Late-Miocene palaeoecological evolution of the Tianshui Basin, NE Tibetan Plateau: Evidence from stable organic carbon isotope record

The spatial and temporal variations and the driving mechanisms of C3/C4 vegetation evolution throughout the Miocene have been a matter of long-standing debate. A continuous lacustrine-fluvial sediment sequence widely exposed in the Tianshui Basin, NE Tibetan Plateau provides great potential for deciphering the C3/C4 vegetation dynamics during the Miocene. Based on the measurements of δ13Corg, C, N, and C/N ratios of organic matter of this sediment sequence, here we present a new perspective on vegetation history in inland Asia during the period from 17.1 to 6.1 Ma. The organic matter preserved in the Tianshui Basin is most likely a mixture of terrestrial and aquatic origins. The lack of correlation between organic δ13Corg and carbonate δ13Ccarb indicates that terrestrial plants made a dominant contribution to the organic matter. The δ13Corg of organic matter shows negative values (between −24.1‰ and −28.3‰) along the sequence from 17.1 to 6.1 Ma, indicating a predominance of C3 plants. More specifically, our results indicate that the δ13Corg values of organic matter show a significant increase by ca. 1.4‰ from −26.6‰ to −25.2‰ from 7.1 Ma onwards (equivalent to ca. 10% increase in C4 plants), reflecting the initial occurrence of C4 plants. The occurrence and expansion of the C4 component in the study area is supposed to be a result of increasing summer precipitation due to the enhanced Asian Summer Monsoon since that time.