Magnetic properties of bottom sediments from Meromectic Shira Lake (Siberia,Russia)

Magnetic properties were studied in bottom sediments of saline meromictic Shira Lake by the methods of static magnetometry and resonance Mössbauer spectroscopy for the first time. All layers of bottom sediments contain nanosized single-domain magnetite particles produced by magnetotactic bacteria. The concentration of magnetite in bottom sediments decreased with depth, reaching a local minimum in the layer corresponding to the minimal level of the lake observed in 1910–1930. It is demonstrated that biogenic magnetite may indicate climate-related changes in the level of Shira Lake, in addition to the other biological and geochemical characteristics.     

The identification and biogeochemical interpretation of fossil magnetotactic bacteria

Magnetotactic bacteria, which most commonly live within the oxic-anoxic transition zone (OATZ) of aquatic environments, produce intracellular crystals of magnetic minerals, specifically magnetite or greigite. The crystals cause the bacteria to orient themselves passively with respect to the geomagnetic field and thereby facilitate the bacteria's search for optimal conditions within the sharp chemical gradients of the OATZ. The bacteria may also gain energy from the redox cycling of their crystals.Because magnetotactic bacteria benefit from their magnetic moments, natural selection has promoted the development of traits that increase the efficiency with which the intracellular crystals impart magnetic moments to cells. These traits also allow crystals produced by magnetotactic bacteria (called magnetofossils when preserved in sediments) to be distinguished from abiogenic particles and particles produced as extracellular byproducts of bacterial metabolism. Magnetofossils are recognizable based on their narrow size and shape distributions, distinctive morphologies with blunt crystal edges, chain arrangement, chemical purity, and crystallographic perfection. This article presents a scheme for rating magnetofossil robustness based on these traits.The magnetofossil record extends robustly to the Cretaceous and with lesser certainty to the late Archean. Because magnetotactic bacteria predominantly live in the OATZ, the abundance and character of their fossils can reflect environmental changes that alter the chemical stratification of sediments and the water column. The magnetofossil record therefore provides an underutilized archive of paleoenvironmental information. Several studies have demonstrated a relationship between magnetofossil abundance and glacial/interglacial cycles, likely mediated by changes in pore water oxygen levels. More speculatively, a better-developed magnetofossil record might provide constraints on the long-term evolution of marine redox stratification.More work in modern and ancient settings is necessary to explicate the mechanisms linking the abundance and character of magnetofossils to ancient biogeochemistry.     

Elongated prismatic magnetite crystals in ALH84001 carbonate globules: potential Martian magnetofossils.

Using transmission electron microscopy (TEM), we have analyzed magnetite (Fe3O4) crystals acid-extracted from carbonate globules in Martian meteorite ALH84001. We studied 594 magnetites from ALH84001 and grouped them into three populations on the basis of morphology: 389 were irregularly shaped, 164 were elongated prisms, and 41 were whisker-like. As a possible terrestrial analog for the ALH84001 elongated prisms, we compared these magnetites with those produced by the terrestrial magnetotactic bacteria strain MV-1. By TEM again, we examined 206 magnetites recovered from strain MV-1 cells. Natural (Darwinian) selection in terrestrial magnetotactic bacteria appears to have resulted in the formation of intracellular magnetite crystals having the physical and chemical properties that optimize their magnetic moment. In this study, we describe six properties of magnetite produced by biologically controlled mechanisms (e.g., magnetotactic bacteria), properties that, collectively, are not observed in any known population of inorganic magnetites. These criteria can be used to distinguish one of the modes of origin for magnetites from samples with complex or unknown histories. Of the ALH84001 magnetites that we have examined, the elongated prismatic magnetite particles (similar to 27% of the total) are indistinguishable from the MV-1 magnetites in five of these six characteristics observed for biogenically controlled mineralization of magnetite crystals.     

