Mineralogical And Geochemical Evidence Of The Presence Of Ancient Paddy Soil Layer In Chuodun Ancient Agricultural Site, Kunshan

Paleosol implicates regional and even global environmental change. Not only the information on natural change, but also a lot of ancient human activities could be clarified from the investigation of paleosol. Systematically study in ancient paddy soil is beneficial for improving our understanding in the origination and development of rice culture, and for supplement of the theory in its formation and evolution. However, it is extremely difficult to obtain a well-preserved ancient paddy soil profile. The Chuodun site is one of the rice field origins, and its finding provided us an opportunity to investigate the geochemical evidence of the presence of paleosol in the site.Paleosol profiles (P-02) within 2 m of the Chuodun site (100 samples) in the vertical direction were sampled every 2 cm, and investigated. Grain size distributions, clay mineral concentrations, magnetic susceptibility, and fixed ammonium were systematically studied, mineralogical and geochemical evidences were provided to prove the existence of the paddy soil. The results obtained were list as follows:1. Distributions of particle size in the P-02 profile ancient paddy soil were measured by the utilization of Laser-size and settlement-centrifugation methods. Compared with clay of the settlement- centrifugation method, it is same trend with clay of Laser particle size method. There are comparable with clay of the two methods .The distribution of clay content in the vertical direction showed that the ancient paddy soil might be located in 160-140 cm of the profile. The grain-size analyses indicated that the grain size composition of the Chuodun paleosol was well correlated with that of the Xiashu loess in Zhenjiang. It could be deduced that the parent material of the Chuodun paleosol might be Xiashu loess.2. In the study, particle size distributions of 100 samples from the Chuodun agricultural site P-02 profile were identified by the settlement-centrifugation method. X-ray diffraction method was used to study the relative mineral composition of each sample. It showed that sand and silt contained a large amount of quartz, and a small amount of feldspar and mica minerals. Illite was the most abundant component in clay minerals, and kaolinite was the second abundant element. Some samples contained a little of montmorillonite. Kaolinite/illite and vertical distribution of illite crystallinity degree showed that there were two maturity processes of the soil in 160-140 cm and 28-0 cm of the profile, respectively.3. Two low magnetic susceptibility zones—160-140 cm and 28-0 cm might be formed under waterlogged condition. Magnetic susceptibility of the particle size, whose diameter was more than 200μm, was well correlated with the whole section magnetic susceptibility. It demonstrated that coarse magnetic minerals have larger contribution to whole section magnetic susceptibility. Hematite was the most abundant component in coarse magnetic minerals contents in 160-140 cm, it indicated that coarse magnetic minerals were generated due to human activities, such as ceramics.4. The vertical distribution of fixed ammonium indicated two low zones—160-140cm and 28-0cm in the P-02 profile. The content of fixed ammonium demonstrated that 160-140cm and 28-0cm in the profile were pale-paddy soil, and modern paddy soil, respectively. Fixed ammonium contents in 160-140cm were well correlated with clay content, illite content and correlation coefficient was 0.61 and 0.57, which implicated that fixed ammonium was in the presence of clay and particularly illite.5. Magnetic susceptibility, total organic carbon, total nitrogen, kaolinite, and clay contents were used to form soil development index (SI). The average of the whole profile of soil development was 0.16. The average of the profile—160-140cm and 28-0cm of soil development were 0.26 and 0.27, respectively. Soil development of 160-140cm and 28-0 cm in the P-02 profile was much higher than the average of the whole profile of soil development, which furtherly showed that there were developed to a certain degree of paddy soil. After our comprehensive analysis of research results of the structure of ancient paddy field, carbonized rice grains, rice phytoliths, pollen and the thesis research.we think that ancient paddy soil might be located in 160-140 cm of the profile.