Sedimentary Characteristics And Environmental Changes Of Aggradation Red Earth In Mid-Subtropics

Quaternary red earth is one of the most important materials for Quaternary research in southern China. Mid-subtropical region in the Middle and lower reaches of the Yangtze River is the appropriate place for research on red earth and environmental changes, because it is in the transitional area from loess to red earth and is sensitive to climatic changes. Based on five typical aggradation red earth sections located in southern Anhui, northern Jiangxi, northern Zhejiang, and middle regions of Zhejiang, this paper studied red earth's regional complexity as well as the common characteristics in the genesis of aggradation red earth mainly by means of sedimentology; selected a typical section and studied its sedimentary environment changes with effective grain-size indicators, methods of geochronology and geochemistry in order to extract information of paleoenvironmental changes. The main findings are as follows:(1) The test results of grain-size indicate that aggradation red earth are homogeneous sediments, mostly belong to silty clay and silty sub-clay. Their grain-size characteristics are generally uniform: different fractions' mean content of section has a order of silt (4 ~ 63μm) (62.49% to 71.12%)> clay (<4μm) (28.75% to 36.33%)> sand (> 63μm)( 0.13% to3.60%) , coarse sand (> 250μm) gets the negligible content of less than1%; all the red earth has small particle size, high mixed degree of particle fractions and poor sorting; grain-size frequency curves mostly are positive offset with single-peak shape and small-tail, some samples contain coarse tail. All the characteristics above resemble north loess and Xiashu loess; characteristics of quartz particles in aggradation red earth indicate aeolian features and strong weathering. It is believed that aeolian red earth existed in southern China and .experienced a strong process of weathering during Quaternary.(2) Some differences were found among five sections. Anhui Xuancheng (XC), Zhejiang Anji (AJ) and Jiangxi Jiujiang (JL) section has higher silt content, lower clay and sand content, coarser particle size, lower mixed class of particle fractions than Zhejiang Pujiang (PF) and Zhejiang Jinhua Tangxi(TX) section. Comparative analysis of XC,JL,PF,TX section shows: From northwest to southeast , typical dust components and secondary clay reduces and increases respectively, the impact of near-genesis coarse particles strengthens; sedimentary genesis of PF and TX have a transformation from short-range to long-range, all the characteristics above reflect the trend that from north to south intensity of the monsoon and sedimentation weakens while weathering strengthens, and also show that the impact on aeolian red earth caused by the continuous strengthening and southward advance of winter monsoon is reinforced.(3) The grain-size characteristics of different layers are different to some extent, which is mainly reflected by the vertical changes that sedimentation strengthens and weathering weakens from bottom to top. The sensitive particle components reflecting changes in sedimentary environment are different between the same layers of different sections as well as between different layers of the single section. The effect of research on sedimentary environment changes differs between different sections.(4) Fractal study shows that the grain-size volume fractal feature of JL red earth is notable between 2μm and 63μm.Differerit layers have distinctive volume fractal dimensions, plinthitic red earth layers' fractal dimensions is higher than brown-yellow and paleosol layers.; Volume fractal dimensions have notable positive correlation with <2μm content, average particle size (valueφ) and standard deviation while negative correlation with 10～63μm. content .Volume fraction dimensions can be used not only to describe the complexity of grain-size distribution, but also to indicate the changes of sedimentation-weathering: Higher volume fractal dimensions corresponds to a period with stronger weathering, higher secondary clay content, lower content of dust components, smaller average particle size, bigger standard deviation, higher mixed class of different fractions, poorer sorting, and vice versa. Compared with other grain-size indicators, the vertical changes of volume fractal dimensions along JL section can reflect the details of the changes in sedimentary characteristics, so it is a good new indicator for the sedimentary environment of JL red earth.(5) The changes of sedimentary environment from JL section were studied based on volume fractal dimensions, ESR geochronology and other environment indicators. Volume fractal dimension of JL red earth reduced by phases from bottom to top, changes of value also can be found within each phase, it shows that the weathering environment had fluctuations and weakened stage by stage since the early Pleistocene to the late Mid-Pleistocene in Lushan area.In the early time of Early Pleistocene (2403.19～1945.63 ka B.P.), weathering environment was significant, there were high frequency and amplitude changes in weathering environment and climate. In the middle and late time of Early Pleistocene (1945.63～785.81ka B.P.), the intensity of weathering weakened, with the establishment of East Asian monsoon circulation, impact of winter and summer monsoon on this area was reinforced, but summer monsoon dominated, volume fractal dimensions and its changing cycle were enhanced, it reflects that the hot and humid condition of climate was strengthening in the background of strong East Asian summer monsoon.In early Mid-Pleistocene (785.81～476.46 ka B.P.), East Asian monsoon circulation strengthened further, both winter and summer monsoon increased significantly, Lushan area was still in an appropriate stage for the development of plinthitic red earth, but the gradual decrease of volume fractal dimensions reflects that the impact of winter monsoon on the area strengthened progressively after "Mid-Pleistocene Climate Transition", sedimentation was reinforced and intensity of weathering weakened gradually.In late Mid-Pleistocene (370.04～257.75 ka B.P.), volume fractal dimensions greatly reduced than before, sedimentation significantly strengthened, intensity of weathering reduced remarkably, the changing cycle and amplitude of volume integral dimensions increased, the control of East Asian monsoon circulation over the dust deposition in this area became more notable, contrast between the intensity of winter and summer monsoon became bigger, the cyclic changes of climate in this area were similar to northern loess regions, but it was still hotter and humider than the Loess Plateau and Xiashu Loess regions. The climate in Lushan area had transferred from humid-hot to dry-cold since Mid-Pleistocene.