Aggradation Red Earth Sediments In Mid-subtropics Of China And Their Recorded Environmental Changes During Quaternary

Global environmental changes and regional responses are widely followed research topics in science areas. Paper concerns about the red earth and environmental changes in south China, chooses the middle and lower reaches of Yangtze River region in mid-subtropical areas of China as the subject, which is the transition zones between south China and north China. This area is a climate change sensitive area. As research carrier, aggradation Quaternary red earth and their paleo-environment records were studied, based on some methods of sedimentology, soil science, environmental geochemistry and chronology. 1460 samples from 7 sections located in Jiangxi, Zhejiang, Anhui, Hunan were analyzed in laboratory, inclusion grain-size measurements, magnetic susceptibility, soil color, PH, main elements, REE, clay minerals, optical microscopic and ESR dating. The main preliminary conclusions are:1. Grain-size distributions of all samples are similar. There is no gravel(>2mm), the range of the grain-size distribution is between 0μm and 250μm generally, silt(4-63μm)is the advantages particle, mean content of all the sections ranges from 48.12% to 71.31%, 10-63μm particle group is the dominated particle, mean content ranges from 26.59% to 45.05%, the first dominated peak appears between 5-6φ(30-15μm), which indicates the high content of dust particle. The content of clay (<4μm) is also high, the average value ranges from 27.57% to 50.1%, which is the secondary dominated particle, and the secondary dominated peak is near 9φ(2μm) or 10φ-11φ(1-0.5μm), which indicates strong weathering process. Grain-size composition, distribution, graphic parameter eigenvalues of aggradation red earth are similar to the north loess, but significantly different from the residual clay which is caused by weathering process in hot and humid weather conditions, which suggests aeolian characteristics from their parent materials and strong weathering process.The grain-size spatial distribution of red earth changes regularly along thesouthern Anhui, Zhejiang north——northern Jiangxi and Zhejiang——Hunan,namely silt content decreases from north to south, but increases in clay content , median particle size and average size become smaller. The silt content and average particle size of Shahe, Jiujiang, Xuancheng, Anji sections respectively become lower and smaller from west to east. These trends correspond with the trend that winter-monsoon gradually weakened and weathering process strengthened southward.2. The red earth elements geochemical characteristics and clay mineral composition of all samples have good consistency. The total content of SiO₂,Al₂O₃ and Fe₂O₃ ranges from 85.19%-92.68%, K₂O,Na₂O,CaO,MgO are leached out in large quantities. The upper continental crust-normalized values for major elements display Ti, Fe and Al enrichment and K,Na,Ca,Mg eluviation. Iron oxide is significantly higher than UCC; there is a significantly negative linear correlation between SiO₂ and (Fe₂O₃ + Al₂O₃), which indicates strong base leaching and desilicification-allitization features. The content of major elements has such a sequence as: SiO₂>Al₂O₃>Fe₂O₃>K₂O>TiO₂>MgO>CaO>Na₂O>MnO>P₂O₅. There are more quartz, mica and kaolin in clay(<4μm), followed by feldspar, hematite and goethite, some samples contain chlorite, illite and illite/smectite mixed layers minerals. The REE total content of red earth ranges from 127.7μg.g^-1 to 275.4μg.g^-1, and average value is 175.1μg.g^-1.REE content of iron-manganese nodules enrichment notabiy, ranges from 385.46μg.g^-1 to 1641.60μg.g^-1, and average value is 823.33μg.g^-1. The REE abundance of "worm-like" white stripe in vermicular red earth is slightly higher than the red matrix. However, the chondrite normalized REE distribution patterns of all the samples are very consistent, the slope is negative, La-Eu curves are steep, Eu-Lu curves are gentler, LREE enrichment-Eu loss.The ⁸⁷Sr/⁸⁶Sr value of representative samples ranges from 0.722001 to 0.727741, the majority of the sample values is greater than 0.725000,¹⁴³Nd/¹⁴⁴Nd value is between 0.722001 and 0.727741. eNd(0) value ranges from -10.54 to -12.66. By contrast, clay REE distribution patterns, Eu/Sm, Sm/Nd, La/Nd, Eu/Eu* (δEu) et al. parameter eigenvalues, discriminant function DF, source index PI values and Sr, Nd isotopic characteristics are all similar with north eolian sediments. The parent materials of aggradation red earth was high mixed by removal, weathering process led to the REE fractionation's complication.3. Take JL red earth section from Jiujiang area as a typcial section, launched ESR study, established the deposition time series and environment evolution from 2.1 Ma to 0.23Ma by seven effective ESR data from the upper section and "grain-size age model", we have an age framework for further exploration of the environmental evolution in the early Pleistocene. According to timing series, vermicular red earth was formed in the 2.1 -0.44 Ma, brown soil deposited at 0.44 - 0.23 Ma.4. Based on the comprehensive analysis of various environmental indicators, the environment evolution features of Pleistocene red earth can be divided into several phases:The middle period of early Pleistocene (the bottom 1_-3 layer of plinthitic layer, 2.1 - 1.75 Ma): climate condition is appropriate in this period in subtropics of China, Overall, the environment is warm and humid. But there are low scope-high frequency oscillations of climate, the warm and humid environment weakened late, the climate was not stable. This period corresponds to Q₁² red earth period stated by Huang Zhen-guo, also can be compared with the appropriate climatic events in W_S-2 period of north Wucheng loess, Olduvai paleomagnetic positive polarity subage event was recorded in this period, after this period, the climate became cooler, at this time, the subtropical environment had the transition feature between the south China and north China, is neither dry and cold like Mid-latitude areas nor humid and hot like Southern China.The late period of early Pleistocene (section 1_-2-3 layers, 1.75-0.78 Ma): recorded three cool periods in 1.75—1.58Ma,1.43—1.15Ma and 0.93-0.78Ma, two hot periods in 1.58—1.43 Ma,1.15-0.93Ma. The intensity of winter-monsoon and summer-monsoon alternated in different phases, low scope-high frequency oscillations of climate still exist. The parent materials became coarser, weathering process weakened. Three cool periods can respectively correspond to 1.7 Ma, "the Qinghai-Tibet Campaign C-screen" uplift,1.2 Ma, 0.8Ma two dry and cold pollen climate events and L₁₅ and L₉ two sandy loess layer formation period. Jaramillo normal subzone event was recorded in late 1.15-0.93Ma. At this time, environmental indicators change obviously, can correspond with "Kunlun-Yellow Movement A screen" uplift.The early period of Mid- Pleistocene (Section 4-5 layers, 0.78 to 0.44 Ma): warm and humid environment, summer-monsoon strengthened, rainfall increased in summer. Peak valley changes of environmental indicators are different from low scope-high frequency oscillations, show high scope-low frequency characteristics, which probably is the signal of Mid-Pleistocene Climatic Transition. Correspond to Q₂¹ red earth period, north S₅ period, sea water intrusion event in North China Plain and sea-level rose in eastern areas. However, this period wasn't the blooming period of the development of "worm-like" reticulations.The late period of Mid-Pleistocene (Section 6-8 layers, 0.44 to 0.23 Ma): climate became dry and cool, the development of vermicular red earth stopped, Xiashu loess sediments are above the section, including two loess layers and a paleosol layer. Loess layers show low magnetic susceptibility, low redness rating, low Rb/Sr ratio, low CIA and low clay content; paleosol layer shows relatively high magnetic susceptibility, Rb/Sr ratio, redness rating, CIA and the clay content. Hydrothermal conditions of paleosol layer is better, which can be compared with the L₄,S₃,L₃ periods of north loess.