| In this paper,the four sections of Huanxian,Xifeng,Changwu and Weinan located on the north-south cross section of the Loess Plateau are selected.The study is about analysing K feldspar and plagioclase formed in the Loess Plateau region of the Quaternary loess deposits through X-ray diffraction.On this basis,probe into the information of paleoclimate and paleoenvironment indicated by K feldspar and plagioclase in loess and paleosol,and try to build up a sensitive proxy index based on the mineralogical principle.The results and conclusions are listed as below:(1)In the Chinese Loess Plateau,the content of K feldspar and plagioclase in loess and paleosol is controlled by the influence of grain size change.K feldspar and plagioclase in loess and paleosol tend to be enriched in coarse grains.(2)The K/P ratio in loess and paleosol is affected by the particle size distribution,and it shows a similar variation.Among them,the K/P ratio of all the fractions with the size of>5?m is relatively close to the whole,but the K/P ratio of all the particles with the size of<5?m is obviously higher than that of the coarse particles>5 pm.In loess,the K/P ratio basically maintains the composition of the original dust in the various fractions of>5?m.In the finely weathered<5?m particles,the loess dust went throught certain chemical weathering in the source area.The difference of chemical weathering of plagioclase and K-feldspar shows the characteristics of the early stage of continental weathering.In paleosol,the grain grade of>5?mn mainly inherited the characteristics of the original dust.The content of plagioclase is relatively decreased in the fine particles<5 ?m,and the K/P ratio is increased.In the Chinese Loess Plateau,the weathering of the paleoso is also in the early stage of continental weathering.(3)The<5?m K/P ratio effectively excludes the effect of grain size change.In loess,the change of the<5?m K/P ratio is mainly controlled by the weathering intensity of the source area,which is mainly affected by source area factors and reflected the material composition of the source area.In paleosol,the change of the K/P ratio of<5?m is mainly influenced by the soil erosion.The<5?m K/P ratio can be used as a good alternative index of weathering soil strength,and it can indicate the intensity of the East Asian summer monsoon circulation to some extent.(4)Results show that the<5?m K/P ratio in each section is higher in palesol than in the underlying loess unit.Spatially,the<5?m K/P ratio of each of the palesol units increased significantly from northwest to the southeast,while the increasing trend was weak in the loess units.The higher<5?m K/P ratio and the steeper S-N gradient in the palesols indicate a significantly strengthened summer monsoon and the stronger S-N gradients in the Loess Plateau during interglacial times.The lower<5?m K/P ratio and weaker S-N gradients in loess units indicate a smaller impact of the summer monsoon and weak S-N climate gradients during glacial times.(5)The<5?m K/P ratio of the loess-paleosol sequence in the Xifeng section shows that the chemical weathering intensity of the inland region of Asia has been gradually decreasing since 1.65Ma.Moreover,fluctuations in glacial and interglacial periods indicate the changes in the chemical weathering in the Loess Plateau,which reflecting the East Asian summer monsoon effect.During the period of 1.65Ma to 0.65Ma,the K/P ratio of<5?m of the Xifeng section of the Loess Plateau in China reflected a gradual decline of the East Asian summer monsoon.Since 0.65Ma,the K/P ratio of<5 ?m mainly reflects the weakened continental weathering in the source area.(6)In the Chinese Loess Plateau,weakened weathering of the Asian continental continent since 1.65Ma may be related to the increase in global ice volume and the uplift of the Qinghai-tibet plateau.Moreover,the changes of global ice volume during glacial and interglacial periods also have a certain effect on chemical weathering in the Loess Plateau. |
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