Content Of Hematite And Goethite In Loess Of Shengshan Island,Eastern China And Evolution Of East Asian Monsoon

The evolution of East Asian monsoon has always been a hot topic in the field of Quaternary paleoclimate research.Hematite and goeiite have direct response to the changes of temperature and precipitation and can be used as a proxy index to quantitatively reflect the changes of the intensity of the East Asian summer monsoon.Diffuse reflectance spectroscopy is an important method for the quantitative determination of hematite and goethite,which can effectively identify many kinds of iron oxides.In addition,the spectral characteristics are not only related to mineral content,but also affected by aluminum substitution phenomenon and matrix effect in hematite and goethite.The loess in eastern China has undergone a relatively intense loess formation process,which has caused a great disturbance to the interpretation of paleoclimate information.Based on the relationship between the content of hematite and goethite and the diffuse reflectance spectrum,the influence of aluminum substitution and matrix effect on the spectral characteristics was further studied in this paper,and on this basis,the quantitative equations of hematite and goethite were determined.Based on the quantitative analysis of hematite and goethite in the loess section of Shengshan Island,eastern China,and in combination with the photoluminescence age,grain size,major elements and magnetic susceptibility,the changes of the East Asian summer monsoon since the Last Glacial Age have been studied.The main understanding is as follows:(1)Aluminum substitution and matrix effect have obvious influence on the spectral characteristics of hematite and goethite.Aluminum substitution has a significant weakening effect on the strength of the first derivative characteristic peaks of hematite and goethite,and the position of the corresponding mineral characteristic peaks has a significant shift.When the two minerals are matrix to each other,increasing the content of hematite can inhibit the first derivative characteristic peak of hematite,while increasing the content of goethite can increase the intensity of characteristic peak of hematite.(2)According to the data of the first derivative,the quantitative function after aluminum substitution and mutual matrix correction is the best.The quantitative effect of hematite and goethite after correction is significantly improved,which is closer to the measured free iron content.There are also local differences in the hematite and goethite content curve of loess profile in Shengshan Island before and after correction.(3)The loess on Shengshan Island has mainly experienced three stages: MIS3,MIS4 and MIS5 a.In MIS3 stage,the overall climate was warm and wet,and the summer monsoon showed a weakening trend,but increased obviously in the later stage,and two rapid cooling events of H4 and H5 occurred,which were consistent with the oxygen isotope change curves of the stalagmite in Hulu Cave and Greenland ice core in Nanjing.In MIS4 stage,the climate was the driest,and the summer monsoon continued to weaken in the early stage,but increased in the later stage,which was in reverse correspondence with the Antarctic ice core.In the later stage,there was an obvious short-term cooling event,which was in correspondence with the stalagmite in Nanjing Hulu Cave,but the H6 event was slightly delayed relative to the Greenland ice core.In MIS5 a period,the summer monsoon was strong,and both MIS3 stage and MIS5 a stage may experience strong chemical weathering,leading to partial dissolution of magnetic minerals.