Research Of Micromorphological And Pedogenic Environment Of Aggradation Red Earth In Jiujiang

As a typical continental sediment in South China,the aggradation red earth is an important sedimentary carrier to reveal the environmental changes of Quaternary.Previous studies on the aggradation red earth have focused on chronology,stratigraphy,weathering intensity and provenance,indicating that the aggradation red earth have undergone moderate to high weathering intensity since the Middle Pleistocene.However,there are few detailed studies on the soil science under this weathering background,and the mechanism of reticulation is still not clear.This article selects the L-HH profile of Haihui town,Jiujiang City,Jiangxi province.On the basis of field investigation,has carried on the sedimentary section to division,is divided into three units from bottom to top(reticulated red soil unit,reticulated yellow brown soil unit,yellowish brown soil unit),six soil layers((1)reticulated red soil layer、(2)reticulated yellow-brown soil layer、(3)Paleosol layer、(4)reticulated yellow-brown soil layer、(5)coating-yellow-brown soil layer and(6)yellow-brown soil layer).A total of 9 pieces of undisturbed soil samples and 159 bulk samples.Introducing soil micromorphology,using the polarizing microscope identification of soil micromorphology,and grain size,chroma characteristics,magnetic susceptibility and constant element,in the study area aggradation type red earth weathering characteristics of basic framework,based on the soil micromorphological characteristics to further explore the pedogenic characteristics and reticulated mechanism of the aggradation red earth.The preliminary conclusions are as follows:(1)The relative abundance of plagioclase is 4～5%,and the grain size is 10～110μm.Muscovite abundance is 1～5%,size is 30～200 μm.Both have secondary minerals illite,vermiculite,iron oxides to goethite,hematite.The illite abundance is 4～12% and the grain size is 10～50 μm.Vermiculite abundance is 1～5%.Goethite,hematite and other components of iron in the soil matrix,the abundance of 2～5%.In general,the clay colloid is mainly(65～80%),and the colloid content is high(50～70%).There are some characteristics of iron infection.The clastic minerals are distributed in the clay matrix,and the grain size is mainly silt-sand size.However,the ratio of clay to debris was the highest in reticulated red soil(8:2 or 7.5:2.5),followed by reticulated yellow-brown soil(7:3),and the lowest in yellow-brown soil(6.5:3.5).(2)The composition of soil pores and pedological features are different greatly.The yellowish brown soil unit((5)～(6))is dominated by plane voids,channel and chamber voids,with rough channel walls and poor connectivity.The pore area of each kind accounts for about 2～15% of the whole slice area.The width of the plane voids is mostly between 6μm and 8μm,and the diameter of the vughs is mostly between 10μm and 30μm.Diffused clay-coating was the main form of pedological features,and thin iron deposition clay coating was found on some channel walls.Yellow or brown granular iron coating with strong extinction,band structure,150～300 μm in diameter.The dark brown Fe masses were 50～200 μm in size and dispersed in the soil matrix.Yellow or yellowish-brown flaky iron coatings is often attached to the channel walls.The reticulated yellowish brown soil unit((2)～(4))is mainly distributed randomly with vughs,chamber voids and channels.The channel wall is smooth and the connectivity is strengthened.The channels area accounts for about 20% of the whole slice area,and the pore diameter is mostly between 30μm and 200μm.The types of pedological features are rich,with Fe masses,ferromanganese nodules and rhizospheric concentric iron-rings scattered in the soil matrix.The channel wall flaps of iron coatings are developed,and the deposition thickness is increased obviously.The reticulated red soil unit((1))is dominated by channel,and the plane voids are not obvious.The channel and chamber voids are scattered.The voids area is about 15～20% of the whole slice area,and the channel diameter is mostly 200～300 μm.The voids wall is smooth,the deposition coating is thickened,and the multi-layer lamination is characterized by microstratification.Most of them are deposited coatings,ferromanganese nodules and diffuse concentric rings,and the micromorphological characteristics are different at different depths.Exist a large number of iron diffusion ring,the outer ring 250 microns to 600 microns in diameter,diffusion and deposition iron coating is development,ferromanganese nodules are decoloring phenomenon.Channel wall deposition of red brown iron and manganese black coating,manganese coating more superimposed on iron coatings,some of the channels are filled with coating,the substrate of granular iron coatings development.(3)The variation of soil micromorphological along different sedimentary units in L-HH profile indicates the variation of soil moisture in the process of pedological features to a certain extent.Soil micromorphological characteristics are well correlated with grain size,chroma,magnetic susceptibility and chemical alteration index(CIA),which is an important soil index to interpret the aggradation red earth formation environment,monsoon evolution signal and reticulation mechanism.The L-HH profile generally records the climate change process from warm-wet to cold-dry.During the formation of reticular red soil unit,the summer monsoon was strong,the contrast between climate and seasons increased,soil voids developed,and iron migration was active.During the weakening period of reticularization,the soil moisture decreased somewhat,and the change of dryness and humidity was still significant,which was a warm and dry environment.At the end of reticularization,the temperature and precipitation were further deteriorated,the environment was dry and cold,and the winter monsoon was strengthened.(4)Micromorphological evidence showed that there was a good relationship between soil voids and its development degree and reticulation degree.The typical units were reticulated,with well-developed soil channel,good connectivity,large decolorization area,clear boundary.The decolorization area is small,the boundary is fuzzy,and the decolorization degree is low in the weak reticulate interval.No reticulated units,thin voids,poor connectivity,is not conducive to reticulation.The physical basis of reticulation is the generation,widening and connection of voids,while the plant root system accelerates the process of reticulation.