Jet-induced rainfall seasonality and C4 migration over East Asia.

The Neogene expansion of C4 grasslands transformed terrestrial ecosystems with marked influence on mammalian evolution, including hominins. However, the asynchronous C4 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 C4 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 million years ago, 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. The most C4-rich ecosystems, as identified by organic carbon δ13C records, tracked this summer rain band, eventually eclipsing the southern CLP margin during the late Pleistocene cooling. Our model refines the East Asian paleomonsoon concept and explains the equatorward migration of extratropical C4 ecosystems, highlighting the tight coupling between regional rainfall seasonality and vegetation.