澳大利亚黄土磁学性质及环境意义

MAGNETIC PROPERTIES OF LOESS DEPOSITS IN AUSTRALIA AND ITS ENVIRONMENTAL SIGNIFICANCES

澳大利亚内陆广大而干旱,发育着大面积沙漠,却没有多少黄土的报道。本文对惟一报道的黄土,新南威尔士州Mackenzie地区Waterholes Creek剖面进行了详细的采样和磁学参数测量,并与中国黄土高原洛川和西峰经典剖面进行了对比。结果表明该剖面磁学性质与中国黄土高原有许多异同。它成土特征强,百分比频率磁化率较高,但是磁化率值较低,硬磁性矿物含量远远高于中国黄土高原。磁性矿物种类与黄土高原相似,强磁性矿物为磁铁矿和磁赤铁矿,弱磁性矿物为针铁矿/褐铁矿和赤铁矿。但是磁赤铁矿和赤铁矿含量明显少,顺磁性矿物占很大的比例,相对含量高于中国黄土高原。而且,这个比例随深度增加而增高。相对而言,磁赤铁矿对氧化与湿润滞水环境比磁铁矿更为敏感,它在中国洛川黄土剖面随磁化率增加而增多;而在澳大利亚剖面随磁化率增加而减少,表明该剖面成土环境是在磁赤铁矿/磁铁矿不稳定条件下进行的。当地现代年均降水量924.2mm,比洛川高出300mm。可能是因为当地土壤湿度超过磁赤铁矿/磁铁矿临界湿度范围,使其不稳定而被改变成为低磁化率的针铁矿,导致磁化率降低。该剖面普遍可见铁锰质从上部被溶解并迁移到下部沉积,形成铁质胶结和铁质结核,说明成土过程发生了较多水分参与的化学风化。这个过程也导致了原生风积物中强磁化率矿物磁赤铁矿/磁铁矿改变成为铁的氢氧化物(如针铁矿褐铁矿)或者顺磁性矿物,使得磁化率降低。尽管该剖面磁化率大部分与粒度正相关,似乎符合阿拉斯加"风速论"的解释。但剖面下部有明显铁质胶结和结核,说明已经发生了铁的化学风化和物质迁移,"风速论"的前提条件已经不能满足,磁化率变化只能从成土过程寻求解答。

There are vast and dry areas of deserts in the interior of Australia, but none of loess section has been reported until 2003, 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, 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 is 924.2mm in MWC area, which is about 300mm 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.