纳米磁铁矿链的仿生合成及其生物矿化意义

本文在不使用任何蛋白质或生物分子的情况下,以四方针铁矿和二价铁离子为铁源仿生合成磁铁矿纳米颗粒。实验 结果表明,在弱碱性条件下,合成的磁铁矿颗粒为35 nm左右的近似立方体,而且这些颗粒能够自发的定向排列,形成类 似趋磁细菌体内的磁小体链状结构。作者认为,由于磁铁矿晶体存在着固有的磁偶极,晶体之间的磁偶极作用力驱动着磁 铁矿颗粒自发组装成定向排列的链状结构。这就揭示了在趋磁细菌体内磁小体的矿化及组装链形成过程中,除了生物蛋白 影响外,磁小体颗粒之间的磁偶极吸引作用也可能是一个重要因素。生物蛋白和晶体化学因素可能在趋磁细菌体内生物矿 化过程中协同起作用。     

中国黄土剖面趋磁细菌的组成特征与生态意义

选择中国的黄土-古土壤序列的超磁细菌作为研究对象，以透射电镜、扫描电镜、生化实验和有机地球化学方法先后研究了从黄土地区陕西段家坡、甘肃西峰以及陕西洛川富集的趋磁细菌，结果表明，虽然趋磁细菌在黄土和古土壤层的形态有所差异，但它们均由细胞膜、细胞质和沿长轴排列的磁小体与气泡组成，在适宜的条件下它能生长出较多的磁小体。磁小体由氧化铁组成，并可从体内排出，使环境中部分Fe^2 转变为Fe^3 ，促使地层磁化率增高，尤其在古土壤层段，中国黄土剖面中趋磁细菌的产量是地表温度、湿度、pH、铁含量的函数，与湖沼中的趋磁菌相比，它在菌体形态、磁小体的数量与排列方式上有很大差距，因而中国西北黄土地区的趋磁细菌是典型的生态物种。     

黄土剖面中趋磁细菌及其磁小体的初步研究

从中国西安市附近段家坡黄土剖面分离到微好氧性的趋磁细菌，经透射电子显微镜Ｈ－７０００ＦＡ和Ｈ－６００ＳＥＭ／ＥＤＡＸ－ＰＶ９１００型分析，黄土层（Ｌ１）中仅发现弧形菌，数量少，每个菌体含２个磁小体；古土壤层（Ｓ０和Ｓｉ）含有大量杆状菌，每个菌体有１５个以上的磁小体，沿细菌长轴呈两排链状排列，且具有向南磁极游动的习性，Ｘ射线能谱分析表明，磁小体中具有高丰度的Ｆｅ和Ｃｏ。趋磁细菌中磁小体的数量在古土壤     

Diagenetic Dissolution of Biogenic Magnetite in Surface Sediments of the Benguela Upwelling System

Intense primary biologic productivity in the surface waters of the Benguela upwelling system provides a high supply of organic matter to the sea floor at the continental slope off Namibia and sustains extreme concentrations of magnetite producing bacteria in the top sediment layers. Biogenic magnetite is thus by far the dominant carrier of the magnetic signal in these deposits also because of a very minor input of terrigenous ferrimagnetic minerals. Reducing conditions in the sediment column cause a selective dissolution of the bacterial magnetite fraction just a few centimeters below the main mineralization horizon. This diagenetic process is documented in detail by high-resolution rock magnetic analyses and transmission electron microscopy. Concentration dependent and grain-size sensitive magnetic parameters, such as susceptibility, laboratory imparted remanences, and hysteresis data, reveal a significant drop in ferrimagnetic mineral content within the upper 10 cm of the sediments accompanied by a gradual downward coarsening of the ferrimagnetic mineral assemblage from primarily magnetic single-domain particles in the top centimeters to multi-domain grains in deeper strata. Electron microscope observations enable both an unequivocal identification of bacterial magnetite on the basis of shape and grain-size and to trace dissolution effects on the biogenic magnetic mineral component to depth. Received: 9 November 1998 / Accepted: 10 October 1999     

Mössbauer hyperfine parameters of iron species in the course of <Emphasis Type="Italic">Geobacter</Emphasis>-mediated magnetite mineralization

