Simultaneous measurements of tidal straining and advection at two parallel transects far downstream in the Rhine ROFI     

Sabine Rijnsburger  Carola M. van der Hout  Onno van Tongeren  Gerben J. de Boer  Bram C. van Prooijen  Wil G. Borst  Julie D. Pietrzak 《Ocean Dynamics》2016,66(5):719-736

This study identifies and unravels the processes that lead to stratification and destratification in the far field of a Region of Freshwater Influence (ROFI). We present measurements that are novel for two reasons: (1) measurements were carried out with two vessels that sailed simultaneously over two cross-shore transects; (2) the measurements were carried out in the far field of the Rhine ROFI, 80 km downstream from the river mouth. This unique four dimensional dataset allows the application of the 3D potential energy anomaly equation for one of the first times on field data. With this equation, the relative importance of the depth mean advection, straining and nonlinear processes over one tidal cycle is assessed. The data shows that the Rhine ROFI extends 80 km downstream and periodic stratification is observed. The analysis not only shows the important role of cross-shore tidal straining but also the significance of along-shore straining and depth mean advection. In addition, the nonlinear terms seem to be small. The presence of all the terms influences the timing of maximum stratification. The analysis also shows that the importance of each term varies in the cross-shore direction. One of the most interesting findings is that the data are not inline with several hypotheses on the functioning of straining and advection in ROFIs. This highlights the dynamic behaviour of the Rhine ROFI, which is valuable for understanding the distribution of fine sediments, contaminants and the protection of coasts.  相似文献   

Variable infiltration and river flooding resulting in changing groundwater quality – A case study from Central Europe     

Konrad Miotliński  Dieke Postma  Andrzej Kowalczyk 《Journal of Hydrology》2012

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Acid groundwater in an anoxic aquifer: Reactive transport modelling of buffering processes     

Gudrun Franken  Dieke Postma  Wilhelmus H.M. Duijnisveld  Jürgen Böttcher  John Molson 《Applied Geochemistry》2009

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Die Entstehung von Trübungszonen im Unterlauf der Flüsse,speziell im Hinblick auf die Verhältnisse in der Unterelbe     

H. Postma  K. Kalle 《Ocean Dynamics》1955,8(4):137-144

Zusammenfassung Im Unterlauf von Flüssen, welche dem Gezeiteneinfluß des Meeres ausgesetzt sind, finden sich häufig Zonen besonders hohen Trübstoffgehaltes. Diese Erscheinung findet ihre maximale Entfaltung dort, wo sich der Mischungsvorgang des Flußwassers mit dem Meerwasser durch das schwache Ansteigen des Salzgehaltes erstmalig zu erkennen gibt. Aus diesem Grunde glaubte man bisher annehmen zu müssen, daß das Auftreten der Trübungszone in erster Linie durch die im Experiment nachweisbare Ausflockung gelöster Kolloide durch den Ionengehalt des Meerwassers hervorgerufen würde, während dynamische Vorgänge hierbei nur eine untergeordnete Rolle spielen sollten. An Trübungsmessungen in Wasser von verschiedenen Punkten aus der Unterelbe wird wahrscheinlich gemacht, daß die Verhältnisse gerade umgekehrt liegen. Durch die eigenartigen Strömungs- und Vermischungsvorgänge in solchen dem Gezeiteneinfluß unterliegenden Ästuarien bildet sich eine Art Sinkstoff-Falle. Diese fängt die Trübungsstoffe aus dem Flußwasser in der Mischzone ab, indem sie diese in einem ständigen vertikalen Kreislauf herumführt, während das Wasser selbst ungehindert ins Meer abfließt. Der chemische Vorgang der Ausflockung dagegen scheint, wenn überhaupt, nur eine ganz untergeordnete Rolle bei der Ausbildung dieser Zonen zu spielen.

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On the development of turbid zones in the lower course of rivers with special consideration of conditions in the lower Elbe

Summary Zones with an extremely high degree of turbidity are of frequent occurrence in the lower course of rivers exposed to tidal influences of the sea. Their maximum development will be found in an area where the mixing process between river water and sea water first becomes perceptible from a slight increase in salinity. For this reason, it was believed that the presence of turbid zones should, in the first place, be attributed to the occurrence of dissolved colloids, a process that is caused by ion concentration of sea water the existence of which has been proved by laboratory experiments; dynamic processes were, however, believed to play only a subordinate part in the development of such turbid zones. Turbidity measurements in water taken from various places in the lower Elbe show that, in all probability, the reverse will be true. The peculiar water movement in such tidal estuaries form a sort of trap for suspended matter which catches the turbid matter from the river water in the mining zones and makes them revolve continuously in a vertical circulation while the water itself flows to the sea, without hindrance. The chemical process of precipitation seems, if at all, to exercise but little influence on the development of the turbid zones.

