Pinnoite Deposit Of Da Qaidam Salt Lake: Forming Environment And Associated Hydroclimate Change

Da Qaidam Salt Lake is located at the northern margin of the Qaidam Basin. It lies in an intermountain basin of the southern Qilian mountain belt. As the lake and its catchment are controlled by the climate of prevailing westerlies, the sediment sequence formed in the closed-basin lake is ideal for the study of the evolution history of the westerlies. Da Qaidam Salt Lake is also known to its boron deposit. Although investigation to the deposit started since 1950 s, the pinnoite deposit in the subbottom sequence of the salt lake is yet not studied until the last several years. Investigation on the sediments deposited below the pinnoite layers so far is not carried out even though it is extremely important for the understanding of the forming environment of the ore deposit. The thesis focused on the study of subbottom sediment section, DCD03, located at the middle of the salt lake(37°50′26″N, 95°15′48″E). Carbonate composition, Loss-on-ignition, and grain size are used as paleoenvironmental proxy records. Synthesized analyses of the proxy records are carried out for the reconstruction of lake level, water chemistry, and paleoenvironmental conditions prior to the formation of the pinnoite deposit. The study aims(1) to better understand the forming environment of the pinnoite layers associated with regional hydroclimate change, and(2) to find more evidence for unravelling the mechanism of the pinnoite formation.Three distinct sediment units are identified for the sediment section of the DCD03 based on lithological feature, carbonate composition, and Loss-on-ignition. Unit I is the grayish-yellow silty clay(423~623 cm) deposited approximately during the period of 6600-9700 a BP. Unit Ⅱ is the gray black silt mixed with nodules(407~423 cm) formed roughly between 6300 and 6600 a BP. Unit Ⅲ is the pinnoite deposit(395~407 cm) deposited during the time period between 6000 and 6300 a BP. The result shows that Da Qaidam Salt Lake had been a non-salt lake facies sedimentary environment that dominated by detrital deposit for a long period of time before subbottom pinnoite deposited, until small amount of gypsum began to form in upper sediment(20 cm) of UnitⅠ. After that, it rapidly went into sulfate-type salt lake sedimentary stage(Ⅱ) and subbottom pinnoite sedimentary stage(Ⅲ). The increase of organic matter LOI in subunitⅠ1,Ⅰ2 andⅠ3 implies a steady increase of air temperature, resulting in precipitation increase in mountainous area and evaporation enhancement in lake region. Consequent changes in water balance and water chemistry of the lake may in a way associate with the sedimentary conditions favorable for the deposition of the pinnoite layers which are AMS 14 C dated to 6300-6000 a BP、4150-3600 a BP.