Stratigraphic Sequenceand Weathering Pedogenesis Of The Loess In Mituosi Profile In Yunxian In The South Of Qinling Mountain

As the important information carrier which recorded the past environmental changes, the loess has been used in the study of the evolution of ancientclimate in the global change, especially the concerned record after the Last Glacial Maximum. Located in the south of Qinling Mountain, the Hanjiang River up stream belong to the north rimof northern subtropics and is very sensitive to climate change. Although covered a thick layer of eolian loess in the Hanjiang River upstream, it is limited about the research for the loess deposition.On the basis of fieldinvestigation, Mituosi profile (MTS) was choiced as the object of the study, which in the left bank of Hanjiang River in Yunxian.The physical and chemical propertiesof sediment samplesincluding grain-size distribution, concentrations of chemical elementsand micromorphological features including morphology of quartz, aggregate microstructure, pores, coarse particles, pedologicalfeatures were tested. The stratigraphic age was obtained in the way of optically stimulated luminescence. Based on above experimental data, we studied the stratigraphic sequences, composition and microstructure, weathering pedogenesis of the loess as well as the response to climate change, and obtain the main results as follows:(1) The stratigraphic sequence of the loess in the first terrace of the upper Hanjiang River wasthe top soil(TS)�'Holocene loess(Lo)�'paleosol layer(S0)�'transition layer(Lt)�'Malan loess (Li)�'loess and sand interlaced layer(JH)�'luvial sediments (AL-1) from top to bottom. Combining theoptically stimulated luminescence data, the age frameworks of the loess were established along the upper Hanjiang River. The bottom age of the loess and sand interlaced layer was about 55000 aB.P. and the top boundary age was 25000 aB.P.. The top boundary age of Malan loess (L1) was 11500 aB.P.. The bottom and top ages were about 8500 aB.P.,3100 aB.P. relatively.(2) It is proved the sediments on the first terrace in upper Hanjiang River were the eolian loess with the grain-size distribution, concentrations of chemical elementsand micromorphological features. The loess in the interlaced layer (JH-t) were eolian because its compositions were similar to the Malan loess, but the sand layer (JH-s) was aquiferous as its trait was close to fluvial sediment below. Influenced by the ancient climate change, the grain-size distribution, concentrations of chemical elementsand micromorphological featuresamong the stratigraphic sequences had the differences. It was showed concretely as follows:①The grain-size distribution of the loess in every stratum was given priority with silt. And the grain-size was more finer in the paleosol (So) than that in the loess (L0/Lt/L1). Its grain-size in loess layer was more coarse. What’s more, the order of the mean grain size was So<TS<Lt<L0<L1<JH-t<JH-s. ②The concentrations of chemical elements in the profile had very low variation, which is reflected the consistent composition and strong combination of the loess in the same area. Easymigratory elements (Ca, Sr et al) and stable elements (Al, Cu et al) showed oppsite changes along the sequence. The easy migratory elements presented the low content in the paleosol So, and the high content in the loess (L0/L1). But the stable elements wereon the contrary.③ Among the micromorphological features, the quartzgrains mainly were subangular or subrounded, with dish-shaped pits, pockmarked pits that were characteristics of aeolian origin. Conchoidal fracture and triangle marketc of the aqueous sign were observed on the sands of the interlaced layer, which indicated different origin from the loess above. The subangularor angular quarz grains increased and roundness became worse from S0�'Lt�'L0�'L1. Siliceous deposition and SiO2 dissolution or etching which were chemical characteristics increased apparently on the surface of quartzgrains of paleosol So. The connectionsof skeleton grains along the profile were flocculent cementation�'weak inlaid and semi-cemented stucture�'clot cementation�'inlaid and semi-cemented stucture�'supported weak cementation�' supported weak cementation�'single particle contaction. The pores of the loess were mainly complex stacked forms and mostly were cystic or bubble shapes. What’s more, the sort of porosity was S0>Lt>L0>L1. The coarse particles increased, while the roundness and the value of C/F10μmdecreased in turn from S0�'Lt�'L0�'L1. The pedological features of MTS loess were mainly secondary clay, Fe-Mn disseminated forms or Fe-Mn coagulation group. And the secondary calcite was not seen in the profile.