intent here is to date and quantify the radiation of C4 grasses across the Americas by detailed sampling of paleosols and fossil teeth from sections in New Mexico, USA, and in La Rioja and San Juan Provinces in NW Argentina. If the C4 expansion is related to onset or strengthening of the Asian Monsoon, as has been previously suggested, then we would not expect expansion in the Americas and the Old World to be synchronous. If the expansion is synchronous, then some other explanation involving a global change, such as reduction in atmospheric pCO2 or other causes, must be invoked.
