Preliminary Research On Provenance Of Quaternary Red Earth In Dongting Lake Area

This paper selects the Dongting Lake area as the study area, the types of the red earth and their distribution was investigated preliminarily, the author chose one kind of Quaternary red earth with uniform texture which widely distributed in this area as the research object,122samples from10profiles was collected, based on the field observations and the sectional structure, samples were divided into four kinds-yellow-brown earth, reticulate yellow brown earth,homogeneous red earth and reticulated red earth. Grain-size, quartz grain-size and REE features of the four kinds samples were tested, the purpose, on one hand, is to reveal causes and sedimentary environments of the Quaternary red earth through grain size characteristics analysis, on the other hand, is to discuss the sources of the Quaternary aggradation red earth in the Dongting Lake area preliminarily by means of comparing the physical and chemical features of the Quaternary aggradation red earth here to those of the northern loess, Xiashu loess, local sandstone weathering crust, lacustrine sedimentary and Yangtze River sediments. The main results as follows:(1) Grain-size characteristics of Quaternary red earth in the Dongting Lake areas are in good agreement:sand (>63μm) content is the least, its average value0.12%～2.11%; The average content of clay (<4μm) ranges from27.52%to36.91%; while silt (4-63μm) was the dominant grain size, with an average content of62.22%-72.17%, the particles smaller with poor sorting; Grain-size frequency curves mostly are single-peak shape with thin-tailed, these characteristics of grain size are similar to those of northern loess, Xiashu loess and eolian red earth but different to those of fluvial red earth and weathering curst in Dongting Lake area, all these show the Aeolian features of the Quaternary red earth in the Dongting Lake areas and this kind of red earth here belongs to aggradation red earth in the subtropical. And also, particle size distribution of quartz grains also reflects the aeolian features of the Quaternary red earth.(2) Along with the winter dominant wind (northerly wind) from north to south, Md value, Mcp (cp value) of Quaternary aggradation red earth in Dongting Lake area increases (thinning),<2μm component content increased slightly, the wind group (10-63μm) content decreasing trend implies not only the dust which formed Quaternary aggradation red earth comes from the north and weathering intensity of the parent material increases southward, but also explain the formation of the parent material comes from the upwind area of the northerly winds. On a larger scale, grain size spatial trend of loess-Xiashu loess-Jiujiang red earth-Dongting Lake area red earth also shows there are source connections among Quaternary aggradation red earth in Dongting Lake area, Jiujiang red earth(JL), northern loess and Xiashu loess (XG). In addition, based on the wind tunnel experimental results, and take the distance from south to north in Dongting lake area into consideration, Quaternary aggradation red earth here contains a small amount of clay>30μm particle fractions, we can draw a conclusion that Quaternary aggradation red earth in Dongting Lake area is also affected by near-source material in its deposition process.(3) The total REE of Quaternary aggradation red earth in Dongting Lake area has a wider range from96.10to434.37μg/g, the average content171.12μg/g. Compare the REE content of4kinds of samples to each other, the REE content of reticulated red earth and homogeneous red earth are less, but their coefficient of variation are much bigger, which means the REE fractionation of reticulated red earth and homogeneous red earth are more significant. Meanwhile,4kinds of samples have a consistent Chondrite-normalized REE rare earth distribution patterns:slope is negative, La～Eu curve is steep, and Eu-Lu curve is relatively flat, Eu negative anomaly, heavy rare earth (HREE) fractionation, and light rare earth (LREE) relative enrichment, display material composition uniformity. Eigenvalues results show that provenance of the homogeneous yellow-brown earth, reticulated yellow brown earth are different form that of the homogeneous red earth and reticulated red earth; Eu/Eu*value shows this kind of difference within four samples, Ce/Ce*ratio indicates parent material of Quaternary aggradation red earth in Dongting Lake area suffered strong pedogenesis.(4) A comparative study showed that the total REE of the aggradation red earth in Dongting Lake area is similar to that of standard Chinese loess, reticulated red earth in other mid-subtropical area and local sandstone weathering crust; REE distribution patterns of the aggradation red earth here are not only similar to that of standard Chinese loess and Xiashu loess, but also comparable to that of local sandstone weathering crust, Dongting lake sediments and Yangtze River sediments; REE characteristic parameter values of the aggradation red earth in Dongting lake area are close to that of the northern loess, Xiashu loess, local sandstone weathering crust and Dongting lake sediments, the combination of these features speculate the material source of aggradation red earth in Dongting Lake area are not only related closely to the northern loess, Xiashu loess, but also related to local sandstone weathering crust, lake sediments, the Yangtze River sediments.(5) Use Fisher discriminant method to study provenance similarity of Quaternary aggradation red earth in Dongting Lake area, the results show that62.5%of the samples’provenance similar to the northern loess and Xiashu loess and most of these samples are located on upper part of the sections, further analysis indicate that the provenance of yellow-brown earth is closer to the northern loess and Xiashu loess than that of the other three kinds of samples;37.5%of the samples’provenance similar to local sandstone weathering crust, Dongting lake sediments and Yangtze river sediments, most of these samples are located on lower part of the sections, reticulated red earth is the main part. Use Fisher discriminant method again to analyze35samples whose provenance similar to the northern loess and Xiashu loess and21samples whose provenance related to local sandstone weathering crust, Dongting lake sediments and Yangtze River sediments separately, the result implies the provenance of35samples similar to northern loess more than Xiashu loess, and that of the21 samples related to local sandstone weathering crust closely rather than Dongting lake sediments and Yangtze river sediments.In summary, the particle size characteristics of the Quaternary red earth in Dongting lake area are similar to that of the northern loess and Xiashu loess, which indicates Aeolian features of the Quaternary red earth here and its belong to subtropical aggradation red earth; grain size spatial variations of the aggradation red earth and comparative studies in REE features explain that the provenance of aggradation red earth in Dongting lake area is not only related to northern loess and Xiashu loess, but also affected by local sandstone weathering crust, Dongting lake sediments and Yangtze River sediments, further provenance discrimination explain the provenance similarity between aggradation red earth and long-distance transport material are much higher.