The occurrence of Greigite in sediments from Loch Lomond

Greigite has been identified in the sediments of Loch Lomond using X-ray diffraction. Greigite is the dominant magnetic mineral in the oldest sediments recovered, while magnetite predominates in the younger sediments which had previously been used for palaeomagnetic secular variation studies. A layer of sediment in between these two magnetic mineral regimes has very low magnetic concentrations, probably as a consequence of magnetite dissolution associated with sulphide rich pore-waters produced at the time of a marine incursion. The greigite largely oxidises once the sediment is exposed to air, but if freeze dried the greigite becomes surprisingly stable. Following freeze drying the greigite can be heated to 280°C in air before it alters and loses its strong ferrimagnetic properties.     

Geochemical control of magnetite dissolution in subarctic lake sediments and the implications for environmental magnetism

Geochemical and mineral magnetic studies of lake sediments from the Kårsa valley in north Sweden demonstrate that down-core variations in magnetic properties have been determined by post-depositional magnetite dissolution rather than by sediment source changes. A low sediment accumulation rate during times of climatic amelioration combined with a higher organic carbon content produced intense reducing conditions with almost complete magnetite loss. An increase in the sedimentation rate and a reduction in the organic carbon content, primarily due to glacier reformation, allowed magnetite preservation. Thus the mineral magnetic profiles record lake sediment palaeoredox conditions brought about by climate change. This new interpretation of mineral magnetic transformations in freshwater sediments may have far reaching implications for other mineral magnetic studies concerned with the alternation between glacial (stadial) and interglacial (interstadial) episodes during the Quaternary period.     

Magnetic mineralogy and its implication of contemporary coastal sediments from South China

Magnetic parameters have been widely used as a rapid, cost-effective, and non-destructive method. To assess whether their magnetic properties reflect the development history in the Pearl River Delta, three sediment cores collected from South China coastal waters were selected for magnetic research. The results indicate that the predominant ferrimagnetic mineral is magnetite, with an additional smaller amount of surprisingly high-coercive greigite. Fine-grained superparamagnetic and single-domain particles of magnetite and the relative contribution of greigite are increased when the total concentration of ferrimagnetic minerals is higher. Three well-known stages of the Chinese history, i.e., iron smelting, cultural revolution, and the latest phase of opening and reforming were identified in the vertical variation of magnetic concentration parameters. Although the record of the three cores is not completely consistent, the results point out that magnetic concentration parameters can reflect the development and pollution history in surrounding coastal areas.     

磷灰石对湖泊沉积物中水铁矿稳定性的制约

通过在滇池开展原位实验,研究探讨了湖泊沉积物中磷灰石制约水铁矿分解和转化的机制,以及二者共存时的环境效应。结果表明:将水铁矿放置到沉积物中1个月,矿物保持稳定;放置时间达到3个月时,添加磷灰石实验中水铁矿发生了显著物相转变。冬天(12—2月)实验中,转化产物随深度的变化趋势为针铁矿+磁(赤)铁矿→针铁矿+纤铁矿→针铁矿;夏天(6—9月)实验中,转化产物随深度的变化趋势为针铁矿+纤铁矿+磁(赤)铁矿→针铁矿+纤铁矿→未转化。透射电镜分析结果显示冬天实验中生成的磁性铁氧化物为纳米磁铁矿和磁赤铁矿,夏天实验中产生的则主要为纳米磁铁矿。X射线光电子能谱分析结果显示冬天表层实验样品具有较高P含量。分析表明的湖泊沉积物中磷灰石促进水铁矿转化的过程为:(1)微生物促进磷灰石溶解;(2)磷灰石溶解释放的P促进铁还原菌生长;(3)铁还原菌促进水铁矿还原;(4)水铁矿还原产生的溶解态Fe²⁺催化水铁矿向针铁矿、纤铁矿和磁铁矿转化。冬天及沉积氧化-还原界面最适宜磷灰石分解菌和铁还原菌生长,水铁矿的转化和P释放能力也更强,相应地内源磷释放的风险也更大。     

