| As a typical continental deposit in South China,the aggradation red earth is a good geological carrier for Quaternary paleoenvironment study in the middle subtropical region of China.The typical profile of the aggradation red earth is usually composed of the gravel layer,the reticulated laterite layer,the homogeneous laterite layer,the yellowish brown soil layer and other sedimentary units from the bottom to the top.There have been a lot of research results in the aspects of sedimentary characteristics,genesis,physical and chemical properties,formation age,environmental records and so on,but there are still controversies on the provenance,the mechanism of the reticulation and so on.In this paper,20 samples of aggradation laterite in the middle subtropical region are selected.The samples are separated into six fractions,i.e.,<2?m,2-4?m,4-10?m,10-32?m,32-63?m and>63?m by sedimentation and sieve analysis.The REE composition of the bulk samples and six fractions are measured by ICP-MS The REE fractionation characteristics of various samples were analyzed,and then the weathering characteristics and the provenance information of the aggradation red earth were interpreted,which laid a foundation for further understanding the development environment of the aggradation red earth.The preliminary conclusions are as follows:(1)The content of REE in all kinds of aggradation red earth samples is slightly different,but the distribution pattern is similar.The?REE of the bulk samples of reticulated laterite,reticulated yellowish brown soil,homogeneous laterite and yellowish brown soil varied from 105.64?g/g to 244.95?g/g,with the mean values of154.96?g/g,168.71?g/g,156.16?g/g and 224.47?g/g,respectively.The normalized curve of REE chondrite shows a"L"pattern with a negative slope.The curve between La and Eu is steep,the curve between Gd and Lu is gentle,and Eu is negative.According to the characteristics of light and heavy rare earth elements,the LREE/HREE value of yellowish brown soil is the highest(9.64),followed by homogeneous laterite(9.06)and reticulated yellowish brown soil(8.29),and reticulated laterite is the lowest(7.14).Further analysis shows that the LREE leaching degree of reticulated laterite,reticulated yellowish brown soil and homogeneous laterite samples is significantly higher than that of yellowish brown soil,so the LREE/HREE value indicates that there are differences in the weathering degree between samples,among the four types of samples,the weathering degree of reticulated laterite is the strongest,and the weathering degree of yellowish brown soil is the weakest.(2)The REE test of 20 samples showed that the REE had grain fractionation.As a whole,the content of REE in the 2-4?m particle group is the highest;the content of REE in the<2?m,4-10?m and 10-32?m particle group is relatively low;the content of REE in the 32-63?m and>63?m particle group is the largest,especially in the two types of reticulated samples(>63?m particle group content of REE is between 25.71?g/g?596.88?g/g),which means that the provenance of the coarse-grained components of all kinds of samples is complex.At the same time,the difference of REE content in the grain group of>63?m between reticulated laterite and reticulated yellowish brown soil is closely related to the abnormality of Ce content.The former changes from 12.34?g/g to 363.31?g/g,and the latter from 133.45?g/g to 392.72?g/g,suggesting that the geochemical behavior of Ce may be related to the reticular process.?CE is used to measure the abnormal state of Ce element.The?CE value of components>63?m in yellowish brown soil is the lowest,with an average value of 0.94;that of homogeneous laterite is the highest,with an average value of 5.91;that of reticulated laterite and reticulated yellowish brown soil is 3.02 and 2.75,respectively.Based on the principle that Ce is a variable valence element and is easier to be rich in high valence state under strong oxidation environment,it is preliminarily inferred that?Ce value is related to the redox environment of the sediment.The degree of?Ce anomaly in homogeneous laterite is high,and the soil oxidation is the strongest;the degree of weathering of yellowish brown soil is weak,and the degree of?Ce anomaly is not obvious;the environment of oxidation-reduction alternation may be between reticulated laterite and reticulated yellowish brown soil.The REE test results of some samples of"red matrix"and"white reticulum"also showed that the?Ce values of red matrix samples were higher than those of white reticulum samples,and the white reticulum should be the product of local reduction environment.(3)The comparison of?Eu,Eu/Sm,Sm/Nd,La/Nd and other parameters with provenance indicating significance shows that the values of?Eu,Eu/Sm,Sm/Nd of the four types of samples are similar,but there are some differences in La/Nd,and the values of reticulated laterite(1.38)and homogeneous laterite(1.40)are significantly higher than those of reticulated yellowish brown soils(1.27)and yellowish brown soils(1.23),which may indicate that the source of the first two types of samples is not exactly the same as that of the latter two types of samples.The different grain fraction REE test shows that,the?Eu values of all kinds of samples in the grain fractions of<2?m and 2-4?m are very consistent,while the?Eu values in the grain fractions of 4-10?m,10-32?m,32-63?m and>63?m are different,which shows that the provenance of aggradation red earth in the grain fractions of<2?m and 2-4?m is consistent,and?Eu is basically free from weathering interference,but there are provenance differences in the grain fractions of 4-10?m,10-32?m,32-63?m and>63?m.In view of the fact that silt fraction(4-63?m)is the dominant grain size of the aggradation red earth,this paper uses the source index(PI)discrimination method,Based on the characteristic parameters of REE,such as?Eu,(Sm/Nd)_N,(Nd/Eu)_N,(Nd/Yb)_N,(Eu/Sm)_N,(Gd/Ho)_N,(Eu/Gd)_N,etc.,the end element 1 and end element 2 are selected from the Loess and Yangtze River Sediments in the north of China,and the PI values of 4-10?m,10-32?m,32-63?m grain size fractions of the aggradation red earth are calculated.The results are between 0.42 and 0.56,which belong to the mixed source.In contrast,the source of the yellowish brown silt is more affected by the remote source material,PI value is 0.42-0.51,while the reticulated laterite and homogeneous laterite are more affected by the near source material,PI value is 0.50-0.56.The?Eu of the coarse grains>63?m in all kinds of samples varies greatly,which is mainly affected by the near source materials,and the source and composition of the coarse minerals are complex.This is consistent with the differences of Ce element and REE characteristics among the samples.The particle fractionation of REE can separate the weathering and provenance signals,which provides a new research perspective for the study of the weathering and provenance information of REE.(4)The characteristics of rare earth elements in the bulk samples and different grain fractions samples of the aggradation red earth record the Quaternary environmental evolution information in the central subtropical.The characteristics of REE leaching,Ce enrichment,?Eu differentiation and PI>0.50 indicate that the source of REE is complex,and the weathering is strong,which is formed in the environment of redox or high oxidation,indicating that the climate of Middle Pleistocene in the middle subtropical region of China is generally humid or warm.The REE leaching degree of the yellowish brown soil is relatively weak.Compared with the reticulated laterite and the homogeneous laterite,the remote provenance information is increased,which indicates that the climate in the middle subtropical region has turned cool since the late Middle Pleistocene,the East Asian winter monsoon has strengthened and the inland aridity of Asia has intensified.Therefore,the contribution of the remote dust is increased in the yellowish brown deposit in the south of the Yangtze River. |
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