Transformation relationship among different magnetic minerals within loess-paleosol sediments of the Chinese Loess Plateau

The dominant magnetic minerals and carriers of magnetic signals within the Chinese Loess Plateau are magnetite, maghemite, hematite, and goethite. In this study, we investigated the provenance and evolution of magnetic minerals during loess pedogenesis, using X-ray diffraction (XRD) and optical and electron microscopy, including field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM). Our results reveal that single- and multiphase mineral assemblages among magnetic minerals in the loess-paleosol sequence have been formed. Partial oxidation of coarse eolian magnetite has occurred in the desert source area and the oxidation degree is enhanced after deposition of the dust upon the Chinese Loess Plateau. This mode of origin resulted in a microtexture consisting of an inner magnetite core surrounded by a hematite rim, and strongly affected the magnetic characteristics of the loess. Goethite coexists with hematite in the loess and paleosol, and nanometer-scale hematite is formed upon goethite rims via dehydration. Our study provides direct mineralogical evidence of the magnetic record and paleoclimatic implications of the loess-paleosol sequence of the Chinese Loess Plateau. Supported by National Natural Science Foundation of China (Grant Nos. 40772032 and 40573054) and National Basic Research Program (Grant No. 2007CB815603)     

Magnetic properties and paleoclimatic implications of loess-paleosol

Detailed rock magnetic investigations and X-ray diffraction (XRD)were carried out on loess-paleosol sequences of the last interglacial-glacial at Znojmo section in Czech Republic. The results indicate that pedogenesis causes susceptibility enhancement in the paleosols, which is similar to that observed in the Chinese Loess Plateau. k-T curves, IRM, and XRD show that magnetite is the dominant magnetic mineral in the loess-paleosol sequences at the Znojmo section, while maghemite, hematite, and pyrite/pyrrhotite are minor minerals. Measurements of anisotropy of magnetic susceptibility (AMS) indicate that the magnetic lineation is smaller than the foliation. The susceptibility ellipsoids are oblate and the directions of the maximum principal axes (Kmax) are distributed randomly, and cannot be used to determine the paleo-wind direction.     

甘肃西山坪遗址岩石磁学性质及其研究意义探讨

对甘肃西山坪遗址剖面样品进行磁化率、频率磁化率、热磁分析、非磁滞剩磁和等温剩磁等一系列岩石磁学实验测试,分析环境磁学参数特征变化,试图探讨西山坪遗址研究意义,为今后深入研究该沉积序列蕴含的古环境记录提供了岩石磁学基础.结果表明：西山坪遗址剖面沉积物中磁性矿物主要以低矫顽力的磁铁矿和磁赤铁矿为主,高矫顽力磁性矿物赤铁矿含量极其有限,越向剖面表层,强磁性矿物含量逐渐增加.磁性矿物颗粒表现出黄土的典型特征,以单畴、准单畴为主,含有一定量的超顺磁颗粒,颗粒度越向剖面表层越细.从物源上看,遗址沉积物主要来自近源的黄土风尘堆积,主要是风成堆积,并混合大量人类活动信息,给今后综合研究岩石磁学与人类活动特征的关系有一定借鉴意义.     

黄土高原西北缘末次冰期晚期以来黄土沉积物的岩石磁学性质

以黄土高原西北缘的靖远和古浪剖面(包含黄土层L1上部和古土壤层S0)作为研究对象,选取代表性样品进行磁化率、频率磁化率、热磁曲线、等温剩磁获得曲线和磁滞回线等测定.结果表明,靖远和古浪L1黄土和S0古土壤具有相似的岩石磁学特征.磁性矿物含量相对较低,载磁矿物均以磁铁矿为主,同时含有磁赤铁矿和赤铁矿,且S0古土壤中的磁赤铁矿含量要高于L1黄土.靖远和古浪剖面L1黄土和S0古土壤的亚铁磁性矿物平均粒度都分布在准单畴(PSD)范围,并且L1黄土平均粒径比S0古土壤更靠近多畴(MD)范围.通过与黄土高原东部黄土剖面L1黄土和S0古土壤的磁化率对比分析,认为导致靖远和古浪S0古土壤磁化率增强的主要因素是气候,但是,研究区受物源影响较大,来自粉尘源区的粗颗粒磁性矿物对磁化率贡献不可忽略.     

