Sediment Weathering Process And Driving Mechanism Since Late Pleistocene In Golmud River Basin

Located in the southeast margin of Qaidam Basin,the Golmud River catchment is surrounded by the Qilian Mountains,the East Kunlun Mountains and the Altun Mountains,and has a wide variety of geomorphic types（Such as glacier landform,river landform,alluvial fan landform）.Golmud River carries a large amount of detrital material and element ions into Qarhan Salt Lake throughout the year and becomes one of the sediment sources of the downstream lake.Qarhan Salt Lake,as the Quaternary sedimentary center,has recorded a large amount of paleo-environmental information.Therefore,a systematic study of the weathering process in the upper and lower reaches of Golmud River catchment from the perspective of the whole catchment can reveal the relationship between regional dynamic weathering,westerly and monsoon circulation system and global climate change,explore its driving mechanism,and provide theoretical basis for the salting evolution of the lower reaches of the salt lake.On the basis of previous studies on the sediments from sediment core（ISL1A）in Qarhan Salt Lake,the magnetic susceptibility index of the samples from Qarhan lacustrine core（ISL1A）was analyzed,and the weathering characteristics of the sediments from Nachitai（NCT）and Xiaogangou（XGG）sections in the middle and upper reaches of Golmud River catchment were analyzed by using climate substitute indexes such as magnetic susceptibility and geochemical elements.The relationship between climate change and weathering intensity in this region since late Pleistocene is discussed,and compared with the records of Guliya ice core and other related climate indicators,the following conclusions are drawn:（1）The depth and age frames of the above profiles were constructed by linear interpolation.Based on the sampling depth,the NCT profile was estimated to be formed at 98.93～6.65 ka BP,and the XGG profile was estimated to be formed at 97.64～10.64 ka BP.The bottom age of the ISL1A core in Qarhan Salt Lake was deduced to be 92.70 ka BP.The top age is 9.29ka BP,indicating that these sedimentary sequences were deposited between late MIS5stage and Early Holocene.（2）The data analysis of major geochemical elements showed that the elements in NCT and XGG profiles were mainly SiO₂,Al₂O₃and CaO,and the average sum of their content was more than 80%.The average value of other major elements（Na₂O,K₂O,Mg O and Fe₂O₃）was less than 3%.In the NCT and XGG sections,the composition of SiO₂and Al₂O₃is close to the upper continental crust（UCC）,and the content of CaO is enriched.The other elements are located below the upper continental crust（UCC）,showing deficit.The content of trace elements varies with the change of dry and wet climate.In warm and wet climate,the leaching loss of Sr,Li and B leads to the increase of Rb/Sr.When the climate is dry and cold,no obvious migration process of elements occurs.（3）Both NCT and XGG sections had experienced weak chemical weathering after deposition.According to the comparative analysis of weathering parameters of constant elements,the weathering strength of NCT and XGG sections,the weathering strength is related to the dry and wet change of climate,but the age of NCT section Marine isotope stages 5a（MIS5a）and late MIS3 is consistent with the glacial age,indicating that the weathering strength in this stage is also affected by physical weathering.（4）The abnormal correlation between magnetic susceptibility and Rb/Sr in the ISL1A core samples of Qarhan Salt Lake,the rind of Golmud River indicates that the addition of a large number of aeolian materials in the lacustrine sediments in arid areas can significantly affect the correlation between magnetic susceptibility and Rb/Sr values,and can also be used to distinguish aeolian signals in the lacustrine sediments in arid areas.（5）Since the late Pleistocene,the materials in the Golmud River catchment have experienced multi-stage cycles of transport-sediment-transport in each reach,indicating the complexity of material transport.The weathering process of the basin is closely related to the climatic cycles of glacial-interglacial periods,and in the climatic humid stage（MIS2,MIS3a,MIS5）,The sediment weathering in the Golmud River catchment is mainly driven by the monsoon,mainly chemical weathering,and a large number of salt-forming ions accumulate in the downstream.In the climatic drought stage（MIS4,MIS3b）,the physical weathering is strong,and the westerly is strong.The sediment migration weathering is mainly carried by the wind.