Boron Isotope Fractionation Of Carbonates In Salt Lakes And Its Tracer To The Evolution Of The Qarhan Salt Lake

The boron isotopic compositions of marine carbonates have been widely used to calculate the past ocean p H values,which is an effective means for estimating paleo-atmospheric CO₂concentrations and reconstructing global climate change.Abundant carbonates deposit in salt lakes.However,studies on boron isotope geochemistry of brine-carbonate systems in terrestrial salt lake systems are rare,and the question that how the boron isotopic compositions of carbonates in lakes respond to the sedimentary environment is still inconclusive.To remedy this,a boron isotope analysis was performed on autogenetic carbonates from the ISL1A core with precise chronological control in Qarhan Salt Lake?QSL?in this study.Combined with the reported geochemical data of the lake brines,the boron isotope fractionation of brine-carbonate systems in salt lakes was discussed in detail.Furthermore,the boron isotopic compositions(?¹¹B)of carbonates in the ISL1A were applied to trace the paleohydrological conditions of the QSL.At the same time,the micropaleobios,mineral assemblages,particle size of the sediments and strontium isotopic compositions of the carbonates were analyzed to deepen the understanding of the evolution process of the QSL.The conclusions are as follows:1.At the early stage of a lake evolution,p H controls the degree of boron isotopic fractionation between carbonate and lake water.The p H values increase with the increase of salinity,and then,the?¹¹B values of the carbonates increase accordingly.When the lake evolves to a sulfate or chloride-type salt lake,along with the evaporation process,the salinity and?¹¹B values of brines increase,while the p H and p K_Bvalues of brines decrease.Due to the simultaneous decrease of p H and p K_B,the?¹¹B values of the carbonates are mainly affected by the?¹¹B values of the brines,rather than p H.Boron isotopic fractionation in a brine-carbonate system conforms to the Rayleigh fractionation model.Therefore,regardless of the early or late stage of lake evolution,the?¹¹B values of carbonates increase with the increase of salinity.Unlike the?¹¹B values of marine carbonates,which is a good indicator of seawater p H value,the?¹¹B values of carbonates in lakes are only influenced by p H values at the early stage of the lake,and have limited influence by p H values in the sulfate or chloride-type salt lakes.2.The ?¹¹B values of carbonates in the ISL1A core range from-11.3‰to+9.94‰.The?¹¹B values?-11.3‰to-4.15‰?of the lower section are significantly lower than those?-4.25‰to+9.94‰?of the upper section.The?¹¹B values of carbonates significantly increase at 46.9 m.This change clearly indicates a sudden change in the salinity of the QSL.Meanwhile,the ostracod assemblages indicate that the Qarhan paleolake was a shallow freshwater-brackish lake before?49.8 ka?46.7m?.In detail,the ostracod was Limnocythere inopinate at 46.9?46.7 m,indicating that the salinity of lake water was less than 14‰.Then,the ostracod experienced a sudden extinction at 46.7 m.Therefore,the micropaleobios also indicate a rapid salt formation in the QSL.Previous studies speculated that the sudden formation of the halite in the QSL was related to the sudden increase in paleohydrological supply.However,the content of detrital minerals and the particle sizes did not show a sudden increase,which disproved the sudden increase of the runoff in this study.3.The ⁸⁷Sr/⁸⁶Sr of carbonates in the ISL1A core were distributed between0.71049?0.71178.In detail,the Sr isotopes of carbonates ranged from 0.71071 to0.71116 before halite precipitation,which were significantly lower than those of the brines?0.71120?0.71201?and carbonates?>0.7110?from the upper evaporite layer in the QSL.The comparison shows that the ⁸⁷Sr/⁸⁶Sr of the lake water experienced an obvious increase when the QSL were formed.As a result of the weakening chemical weathering ratio when the halite precipitation,the change of water sources should be the main reason that increase the ⁸⁷Sr/⁸⁶Sr ratio in the QSL.Combined the Sr isotopes of different geological bodies with the boron isotopes of carbonates,ostracod,particle size,and mineral assemblages of sediments from the ISL1A core,it is inferred that the sudden formation of the QSL may be related to the rapid withdrawal of waters from the Qilian Mountains.