中文摘要：

植物叶蜡正构烷烃组分能够在地质体中长久保存，并记录历史时期植被变化的信息。本研究通过对黄土高原中部董志塬西峰地区赵家川剖面古土壤中正构烷烃组分的分析，讨论7～2Ma期间黄土高原植被变化的植物叶蜡组分变化特征及其对气候变化的响应。结果表明：从7．0Ma至3．4Ma，正构烷烃C31／C27和C31／C29比值处于相对低值的波动范围，范围分别为0．5～1．9和0．6～1．3，表明该时期以木本植物占优势的生态格局；在3．4Ma前后，古土壤中保存的古植被叶蜡正构烷烃组分的C31／C27和C31／C29比值有一个明显的增加，分别从0．8变化到2．9，从0．7变化至1．6，即古生态系统中草本植物的相对比例突然增加；从3．4Ma至2．0Ma，正构烷烃C31／C27和c31／C29比值处于相对高值的波动范围，范围分别为1．7～3．3和1．1～1．6，指示该时期木本植物减少，草本植物增加的生态格局。我们推测这个生态系统的变化可能响应晚新生代3．5Ma左右的中国内陆干旱化，并形成了自此以后黄土高原塬面以草原或森林草原为主的自然生态格局。

英文摘要：

Plant n-alkanes from leaf waxes can be preserved in the geological record for a long time and register the information of the paleovegetation changes during historical periods. Here, we analysis n-alkanes component of paleosol from the Zhaojiachuan profile located at the Xifeng district of Dongzhi Yuan in the central of the Chinese Loess Plateau, and discuss leaf wax components and their response to climatic changes when the vegetation changes had occurred during 7 -2Ma. The results showed that the ratios of C31/C27 and C31/C29 of leaf n-alkanes were fluctuating at relatively lower values from 7.0Ma to 3.4Ma, ranging 0.5 - 1.9 and 0.6 - 1.3, respectively. These values indicate that this period was dominated by woody plants. The ratios of C31/C27 and C31/C29 of leaf n-alkanes displayed an abrupt increase at about 3.4Ma,varying from 0. 8 to 2.9 and from 0.7 to 1.6,respectively. Thus,the relative proportions of the herb increased suddenly in the ancient ecosystem,with the ratios of C31/C27 and C31/C29 of leaf n-alkanes were fluctuating at relatively higher values from 3.4Ma to 2.0Ma, ranging from 1.7 - 3.3 and 1.1 - 1.6 respectively. These data suggest that the ecological pattern is with the decrease of woody plants and the increase of herb during this period. We infer that the change of ecosystem might have responded the aridity in the Asian interior in Late Cenozoic about 3.5Ma,and formed the ecological pattern relying mainly on the grassland or forest grassland at the loess plateau after this.