摘要
对长武和宜川晚第四纪黄土-古土壤序列的高分辨率铁含量分析表明，不同时期的风尘在化学成分上具有较明显的区别。在排除成壤过程中CaCO3淋溶和淀积的相对影响后，研究剖面的铁含量基本反映了原始风尘的铁含量。已有研究表明，西部源区的气溶胶和现代地表流沙中铁的相对含量显著高于东部源区。本文认为，风尘铁含量的变化反映了西部沙漠风尘对黄土堆积的相对贡献。由于西风带北支是西部源区风尘的主要搬运者，风尘中全铁含量可能反映了西风带北支气流的强度，并明显存在着千年尺度的变化。
The loess-soil sequences in northern China is a near continuous climate record of the Quaternary, which contains signal related to the conditions of the source areas, the transporting winds and the climate conditions in the Loess Plateau region.Meteorological analyses and study on modem dust activities have demonstrated that the aeolian dust forming the loess-soil sequence in China is mainly originated from the desert lands in northern and northwestern China through the transportation of the northwesterly winter monsoon and a northern branch of the Northern Hemisphere westernes. Available data indicated that the chemical composition of dust fromdifferent parts of the deserts is variable and that iron is potentially one of the important tracer elementS for differentiating the dust sources. In this study, iron content of 477 high-resolution samples from Changwu and 160 samples from Yichuan at the central Loess Plateau was measured to determine the variations of aeolian iron in the Late Pleistocene. The yielded timeseries display a series of millennial changes inlaid in the slow trends attributable to the Earth's orbital forcing. Examination on the relationship between iron content and grain-size of weakly weathered loess samples reveals that iron content is in poor correlation with grain-size, suggesting that grainsize sorting is not a dominant factor influencing the iron content. Previously Published chemical data on modern mineral aerosol and surface dune sand from the potential dust source areas indicate that iron concentration in western desertS is significantly higher than for the eastern deserts. We therefore interpret that the variations in iron content of loess mainly reflect the relative contribution of the western desert to the loess deposition at the middle reaches of the Yellow River. As the northern branch of the westernes is the main carrying wind for dust from western sources, the variations of aeolian iron recorded in loess would primarily reflect the strength of the northern branch of the westernes. If thes was the case, the aeolian iron in Chinese loess may be regarded, to some extent a proxy of the zonal westerlies wind. For confirming this inference, examination on longer timeseries at the orbital timescale and comparison with other proxy of westerly wind are necessary. The resultS also provide the following information. (1) the paleosols in Chinese loess are typical accretionary soils. Aeolian dust deposition and Pedogenesis are, indeed,competing processes at all bine, and the presence of a paleosol simply indicates that the latter process was predominant. For some accurate analyses, the loess underlying a paleosol cannot be regarded as the parent matorials of the soil. (2) loess is not a homogenous materials. Composition of the aeolian dust did change through bine. These characterishcs must be taken into consideration in some accurate comparison betWeen loess and paleosols. (3) the variations of the dust composition, such as the total iron content is not only related with the climate conditions in the Loess Plateau region (such as the dissolution and reprecipitation of carbonate), but also with sedimentary causes. Total Fe2O3 can therefore not be used as an appropriate proxy for addressing the paleoclimate changes in the LOess Plateau region.
出处
《第四纪研究》
CAS
CSCD
北大核心
1999年第1期41-48,共8页
Quaternary Sciences
基金
国家自然科学基金!49725206
关键词
晚第四纪
风尘
物质成分
古环境
环境意义
loess-soil sequence, aeolian dust, westerlies, abrupt changes
