| The intense uplift of the Tibetan Plateau since Cenozoic has remarkable effects on the regional and global climate,which could be viewed as the driver and amplifier of global climatic changes.Thus the uplift of the Tibetan Plateau and its environmental effects is one of the long-standing problems in geoscience.The thick sediments in the internal and surrounding basins of the Tibetan Plateau are derived from uplifted landmass after a series of surface processes involving weathering,erosion,transportation and deposition.During the above processes,the changes in climate or environment will alter the physical or chemical characteristics and compositions of sediments.Therefore,the sedimentary records of the basin can well reconstruct the history of regional paleoenvironment evolution.The Jiudong Basin,located in the northeastern margin of the Tibetan Plateau,is an important Cenozoic sedimentary basin in the Hexi Corridor.There is a thick layer of fluvial and lacustrine sediment deposited in the basin since the Quaternary and these deposits preserved substantial information about the climatic changes.In this study,we analyzed the clay mineralogy of the core sediments(DWJ)in Jiudong Basin to reconstruct the regional paleoenvironment change.The study found that:1.The clay minerals of the sediments includes illite(I),chlorite(C),smectite(S)and kaolinite(K),all of which were detrital originated and reflected the paleoenvironment of their source areas.2.The clay minerals of the sediments in core DWJ are dominated by illite,while the smectite,chlorite,and kaolinite has low percentage,suggesting the paleoenvironment was arid and with phase changes.3.Based on the clay mineral contents,crystallinity,chemical index and ratios of clay minerals,we divided the paleoenvironment into four stages:Stage A(1.7~1.3 Ma).In this stage,the content of illite is high while the content of smectite and crystallinity of illite have low values.Further,the ratio of(S+K)/(I+C)shows a significant decreasing trend,which is consistent with the trend of smectite.These characteristics indicate the source areas of sediments in Jiudong Basin were dominated by arid climate during this stage.Stage B(1.3 to 1.0 Ma).In this stage,the content of illite is less than that in the stage A,but the content of smectite,crystallinity of illite,chemical index of illite and the ratio of(S+K)/(I+C)all show increasing trends.These characteristics indicate the source areas of sediments in Jiudong Basin became humid compare with former stage,resulting in a notable chemical weathering and hydrolysis processes.Stage C(1.0-0.3 Ma).In this stage,both the illite and chlorite shows significant increase,and accompanied by the decrease of smectite,kaolinite,crystallinity of illite,chemical index of illite and the ratio of(S+K)/(I+C).These characteristics indicate the source areas of sediments in Jiudong Basin changed to arid environment again.Stage D(0.3 to 0.16 Ma).In this stage,the content of the illite decreased while the content of smectite increased.Both the crystallinity and chemical index of illite decreased and the kaolinite and the ratio of(S+K)/(I+C)increased.These characteristics indicate the source areas of sediments in Jiudong Basin was more humid than the previous stage.The clay mineralogy of the sediments in Jiudong Basin reflects its source areas were characterized by an arid environment,but with phase changes since ca.1.7Ma B.P.Comparing with the regional environment records,we consider that the drying environment in the Jiudong basin was controlled by the aridity of the Asian inland.The wetting stage was influenced by the East Asian summer monsoon,and the enhanced summer monsoon led to the westward migration of its western boundary,by reference,more precipitation in the source areas of Jiudong Basin. |
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