Compositions And Eolian Dust Contribution Of Recent Lake Sediments In Kala Kul Lake

Lakes are sensitive to climatic and environmental changes, so that the compositions of their sediments play important roles in the study of the evolution of past climate and environment at various geographical settings. Kala Kul Lake?38°25.32'–38°27.57'N, 75°02.27'–75°04.17'E, 3661 m above sea level? is located in the eastern part of the Pamir Plateau and is a hydrologically-open freshwater lake mainly fed by glacial meltwater from the Muztagh Ata. The climate of the lake catchment is dominated by northern hemisphere mid-latitude westerly jet carrying both rain and dust in summer. Owing to little human activity, lake sediment in Kala Kul Lake has been considered to be ideal for reconstructing regional climate and environment history, and even for glacial expansion and retreat. However, until now little is known about the compositions and sources of the lake sediments, as well as their relationship with the westerly climate and its associated eolian dust contribution.Time-series sediment trap has been successfully used to investigate the modern processes of fluxes and compositions of settling particles at seasonal and interannual time sacles. It has been deployed routinely in oceans and foreign large lakes, but little relevant work has been carried out in inland lakes in China yet besides a deployment in Lake Qinghai since 2010.Kala Kul Lake is chosen as the study site for this thesis. Thirty samples of surface sediments?topmost 2 cm? were collected in June to July 2014 across Kala Kul Lake with the water depths ranging from 0.5 m to 18.5 m. Meanwhile, one river sediment and four wind-driven sand samples were collected surrounding the catchment. Additionally, we drilled a KL-4 core in the center of the lake and core sediments of the the top 200 centimeters were used for this work. A McLane Mark 8-13 time-series sediment trap was deployed at the deposional center in Kala Kul Lake from July 2013 to May 2014 for collecting seasonal sediments in order to understand seasonal variations in fluxes and compositions of modern lake sediments. To monitor the hydrological conditions, a CTD?Conductivity–Temperature–Depth? instrument was fixed with the trap.Through the compositions and temporal-spacial variations of grain size, mineralogy, major and trace elements of these sediment samples, the main conclusions were drawn as the followings:1. In Kala Kul Lake, surface sediments can be classified as three types, characterized by fine sand, mixture of fine, clayey silt and clayey silt, and respectively. High contents of albite and biotite but low clays in the fine detrital indicate weak weathering within the catchment, further supported by their low CIA values. Chemical compositions of Kala Kul lake surface sediments are closely related with grain size and lake water depth. SiO₂ contents of the sediments are positively correlated with grian size, whereas higher Al₂O₃, Fe₂O₃, K₂O and MgO contents in clayey silt at deep lake have a negative correlation with grain size, as same as trace elements of Rb, Ni and Ti; Ca is closely correlated with Sr owing to their similar geochemical behaviors. According to the dust climate conditions and the grain size distribution of eolian dust from the Muztagh Ata glacials, there are two major sources for the Kala Kul lake sediments, i.e., the fine sand and coarse silt around 50 ?m size near lake shore are mainly the products of physically eroded rocks from the catchment itself after sorted by strong wind, whereas the well-sorted fine and clayey silt dominated in the deep water area might stem from eolian dust by the westerly jet during the summer. The fine and clayey silt in deep lake therefore is potential for reconstructing the history of eolian dust associated with the westerly jet in central Asia.2. The fluxes of settling particles in Kala Kul Lake from July 2013 to May 2014 show distinct seasonal variation, whose compositions are basically detrital minerals, diatoms, and minor authigenic carbonates. The high fluxes of settling particles mainly occured in August to September, whereas the low fluxes mainly occured in the frozen seasons?November to next April in the following year?. High fluxes during the summer might also be contributed to high eolian dust input associated with the westerly jet, though further evidences are needed.3. Sediments at the top 200 cm core depth from Kara Kul Lake are largely consisted of clayey silt. Following the average grain size distributions of both the modern sediment from the site and recent eolian dust carried by the westerly jet, the top 200 cm core can be divided into nine stages during the past 1400 years, i.e. four stages with coarser grain sizes and five stages with finer grain sizes relative to modern eolian dust ones. The four stages with coarser grains have experienced stronger westerly jet climate comparied with the modern conditions, resulting in more component of 8–32 ?m grains from eolian dust, and vice versa.These results show two major sources of lake sediment in Kala Kul Lake and highlight the contribution of eolian dust associated with the mid-latitude westerly jet to the lake at the modern time and in the past, which is important to withdraw the information of climatic and environmental conditions from the lake sediment. Further investigations are on the way.