陕西洛川黄土的粒度分维值及其意义

GRAIN-SIZE FRACTAL DIMENSION OF LOESS-PALEOSOL AND ITS SIGNIFICANCE

对陕西洛川剖面 15 80个黄土样品的粒度数据分析表明 ,黄土的基本粒级 ,即在 2～ 5 0 μm的标度范围内 ,粒度分布具有分形特征。黄土的粒度分维值在 2 16～ 2 5 1之间 ,平均值为 2 32 ;而古土壤的粒度分维值在 2 2 3～ 2 5 1之间 ,平均值为 2 41;粒度分维值对黄土与古土壤的形成环境的演变 ,有良好的指示意义。粒度分维值显示 ,2 5 0万年以来 ,东亚季风不断增强 ,变化周期和变化幅度逐渐变大。在早更新世 ,季风较弱 ,变化周期短 ,变化幅度小 ,表现为 2～ 4万年的周期 ;而在晚更新世 ,季风较强 ,冬季风和夏季风相互消长 ,变化周期长且明显 ,变化幅度大 ,表现出 10万年的变化周期。

The grain size distribution of loess is considered as an important proxy index describing the East Asia paleomonsoon climatic variations recorded by Chinese loess deposits during the Quaternary. In this paper, the grain size of 1580 samples in the Luochuan loess paleosol sequence has been measured. Based on the fractal theory, the fractal dimension ( D ) of the grain size has been calculated. Results show that the grain size distribution has fractal characteristics. The D values in loess are between 2 16 and 2 51, with an average of 2 32, a little lower than those in paleosols which are 2 23-2 51, with an average of 2 41. Similar to other climatic indicators, the D values are strictly related to the loess paleosol cycles of the sequence, suggesting that this parameter could also serve as an index for monsoon climatic changes in the Loess Plateau. The D values show that the East Asia monsoon has gradually strengthened since 2 50 Ma B.P. with the amplitude of climatic fluctuations increasing and the frequency reducing. The monsoon strength is weaker during the Early Pleistocene and displays high frequency, low amplitude changes with 20- 40 kyr cycles. While during the Late Pleistocene the monsoon is stronger and shows low frequency, high amplitude changes with 100 kyrcycles. From this point of view, the D value could be a new indicator for evaluating the strength of the East Asia monsoon circulation and indicating the evolution of environment in which loess and paleosols formed.