贵州省麦岗水库沉积物磁性特征及其与粒度的关系

对贵州省麦岗水库沉积物环境磁性特征的研究表明,亚铁磁性矿物主导了沉积物矿物磁性特征,但同时也存在反铁磁性矿物等其他矿物,超顺磁颗粒在沉积物中广泛存在.在所选矿物磁性参数中,χlf、χfd、SOFT与粒度不相关;χARM、SIRM、F300与粒度相关,但相关系数不高;χARM/χlf、χARM/SIRM和粒度显著相关,可以作为粒度的代用指标.研究结果显示,磁性参数确实可以作为粒度的代用指标.但对比研究表明,在不同沉积环境,甚至相似沉积环境的沉积物中,矿物磁性参数和粒度的关系可能不同,在特定沉积环境中,利用磁性参数作为粒度的代用指标应该在充分研究的基础上进行,使研究结果更为可靠.     

Effects of bottom sediments on the survival of Enterobacter aerogenes in seawater

The survival of the coliform bacterium Enterobacter aerogenes in seawater was greatly affected by the underlying bottom sediments. The presence of sediments which contained higher concentrations of organic nutrients than the overlaying seawater prolonged the survival and even enabled growth of the bacterial cells. Results obtained from chemical analysis indicated that sediments of fine-grained, silt-clay type contained highest amounts of organic nutrients as compared with sediments with larger particle sizes such as silt and sand grains. Thus, nutrient contents of the sediments were found to be inversely proportional to sediment particle size. The degree of prolonged survival of E. aerogenes was also found to be inversely proportional to the size of the sediment particles presented. Release of the bound nutrients which became available to cells of E. aerogenes from sediments by action of waves and human activities can explain, at least in part, why high counts of coliform bacteria are obtained in coastal and beach waters.     

Palaeomagnetic secular variation studies of Finnish lake sediment and the carriers of remanence

Palaeomagnetic and mineral magnetism measurements have been carried out on two cores from Lake Vuokonjarvi in Finnish Karelia. The sediment probably covers 5000 years of continuous deposition at a mean sedimentation rate of about 0.8 mm/yr.The magnetic declination exhibits fluctuations of similar amplitude(～20°) and character to those recorded in northern England and northern Ireland. Magnetic inclination variations are of higher amplitude(～15°) than those found in Britain. Matching the palaeomagnetic patterns with the dated British master curves permits an estimate of the rate of deposition of the Finnish sediments, which is suggested to be more reliable than estimates from radiocarbon dating of the Vuokonjarvi sediment.The stable natural remanence is shown to be carried by fine-grained magnetite and titanomagnetite grains and to have grown by post-depositional alignment during a period of the order of 100 years. Laboratory dehydration of the sediment results in loss of around 40% of the stable natural remanence. Such behaviour is also found in lake sediments from central and southern Europe and should be considered in interpreting palaeomagnetic data from dried out lake sections and ocean cores.     

Paleomagnetic results from lakes Victoria and albert,Uganda

Summary Unfortunately most of the cores taken from Salisbury Channel, Lake Victoria which contained discrete pieces of wood suitable for¹⁴C dating were organic-rich and therefore showed a considerable scattering of the remanent magnetic direction plots resulting in poorly defined oscillations. However, the cores from Pilkington Bay as well as core C5 from Salisbury Channel, Lake Victoria and the cores from Lake Albert show well-defined paleodeclination and paleoinclination logs. Several cores from Pilkington Bay and Lake Albert with some¹⁴C control can be used to verify the isochronous correlation of the paleodeclination and paleoinclination oscillations. Core C5, from Salisbury Channel, Lake Victoria, which did not contain any discrete pieces of plant or shell material has been dated by correlation with other cores which had¹⁴C control. This core which shows the best-defined paleodeclination and paleoinclination logs of this study would appear to have bottomed in sediments in excess of 6000 years BP age and with the work of Maley et al. (1990) from the Cameroons provides a complete record of the Holocene for Central Africa.     

新疆泥火山发育区场地震陷灾害机理探讨

粒度组分是影响黄土震陷的重要因素之一.通过采集新疆艾其沟泥火山泥质碎屑喷出样品和泥火山发育区域出露的沉积剖面泥质碎屑样品,利用激光粒度分析测试的实验方法对样品进行粒度测试,分别对样品中值(Md)、均值(Mz)、标准偏差(σφ)、偏度(SK1)、峰度(KG)等粒度参数进行分析.研究结果显示:艾其沟泥火山泥质沉积物粒度组分...     

