Post-glacial Environmental Evolution Of Lake Huangqihai, Inner Mongolia: An Investigation On Loss-on-ignition

Lake-level reconstruction for closed-basin lakes in China may provide key-site information important for the study of Asian monsoon evolution of the Holocene. Lake Huangqihai is a hydrologically closed lake, located in the semiarid area of Inner Mongolia. Past changes in the lake-level and water chemistry are closely related to the changes of catchment precipitation, which was controlled mainly by monsoon circulation. Three sediment cores were for the first time retrieved from the middle of the lake. The coring was successfully operated on the winter ice-cover of the lake in 2000 and the pioneer practice of the "off-the-ice" coring proves to be very useful and economic for taking quality cores from lakes in northern China.A multidisciplinary investigation has been carried out on core HQH4, one of the three 11-m-long cores. Results from previous investigation include lithostratigraphy, radiocarbon dating, paleoecology of Ostracoda, stable isotopic record of ostracod shells, total nitrogen and organic carbon, C/N ratio, 81 C of organic matter etc. This thesis study has focused on the investigation of loss-on-ignition (LOI) and Ruppia L. macrofossil for the reconstruction of the lake level and post-glacial environmental change.The results indicate that Lake Huangqihai was in cold and arid climate conditions between 16-10.2 ka BP with ice cover for much of the years, which led to the organic and carbonate production much lower than in the Holocene. An abrupt increase of seasonal inflow occurred at about 10.2 ka BP, as suggested by a layer rich in Ruppia L. seed, in situ dropped from the rooted aquatic plant and by the deposition of well laminated lacustrine sediments. The lake level thereafter became deeper than 4 m so that Ruppia L. plants were no longer growing in the middle of the lake. The data of LOI shows that organic productivity increased markedly from the beginning of the early Holocene. This, along with the evidence of increased carbonate production, is most likely resulted from a substaintial increase of summer temperature.In 10200—6800a BP, LOI distinct fluctuation reflects remarkable changes of climate and environment shifted with characters of warm and dry, cold and wet, warm and wet, and coldand wet. In 6800-3800 a BR, the high and stable LOI value indicates a stable environment with warm and wet climate and higher productivity of authigenic carbonate and organic matter. From about 3800a BP., the lake level declined and the regional climate began to cold. At 1960s, human activities caused the lake's ecological conditions unbalanced and damaged the fishery industry of the lake.