| The loess magnetic susceptibility change mechanism is a hot and difficult problem in current paleoenvironment study, so there are great significances for the resove of this problem. Recent years, studies of loess magnetic properties are mainly concentrated in the northern hemisphere, but very few in the other one. In Australia and New Zealand, especially the later, loess distributes widely, and this provides a valuable record of loess study in the southern hemisphere. Therefore, this study is based on the magnetic properties of loess from Australia and New Zealand, in order to explore the similarities and differences of magnetic susceptibility change mechanism between southern hemisphere and northern hemisphere.There are vast and dry areas of deserts in the interior of Australia, but none of loess section has been reported until2003, Mackenzie’s Waterholes Creek (MWC) profile in New South Wales was first studied as loess. In this paper, various measurements of magnetic parameters from MWC were carried out, and comparison was made with those of Luochuan profile, which lies in the central Chinese Loess Plateau (CLP). The types of magnetic minerals are similar to the CLP:magnetite, maghemite, limonite and hematite. However, the magnetic properties of Australian aeolian sediment show quite different. Frequency-dependent magnetic susceptibility (χfd%) is rather high, while values of magnetic susceptibility is low. The relative content of hard (paramagnetic)-magnetic components is much higher than CLP, whereas the content of maghemite and hematite is obviously much less. Furthermore this proportion increases along with the depth. Maghemite is more sensitive than magnetite under pedogenic condition of experiencing humid and stagnant watered environment. Maghemite increases along with enhancement of magnetic susceptibility in Luochuan section; however, decreases with that in MWC profile. This very likely indicate that the pedogenic process in MWC took place under environment that maghemite/magnetite are unstable. The modern annual precipitation is924.2mm in MWC area, which is about300mm higher than that in Luochuan. That extra moisture may lead MWC exceeding stable range for maghemite/magnetite. Those minerals therefore gradually converted to more stable goethite under the condition, causing magnetic susceptibility to decrease. Fe and Mn nodules are commonly observed in the lower part of section, implying pedogenic chemical weathering happened that Fe was dissolved and migrated downward and enriched in certain position, indicating that wind-blown maghemite/magnetite under such moisture condition were transformed to iron hydroxides, such as goethite/limonite, or paramagnetic minerals, resulting in magnetic susceptibility reduced. Although the susceptibility from the most of section shows a general positive relation to the particle size, it seems fit well to the ’wind vigor’Alaskan mode. However, Fe chemical weathering and moved downward happened in MWC suggest that the precondition for ’wind vigor’ mode cannot be satisfied, the susceptibility variation can only find a solution from pedogenesis.The measurements and results of Dashing Rocks profile suggest that the types of magnetic minerals are also similar to the CLP, but there are great differences in the composition:most minerals are hard-magnetic, such as goethite, the content of paramagnetic minerals is high, and the content of soft magnetic minerals is very low and they only exist in the upper part. Hard and paramagnetic minerals increase along with depth, and the soft magnetic minerals almost disappear in the bottom. In Dashing Rocks section, the annual rainfall is550mm, there are gammates in the bottom of paleosol, and Fe and Mn nodules are commonly observed in the section, implying pedogenic chemical weathering happened that Fe was dissolved and migrated downward and enriched in certain position The overall magnetic susceptibility value is very low, maybe relate to the source and the transformation of the soft magnetic mineral in the excessively moist environment caused by the excessive rainfall. The relationship between magnetic susceptibility and pedogenic strength is differ from positive and negative ones in CLP and Siberia, respectively. It shows no relationship. This maybe relate to the different strength of the magnetic mineral transformation in different thickness and at different times.Overall, there are similarities between Australia and New Zealand loess magnetic susceptibility change mechanism, but is differ from that of CLP. It is closely related to its geography, climate and other natural conditions, the soil moisture is too high that caused the varying degrees dissolution and transformation of magnetic mineral, is the main factor leading to magnetic susceptibility changed in Waterholes Creek profile, while in Dashing Rocks profile, the loess resource is also an influencing reason expect the above one. |
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