Boron Isotopic Characteristics Of Borehole SG-1b In Jianshan Of Western Qaidam Basin And Its Paleoclimatic Implications

Because of the large relative mass difference between ¹⁰B and ¹¹B,the boron isotope group has a very good advantage in reflecting its geological environment and geological process.In recent decades,boron isotopes have been successfully applied to paleoocean and paleoclimate environment reconstruction,pollutant tracing and chemical weathering research.Salt lake brine in Qaidam Basin contains abundant boron,lithium,potassium and other salt mineral resources.Therefore,it is an ideal natural experimental site for studying boron isotopes.Furthermore,the continuous and complete Cenozoic sediments deposited in Qaidam Basin provide good research materials for studying the fractionation characteristics of boron isotopes,controlling factors and the possibility of reacting with long-time scale paleoclimate and environment.Based on the SG-1b borehole obtained from the top of the Jianshan anticline in the Western Lake Basin of Qaidam Basin,the element content and boron isotope of the water-soluble components in the borehole sediments were analyzed.Combination with the lithological changes,lithofacies characteristics of SG-1b borehole and the published results of paleomagnetic age,mineral content and grain size analysis of borehole sediments,we discussed the the hydrochemical characteristics of the paleolake in the Jianshan during the period of 7.3-1.6 Ma,the characteristics and controlling factors of B isotope fractionation during the evolution of the ancient lakes,and the evolution process of the ancient lakes in the Alkali Mountains indicated by the changes of boron content and boron isotope.The results show that:?1?The Lake salinity curve reconstructed by soluble salt ions corresponds well with the concentration and evolution of ancient lakes revealed by the study of evaporites and carbonate minerals in SG-1b borehole,which fully shows that the soluble salt ions in lake sediments can well trace the evolution of lakes.The concentration and combination of soluble salt ions,together with the changes of evaporite and carbonate minerals,indicate the continuous concentration and salinization process of the paleolake environment from deep to semi-deep lake to shallow lake and coastal lake.?2?The values of?¹¹B in SG-1b borehole sediments are positively correlated with the contents of B³⁺,Ca²⁺,Mg²⁺,Cl^-and Na⁺.The gradual increase of sediment B content and?¹¹B values from the bottom of borehole and the sharp increase in later period are consistent with the changes of water-soluble ion content and mineral and grain size,which indicates that the lake sediment B content and?¹¹B values since late Miocene in Qaidam Basin can well reflect the evolution process of climate and lake water in the study area.?3?The comparative study of boron content and isotope variation in borehole sediments with mineral characteristics,ion content and grain size variation shows that the variation of boron content can well reflect the change of lake water salty,and the variation of boron isotope value is affected by the changes of lake water salty,mineral composition and provenance.The comprehensive analysis of the above indicators,combined with the lithological characteristics of boreholes,indicates the continuous aridity of climate and the gradual increase of salty of lake water since 7.3 Ma,and the intensification of the process of aridity and lake water concentration since 3.3 Ma.?4?The variation of boron content and isotope in SG-1b borehole is generally consistent with the global oxygen isotope record in deep sea,which indicates that global cooling since the late Miocene has an important influence on the climate and environment evolution in the Jianshan area of Qaidam Basin.The growth of the Jianshan anticline has also played an important role in the evolution of the paleolake in the Jianshan area.Based on the comprehensive index analysis results,we divided the evolution of the Jianshan paleolake and the corresponding boron isotope characteristics into three stages since the late Miocene.Phase?:During the 7.3⁴.5 Ma?723³50 m?,the Jianshan anticline is in a deep to semi-deep lake environment,with relatively stable hydrochemical properties and weak hydrodynamic conditions.The value of?¹¹B in lake water may be affected by the adsorption of clay minerals and the co-precipitation of carbonate minerals.Although the value fluctuates,it is generally lower and shows an upward trend in the later period;while the content of boron is relatively stable,but it is generally lower and shows a slightly upward trend.During this period,the lake sedimentary process in the Jianshan anticline area was mainly controlled by climate change.Phase?:During the 4.5³.3 Ma?350²00 m?,the rapid change of hydrodynamic environment from deep-semi-deep lake environment to turbulent shallow lake environment caused by climate drought and the growth of Jianshan anticline,as well as the increase of coarse-grained debris content in sediments and the relative decrease of clay minerals,aragonite and iron dolomite content,may lead to the decrease of the?¹¹B value of lake water affected by clay minerals and carbonate minerals during this period.In general,the value of?¹¹B decreased,while the content of boron increased slightly compared with the previous stage.Phase?:During the 3.3¹.6 Ma?200⁰ m?,the content of B and the value of?¹¹B increased sharply at the same time,reflecting the rapid aridity of climate and the significant increase of lake salty in the Jianshan area at this stage.During this period,the Jianshan anticline continued to uplift and the hydrodynamic conditions of the lake were further enhanced due to the further influence of tectonic activities.Therefore,we speculate that the increase of the value of?¹¹B during this period may be due to the increase of Lake salinity,the increase of fine debris in borehole caused by the sedimentary differentiation of the two wings of the anticline caused by the movement of borehole to the southwest relative to the top of the anticline,the obvious increase of the contents of halite,aragonite and iron dolomite in boreholes,and the significant development of gypsum in some layers.