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 a t Lingtai (35 degrees N), the Loess Plateau. Stable carbon isotopic composi tion indicates that (i) C-4 plants might be present at least by 7.0 Ma; (ii ) C-4 plants expanded gradually between similar to 4.0 and similar to 3.2 M a, and their biomass fraction was up to 50%; and (iii) the biomass of C-4 v egetation since similar to 2.0 Ma seems to have been decreased to the level (about 20%) before 4.0 Ma. C-4 plant expansion at Lingtai cannot be fully understood with the "global C-4 expansion" model because it occurred much l ater up to 3.0 Ma than in Pakistan, which indicates that some changes in th e regional climatic system may also contribute to C-3/C-4 shift except chan ges in atmospheric CO2 concentrations and temperature. The latitudinal zone for C-3/C-4 transition seems to move southwards slightly in East Asia, com pared with the case in North America where 37 degrees N is the ideal bounda ry for the C-4 expansion.