高分辨率洛川剖面黄土磁性地层学

洛川剖面作为经典的黄土剖面,磁极性地层界线的具体位置还不清楚.通过对洛川黄土-古土壤序列以10 cm间隔的古地磁研究,厘定了磁极性界线的具体位置：（1）MB (Matuyama-Brunhes) 界线位于53 m处,S8的上部.（2）Jaramillo位于67.8~72.5 m,上界在S10的上部,其下界位于L13的顶部.（3）Olduvai位于107.2~114.2 m,上界位于S25的顶部,相当于WS2的上部,其下界位于L29的上部,相当于WL2的上部.4）并未检测到Matuyama的底界,可能位于红粘土内.     

Loess magnetic properties in the Ili Basin and their correlation with the Chinese Loess Plateau

Over the past two decades, magnetoclimatological studies of loess-paleosol sequences in the Chinese Loess Plateau (CLP) have made outstanding achievements, which greatly promote the understanding of East Asian paleomonsoon evolution, inland aridification of Asia, and past global climate changes. Loess magnetic properties of the CLP have been well studied. In contrast, loess magnetic properties from outside the CLP in China have not been fully understood. We have little knowledge about the magnetic properties of loess in the Ili Basin, an intermontane depression of the Tianshan (or Tien Shan) Mountains. Here, we present the results of rock magnetic measurements of the Ili loess including mass magnetic susceptibility (χ) and anhysteretic remanent magnetization (ARM), high/low temperature dependence of susceptibility (TDS) and hysteresis, as well as X-ray diffraction (XRD) for mineral analysis. Based on the comparison with loess-paleosol sequences in the CLP (hereafter referred to as the Chinese loess), we discuss the possible magnetic susceptibility enhancement mechanism of the Ili loess. The results show that 1) the total magnetic mineral concentration of the Ili loess is far lower than that of the Chinese loess, though they have similar magnetic mineral compositions. The ferrimagnetic minerals in the Ili loess are magnetite and maghemite, and the antiferromagnetic mineral is hematite; XRD analysis also identifies the presence of ilmenite. The ratio of maghemite is lower in the Ili loess than in the Chinese loess, but the ratios of magnetite and hematite are higher in the Ili loess than in the Chinese loess. 2) The granularity of magnetic minerals in the Ili loess, dominated by pseudo-single domain (PSD) and multi-domain (MD) grains, is generally much coarser than that of the Chinese loess. Ultrafine pedogenically-produced magnetic grains have a very limited contribution to the susceptibility enhancement. Rather, PSD and MD particles of magnetite and maghemite are the main contributors to the enhancement of susceptibility in the Ili loess. 3) The susceptibility enhancement mechanism for the Ili loess is complicated and superimposes both a wind velocity/vigor model (Alaskan or Siberian model) and the in situ ultrafine grain pedogenic model; the former might play an important role in the Ili loess. 4) Magnetic susceptibility enhancements of the Ili loess are related not only to the eolian input of the source area, but also to the local climate, landform, and geological background. Therefore, great care should be taken when reconstructing paleoclimate using magnetic susceptibility data from the Ili loess.     

Spatial variation in soil resistance to flowing water erosion along a regional transect in the Loess Plateau

The factors influencing soil erosion may vary with scale. It remains unclear whether the spatial variation in soil erosion resistance is controlled by regional variables (e.g. precipitation, temperature, and vegetation zone) or by local specific variables (e.g. soil properties, root traits, land use, and farming operations) when the study area enlarges from a hillslope or catchment to the regional scale. This study was performed to quantify the spatial variations in soil erosion resistance to flowing water under three typical land uses along a regional transect on the Loess Plateau and to identify whether regional or local specific variables are responsible for these changes. The results indicated that the measured soil detachment capacities (D_c) of cropland exhibited an irregular trend along the regional transect. The D_c of grassland increased with mean annual precipitation, except for two sites (Yijun and Erdos). The measured D_c of woodland displayed an inverted ‘U’ shape. The changes in rill erodibility (K_r) of three land uses were similar to D_c, whereas no distinguishable trend was found for critical shear stress (τ_c). No significant correlation was detected between D_c, K_r and τ_c, and the regional variables. The spatial variation in soil erosion resistance could be explained reasonably by changes in soil properties, root traits, land use, and farming operations, rather than regional variables. The adjustment coefficient of K_r for grassland and woodland could be well simulated by soil cohesion and root mass density (R² = 0.70, P < 0.01), and the adjustment coefficient of critical shear stress could be estimated with aggregate stability (R² = 0.57, P < 0.01). The results are helpful for quantifying the spatial variation in soil detachment processes by overland flow and to develop process‐based erosion model at a regional scale. Copyright © 2015 John Wiley & Sons, Ltd.     