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Sur le développement des zones de turbidité dans les cours inférieurs des fleuves et particulièrement sur les conditions dans le cours inférieur de l'Elbe

Résumé Dans les cours inférieurs des fleuves soumis à l'influence des marées se trouvent souvent des zones de turbidité, dont le maximum se développe aux endroits où la faible augmentation de la salinité révèle, pour la première fois, le mélange de l'eau fluviatile avec l'eau de mer. Pour cette raison, on supposait jusqu'ici que la présence des zones de turbidité soit produit, en premier lieu, par suite de la précipitation de colloides dissous, par l'eau de mer, un phénomène dont l'existence est prouvée dans des expériments de laboratoire; aux procès dynamiques on n'attribuerait, sous ce rapport, qu'un rôle secondaire. Des observations effectuées à plusieurs échantillons d'eau recueillis à des endroits divers dans le cours inférieur de l'Elbe montrent, au contraire, que ce sont probablement des phénomènes dynamiques, auxquels il faut attribuer la présence des régions de turbidité. Les mouvements d'eau particuliers dans de tels estuaires à marée produisent une sorte de »piège de matières suspendues«. Ce »piège« attrappe les sédiments amenés par les fleuves et les engage dans une circulation verticale pendant que les eaux fluviatiles poursuivent sans entravement leur chemin vers la mer. Le procédé chimique de la précipitation ne semble donc exercer qu'une très faible influence sur la formation des zones de turbidité.

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Fast transformation of iron oxyhydroxides by the catalytic action of aqueous Fe(II)     

Hanne D. Pedersen  Dieke Postma  Ole Larsen 《Geochimica et cosmochimica acta》2005,69(16):3967-3977

Iron oxides may undergo structural transformations when entering an anoxic environment. These transformations were investigated using the isotopic exchange between aqueous Fe(II) and iron oxides in experiments with ⁵⁵Fe-labelled iron oxides. ⁵⁵Fe was incorporated congruently into a ferrihydrite, two lepidocrocites (#1 and #2), synthesised at 10°C and 25°C, respectively, a goethite and a hematite. The iron oxides were then submerged in Fe²⁺ solutions (0-1.0 mM) with a pH of 6.5. In the presence of aqueous Fe²⁺, an immediate and very rapid release of ⁵⁵Fe was observed from ferrihydrite, the two lepidocrocites and goethite, whereas in the absence of Fe²⁺ no release was observed. ⁵⁵Fe was not released from hematite, even at the higher Fe²⁺ concentration. The release rate is mainly controlled by characteristics of the iron oxides, whereas the concentration of Fe²⁺ only has minor influence. Ferrihydrite and 5-nm-sized lepidocrocite crystals attained complete isotopic equilibration with aqueous Fe(II) within days. Within this timeframe ferrihydrite transformed completely into new and more stable phases such as lepidocrocite and goethite. Lepidocrocite #2 and goethite, having larger particles, did not reach isotopic equilibrium within the timeframe of the experiment; however, the continuous slow release of ⁵⁵Fe suggests that isotopic equilibrium will ultimately be attained.Our results imply a recrystallization of solid Fe(III) phases induced by the catalytic action of aqueous Fe(II). Accordingly, iron oxides should properly be considered as dynamic phases that change composition when exposed to variable redox conditions. These results necessitate a reevaluation of current models for the release of trace metals under reducing conditions, the sequestration of heavy metals by iron oxides, and the significance of stable iron isotope signatures.  相似文献   

Geometrie und Kinematik der Taunuskammüberschiebung — Beitrag zur Diskussion des Deckenproblems im südlichen Schiefergebirge     

