The Physical And Chemial Characteristics Of Late Pleistocene Hard Clay In The Southern Flank Of The Yangtze Delta And Its Paleoenvironment Significance

The hard clay layer in the Late Pleistocene strata of the Yangtze River Delta serves as the most potential layer for paleoenvironmental research in the coastal and continental shelf regions,which has rich,diverse,and superimposed paleo-environmental information,and its research helps to explore the mechanism of interaction between regional climate changes and sea-level changes,revealing the regional response of late Quaternary sedimentary strata in the eastern China in the context of global change.Previous researches have been done on its age,causes,and sources.However,there is a lack of optical stimulated luminescence dating of hard clays,and the study of the later-generation processes and magnetic characteristics of hard clay was not thorough enough.There is also a difference in the regional response of hard clay to climate change.Therefore,the grain size,elemental geochemistry and environmental magnetic parameters were used to analyze the physicochemical characteristics of the hard clays of the borehole FD and TL in the southern flank of the Yangtze River Delta.Combined with the results of the optical stimulated luminescence dating,the origin,source and ancient environment significance of the clay layer were discussed.The main findings are as follows:?1?The clay samples of FD and TL were mainly silt granules?4⁶3?m?,belonging to clay silt.The content of particles>63?m was very small,mainly fine particles.The overall sample separation was poor,the particle size distribution was positively skewed,and the kurtosis value was of moderately narrow kurtosis.The frequency curves of the FD sample showed bimodal distribution,and that of the TL sample were mainly bimodal during the TL-1 and TL-2 phases,while the TL-3 phase exhibited multimodal distribution.?2?The chemical compositions of hard clay of FD and TL pole were mainly SiO₂,Al₂O₃ and Fe₂O₃.The majority of samples have undergone moderate chemical weathering effect under warm and humid conditions,in the stage of early Na and Ca removal,some samples have undergone strong chemical weathering effect under hot and humid conditions,entering the stage of mid-term K removal.Comparing the two hard clay samples with other sediment types,the chemical weathering strength from large to small was:Xuancheng Eolian laterite>FD core upper segment>TL-2>TL-1?TL-3>FD core lower segment>Zhenjiang Xiashu loess?Luyuan shale>Luochuan loess>Luochuan paleosol>UCC.?3?The content of ferrimagnetic minerals in the sediments of the upper FD core was relatively high,containing more fine-grained magnetic minerals.The absolute content of the high-coercive minerals was high in lower FD.The content of ferrimagnetic and fine-grained magnetic minerals in the sediments of the TL-1 stage were higher than that in TL-2 and TL-3.The samples of the FD core have much weakly magnetic substance,indicating that the minerals were mainly incomplete antiferromagnetic minerals like goethite.The main magnetic bearing mineral in the TL core hard clay was magnetite,which may contain a small amount of maghemite.There was a small amount of incomplete antiferromagnetic minerals in the TL-1 stage.The Day plot showed that the magnetic particles of the two drilled samples were dominated by coarse quasi-monodomains.?4?Hard clay has similarities with other typical eolian sediments in terms of particle size,elemental composition,particle size parameters,and frequency distribution characteristics.Particle size CM maps and discriminants are also consistent with characteristics of eolian sediments,indicating that hard clay is wind-forming.Its transporting power is smaller than that of loess and paleosol and is similar to that of the Xiashu loess.?5?The particle-standard deviation curves of the samples showed bimodal or even multimodal distribution,indicating the multiple sources.The wind field data from the past 1000 years showed that the northwest winds prevailed in the high altitude regions,the northerly winds prevailed in winter and autumn near the ground,and the easterly winds prevailed in the spring and summer in the study area,providing possibilities for various sources of hard clay.Therefore,the hard clay should be the result of mixed accumulation of multi-source materials.In addition to receiving dust particles transported from the inland regions of the northwestern China through intense winter monsoon as dust storms,it also received terrestrial sediment particles from the continental shelf of the East China Sea transported by low-level winds.?6?The particle size composition and Rb/Sr values of hard clay<4?m?grain?of TL core were in good agreement with the evolution of the oxygen isotopic and deep oxygen isotope curves of the Guriya ice core,and the clay component was more clearly indicated.However,the magnetic susceptibility index lacked accuracy,so caution must be exercised when using the susceptibility index to reveal paleoclimate.?7?The bottom age of TL hard clay was about 120 ka BP,which is consistent with the paleomagic Blake event?108 ka BP?.Therefore,it can be considered that the hard clay layer is the sign layer of the late Pleistocene bottom boundary in the region.The hard clay layer was formed in the early stage of the late Pleistocene,corresponding to the 5d ice stage of the oxygen isotope curve in the deep ocean,and the indicators changed significantly at this stage,showing that this area was particularly sensitive to the climate response of the 5d ice age period.A strong transgression occurred in the late stage of MIS 3 in this area.The seawater drowned the hard soil layer,and the accumulation of hard clay ended.Hard clay reduction reactions began in the infiltration and leaching of the seawater,turning the original Fe³⁺to Fe²⁺,the soil color from the previous yellow-brown,brown to yellow-green and dark green.