Paleoenvironment Evolution Process Of West Jilin In Late Holocene

West Jilin locates in sub-humid continental monsoon climatic district, and is a part of forest-steppe eotone in the Northeast China Plain. The special location determines that it is a sensitve area for the changing of environment and climate.Since late Holocene period, the paleoenvironment and paleoclimate in West Jilin had changed between warmth, wetness and aridity several times for the effection of East Asia monsoon. The changing information could be obtained from the stable carbon-isotope in paleosol carbonate and biomass of C3, C4 plants. This is because carbon-isotope fractiontion of soil carbonate is mainly controlled by the composition of soil CO2, which is determined by the biomass of C3 and C4 plants. So, vary δ13C values of soil carbonate indict different proportion of C3 and C4 plants when soil was formation, and high δ13C values express high proportion of C4 plants.Now, many researches indict the pathway of C4 photosynthesis is different from C3 plants'. C4 plants have higher ratio of CO2 vtilization than C3 plants, so they can develop in hot, aridity environment. C3 plants mainly grow in warmth, wetness environment. Furthermore, C4 plants also show better adaption capability to the saline-alkal environment comparing with C3 plants. The rules of C3, C4 plants could be well validated by the δ13C values of mordern soil. So, biomass of C3, C4 plants andδ13C values of soil section can be used to indicated the changing process of local paleoenvironment.In Jiangjiadian paleosol section, theδ13C values and C3, C4 biomass show there was warmth,wetness when the paleosol growed in West Jilin. During this period, the proportation of C3 plants achieved maximum and C4 plants correspondly declined minimum. After the paleosol devolopment, the environment of West Jilin gradually changed into aridity and the proportion of C4 plants also begined to increasing.According age-dating of paleosol, pollen analysis, grain size analysis and theδ13C values of soil carbonate, paleoenvironment envolution process of West Jilin in late Holocene was rebuilt and divided into two periods. In each period, the paleoenvironment changed from warmth, wetness to aridity.The first period is 3500～2800～2100 aB.P.. In early time (3500～2800 aB.P.), the environment of West Jilin was appropriate, which suited for paleosol development. Later (2800～2100 aB.P.), the environment had gradually changed into aridity. Adaption aridity environment, paleosol in West Jijin stopped to growth and biomass of C4 plants begined to increasing.The second period of environment envolution in West Jijin is 2100～1200～0 aB.P., which also can be divided into early period (2100～1200 aB.P.)and later period (1200～0 aB.P.). In early time, there was warmth, wetness in West Jijin, which leaded to paleosol development again. At this time, biomass of C4 plants achieved the minimum in Jiangjidian soil section. After 1200 aB.P., palaeoclimate swifted to aridity quickly and proportion of C4 plants correspondly increased again.In West Jilin, this trend has lasted until the mordern time.