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Thermally unstable maghaemite and its palaeoclimatic significance in Chinese loess

Chinese Science Bulletin 1999年第S1期44卷 64-69页

作者： Paul HESSE Tim ROLPH Department of Physical Geography Macquarie University NSW 2109 Australia Department of Geology University of Newcastle NSW 2308 Australia

Magnetite and maghaemite are the two main contributors to magnetic susceptibility in theChinese loess/palaeosol sequence. Due to the close similarity in physical properties, earlyinvestigators treated them together as "magnetite". Utilising the property of thermally instability ofmaghaemite, we have been able to separate it from magnetite in loess and palaeosol samples from theupper parts of the Xifeng and Luochuan sections on the central Chinese Loess Plateau and compare itsvariation with depth and horizon. Our results indicate that thermally unstable maghaemite is relativelyenriched in both 1) weakly weathered loess units and 2) towards the west, the direction of dust supply,suggesting that maghaemite is at least partially (coarse grains) of detrital aeolian origin while somepedogenic maghaemite (extra fine grains) occurs in the palaeosols. The physical meaning of magneticsusceptibility, therefore, needs to be reconsidered as a synthetic indicator reflecting thepalaeoenvironment for both source area and deposition area. Thermally unstable goethite is a minormagnetic component of palaeosol samples, but it may play a significant role in determiningcharacteristic magnetic remanence of weakly weathered loess samples, especially when one appliesalternating-field cleaning.

关键词： maghaemite goethite magnetic susceptibility Chinese loess and palaeoclimate

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Effect of Reduction Associated with Organic Matter Decomposition on Magnetic Properties of Red Soils

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Pedosphere 2003年第2期13卷 103-110页

作者： DONGRUIBIN ZHANGWEIGUO LUSHENGGAO YULIZHONG YUJINYAN DepartmentofEnvironmentalengineering NanchangUniversityNanchang330029(China) StateKeyLaboratoryofEstuarine&CoastalResearch EastChinaNormalUniversityShanghai200062(China) CollegeofEnvironmentalandResourceSciences zhejiangUniversityHangzhou310029(China)

Five soils derived from different parent materials were sampled from red soilregion of southern China and studied by magnetic methodology to understand to what extent ironreduction would affect soil magnetic properties and how iron reduction would affect the magneticminerals in soils. Reduction associated with organic matter decomposition strongly affected soilmagnetic parameters at low pH. The losses of original soil magnetic signals in terms of magneticsusceptibility (X), 'Soft' isothermal remanent magnetization ('Soft' IRM), anhysteretic remanentmagnetization (ARM) and saturation isothermal remanent magnetization (SIRM) at pH 4～6 during thetwo-month saturation treatment, were 66%～94%, 54%～90%, 64%～95% and 33%～83%, respectively. Thesechanges were interpreted as a consequence of substantial dissolution of maghaemite and haematite inthe soils. At pH 10, however, there was no significant magnetic change observed. Moreover, stablesingle domain soil maghaemite grains were also sensitive to reduction, which suggested that bothpedogenic and detrital maghaemite were not stable in acid and reducing environments. Goethite,instead, was the most stable iron form under reducing conditions.

关键词： goethite haematite maghaemite magnetic susceptibility reduction

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