Geochemical Characteristics Of B And Cl Isotopes In Soils Of Daigu Geomorphic Area

Because of the consistency of geochemical origin,the unidirectionality of isotope fractionation and the carrying information,B and Cl isotopes can differentiate significantly in regions or processes.They can be used as in-situ or process markers to study the source of tracers and the law of environmental migration and transformation.In the process of rock weathering and soil formation,B and Cl isotope fractionation is easy to occur.Under the influence of different materials and conditions,unique B and Cl isotope characteristic values will be produced.DaiGu landform is a peculiar landform in Yimeng Mountains.DaiGu landform is a new type of landform formed by the interaction of structure,deposition,weathering and denudation as well as various internal and external forces.During the long marine sedimentary process of DaiGu landform formation,a large number of sedimentary layers were formed in DaiGu landform area under the action of flowing water,weathering and gravity.In this paper,the sedimentary profiles of Tianbaoshan National Natural Forest Farm in Linyi City were selected as the research objects.Artificial longitudinal excavation was used.The depth of the profiles was 275 cm.55 soil samples were collected for the study of B and Cl isotope geochemical characteristics and environmental significance.The geochemical characteristics of B and Cl isotopes in sediment profiles in Daigu geomorphic area are revealed by studying the changes of B and Cl isotope composition and geochemical elements in sediment samples,which provides a reliable scientific basis for further study of B and Cl isotope geocycling,migration and transformation of B and Cl isotopes in plants and their biofractionation mechanism.Through this study,the following results are obtained:（1）In this study section,the main negative correlation between pH and B content was found,especially at 130-245 cm and 250-275 cm.There was a negative correlation between pH andδ¹¹B at 10-80 cm,130-245 cm,and a positive correlation between pH andδ¹¹B at 85-125 cm and 250-275 cm.The soil profile is weak alkaline,and the range of pH varies from 7.49 to 7.93.The change of pH affects the form of B element adsorption in soil.With the increase of pH,the relative content of B（OH）₄^-in soil solution increases,and the content of B（OH）₃ and B（OH）₄^-in soil solution changes,leading to the fractionation of B isotope.（2）The distribution of B element is closely related to the content of organic matter.There is a positive correlation between the content of B and organic matter,especially at 0-75 cm,130-185 cm and 190-275 cm.There was also a strong positive correlation between organic matter andδ¹¹B,which showed a strong correlation at0-25 cm and 190-275 cm.Without considering other factors,the variation of the value ofδ¹¹B may be due to the chelation of organic matter with different forms of B and the difference of organic matter type,which results in the difference of the enrichment of ¹¹B and ¹⁰B in the process of B isotope fractionation.（3）The results show that montmorillonite and illite are the main types of clay minerals adsorbing B elements,and the increase of chlorite relative content can also cause the increase of B content to a certain extent.Different types of clay minerals have different charges,which determines the amount of B（OH）₄^-adsorbed.At the same time,the surface of clay minerals will adsorb B（OH）₃,which leads to the change of B content with the relative content of various clay minerals,resulting in different B isotope fractionation.At 10-130 cm,weathering sedimentation is obvious,135-275cm,and weathering is weak.The variation range ofδ¹¹B is-2.10‰⁺1.99‰and+3.07‰⁺7.78‰respectively.The main soil-forming rocks are granite and basalt.（4）Particle size is also one of the factors contributing to the difference of B adsorption.The results show that in this profile,the grain size composition is mainly sand>silt>clay.The relative content of clay decreases with depth,and the relative content of sand increases.A large increase in the relative content of silt also causes the increase of B content.The particle size affects the adsorption of B and the fractionation of B isotopes through the difference of particle size and permeability.The smaller the particle size and the stronger the adsorption,and the smaller the particle size and the worse the permeability.（5）The variation range of B content is 64.37-131.30 mg/kg,and the change trend of B content is increasing from surface to deep layer of sedimentary profile.The variation range ofδ¹¹B is-10.02‰⁺10.73‰,at 5-80 cm,the value ofδ¹¹B is mainlyδ¹¹B<0,and varies greatly.At 85-275 cm,there is a certain correlation betweenδ¹¹B>0,andδ¹¹B content,mainly positive correlation.At 10-40 cm,125-245 cm and250-275 cm,the change of B content was consistent with that ofδ¹¹B,showing a strong positive correlation;at 45-125 cm,it showed a negative correlation.（6）By analyzing the chromaticity,magnetic susceptibility,B content and the numerical characteristics ofδ¹¹B,it is concluded that the change of luminance（L*）value first decreases and then increases,the change of redness（a*）value first decreases and then increases,the change of yellowness（b*）value first decreases and then increases,and the change of frequency magnetic susceptibility first increases.The trend of change decreases further.Through comparative analysis,it is concluded that the cold and dry soil-forming process has the geochemical characteristics of low frequency susceptibility and lowδ¹¹B,while the warm and wet soil-forming process has the geochemical characteristics of high frequency susceptibility and highδ¹¹B.From the surface to the deep,the contents of Ca,Sr,Ba,Cu,Pb and Fe increased as a whole,which is consistent with the overall variation characteristics of B content and the value ofδ¹¹B.Andδ¹¹B can indicate the vertical variation characteristics of Ca,Sr and Ba contents to some extent.（7）Cl content ranged from 5.72 mg/kg to 42.67 mg/kg.The change of Cl content showed a trend of decreasing first and then increasing with the increase of depth.The variation range ofδ³⁷Cl is-0.20‰⁺4.36‰,and the variation ofδ³⁷Cl shows an increasing trend on the whole.The correlation between Cl content andδ³⁷Cl is as follows:at 5-80 cm,130-80 cm,there is a negative correlation;at 85-125 cm,235-275 cm,there is a strong positive correlation.