Detrital and pedogenic magnetic mineral phases in the loess/palaeosol sequence at Lingtai (Central Chinese Loess Plateau)

A detailed rock magnetic investigation of loess/palaeosol samples from the section at Lingtai on the central Chinese Loess Plateau (CLP) is presented. Thermal demagnetisation of isothermal remanent magnetisation (IRM) and Curie temperature measurements suggest the presence of magnetite, maghemite and hematite as remanence carrying components. Bulk and grain size fractionated samples have been analysed using coercivity spectra of remanence acquisition/demagnetisation curves, which identify four main remanence carriers in different grain size fractions of loesses and palaeosols. A linear source mixing model quantifies the contribution of the four components which have been experimentally derived as dominating endmembers in specific grain size fractions. Up to two thirds of the total IRM of the palaeosols are due to slightly oxidised pedogenic magnetite. Two detrital components dominate up to 90% of the IRM of the loess samples and are ascribed to maghemite of different oxidation degree. Detrital hematite is present in all samples and contributes up to 10% of the IRM. The iron content of the grain size fractions gives evidence that iron in pedogenically grown remanence carriers does not originate from the detrital iron oxides, but rather from iron-bearing clays and mafic silicates. The contribution of pedogenic magnetite to the bulk IRM increases with the increasing degree of pedogenesis, which depends in turn on climate change.     

磁性矿物的磁学鉴别方法回顾

磁性矿物的组成和颗粒粒径的分布决定了岩石或沉积物的基本磁学性质及其所携带的天然剩磁在地质时期内的稳定性.由于自然界中磁性矿物的复杂性和多样性，如何有效的确定磁性矿物的成分和粒径分布一直是岩石磁学的基础和难点.本文对确定磁性矿物成分和粒径分布的常见方法进行了较为详细的总结和分析，并对常见磁性矿物在高温和低温下的磁学性质进行了详细的论述.最后对如何应用综合方法判定样品中的磁性矿物种类以及粒度分布进行了探讨.     

泥河湾盆地大长梁剖面河湖相沉积物的岩石磁学性质

以热磁分析为主,对中国北方泥河湾盆地更新世河湖相地层中灰绿色粉砂和灰黄色粉砂/细砂两种典型沉积物进行了详细的岩石磁学研究,有效确定了这两类沉积物中磁性矿物的种类、粒度特征以及加热过程中磁性矿物的变化过程和产物,并对其包含的古环境意义进行了初步探讨.灰绿色粉砂样品主要含有磁铁矿和赤铁矿两种磁性矿物,磁性相对较弱,颗粒相对较细;灰黄色粉砂/细砂主要含有磁铁矿和磁赤铁矿,磁性相对较强,颗粒相对较粗.在氩气环境中经700℃加热处理后,这两种沉积物中的绿泥石都分解,并生成超细粒（处于超顺磁和单畴颗粒区间）的磁铁矿,导致磁化率大幅升高.因此泥河湾盆地沉积物的热磁特征可以用来检测样品中绿泥石的相对含量,进而反映该地区化学风化作用强度的变化.此外,灰绿色粉砂样品中绿泥石含量比灰黄色粉砂/细砂样品的含量高,在氩气中加热后,磁化率升高幅度也较高,可能反映了化学风化相对较弱的沉积环境.     

