The Sediment Record Of Lake Dalongchi And Lake Gahai For The Past 2000 Years

Sediments in closed and semi-closed lakes preserve rich information on regional environmental and climatic changes, being ideal materials for palaeoclimate and palaeoenvironment researches. Climatic change in the past millennium reconstructed by lake sediments in northwest China suggests that the Medieval Warm Period (MWP,800-1300AD) and Little Ice Age (LIA,1500-1800AD) are commonly identified, whereas the climate regime (regional precipitation and humidity) is different at different sites.In this thesis, two cores were selected to reconstruct the environmental histories during the past 2000 years in Tianshan area and 400 years in the northeastern margin of Tibet Plateau respectively. DLC12B (110cm) was taken from Lake Dalongchi Xinjiang and GHC1 (50cm) from Lake Gahai in Qaidam Basin. The chronology of DLC12B core was established based on 210Pb and AMS 14C dating. GHC1 core was dated by 210Pb and 137Cs. Environmental proxies of particle size, carbonate content, magnetic susceptibility, organic matter content and element content were analyzed for both cores. The climatic and environmental change in the past 2000 years recorded by Lake Dalongchi suggest it gradually became drier during 193-800AD. During the period 800-1300AD, the MWP, the carbonate content was slightly higher than the average without significant fluctuations, but the particle size was small, indicating high lake level which might be induced by increased meltwater. After the MWP (1300-1850AD), the average particle size increased, indicating the reduced meltwater and drop of lake level with high carbonate content. Since 1850AD, each proxy fluctuated obviously, indicating an unstable climate toward gradual drying. The carbonate content of Lake Gahai sediment reflects the regional humidity changes, and the coarse particle component (>66.9μm) was mainly transported to the lake by wind, whose content could reflect regional dust activity history. The past 400 years environmental change could be separated into 3 phrases. In 1633-1750AD, the climate and environment were unstable with wet and dry fluctuations. Increased coarse particles indicated strong regional winds, consistent with the Maunder minimum solar activity period. Therefore, it was inferred that variations of solar activity might have influenced the climate change in Delingha area. In 1750-1950AD, regional humidity increased as indicated by decreased carbonate content. In 1950-2010AD, dramatic changes of proxies suggested increased regional precipitation but decreased humidity as result of the strong evaporation. Since 1974AD, the sharp rise in coarse particle component exhibited strong dust storm events.Compared with pollen records from lake sediments in the arid region and precipitation reconstructions from tree-rings in high mountains, our restults of Lake Dalongchi and Lake Gahai suggested that the climate was dry during the LIA in Dalongchi region. It was not consistent with the cold-wet climate condition in adjacent dry basin. However, the climate record in Lake Gahai showed the same variation trend with other records in local mountainous regions. There were signs of turning into wetter conditions in the latest 400 years in both lakes.