Amorphous ferric iron species (ferrihydrite or akaganeite of <5 nm in size) is the only known solid ferric iron oxide that can be reductively transformed by dissimilatory iron-reducing bacteria to magnetite completely. The lepidocrocite crystallite can be transformed into magnetite in the presence of abiotic Fe(II) at elevated pH or biogenic Fe(II) with particular growth conditions. The reduction of lepidocrocite by dissimilatory iron-reducing bacteria has been widely investigated showing varying results. Vali et al. (Proc Natl Acad Sci USA 101:16121–16126, 2004) captured a unique biologically mediated mineralization pathway where the amorphous hydrous ferric oxide transformed to lepidocrocite was followed by the complete reduction of lepidocrocite to single-domain magnetite. Here, we report the ⁵⁷Fe Mössbauer hyperfine parameters of the time-course samples reported in Vali et al. (Proc Natl Acad Sci USA 101:16121–16126, 2004). Both the quadrupole splittings and linewidths of Fe(III) ions decrease consistently with the change of aqueous Fe(II) and transformations of mineral phases, showing the Fe(II)-mediated gradual regulation of the distorted coordination polyhedrons of Fe³⁺ during the biomineralization process. The aqueous Fe(II) catalyzes the transformations of Fe(III) minerals but does not enter the mineral structures until the mineralization of magnetite. The simulated abiotic reaction between Fe(II) and lepidocrocite in pH-buffered, anaerobic media shows the simultaneous formation of green rust and its gradual transformation to magnetite plus a small fraction of goethite. We suggested that the dynamics of Fe(II) supply is a critical factor for the mineral transformation in the dissimilatory iron-reducing cultures.     

Magnetic properties of surface sediments as proxies of recent anthropogenic pollution in the Anllóns riverbed (NW Spain)

A study of surface sediments from the Anllóns riverbed (northwestern Spain) was conducted to assess the use of their magnetic properties as proxies of recent trace element contamination (i.e., As, Zn, Pb, Cr, Ni and Cu). Thermomagnetic curves, hysteresis parameters, specific magnetic susceptibility χ, frequency-dependent magnetic susceptibility κ _FD%, magnetic remanence and saturation isothermal remanent magnetization ratios (i.e., S and SIRM/χ) reveal the presence of mostly fine-grained magnetite and maghemite in these samples. The Tomlinson Pollution Load Index (PLI), as well as the magnetic parameters χ, κ _FD%, SIRM/χ and S-ratios, vary more or less in agreement to each other when they are sequentially arranged throughout a 30-km-long transect that goes from the town of Carballo to the river mouth in Ponteceso. From that profile, it is possible to identify an anomaly of χ that lies between two small peaks of the PLI and seems to define a contaminated area probably linked to a diffuse low-impact source of pollution. There is also a conspicuous peak of the PLI that is likely associated to a nearby gold mine. The variations of the κ _FD%’s along the transect are the ones that best reflect the changes in the PLI values, regardless of the level of contamination. This result suggests that adsorption is the main mechanism of trace elements removal by Fe oxides in these samples. Direct signal analyses, used to decompose the isothermal remanent magnetization curves, reveal the presence of four magnetic fractions, with varying relative concentrations all over the profile, namely single-domain magnetite, close to the superparamagnetic threshold (SD/SP) and/or maghemite (\(\log B_{1/2}\) ca 1.2), single-domain magnetite (\(\log B_{1/2}\) ca 2), hematite (\(\log B_{1/2}\) ca 2.5) and goethite (\(\log B_{1/2}\) ca 3.5). Integrating all the available information, it was sketched out a likely sequence of events that included magnetic minerals neoformation in parent soils, their subsequent chemical transformations and the contamination episodes that affected the region.     

New Data on the Composition of Magnetic Minerals from Paleosols of Southern Tajikistan

A dependence of the magnetic susceptibility () on the alternation of paleosol and loess has been revealed for loess sections of southern Tajikistan: the value increases in paleosols and decreases in loesses. The magnetic signal intensification in the paleosol is caused by an elevated content of ferromagnetics. New electron-microscopic data on magnetic minerals from the paleosol have demonstrated that the principal ferromagnetic responsible for high values of the paleosol is biogenic fine-dispersed magnetite. Its particles vary from 0.0n to 0.n m in size and exhibit crystallochemical characteristics typical for this mineral. Under pedogenetic conditions and participation of Fe-reducing bacteria, the activity of biomineralization is controlled by paleoclimatic variations.     

