Jet-Induced Rainfall Seasonality and C4 Expansion over East Asia

The Neogene expansion of C ₄ grasslands transformed terrestrial ecosystems with marked influence on mammalian evolution, including hominins. However, the asynchronous C ₄ expansion on different continents makes it difficult to identify the environmental drivers, especially for higher latitudes. Here we show that rainfall seasonality governed extratropical Plio-Pleistocene C ₄ distributions in East Asia. Rainfall oxygen isotope ratios and clumped isotope soil temperatures exhibit coupled variations on the Chinese Loess Plateau (CLP) from 7 to 2.5 Ma, indicating more spring rain during warmer times when the subtropical westerly jet was further poleward, and more concentrated summer rain under cooler climates. We attribute these changes to meridional shifts of a summer rain band on orbital and longer timescales. Organic carbon isotope records reveal that the most C ₄ -rich ecosystems tracked this summer rain band, eventually eclipsing the southern CLP margin during late Pleistocene cooling. Our model refines the East Asian paleomonsoon concept and explains the equatorward migration of extratropical C ₄ ecosystems, highlighting the tight coupling between regional rainfall seasonality and vegetation.