Iron Oxide Content And Palaeoclimate Significance Of Wind-dust Accumulation In The Eastern Yangtze River Delta

The accumulation of wind and dust in eastern China is an important part of the Quaternary loess in China.The palaeoenvironment information contained in the dust is of great significance to reveal the regional response of the late Quaternary sedimentary environment and global changes in eastern China.Hematite(Hm)and goethite(Gt)are important products of soil formation,and the changes of content are related to the changes of wet or dry climate.The development of modern reflection spectroscopy technology makes it possible to quickly and semi-quantitatively retrieve iron oxide content based on diffuse reflection spectroscopy.Previous related research was mainly carried out in the northwestern part of China.Although preliminary work has been carried out in the Xiashu loess in the east,the depth of the research needs to be explored,and related research needs to be recorded to the island loess and hard clay.This article selects island loess and hard clay as research objects,and explores the influencing factors of its diffuse reflection spectrum,analyzes the characteristics of diffuse reflectance spectrum,establishes a calibration equation between spectral parameters and iron oxide content,and inverts Hm and Gt records of relevant profiles.At the same time,the chronological framework established by elemental geochemistry,magnetic characteristics analysis,and photoluminescence was used to explore the paleoclimatic evolution information contained in island loess and hard clay and its response to global climate change.The main points are as follows:(1)Photoluminescence dating results of Qingpu core(QP)hard clay show: There are three photoluminescence age samples at depths of 25 m,26m,and 27 m.The test results are 25.5±1.6 ka,47.1±3.0 ka,and 107.0±4.9ka,respectively.Therefore,the QP hard clay studied in this paper is the second hard clay from the Late Pleistocene.Like the eastern Sheshan Island(SSD)loess,it has been formed since the late Pleistocene.(2)Iron oxide is the main factor affecting the diffuse reflection spectrum.A calibration equation for spectral parameters and iron oxide content is established in this paper,except for the Gt calibration equation in QP hard clay,all other calibration equations have passed self-test and free iron test,indicating that the quantitative results are reliable.The applicability of the Gt calibration equation in hard clay is presumed to be that Hm interferes with Gt more significantly,or it has Gt with larger particles,or it is affected by matrix effects and aluminum substitution,which affects the accuracy of the equation.(3)Elemental geochemical analysis shows that the climate experienced by the QP and SSD profiles is relatively humid.Both calcium-containing and sodium-containing silicate minerals are preferentially removed from the parent material,while potassium-containing minerals are relatively residual,and both are in moderate weathering stage.The comparison of the total weathering intensity is as follows: UCC <Luochuan loess <Zhenjiang xiashu soil ? Luochuan ancient soil<Land source shale <QP <SSD loess <Xuancheng aeolian red earth.(4)The hard clay has a higher Gt content and a significantly lower magnetic susceptibility.It is speculated that due to the effect of transgression,the hard soil layer that was originally in an oxidizing environment is in a reducing environment,and some magnetite and maghemite are converted to weak magnetism.Minerals produce a large amount of goethite,resulting in a decrease in magnetic susceptibility and a change in the hue of the sediment.(5)The average contents of Hm,Gt,and Hm / Gt in the SSD loess are 0.3%,1.1%,and 0.27,respectively.The average contents of Hm,Gt,and Hm / Gt in QP hard clay were 0.1%,1.17%,and 0.1,respectively,and the average content of free iron(Fed)was 1.8%.SSD loess and QP hard clay contain MIS 5 ~ 3 paleoclimate information: about 107 ~ 89 ka BP period corresponding to MIS5 stage,the climate is humid,the weathering effect is strong,and there is a dry period fluctuation in a humidenvironment;about 89 The period of ~ 59 ka BP corresponding to MIS4,the climate is dry and cold,and the weathering is weak.At this time,global glaciers develop,the sea surface gradually declines,the area of the continent increases.About 59 ~ 34 ka BP corresponding to MIS3.At this stage,the overall climate is humid and weathering is strong.The period of about 34 ~ 25.5 ka BP corresponding to the later stage of MIS3 a,and the climate is relatively humid.At this time,the QP area is affected by transgression,which causes the color tone of hard clay to change and shell debris.The high values of Hm / Gt at 39 ~ 36 ka BP,46 ± 4 ka B.P.,and 69 ± 3 ka B.P.may be the response to important cold events in the 35.9ka BP,50 ka BP,and 66 ka BP periods.