A high-resolution loess section in the western Chinese Loess Plateau, Yuanbao Section, was sampled for organic carbon isotopic analyses. The soil organic carbon isotope (δ 13 C org ) varied between −22.6‰ and −27.5‰ during the last glacial at the section. During the last interstadial, the δ 13 C org values were more negative than those in both early and late periods of the last glacial by 4‰. The isotopic composition indicates a coupled response of the pure C 3 plants to the temperature, precipitation and the concentration of atmospheric CO 2 . Decrease in temperature and the atmospheric CO 2 concentration from the last interstadial to Last Glaicial Maximum (LGM) caused the organic carbon isotopes to become positive by 1.5‰-2.0‰. The amplitude of 4‰ in the δ 13 C org variation during the last glacial should be mainly caused by the precipitation change.Therefore, the δ 13 C org variations of the Yuanbao Section during the last glacial period documented the large-amplitude fluctuation of the monsoon precipitation, which is estimated to be 250-310 mm more during the last interstadial than that in the LGM, and 100 mm more than that during early last glacial. The rapid changes of the monsoon precipitation on millennial scale during the last glacial have also been recorded in the isotopic variations in Yuanbao loess section. As the isotopic composition varies complicatedly as shown in the loess-paleosol sequence, it cannot be simply attributed to the abundances of C 3 and C 4 plants or be used as an indicator of the summer monsoon variations.Keywords: western Chinese Loess Plateau, last glacial, organic carbon isotopes, C 3 plants.The last glacial period is characterized by a cold and dry climate with low atmospheric CO 2 concentration. The relatively arid climate and low CO 2 concentration are favorable to the growth of C 4 plants, but the low temperature limits the development of this type of plants. Researches in low-latitude Africa and equator Columbia suggest that C 4 plants expanded during the last glacial period in those regions [1][2][3] . The organic carbon isotope records from two lakes in Mesoamerica indicate a reverse trend of the C 4 plants in the last glacial [4] . Researches on organic carbon isotopes (δ 13 C org ) in Chinese loess also suggest low abundances of C 4 plants during the last glacial [5][6][7][8] . The expansion of C 4 plants in low latitudes during the last glacial was interpreted as the response of C 4 plants to the low atmospheric CO 2 concentration or the increasing aridity or to both. In Chinese loess records, the reduction of C 4 plants in last glacial could be attributed to either low temperatures or increased precipitation during the winter as the climate condition was not sufficient to drive the expansion of C 4 plants though the CO 2 concentration was low [5 -7] . The δ 13 C org record in European loess [9] showed that the C 3 plants had been dominant throughout the whole last climatic cycle. The more positive δ 13 C org values during the last glacial were the response of C 3 plan...
