关键词: 厚田沙地, 沙丘砂-砂质古土壤序列, 末次冰期, 粒度分维特征, 季风演变

Abstract:

Fractal theory can be used to reveal the fractal features of many geographic phenomena, and the composition of sediment grain size has been successfully applied to the study of the evolution of geographic environments. The fractal dimension has widely been used as a new grain size index, which is consistent with the environmental changes reflected in the traditional analysis of grain size and composition; however, whether the fractal dimension can also reveal environmental changes in the Poyang Lake area in the mid-subtropics has not yet been determined. This study analyzes the fractal dimension characteristics of Houtian sandy land based on the results of grain size and the power of the exponential function relation method in fractal theory. A series of dune sand-sandy paleosol sequences were developed intermittently on the terraces of the lower reaches of the Ganjiang River. Based on multiple comprehensive investigations, the Houtian section, with rich sequences and a relatively continuous deposition, was selected in Houtian sandy land, Xinjian County, and Nanchang City. With the research carried out, Optically Stimulated Luminescence (OSL) age and grain size tests were completed. A comparison of the fractal dimensions with clay, average grain size, winter and summer monsoon intensity-sensitive grain size, and the Nanjing Hulu Cave stalagmite oxygen isotope gave the following results: 1) Combined with the results of OSL dating and deep-sea oxygen isotope and stratigraphic characteristics, an age-depth framework was constructed based on the segmented sedimentation rate interpolation. The sand dune-sandy paleosol sequence of the Houtian section was mainly formed during the last glacial period (14.9-77.0 ka). The entire section has a good fractal structure, with the dune sand's fractal dimension at 2.04-2.62 (average 2.34) and the sandy paleosol at 2.24-2.70 (average 2.51). 2) The fractal dimension is positively correlated with the summer monsoon intensity-sensitive grain size, negatively correlated with the winter monsoon intensity-sensitive grain size, and closely related to the standard deviation (whereby the smaller the standard deviation, the smaller the fractal dimension). Medium silt, coarse sand, and winter monsoon intensity-sensitive grain size content are higher during the developmental period of the dune sand, and the average grain size is thicker. The standard deviation is smaller, the sorting is better, the degree of self-organization is higher, and the fractal dimension is smaller during the development period of the sandy paleosol. Due to the warm and humid climate, the weathering pedogenesis is stronger; the content of clay, fine silt, and summer monsoon intensity-sensitive grain size has increased significantly; the average grain size is finer; the standard deviation is larger; the sorting is worse; the self-organization is lower; and the fractal dimension is significantly larger. As a result, clay and fine silt formed by weathering sedimentation have the most significant impact on the fractal dimension. 3) The fractal dimension shows some alternating peak-valley cycles in the vertical direction. The peak values correspond to the early MIS2, MIS3c, and MIS3a stages, indicating a strong summer monsoon and a warm and humid climate; the valley values correspond to the late MIS2, MIS3b, and MIS4 stages, indicating that the winter monsoon is strong, and the climate is dry and cold. The results indicated that three climate warming cycles have occurred in the Houtian sandy land. At the same time, the fractal dimension reveals that the H5 and H6 events, which occurred in the HTS3b and HTS4 stages, show that the winter monsoon is the strongest and the summer monsoon is the weakest. Further, the sequence of aeolian sand deposition in the Poyang Lake area is practically synchronized with global climate change and extreme cold weather events.

Key words: Houtian sandy land, Dune sand-sandy paleosol sequence, the Last Glacial Period, Characteristics of grain size fractal, Monsoon evolutions