Dating deformation using crushed alkali feldspar: 40Ar/39Ar geochronology of shear zones in the Wyangala Batholith,NSW, Australia

Diffusion parameters have been estimated for K-feldspar in and adjacent to mylonite shear zones in the Wyangala Batholith. The parameters obtained suggest that deformation during mylonitisation would have caused argon systematics to reset because diffusion distances were reduced by cataclasis, deformation and/or recrystallisation. However, the mineral lattice remained sufficiently retentive to allow subsequently produced radiogenic argon to be retained. ⁴⁰Ar/³⁹Ar geochronology is thus able to constrain operation of these biotite-grade ductile shear zones to the period from ca 380 Ma to ca 360 Ma, at the end of the Tabberabberan Orogeny.     

Résultats préliminaires sur la sédimentation pélagique de l'Atlantique tropical au Crétacé et au Tertiaire (plateau de Demerara,Leg ODP 207)

Five sites located on a bathymetric transect of the distal Demerara Rise were studied by ODP Leg 207. Albian sediments of essentially terrigenous nature (clay, siltstone, sandstone) are the oldest drilled stratigraphic levels and form apparently the top of the synrift sequence. They are overlain by Cenomanian to Santonian finely laminated black shales, rich in organic matter of marine origin, which accumulated on a thermally subsiding ramp. Early Campanian hiatuses are thought to be the result of final disjunction of Demerara Rise (South America) from Africa and the onset of deep water communication between the two Atlantic basins (south and central). The overlying Uppermost Cretaceous–Oligocene chalk includes rich and diversified calcareous plankton assemblages, as well as two radiolarian-rich intervals (Late Campanian and Middle Eocene). A complex erosional surface developed during the Late Oligocene–Early Miocene. Sedimentation was impeded since then on the intermediate and deep sites of Demerara Rise, possibly due to the action of deep submarine currents. To cite this article: T. Danelian et al., C. R. Geoscience 337 (2005).     

Spatial and Temporal Distribution of Oxidation Events Occurring Below the Sediment-Water Interface

Abstract. Irrigation and bioturbation lead to transport of dissolved oxygen into anoxic sediments. The depthdistribution of local oxygen input and the total time of oxygen presence was measured at randomly sampled locations within anoxic sediments, originating from the North Sea.
In the laboratory, continuous redox potential records, displaying a transient increase and decrease when in contact with oxygen for a limited time, were used to record oxidation events'. Oxygen microelectrodes were used to confm the presence of oxygen. Measurements were performed for 180 d at 230 locations in the top 6 cm of sediment cores containing the natural macrofauna community.
4783 oxidation events m^-2· d^-1 were recorded within the upper 6 cm of the sediment. The number and duration of oxidation events declined with sediment depth below the oxic zone. Oxygen was present in the anoxic sediment, as determined from redox potential measurements, ≥ 6 h per day at 10% of the randomly chosen locations within the upper 3 cm. The overall distribution of oxidation events and their duration suggest that local, pulsed additions of oxygen by irrigation may be sufficient to maintain an oxidised sediment layer ( sensu J orgensen amp; R evsbech , 1989). Oxic environments along burrow walls rapidly fluctuate between oxic and anoxic conditions. Using oxygen microelectrodes the presence of oxygen (oxic conditions) in these halos was found to range from 2 to 12 h per day. Continuous redox measurements show that oxidised conditions fluctuate with the oxygen pulses and display the same durations, although these may range up to 21 h per day. Oxic and oxidised sediment volumes are estimated to represent < 1% and 3.7%. respectively, of the anoxic sediment to 6 cm depth. Recognition of temporal variability and spatial heterogeneity of sedimentary conditions may prove valuable for future conclusions drawn in other research fields.     

Imaging spectroscopy as a tool to study sediment characteristics on a tidal sandbank in the Westerschelde

This paper focuses on the use of imaging spectroscopy for the mapping of sediment characteristics on a tidal sandbank in the Westerschelde, called the Molenplaat. On June 8, 2004, during low tide, a HyMap™ scanner recorded the Molenplaat at 4 m pixel resolution. The hyperspectral data were radiometrically calibrated, geometrically corrected, and atmospherically corrected to give apparent surface reflectance data. On the calibrated and corrected dataset a supervised binary classification was performed, based on linear discriminant analysis. Simultaneous to the flight, 25 sediment samples were collected in the field and analysed in the lab to define the median grain size, the water content, the total organic matter content and the chlorophyll-a concentration. These four parameters play a crucial role in sediment stability and macrofaunal habitat definition. Prior to the classification, a feature selection, based on sequential floating forward search (SFFS), was performed. For each of the four parameters two to three bands were retained for the classification. These bands were most frequently selected in the visible and near infrared parts of the spectrum, except for the organic matter content where also SWIR bands were used. The overall classification accuracy was highest for the water content (88%), the median grain size (88%) and the chlorophyll-a concentration (84%). The organic matter content, for which three instead of two classes were distinguished, scored somewhat lower but still reached 80%. The classifications were limited to a small number of classes in order to obtain reliable statistics with a small number of training samples. The spatial patterns in the classified images indicated that the four parameters under study are highly correlated. In most cases coarse sediment coincided with dry conditions, low organic matter and low concentrations of chlorophyll-a. The wet and muddy parts of the Molenplaat were in general characterised by a notably higher amount of organic matter and chlorophyll-a. The individual classification results for the median grain size, the water content and the chlorophyll-a concentration were combined to generate a sediment ecotope map. The presented study illustrates how airborne hyperspectral data can be used to achieve accurate classified maps of intertidal sediment ecotope types, applying feature selection and a binary classification approach.     