兰州盆地渐新世沉积物岩石磁学性质探究

我国西北地区出露大量富含哺乳动物化石的"第三纪红层", 探明这些地层的岩石磁学特征对进一步开展磁性地层、环境磁学和古气候学研究具有重要意义. 本文对青藏高原东北缘兰州盆地渐新世地层进行了详细的岩石磁学研究, 分别确定了咸水河组下部砂岩和泥岩中磁性矿物含量、种类及其在加热过程中的转变; 并结合X射线衍射, 漫反射光谱和粒度分析等非磁学手段, 揭示了磁性矿物种类与沉积环境的关系. 结果表明, 剖面底部和顶部的砂岩为河道沉积, 磁性矿物含量较低, 主要为高矫顽力的赤铁矿和针铁矿; 中部泥岩为湖相沉积, 指示了一个相对稳定的静水沉积环境, 其磁性矿物含量比砂岩要高, 不仅含有高矫顽力的赤铁矿和针铁矿,也含有低矫顽力的磁铁矿. 在岩石磁学和环境磁学结果的基础上,结合野外考察和古生物证据, 我们推测兰州盆地在晚渐新世为半湿润半干旱的气候条件, 这为赤铁矿的生成提供了有利条件, 导致盆地中富含赤铁矿的红色泥岩广泛发育.     

渤海南部莱州湾Lz908孔沉积物的岩石磁学性质

亚洲大陆边缘海和陆表海在区域的物质和能量交换以及区域气候与环境演化过程中扮演了关键角色.磁性地层学和环境磁学方法是建立年代框架和环境演变序列的有效手段,但是,由于该地区边缘海和陆表海沉积物中磁性矿物来源十分复杂,磁性地层学和环境磁学研究的重要基础是要精细地解译沉积物的岩石磁学性质.为此,本文利用渤海南部莱州湾Lz908孔与钻孔附近的现代沉积物样品进行了详细的岩石磁学对比研究.结果显示,渤海南部沉积物中的磁性矿物主要是较粗颗粒(较大的准单畴至多畴)磁铁矿,还有少量磁赤铁矿,部分沉积物还含有赤铁矿和针铁矿,其中磁铁矿是特征剩磁的主要载体;莱州湾现代河流-海洋沉积物和钻孔样品之间的磁性特征无显著差异,说明莱州湾沉积物堆积之后尚未经历明显的沉积后期改造.     

Two pedogenic models for paleoclimatic records of magnetic susceptibility from Chinese and Siberian loess

The magnetic susceptibility (MS) of Chinese loess showing a general proportional relationship to pedogenic grade has been widely recognized and used for reconstruction of paleoclimate by Quaternary scientists. The in-situ pedogenic enhancement of ferrimagnetic content is normally believed to be the main reason for the increase of susceptibility in soil units. However, this pattern of high magnetic susceptibility in palaeosols, and low values in loess, are not replicated in some loess deposits. Siberian loess deposits display a completely opposite susceptibility behavior: high values in loess and low values in palaeosols. This inverse relationship has been explained by the idea that magnetic susceptibility is reflecting the magnitude of an aeolian ferrimagnetic component of consistent mineralogy, the grain size of which is related to average wind velocity. Our magnetic study of Siberian samples in this paper suggests that there are notable differences in magnetic properties between Siberian loess and developed palaeosols, not only in magnetic grain-size and concentration but also in magnetic mineralogy. This evidence is difficult to explain fully through variation in wind strength alone, but implies that the low magnetic susceptibility values in the Siberian paleosol units are a reflection, at least in part, of the alteration of the ferrimagnetic content by post-depositional processes. The Loess Plateau is a very arid area where potential evaporation is always higher than precipitation; pedogenesis occurs under dry oxidising conditions. The Siberian Kurtak region is located on the edge of the tundra where it is always wet and saturation during interglacials will lead to a reducing pedogenic environment. Ferrimagnetic minerals under this condition will be destroyed, resulting in lower magnetic susceptibility. Therefore, great care should be taken when using susceptibility values for paleoclimatic reconstruction.     

雅鲁藏布大峡谷格嘎古堰塞湖沉积岩石磁学性质指示的溃决事件

末次冰期时,在雅鲁藏布大峡谷入口处发育了一个古冰川堰塞湖,称为格嘎古堰塞湖.该堰塞湖是认识冰川阻江地貌效应的重要地质证据.古冰川堰塞湖可能会发生多次溃决洪水,通过堰塞湖沉积的分析可以获取堰塞期间的水文事件的信息.本文通过对古堰塞湖沉积39个样品多种磁学参数的详细分析发现,湖相沉积中的砂层低频磁化率（Xlf）为291×10-8m3/kg,泥质沉积的Xlf仅为42×10-8m3/kg;而非磁滞剩磁磁化率（XARM）和饱和等温剩磁（SIRM）的差别则较小.进一步的X-T、Loop和IRM研究以及XARM/Xlf表明,砂层中包含较多的粗颗粒磁铁矿是Xlf较高的主要原因,说明砂层中的磁性矿物没有经过风化破坏,应为近源沉积,并且快速埋藏,指示了溃决事件.本文的结果表明,利用岩石磁学性质以检测古堰塞湖溃决事件是一种可行的新方法.     