Dr. Onno Oncken 《International Journal of Earth Sciences》1988,77(2):551-575

Zusammenfassung Gefügeanalysen in Bewegungsbahnen und bilanzierte Profile erlauben eine Neubewertung der Taunuskammüberschiebung und der begleitenden Struktureinheiten. über die schon länger bekannte Tatsache einer die Faltung und Schieferung überdauernden NW-gerichteten Schuppenstapelung wird deutlich, da\ der Transport des Oberschiebungskörpers im späten Verlauf in westliche Richtung umschwenkt. Damit verbunden ist eine zunehmende Konzentration der Deformation auf die Bewegungsbahnen. Der Ablauf korreliert mit einer retrograden Gefügeentwicklung in den Scherzonen (von Myloniten zu Kataklasiten, vor allem an der Front bzw. Basis des überschiebungskörpers) ausgehend von anchimetamorphen bis grünschieferfaziellen Bedingungen. Diese, stratigraphische Grenzen anscheinend schneidende Metamorphose mit synkinematischem Höhepunkt setzt eine stärkere tektonisch gesteuerte überlagerung der nur ca. 3 km mächtigen devonischen Abfolge am Schiefergebirgssüdrand voraus. Diese Zusammenhänge und die aus Profilen und ihrer Bilanzierung gewonnene Einsicht in die interne Geometrie der Taunuskamm-Soonwald-Einheit legen eine Neuinterpretation nahe: Die Einheit erscheint insgesamt als parautochthoner Deckenkörper (Transport > 8 km an der Front) mit der Internstruktur eines teleskopartig zusammengeschobenen und gestapelten »hinterland-dipping-duplex«-Systems, das vom kristallinen Fundament der nördlichen Mitteldeutschen Schwelle in das nördliche Vorland hinein abgeschert wurde.

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The analysis of structures in shear zones and of balanced cross sections allows a new evaluation of the »Taunuskamm«-thrust at the southern border of the Rheinische Schiefergebirge. Besides the long known fact of NW-directed imbrication which outlasts folding and cleavage formation, tectonic transport appears to switch to the west during late stage movement. This development is related to increasing concentration of deformation along the shear zones and is moreover accompanied by retrogressive conditions within the shear zones (from mylonites to cataclasites) at the front and at the base of the thrust body, starting from very low grade to low grade conditions. This synkinematic metamorphism which cuts across stratigraphic boundaries presupposes a tectonic superposition or stacking of the mere 3 km of Devonian sediments at the southern rim of the Rheinische Schiefergebirge. These relationships and the results of balancing lead to a new interpretation of the »Taunuskamm-Soonwald«-unit: the unit seems to be an parautochtonous nappe body (over 8 km of thrusting at the front) with the internal structure of a hinterland dipping duplex system which has been sheared of from the crystalline basement of the northern Mid-German Crystalline Rise, and has been telescoped into the northern bassin.

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Résumé L'analyse des structures dans les couloirs de cisaillement («shear zones») et dans les profils équilibrés («balanced cross sections») permet une nouvelle évaluation du chevauchement du «Taunuskamm» à la bordure sud du Massif Schisteux Rhénan. Outre le fait, déjà connu, d'une imbrication vers le NW qui a persisté après la formation du plissement et de la schistosité, il s'avère que le transport tectonique de la masse chevauchante s'est infléchi vers l'ouest pendant sa phase finale. Ce processus s'est accompagné d'une concentration de la déformation dans les couloirs de cisaillement, ainsi que d'une dégradation des structures (de mylonites à cataclasites) au front et à la base du corps charrié dans des conditions (pt) qui vont de l'anchimétamorphisme au faciès des schistes verts. Ce métamorphisme syncinématique, qui transgresse les limites stratigraphiques, implique un empilement tectonique de la série dévonienne, peu épaisse (3 km) au bord sud du Massif Schisteux Rhénan. Ces dispositions ainsi que l'équilibrage des coupes transversales conduisent à une nouvelle interprétation de l'unité du Taunuskamm-Soonwald: il s'agit d'une nappe de charriage parautochtone (flèche de 8 km au front de la nappe) présentant la structure interne d'un système de «hinterland dipping duplex» qui a été décollée du socle cristallin du Seuil de l'Allemagne Centrale et a été télescopée dans le bassin situé au nord.

