CARBON ISOTOPE RECORDS FROM THE LOESS-PALEOSOL SEQUENCES FOR C3/C4 PLANT VARIATIONS IN THE LOESS PLATEAU DURING THE LAST 150 KA BP: IMPLICATION OF VEGETATION RESPONSE TO CLIMATIC CHANGES

It is important to understand which is the major factor to drive ecological changes on a regional scale since our atmospheric CO2 level is increasing because human activities. In this presentation, the analyses of carbon isotope of organic matter, grain size, carbonate, and magnetic susceptibility have been carried out on the samples from the loess-paleosol sequences at different sites in Chinese Loess Plateau for the histories of C3/C4 plant abundance and climate since the last interglacial. The following conclusions emerge from the analytic data. First, the fact that the flora are characterized by a mixture of C3/C4 grasses during relative warm periods except for the last interglacial (equivalent to Eemian in Europe), and by a C3 grass domain during relative cold periods of the last 150 ka BP indicates that temperature is the major factor to induce changes in C3/C4 abundance. This conclusion is also supported by an increasing trend of C4 plant abundance for a given period from the cold northwest to the warmest southeast of the Loess Plateau. Second, the C4 plant abundance during warm periods increase with seasonal intensity of precipitation and/or dryness, implying that the East Asian monsoons play an important role in the plant photosynthetic type when the temperature is high enough to grow C4 plants. Finally, a great contrast in the C3/C4 plant abundance is presented between Holocene and the Eemian, when atmospheric CO2 level was high for both of the periods. The Holocene flora is a mixture of C3/C4 grasses in the plateau. However, the Eemian flora become into a C3 plant domain only in the southeastern plateau. Since the Eamian climate is the warmest in the last 150 ka BP, it can be deduced that forest had fully covered on the southeastern plateau due to reducing of the seasonality of precipitation during that period.