Rock magnetic analyses as a tool to investigate diversity of drift pumice clasts: An example from Japan's 2021 Fukutoku-Oka-no-Ba (FOB) eruption

The crystallization of nanolites within magma chambers has recently raised a strong interest due to their impact on increasing melt viscosity and triggering magmatic eruptions. In 2021, the Fukutoku-Oka-no-Ba (FOB) underwater eruption produced large quantities of pumices that eventually formed rafts drifting at the surface of the ocean to the East coasts of Japan. Pumices collected along the shore shortly after grounding show various colors, microscopic and Raman analyses made by Yoshida et al. (Island Arc, 31, 1, 2022) revealed the presence of magnetite nanolites in some of them. In this study, we explore the magnetic properties of a batch of pumices of different colors from the FOB eruption, aiming to refine characterization of iron oxide nanolites. We used various analytical techniques such as SEM and FEG-SEM observations, EDS-X analyses, and rock magnetic experiments, including thermomagnetic analyses, hysteresis curves, coercivity analyses and FORC measurements. Our findings reveal that the iron oxides present in the FOB samples are Ti-magnetite, with minor amounts of Mg and Al. The magnetic crystals show a wide range of sizes, from extra small iron oxide nanolites (ESION) in the pumices with the lighter colors, to more bulky grains reaching the micrometer size in some of the dark color samples, significant diffusion is inferred in that case. Consequently, the magnetic characterization of iron oxide crystals within the Fukutoku-Oka-no-Ba pumices reveals varying stages of nucleation, dissolution, growth, and diffusion processes, providing evidences for the heterogeneous state of the magma during the eruption.     

Loess in the Czech Republic: Magnetic properties and paleoclimate

Summary We have studied 454 oriented samples from seven loess outcrops in the Czech Republic for comparison and correlation of the magnetic properties with those of the loess profiles in China and Central Asia. Three sections at Sedlec (Prague), Zeměchy and Dolní Věstonice cover the time span of the last glacial-interglacial cycle. Loess from the middle Pleistocene period including pedocomplexes V and VI was studied at Sedlešovice (Znojmo) and Karlštejn. Loess of uncertain — possibly lower to middle Pleistocene age — was sampled at Sedlec near Mikulov. At Červeny Kopec (Red Hill, Brno) we investigated the oldest loess near the Matuyama/Brunhes (M/B) boundary. The characteristic remanence (ChRM) determined after magnetic clearning is of normal polarity throughout all sections except at Červeny Kopec. There, two polarity changes from reversed to normal were found within two sections initially thought to represent two successive stratigraphic intervals. However, the marked similarities between lithological, susceptibility, declination and inclination profiles lead us to suggest that the two polarity changes are in fact one and the same. The two sections are therefore laterally equivalent and both have recorded the M/B field reversal. The susceptibility variations were used to mutually correlate the Czech sections and to construct a composite profile across the present day loess outcrops in the Czech Republic. They also serve as a paleoclimate proxy which can be compared and correlated with the paleoclimatic records observed in the Chinese and Central Asian loess. The origin of the low field susceptibility variations was studied by measuring the frequency dependence of susceptibility which revealed that the enhancement within the pedocomplexes is controlled by the amount of fine-grained ferromagnetic minerals present.     

苏北盆地兴化1#钻孔沉积物的岩石磁学特征

对苏北盆地兴化1#钻孔岩心沉积物进行了质量磁化率、频率磁化率、变化曲线、等温剩磁和磁滞回线等参数的测试分析,结果表明,粘土质沉积物中的主要载磁矿物为磁铁矿和赤铁矿;砂质沉积物中的磁性矿物除了磁铁矿和赤铁矿外,还含有少量的磁赤铁矿和针铁矿.针铁矿普遍存在于钻孔下部的样品中.整个钻孔沉积物中的磁性矿物颗粒都是介于单畴和多畴之间的准单畴颗粒,但粘土质沉积物中的磁性矿物颗粒更趋近于单畴颗粒,而砂质沉积物中的磁性矿物颗粒更趋近于多畴.根据不同的磁性矿物组合特征,选择合理的温度和磁场强度区间对古地磁退磁结果进行分析,得到沉积物可靠的特征剩磁方向,为古地磁年代学提供依据.     

