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1.
Magnetofossils in the sediment of Lake Baikal, Siberia 总被引：1，自引：0，他引：1
A multidisciplinary approach involving rock-magnetics, transmission electron microscopy, and X-ray diffraction was used to identify a biogenic magnetite component in the Lake Baikal, Siberia, sedimentary magnetic record. The distinctive biogenic component to the magnetic record occurs as chains of single-domain, elongate hexagonal and tear-drop cone-shaped magnetite particles. These magnetofossils are inferred to be magnetosomes produced by magnetotactic bacteria living in the surficial sediment throughout Lake Baikal. Postdepositional reduction diagenesis results in the loss of the fine-grained magnetofossils at depth. In addition, this study shows that the fine-grained magnetofossils are removed by the process of storage diagenesis during long periods (21 months) of core storage, which results in a change to a coarser grained, slightly higher coercivity bulk magnetic mineral assemblage. Although the Lake Baikal sedimentary magnetic record has several distinct and complex components, by determining their origin this study has shown that the sediments are well suited for environmental magnetic study. 相似文献
2.
Rock magnetic criteria for the detection of biogenic magnetite 总被引：15，自引：0，他引：15
Bruce M. Moskowitz Richard B. Frankel Dennis A. Bazylinski 《Earth and Planetary Science Letters》1993,120(3-4):283-300
We report results on the magnetic properties of magnetites produced by magnetotactic and dissimilatory iron-reducing bacteria. Magnetotactic bacterial (MTB) strains MS1, MV1 and MV2 and dissimilatory iron-reducing bacterium strain GS-15, grown in pure cultures, were used in this study. Our results suggest that a combination of room temperature coercivity analysis and low temperature remanence measurements provides a characteristic magnetic signature for intact chains of single domain (SD) particles of magnetite from MTBs. The most useful magnetic property measurements include: (1) acquisition and demagnetization of isothermal remanent magnetization (IRM) using static, pulse and alternating fields; (2) acquisition of anhysteretic remanent magnetization (ARM); and (3) thermal dependence of low temperature (20 K) saturation IRM after cooling in zero field (ZFC) or in a 2.5 T field (FC) from 300 K. However, potentially the most diagnostic magnetic parameter for magnetosome chain identification in bulk sediment samples is related to the difference between low temperature zero-field and field cooled SIRMs on warming through the Verwey transition (T ≈ 100 K). Intact chains of unoxidized magnetite magnetosomes have ratios of δFC/δZFC greater than 2, where the parameter δ is a measure of the amount of remanence lost by warming through the Verwey transition. Disruption of the chain structure or conversion of the magnetosomes to maghemite reduces the δFC/δZFC ratio to around 1, similar to values observed for some inorganic magnetite, maghemite, greigite and GS-15 particles. Numerical simulations of δFC/δZFC ratios for simple binary mixtures of magnetosome chains and inorganic magnetic fractions suggest that the δFC/δZFC parameter can be a sensitive indicator of biogenic magnetite in the form of intact chains of magnetite magnetosomes and can be a useful magnetic technique for identifying them in whole-sediment samples. The strength of our approach lies in the comparative ease and rapidity with which magnetic measurements can be made, compared to techniques such as electron microscopy. 相似文献
3.
Abstract Characterization of magnetic particles, and magnetostratigraphic dating, of Holocene and Pleistocene shallow-water carbonates (the Ryukyu Group) in the Ryukyu Islands was carried out to infer the age of reef formation. Magnetic particles from these strata are dominated by fine-grained, single-domain magnetite/maghemite (40–140 nm in length). The magnetite crystals exhibit the size and morphologies characteristic of the magnetite formed by magnetotactic bacteria. No large multidomain grains were seen in transmission electron microscopy observations. The widespread presence of single-domain magnetite in the Ryukyu Islands suggests that bacterial magnetites carry depositional remanent magnetization, which is stable enough for magnetostratigraphic dating and thereby for elucidating the evolution of shallow-water carbonates. The polarity-reversal sequence seen in a core taken from the Ryukyu Group correlates with the timescale of the Matuyama chron, including the Jaramillo subchron, Kamikatsura or Santa Rosa Excursions, through the Brunhes chron. These magnetostratigraphic results imply that the time of reef initiation in the Ryukyu Islands of the northwestern Pacific was earlier than in the Great Barrier Reef of the southwestern Pacific, where it started after the Brunhes–Matuyama boundary. 相似文献
4.