Altersbeziehungen zwischen den Metamorphosezyklen und den orogenen Bewegungen im östlichen Teil des nordrhodesischen Grundgebirges

Zusammenfassung Im Irumiden-Gebirge (E.Ackermann 1950) Nordrhodesiens konnten mit Hilfe von Gefüge-Auswertungen zwei altersverschiedene tektonische Verformungspläne mit jeweils voneinander abweichenden Richtungen herausgestellt werden.Jedem der orogenetischen Abläufe kann ein spezifischer Metamorphose-Zyklus zugeordnet werden.Für die erste Metamorphose ist eine Plagioklas-Blastese mit vermutlicher Natrium- und eventuell auch Kalziumzufuhr, für die letzte eine sehr kräftige Kalizufuhr von größter Bedeutung. Die Plagioklas-reichen Gneise, die Endprodukte der ersten Metamorphose, sind in kleineren reliktischen Komplexen über das gesamte Gebiet verbreitet, während sich die Kalifeldspat-Porphyroblasten-Gneise und Gneis-Granite auf ENE und NE gerichtete und in dieser Richtung vielfach gelängte und durchbewegte Züge verteilen. Zur Sprossung von sehr großen idiomorphen Mikroklinen und zu großen KF-Anreicherungen kommt es zu beiden Seiten der irumidisch-ENE-gerichteten Scheitelzonen (siehe E.Ackermann und A.Forster 1960). Innerhalb dieser hochmetamorphen Gneise und Gneisgranite treten die erstgenannten Plagioklas-betonten Gneise als reliktische, unverdaute Schollen auf. Die Plagioklase und wenigen Kalifeldspäte der älteren Feldspatgeneration werden zusammen mit ihren Biotitsäumen in den bis zu Kinderkopf-großen jüngeren Mikroklinporphyroblasten der sogenannten Mkushi-Gneise eingeschlossen.

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In the Irumide-Mountains of Northern Rhodesia, petrofabric analysis shows two intersecting systems of deformations being of different age. Each orogeny can be traced to a specific cycle of metamorphism. The earlier metamorphism was connected with a blastesis of plagioclase and with probable influx of sodium and calcium, the later-one with a strong addition of potassium. Final products of the first cycle occur (as small relict complexes of plagioclase-gneisses) within the whole area of highly metamorphous gneisses of the second phase. Gneisses containing porphyroblasts of potassium feldspars and granitic gneisses strike ENE and NE. At both flanks of the ENE-striking anticlinoria, blastesis of large euhedral microcline crystals and enrichment of potassium feldspars occur. Plagioclases and potassium feldspars of the older generations are found (as big inclusions) within the porphyroblasts of microcline belonging to the younger generation.

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Résumé Dans le massif des Irumides (E.Ackermann 1950) en Rhodésie du Nord on trouve deux systèmes de déformation d'âge différent avec des directions distinctes et un cycle spécifique de métamorphoses. Pour la première métamorphose on constate une blastèse à plagioclase avec probablement un apport de Na et éventuellement de Ca—pour la dernière un fort apport de K. Les produits finaux du premier cycle, sous la forme de complexes résiduels à base de gneiss plagioclasiques, se rencontrent dans toute le région des gneiss fortement métamorphiques de la seconde phase. Des gneiss à porphyroblastes de feldspath de potasse et des granites gneissiques ont une direction ENE et NE. Une blastèse de grands microclines idiomorphes et de grands enrichissements en feldspath de potasse se sont effectués sur les flancs des zones de crête irumidiques de direction ENE. Les plagioclases avec leurs bordures à biotite de la génération plus ancienne sont entourés par des porphyroblastes à microcline dans les gneiss de Mkushi.

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The effect of human activity on radiative forcing of climate change: a review of recent developments

Human activity has perturbed the Earth's energy balance by altering the properties of the atmosphere and the surface. This perturbation is of a size that would be expected to lead to significant changes in climate. In recent years, an increasing number of possible human-related climate change mechanisms have begun to be quantified. This paper reviews developments in radiative forcing that have occurred since the second assessment report of the Intergovernmental Panel on Climate Change (IPCC), and proposes modifications to the values of global-mean radiative forcings since pre-industrial times given by IPCC. The forcing mechanisms which are considered here include those due to changes in concentrations of well-mixed greenhouse gases, tropospheric and stratospheric ozone, aerosols composed of sulphate, soot, organics and mineral dust (including their direct and indirect effects), and surface albedo. For many of these mechanisms, the size, spatial pattern and, for some, even the sign of their effect remain uncertain. Studies which have attributed observed climate change to human activity have considered only a subset of these mechanisms; their conclusions may not prove to be robust when a broader set is included.     

Blood oxygenation in shortfin eels during swimming and hypoxia: Influence of the root effect

Blood samples were withdrawn from dorsal aortic cannulae in shortfin eels before, during, and after acute exposure to hypoxia and to forced swimming. Both hypoxia (PIO₂ < 40 mm Hg) and forced swimming at 24 cm s^‐1 abolished apnoeic periods and increased ventilation. The fall in PaCO₂with hyperventilation, raised pH and thus increased the oxygen carrying capacity of arterial blood. The Root effect influences shortfin eel blood up to pH 8.0, at 17°C and has an effect on arterial oxygen content. It cannot be responsible for all of the increased oxygen content during swimming and in recovery from swimming and hypoxia; ventilatory and/or circulatory changes must account for the rest. Hypoxia, but not swimming, produced a rise in dorsal aortic blood pressure. The bradycardia of hypoxia was deeper than that which accompanies apnoeic periods in air‐saturated water.     

An overview on source rocks and the petroleum system of the central Upper Rhine Graben

International Journal of Earth Sciences - The petroleum system of the Upper Rhine Graben (URG) comprises multiple reservoir rocks and four major oil families, which are represented by four distinct...