本文对汉江上游黄土的常量元素含量及相关地球化学参数CIA、Na/K、淋溶系数、退碱系数、残积系数等进行了系统分析。结果显示:①汉江黄土的主要化学成分为SiO_2、Al_2O_3和Fe_2O_3,三者含量总和达767.3 g/kg;常量元素含量大小排序为SiO_2>Al_2O_3>Fe_2O_3>K_2O> MgO>Na_2O>CaO。风化成壤过程中Na、Ca、Mg、Si发生不同程度的迁移淋溶,而Fe、Al、K相对富集。②其风化成壤强度呈现从马兰黄土L1过渡性黄土Lt古土壤S0逐渐升高、全新世黄土L0又降低的规律,记录了该区域气候经历了末次冰期(55.0-15.0 ka BP)冷干、早全新世(15.0-8.5 ka BP)增温增湿、中全新世(8.5-3.1 ka BP)达到最暖湿,晚全新世(3.1-0.0 ka BP)降温变干的演变过程。③汉江黄土与洛川、巫山、下蜀黄土的元素组合特征高度一致,不同地区常量元素(CaO除外)含量十分接近且UCC标准化值变幅均小于0.25,这暗示了它们风化之初具有相似的风成沉积基础;但不同区域黄土的化学风化强度差异明显,大致呈现洛川黄土<汉江黄土<巫山黄土<下蜀黄土的趋势,与中国现代季风气候的空间变化规律相吻合,即不同地区黄土风化程度差异主要是东亚季风变化影响的结果。
英文摘要:
Field investigations were carried out along the upper reaches of the Hanjiang River, a major tributary of the Yangtze River in China. Three loess profiles on the first river terrace were selected for a detailed study. The characteristics of major elements and chemical weathering were analyzed systematically. Results are shown as follows. (1) The major elements are dominated by SiO_2, Al_2O_3 and Fe_2O_3, the total average content of which is 767.3 g/kg, in the loess profiles of the region. The contents of the major elements are ranked in the following order:SiO_2>Al_2O_3>Fe_2O_3>K_2O>MgO>Na_2O>CaO. The elements Na, Ca, Mg and Si are leached to different degrees and Fe, Al and K relatively enriched in the chemical weathering processes. (2) The weathering and pedogenic intensity was the weakest in the Malan loess L1, increased slightly in the transition loess Lt, became the strongest in the paleosol S0, and decreased again in recent loess L0. The results reveal that the climate was cold-dry in the Last Glacial (55.0-15.0 ka BP), turned into warm-wet gradually in the Early Holocene (15.0-8.5 ka BP), became the warmest and wettest in the Mid-Holocene Climate Optimum (8.5-3.1 ka BP), and turned into cool and dry in the Late Holocene (3.1-0.0 ka BP). (3) Based on the comparison with Luochuan, Wushan and Xiashu loess, we can find that the major elemental compositions and the UCC-normalized patterns of the loess are similar with those of Luochuan, Wushan and Xiashu loess to a higher degree, suggesting a similar sedimentary background of aeolian loess. But the chemical weathering intensity of these loess deposits changes significantly in different parts of China, with the sequence of Luochuan loess < loess in the Hanjiang River < Wushan loess < Xiashu loess. The changing trend is well consistent with the modern climate change in China, which is the response to the East Asian monsoon.