Measurement of the effect of particle size variation on the detrital remanent magnetization to anhysteretic remanent magnetization ratio in some abyssal sediments

Deposition experiments have been carried out to measure the effect of particle size variation on the relationship between detrital remanent magnetization (DRM), anhysteretic remanent magnetization (ARM), and geomagnetic field intensity in sediments.Foraminiferal ooze from a box core taken in the Columbian Basin south of Jamaica was separated into several particle size ranges and redeposited in the laboratory in known magnetic fields. The intensity and alternating field (AF) demagnetization characteristics of the DRM and a subsequently applied ARM were compared for the various particle size ranges.The results show a variation of DRM/ARM ratios with particle size. The DRM intensities and directions indicate that particle sizes greater than 38 μm do not contribute significantly to the DRM of the total sediment. ARM intensities for larger particle sizes and particle size analysis of the whole sediment indicate that the fraction greater than 38 μm does make a significant contribution to the total ARM of a sample. Use of the DRM/ARM ratio in experimental measurements of magnetic paleointensity indicates that the method is unsatisfactory for sediments having a significant fraction of magnetic particles larger than 38 μm. It is also shown that, for sediments having a significant fraction of high-coercivity magnetic grains, the relative orientation of the ARM and DRM will affect ARM intensities, making necessary the use of corrected ARM and DRM intensities for ratio calculations.     

2001年3月2日磁通量传输事件特性的研究

2001年3月2日11:00 至11:15 UT 期间,Cluster Ⅱ在南半球极尖区晨侧附近磁鞘内探测到3个通量传输事件（简称FTEs）. 本文利用Cluster Ⅱ星簇4颗卫星观测到的磁场和等离子体资料研究了这些通量传输事件的磁场形态和粒子特征. 并利用它们探测到的空间磁场梯度资料由安培定律直接求出星簇所在区域的电流分布. 结果指出：（1）BY占优势的行星际磁场结构在磁层顶的重联可以在极尖区附近发生;（2）FTEs通量管形成初期内外总压差和磁箍缩应力不一定平衡,达到平衡有一发展过程;（3）FTEs通量管截面在L M平面内的线度约为1.89RE;（4）FTEs通量管中等离子体主要沿轴向场方向流动,整个通量管以慢于背景等离子体的速度沿磁层顶向南向尾运动;（5）FTEs通量管中不仅有轴向电流,也存在环向电流. 轴向电流基本沿轴向磁场方向流动. 轴向和环向电流在管内均呈体分布,因而轴向电流产生的环向磁场接近管心时不断减小到零,而环向电流生成的轴向场则不断增大到极值;（6）在通量管的磁鞘部分观测到磁层能量粒子流量的增强,这表明通量管通过磁层顶将磁鞘和磁层内部连通起来了.     

Anisotropy of magnetic susceptibility in welded tuffs: application to a welded-tuff dyke in the tertiary Trans-Pecos Texas volcanic province,USA

Consideration of published anisotropy of magnetic susceptibility (AMS) studies on welded ignimbrites suggests that AMS fabrics are controlled by groundmass microlites distributed within the existing tuff fabric, the sum result of directional fabrics imposed by primary flow lineation, welding, and (if relevant) rheomorphism. AMS is a more sensitive indicator of fabric elements within welded tuffs than conventional methods, and usually yields primary flow azimuth estimates. Detailed study of a single densely welded tuff sample demonstrates that the overall AMS fabric is insensitive to the relative abundances of fiamme, matrix and lithics within individual drilled cores. AMS determinations on a welded-tuff dyke occurring in a choked vent in the Trans-Pecos Texas volcanic field reveals a consistent fabric with a prolate element imbricated with respect to one wall of the dyke, while total magnetic susceptibility and density exhibit axially symmetric variations across the dyke width. The dyke is interpreted to have formed as a result of agglutination of the erupting mixture on a portion of the conduit wall as it failed and slid into the conduit, followed by residual squeezing between the failed block and in situ wallrock. Irrespective of the precise mechanism, widespread occurrence of both welded-tuff dykes and point-welded, aggregate pumices in pyroclastic deposits may imply that lining of conduit walls by agglutionation during explosive volcanic eruptions is a common process.     