Magnetic grains isolated from magnetococcoid bacterial cells were studied by means of transmission electron microscopy, electron diffraction and electron microprobe analysis. Observed in situ the magnetic grains are each surrounded by an organic membrane and are usually found in a random array although “chains” are also seen. Electron diffraction confirms the magnetite mineralogy and provides additional evidence in favor of vacancies in the structure. Electron microprobe analysis shows the magnetite to be slightly titaniferous. Electron microscopy indicates that the grains, rather than being flake shaped, are parallelepiped crystals with a mean length of99.3 ± 8.7nm, a mean width of62.3 ± 6.1nm yielding a width-to-length ratio of 0.63. These data support the contention that the magnetic bacterial grains are single-domain crystals capable of producing a natural remanent magnetization in sediments. 相似文献
5.
Jacques Jedwab 《Earth and Planetary Science Letters》1980,49(2):551-564
Four hundred membrane filters from the Atlantic and Pacific deep waters have been scanned for coloured and opaque particles by a method integrating the light and the scanning electron microscopes, the electron microprobe and the micro-X-ray diffraction camera. About 40 different types of particles have been found. A tentative classification of the particles and of their morphological and chemical varieties is presented, according to their anthropogenic or natural origins. From this point of view, the particles fall into five groups: (1) particle species exclusively produced by man's activities (like brass, cobalt aluminate); (2) particles exclusively produced by natural processes (like amphibole, ilmenite); (3) particles rarely produced by natural processes, but massively so by man (like metallic copper, chromium oxide); (4) a large group of particles massively produced by nature and by man (like magnetite spherules, malachite); (5) a very small group of particles whose origin could not be attributed at present. Very few entirely new compounds from the point of view of continental mineralogy have been encountered. 相似文献
6.
Joseph L. Kirschvink 《Earth and Planetary Science Letters》1982,59(2):388-392
Previous paleomagnetic research has shown that the concentration of fine-grained magnetite preserved in Miocene marine clays of western Crete drops during two successive geomagnetic transitions, although the mechanism responsible for this effect was not known. Magnetite precipitation by magnetotactic bacteria offers a straightforward explanation for this decrease. Reduced field strengths during a reversal will strain the competitive advantage of having intracellular magnetite and will force bacteria to precipitate correspondingly more magnetite if they are to maintain the same alignment in the geomagnetic field. Their ability to do this is limited by several magnetophysical constraints which should act to reduce the numbers of magnetite-precipitating cells, particularly in fields of less than about 12% of the present level. This would lead to a decrease in the supply of bacterial magnitude to the sediments. Recovery of intact magnetotactic bacterial fossils may lead to a new method for estimating paleointensities from sedimentary rocks. 相似文献
7.
Anhysteretic remanent magnetization (DC field = 1 Oe, peak AC field = 1000 Oe) was given to an artificial sediment consisting of a kaolinite matrix and a 0.03% magnetic fraction of needle-shaped magnetite grains. The mean grain size of the clay flakes and the magnetite needles was approximately 0.5 μm. This sediment was subjected to plane strain while maintaining constant volume. The axis of maximum compression was shortened by as much as 331/3%, which produced no significant change in the direction of magnetization with a decrease in intensity of 28%. A continuous deformation model, in which the magnetite needles are embedded in a plastically deforming linearly homogeneous medium, failed to explain these results. A discontinuous deformation model was more successful. In this model, deformation occurs mainly in shear zones bounding blocks which translate along the shears. Only those magnetite grains in the shear zones rotate, and from symmetry considerations no net change in the direction of magnetization is predicted. Because magnetite grains rotate in opposite directions in complementary sets of shears, the model predicts a decrease in intensity comparable to that which was observed. 相似文献
8.
