CHANGES IN THE COMPOSITION OF LATE PLEISTOCENE AEOLIAN DUST AND THE ENVIRONMENTAL SIGNIFICANCE
晚第四纪风尘物质成分的变化及其环境意义

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 t