Kordillerisch-atlantische Wechselbeziehungen

Ohne ZusammenfassungEtwas erweiterte Wiedergabe eines auf der Atlantiktagung der Geologischen Vereinigung am 8. Januar 1939 gehaltenen Vortrages.     

Aragonitische Lumachellen im bituminösen Wealden des Emslandes

In the Wealden beds of the Dalum oil pool (NW-Germany), cycles of the following composition have been deposited especially on synsedimentary uplifts: Bituminous shales (at the base) — shales with cyrena layers — cyrena coquinas (at the top). The cyrena and gastropod shells are preserved in aragonite, if they are embedded in shale or surrounded by oil, which stopped the diagenesis. Water saturated coquinae, poor in clay, are always calcitisated. As a consequence, the transformation aragonite → calcite should have been performed by solution and recrystallisation. Only in aragonitic shells, organic structures are visible by fluorescence microscopy. In the bituminous shales, admixtures of Mg-calcite have been preserved sporadically.     

Das Gebiet des Yangtze-Deltas mit den Nanking-Bergen

Ohne Zusammenfassung     

Physiography and deposition on a distal deep-sea system: The Valencia Fan (Northwestern Mediterranean)

The Valencia Fan developed as the distal fill of a deep-sea valley, detached from the continental slope and the main sedimentary source. A survey of side-scan sonar, Sea Beam and reflection seismics shows that the sediment is largely fed through the Valencia Valley. The upper fan comprises large channels with low-relief levees, and the middle fan has sinuous distributary channels. Depositional bedforms predominate on the valley floor and levees, and erosional bedforms are common in the valley walls. A change to slope on the fan apex and the presence of volcanoes on the upper fan are the main factors influencing fan-growth pattern.     

Filter-feeding ascidians (Ciona intestinalis) in a shallow cove: Implications of hydrodynamics for grazing impact

The grazing impact by a dense population of filter-feeding ascidians Ciona intestinalis on horizontally flowing water (driven by density circulation) in a shallow cove (Kertinge Nor, Denmark) has been described and quantified by means of a simple one-dimensional numerical model. The agreement between observations and modelled predictions was satisfactory. The applied numerical model has the following analytical solution in the idealized case: Cx = C₀e^−(fx/Y₂), where C_x = algal concentration at a downstream distance x, C₀ = initial concentration, f = F/v_c; F = area specific population filtration rate; v_c = current velocity; Y₂ = depth of mixed layer below halocline. The numerical model quantifies the actual grazing impact while the analytical model illustrates the governing physics in well-known terms. To describe situations with no current (i.e. stagnant water), we performed simulation studies in the laboratory and measured vertical profiles of algal cells over filter-feeding C. intestinalis. The results showed that phytoplankton became reduced in a near-bottom water layer of 20–30 cm thickness. Such water layers may develop in stagnant water (calm days and no advective currents), thus uncoupling the pelagic food and the filter feeders which within a short time will experience extremely meagre food conditions.     

Spectre of overcapacity

The freight markets went into the doldrums again in 1981 after having withstood the impact of industrial recession astonishingly well into the early months of the year thanks to expanding coal traffic and long port delays for bulkers. Oil imports of the main consuming areas as well as the raw material intake of the steel industries shrank considerably, while movements of other goods were still expanding. On the other hand, the world merchant fleet started to grow again at an accelerated pace. Transport capacities were, as a consequence, less employed; laid-up shipping increased rapidly, and freight-rate levels declined sharply in most market sectors. These developments are analysed in detail in the annual review of maritime transport by Dr Hans Böhme, of the Kiel Institute of World Economy, and published, as ‘Weltseeverkehr: Vor einem neuen Tonnageüberschuß?’ in the Institute's bi-annual, Die Weltwirtschaft. The author summarizes the review below.     

High nutrient pulses, tidal mixing and biological response in a small California estuary: Variability in nutrient concentrations from decadal to hourly time scales

