C3/C4 vegetation evolution over the last 7.0 Myr in the Chinese Loess Plateau:: evidence from pedogenic carbonate δ13C

The stable carbon and oxygen isotopic compositions of soil carbonate were measured on an eolian loess and red clay sequence at Lingtai, the Chinese Loess Plateau. This sequence is composed of 130 m of Tertiary red clay deposits with a basal age of 7.05 Ma overlain by 175 m of Pleistocene loess. In the field we identified ca. 110 carbonate nodule horizons in the red clay and 27 nodule horizons in the loess. These carbonate nodule horizons are formed by leaching and re-precipitation of carbonate from the eolian material. The delta(13)C record of soil carbonate indicates a major expansion of C-4 plants at ca. 4.0 Myr in the Loess Plateau. This event is comparable in timing with the expansion of C-4 plants in northern North America (Cerling et al., 1997. Nature 389, 153-158) but is ca. 3 million years later than the C-4 biomass expansion in Pakistan (Quade et al., 1989. Nature 342, 163-166). The pedogenic characteristics of the soils and the delta(18)O record in the red clay suggest that the C-4 plant expansion in the Loess Plateau was not driven by local climatic changes, which may support Cerling et al.'s (1997) assertion that the decline of atmospheric CO2 levels in the Neogene is responsible for this global vegetation change. Our record also implies that the Tibetan Plateau could have been uplifted to a critical height in the late Miocene, thus resulting in the formation of the atmospheric Great East-Asia Trough. (C) 2000 Elsevier Science B.V. All rights reserved.