| The major objective of the research is to select a natural sediment profiles, which has two different kinds of sedimentation type, and the upper for fluvial facies sedimentation, lower for the oxbow lake deposition. We call this section Jijitan profile (JJT,40°6’46"N, 114°26’54"E), it is located in the middle part of Nihewan Basin, south bank of the Sanggan River, with an altitude of 850 m. We did the experiment analysis for JJT profile of the soil, and got high resolution pollen data and charcoal content to reconstruct the environment’s change, and find out the spoor of human activity. Eight pieces of muddy sample were chosen for AMS14C dating. After calibration, we get that 65-70cm is 2117 cal yr BP, and 560-570 cm is 11932 cal yr BP. It shows that Profile formed in the late of the Last Interglacial Period to 2000 cal yr BP. The pollen in the oxbow lake deposits mainly came from local and regional pollen by wind, as well as the rivers carried pollen from the south mountain region.According to the pollen analysis, section reveals that changes in climate and vegetation can be divided into three main stages:The first stage (13000-10000 cal yr BP), in the earlier period, tree pollen content is higher, while herb pollen content is less, and it indicates that the study area’s climate is mild and humid for a period of time before the YD event. During the period of YD event, the forest began to retreat, and grassland to expanse, showing that the climate began to get cold and dry, and annual precipitation decreased obviously. This stage contains a lot of aquatic Cyperaceae, indicating a shallow water environment, which proved the oxbow lake shrink. The sampling point is closer to the edge of the lake. The vegetation type in the study area mainly is forest grassland.The second stage (10000-5500 cal yr BP). The climate warmed up gradually. In the earlier period, the climate is mild. Before the development of forest vegetation, its first growth is some pioneer plant, such as Betulaceae. With the coming of the Middle Holocene, the climate is gradually warming, and humidity is further increased, in the same time the mixed broadleaf-conifer forest fully developed. The most suitable period of the climate in the study area maybe is 7600-6000 cal yr BP. Influenced by human activities, the area of forest vegetation has a fluctuation change. About 6400 cal yr BP, because of the destruction of human, the forest area reduced. Around 6400 cal yr BP, forest vegetation restoration, which may be due to human activity weakened or migration. At this point, and it may be another reason for the humidity increased, the lake area expanded, tree pollen content increased.The third stage(5500-2000 cal yr BP). Pinus trees gradually reduces, which was replaced by grassland vegetation, such as Artemisia, Chenopodiaceae, etc. It indicates the climate began to be cold and dry. The forest was almost disappeared, and only a small piece of broadleaf brush grew in some place which had better hydrothermal condition. In historical period, human activities strengthen, and the climate and vegetation condition tends to be modern. The comparison of climate change between Nihewan basin and other regions during the Middle and Late Holocene shows that the East Asian monsoon recession was synchronous. |
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