| The Tibetan Plateau(TP)is the critical region for the intersection and interaction between westerly and monsoon circulation system,which exerts significant influence on the climate of Asia,and even the global atmospheric circulation.Systematical research for the evolution,variation characteristics and driving mechanism of westerly and monsoon is the key to reveal the environmental changes of TP.The lack and even blank of multi-source information in TP limits the development of paleoenvironment study in TP.The paleoenvironment study of TP loess is of great significance to understand the evolution of monsoon and westerly,and provides scientific guidance for predicting the climate change of TP in the future.The Yarlung Zangbo River(YZR)basin is a major feature of the southern TP,loess deposits are sporadically distributed in this region.Previous studies on the chronology and sedimentology of loess in the YTR basin has focused on comprehensive studies,while the paleoclimate study of loess in this region is limited.Therefore,few paleoenvironment proxies of loess have been applied to paleoclimate studies and their paleoenvironment significance is still unclear.Here,we present the results of magnetic susceptibility,grain size,color,geochemical elements,rock magnetism and diffuse reflectance spectrum(DRS)of the XTM loess,summarize the physical and chemical characteristics,discuss the possible magnetic susceptibility enhancement mechanism of XTM loess,preliminarily determine the paleoenvironment significance of grain size,color and the ratio of trace elements.The following results and new findings have been concluded:(1)The alternation of loess and paleosols is also clearly expressed in the magnetic susceptibility records.Paleosols are characterized by consistently lower susceptibility values compared with the loess horizons.Values of χfd% indicate pedogenically produced ultrafine ferrimagnetic minerals do not play a significant role in accounting for χlf variations of the XTM loess.a* and b* are lower in the loess units than paleosol units,and L * is the opposite.The median grain size of loess is generally coarse,mainly consists of coarse silt and fine sand.Compared with XTMB section,the leaching and enrichment degree of trace elements in XTMA section is more obvious.The fluctuation of Sr is the most apparent in XTMA section,and the trend of other trace element curves is basically same except Mn in XTMB section.(2)The magnetic minerals in XTM loess are dominated by multidomain(MD)and pseudo-single domain(PSD)magnetite.The change mechanism of magnetic susceptibility for the XTM loess is complicated and superimposes both the “wind vigor”model and the “in situ gleization” model.The higher magnetic susceptibility of loess is attributed to stronger and more frequent wind during the glacial period.The decrease of magnetic susceptibility is resulted from the transformation from magnetite to iron hydroxides and iron sulphides.(3)The single sample analysis on grain size of XTM loess indicates four components(C1,C2,C3 and C4)can be identified for XTMA section.C1 records the differences about analysis methods of different laser systems.C2 is closely linked to pedogenesis.The fluctuation of C3 can be interpreted as the change of winter upper westerlies intensity.C4 reflects the intensity of surface wind.(4)The correlation between a* and Hm and between a* and Hm/Gt is weak as a result of a* is affected by various factors.Therefore,a* cannot serve as the effective proxy of paleoenvironmental evolution.b* and Gt exhibits a positive linear correlation,and corresponds well with loess-paleosol units,so b* is an excellent proxy for revealing the intensity of pedogenesis.Compared with XTMB section,the leaching and enrichment degree of trace elements in XTMA section is more obvious.Hence,Rb/Sr can be interpreted as an indicator of weathering intensity. |
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