Mineralogical and isotopic properties of inorganic nanocrystalline magnetites

Inorganic magnetite nanocrystals were synthesized in an aqueous medium at 25°C, atmospheric pressure, ionic strength of 0.1 M, oxygen fugacity close to 0, and under controlled chemical affinity, which was maintained constant during an experiment and varied between different experiments. The total concentration of iron in the initial solutions, with Fe(III)/Fe(II) ratios of 2, was varied in order to measure the role of this parameter on the reaction rate, particle morphology, and oxygen isotopic composition. The reaction rates were followed by a pHstat apparatus. The nature and morphology of particles were studied by transmission electron microscopy and electron energy loss spectroscopy. Fractionation factors of oxygen isotopes were determined by mass spectrometry after oxygen extraction from the solid on BrF₅ lines. At low total iron concentrations, goethite and poorly crystalline iron oxides were observed coexisting with magnetite. At higher concentrations, euhedral single crystals of pure magnetite with an average characteristic size of 10 nm were formed, based on a first-order rate law with respect to total iron concentration. These results confirm that, under high supersaturation conditions, low-temperature inorganic processes can lead to the formation of well-crystallized nanometric magnetite crystals with narrow size distribution. The observed oxygen isotope fractionation factor between magnetite crystals and water was of 0-1‰, similar to the fractionation factor associated with bacterially produced magnetite. We suggest that the solution chemistry used in this study for inorganic precipitation is relevant to better understanding of magnetite precipitation in bacterial magnetosomes, which might thus be characterized by high saturation states and pH.     

Characterization of magnetite particles in shocked quartz by means of electron- and magnetic force microscopy: Vredefort, South Africa

Submicroscopic opaque particles from highly shocked granite-gneisses close to the core of the Vredefort impact structure have been investigated by means of micro-analytical techniques with high spatial resolution such as electron diffraction, orientation contrast imagery and magnetic force microscopy. The opaque particles have been identified as nano- to micro-sized magnetite that occur in several distinct modes. In one sample magnetite occurs along relict planar deformation features (PDFs) in quartz, generally accepted as typical shock lamellae. The magnetite particles along shock lamellae in quartz grains virtually all show uniform crystallographic orientations. In most instances, the groups of magnetite within different quartz grains are systematically misorientated such that they share a subparallel <101> direction. The magnetite groups of all measured quartz grains thus appear to have a crystallographic preferred orientation in space. In a second sample, orientations of magnetite particles have been measured in microfractures (non-diagnostic of shock) of quartz, albite and in the alteration halos, (e.g. biotite grains breaking down to chlorite). The crystallographic orientations of magnetite particles are diverse, with only a minor portion having a preferred orientation. Scanning electron microscopy shows that magnetite along the relict PDFs is invariably associated with other microcrystalline phases such as quartz, K-feldspar and biotite. Petrographic observations suggest that these microcrystalline phases crystallized from locally formed micro-melts that intruded zones of weakness such as microfractures and PDFs shortly after the shock event. The extremely narrow widths of the PDFs suggest that heat may have dissipated rapidly resulting in melts crystallizing relatively close to where they were generated. Magnetic force microscopy confirms the presence of magnetic particles along PDFs. The smallest particles, <5 μm with high aspect ratios 15:1 usually exhibit intense, uniform magnetic signals characteristic of single-domain magnetite. Consistent offsets between attractive and repulsive magnetic signals of individual single-domain particles suggest consistent directions of magnetization for a large proportion of particles. Received: 16 November 1998 / Accepted: 17 May 1999     

长江与黄河河口沉积物磁性特征对比的初步研究

根据2001年8月和9月分别采自黄河与长江河口沉积物样品的磁性测量和粒度分析,探讨长江和黄河河口沉积物的磁性特征及其差异。长江河口沉积物中亚铁磁性物质的含量高于黄河口,但长江口与黄河口沉积物中都是亚铁磁性矿物主导了样品磁性特征,亚铁磁性矿物晶粒都以假单畴-多畴为主。相比黄河口沉积物,长江口沉积物不完整反铁磁性物质对磁性特征的的贡献较小。长江与黄河河口的这种磁性特征主要反映了不同的沉积物来源的控制影响。此外,无论是长江口还是黄河口沉积物,磁性参数χARM、χfd%与沉积物细粒级组分存在显著的相关性,表明这两个参数作为粒度的代用指标具有普遍性。     

