Characteristics And Mechanism Of Reticulation Of Laterite In Southern China

The reticulate laterite widely distributed in the south of China contains the information of the formation and evolution of the Earth’s rings,and is a good carrier for the study of Quaternary climate change.The distinctive reticulate structure has attracted the attention of many scholars.Up to now,the predecessors have done much research on its formation age,material source,physical and chemical properties,environmental significance and so on,have achieved fruitful results.However,there are still some problems to be solved:the formation mechanism of the reticulate is controversial,and the study of the reticulate morphology is relatively weak,especially the quantitative expression of the reticulate morphology,whether the reticulate morphology has an indicative significance to the climate.Based on the previous research and field investigation,the reticulate laterite profiles in Langxi,Xuancheng,Anhui Province and Tangxi,Zhejiang province are selected in this paper.Using high resolution digital camera to take original profile pictures in the field,using Photoshop and Pores and Cracks Analysis System（PCAS）to segment the sample square,extract the mesh and calculate the mesh number,average area,maximum area,ensity,average length,average width,average perimeter,maximum length,maximum width,fractal dimension and so on,the morphological characteristics of reticulate pattern and its dynamic changes on the profile were expressed quantitatively.The red matrix was separated from the white mesh at the two locations and the geochemistry,chroma,granularity,magnetism,p H value and water content were tested and analyzed respectively.By using principal component analysis and correlation analysis,the formation mechanism of reticulation was discussed and the conceptual model of reticulation was constructed.The main conclusions are as follows:（1）In morphological characteristics,the meshes in the upper part of the Langxi section（230～440 cm）are small in area,small in number and low in density,and small and sparse in density,while the meshes in the middle section（440～740 cm）are larger in area and higher in density,the quantity fluctuation is reduced（but more than the upper）,the mesh is large and sparse,the length,width,perimeter and area of the lower mesh（740～840 cm）are increased,the density is increased,the mesh is thick and relatively dense.With the deepening of the section depth,the complexity of the reticulate pattern gradually increases.In the Tangxi section,there are more meshes in the upper part（50～90 cm）,smaller in size and area,lower in density,denser and smaller in development,no obvious changes in the middle part（90～290 cm）,and a gradual decrease in the number of the lower meshes（290～370 cm）,with the increase of the size and area,the density fluctuation increased,and the reticulate development was sparse and coarse.With the deepening of the section depth,the reticulate pattern becomes more complex.（2）The physicochemical characteristics of red matrix and white reticulate in the Langxi and Jinhua sections are basically the same.In terms of geochemistry,Si O₂,Al₂O₃ and Fe₂O₃ were the main components,and soluble components such as K₂O,Na₂O,Ca O and Mg O were strongly leached.The contents of Fe₂O₃and Mn O in the white reticulate matrix were significantly lower than those in the red matrix,while the contents of Si O₂ were higher than those in the red matrix.The difference in element composition between the two groups indicates that the elements of the homologous substances in the later stage of the soil formation process are differentiated during the chemical weathering process.The content of TOC in white reticulate matrix was higher than that in red matrix,and the content of TOC in red matrix and white reticulate matrix decreased with the decrease of the depth of profile.There is a significant difference between red and white reticulate color,the former is high a*and b*,low L*,while the latter has the characteristics of low a*and b*,high L*.As the depth of the section deepens,the a*fluctuation of the red matrix increases and the L*fluctuation decreases,while the a*and b*curves of the white reticulate matrix are very similar,and the a*and b*fluctuations decrease in general,while the L*fluctuations increase.The color difference value（ΔE）of both decreased from the bottom to the top of the profile.Compared with red matrix,the content of fine particles in white reticulate was lower and the particle size was coarser.The content of fine clay in red matrix and white reticulate increased along the depth of profile.The content of magnetic minerals in red matrix is higher than that in white reticulate matrix,and the former is mainly antiferromagnetic minerals,the content increases with the depth,while the latter is lower.p H showed that both white reticulate and red matrix were acidic,and the water content in white reticulate was higher than that in red matrix.（3）The weathering parameters,such as CIA,BA,SA,SAF,Na₂O/K₂O and so on,revealed that the weathering of red matrix and white reticulate experienced medium-intensity chemical weathering,and the weathering intensity decreased gradually from the bottom to the top of the profile,the climate changes from warm and wet to cold and dry in the course of soil formation.（4）Among the morphological parameters,there are some correlations between the morphological indexes and physical and chemical indexes.Principal component analysis of Langxi and Tangxi profiles showed that the variation of morphological indexes of reticulate was greater than that of physical and chemical indexes.There are some correlations among the morphological indexes,principal component scores and weathering indexes,which may indicate that the meshwork is formed under the humid and hot climate background.The development of reticulate pattern is affected by climate,but the index of reticulate pattern is not significant to climate,and it is difficult to reflect the fluctuation of climate.The quantitative study of reticulate pattern is helpful to enrich our knowledge of reticulate characteristics.（5）The difference of material base in the upper and lower parts of the weathered profile is the internal factor that influences the development of reticulate,and the reticulate red soil is the product of the humid and hot climate environment,and the alternating dry and wet climate conditions are the external driving force of the development of reticulate,the difference of local environment and the feedback mechanism lead to the fluctuation of morphological and physicochemical characteristics in the vertical direction of the profile.