Biogenic and lithogenic magnetic minerals in Atlantic and Pacific deep sea sediments and their paleomagnetic significance

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.

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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.

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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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Dynamic response of 2D and 3D block foundations on a halfspace with inclusions

The present study aims to demonstrate how the dynamic behaviour of structures is affected by local inhomogeneities inside the soil. Particularly, the influence of block-shaped elastic inclusions directly beneath a vertically vibrating block foundation is considered. Comparisons between a 2D and a 3D modelling of the soil—structure system are made. It can be observed that, depending on the stiffness of the inclusion and especially on the excitation frequency, the dynamic response of the foundation may either increase or decrease.     

High-frequency climatic oscillations during the last deglaciation as revealed by oxygen-isotope records of benthic organisms (Ammersee,southern Germany)

High-resolution oxygen-isotope records of benthic ostracods and molluscs from Ammersee, southern Germany, show high-frequency climatic changes during the last deglaciation and parallel in great detail published faunal and floral variations reconstructed from Norwegian Sea sediments and isotope variations in Greenland ice cores. The marine and the terrestrial records give evidence of a synchronous late glacial climatic development in Greenland, NW- and Mid-Europe. However,¹⁴C-ages of the supraregional climatic events and of two tephra layers in the marine sediments of the northeastern Atlantic Ocean are significantly older than the¹⁴C-ages of the corresponding horizons on land. These differences strongly suggest that major short-term events have affected the exchangeable carbon on earth during the dramatic environmental changes related to the deglaciation and in particular have affected the CO₂-distribution within the ocean and between ocean and atmosphere. Dating methods independent of climatic variations and of the global carbon budget should be given priority to refine the timescales of the marine and atmospheric processes during the last deglaciation.This is the fourth paper in a series of papers published in this issue on high-resolution paleolimnology. These papers were presented at the Sixth International Palaeolimnology Symposium held 19–21 April, 1993 at the Australian National University, Canberra, Australia. Dr. A. F. Lotter and Dr. M. Sturm served as guest editors for these papers.     

How reliable is the210Pb dating method? Old and new results from Switzerland

We present a historical overview of applications of²¹⁰Pb dating in Switzerland with a special emphasis on the work performed at the University of Bern. It is demonstrated that the average specific activity of²¹⁰Pb in the lower atmosphere is very constant and does not show seasonal variations. We then concentrate on new results from Lobsigensee, a very small lake, and on published and new data from Lake Zurich. Several²¹⁰Pb profiles from these lakes show obvious disturbances and a disagreement of the resulting sedimentation rate when compared to that for the 23 years defined by¹³⁷Cs peaks of 1986 (Chernobyl) and 1963 (bomb fallout).A mean sedimentation rate of about 0.14 g cm^–2 y^–1 is found in the oxic and suboxic center part of Lake Zurich. In the oxic locations, the²¹⁰Pb flux to the sediments was close to the atmospheric input of about 1/60 Bq cm^–2 y^–1. In other parts of the lake a significant deficit in the inventory of²¹⁰Pb was found in the sediments. This could be due to a chemical redissolution of²¹⁰Pb together with Mn under reducing conditions. In contrast, in the suboxic part of the lake (135 m depth) the flux of²¹⁰Pb was about twice the atmospheric input. This excess is not caused by allochthonous contributions and is tentatively explained by the transport of sediment material resulting from small slides at the very steep lake shores or more probably by reprecipitation of²¹⁰Pb together with Mn when the conditions in the lake water become locally and seasonally more oxidizing. Dissolved²¹⁰Pb may migrate from locations with reducing conditions and reprecipitate under more oxic conditions. Indeed, a correlation of Mn and²¹⁰Pb in sediments of Lake Zurich was found.This is the first of a series of papers to be published by this journal following the 20^th anniversary of the first application of²¹⁰Pb dating of lake sediments. Dr P. G. Appleby is guest editing this series.     

Suevite of the ries crater,Germany: Source rocks and implications for cratering mechanics

Suevites are impact breccias with a montmorillonitic matrix that contains shocked and unshocked mineral and rock fragments from the crystalline basement, glass inclusions and a small amount of sedimentary clasts. Data are given of the modal composition of fall-out suevites (deposited at isolated points around the crater) and crater suevite (forming a layer below post-impact lake sediments in the crater cavity). Fall-out suevites contain aerodynamically shaped bombs which are absent in crater suevite. Taking into account not only large glass fragments and bombs, but also the finer fractions, the glass content of fall-out and crater suevites amounts to 47 and 29 vol%, respectively. Crystalline clasts in suevites consist of all igneous and metamorphic rock types that constitute the local basement which consists of an upper layer of igneous rocks (mainly granites) and a lower series of gneisses and amphibolite. Based on a collection of 1 200 clasts from 13 suevite occurrences the average crystalline clast population of suevites was determined. Suevites contain on the average 46 % igneous and 54 % metamorphic clasts. In constrast, weakly shocked and unshocked crystalline ejecta of the Ries structure consist of 82 % igneous and 18 % metamorphic rocks. From 138 analyses of crystalline rock samples average compositions of the major rock types were calculated. Comparison of these averages with the average glass composition leads to the conclusion that suevite glasses were formed by shock melting of gneisses in deeper levels of the basement. Suevite matrices consist in most cases of 80 to 90 % montmorillonite, in special cases of celadonite. Chemical analyses are given of some matrices and montmorillonite formulas calculated. It is supposed that montmorillonite was formed by early hydrothermal alteration of rock flour or fine glass particles. In the latter case the original glass content of suevites was higher than at present. Of all ejecta from the Ries crater only crystalline rocks contained in suevites occur in all stages of shock metamorphism up to complete fusion. The overwhelming majority of the ejecta from the sedimentary sequence (about 580 m) show no indications of shock pressures above 10 GPa. The same holds true for crystalline megablocks and breccias around the crater which consist mainly of granites from upper levels of the basement. We assume that the Ries impact can be approximated by a deep-burst model: The projectile penetrated through the sedimentary cover into the basement in such a way that the highest pressures and temperatures developed within the gneiss complex below the upper, predominately granitic layer and that rocks of the sedimentary sequence experienced weak shock compression. Numerical data are given for such a model of the Ries impact on transient crater geometry and volumes of vaporized, melted, shocked and excavated rocks. Fall-out suevites are supposed to have been lifted from the central zone by an expanding plume of vaporized rocks and deposited as fluidized turbulent masses outside the crater whereas the main mass of crater suevite was not removed from the crater cavity.     

