Mid-Miocene C-4 expansion on the Chinese Loess Plateau under an enhanced Asian summer monsoon

Atmospheric CO2 starvation, aridity, fire and warm season precipitation have all been proposed as major contributors to C-4 plant expansion during the Late Miocene. However, the driving factors responsible for the distribution of C-4 plants in the early and mid-Miocene still remain enigmatic. Here we report pedogenic carbon and oxygen isotope data (delta C-13(pedo), delta O-18(pedo)), along with magnetic susceptibility (MS) results, from the Zhuang Lang drilling core on the Chinese Loess Plateau (CLP). Elevated delta C-13(pedo). values ( > -5 parts per thousand) signal a prominent C-4 expansion and substantially increased delta O-18(pedo). and MS values indicate enhanced Asian summer monsoon (ASM) precipitation. Both of these conditions are observed during the Mid-Miocene Climatic Optimum (MMCO), 14.5-17 million years ago. The marked increase in C-4 plants, associated with warm temperatures and increased precipitation, strongly suggests the control of an enhanced ASM on C-4 expansion on the CLP during the MMCO. This finding contrasts with the late-Miocene C-4 expansion associated with cooling and drying conditions observed in low latitudes and argues for regionally specific control of C-4 plant distribution/expansion.