Late Pleistocene Pedogenesis And Palaeoclimatic Records From The Shengshan Island Loess In The East China Sea

As a important geological carrier for paleoenvironmental research,the Chinese loess-paleosol sequence records the evolution information of the East Asian monsoon,which is of great instructive significance for understanding the driving mechanism of Quaternary climate change.The Shengshan Island is located in the subtropical seas of China,and it is an area with a special geographical location in the study of paleoenvironmental evolution.The bedrock of the Shengshan Island is mainly acid granite,and thin layers of aeolian loess are deposited on the granite weathering crust.These loess become ideal materials for reconstructing the evolution of paleoenvironment in the subtropical regions of China.However,due to the differences in geological background,physiognomy evolution,climatic conditions,material sources and many other factors among different regions,the climate proxy indicators applicable to the loess of the Loess Plateau in northern China may not be applicable to the loess of the Shengshan islands in southern China.This study takes the loess of the Shengshan Island in the East China Sea as the research object,establishes the age frame of the Shengshan loess,and selects suitable paleoclimate proxies(environmental magnetism,quartz single mineral,organic carbon isotope and clay minerals),and focuses on the evolution of the East Asian monsoon in the Late Pleistocene.The main conclusions of this study are as follows:1)There is a good correlation among the magnetic indexes of the Shengshan loess,which can be used as a great proxy for paleoclimate.The environmental magnetic parameters show that the magnetic minerals formed in the two periods(55?40ka B.P.and 75?67ka B.P.)are mainly weakly magnetic antiferromagnetic minerals(hematite,goethite),and during this period(67?55ka B.P.),intense pedogenesis produced a large number of superparamagnetic particles(magnetite and hematite).2)In the Shengshan loess section,the change trend of?¹³C after correction of humus removal by weak alkali method is roughly similar to that of the original,but its value is 3‰negative than the original.Using the corrected carbon isotopic composition,it is estimated that the average proportion of C₃ plants in the region exceeds 88.5%,occupying the position of absolute dominant species.Estimates of plant mass using carbon isotopes from the whole sample would underestimate the abundance of C₃ plants in the area.3)There is strong weathering and pedogenesis in the subtropical regions of eastern China.One of the manifestations is that the quartz grain size has increased significantly since the deposition of the Shengshan loess,while the grain size of the whole sample is relatively uniform,which may be related to the fragmentation of the particles due to weathering and soil formation in the later period of deposition;The second is that the?¹³C value of the Shengshan loess carbon isotopic composition after the removal of humus is between-26.51 to-24.79‰,while the original organic carbon isotopic composition is significantly heavier,between-24.56 to-20.78‰.In addition to the carbon isotope fractionation caused by plant species,there are also strong microbial activities in the profile.The selective utilization of carbon isotopes by these microorganisms in warm and humid environments also causes carbon isotope fractionation.The third manifestation is that the average content of kaolinite in the profile is more than 10%,and kaolinite is often formed in a high temperature and humid strong weathering environment,indicating that the Shengshan loess experienced strong weathering and soil formation during the formation period.4)The results of the climatic proxies for the Shengshan loess are consistent with other regional climate records(loess on the China Loess Plateau,deep-sea sediments,etc.):In the middle of the late Pleistocene(75?60ka BP),the East Asian summer monsoon and the winter monsoon showed a positive phase relationship,and both the summer monsoon and the winter monsoon showed an intensified phase relationship.In the middle and late late Pleistocene(60?40ka BP),the intensity changes of the East Asian summer monsoon and winter monsoon showed an obvious negative phase relationship.At this time,the summer monsoon weakened and the winter monsoon continued to strengthen.In a word,in the middle and late Pleistocene,the East Asian winter monsoon and the summer monsoon showed an inconsistent phase relationship.The reason is that the driving mechanisms of the winter and summer monsoons are different.The intensity of the summer monsoon is affected by the variation of the summer solar radiation in the mid-latitudes of the northern hemisphere,while the winter monsoon is driven by the amount of ice sheet and ice volume.