塔里木盆地西北缘库孜贡苏剖面晚白垩世-早中新世沉积物岩石磁学研究

对塔里木盆地西北缘库孜贡苏剖面晚白垩世-早中新世沉积物进行了热退磁及岩石磁学研究,结果表明岩石热退磁及岩石磁学特征随沉积环境可分为三种类型:潮下、台地边缘浅滩相岩石主要磁性矿物为磁铁矿及少量针铁矿、磁赤铁矿,磁性矿物含量较少、颗粒较小（假单畴）,其天然剩磁强度较小,一般小于1×10-2 A/m,在250℃~500℃能获得稳定特征剩磁方向,特征剩磁由磁铁矿携带;潮间、潮上带岩石主要磁性矿物为磁铁矿,〖JP2〗并含有少量磁赤铁矿、赤铁矿、针铁矿,磁性矿物颗粒为假单畴和多畴,天然剩磁强度一般在1×10-2 ~1 A/m之间,在250℃~580℃能获得稳定特征剩磁方向,特征剩磁由磁铁矿携带;河湖相岩石主要磁性矿物为磁铁矿、赤铁矿,并含有少量磁赤铁矿、针铁矿,磁性矿物含量较多、颗粒较小（假单畴）,天然剩磁强度一般在1×10-1 A/m以上,多数样品特征剩磁由赤铁矿携带,少数由磁铁矿与赤铁矿共同携带。岩石磁学研究对于在沉积环境复杂剖面进行古地磁研究具有重要的意义。     

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。     

青藏高原腹地湖泊沉积物磁化率及其环境意义

通过青藏高原腹地可可西里边缘地区BDQ0608 钻孔岩芯分析,表明其岩性主要为浅绿色湖相沉积物,其中夹杂部分较薄的氧化色层段.热退磁表明:BDQ0608钻孔中磁性矿物主要有磁铁矿、磁赤铁矿、针铁矿和胶黄铁矿,赤铁矿表现不太明显,其组分含量直接控制磁化率值的大小;并且对样品进行了磁化率、粒度、总有机碳及色度的测定.磁化率...     

Low temperature magnetism and Mössbauer spectroscopy study from natural goethite

In this work a magnetic characterization was made of natural goethite from Burkina Faso, Africa, by using low temperature magnetization curves, hysteresis loops, Mössbauer spectroscopy at room temperature and 4.2 K, and AC susceptibility from 10 to 400 K. The samples are from two distinct geological sites that underwent different weathering processes. All measurements point to the occurrence of typical high coercivity goethite. Through Mössbauer spectroscopy sample BL44, from Gangaol, northeast Burkina Faso showed relaxation effects due to a wide distribution of grain size, including superparamagnetism threshold. AC susceptibility also supports this interpretation. The sample BL50 from Bonga in Burkina Faso is associated with lateritic Ni and in addition to goethite this sample also contained magnetite, as determined by Verwey transition in low temperature measurements as well as a small content of hematite identified by Mössbauer spectroscopy.     

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.     

Environmental magnetic studies on surface sediments: a proxy for metal and hydrocarbon contamination

Visakhapatnam is one of the major port cities, and it is developed into a hub of many large- and medium-scale industries. Due to growing industrialization and urbanization, coast is vulnerable to both organic and inorganic micro-pollutants. Twenty-five surface sediments were collected along the Visakhapatnam coast for the measurement of texture size, petroleum hydrocarbons, trace metals and environmental magnetic parameters. The percentage of coarser particles was more in the northern region, whereas the percentage of fine particles was increased toward south. Elevated levels of petroleum hydrocarbons and trace metals were attributed due to marine and land-based sources, in particular, those were due to shipping activities, treated and partially treated sewage and industrial wastes. The concentrations of trace metals, petroleum hydrocarbons and magnetic minerals were decreased from nearshore to seaward. Our results revealed that the magnetic mineralogy is dominated by magnetite with a small proportion of hematite, and the grain size of magnetic minerals was in the range of pseudo-single domain to multidomain nature with detrital origin. From the principal component analysis, the magnetic concentration and mineralogy-dependent parameters co-vary with the heavy metal and PHC concentrations, suggesting that the inputs of magnetic minerals, petroleum hydrocarbons and heavy metals in the Visakhapatnam shelf sediments were derived from the same anthropogenic sources. Thus, the large magnetic dataset can be used to reduce the number of chemical analysis; hence, environmental magnetic parameters were used as a proxy for both organic and inorganic micro-pollutants.     

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.