Soil Formation Process And Environmental Significance Of Libantang Red Earth Section In Xinsheng Basin,Zhejiang Province

Red earth are widely distributed in the middle and lower reaches of the Yangtze River in China,and are receiving increasing attention from the academic community as an important vehicle for reconstructing the climatic environment.In this paper,a typical red earth profile on Pliocene basalt in the Xinsheng Basin was selected and a total of75 samples were collected.By analyzing the grain size,environmental magnetism and clay minerals of the red earth profile,and by combining the current research status of relevant scholars in the region,the process of laterite formation in the Xinsheng Basin and its environmental indication significance were explored,and the following points were realized:(1)Through field observation and indoor experimental analysis,the red earth profile of Libantang in the Xinsheng Basin was divided from top to bottom into:brownish-yellow soil layer(0-150cm)-deep reticulated red earth layer(150-590cm)-shallow reticulated red earth layer(590-750cm).The grain size distribution characteristics show that the grain size in all layers is dominated by chalky sand,followed by clay particles and least sand particles.The fluctuating trend of grain size components from the top of the profile downwards reflects the complex origin of the sediments in the profile and the unstable depositional environment;there are mixed components of jumping and rolling,hostage components and secondary grain size components in the profile,reflecting that the reticulated laterite in the profile is formed by alluvial and flooding.(2)The magnetic parameter characteristics can be used as a good paleoclimate indicator.The ambient magnetic parameters of the red earth profile of the Libantang show that the magnetisation rate is less influenced by superparamagnetism and the profile is dominated by ferrous magnetic minerals.The brownish-yellow clay layer(0-150cm)intensifies soil formation and produces the secondary strong magnetic minerals-maghemite;the magnetic minerals produced in the deep reticulated laterite layer(590-150cm)are mainly weakly magnetic antiferromagnetic minerals(haematite and needle iron ore),while the soil formation at the bottom of the shallow reticulated laterite layer(750-590cm)produces a large amount of ferrous magnetic minerals,and their content gradually decreases as the height The content of these minerals gradually decreases as the height increases.(3)The qualitative and semi-quantitative results of the clay minerals in the red earth section of Libantang show that illite-kaolinite-chlorite and montmorillonite are the main clay mineral compositions in the section,with illite being the most abundant,accounting for 62.19% on average.Kaolinite is 16.95%,and montmorillonite and chlorite have a comparable proportion of about 10%.The clay mineral fraction is dominated by illite,with kaolinite accounting for over 16%,indicating that the profile as a whole was in a warm weathering environment.The relatively high content of montmorillonite in the reticulated red earth layer indicates that the profile was in a lowlying environment and received the influence of alternating wet and dry environments.The crystallinity of illite shows a negative correlation with its chemical index,reflecting changes in the structural stability of illite at different stages of weathering soil formation.(4)The environmental changes reflected in the red earth profiles of the Xinsheng basin are divided into three main phases.In the early to mid-Pliocene,after the basaltic eruption,the climate was warm and wet and soil formation increased;in the late Pliocene,the climate became colder,the illite content in the profile increased,the magnetic mineral grains became coarser,and under the influence of the Quaternary environment,paleosols and weathered crusts that were previously higher up were washed away and deposited there;in the Pleistocene,reticulated laterites developed,the profile was in a low-lying area,and the overall expression of the Pleistocene period,reticulated laterites developed,the The profile is in a low-lying zone and shows an overall alternating wet and dry climatic environment,which is the main reason for the formation of the reticulated laterite;on the basis of the reticulated laterite as a parent body into the Holocene,the climate changed to warm and wet,and the upper part of the profile was modified by the climatic environment of the time,indicating a global warming trend.