Can a pollution event be detected using a single biological effects monitoring method?

The trends of malformation prevalence in embryos of dab, Limanda limanda, in the southern North Sea after the year 1990 mirrored the drop in major pollutants in the rivers draining into the German Bight. Despite this general decline, we detected a pollution event in the southern North Sea in winter 1995/1996 employing the prevalence of malformations in pelagic dab embryos as an indicator. An abrupt rise in malformation prevalence in the embryos of dab, corresponded to a dramatic increase in DDT levels in parent fish from the same area, indicating a hitherto unnoticed introduction of considerable quantities of DDT into the system.     

Modelling the variability of midlatitude storm activity on decadal to century time scales

The output of several multi-century simulations with a coupled ocean–atmosphere general circulation model is examined with respect to the variability of global storm activity in winter on time scales of decades and longer. The frequency of maximum wind speed events within a grid box, using the lower limits on the Beaufort wind speed scale of 8 and 10 Bft as thresholds, is taken as the characteristic parameter. Two historical climate runs with time-dependent forcing of the last five centuries, one control simulation, and three climate change experiments are considered. The storm frequency shows no trend until recently. Global maps for the industrially influenced period hardly differ from pre-industrial maps, even though significant temperature anomalies temporarily emerge in the historical runs. Two indicators describing the frequency and the regional shift of storm activity are determined. In historical times they are decoupled from temperature. Variations in solar and volcanic forcing in the historical simulations as well as in greenhouse gas concentrations for the industrially influenced period are not related to variations in storm activity. Also, anomalous temperature regimes like the Late Maunder Minimum are not associated with systematic storm conditions. In the climate change experiments, a poleward shift of storm activity is found in all three storm track regions. Over the North Atlantic and Southern Ocean, storm activity increases, while it decreases over the Pacific Ocean. In contrast to the historical runs, and with the exception of the North Pacific storm frequency index, the storm indices parallel the development of temperature, exceeding the 2 σ-range of pre-industrial variations in the early twenty-first century.     

Improved magnetic anomalies of the Antarctic lithosphere from satellite and near-surface data

The Antarctic magnetic anomaly map compiled marine and airborne surveys collected south of 60°S through 1999 and used Magsat data to help fill in the regional gaps between the surveys. Ørsted and CHAMP satellite magnetic observations with greatly improved measurement accuracies and temporal and spatial coverage of the Antarctic, have now supplanted the Magsat data. We combined the new satellite observations with the near-surface survey data for an improved magnetic anomaly map of the Antarctic lithosphere. Specifically, we separated the crustal from the core and external field components in the satellite data using crustal thickness variations estimated from the terrain and the satellite-derived free-air gravity observations. Regional gaps in the near-surface surveys were then filled with predictions from crustal magnetization models that jointly satisfied the near-surface and satellite crustal anomalies. Comparisons in some of the regional gaps that also considered newly acquired aeromagnetic data demonstrated the enhanced anomaly estimation capabilities of the predictions over those from conventional minimum curvature and spherical harmonic geomagnetic field models. We also noted that the growing number of regional and world magnetic survey compilations involve coverage gaps where these procedures can contribute effective near-surface crustal anomaly estimates.     

Trace element zoning in pyroxenes from ODP Hole 735B gabbros: diffusive exchange or synkinematic crystal fractionation?

Major and trace element profiles of clinopyroxene grains in oceanic gabbros from ODP Hole 735B have been investigated by a combined in situ analytical study with ion probe, and electron microprobe. In contrast to the homogeneous major element compositions, trace elements (REE, Y, Cr, Sr, and Zr) show continuous core to rim zoning profiles. The observed trace element systematics in clinopyroxene cannot be explained by a simple diffusive exchange between melts and gabbros along grain boundaries. A simultaneous modification of the melt composition is required to generate the zoning, although Rayleigh fractional crystallization modelling could mimic the general shape of the profiles. Simultaneous metasomatism between the cumulate crystal and the porous melt during crystal accumulation is the most likely process to explain the zoning. Deformation during solidification of the crystal mush could have caused squeezing out of the incompatible element enriched residual melts (interstitial liquid). Migration of the melt along grain boundaries might carry these melt out of the system. This process named as synkinematic differentiation or differentiation by deformation (Natland and Dick in J Volcanol Geotherm Res 110(3–4):191–233, 2001) may act as an important magma evolution mechanism in the oceanic crust, at least at slow-spreading ridges.