LAUSR

Abstract C4-grasslands are known to have first expanded globally in the Late Miocene, and recent research has hinted at a second expansion phase during the Early Pliocene; however, the geographic extent of this second event and its driving force are debated. In this study, we present organic carbon-isotopic evidence from a high-resolution drillcore in North China spanning the interval from ∼8.0 Ma to the present. A prominent positive δ13Corg shift at ∼4.1 Ma marks C4-grassland expansion on the North China Plain, and rapid oscillations of >6‰ from ∼3.2 to 2.2 Ma imply large shifts between C3- and C4-dominated floras. Our δ13Corg profile mirrors carbon isotope shifts in loess-red clay deposits of the Chinese Loess Plateau, confirming the regional nature of this event. Furthermore, coeval carbon isotope records from Africa, North America, and South America provide evidence that the Early Pliocene C4-grassland expansion was global in scale and temporally distinct from the Late Miocene event. Whereas the Late Miocene event may have been related to aridification and altered fire regimes, this mechanism cannot account for the Early Pliocene expansion owing to contemporaneous increases in humidity at many Northern Hemisphere sites. We hypothesize that C4-grassland expansion in the Early Pliocene was triggered by falling atmospheric CO2 levels, as evidenced by multiple pCO2 proxy records and supported by quantum photosynthetic efficiency modeling.