Elkhorn Slough is a small estuary in Central California, where nutrient inputs are dominated by runoff from agricultural row crops, a golf course, and residential development. We examined the variability in nutrient concentrations from decadal to hourly time scales in Elkhorn Slough to compare forcing by physical and biological factors. Hourly data were collected using in situ nitrate analyzers and water quality data sondes, and two decades of monthly monitoring data were analyzed. Nutrient concentrations increased from the mid 1970s to 1990s as pastures and woodlands were converted to row crops and population increased in the watershed. Climatic variability was also a significant factor controlling interannual nutrient variability, with higher nutrient concentrations during wet than drought years. Elkhorn Slough has a Mediterranean climate with dry and rainy seasons. Dissolved inorganic nitrogen (DIN) concentrations were relatively low (10–70 μmol L⁻¹) during the dry season and high (20–160 μmol L⁻¹) during the rainy season. Dissolved inorganic phosphorus (DIP) concentrations showed the inverse pattern, with higher concentrations during the dry season. Pulsed runoff events were a consistent feature controlling nitrate concentrations during the rainy season. Peak nitrate concentrations lagged runoff events by 1 to 6 days. Tidal exchange with Monterey Bay was also an important process controlling nutrient concentrations, particularly near the mouth of the Slough. Biological processes had the greatest effect on nitrate concentrations during the dry season and were less important during the rainy season. While primary production was enhanced by nutrient pulses, chlorophyll a concentrations were not. We believe that the generally weak biological response compared to the strong physical forcing in Elkhorn Slough occurred because the short residence time and tidal mixing rapidly diluted nutrient pulses.     

Eine einfache Methode zur Messung der Tagessummen ultravioletter Globalstrahlung

Zusammenfassung Tagessummen der Globalstrahlung von Wellenl?ngen unter 345 nm k?nnen mit dünnen Pl?ttchen eines leicht erh?ltlichen Harzkunststoffes, Plexiglas, Typ G [Abk. P (G)] von 0,8 mm Dicke direkt integriert werden. Die optischen Dichte?nderungen des P (G) im Spektralbereich zwischen etwa 340 und 390 nm sind den UV-Dosen proportional und k?nnen mittels einfacher densitometrischer Apparate gemessen werden. Verschiedene physikalische und phototropische Eigenschaften des P (G) wurden geprüft, insbesondere die Abh?ngigkeit der Photodegradierung von der Temperatur. Der Temperatureinflu? wird durch Reduktion der Me?werte auf 45° C ausgeschaltet; hierzu dient eine Formel mit einem empirisch bestimmten Temperaturkoeffizienten, in welcher die w?hrend der Bestrahlung erreichte maximale Lufttemperatur benützt wird. Die P (G)-Pl?ttchen wurden mittels einer Normalstrahlungsquelle geeicht und damit wurde der Eichfaktor bestimmt. Die Fehlerquellen und die Genauigkeit der unter verschiedenen Bedingungen gemessenen UV-Dosen werden analysiert.

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Summary The daily sums of global radiation of wavelengths below 345 nm can be directly integrated by means of thin platelets of an easily obtainable acrylic plastic, Plexiglas, type G [abbr. P (G)] of 0.03 inch thickness. The optical density changes of P (G) in the spectral range between approximately 340 and 390 nm are proportional to the UV-doses and can be measured with simple densitometric instruments. Various physical and phototropic properties of P (G) were tested, in particular the dependence of the photodegradation on temperature. This temperature effect is removed by reduction of the measurements to 45° C; for this purpose a formula containing an empirically determined temperature coefficient is given in which the maximum air temperature during irradiation is used. The P (G)-platelets were calibrated by means of a standard irradiance source, and the calibration factor was determined. The error sources and the accuracy of the UV-doses under various conditions of measurement are analyzed.

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Résumé Il est possible d'intégrer directement les sommes journalières du rayonnement global ayant une longueur d'onde de moins de 345 nm. Pour ce faire, on utilise des lames minces (0,8 mm d'épaisseur) d'une substance synthétique du commerce: le Pléxiglas Type G [abrégé par P (G)]. Les modifications de la densité optique du P (G) sont, dans les lignes spectrales comprises entre 340 et 390 nm proportionnelles aux doses d'ultra-violets re?ues. Elles peuvent être mesurées au moyen d'un densitomètre simple. On a examiné différentes propriétés physiques et phototropiques du P (G), en particulier la relation existant entre sa dégradation et la température. L'influence de la température peut alors être éliminée par une réduction à 45° C des valeurs mesurées. Pour ce faire, on utilise une formule contenant un coefficient de température déterminé empiriquement. Le maximum des températures de l'air atteint durant l'exposition entre également dans la dite formule. Les lames de P (G) ont été étalonnées au moyen d'une source de rayonnement normale et on en a déduit un facteur d'étalonnage. On analyse enfin les sources d'erreurs et la précision des doses d'ultra-violets recueillies dans diverses conditions.

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Mit 6 Textabbildungen

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Die vorliegende Untersuchung wurde durch Subventionen des U. S. Weather Bureau, E.S.S.A., erm?glicht.     

