The Environment Geological Information In The Sediments Of Diexi Ancient Dammed Lake On The Upstream Of Mingjiang River In Sichuan Province, China

The important information with ancient climatic evolution of sensitive reflection was kept in the ancient dammed lake sediments in the upstream of Minjiang River. Among them the ancient dammed lake lacustrine sediments thickness is up to more than 200 meters, and it has kept the six terraces which have taken the lacustrine sediments as base. The 14C dating showed that it was formed in about 2 million years ago and disappeared in about 1 million years ago, that is, in the era of the last period of the end of the ice, going through periods of warming to Younger Dryas cold period of the stage. Regardless of the scale and sediment from the sediment record of the time information, geological environmental information should be very rich,for the study of the changes of climate and environment to provide a very valuable information in the region since 2 million years.The Diexi ancient dammed lakes of upstream Minjiang River has been in-depth study in this paper which include distributed characteristic of ancient dammed lakes, Structural Characteristics of sediment deposition, sediment indicators of environmental information, and the 14C dating of forming and disappearing of the dammed lake, etc. Based on these test results with the combination of a series of ancient dammed lakes characteristic in the upstream of Minjiang River, the paleoenvironment and palaeoclimatic evolution of Diexi region were analyzed since 20,000 years. Through this study the following main results:(1) The Diexi ancient dammed lake lithofacies characteristics, characteristics of spatial distribution, as well as structural characteristics of the material composition of many aspects, such as proof of the Diexi ancient sediments are lacustrine sediments, which is different from the loess sediments, glacial lake sediments, and closed lake sediments. Through particle analysis experiment of the lacustrine sediments(64 test samples), these sediment grain's diameters are many in (0, 0.075mm), belong to the powder and clay grain; Sediment laminae thickness is between 2-5cm; The dark laminae of the ancient dammed lake sediment line layer represents the sufficient rainwater in spring and summer to deposit, dry deposition of light-colored representative of autumn and winter; The ancient dammed lake sediment thick particle color of sediment is relatively dark, detailed particle color is relatively shallow.(2) Based on the result of granularity analysis(33 test samples), carbon oxygen isotope analysis(20 test samples), and spore's powder analysis(33 test samples), the whole Tuanjie village sediment profile is divided into 7 environment time intervals: The first stage (22 ~ 20.8 ka BP) : elevation range:2141~ 2130m, it is to be forest grassland vegetation types, the climate environment is from frigidity dry turn into warm wet process in the period; The second stage (20.8～17.7ka.BP):elevation range:2141～2177m, it is to be grassland forest vegetation types, the climate environment is from warm wet turn into frigidity dry process in the period; The third stage (17.7～16.4ka.BP):elevation range:2177～2197m, it is to be mingled forest grasslands vegetation types, the climate environment is from frigidity dry turn into warm wet process in the period; The fourth stage (16.4～13.4ka.BP):elevation range:2197～2224m, it is to be evergreen needle leaf forest vegetation types, the climate environment is from warm wet turn into warm semi-dry process in the period; The fifth stage (13.4～11.9ka.BP):elevation range:2224～2281m, it is to be needle leaf forest and mingled forest vegetation types, the climate environment is from warmer lilted wet turn into cold dry process in the period; The sixth stage (11.9～10.3ka.BP):elevation range:2281～2299m, it is to be needle leaf forest and part mingled forest vegetation types, the climate environment is from cold dry turn into frigidity wet process in the period; The seventh stage (10.3～10ka.BP):elevation range:2299～2310m, it is to be needle-broad leaf mingled forest vegetation types, the climate environment is from frigidity wet turn into warm dry process in the period. The information has pointed out that has gone through changes of the environment about 7 times during 10kaBP-22kaBP in the Diexi area.(3) The Diexi ancient dammed lake preserved in lacustrine sediments of the base 6 for the terrace, which the existence of six terrace shows the ancient dammed lake outburst had occurred in six of the corresponding deposition process. According to the dating and palynological test data, get the following laws to the palaeoclimate and ancient environment in this area since in 10kaBP: 1) The Diexi ancient dammed lake begins to burst since 10ka.BP, disappear after being divided into about 6 times burst , every time after all possible efforts from climate change; 2) Among the 10-8.4kaBP, the climate environment is from warm dry turn into warm-cold little wet process in the period; 3) Among the 8.4～5.5kaBP, the climate environment is from warm-cold little wet turn into warm semi-dry process in the period.(4) Through comparing the ancient environment and palaeoclimate of adjacent area of edge of the east of Qinghai-Tibet Plateau with discovering: Diexi region and adjacent Ruoergai area in the same period of time characterized by the existence of differences in climate and environment, but demonstrated unanimously generally in these regional climate in 2 million years it is frigidity and dry; These regional climate demonstrated unanimously generally that warm and wet in 10,000 years. These evidences prove on the edge of the east of Qinghai-Tibet Plateau, has these two time intervals, the regional climate is identical, the influence that it should be the great environmental macroclimate plays a leading role.