| The Quaternary reticulate red soil in southern China is the product of specific climate in specific period.The reticulate red soil contains abundant environmental information,and its in-depth study can help to reconstruct the paleogeographic environment.In this paper,the following results are obtained by measuring and analyzing the age,total particle size,quartz particle size,environmental magnetism,paleomagnetism and chromaticity index of Tongling reticulate profile in the middle and lower reaches of the Yangtze River.Preliminary studies have shown that:(1)According to the profile of intuitive color changes and the shape of the textures,from top to bottom,Tongling profile is divided into reticulated Xiashu loess layer,purple red reticulate layer,typical reticulate red soil layer and yellow reticulate layer.According to the results of ESR and paleomagnetism,the age of Tongling is slightly earlier than 0.9 Ma.(2)From the particle size of the components of view,(4～63μm)as the first dominant grain size,the variation of the whole profile content was between 58.5%and 74.2%,the average value was 65.24%.The content of cosmid(<4pm)is slightly smaller than this,and the content is between 23.35%and 40.5%,with a mean value of 33.05%.The secondary cosmid fraction(<2μm)is about 20%this value is much higher than the northern loess.Sand(>63μm)is the least,ranging from 0.02%to 5.91%,with a mean of less than 2%.(10～50μm)content of 31.68%～50.58%,the average value of 40.91%,this value and Jiangxi Jiujiang,Shahe,Zhejiang Anji,Anhui Xuancheng content of little difference,this has some similarity to the northern loess.The full-size particle size distribution curve showed a bimodal and multimodal distribution,showing complex and diverse genesis and material sources.The results show that the clay particles in the whole sample are much larger than that of quartz and the coarse silt 10～50μm contained in the quartz is obviously larger than that of the whole sample.It is shown that the grain size of the quartz particles is larger than that of the whole sample.Because the quartz particles can reflect the particle size of the original dust,it is tentatively speculated that the later period of the red soil has experienced a strong weathered and soiling effect,so that the fine particles in the reticulate red clay are morereflecting the late causes.C-M diagram of the projection area,reflects the late weathering on the red clay particle size transformation.(3)The x value of the lower three layers of Tongling section is very low,ranging from 7.7 to 15.33 x 10-8mkg-1,and the top of the Xiashu loess layer is the highest and the maximum value is 129 x 10-8m3kg-1.From the magnetic parameters HIRM,L-ratio and S-ratios,which represent the magnetic mineral mineral types,the lower three layers of the sections are mainly antiferromagnetic minerals,and they are hematite and goethite usually.The high values of χ,SIRM and S-ratios in the reticulite loess layer indicate that the layer contains a large amount of ferrimagnetic minerals,and the particle type is mainly SP and SD.Thermo-magnetic curve saturation temperature of 630-650℃,and it can be speculated that containing hematite,the analysis results of the hysteresis loop also show that the highly developed reticulate red soil layer is more difficult to magnetize.With the increase of the magnetic field strength,the magnetization intensity increases slowly,and the magnetic field strength is still no satration at 1000mT.Furthermore,the incomplete antiferromagnetic minerals with weak magnetic properties in the intron layer of Tongling section are explained,which is consistent with the result of thermal magnetism.(4)The chromaticity index of the red soil profile of Tongling has fluctuated greatly with depth.The dramatic changes in the chromaticity index of the profile may be related to the climate change events in the mid-renewal period,the strong winter and summer monsoon makes the dry and wet,alternation of the climate in this area more obvious.Because the content of hematite in red soil is closely related to the effect of weathering and soil formation,the typical reticulate red soil layer and magenta reticulate layer have high a*value,which indicates that the two layers of weathered soils are stronger.The results of particle size and magnetism are more consistent.(5)The particle size composition indicates that the Quaternary red soil in Tongling area has obvious dust characteristics,from top to bottom material source has changed,but the material factors on the particle size is not significant,from the study of this article,mainly by the impact of weathering.The results show that there is a certain degree of weathered in the strata of Tongling section,and the typical reticulate red soil layer and purple red reticulate layer have the strongest soil effect.The variation of magnetic parameters and chromaticity index with depth shows that the three layers of magnetic minerals in the lower part of the section are mainly hematite and goethite,and the two minerals are the most abundant in the middle,and thecorresponding weathering is the strongest,Climate high temperature and humidity.The average grain size of the loess layer is reduced,the content of hematite and goethite decreases,the color a*and b*are gradually decreased,and the weathering effect is weakened,indicating that the later development of reticulate red soil to dry and cool excessive.(6)Through the paleomagnetic measurements,it is found that the B/M boundary is located in the middle and lower part of the typical reticulate red soil layer(6.7m).According to the previous studies,it can be determined that the corresponding age is about 0.78Ma,the yellow mesh layer(7.8m)The results show that the particle size,magnetism and chromaticity index of Tongling red clay are strongly changed during this period.The results show that the particle size increases from 10 to 50 μm,and the average particle size becomes coarse and the magnetic parameters HIRM increases,S_ratio decreases,indicating that the hematite content increased,the color value of the dramatic changes in these signals is the response to the Pleistocene revolution,that is,Tongling region of the Pleistocene climate transition period of about 0.9Ma. |
