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 C-3/C-4 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 delta C-13(org), 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 delta C-13(org) and carbonate delta C-13(carb) indicates that terrestrial plants made a dominant contribution to the organic matter. The delta C-13(org) of organic matter shows negative values (between -24.1 parts per thousand and -28.3 parts per thousand) along the sequence from 17.1 to 6.1 Ma, indicating a predominance of C-3 plants. More specifically, our results indicate that the delta C-13(org) values of organic matter show a significant increase by ca. 1.4 parts per thousand from -26.6 parts per thousand to -25.2 parts per thousand from 7.1 Ma onwards (equivalent to ca. 10% increase in C-4 plants), reflecting the initial occurrence of C-4 plants. The occurrence and expansion of the C-4 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. (C) 2014 Elsevier Ltd. All rights reserved.