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H. Vali T. von Dobeneck G. Amarantidis O. Förster G. Morteani L. Bachmann N. Petersen 《International Journal of Earth Sciences》1989,78(3):753-764
The carrier of the natural magnetization of deep sea sediments was characterized by mineralogical, electron microscopic, and rock magnetic investigations. Magnetic single domain (SD) and pseudo single domain (PSD) particles which are most important for the stable remanent magnetization were separated from the magnetic »coarse fraction« and concentrated as magnetic »fine fraction«. The magnetic coarse fraction consists of lithogenic magnetite and titanomagnetite, which often contains exsolution-lamellae of ilmenite. Both minerals are partially maghematized and occur isolated in the sediment or embedded in rock particles, in regionally different concentrations. The magnetic fine fraction consists of lithogenic magnetite and titanomagnetite and biogenic magnetite (magnetofossils = fossil bacterial magnetosomes), the latter generally maghematized.A graphical method is described which allows the classification and characterization of the magnetic fine fraction by demagnetization of the anhysteretic remanent magnetization (ARM) of whole sediment samples. Three groups with different magnetic properties can be distinguished, characterized by three ARM type-curves: Type A curves are associated with sediments from abyssal plaines. They show nearly identical ARM properties and are typical for magnetofossils.Type B curves are produced by sediments from the vicinity of volcanic regions. Their shapes are variable to a certain degree and indicate two lithogenic magnetic phases.Type C curves are found for sediments from submarine ridges and regions with input of terrigenous detritus. These curves have the largest deviations among each other indicating a magnetic multi-phase assemblage including magnetofossils.
Zusammenfassung Die Träger der Magnetisierung von Tiefseesedimenten wurden mineralogisch, elektronenmikroskopisch und gesteinsmagnetisch untersucht. Magnetische Eindomänen (SD) und Pseudo-Eindomänen (PSD) Partikel, die für eine stabile remanente Magnetisierung wichtig sind, wurden als magnetische »Feinfraktion« von der magnetischen »Grobfraktion« abgetrennt. Die magnetische Grobfraktion besteht aus lithogenem Titanomagnetit und Magnetit die teilweise maghemitisiert sind und teilweise auch Ilmenit-Entmischungslamellen aufweisen. In regional unterschiedlichen Konzentrationen liegen sie frei im Sediment oder in silikatischer Matrix eingebettet vor. Die magnetische Feinfraktion besteht sowohl aus lithogenem Titanomagnetit und Magnetit, als auch aus biogenem Magnetit (Magnetofossilien = fossile bakterielle Magnetosomen); letzterer ist größtenteils maghemitisiert.Es wird eine grafische Darstellungsmethode beschrieben, die anhand von Untersuchungen der anhysteretischen remanenten Magnetisierung (ARM) von Sedimentproben eine Charakterisierung der magnetischen Feinfraktion erlaubt. Es lassen sich dadurch drei Gruppen mit unterschiedlichen magnetischen Eigenschaften unterscheiden, die durch drei Gruppen von ARM-Kurventypen charakterisiert sind.Kurventyp A wird bei Sedimenten aus Tiefsee-Ebenen beobachtet. Die ARM-Daten sind nahezu identisch und zeigen ein Verhalten, wie es für Magnetofossilien typisch ist. Kurventyp B tritt bei Sedimenten aus dem Einzugsbereich vulkanischer Gebiete auf. Er zeigt eine größere Variation und die Form der Kurven spricht für ein System aus zwei lithogenen magnetischen Komponenten.Kurventyp C gehört zu Sedimenten aus submarinen Rücken und dem Einzugsgebiet terrigener Schüttungen. Die Kurvenverläufe sind uneinheitlich und sprechen für ein magnetisches Mehrkomponenten-System mit Beteiligung von Magnetofossilien.
Résumé Les minéraux porteurs du magnétisme dans les sédiments de mer profonde ont été explorés par les méthodes de la minéralogie, de la microscopie électronique et du magnétisme des roches. Les particules qui correspondent à un domaine magnétique unique (SD) et pseudo-unique (PSD), significatives pour un magnétisme rénanent stable, ont été concentrées comme «fraction magnétique fine» après séparation de la «fraction magnétique grossière». Cette dernière consiste en magnétite et titanomagnétite lithogéniques, qui renferment souvent des lamelles d'exsolution d'ilménite. Ces deux minéraux sont partiellement maghémitisés; ils se présentent isolés ou inclus dans des fragments de roches, avec des concentrations régionales diverses. La fraction magnétique fine consiste en magnétite et titanomagnétite lithogéniques, ainsi qu'en magnétite biogénique (magnétofossile = magnétosome fossile bactérien), cette dernière ordinairement maghemitisée.Les auteurs présentent une méthode graphique qui permet de caractériser la fraction magnétique fine à partir de l'examen du magnétisme rémanent anhystérique (ARM) de l'échantillon de sédiment. Cette méthode permet de distinguer trois groupes de propriétés magnétiques différentes, caractérisés par trois types de courbes ARM. Les courbes de type A caractérisent les sédiments de plaines abyssales; elles montrent des propriétés ARM presque identiques et sont typiques pour les magnétofossiles. Les courbes de types B sont fournies par les sédiments voisins des régions volcaniques; leurs formes varient dans une certaine mesure et indiquent un système à deux composants magnétiques lithogéniques. Les courbes de type C correspondent aux sédiments des crêtes sous-marines et des régions à apports terrigènes; ces courbes présentent entre elles des différences plus marquées, ce qui indique un système magnétique à composants multiples, comportant des magnétofossiles.