Pedogenic factors affecting magnetic susceptibility of the last interglacial palaeosol S1 in the Chinese Loess Plateau

The magnetic susceptibility has been used as a quantitative or semi‐quantitative proxy for reconstructing the summer monsoon intensity in the Chinese Loess Plateau based on extensive studies on climatic or/and environmental mechanisms producing the magnetic susceptibility signatures. However, the precise nature of the link between past climates and the susceptibility signatures has remained uncertain primarily due to lack of our understanding in the ?nalizing and preserving processes of the signatures. This paper attempts to examine the reliability or acceptability of this summer monsoon proxy from non‐magnetic perspectives of soil‐forming processes. We chose nine sections along two transects: one across the western part of the Chinese Loess Plateau and another across the eastern part. Several conclusions can be drawn from our analytical data. First, clay translocation within the S1 palaeosol pro?les, as indicated by ?eld‐observed clay coatings on ped faces in Bt and Bk horizons and demonstrated by laboratory‐analysed clay contents, must have moved some of the magnetic minerals downward so that the susceptibility re?ects only the post‐translocation distribution of the magnetic‐susceptibility‐producing minerals. Second, the best‐developed palaeosol S1S3 at most of the sections studied is not expressed by the magnetic susceptibility because this palaeosol developed in underlying coarse loess (L2) and coarse textures tend to lower the susceptibility. Third, carbonate concentration is normally negatively correlated with the magnetic susceptibility or simply suppresses the magnetic susceptibility peak when the susceptibility enhancement exceeds the carbonate dilution effect. To conclude, extreme caution must be observed when using magnetic susceptibility signatures to retrieve high‐resolution records of the last interglacial palaeoclimate in the Chinese Loess Plateau. Copyright © 2004 John Wiley & Sons, Ltd.     

Magnetic properties and their paleoclimatic implications revealed from the last glacial eolian sedimentary sequence in Pengze, Jiangxi

The sand-hill section in Hongguang, Pengze County, Jiangxi Province is composed of eolian sand beds interbedded with 12 weakly-pedogenetic paleosols, and this sedimentary sequence was formed from about 25 to 13 kaBP. A detailed rock-magnetic study demonstrates that the magnetic minerals in the sequence are dominated by multi-domain magnetite grains in addition to small amounts of maghemite and hematite. Compared with the eolian sand beds, the paleosols are characterized by smaller magnetic particles, higher proportion of low-coercivity minerals and markedly increase in the magnetic mineral content. In the section, various magnetic parameters (K, ARM, SIRM and S ratios) display a roughly gradual increase from eolian sand beds to the overlying paleosols but an abrupt decrease from paleosols to the overlying eolian sand beds. The content of low-coercivity magnetic minerals shows the lowest values in the second eolian sand bed from the bottom of the section, and from the bed upwards it gradually increases on the whole in the eolian sand beds. These magnetic properties reflect regional climatic changes during the last glacial maximum.     

福建三明地区被污染土壤的磁学性质及其环境意义

对福建三明某钢铁厂和火电厂附近的污染表土样品进行了多参数的岩石磁学测试分析,包括χ T曲线、磁滞回线、等温剩磁获得曲线等. 三明地区污染表土中的磁性矿物有磁铁矿、赤铁矿和磁黄铁矿. 样品中磁性矿物的平均粒度较粗,为较大的准单畴,甚至多畴,粒度明显大于成土作用所产生的磁性颗粒. 粗粒的磁铁矿颗粒是污染物的主要磁性组分. 虽然磁化率测量可以作为一种简单、快速而且廉价的检测污染土壤的方法,但同时辅以必要的岩石磁学测量将有利于提取更多的污染信息. 对于低磁化率的污染土壤,亚铁磁性硫化物的存在可以作为土壤可能被污染的证据之一.