Anisotropy of magnetic susceptibility (AMS) has been used to interpret flow directions in ignimbrites, but no study has demonstrated
that the AMS fabric corresponds to the flow fabric. In this paper, we show that the AMS and strain fabric coincide in a high-grade
ignimbrite, the Nuraxi Tuff, a Miocene rhyolitic ignimbrite displaying a wide variability of rheomorphic features and a well-defined
magnetic fabric. Natural remanent magnetization (NRM) data indicate that the magnetization of the tuff is homogeneous and
was acquired at high temperatures by Ti-magnetite crystals. Comparison between the magnetic fabric and the deformation features
along a representative section shows that AMS and anisotropy of isothermal remanent magnetization (AIRM) fabric are coaxial
with and reproduce the shape of the strain ellipsoid. Magnetic tests and scanning electron microscopy observations indicate
that the fabric is due to trails of micrometer-size, pseudo-single domain, magnetically interacting magnetite crystals. Microlites
formed along discontinuities such as shard rims and vesicle walls mimicking the petrofabric of the tuff. The fabric was thus
acquired after deposition, before late rheomorphic processes, and accurately mimics homogeneous deformation features of the
shards during welding processes and mass flow. 相似文献
9.
10.
Hkon Fischer Giovanni Mastrogiacomo Jrg F. Lffler Rolf J. Warthmann Peter G. Weidler Andreas U. Gehring 《Earth and Planetary Science Letters》2008,270(3-4):200-208
The magnetic characteristics of intact magnetosome chains in Magnetospirillum gryphiswaldense bacteria were investigated by means of static and dynamic magnetic analyses and ferromagnetic resonance spectroscopy. The nano-sized magnetosomes are generally in a stable single-domain state, but magnetosomes smaller than 30 nm characteristic of superparamagnetic magnetite particles were also found. Alternating current (AC) susceptibility indicates that all magnetosomes are blocked below 150 K. At room temperature the anisotropy of M. gryphiswaldense is dominated by the shape of the magnetosome chains. Low-temperature ferromagnetic resonance (FMR) spectroscopy indicates that this dominant shape anisotropy can affect the detection of the Verwey transition at 100 K. The static and dynamic magnetic analyses show that the Verwey transition is smeared and that our magnetotactic bacteria fail the Moskowitz test. This failure is explained by the biomineralization of non-stoichiometric magnetosomes. This interpretation is based on the increase in high-field susceptibility and the distinct peak in the out-of-phase component of the AC susceptibility below 50 K. These results are attributed to freezing of spins associated with defect structures in the core and at the surface of nano-sized magnetosomes. The results obtained from M. gryphiswaldense demonstrate that intrinsic properties of nano-sized magnetosomes are significantly influenced by non-stoichiometry and by the anisotropy excited from their arrangement in the bacteria. 相似文献
11.
Experiments were undertaken to determine the feasibility of tracing sediment movement in interrill overland flow. Crushed magnetite was introduced as a source-line 10 cm wide by 8 m long on a runoff plot 18 m wide by 29 m long located in southern Arizona. Initial magnetic susceptibilities along this source line, and along three transects located 0·25, 2·95 and 5 m downslope of the source-line, were measured. Movement of the magnetite in response to three rainfall simulation experiments was monitored. During the first two experiments, overland flow discharge was sampled at miniature flumes located along two cross sections on the plot downslope of the source-line, and at a supercritical flume at the plot outlet. Magnetic susceptibilities along the source-line and transects were measured after all three experiments. Results show that the magnetite moves very early in the experiments and that it reaches one of the flumes 2 m downslope of the source-line in 3 min. Most of the tracer moves a very short distance: 29·7 per cent is deposited within 25 cm of the source-line and only 2·2 per cent is deposited 2·95 m away. The deposition rate appears to decrease exponentially away from the source-line. Very little magnetite is recorded in the flow through the miniature flumes: in general it makes up less than 1 per cent of the total sediment load. No temporal pattern in these percentages is observed. Magnetite appears to be an effective tracer of sediment movement in interrill overland flow, though its higher density than natural soil may affect its detachment and transport. 相似文献
12.
13.
研究不同区域黄土颗粒和孔隙大小、形状和排列的定量差异,针对甘肃兰州、陕西西安和山西运城的原状黄土的电镜照片进行定量分析。首先应用Autocad描绘获得颗粒和孔隙的边界,拉开各对象后填充,然后导入到图像分析软件中获得各对象的长轴,短轴、周长、面积和长轴方向角;其次应用得到的参数计算反映对象大小、形状和定向排列的定量参数;最后比较不同区域原状黄土中颗粒和孔隙的定量参数差异。结果表明:甘肃兰州黄土颗粒的平均直径最大,形状最偏离圆形,三个区域颗粒排布在空间分布均匀;甘肃兰州黄土的孔隙平均直径与陕西西安的接近,形状更偏离圆形;颗粒的平均直径接近于孔隙的平均直径,颗粒形状更接近于圆形。 相似文献
14.