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Near-surface electromagnetic, rock magnetic, and geochemical fingerprinting of submarine freshwater seepage at Eckernf?rde Bay (SW Baltic Sea) 总被引：1，自引：1，他引：0
Submarine groundwater discharge in coastal settings can massively modify the hydraulic and geochemical conditions of the seafloor.
Resulting local anomalies in the morphology and physical properties of surface sediments are usually explored with seismo-acoustic
imaging techniques. Controlled source electromagnetic imaging offers an innovative dual approach to seep characterization
by its ability to detect pore-water electrical conductivity, hence salinity, as well as sediment magnetic susceptibility,
hence preservation or diagenetic alteration of iron oxides. The newly developed electromagnetic (EM) profiler Neridis II successfully realized this concept for a first time with a high-resolution survey of freshwater seeps in Eckernf?rde Bay
(SW Baltic Sea). We demonstrate that EM profiling, complemented and validated by acoustic as well as sample-based rock magnetic
and geochemical methods, can create a crisp and revealing fingerprint image of freshwater seepage and related reductive alteration
of near-surface sediments. Our findings imply that (1) freshwater penetrates the pore space of Holocene mud sediments by both
diffuse and focused advection, (2) pockmarks are marked by focused freshwater seepage, underlying sand highs, reduced mud
thickness, higher porosity, fining of grain size, and anoxic conditions, (3) depletion of Fe oxides, especially magnetite,
is more pervasive within pockmarks due to higher concentrations of organic and sulfidic reaction partners, and (4) freshwater
advection reduces sediment magnetic susceptibility by a combination of pore-water injection (dilution) and magnetite reduction
(depletion). The conductivity vs. susceptibility biplot resolves subtle lateral litho- and hydrofacies variations. 相似文献
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We combine environmental magnetism, geochemical measurements and colour reflectance to study two late Quaternary sediment cores: GeoB 4905‐4 at 2° 30′ N off Cameroon and GeoB 4906‐3 at 0° 44′ N off Gabon. This area is suitable for investigating precipitation changes over Central and West Africa because of its potential to record input of aeolian and fluvial sediments. Three magnetozones representing low and high degree of alteration of the primary rock magnetic signals were identified. The magnetic signature is dominated by fine‐grained magnetite, while residual haematite prevails in the reduced intervals, showing increase in concentration and fine grain size at wet intervals. Our records also show millennial‐scale changes in climate during the last glacial and interglacial cycles. At the northern location, the past 5.5 ka are marked by high‐frequency oscillations of Ti and colour reflectance, which suggests aeolian input and hence aridity. The southern location remains under the influence of the Intertropical Convergence Zone and thus did not register aeolian signals. The millennial‐scale climatic signals indicate that drier and/or colder conditions persisted during the late Holocene and are synchronous with the 900 a climatic cycles observed in Northern Hemisphere ice core records. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
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C. Franke G. M. Pennock M. R. Drury R. Engelmann D. Lattard J. F. L. Garming T. von Dobeneck M. J. Dekkers 《Geophysical Journal International》2007,170(2):545-555
In paleomagnetic and environmental magnetic studies the magnetomineralogical identification is usually based on a set of rock magnetic parameters, complemented by crystallographic and chemical information retrieved from X-ray diffraction (XRD), (electron) microscopy or energy dispersive spectroscopy (EDS) of selected samples. While very useful, each of these supplementary techniques has its limitations when applied to natural sample material which are related to low particle concentrations (down to the ppm range in marine sediments) and very fine grain sizes (down to the nm scale). Therefore, meaningful application of such techniques depends on sample quality. Electron backscatter diffraction (EBSD) of individual grains in scanning electron microscopy (SEM) enables mineralogical identification of grains down to ∼0.2 micrometer and is particularly powerful when combined with EDS. In this study, we show the merits of EBSD for rock magnetic investigations by analyzing titanomagnetites and hemoilmenites of various compositions and submicron lamella of titanomagnetite–hemoilmenite intergrowths. Such particles often occur in natural marine sediments where EDS often has a semi-quantitative character and compositionally similar intergrowths may be difficult to distinguish. With the mineralogical information provided by EBSD unambiguous identification of spinel-type and trigonal oxides is obtained. Optimal EBSD patterns are gathered from smooth, polished surfaces, but here we show that interpretable EBSD patterns can be obtained directly from the surface of unconsolidated, so called 'non-embedded' particles from marine sediments. This information enhances the interpretative value of rock magnetic parameters. 相似文献
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