Segmentation and morphotectonic variations along a slow-spreading center: The Mid-Atlantic Ridge (24°00′ N– 30°40′ N)

Analysis of Sea Beam bathymetry along the Mid-Atlantic Ridge between 24°00 N and 30°40 N reveals the nature and scale of the segmentation of this slow-spreading center. Except for the Atlantis Transform, there are no transform offsets along this 800-km-long portion of the plate boundary. Instead, the Mid-Atlantic Ridge is offset at intervals of 10–100 km by nontransform discontinuities, usually located at local depth maxima along the rift valley. At these discontinuities, the horizontal shear between offset ridge segments is not accommodated by a narrow, sustained transform-zone. Non-transform discontinuities along the MAR can be classified according to their morphology, which is partly controlled by the distance between the offset neovolcanic zones, and their spatial and temporal stability. Some of the non-transform discontinuities are associated with off-axis basins which integrate spatially to form discordant zones on the flanks of the spreading center. These basins may be the fossil equivalents of the terminal lows which flank the neovolcanic zone at the ends of each segment. The off-axis traces, which do not lie along small circles about the pole of opening of the two plates, reflect the migration of the discontinuities along the spreading center.The spectrum of rift valley morphologies ranges from a narrow, deep, hourglass-shaped valley to a wide valley bounded by low-relief rift mountains. A simple classification of segment morphology involves two types of segments. Long and narrow segments are found preferentially on top of the long-wavelength, along-axis bathymetric high between the Kane and Atlantis Transforms. These segments are associated with circular mantle Bouguer anomalies which are consistent with focused mantle upwelling beneath the segment mid-points. Wide, U-shaped segments in cross-section are preferentially found in the deep part of the long-wavelength, along-axis depth profile. These segments do not appear to be associated with circular mantle Bouguer anomalies, indicating perhaps a more complex pattern of mantle upwelling and/or crustal structure. Thus, the long-recognized bimodal distribution of segment morphology may be associated with different patterns of mantle upwelling and/or crustal structure. We propose that the range of observed, first-order variations in segment morphology reflects differences in the flow pattern, volume and temporal continuity of magmatic upwelling at the segment scale. However, despite large first-order differences, all segments display similar intra-segment, morphotectonic variations. We postulate that the intra-segment variability represents differences in the relative importance of volcanism and tectonism along strike away from a zone of enhanced magma upwelling within each segment. The contribution of volcanism to the morphology will be more important near the shallowest portion of the rift valley within each segment, beneath which we postulate that upwelling of magma is enhanced, than beneath the ends of the segment. Conversely, the contribution of tectonic extension to the morphology will become more important toward the spreading center discontinuities. Variations in magmatic budget along the strike of a segment will result in along-axis variations in crustal structure. Segment mid-points may coincide with regions of highest melt production and thick crust, and non-transform discontinuities with regions of lowest melt production and thin crust. This hypothesis is consistent with available seismic and gravity data.The rift valley of the Mid-Atlantic Ridge is in general an asymmetric feature. Near segment mid-points, the rift valley is usually symmetric but, away from the segment mid-points, one side of the rift valley often consists of a steep, faulted slope while the other side forms a more gradual ramp. These observations suggest that half-grabens, rather than full-grabens, are the fundamental building blocks of the rift valley. They also indicate that the pattern of faulting varies along strike at the segment scale, and may be a consequence of the three-dimensional, thermo-mechanical structure of segments associated with enhanced mantle upwelling beneath their mid-points.     

The ocean/continent transition along a profile through the Lofoten basin,Northern Norway

The Cenozoic margins of the Norwegian-Greenland Sea offer ideal conditions for passive margin studies. A series of structural elements, first observed on these margins, led to the concept of volcanic passive margins. Questions still remain about the development of such features and the location of the boundary between oceanic and continental crust. Despite the thin sediment cover of the margins, seismic reflection data are not able to image the deeper structures due to the occurrence of igneous rocks at shallow depth.This paper presents a 320-km long profile perpendicular to the strike of the main structural units of the Lofoten Margin in Northern Norway. A geological model is proposed, based on observations made with ocean bottom seismographs, which recorded seismic refraction data and wide angle reflections, along with a seismic reflection profile covering the same area. Ray-tracing was used to calculate a geophysical model from the shelf area into the Lofoten basin. The structures typical of a volcanic passive margin were found, showing that the Lofoten Margin was influenced by increased volcanic activity during its evolution. The ocean/continent transition is located in a 30-km wide zone landwards of the Vøring Plateau escarpment.The whole margin is underlain by a possibly underplated, high velocity layer. Evidence for a pre-rift sediment basin landwards of the escarpment, overlain by basalt flows, was seen. These structural features, related to extensive volcanism on the Lofoten Margin, are not as distinct as further south along the Norwegian Margin. Viewed in the light of the hot-spot theory of White and McKenzie (1989) the Lofoten Margin can be interpreted as a transitional type between volcanic and non-volcanic passive margin.