Carlos A. Vasquez Facundo F. Sapienza Agustín Somacal Sabrina Y. Fazzito 《Studia Geophysica et Geodaetica》2018,62(2):339-351
A phenomenological model based on a linear relationship between the magnetic coercivity field and the reciprocal of the grain diameter is applied to explain the anhysteretic remanent magnetization (ARM) imparted to artificial samples with different concentrations of a very well characterized magnetite powder. By analyses of scanning electron microscopy images, the spherically shaped single domain synthetic magnetite is found to follow a lognormal grain size distribution with ~86 nm of mean diameter. The proposed model, fitted to ARM measurements up to a peak alternating field of 100 mT, yields a very good agreement. The coercivity behaviour predicted by micromagnetism theory disagrees with the experimental results of this work. A likely explanation for the discrepancy is that the magnetite particles, which consist of a mixture of grains in coherent rotation and curling modes, produce similar observations as domain processes. 相似文献
15.
A Precambrian metadolerite dyke has two distinct types of remanence carriers; those with medium/high coercivities (unblocking fields of 20–120 mT) and those with low coercivities (unblocking fields of <15 mT). Optical examination reveals numerous submicron probably opaque inclusions in the plagioclase feldspar and also large opaque grains consisting of coarse oxidation-exsolution intergrowths of magnetite and ilmenite. All opaque phases have been examined using transmission electron microscopy together with microanalysis and electron diffraction. The submicron inclusions in the plagioclase are titanomagnetites(0 < x ≤ 0.14) with a size range between about 0.01 and 0.5 μm and axial ratios between 1 (equidimensional) and about 0.3. Many of these inclusions fall in the single-domain field but some are probably pseudo-single-domain. The large opaque grains contain almost pure magnetite and ilmenite and show no fine-scale exsolution; the magnetite regions of the intergrowths are of multidomain size and reveal multidomain structure under Lorentz electron microscopy. There are also some primary ilmenites containing very fine exsolved haematite, and there are very fine plates of ilmenite and very elongate needles of magnetite within the augite. Experiments on artificial samples containing very carefully prepared separates of plagioclase and large opaque grains show that the pure plagioclase acquires a remanence with unblocking fields of 20–140 mT and blocking temperatures of 390–590°C and the large opaque grains acquire a remanence with unblocking fields of less than 15 mT but a wide range of blocking temperatures up to about 570°C. It is concluded that the medium/high coercivity component of remanence in the rock is carried largely or possibly entirely by the submicron magnetites within the plagioclase and that the low coercivity component is carried largely or entirely by the multidomain magnetites in the large opaques. The contribution of the magnetite needles in the augite is uncertain as the rock does not contain any detectable component of remanence with the extremely high coercivities expected from their very elongate shape. 相似文献
16.
Biosorption of heavy metals by a marine bacterium 总被引：1，自引：0，他引：1
Heavy metal chelation property of exopolysaccharide produced by Enterobacter cloaceae, a marine bacterium, isolated from the West Coast of India, is reported in this paper. The exopolysaccharide demonstrated excellent chelating properties with respect to cadmium (65%) followed by copper (20%) and cobalt (8%) at 100 mg/l heavy metal concentration. However, it could not chelate mercury. A comparative study of the percentage biosorption of the above mentioned metals is presented here. 相似文献
17.
