Variations of organic carbon isotopic composition and its environmental significance during the last glacial on western Chinese Loess Plateau

摘要

A high-resoiution loess section in the western Chinese Loess Plateau, Yuanbao Section, was sampled for organic carbon isotopic analyses. The soil organic carbon isotope (delta~3C_(org)) varied between -22.6 percentc and -27.5 percente during the last glacial at the section. During the last interstadial, the delta~(13)C_(Org) values were more negative than those in both early and late periods of the last glacial by 4 per thousand. 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 per thousand-2.0 per thousand. The amplitude of 4 per thousand in the delta~(13)C_(org) variation during the last glacial should be mainly caused by the precipitation change. Therefore, the delta~(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 compli-catedly 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.