④The chroma characteristic, clay content, the medium-coarse sand content of >100μm, concentrations of chemical elementsin the paleosol of 150～190cm all showed abnormal changes in value and had some differences with the paleosol up and down. What’s more, the roundness of quarz in this layer became worse, the voids increased and cementation also became worse, the coarse particles also increased. It is suggested the climate happened some change that lead to the sediments’properties had more changes.(3) The chemical weathering of the loess in the upper Hanjiang River was in the stage of primary transition to medium, with Ca、Na loss seriously and potassium feldspar changed a little. The order of weathering of the sequence was So>Lt>L0>L1 and it is performanced Specificlyas follows:①The magnetic susceptibility and total iron, free iron, amorphous iron etc all had a good corresponding relation with their values sort was S0>Lt>L0>L1, Which reflected the weatheringof the paleosol So was most strongest. ②The chemical index of alteration CIA and CPA both indicated the chemical weathering of the loess in mituosi was in the stage of primary transition to medium. The seconday weathering products mainly were montmorillonite or illite and it has not reached the strong stage with mainly contained the kaolinite or gibbsite. ③The orders of the three chemical weathering parameters K/Na、Fe/Mg、Rb/Sr were in TS> S0>Lt>L0>L1 and the silicon aluminum coefficient (Sa) contraried to the above four index, which all indicated the weathering of the paleosol So was stronger than the loess (L0/Lt/L1). In addition, It indicated a weak weathering in 150-190cm of the paleosol and reflected the ancient climate had some changes in this period with the abnormal values of weathering indicators of magnetic susceptibility, iron forms, CIA, K/Na, Fe/Mg, Rb/Sr, Sa.(4) The four stages in the evolution of the ancient climate after about 12500 aB.P. in the upper Hanjiang River were:the last glacial maximum (25000-11500 aB.P.), the early Holocene (11500～8500 aB.P.), the middle Holocene (8500～3100 aB.P.), the late Holocene (3100 aB.P.～0). The climate conditions were cold-dry�'cool-dry�' warm-humid�'cold-dry in turn. Besides during the Holocene warm period it existed multiple climate fluctuations, and the weak weathering in 150-190 cm of the paleosol recorded directly the cold event at 5.5 ka.(5) Compared the stratigraphic sequences, grain-size distribution,composition of chemical elements and degree of weathering of the loess in north and south Qinling Mountain, it had obtained some similarities and differences as follows:①The stratigraphic sequences of the loess in two area were consistant with TS�'L0�'S0�'Lt�' L1 from top to bottom. The differences were that the sedimentary under L1 in upper Hanjiang River was the eolian loess and alluvial sand interlaced layer, but it was the loess under L1 in Guanzhong area. The age frameworks of both regions were consistant and the ages of L1, So were also same. ②The grainsize of the loess in the two regions were both mainly composed of silt especially the coarse silt. Their soil texture both was silty clay loam. The particle of Soin the upper Hanjiang River was more fine-grained than the loess-poleosol in Guanzhong area and that of L1in Hanjiang upstreamwas more coarse.This maybe related to the loess of Hanjiang upstream mixed with nearly source materials.③The chemicalelements of the loess in two areas had low variable coefficients, while the compositionwas consistent and uniform. The active elements (Ca, Mg, Sr, etc) of the loess in Hanjiang upstream had reduced than that in Guanzhong area, but stable elements (Al, Mg, Sr, etc) increased relatively. It is reflected that the unstable compositions of the loess in the south of Qinling Mountain had been leached more, where it experienced the stronger leaching.④The weathering indicatorsincluded magnetic susceptibility, chemical index of alteration (CIA), element ratios (K/Na, Ca/Mg, Rb/Sr), eluvial coefficient, vresidual indexshown that the weathering of the paleosol were more stronger than the loess. It is reflected that the loess experienced similar sedimentary process in north and south Qinling Mountains. Howeverthe weathering of the loess inHanjiangRiver upstream was stronger with its higher weathering indexes, which testified the ancient climatewas more moist in south Qinling Mountain.