Baoji pou mian S₅ gu tu rang ci hua lu bian hua ji zhi

Translated title of the contribution: The Magnetic mechanism of S₅ paleosol in Baoji section

Xuelian Guo, Xiuming Liu, Hui Guo, Qu Chen, Pingyuan Li, Mingming Ma, Zhi Liu

    Research output: Contribution to journalArticlepeer-review

    Abstract

    The magnetic susceptibility(MS)has been widely used as a proxy of summer monsoon,whicla is Daseu on an important fact that the MS achieves the highest value in the most developed paleosol S5 ,while lower values in less weathered loess layer in the Central Chinese Loess Plateau(CLP). However,in Baoji loess section, the MS of Ss is the lowest value within the top 6 paleosols( S0, S1 ...S5), which contradicts above normal situation. Baoji(34°25 'N, 107°07'E) is located in the southern edge of CLP, north foot of Qinling Mountains. The present climate is warm temperate semi-humid with mean annual precipitation (MAP) of more than 700ram ( up to 1100mm in Qinling areas) ,the rain distribution is uneven (60% rain falls between July and Sep. ), and mean annual temperature (MAT)of 12℃ (25.4℃ in July). Pedogenic condition in Baoji,such as precipitation,is better than the Central Loess Plateau,for example MAP 654mm in Xifeng (35°46'N, 107°41 'E) and MAP 555mm in Lingtai ( 35°33'N, 107°31'E). In order to find out its mechanism,the samples were collected from the Baoji loess-paleosol sequence since S5, with sampling interval 30cm. The results of rock magnetism and geochemical measurements for Baoji S5 and S3 samples show, that ratio Rb/Sr of Ss samples was higher than that of S3, indicating that S5 was a better-developed paleosol unit, stronger chemical weathering than S3. The climate condition during S5 period was confirmed to be the warmest and wettest for the last O. 5Ma. The S3 paleosol was formed under oxidation environment, under such pedogenic conditions, maghemite and hematite were formed simultaneouly, the anti-ferromagnetic minerals are mainly hematite, superparamagnetic(SP) maghemite is the main contributor to the magnetic properties. However, the contents of ferrimagnetic minerals in S5 samples are lower than that of S3 samples. With pedogenic developing,the proportion of ferrimagnetic component reduces in S5 while the content of anti-ferromagnetic minerals ( mainly goethite) increases. There are amounts of dark brown Fe-Mn film on the cranny surface of Ss paleosol, which indicates where the iron in S1 went to and why MS in S5 becomes lower. This phenomenon seems to suggest different pedogenic condition between S3 and S5 development. S3 pedogenic environment was under oxidizing conditions: so that extra fine grain ferrimagnetic minerals produce with pedogenic development; while S5 development in Baoji was under condition of moisture over much, pedogenesis actually occurs under waterlogged conditions. So that fine- grained magnetite ,maghemite and hematite are gradually dissolved, and finally converted into goethite and limonite and other weak magnetic minerals. Such transformation of magnetic minerals ,from high MS minerals( magnetite and maghemite) to low MS minerals (such as goethite)due to over moisture (gleying) , results in the lower MS value for Baoji S5 paleosol.
    Translated title of the contributionThe Magnetic mechanism of S₅ paleosol in Baoji section
    Original languageChinese
    Pages (from-to)785-794
    Number of pages10
    JournalDisiji Yanjiu
    Volume32
    Issue number4
    DOIs
    Publication statusPublished - 2012

    Keywords

    • S₅ paleosol
    • magnetic susceptibility
    • magnetic mechanism
    • Baoji section

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