大同盆地高砷地下水系统沉积物环境磁学特征

为了研究高砷含水层沉积物中砷与磁性矿物的关系, 对大同盆地高砷地区含水层钻孔样品进行了环境磁学和地球化学分析, 结果表明亚铁磁性矿物是决定含水层沉积物磁性特征的主要磁性矿物.通过对比砷和磁性参数在垂向上的变化趋势发现, 高砷含量往往对应着低的饱和等温剩磁(SIRM) 值, 揭示出砷和顺磁性矿物之间也存在联系.沉积物砷含量与磁性参数的相关系数均小于0.5 (α=0.05), 表明砷和亚铁磁性矿物及不完整反铁磁性矿物之间不存在明显的联系.在所有磁性参数中磁化率(χ)和砷含量之间相关性最显著, 相关系数均在0.4 (α=0.05) 左右, 低的相关系数与亚铁磁性矿物和不完整反铁磁性矿物对顺铁磁性矿物的稀释有关.该地区高砷地下水的形成可能与水铁矿及纤铁矿等砷的赋存矿物在还原条件下的还原溶解有关.      

Magnetic properties of light and dark sediment layers from the japan sea: Diagenetic and paleoclimatic implications

Rock magnetic/paleoclimatic/diagenetic relationships of sediments spanning the last 0.78 Ma have been investigated using samples collected from light and dark layers recovered at ODP Sites 794 (Yamato Basin) and 795 (Japan Basin). Rock-magnetic parameters (K, K_fd, ARM, SIRM, S-ratio) are shown to reflect diagenetic processes and climate-related variations in the concentration, mineralogy and grain-size of the magnetic minerals contained within the sediments. The magnetic mineralogy is dominated by ferrimagnetic (magnetite-type) minerals with a small contribution made by hematite and iron sulphides such as pyrrhotite and/or greigite. Magnetic mineral concentration and grain size vary between light and dark layers with the former characterized by a higher magnetic content and a finer magnetic grain size. Magnetite dissolution, related to sulfate reduction due to bacterial degradation of organic matter, is the process responsible for the magnetic characteristics observed in the dark layers, testifying to the reducing conditions in the basin. Variations in the rock magnetic properties of the sediments are strongly correlated with global oxygen isotope fluctuations, with glacial stages characterized by a lower magnetic mineral content and a coarser magnetic grain size relative to interglacial stages. Major downcore changes in the magnetic properties observed at Site 794 can be related to changes in the oceanographic conditions of the basin associated with the flow of the warm Tsushima Current into the Japan Sea at about 0.35–0.40 Ma ago.     

Investigating the potential of using environmental magnetism techniques as pollution proxy in urban road deposited sediment

The use of mineral magnetic techniques as pollution proxy for road deposited sediment was explored using various statistical approaches. Standard techniques were adopted for measurement of mineral magnetic and geochemical parameters. The analyses of magnetic parameters revealed that the samples were dominated by ferrimagnetic minerals and multidomain grains. This implied that the magnetic fractions in the samples might be of anthropogenic origin. Results also indicate that the samples were dominated by low coercive, magnetically soft minerals. Thermomagnetic curves confirmed magnetite as the remanence bearing magnetic mineral having a Curie point temperature of ~580 °C. The strong association observed between magnetic susceptibility, susceptibility of anhysteric remanent magnetization and saturation isothermal remanent magnetization and aluminum, titanium, manganese, iron, chromium and lead demonstrated that these metals occurred as ferrimagnetic particles of technogenic origin resulting from vehicular sources. Assessment of pollution status of the road deposited sediment identified silicon and lead as the priority pollutants of concern. Generally, pollution load index was <1 (mean, 0.66 ± 0.14), indicating that the samples were not polluted in the overall, but the metals were in the buildup stage requiring constant monitoring. The sources of pollutants from principal component and cluster analyses identified the sources of pollution to be mainly from vehicular emissions such as brake linings, exhaust materials, tire wear, corroded metal parts, abrasion of lubricating oil and road construction materials. This study found that mineral magnetic techniques offer great potential as pollution proxy for soil pollution studies.     

