中文摘要：

相对黄土高原东部地区而言，处于六盘山以西的陇西黄土高原，其末次冰期以来陆地生态系统中C3／C4植物相对丰度变化的研究，不仅已开展研究工作的剖面数量相对较少，所得到的结果也不如黄土高原东部地区那样一致。本文对陇西黄土高原东南部边缘区的张家川剖面末次冰期以来的黄土／古土壤地层进行总有机质碳同位素的分析测试，得到的结果表明该剖面全新世古土壤的总有机质碳同位素相对末次冰期马兰黄土更为偏正，这一结果与黄土高原东部地区数个剖面的研究结果是一致的，表明该区域末次冰期向全新世转化过程中，C4植物相对丰度有一定程度的上升。进一步将张家川剖面的研究结果与陇西黄土高原其他几个研究剖面进行了对比，发现陇西黄土高原末次冰期以来陆地生态系统中C3／C4植物相对丰度变化情况要比黄土高原东部地区更加复杂，很可能在该区域内部还存在一些地区之间的差异，该区域东部和南部地区全新世相对于末次冰期C4植物相对丰度有一定程度的增加，而该区域西北部地区则很可能末次冰期至全新世都以C3植物占绝对优势。由于大量现代C3植物的研究结果表明，其碳同位素组成的变化主要响应于降水量的变化，因此，来源于C3植物为绝对优势植被的黄土有机质碳同位素很可能是一个潜在的古降水量的指示器，陇西黄土高原黄土／古土壤序列的有机碳同位素研究还值得进一步加强。

英文摘要：

Stable organic carbon isotopic composition（δ13Corg. ）of more than 10 Chinese Loess profiles have been studied to indicate the relative abundance of C3/C4 plants in the Chinese Loess Plateau （CLP）since the Last Glacial（LG） , while only few studies have been conducted in the western part of the CLP （ Longxi Loess Plateau） , and the variations of the relative abundance of C3/C4 plants since the LG in the western CLP has not been well studied. Zhang Jiachuan loess profile （34°59＇N, 106°15＇E） is located in the northwestward of the Zhang Jiachuan county town,the northeastern part of Tianshui City in the south eastern Gansu Province,about 368km away from the provincial capital Lanzhou City. The top part of the profile （ ca. 1 m） is disturbed by modern agricultural activities. The stratigraphy of this profile below the top part along the depth is the loess layer of the Late Holocene （ca. 0.54m） , the Holocene paleosol layer（ ca. 0.54m to 2.70m） , and Malan loess layer accumulated during the LG （below ca. 2.7m）. Totally, 165 loess samples were collected with the density of 2cm per sample from this profile, except the top disturbed part. Four samples for radio-carbon（14C） dating were collected from depth of 54cm, 130cm, 190cm and 240cm, and the dating results indicated that the sampled profile do cover the LG and the Holocene periods. Magnetic susceptibility and δ13 Crg. of the 165 loess samples have been measured indoor. For δ13Corg. , the results of the standard materials（ n = 15 ）demonstrated that the standard deviation of the instrumental measurement is ≤±0. 1‰ （VPDB）. The obtained δ13Corg. values from Zhang Jiachuan profile are ranged from -21.5‰ to -23.4‰ with an average value of -22.6‰. The δ13Corg. values of the paleosol developed during Holocene is more positive than those of the loess accumulated during the LG, indicating that C4 plants relative abundance was increased from the LG to the Holocene in the study site which is similar to the resu