Pedogenic carbonate isotope record of vegetational evolution since late Miocene in Loess Plateau

Isotopic analyses for paleovegetational evolution have been carried out on samples of the pedogenic carbonate nodules in the Red Clay-Loess sequence at Lingtai (35°N), the Loess Plateau. Stable carbon isotopic composition indicates that (i) C₄ plants might be present at least by 7.0 Ma; (ii) C₄ plants expanded gradually between ∼4.0 and ∼3.2 Ma, and their biomass fraction was up to 50%; and (iii) the biomass of C₄ vegetation since ∼ 2.0 Ma seems to have been decreased to the level (about 20%) before 4.0 Ma. C₄ plant expansion at Lingtai cannot be fully understood with the “global C₄, expansion” model because it occurred much later up to 3.0 Ma than in Pakistan, which indicates that some changes in the regional climatic system may also contribute to C₃/C₄ shift except changes in atmospheric CO₂, concentrations and temperature. The latitudinal zone for C₃,/C₄ transition seems to move southwards slightly in East Asia, compared with the case in North America where 37°N is the ideal boundary for the C₄, expansion.