赤道东太平洋C-C区硅质沉积物的磁性特征及细菌合成磁铁矿

对分布于赤道东太平洋克拉里昂和克里帕顿断裂带之间 (C-C区 )的硅质沉积物所作的环境磁学研究表明,沉积物的磁性特征由亚铁磁性矿物主导,亚铁磁性矿物颗粒以单畴 (SD)和超顺磁 (SP)为主。磁性参数和透射电镜(TEM )分析表明,该区沉积物中存在着细菌合成的磁铁矿,并观察到磁铁矿存在不同程度的溶解。这一发现是微生物参与C-C区铁元素循环的直接证据。不完整反铁磁性矿物在氧化性较强的西区沉积物中含量较高,且随着纬度升高而增加,显示随着远离赤道,沉积物氧化性趋强.     

A magnetic investigation of a loess/paleosol sequences record in Ili area

The pedogenic ferrimagnetic minerals have been considered to be the cause of magnetic susceptibility enhancement in loess deposits distributing in the Chinese Loess Plateau and Central Europe, while “wind intensity” mechanism is proposed to be responsible for the magnetic susceptibility enhancement of loess in Alaska and Siberia. However, the magnetic enhancement mechanism is still open for loess in Ili valley, Xinjiang Uygur Autonomous Region, China. To understand this, we conducted a rock magnetic investigation on Axike section that is located in Ili valley. Results show that transitional stage from magnetic (χ_lf) trough to magnetic (χ_lf) peak corresponds to soil units. The PSD and MD-grained magnetite dominate the magnetic properties of AXK sequences, and the main factor affecting magnetic properties is the concentration of ferrimagnetic fraction. For the “pedogenesis enhancement” and “wind intensity” model, it seems hard to explain the magnetic susceptibility enhancement in this area. For the concentration of fine-grained magnetite in magnetic mineral shows positive relationship with the intensity of pedogenesis, the magnetic parameters of loess deposit in Ili valley can be used to recover paleoclimatic variations.     

Mineral magnetic properties of Holocene lake sediments and soils from the Kårsa valley,Lappland, Sweden,and their relevance to palaeoenvironmental reconstruction

A mineral magnetic study of lacustrine sediments and soils from the Kårsa valley in Lappland, Sweden was undertaken to estimate the potential of mineral magnetic techniques in elucidating the variability of climate, recorded by glacier fluctuations and soil development. Sediment cores from two lake basins receiving sediment input from a glacial outflow stream, and their respective catchments, show a mineral magnetic record that can be interpreted in terms of glacial activity and soil development. Dissolution of magnetite is indicated during periods of climatic amelioration and causes a high S-ratio. During periods of glacial advance (and colder, wetter climate) the minerogenic sediment supply is increased and magnetite is preserved in the sediment, resulting in a low S-ratio. Variations in the S-ratio can then be used as a proxy-climate indicator.     

石英尾砂对濠河上游沉积物污染的磁学响应

濠河上游沉积物和石英尾砂磁学参数及SiO₂质量分数(w(SiO₂))分析结果表明,沉积物中w(SiO₂)高达80%以上,说明沉积物已被石英尾砂严重污染。磁测结果显示,沉积物与石英尾砂的磁学特征基本相同,这2类样品中的主要磁性矿物是磁铁矿,含量较低,磁性矿物粒径较宽,主要为0.1~5 μm;这主要是大量石英尾砂进入濠河后,在其稀释作用下,沉积物磁性减弱,使磁性矿物的粒径变粗。沉积物中w(SiO₂)与磁学参数(χ、SIRM和χ_ARM)存在负相关关系(-0.83≤R≤-0.61),表明磁学参数能反映石英尾砂对沉积物的污染程度,其值越低说明石英尾砂对濠河污染越严重。     

Physical and chemical steady-state compaction in deep-sea sediments: Role of mineral reactions

