Boron Isotopic Characteristics And Its Geological Significance In The Sediments Of East Taijnar Salt Lake,Qaidam Basin

Boron（B）isotopes have been widely used to study the formation and evolution of sediments due to their significant fractionation characteristics,and there have been a large number of successful cases in the fields of paleoclimate reconstruction,pollution source tracing and deep hydrothermal source tracing.The sedimentary strata of salt lakes are mainly composed of rock salt layers and the interlayers dominated by debris.The salt layers are mainly composed of salt minerals,and it is difficult to use traditional sedimentary stratigraphy analysis methods,such as palynology,¹⁴C,U/Th,etc.Boron isotope analysis can overcome these problems,which is expected to carry out paleoclimate reconstruction in the field of salt lake paleoclimate.In this paper,we take the East Taijnar Salt Lake（Dongtai Salt Lake）in the central and southern Qaidam Basin as the research subject,and analyze the main and trace elements,X-ray diffraction（XRD）and isotope analysis of sedimentary strata in the lake area to study the dynamic changes of element and mineral composition,lake salinity and boron isotope in the Salt Lake area.The results show that the sedimentary environment of Dongtai Salt Lake has been significantly affected by climate change since the late Pleistocene,and the contents of salt elements and minerals andδ¹¹B（boron isotope ratio）in the sediments have significantly changed.Based on the research results of geochronology and element geochemistry of Dongtai Salt Lake,the climate evolution characteristics of Dongtai Salt Lake since late Pleistocene were analyzed.The results show that the sedimentary environment of the Salt Lake has undergone three evolutionary stages,specifically:（1）Late Pleistocene to Middle Holocene:climatic drought periodThe analysis results show that the mineral composition in the section of 13.4～7.9m is mainly stone salt,the content of which is more than 90.0%,and the rest is mainly gypsum,carbonate and clastic minerals.The average content of B is 160.81μg/g,and the mean value ofδ¹¹B is-5.18‰in the range of-3.13～-8.52‰.Results show that the climate in the area drought that ling,the Nalinggele river as the main supply of Dongtai Salt Lake river in the small runoff,terminal lake recharge is far less than evaporation,in that lake rock salt mine a large amount of precipitation.At the same time,due to the decrease of river runoff and transport capacity,the content of detrital minerals and carbonates in Salt Lake sediments is low.In addition,theδ¹¹B value at this stage is low because the boron isotope fractionation between salt lake brine and stone salt minerals is small.In general,the Salt Lake was in the evolution stage of chloride type.（2）Middle Holocene to Late Holocene:climatic humid periodIn the sedimentary stratigraphic profile at the depth of 7.9-0.4 m,the mineral composition is dominated by clastic minerals,accounting for～69%,and the content of gypsum,carbonate and halite is low（<10%）.The average B content is 410.81μg/g,and theδ¹¹B value is in the range of-7.12 to 7.93‰,with an average of-1.13‰.Compared with the results of 13.4-7.9 m,at this time,the climate in the study area was humid,the runoff of the Nalinggele river was large,and the transport capacity was strong,which led to a rapid increase in the content of debris minerals in the salt lake sediments,the water in the lake area was desalted,the content of stone salt minerals was sharply reduced,and the brine changed from acidic environment to alkaline environment,and the clay minerals in the alkaline environment were mainly adsorbed ¹¹B,¹⁰B is enriched in brine,so B is reverse fractionated and occurs in sediments,and theδ¹¹B value in sedimentary strata is large.In general,the Salt Lake was in the sulphate evolution stage during this period.（3）Late Holocene to present climatic drought periodIn the sedimentary stratigraphic profile with a depth of 0.4～0.1 m,the mineral composition is mainly stone salt,accounting for more than 90%,and the rest is mainly gypsum,carbonate and clastic minerals.The average B content in the sediment is significantly reduced to 5.82μg/g,resulting in theδ¹¹B value not measured.Because the interval is located in the surface of the lake area,stone salt minerals are formed in large quantities,which indicates that it is related to the recent continental desert climate of Qaidam Basin.Due to the thin range,the element content,mineral composition and boron isotope characteristics of the sediments are not clear,which is difficult to be used to reveal the sedimentary environment of the lake area.In general,the Salt Lake in this period is in the evolution stage of chloride salt lake.In conclusion,Dongtai Salt Lake has undergone reverse（chloride-sulfate type）and forward（sulfate-chloride type）evolution of Salt Lake deposition since the late Pleistocene.The lake water is in the process of evaporation and concentration under the influence of cold and dry climate from the Late Pleistocene to the middle Holocene.Under the influence of warm and wet climate conditions in the subsequent period,the lake water was in the process of desalination.In the late Holocene,the lake affected by evaporation,the corresponding relationship between sediment and climate change is weak,on the whole,in the process of evaporation concentration.