Magnetic anisotropy of laboratory materials in which magma flow is simulated

Summary The magnetic anisotropy of lava flows has been simulated using plaster of Paris containing about 2% of nickel filings. The ellipsoid of magnetic anisotropy was oriented with its long axis close to but tilted with respect to the flow direction and its intermediate axis in the flow plane perpendicular to the flow direction; the same alignment was obtained with four different experimental conditions. This result is at variance with observations on basalt flows in which the long axis of the magnetic ellipsoid is perpendicular to the flow direction. It appears likely that the favoured alignment of elongated grains in a fluid or semi-fluid flow is sensitive to physical parameters which have not yet been examined adequately. Reliable magnetic determinations of flow directions in lavas must await clarification of this problem.     

Hysteresis characteristics of subaeral deposits in the Baikal region

The results of rock magnetic studies in four subaeral cis-Baikalian sections of different ages and geneses are described in detail. The similarities and distinctions in the formation of the magnetic properties of the sediments are established based on the analysis of the magnetic hysteresis parameters. The mechanisms responsible for the dissimilarity of the magnetic characteristics of the rocks in the different sections are identified. The variations in the composition, concentration, and particle size of the grains of magnetic fraction as a function of climate, geomorphologic factors, and volcanism are demonstrated.     

Textural remanence: A new model of lunar rock magnetism

In reexamining the accumulated magnetic data on lunar rocks, several common patterns of magnetic behavior are recognized. Their joint occurrence strongly suggests a new model of lunar rock magnetism, which appeals only to partial preferred textural alignment of the spontaneous moments of magnetic grains, without requiring the existence of ancient lunar magnetic fields. This magnetic fabric, mimetic to locally oriented petrofabric, gives rise to an apparent “textural remanent magnetization” (TXRM). In order to account for the observed intensity of “stable remanence” in lunar rocks, only a minute fraction (10^?3 to 10^?5) of the single-domain iron grains present need be preferentially aligned. Several mechanisms operating on the lunar surface, including shock and diurnal thermal cycling, appear adequate for producing the required type and degree of magnetic alignment in all lunar rock classes. The model is supported by a wide variety of direct and indirect evidence and its predictions (e.g. regarding anisotropic susceptibility and remanence acquisition) can be experimentally tested.     

Magnetic characterization and Mössbauer spectroscopy of magnetic concentrates from Greek lake sediments

Bulk magnetic properties of three Greek lake sediments indicated that the main magnetic constituent was predominantly pseudo-single-domain magnetite. This was confirmed by successfully extracting and directly identifying the magnetite. The majority of the magnetite in the magnetic concentrates is present as grains of about 2–3 μm in size, the overall size ranging from < 1to～ 10 μm with occasional grains up to ～ 20 μm. The grains are roughly equidimensional and angular in form indicating that they are of primarily detrital origin, although an authigenic origin for the very fine grains must be a possibility. Mössbauer spectra of the magnetic concentrates suggest that either the magnetite is slightly non-stoichiometric to a similar degree in the three lakes, or contains impurity ions. Haematite was also found in the concentrates but its abundance is only ～10–15% that of magnetite and its is not detectable in any of the magnetic properties of the sediments.     

贵州红枫湖沉积物生物可利用磷分布特征及其与粒径的关系

应用化学提取法分析了红枫湖主要出入湖河口及湖心沉积物生物可利用磷(BAP)的含量,并探讨了BAP空间分布与总磷(TP)和粒度组成之间的关系.研究结果表明,各形态BAP含量顺序为:藻类可利用磷(AAP)NaHCO3可提取磷(Olsen-P)水溶性磷(WSP)易解吸磷(RDP).沉积物柱芯中BAP迅速降低,剖面变化比TP更为明显.各形态BAP与TP显著相关,除RDP与AAP外,其它形态BAP之间也显著相关.Olsen-P是评价红枫湖沉积物磷的生物有效性的最佳指标.红枫湖沉积物颗粒组成以粘土及粉砂为主,湖心沉积物比河口粒度小.表层(0-5cm)沉积物中Olsen-P和AAP的含量与细组分(粘土)的比例呈正相关,而RDP、WSP与细组分呈负相关性,表明沉积物细组分对深水湖泊富营养化的重要性.     

