Clay Minerals And Palaeoenvironment Changes Of Dalianhai Core Since Late Glacial In The Gonghe Basin, Tibet Plateau

A 40.92m long core (DLH) was drilled in a terminal lake, Dalianhai, of Shazhuyu River in Gonghe Basin, Northeast Tibet Plateau. The preliminary chronology of the core was established by AMS ¹⁴C dating on 10 plant remains from the core sediment. In combination with ¹³⁷Cs timescale of previous study, the carbon reservoir effect of 380a was deducted to give the final credible chronology of DLH core since the late glacial. On the basis of this reliable chronology, the evolutionary history of palaeoclimate and palaeoenvironment in Gonghe Basin since the late glacial and Holocene is reconstructed by clay mineral (200 samples with 70a resolution) and other proxies as grain size (200 samples with 70a resolution) , carbonate content (1,029 samples with resolution of 15 a), Petrological face, sedimentary rate and lake retraces as well.The contents, crystallities and ratios of illite, chlorite and smectite can reflect the characters of paleoclimate and paleoenvironment changes very well, whereas, kaolinite doen't obviously. By the mult-proxies analyses of clay minerals with grain size, carbonate, Petrological face and sedimentation together, the history of paleoclimate and paleoenvironment changes of Gonghe basin can be partioned the following stages: the late deglacial, early-middle Holocence and late Holocence. During the deglacial(14.5-11.3 cal ka BP) , the climate was generally dry and cold, with strong physic weathering, and the lake levelwas low with less hydrophytes as indicated by the high content of illite and chlorite and their crystallity, low content of smectite and the high ratios of smectite/(illite+chlorite) and kaolinite/smectite.. However, during this period an abruput climatic change occurred from 12.9-11.3 cal ka BP, where the temperatue and humity were becoming higher, which may correspond to Bφlling-Allerφd events. During the early-middle Holocences (11.3-4.5cal ka BP) , the contents and crystallities of illite and chlorite were low, the content of smectite was high, and the ratios of smectite/(illite+chlorite) and kaolinite/smectite were high, therefore, it indicates that increasing temperature and humidity, which reached Climatic Optimum at about 6.0 cal ka BP. Chemical weathering became stronger, lake water table was rising. At the beginning of this period, lake water was shallow and suitable for plants growth, much peat accumulated. But, in the time from 11.3-10.0 cal ka BP the climate beame cold and lasted for a about 1.0 ka, which may be a transition period. However, after the Climatic Optimum, from 6.0-4.5 cal ka BP the climate was very dry. During the late Holocence (4.5-0.0 cal ka BP) , the climate was similar to that of the deglacial. The cliamate was dry and cold, physical weathering prevailed, the lake water table became shallow again, less aquatic plant grew around lake shore and less peats accumulated.Compared with pollen of this core, clay minerals can reflect climate and environment very well just like pollen. Generally, they both have the same clamitic and environmental phases in long time intervals, especially in middle and late Holocene. In other stages, the change of clay minerals lagged after that of pollen about 1.0 ka.High lake levels of Dalianhai, Gonghe Basin occurred about 25-10 ka of the Last Glacial and Deglacial, and about 8.0-5.0 ka BP of Holocene.In comparison with other lake records and monsoon proxies of Tibet plateau, the records of Dalianhai evidently confirms that the consistency amongst all these records in large phases. During the Bφlling-Allerφd period, the humid climate of Gonghe Basin corresponded with the strengthening of Monsoon in other regions of Tibet Plateau. When the Young Dryas event occurred, the climate began to dry at many sites in the plateau. From early Holocence, Monsoon of the plateau began to strengthen and ameliorated at middle Holocence, which corresponreded to the expansion of forests and humid climate in the mountains around the Gonghe Basin. In late Holocene, the climate of Gonghe basin was dry and cold, that is constent with the flinches of Monsoon, forests and humity. Indian Monsoon played importamt role in climatic and invironmental changes of this area. However, the detail effects of Indian Monsoon and East Asian Monsoon on the plateau and their mechanisms are to be researched.