Multiple origins of magnetic minerals in soils—Discrimination between pedogenic and anthropogenic magnetism
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    Abstract:

    To study multiple origins of magnetic minerals in soils, comparison was made between paeosols in the Loess Plateau, China and urban soils in Shanghai in magnetic properties. Results show the mean values of magnetic susceptibility (χlf) and frequency-dependent magnetic susceptibility (χ fd %) of the paleosols in the Loess Plateau are 108×10-8 m3 kg-1 and 11.4%, respectively; and the two are positively and significantly correlated (p≤0.01), suggesting that the magnetism enhancement in the soils is mainly attributed to the formation of pedogenic superparamagnetism (SP) grains. Significant positive correlations of χlf with free iron (Fed), free iron degree (Fed/Fet) and clay content also suggest that the magnetism enhancement in the soils is closely related to pedogenesis. Magnetism of the urban soils in Shanghai is unusually enhanced, with χlf ranging from 127 to 1 959×10-8 m3 kg-1, and χ fd % being mostly <2.0%, indicating little SP grains formed, which suggests that the magnetic enhancement of the urban soils is not caused by pedogenesis but by anthropogenic activities. The study of magnetic parameters of their combinations, such as anhysteretic remanent magnetization(χarm), anhysteretic remanent magnetization/saturated isothermal remanent magnetization (χarm/SIRM),χarmlf and SIRM/χlf, reveals that the urban soils are dominated by multidomain (MD) and stable-single-domain (SSD) grains; while the paleosols in the Loess Plateau by superparamagnetism (SP) grains. The magnetic parameters of soft isothermal remanent magnetization (SOFT), hard isothermal remanent magnetization (HIRM), SOFT percentage (SOFT%), HIRM percentage (HARD%) and IRM300 mT/SIRM (F300 mT) indicate that the urban soils are dominated by ferrimagnetic minerals, with low coercivity and high magnetic soft fraction and the paloesols in the Loess Plateau are also dominated by ferrimagnetic minerals, which are, however, higher in HARD%. The magnetic parameters and their combinations are effective means to discriminate between pedogenic and anthropogenic magnetism in soils.

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Pan Yun, Hu Xuefeng. Multiple origins of magnetic minerals in soils—Discrimination between pedogenic and anthropogenic magnetism[J]. Acta Pedologica Sinica,2010,47(2):206-215.

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