【作者】 马明明；

【导师】 刘秀铭；

【作者基本信息】 兰州大学 ， 自然地理学， 2012， 硕士

【摘要】 世界黄土磁化率变化机制问题是目前黄土古环境研究的热点和难点,问题的解决对于黄土古环境研究具有重大的意义。近些年来对于黄土磁学性质的研究工作主要集中在北半球,而对于南半球的研究甚少。澳大利亚和新西兰发育的黄土堆积为南半球的黄土研究提供了宝贵的记录,基于此,本文对南半球澳大利亚和新西兰黄土样品进行了详细的岩石磁学测量分析,以此来探讨南半球黄土与北半球黄土磁学性质的异同及磁化率变化机制的差异。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导。本文对唯一报导的黄土,新南威尔士州Mackenzie地区Water holes 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经典剖面进行了对比。结果表明它成土特征强,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,磁赤铁矿,弱磁性矿物为针铁矿/褐铁矿,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。当地现代年均降雨量924mm,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。此外,对新西兰南岛Timaru地区Dashing Rocks黄土剖面的磁学参数测量结果表明,该剖面所含磁性矿物种类与黄土高原相似,但是在组成上有很大的差别：磁性矿物主要以硬磁性矿物(针铁矿)为主导矿物,顺磁性矿物含量很高,软磁性矿物(磁铁矿)含量很少,并且只存在于剖面的上半部分。剖面自上而下随着深度的增加,硬磁性和顺磁性矿物含量越来越高,到了剖面底部软磁性矿物已经几乎消耗殆尽。在Dashing Rocks剖面,年降水量为550mm左右,剖面发育有网纹,古土壤底部还发育有脆盘结构,说明曾经历过强烈的淋溶作用,剖面中广泛存在着铁锰结核,说明磁性矿物曾经发生了化学变化。剖面整体磁化率值很低,其原因可能与物源及沉积后期所经历的过度湿润环境所造成的软磁性矿物的溶解转化有关。黄土—古土壤层与磁化率值高低的对应关系,既不是中国黄土高原的正相关,又不同于西伯利亚、阿拉斯加的反相关,而是表现出不相关的关系。这种不相关可能是由于不同土层所经历的磁性矿物改造作用的强度不同所导致。总的说来,澳大利亚和新西兰黄土磁化率变化机制具有相似之处,但是与中国黄土高原完全不同,磁化率的这种变化与其地域、气候等自然条件密切相关,土层的有效湿度过高,造成磁性矿物发生不同程度的溶解转化,是导致Waterholes Creek剖面磁化率变化的主要因素,而Dashing Rocks剖面除了上述主要因素外,还可能与其物源有一定的关系。更多还原

【Abstract】 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.更多还原