Abstract

Plants using the C-4 photosynthetic pathway, commonly tropical and subtropical grasses, increased in abundance in East Asia during the late Cenozoic. Determining the exact timing and likely factors leading to this major vegetation change requires region-specific studies. Here variations in pyrogenic carbon mass accumulation rate (PyC-MAR) and isotope composition (delta C-13(PyC)) from an similar to 7-m.y.-long depositional sequence from the central Loess Plateau, China, suggest increased biomass burning and an increased contribution to combusted material from C-4 taxa from 2.6 Ma. Changes in the composition of PyC after 0.6 Ma likely reflect the effects of lower temperatures, particularly during glacial periods, and changes in seasonality of precipitation. Increased PyC-MAR without concomitant changes in delta C-13(PyC) at ca. 0.15 Ma appears to indicate a decoupling of feedbacks between changes in climate, fire regime, and vegetation, and may mark the onset of anthropogenic burning in the region. These new data suggest that C-4 taxa were present on the Loess Plateau from at least the late Miocene, rising to prominence at ca. 2.6 Ma following changes in climate and, critically, an increase in biomass fires.