Varve Chronology And Late Holocene Environmental Changes In Sugan Lake, Northern Qaidam Basin

Late Holocene is one of the important study periods of IGBP-PAGAES. High resolution climate records and its relative climate change studies are also highly focused by CLIVER and other programs during the past 2,000 years. Chaidam Basin is located along the boundary between Tibet Plateau and Northwest arid area so that the higher/cold and dry climate become the climate characters in this area therefore it is very sensitive to the climate and environmental changes. This paper discusses the research carried out on the Lake Sugan, in the northern margin of Chaidam Basin. The climate and environmental changes in Lake Sugan drainage area over the past 2670 years were studied using the varve dating of the well preserved lamination sediments in core SG03I taken from the center of Lake Sugan, the study of varve formation in modern lake system and modern process study of carbonate and oxygen isotopes of the water within the drainage area, and according to the main index of the stable isotopes of carbonate sediments synthesized other index analysis. Concerned with other records from this area, the forcing mechanics of Late Holocene climate changes in this area is discussed and the following conclusions were drawn:1. Detailed examination of sedimentary cores retrieved from Sugan Lake in the northern Qaidam Basin of northwest China's Tibetan Plateau reveal that fine laminated beddings form in the sediments where water depth exceeds 3 m. Seasonal surface sediments trapped at the bottom of the lake suggest that sediments deposited during summer and autumn are mainly light colored monohydrocalcites, while those deposited in winter are dark organic matter, indicating that varve layers form under modern limnological conditions. Continuous varve sediments comprising four types have accumulated in the upper 5.5 m of Core SG03I from the center of the lake. All types exhibit clear seasonality indicative of annual deposition. Varve counts correspondence with 210Pb dates on recent sediments in the upper core suggest the continuous varves of the upper 5.5 m of the core formed in the late Holocene (2670 B. P). The Sugan Lake varve sequence is the first demonstration of annually laminated sediments reported in arid western China. Using this varve dating results, we calibrate the other dating methods (radiocarbon dating and U-series dating) which were carried out in Lake Sugan. It was found that in the Little Ice Age with clastic varve type, radiocarbon dating has 2630 years radiocarbon reservoir effect, while beyond the LIA, the radiocarbon reservoir changed as the time passed. Influenced by the exterior source input of the detrital matters, U-series dating using aragonite is not suitable to the study of Lake Sugan sediment and a further study is required.2. The isotopes study of precipitation-groundwater-lake water system in Lake Sugan drainage area indicates that in modern climate system, the precipitation in Lake Sugan drainage area is mainly supplied by the water vapor carried by Westerly. Water in Lake Sugan is supplied by the groundwater and stream infiltration from shallow groundwater and all of them are supplied by the melt-water of Qilian Mountain. Inthis situation the hydrological changes of the lake is the primary reason for theδ¹⁸O changes in lake water.δ¹³C_TDIC of lake water reflects the exchange time between the lake water TDIC and CO₂ in the atmosphere and its changes indicate the water volume changes in a relative large time scale. Oxygen and carbon isotope of the carbonate sediments separately indicate lake waterδ¹⁸O andδ¹³C_TDIC,δ¹⁸O indicates the volume of the input water which is a good index of Lake water level changes;δ¹³C_TDIC indicates the degree of the exchange between the lake and the atmosphere and indirectly indicates the ice cover time controlled by the temperature.3. Environmental changes in Lake Sugan drainage area during the past 2670 years reconstructed by mul-index synthesis analysis indicates that during 679-200B.C. the climate appears relatively warm and humid; during the period from 200B.C.-800A.C hydrodynamics has become weak and the lake water level lowers down. During 280-200B.C. occurrs a cold and dry event; during 800-1200A.D. it is the MWP of Lake Sugan area where climate appears warm-arid and stable; during 1200-1830A.D., the Little Ice Age, appears abrupt event of hydrological condition where we come across extreme negative oxygen isotope phenomenon which indicates that the lake water level is relatively high suggesting the cold and humid climate. But during the 1400-1550A.D. occured a dry event under the cold climate conditionThe carbonateδ¹⁸O records of Lake Sugan and the Dunde ice core are under the same climate system and both of them incidentally show the same increasable trends over the past 2,000 years. Accumulation of Dunde ice core and the thickness of the varve show a good correlation. However, as a drainage area system, high temperature leads to the increase of melt-water while the increase of precipitation will increase the lake water level which leads to more negative results of lacustrine isotope. The thicknesses of annual varve layers suggest the increase of the sediment flux. Therefore oxygen isotope in Lake Sugan and the oxygen isotope in Dunde ice core are two different records within the same climate; when the tree ring records from Qilian Mountain-Chaidam Basin were compared with varve records in Lake Sugan, it was found that the tree ring records did not show the distinct phases of low frequency changes as found in Lake Sugan, but showed the coherence on recording the abrupt climate change. For example, both tree ring and Lake Sugan records recorded abrupt changes in 400A.D., 800 A.D. 1200 A.D. and 1400-1550 A.D.. While comparing Lake Sugan with its nearby lakes, it is found that carbonate isotope records in Hala Lake is almost the same as in Lake Sugan. The oxygen isotope of carbonate sediments from Qinghai Lake at around 1200A.D. shows the opposite trend different from the one in Lake Sugan but the chronology is not synchronous. They probably recorded the different sedimentary responses of the two climatic systems.4. The Oxygen isotope records of Lake Sugan carbonate shows the same synchronous increased trend as shown by winter monsoon and westerly during the past 2700, and isotope negative and the increase in the thickness of the varve seen during 1200-1830A.D. in the Little Ice Age, correspond to the prevalence of the westerly but it is not synchronous. The prevalent winter monsoon and westerly might be one of the reasons why the Little Ice Age in Lake Sugan is humid. The solar activities have two stages, before 1280A.D. the stable period and after it the decreasing period of solar activities which correspond the two main periods of Lake Sugan sedimentary records. Therefore, the solar activities may be the primary forcing factor of climate changes in Lake Sugan area.