Marine sediments typically exhibit steep porosity gradients in their uppermost centimeters. Although the decrease in porosity with depth below the sediment-water interface is primarily due to compression arising from the accumulation of overlying sediment, early diagenetic mineral dissolution and precipitation reactions may potentially also affect the porosity gradient. Here, we present a steady state compaction model, based on the mass and momentum conservation of total fluid and solid phases, in order to quantify the relative contributions of mineral reactions and physical compaction on porosity changes. The compaction model is applied to estimate hydraulic conductivity and compressive response coefficients of deep-sea sediments from measured porosity depth profiles. The results suggest an inverse relation between the compressive response coefficient and the lithogenic content of marine sediments. For deep-sea sediments exhibiting high rates of dissolution of siliceous shell fragments, the compaction model ignoring mineral reactions overestimates the hydraulic conductivity and compressive response coefficients. In contrast to non-compacting porous media, mineral dissolution in surficial sediments can lead to lower porosity. However, as illustrated for a deep-sea sediment in the equatorial Atlantic characterized by extensive dissolution of calcareous shell fragments, the effect of mineral dissolution and precipitation reactions on porosity gradients is, in most cases, negligible.     

长江三角洲晚新生代沉积物磁性特征和磁性矿物及其指示意义

笔者通过对长江三角洲平原晚新生代钻孔SG7孔沉积物的粒度和磁性分析,揭示磁性矿物类型随时间的演变,探讨新构造运动、气候、海平面等对本区沉积物源、沉积环境的耦合作用。研究结果显示,上新世磁性强弱相间,且达到全剖面最强,磁性矿物为磁铁矿、磁赤铁矿、针铁矿和黄铁矿,反映本区气候暖湿,发育山间湖泊,沉积物主要来自周边白龙港玄武岩的风化产物;早更新世早期,磁性为全剖面最弱,以磁铁矿为主,反映在构造沉降作用下,发生物源改变,即主要来自贫铁的中酸性浅成岩或喷出岩,同时沉积物主要为冰期的河道相沉积,反映山间河流/冲积扇环境;早更新世晚期至晚更新世末,磁性显著增强且和沉积物粗细变化一致,粗颗粒沉积物中磁性矿物以变质岩来源的粗粒磁铁矿为主,细粒沉积物中磁性矿物见针铁矿、赤铁矿和黄铁矿,反映古地理环境逐渐向冲积平原、滨海平原演变;自晚更新世晚期出现细粒磁赤铁矿和磁铁矿,反映长江上游物源的加入。全新世沉积物磁性较更新世泥质沉积物显著增强,反映长江河口、三角洲环境。     

热带西太平洋沉积物的环境磁学特征对东亚冬季风的响应

西太平洋暖池(Western Pacific Warm Pool,WPWP)指位于热带太平洋中、西部(包括南海南部和苏禄海)年平均水温超过 28 ℃的广大海域。由于巨厚的表层暖水覆盖,西太平洋暖池成为全球热量和水汽交互的重要源区,对驱动温盐环流、调节全球气候变化具有重要作用。东亚冬季风(EAWM)是全球气候系统中最活跃的组成部分之一,它可能会通过寒潮侵入热带地区,引起深层对流,以此加强暖池区的对流活动和降水异常,从而影响赤道地区的潜热释放。但地质历史时期西太平洋暖池与东亚冬季风的相互作用关系尚不明确。由于东亚冬季风携带的风尘中往往包含有大颗粒的高矫顽力磁性矿物,我们可以通过沉积物中磁性矿物的组合、含量、颗粒大小和形态变化,分析不同时间尺度上气候环境变化和风尘物质的输入情况,进而反演东亚冬季风的强度变化。本文对取自热带西太平洋B10钻孔的岩心样品进行了环境磁学测试,以揭示地质历史时期西太平洋暖池沉积物环境磁学特征对东亚冬季风的响应。实验结果表明,沉积物中的主要载磁矿物为低矫顽力的磁铁矿,属于亚铁磁性矿物,并含有少量高矫顽力磁性矿物。沉积物中的磁性颗粒以准单畴(PSD)颗粒为主。高矫顽力的磁性矿物含量和细颗粒磁铁矿相对含量在冰期和间冰期呈现出显著相对变化,对东亚冬季风的变化有敏感响应:冰期沉积物中高矫顽力矿物含量增多,磁性颗粒粒径变大;间冰期沉积物中高矫顽力矿物含量降低,磁性颗粒粒径变细。在干燥、寒冷的冰期,由风尘携带而来的高矫顽力磁性矿物相对含量增加,沉积物中的磁性颗粒粒径变大,反映冰期东亚冬季风强度增大;在气候温暖湿润的间冰期,风尘的输入量较小,由风尘携带的高矫顽力磁性矿物含量相对较低,沉积物中的磁性颗粒粒径变小,反映间冰期东亚冬季风的强度减弱。     