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Precambrian banded iron formations - physicochemical conditions of formation | Y.P. Mel'nik (Editor), 1982. (Translated from Russian by D.B. Vitaliano.) Developments in Precambrian Geology 5, Elsevier,Amsterdam, xii + 316 pp., US$72.25, Dfl.170.00 (hardcover)     

David Bridgwater  Dieke Postma 《Earth》1984,21(4):306-307

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Sorption of phosphate onto calcite; results from batch experiments and surface complexation modeling   总被引：3，自引：0，他引：3  

Helle Ugilt Sø  Dieke Postma  Flemming Larsen 《Geochimica et cosmochimica acta》2011,75(10):2911-2923

The adsorption of phosphate onto calcite was studied in a series of batch experiments. To avoid the precipitation of phosphate-containing minerals the experiments were conducted using a short reaction time (3 h) and low concentrations of phosphate (?50 μM). Sorption of phosphate on calcite was studied in 11 different calcite-equilibrated solutions that varied in pH, P_CO2, ionic strength and activity of Ca²⁺, and . Our results show strong sorption of phosphate onto calcite. The kinetics of phosphate sorption onto calcite are fast; adsorption is complete within 2-3 h while desorption is complete in less than 0.5 h. The reversibility of the sorption process indicates that phosphate is not incorporated into the calcite crystal lattice under our experimental conditions. Precipitation of phosphate-containing phases does not seem to take place in systems with ?50 μM total phosphate, in spite of a high degree of super-saturation with respect to hydroxyapatite (SI_HAP ? 7.83). The amount of phosphate adsorbed varied with the solution composition, in particular, adsorption increases as the activity decreases (at constant pH) and as pH increases (at constant activity). The primary effect of ionic strength on phosphate sorption onto calcite is its influence on the activity of the different aqueous phosphate species. The experimental results were modeled satisfactorily using the constant capacitance model with >CaPO₄Ca⁰ and either >CaHPO₄Ca⁺ or > as the adsorbed surface species. Generally the model captures the variation in phosphate adsorption onto calcite as a function of solution composition, though it was necessary to include two types of sorption sites (strong and weak) in the model to reproduce the convex shape of the sorption isotherms.  相似文献   

Rapid extensive erosion of the North Alpine foreland basin at 5–4 Ma     

Charlotte E. Cederbom  Peter van der Beek  Fritz Schlunegger  Hugh D. Sinclair  Onno Oncken 《Basin Research》2011,23(5):528-550

An extensive low‐temperature thermochronology study of the Swiss part of the North Alpine Foreland Basin has been conducted with the aim of deciphering the late Neogene basin development. Apatite fission‐track (AFT) ages from wells located in the distal and weakly deformed Plateau Molasse reveal rapid, km‐scale erosion with an onset in early Pliocene times. The distribution of erosion implies that there was a strong gradient in late Miocene deposition rates along the strike of the basin, with an increase towards the northeast. Additionally, renewed tectonic activity and km‐scale out‐of‐sequence thrusting during Plio‐Pleistocene times is indicated by AFT data from wells within the thrusted, proximal Subalpine Molasse. Several different mechanisms driving late Neogene basin erosion and accelerated erosional discharge from the European Alps have been considered in the literature. Based on our AFT results, we reevaluate previously published hypotheses, and suggest that a change in climate and/or drainage reorganisation coincided and possibly interacted with preexisting tectonic and geodynamic forces in the Alpine region.  相似文献   

Morphodynamics and sedimentary structures of bedforms under supercritical‐flow conditions: New insights from flume experiments     

Matthieu J.B. Cartigny  Dario Ventra  George Postma  Jan H. van Den Berg 《Sedimentology》2014,61(3):712-748

Supercritical‐flow phenomena are fairly common in modern sedimentary environments, yet their recognition and analysis remain difficult in the stratigraphic record. This fact is commonly ascribed to the poor preservation potential of deposits from high‐energy supercritical flows. However, the number of flume data sets on supercritical‐flow dynamics and sedimentary structures is very limited in comparison with available data for subcritical flows, which hampers the recognition and interpretation of such deposits. The results of systematic flume experiments spanning a broad range of supercritical‐flow bedforms (antidunes, chutes‐and‐pools and cyclic steps) developed in mobile sand beds of variable grain sizes are presented. Flow character and related bedform patterns are constrained through time‐series measurements of bed configurations, flow depths, flow velocities and Froude numbers. The results allow the refinement and extension of some widely used bedform stability diagrams in the supercritical‐flow domain, clarifying in particular the morphodynamic relations between antidunes and cyclic steps. The onset of antidunes is controlled by flows exceeding a threshold Froude number. The transition from antidunes to cyclic steps in fine to medium‐grained sand occurs at a threshold mobility parameter. Sedimentary structures associated with supercritical bedforms developed under variable aggradation rates are revealed by means of combining flume results and synthetic stratigraphy. The sedimentary structures are compared with examples from field and other flume studies. Aggradation rate is seen to exert an important control on the geometry of supercritical‐flow structures and should be considered when identifying supercritical bedforms in the sedimentary record.  相似文献