The Expansion of C4 Grasses in the Americas

古土壤碳酸盐稳定碳同位素显示，约7.3 Ma开始，C4热带牧草迅速扩展到印度河-恒河平原的洪泛平原。大约在同一时间，C4草开始在东非扩张。来自美国中西部化石牙齿的碳同位素证据表明，那里也出现了C4草的最新中新世(8至6 Ma)。我们在这里的目的是通过对美国新墨西哥州以及阿根廷西北部的拉里奥哈和圣胡安两省的古土壤和化石牙齿进行详细采样，来确定整个美洲C4草的辐射时间和数量。如果C4的扩张与亚洲季风的爆发或加强有关，就像以前所说的那样，那么我们预计美洲和旧世界的扩张不会同步。如果扩张是同步的，那么必须援引一些涉及全球变化的其他解释，例如大气中二氧化碳的减少或其他原因。

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.