黄土高原粉尘堆积源区环境及风化历史初步研究

In this study, we examined the surface features of quartz grains, the quartz oxygen isotopic ratios and the mineralogical compositions of the loess - paleosol - red clay sediments systematically. The surface features of quartz grains do not show significant changes of the dust deposits through the past seven million years. The particles were mainly created in the process of glacial and frost weathering of high mountains. Then the surfaces were altered in some degree by the flood and wind abrasion. The surface features registered all these processes. The assemblages of surface features changed for four times in the past seven million years, the occurrence ages are: 5.0~4.2MaBP, about 3.6MaBP, about 2.6MaBP and about 0.9MaBP, respectively. This may indicate that there were uplift events of the Tibetan Plateau during those times. The oxygen isotopic compositions of quartz in the sediments represent the oxygen isotopic compositions of the initial dusts because of the stable properties of quartz both physically and chemically. The oxygen isotopic compositions of 4~16um quartz changed significantly at about 2.6MaBP, decreasing from about 19.5%o to about 18.5%o. This decrease of quartz oxygen isotopic ratio suggests that the environments of the dust source areas changed at that time, or the range of dust source area changed at that time. The environmental change may result from the structural evolution of the Tibetan Plateau and global cooling at that time. The coarse fractions (>30μm) of the dust deposits were examined using the EDXA device for mineral identification. The quartz content has a decrease trend during 7~2MaBP, then increase rapidly at about 2MaBP. After 2MaBP, quartz content continues to decrease. The Ca-plagioclase content / quartz content ratio increase at about 3.6MaBP. The ratio shows a peak of 3-6 fold values at about 2.5~1.8MaBP, the cause of this is still unknown. The Ca-plagioclase content / quartz content ratio continues to increase after 1 MaBP. The flowing can be regarded as the conclusion remarks of this study: Some of the red clay sediment of the Chinese Loess Plateau (at least Lingtai and Jingchuan red clays) is eolian in origin. The quartz grains from dust deposits throughout the past seven million yeas showed the clues of glacial and frost processes. This indicates that the high mountains of western China reached a certain altitude to favor the glacial and/or frost processes at least seven millions years before. The weathering intensities of the past seven nnillion yeas have a decreasing trend. In about 5~4.5MaBP, the weathering is relatively weak, and the dust supply is relatively low. At about 3.6MaBP and 2.6MaBP, the dust supply increased significantly. The mineralogical composition, the quartz surface feature and the quartz oxygen isotope composition were influenced by the uplift of the Tibetan Plateau. The Plateau may have reached a certain altitude to generate the arid regions of inland China and favor the glacial and frost weathering. And it underwent a phased uplift, which have uplift events at about 3.6MaBP and 2.6MaBP.