Magnetic minerals are mostly identified by a combination of rock magnetic and microscopic techniques and the ferrofluid method in combination with an optical microscope was applied during the last decades. But today, scanning electron microscopy (SEM) is preferred for the observation of mineral phases because its depth of focus and resolving capability at the same magnification. In this study, we report on a method, which allows ferrofluid application under the SEM. We coated a polished basalt sample containing titanomagnetite grains with high demagnetizing fields, with a colloidal suspension containing magnetite particles (ferrofluid) ranging in size between 11 and 20 nm. Due to large gradients of multidomain grains the ferrofluid particles adhered to their surfaces. Other grains of similar composition present (ilmenohematite) but with low demagnetizing fields do not generate large magnetic gradients and therefore do not attract the colloidal particles. Upon evaporation of the ferrofluid and covering the sample with conducting material the magnetic grains with high demagnetizing field are easily identifiable under the scanning electron microscope. The different mineralogy observed by this method is confirmed by temperature dependent variation of magnetic susceptibility, revealing titanomagnetite and ilmenohematite as magnetic carriers in the basaltic samples. 相似文献
18.
《Earth and Planetary Science Letters》2006,241(1-2):227-233
Direct observation using high resolution transmission electron microscopy reveals that precipitated iron(II) monosulfide, FeS, consists of nanocrystalline mackinawite particles. The individual nanocrystals are laminar rectilinear prisms displaying a continuum of particle sizes from 2 to 5.7 nm in thickness (the direction parallel to the c axis) and from 3 to 10.8 nm in length. The corresponding mean specific surface area is estimated to be to 380 ± 10 m2/g. The d001 of mackinawite nanocrystals obtained from precipitated FeS and freeze-dried FeS by electron diffraction are 5.19 and 5.08 Å, respectively. The effect of water on the nanoparticle structures is indicated by the formation of curved structures and infrequent dislocations in an anhydrous environment. The apparent disorder suggested by absent or weaker lattice spacings is symptomatic of the breakdown of Braggian systematics at these particle sizes. The results confirm that nanoparticulate materials do not behave simply as small fragments of their bulk crystalline counterparts. The results contribute to understanding the behavior of nanoparticulate materials on planetary surfaces and in the biosphere in general and the nature and properties of FeS in anoxic aqueous environments in particular. 相似文献
19.
Stable remanent directions encountered in three oriented lake-sediment cores, covering 6 m of gyttja deposits, reveal declination swings with amplitudes of the order of ± 50°. Anomalously low inclinations, scattered around the horizontal in the upper 2 m of core, is attributed to compressional disturbances of the alignment of magnetic grains during the coring procedure. Four 14C dates obtained (1250, 1940, 4600 and 6330 y B.P.) have enabled a correlation with features ‘c’–‘g’ of the British Geomagnetic Master Curve). For the older part of the curve, features ‘h’ and ‘i’, the correlation is questionable. Magneto-mineralogical parameters and qualitative chemical analysis of individual grains by secondary X-ray emission analysis reveal the presence of pure magnetite grains. Scanning electron micrographs show a dominant proportion of concoidal slightly elongated grains (10–80 μm). A few almost-perfect octahedral magnetite crystals can be related to outcrops of augen gneisses in the vicinity confirming a detrital origin of these grains. 相似文献
20.
Synthetic single crystals of titanomagnetite of nominal composition Fe2.4Ti0.6O4 have been oxidized at 1275°C in controlled gas atmospheres, producing multiphase intergrowths to simulate the natural process of deuteric oxidation. The evolution of the intergrowths was monitored using the conventional techniques of petrology: optical and electron microscopy and X-ray and electron microprobe analyses. In addition, the measured magnetic properties — particularly the temperature-dependence of hysteresis properties — provided further information about the composition and concentrations of magnetic phases, and their domain state, as oxidation proceeded. The evolution of a trellis pattern of ilmenite lamellae, characteristic of the “exsolution” stages of deuteric oxidation, was observed in the oxidized crystals. The interlamellar spinel region consisted of two iron-enriched titanomagnetites, one thought to occur along the lamellar boundaries. The magnetic hardness of both phases was found to be greater than the original homogeneous multidomain titanomagnetite crystals, although neither phase achieved monodomain characteristics, and the stability of thermoremanence (TRM) remained quite low (median destructive fields (MDF) of the order of a few thousend A m?1). The lamellae made little contribution to the total remanence. A sharp rise in magnetic hardness, observed during the post-exsolution stages of oxidation, was due to the presence of fine-grain monodomain magnetite, thought to be distributed within the haemoilmenite matrix, but probably not having been formed by lamellar subdivision. The crystals could now carry an intense and stable TRM with MDFs of many tens of thousands of A m?1. 相似文献