Magnetic properties of sediments in the Bay of Bengal and the Andaman Sea: impact of rapid North Atlantic Ocean climatic events on the strength of the Indian monsoon

The results of a high-resolution mineral magnetic study combined with major element geochemistry analysis, oxygen isotopes and ¹⁴C AMS stratigraphy are reported for deep-sea gravity cores MD77-169 and MD77-180 located in the Andaman Sea and the Bay of Bengal, respectively. Core MD77-169 covers the last 280 kyr and core MD77-180 covers the last 160 kyr. In both cores, rock magnetic parameters indicate that the magnetic assemblage is dominated by pseudo-single domain titanomagnetite grains, with grain-size variations following a strong 23 kyr periodicity. Smaller magnetic grain sizes are observed during periods characterized by a strong summer monsoon. In addition, in core MD77-180, we observe a correlation between magnetic grain size and a chemical index of alteration. This suggests that these magnetic grain-size changes are related to chemical weathering driven by summer monsoon rainfall. A comparison of the GISP2 ice core isotopic record and the magnetic grain-size record of the Bay of Bengal shows that rapid temperature variations documented in the ice core (Dansgaard–Oeschger cycles and Heinrich events), during the last glacial period are also present in the magnetic grain-size record. Heinrich events and cold stadial events are characterized by relatively large magnetic grain sizes. Furthermore, Heinrich events are characterized by lower values of the chemical index of alteration implying a lower degree of chemical weathering related to significantly drier conditions on the continent. We suggest that rapid cold events of the North Atlantic (Heinrich events) during the last glacial stages are characterized by a weaker summer monsoon rainfall over the Himalaya via an atmospheric teleconnection.     

现代黄河三角洲沉积物磁性地层年代框架及环境磁学研究

本文对黄河三角洲Z07孔沉积物进行了系统的磁性地层学和环境磁学研究.通过结合沉积速率和古地磁长期变化数据,我们为该孔建立了较为精确可信的年代框架(1999-03—2006-06A.D.).环境磁学结果表明黄河三角洲沉积物与中国黄土的磁学特征相似,主要载磁矿物为单畴(SD)磁铁矿,超顺磁颗粒(SP)含量也较高.整体上,该孔沉积物磁学参数的变化主要受粒度和含量控制.岩芯磁性参数在2003年前后发生了系统变化.我们认为,黄河自2002年起进行调水调沙工程,黄河下游河道冲刷加剧,形成新的物质来源,河流输入的沉积物粒度变粗,输沙量增加,这一新的物质来源是造成Z07钻孔磁性参数发生显著变化的主因.     

Post-depositional remanent magnetisation in Recent tidal-flat sediments

Palaeomagnetic measurements of 1-m cores and a 9-m Delft core of Recent tidal-flat sediments from the Wash, England have shown that these sediments possess a record of the variation of the Earth's magnetic field. The record compares well with the historic-archaeomagnetic record for the period 0–1000 years B.P. but is offset down the core due to the remanence being of post-depositional origin. A period of at least 100 years is suggested for the alignment of particles during acquisition of the post-depositional remanence. Magnetite has been identified as the major carrier of this remanence.     

Towards the interpretation of Uranus's eccentric magnetic field

Abstract

Coriolis forces stimulate dynamo action in a rapidly-rotating fluid by promoting complexities in the pattern of fluid motions, notably departures from symmetry about the axis of rotation. This pattern and its time variations determine the instantaneous form and temporal behaviour of the magnetic field so produced. Instantaneous magnetic fields will usually exhibit in their broad-scale features approximate alignment with the rotation axis. This is borne out by observations of the magnetic fields of the Earth, Jupiter and Saturn, and it is likely on general grounds that Neptune will be found to have an aligned magnetic field. But, as is shown by laboratory and theoretical studies of thermal convection in rapidly-rotating fluids, for some ranges of rotation speed, rate of heating, etc. certain patterns can occur which in electrically-conducting fluids would produce magnetic fields exhibiting departures from alignment with the rotation axis, which instantaneously could be quite pronounced but would average out to very small values over sufficiently long periods of time. These findings indicate obvious strategies for theoretical studies towards the interpretation of Uranus's eccentric magnetic field (which need not invoke departures from axial symmetry in the thermal, mechanical or electrical boundary conditions of the dynamo region within the planet) and for further observational studies.     

