ENVIRONMENTAL SIGNIFICANCE OF PHYSICOCHEMICAL PARAMETERS OF SEDIMENT AND CLIMATE CHANGES SINCE THE LATE GLACIAL AT QINGHAI LAKE OF TENGCHONG COUNTY, YUNNAN PROVINCE
Qinghai Lake, which located at Tengchong County in Yunnan Province near the border between China and Myanmar (25°08'06"~25°07'44"N,98°34'11"~98°34'26"E),is a small crater lake with an area of 0.21km~2,a maximum water depth of 5.9m and a drainage area of 1.5km~2 (when the water level at 1885m a.s.l.). According to the meteorological data, the annual average temperature is 14.7℃,with a maximum temperature of 30.5℃ and a minimum temperature of - 4.2℃ ; the annual average precipitation is 1425mm, among that the rainfall amount between May and October takes up 84.3%. the annual average potential evaporation amount is 1575mm, therefore, the study area is a tropical plateau humid monsoon climate. A 513cm long sediment core (25°07'54", 98°34'17"),which has been composed by two parallel cores took from Qinghai Lake has been studied. Our purpose is to understand how the grain size, paleomagnetic susceptibility, total organic (TOC) and inorganic (TIC) contents changed and what did these parameters mean. Based on 16 AMS ~(14)C dates, the chronology of the core was established and the age at 513cm depth is 15830a B.P. Totally 513 sample were analyzed and based on the age-depth relationship it was calculated that each the samples analyzed possesses a resolution of 30~50 years. Among the four parameters we discussed,we found that the organic content in some part of the Tengchong core can be as high as 66.60%,e.g. at 123cm ( 835a B.P.) depth in the core, while in some parts the CaC0_3 content can be as low as zero, e.g. at 479~423cm (13560~ 11660 a B.P.) and 382~180cm (9690 ~ 2650 a B.P., excepth at 278~275cm, 5990~5940 a B. P.) depth in the core. We attribute the high TOC content to the very stable climate with not very high amount of precipitation. The extreme low TOC content is attributed to the out flow of the lake water when the input of the water amount excesses the capacity of the lake,due to the strong rainfall. According to the grain-size characteristics, total organic carbon and carbonate contents, paleomagnetic susceptibility and chronology of the sediments from Qinghai Lake, the environment and climatic change history since 15830a B.P. were reconstructed. Phase I ( 15830~ 12790a B.P.) : climate condition was warm-cool, and partial wet. Phase II (12790~4680a B.P.) : warm-humid, higher water level of lakes and precipitation increasing. Phase III ( 4680 ~ 312a B.P.) : warm-humid to warm-dry, precipitation decreasing and the water level of lakes descending. Phase IV (312a B.P.) : reflecting the trend of climate change in recent decades under the global warming condition, mild-humid at prophase, warm-dry at anaphase.