以色列Har Keren沙漠黄土环境磁学特征研究

风成黄土是研究古气候变化的良好载体,在全球陆地分布广泛,而不同区域地理环境差异显著,其记录气候变化的机制亦有所不同。以色列南部黄土区地处亚热带沙漠边缘,为探究该区域黄土磁性特征及其对区域环境的响应机制,对内盖夫沙漠边缘黄土区进行野外考察并测试系统环境磁学及粒度参数。结果表明,Har Keren(HK)剖面中磁性矿物主要为碎屑磁铁矿和磁赤铁矿,磁性矿物含量较少,磁性颗粒主要为粗单畴(SD)和多畴(MD),磁性特征与中国西北干旱区黄土较为相似。HK剖面常温磁学参数整体变化范围较小,仅在剖面亚表层有所升高,可能是受现代成土作用影响。HK剖面中值粒径自底部至顶部呈减小趋势,但粒度整体偏粗,其频率分布曲线显示为正偏态,分选较差,反映出多种搬运方式的混合特征,与典型风成黄土有所差异。

Study on Environmental Magnetic Characteristics of Loess in Har Keren Desert, Israel

[Objective] Aeolian loess, widely distributed over the world, is a good carrier for studying paleoclimate change. However, mechanisms of their recording climate change vary as their geographical environments from region to regions. The loess region in the south of Israel borders, a subtropical desert. To explore mechanisms of loess magnetic characteristics responding to the environment of this region, a field survey was conducted in the loess area on the fringe of the Negev Desert to investigate systems environmental magnetism and particle size composition. [Method] This study had soil samples collected from the area and analyzed for types, contents and particle sizes of magnetic domain particles therein by means of the room temperature and high temperature magnetic methods. Through analysis of particle size composition of the samples, transport process and deposition characteristics of loess were determined. [Result] Results show that the magnetic minerals in the HK profile consisted mainly of clastic magnetite and maghemite, quite low in content. The Dearing plot of the sample shows that the magnetic domain particles in the magnetic minerals were quite coarse and mainly of coarse single domain (SD) and multi-domain(MD). Analysis of particle size composition shows that particles in the HK profile varied in size, tended to be coarse, declining from the bottom to the top in the profile, with frequency distribution curve appearing to be of positive skewness and were hard to be fractionated, exhibiting the feature of a mixture of multiple transport modes, which was somewhat different from that of a typical aeolian loess. [Conclusion] (1) Both the room-temperature magnetic parameters and high-temperature thermomagnetic curve of the HK profile show that the magnetic minerals in the HK (Har Keren) profile samples consist mainly of clastic magnetite and maghemite, quite low in content, quite coarse in particle size, and dominated with coarse SD and MD, which reflects that they are clastic magnetic minerals coming from the source area. The loess in the profile is quite similar to that in the arid region of Northwest China in magnetic characteristics. (2) The room-temperature magnetic parameters of the HK profile, as a whole, vary within a quite narrow range and ascend somewhat only in the sub-surface layer, which may be attributed to the impact of the modern soil forming process. And (3) median particle size of the HK profile displays a trend of declining from the bottom to the top, and the particles tend to coarse on the whole. Its frequency distribution curve appears to be of positive skewness and the particles are hard to be fractionated. Besides, in the bottom of the profile mixed are gravel debris layers. All of the above reflect that the process of near-source-wind-driven transport and deposition has been subjected to the fluvial effect of flows.