Research On The Paleo-Environmental Inofrmation Of Loess-Palaeosols At Luo Hill In North Dabie Mountain

Dabie Mountain is seated in China’s north-south boundary of geography and climate, therefore it is very sensitive to the regional and global changes. To research the palaeoenvironment evolution of the quaternary loess-paleosol deposits from Dabie Mountain aera, we select Luo Hill as research objective because it is by far the best preserved sections in this region, its complete layer record provides a good reliability to research area palaeoenvironment. By extracting the paleo environmental information of the loess-paleosol from Luo Hill, the genetic environment "and ancient environmental changes and provenance of the loess-paleosol are analyzed in detail. This will be an important basis for the study of the responding and feedback mechanism to global change for eastern China. Based on the magnetostratigraphy and optically stimulated luminescence (OSL) dating, the geological age of the Luo Hill section located at North Dabie is researched firstly, then, the genesis patterns and sedimentary environment and source direction of the loess-paleosol of Luo Hill are researched and analyzed by studying the grain-size,magnetic fabric, magnetic susceptibility, geoehemistry elements.According to the results of the magnetostratigraphic, the boundary between the Brunhes normal zone and the Matsuyama reversed zone occur in the depth of1440cm, and the bottom does not appear in the Jaramillo normal subzone, so we infer the geological age of the Luo Hill section could be developed between late Early Pleistocene or early Middle Pleistocene. Then the geological age of the Luo Hill section is presumed by using OSL dating results and B/M boundary age separately as time control point, the results of paleomagnetic research show that the geological age of loess-palaeosols located at North Dabie is about870kaB.P.The single and combined magnetic fabric parameter are tested for the loess-palaeosols from Luo Hill, the results shows that Luo Hill section could be divided into two phases in genesis, the mean values (P, F, L) of the first phase (1480-1600cm) is more than that of the second phase(0-1480cm), but the q value of the first strata section is smaller. The characteristics of mean values reveal that the comparatively second phase is coincident with typical aeolian deposit, however, the first phase belongs to typical hydraulic deposit. In the figure of F-L and P-q and P-F relations of magnetic fabric, the data of eolian deposit are mostly concentrated in the vicinity of the coordinate origin, which indicates that sedimentary dynamic is relatively weaker. While the data of hydraulic deposit are mostly distant from the coordinate origin, which shows its sedimentary dynamic is relatively stronger and stable. Stereographic projection of anisotropy of aximum magnetic principal axis show that the direction of declination of the second phase section is relatively dispersing with inclination of the long axis smaller than15°nd short axis bigger than60°, however, the first phase is relatively concentrating with inclination of the long axis smaller than10°and short axis bigger than80°; The stereographic projection of the K1and K3demonstrate that the wind direction in the comparatively0-1480cm strata section is NW-SE, While the direction of water flowing in the1480-1600cm strata section is SW-NE. The Kmax optimal directions of AMS may be controlled by the uplift of the Luo Hill. Generally, magnetic fabric characters show that the0～1480cm strata section is eolian deposit and the bottom is hydraulic deposit. The change of magnetic fabric in the whole section from the bottom to top shows the variation of deposits genesis, and the sedimentary climate varied from warm humid to cool dry.The paleoenvironment is researched by the characteristic of grain-size composition, grain-size parameter, grain-size curve and the characteristics of quartz microstructure. The results show that the deposits are not enrichment of aeolian basic fraction (10～50um), and possessing larger standard deviation and poorer relative sorting, grain size freqency curve diagram, grain size frequency curves are double or multi-peak and right leaning, the environment discriminant parameters show that this phase is aqueous deposit, and quartz particles are mainly more than200μm, the shapes of quartz sands are mainly angular and subangular, there are high content of crude particles,these are formed under an a very strong dynamic hydraulic deposits and these are typical aeolian deposits and mainly composed of proximal materials transported by local very strong dynamic water flow in the first phase profile, it is very similar to debris flow facies feature in the alluvial fan facies; however, it is mainly made up10～50um that is basic grain-size of eolian deposit in the second phase, and clay content is the second mode group, this stage belongs to muddy silt sediment and possessing smaller standard deviation and better relative sorting, grain size frequency curves are double and left leaning, and these characteristics have a close relation to the loess in North China. The singleness of grain size composition hints the provenance is the common original sources in this stage, and the grain-size parameter and their curves and the environment discriminant parameters all show that this phase belong to eolian deposits. Moreover, quartz has the characteristics of smaller particle size and angular and subangular shapes, so it is typical eolian deposit, and the loess are subjected to strong weathering, on the other hand, these explain indirectly that Dabie Mountains behaves a warm and wet climate characteristics since870kaB.R.The higher value of the magnetic susceptibility of the loess paleosol shows that it is warmer and humid background in Luo Hill since870ka B.P., the curve of magnetic susceptibility displays that the peak and trough values of the magnetic susceptibility are respectively corresponding to the paleosol and loess.Another we conclude that the mean annual temperature is0.91～13.88℃, and the mean annual precipitation is126.46～789.14mm by the four times of the regression equation calculation of the mass susceptibility.The research of geochemical element of the loess-palaeosols from Luo Hill indicates that it is rich in SiO2, Al2O3, Fe2O3,this shows that Luo Hill is characterized with hot humid regional climate, and the climate changed with fluctuation from warm humid in the early stage into cool dry in the late stage since870kaB.P. By the analysis of CIA and Rb/Sr characteristics, we can conclude that the degree of chemical weathering in the aeolian-dust deposits of Dabie Mountain is middle intensity. Based on the enrichment factor, characteristic element and stable trace elements, the provenance of the loess-palaeosols from Luo Hill is analysed by their distribution patterns and correlations, the results of research show that the provenance of Luo Hill loess-palaeosols is the same as the loess of LuoChuan and Nanjing Xiashu, and they come from the northwest aeolian-dust deposit. By comprehensive analysis of the characteristics of magnetic fabric, grain-size and magnetic susceptibility, we conclusively establish the bottom of eolian deposit is originated in870kaB.P. These results are correspondent with area in the middle and lower reaches of Changjiang River, hinting the eolian deposits were the possible response to the Global Change.