Grain-size distribution,morphoscopy and elemental chemistry of suspended sediments of Pindari Glacier,Kumaon Himalaya,India

Abstract

Suspended sediment concentrations in the meltwater of Pindari Glacier were determined at regular intervals in four ablation seasons. The late ablation periods (September 1994 and October 1995) were characterized by a reduced level of sediment concentration, while the sampling periods of early ablation (May 1994 and July 1995) showed very high concentrations of suspended sediment in the meltwater. Grain size distribution shows the dominance of medium and coarse silt fractions of the mean size of the suspended sediments between 4.35 and 5.82 ø. Clay size constitutes about 7% of the total size population. The majority of the samples are poorly sorted, symmetrically to finely skewed and mesokurtic in nature. The grain shows texture of mechanical and chemical origin, in which mechanical texture is predominant on most of the grains. It was observed that the grains were mostly subangular to subrounded in shape with variable size ranges. Bulk sediment chemistry consists mostly (>70%) of the five elements, Si, Al, K, Fe and Mg. Iron (Fe) and Mn are dominant heavy metals and sediments show the elemental abundance in the order of Fe > Mn > Zn > Cu > Ni > Pb. The Chemical Index of Alteration (CIA) of suspended sediments (57) is relatively higher than in the case of average unweathered upper continental crust (~50) indicating a higher degree of weathering due to glacier grinding and crushing action. Quartz is the most dominant mineral, followed by mica, illite, feldspar and kaolinite.     

Particle size characteristics of suspended sediment in hillslope runoff and stream flow

This study examines the particle size characteristics of hillslope soils and fluvial suspended sediments in an agricultural catchment. Samples of surface runoff and stream flow were collected periodically and analysed for the size distributions of the effective (undispersed) sediment. This sediment was subsequently dispersed and the ultimate size distributions determined. The median effective particle size of stream suspended sediment was considerably coarser than the median ultimate particle size, indicating that most of the load included a substantial proportion of aggregates. Moreover, the proportion of fine material (i.e. silt and clay) increased, and the proportion of sand-sized material decreased, with increasing discharge. This decrease in sediment size with increased flow, which is contrary to the traditional assumption of a positive discharge/particle size relationship, is thought to reflect: (i) the influx of silt and clay, predominantly the former, originating on the catchment slopes and brought to the stream by overland flow along vehicle wheelings, roads and tracks; and (ii) erosion of fine material from the channel bed and banks. During large storms, however, the proportion of sand-sized sediment increased during the rising limb of the hydrograph, as a result of the entrainment of coarser source material from the valley floor during overbank flooding. The stream suspended sediment was finer than the catchment soils and considerably finer than material eroding from the catchment slopes during storms. The degree of clay and silt enrichment in the suspended sediments was largely the result of preferential deposition of the coarser fraction during the transport and delivery of sediment from its source to basin outlet. The data from this study confirm that a significant mode of sediment transport in fluvial systems is in the form of aggregates, and that the dispersed sediment size distribution is inappropriate for determining the transportability of sediment by flow. © 1997 by John Wiley & Sons, Ltd.     

Trace element studies in Laucala Bay, Suva, Fiji.

The trace metal distribution in the sediments of Laucala Bay, a coastal lagoon with a barrier reef and significant freshwater input, adjacent to Suva, the capital of Fiji (150,000 people), was studied from the point of view of assessing the significance of anthropogenic sources relative to natural ones. Surface sediments from 25 sites in the Bay were analysed for particle size distribution, organic carbon and major (Si, Al, Fe, Ca, Mg) and trace metal (Mn, Zn, Cu, Pb, Cd, Hg) contents. Suspended sediments from one site and shellfish samples from five sites were also analysed. The sediments were found to be mostly of terrigenous origin. Particle size distribution, organic carbon and major elemental composition of the sediments were generally related to location within the Bay. A significant natural source of trace metals in the sediments is the suspended solids transported into the Bay by rivers. The spatial distribution of trace metals could not be fully explained by the particle size distribution and mineral composition of the sediments. Contributions from anthropogenic sources were also suspected, but the degree of enrichment from such sources is not great at the present time.