Mineral magnetic properties of Late Weichselian-Holocene sediments from the northwestern Baltic Proper

Mineral magnetic measurements were carried out on twelve sediment cores from the northwestern part of the Baltic Proper. The purpose was to use magnetic properties to correlate sediment sequences and to investigate changes in depositional conditions (e.g. rate of sedimentation and redox conditions). A general lithostratigraphy consisting of five units was established based mainly on mineral magnetic properties. Sediment has been deposited from the time of the Baltic Ice Lake (before 10 300 ¹⁴C years BP) to the present, i.e. since the Late Weichselian deglaciation. Five of the cores were also investigated by means of biostratigraphical methods and three by ¹⁴C dates. Based on the general stratigraphy and hiatuses, variations in sedimentation rate were identified. The occurrence of authigenically formed ferrimagnetic greigite (Fe₃S₄), mainly in sediments deposited during the brackish Yoldia Sea and Litorina Sea stages, is implied. These sequences are characterized by low S-ratios and high SIRM/z ratios. Glacial clay, deposited in freshwater during the Baltic Ice Lake stage and the early freshwater phase of the Yoldia Sea stage, seems to be unaffected by diagenetic processes. Gyttja clay, deposited in the Litorina Sea, has significantly lower susceptibility and SIRM than in the underlying clay. Dissolution of magnetic iron oxides (e.g. magnetite and hematite) in an anoxic environment was suggested as an explanation.     

川西高原理县黄土磁学特征及其影响因素

川西高原河谷阶地和断陷盆地广泛分布厚层风成黄土-古土壤序列,目前对其磁学性质变化机制及古气候意义研究还很薄弱。文章对该区理县黄土-古土壤剖面进行了系统的岩石磁学研究,确定了其磁性矿物种类、含量和颗粒大小的特征及其变化规律。通过结合粒度、色度与地球化学参数,进一步探讨了理县黄土-古土壤磁性的主控因素。结果表明:1)理县剖面同时含有强磁性矿物(为磁铁矿和磁赤铁矿)以及弱磁性矿物(为赤铁矿和针铁矿);2)相对黄土层,古土壤层含有更高比例的亚铁磁性矿物;3)成土过程中生成的大量细小强磁性矿物颗粒,是古土壤层S1磁化率增加的主导因素,该模式与黄土高原相似;4)结合色度以及磁学性质,可以较为明确地区分成壤强度;(5)理县黄土剖面物源复杂,磁学特征受到沉积物来源、后期流水和气候的共同作用,利用单一磁学性质(比如磁化率)进行古气候研究会造成多解性。     

长江、汉江沉积物磁学特征比较研究

长江和汉江现代沉积物的磁学特征分析研究结果表明：  1）长江和汉江现代沉积物中铁磁性矿物均以亚铁磁性矿物为主,它们主导了样品的磁性特征。但长江沉积物比汉江沉积物亚铁磁性矿物含量要高;  2)长江和汉江沉积物的亚铁磁性矿物晶粒都以假单畴~多畴为主。相对汉江沉积物,长江沉积物磁性颗粒总体上要比汉江的粗;   3）长江沉积物中的不完全反铁磁性矿物含量比汉江高,但不完全反铁磁性矿物对沉积物磁性参数和SIRM的贡献却比汉江的小;   4）长江上游金沙江段比长江中游（宜昌-武汉）江段沉积物中亚铁磁性矿物含量要高,且长江中游沉积物样品磁性颗粒要比长江上游的偏细,这可能与搬运距离有关;   5）长江样品的磁性参数的变化区间比汉江大,数据间的相关性也比汉江差。这可能与长江较汉江流域范围广、支流多、物源复杂有密切的关系。     

Early diagenesis in anaerobic lake sediments: chemical equilibria in interstitial waters

As part of a study of early diagenesis in anaerobic lake sediments the major ions and P, Fe, Mn, pS, p? and pH were measured in interstitial waters of cores from eutrophic Greifensee in north-central Switzerland. A guide for the interpretation of such data is thermodynamic equilibrium between the dissolved species and mineral phases. The results show that CaCO₃, which precipitates from the lake water, dissolves rapidly in the CO₂-rich interstitial waters resulting in equilibrium with calcite in the top few centimeters of sediment. Siderite, on the other hand, is nearly ten-fold supersaturated 20–30 cm below the surface. Iron monosulfides are metastable in the surface 10–20 cm of sediment and pS and p? results indicate equilibrium between polysulfides and orthorhombic sulfur. Vivianite appears to be limiting the solubility of phosphate below a few centimeters of sediment.The above processes are highly interactive, especially in the case of ferrous iron, with one result being a two order of magnitude decrease in dissolved phosphate in a depth interval of 20cm in the interstitial waters.     

The influence of particle size variations on the magnetic properties of sediments from the north-eastern Irish Sea

This paper considers the magnetic properties of a range of recent and contemporary sediments from the north eastern part of the Irish Sea. Principal component ordinations of the results show a close link between magnetic property variations and particle size. The magnetic properties of a subset of samples, particles sized by a combination of sieving and pipette analysis, confirm that variations in ferrimagnetic (‘magnetite’) grain size parallel those in particle size, despite the fact that the magnetic grains in the fine grades have diameters 1–2 orders of magnitude smaller than those of the particle size fraction in which they occur. This is best explained by postulating that the fine magnetic grains occur in the clay fraction but are present in declining concentrations in the coarser grades up to 4ø as an artefact of the pipette method. Most samples have a biomodal distribution of magnetic minerals, with a coarse mode associated with heavy minerals in the sands or coarse silts, and a fine mode in the clays. Magnetic susceptibility (x) and saturation isothermal remanent magnetization (SIRM) largely pick out the coarse mode where present; anhysteretic remanent magnetization (ARM) largely picks out the fine mode. The results open up the possibility of normalizing samples from these environments for particle size and, more specifically, clay content, by means of ARM or ARM/x values. The most likely source for the uniform and almost exclusive stable single domain magnetite, which dominates the magnetic properties of the clays, is thought to be bacterial magnetosomes. The measurements as a whole do not appear to hold much promise for discriminating sediment source types.     

Characterization of diamictons on the basis of their mineral magnetic properties in Murchisonfjorden,Nordaustlandet, Svalbard

Different grain-size fractions of 12 lithologically defined and 2 non-defined Weichselian glacial stage diamicton units from Nordaustlandet, Svalbard were studied for their mineral magnetic properties. The applied measurements included magnetic susceptibility (χ), anhysteretic remanent magnetization (ARM), saturation isothermal remanent magnetization (SIRM), ‘backfield’ isothermal remanent magnetization (IRM) and several of their parametric ratios. The results indicated a poor correlation between the magnetic fingerprint of dry-sieved and wet-sieved diamicton samples. The dry-sieved samples had higher χ, ARM and SIRM values, suggesting a higher concentration of ferrimagnetic grains. This was probably because washing of the samples resulted in the loss of the most fine-grained ferrimagnetic material. Both sieving methods seem to be capable of discriminating different diamictons, but they cannot be used contemporaneously. The results only partially support previous findings of the chronostratigraphic correlation of diamictons. They indicate that in suitable conditions and within a limited area it is possible to discriminate (and group) glacigenic sequences based on the magnetic signature of their fine-grained fractions in the sample. For example and based on the mineral magnetic fingerprint, a diamicton unit that was described from the marine core (MUR 09/sor1/125–150) obtained from Isvika region most probably represents diamicton material that were derived from provenance areas related to the Middle Weichselian glacial stage. It was also noted that certain mineral magnetic parameters, such as SIRM/χ and SIRM/ARM ratio, had greater potential in discriminating diamicton units than others.