Emission of nitrous oxide from temperate forest soils into the atmosphere

N₂O emission rates were measured during a 13-month period from July 1981 till August 1982 with a frequency of once every two weeks at six different forest sites in the vicinity of Mainz, Germany. The sites were selected on the basis of soil types typical for many of the Central European forest ecosystems. The individual N₂O emission rates showed a high degree of temporal and spatial variabilities which, however, were not significantly correlated to variabilities in soil moisture content or soil temperatures. However, the N₂O emission rates followed a general seasonal trend with relatively high values during spring and fall. These maxima coincided with relatively high soil moisture contents, but may also have been influenced by the leaf fall in autumn. In addition, there was a brief episode of relatively high N₂O emission rates immediately after thawing of the winter snow. The individual N₂O emission rates measured during the whole season ranged between 1 and 92 g N₂O-N m^–2 h^–1. The average values were in the range of 3–11 g N₂O-N m^–2 h^–1 and those with a 50% probability were in the range of 2–8 g N₂O-N m^–2 h^–1. The total source strength of temperate forest soils for atmospheric N₂O may be in the range of 0.7–1.5 Tg N yr^–1.     

Ore paragenesis of recent hydrothermal deposits at the Cocos-Nazca plate boundary (Galápagos Rift) at 85‡ 51' and 85‡ 55' W: Complex massive sulfide mineralizations,non-sulfidic mineralizations and mineralized basalts

Complex massive sulfide ores (»black smokers«) as well as mineralized basalts and unconsolidated sediments were sampled during the German research expedition »Garimas 1« in the eastern portions of the Galapagos Rift at 85 50' W longitude. The massive sulfide ores are characterized by a porous, layered, compositional texture with typical colloidal and/or gel textures. Local occurrences of organism remains (worm tubes), which are embedded in the ore, were also found. The composition of the massive sulfide ores exhibits a considerable range, from iron sulfide ores (pyrite, melnikovite-pyrite, marcasite) to copper-rich ores (chalcopyrite), and/or zinc-dominated ores (sphalerite, wurtzite, schalenblende). Additional new types of hydrothermal mineralizations were found, which consist primarily of manganese hydroxide, limonite or Fe-rich smectite (Fe-rich »protosmectite«) together with limonite and opaline silica. Hydrothermally altered basalts may be characterized by significant sulfide contents (primarily pyrite) and, locally, by hematite. Sediments in the vicinity of the »black smokers« frequently contain fragments of complex massive sulfide ores. With increasing distance from the »black smokers« these massive sulfide fragments exhibit complete replacement and pseudomorphism by limonite. Some replacement and pseudomorphism of microorganisms in unconsolidated sediments by limonitic material is also observed. In places, the unconsolidated sediments contain manganese hydroxide mineralizations of hydrothermal origin.

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Zusammenfassung Anlä\lich der deutschen Forschungsfahrt »Garimas 1« wurden im östlichen Teil des Galápagos-Riftes bei 85 50' W vor allem komplexe massive Sulfiderze (»Schwarze Raucher« — »Black Smokers«) gewonnen, daneben au\erdem vererzte Basalte sowie unverf estigte Sedimente. Die massiven Sulfiderze sind durch einen porösen, lagig bis krustigschaligen Aufbau gekennzeichnet und weisen charakteristische Kolloidalgefüge bzw. Geltexturen auf. Stellenweise liegen Organismenreste (z. B. Wurmröhren), eingebettet in Erz, vor. Die Zusammensetzung der jeweiligen massiven Sulfiderze schwankt erheblich und reicht von praktisch nur aus Eisensulfid (Pyrit, Melnikovitpyrit, Markasit) beste-henden Bildungen bis zu solchen, die reich an Kupfersulfid (Kupferkies) und/oder Zinksulfid (Zinkblende, Wurtzit, Schalenblende) sind. Als weitere und neue Typen hydrothermaler Bildungen bzw. von »Schwarzen Rauchern« konnten hydrothermale Mineralisationen nachgewiesen werden, die vorherrschend entweder aus Manganhydroxid, aus Limonit oder aus eisenreichem Montmorillonit (Fe-reicher »Protosmektit«) mit Limonit und amorphem SiO₂ bestehen. Die alterierten Basalte können stellenweise durch eine reichliche Sulfidführung (vor allem Pyrit) ausgezeichnet sein, wobei gelegentlich auch Eisenglanz beibricht. Die Sedimente aus der Nachbarschaft der »Schwarzen Raucher« enthalten öfters zahlreiche Bruchstücke der komplexen Massivsulfiderze. Diese Sulfiderz-Bruchstücke lassen häufig mit zunehmender Entfernung von den »Schwarzen Rauchern« starke bis völlige Verdrängung und Pseudomorphosierung durch Limonit erkennen. Die in diesen Lockersedimenten vorliegenden rezenten Mikroorganismen zeigen stellenweise Verdrängung und Pseudomorphosierung durch Limonit. In den Lockersedimenten treten Ausscheidungen von Manganhydroxid hydrothermaler Herkunft auf.

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Résumé Dans la partie est de la dorsale des Galapagos, par 85 50' de longitude ouest, l'expédition allemande »GARIMAS I« a récolté des minerais sulfurés complexes massifs (minerais de »fumeurs noirs«), ainsi que des basaltes minéralisés et des échantillons de sédiments meubles. Les sulfures massifs présentent un aspect poreux lité à écailleux et montrent des structures colloÏdales caractéristiques. Localement des traces d'organismes (p. ex. des trous de vers) sont inclues dans le minéral. Les sulfures massifs présentent diverses compositions depuis des formations constituées uniquement de sulfures de Fe (pyrite, pyrite »colloÏdale«, marcasite) jusqu'à d'autres qui sont riches en sulfures de cuivre (chalcopyrite) et/ou de zinc (blende, wurtzite, blende zonaire). De plus, d'autres minéralisations hydrothermales, nouvelles dans l'environnement des fumeurs noirs, ont été mises en évidence: elles sont constituées soit d'oxydes et hydroxydes de Mn, soit de limonite ou de montmorillonite riche en fer (»protosmectite« ferreuse) avec de la limonite et silice amorphe. Les basaltes hydrothermalisés peuvent montrer localement une teneur élevée en sulfure (surtout de la pyrite) éventuellement accompagné d'hématite. Les sédiments recueillis au voisinage des fumeurs noirs renferment souvent de nombreux fragments de sulfures complexes massifs. Au fur et à mesure qu'on s'éloigne des fumeurs, ces fragments sont progressivement et mÊme complètement pseudomorphosés en limonite. Cette pseudomorphose limonitique affecte également, par endroits, les micro-organismes récents inclus dans les sédiments meubles. Dans ces derniers ont lieu des formations d'hydroxydes de manganèse d'origine hydrothermale.

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Pre-Hercynian mantle lead transfer to basement rocks as indicated by lead isotopes of the Schwarzwald crystalline,SW-Germany

Lead isotopes of K-feldspars from five granites of the SE-Schwarzwald and from metamorphites are positively correlated in 207/204 as well- as 208/204- vs. 206/204-diagrams. The linear alignments may be due to correlated laststage lead isotope evolution (lead-lead isochron) and result in a secondary isochron model age of nearly 3 Ga for the Southern Schwarzwald basement. This calculation implies a long-lasting undisturbed lead isotope evolution in the Schwarzwald basement since the Archaean. This is not supported however by geochronological studies. On the other hand the data together with U/Pb-analyses of whole rock samples from metamorphites are consistent with pre-Hercynian mantle lead addition to the basement. This presumably happened during early Paleozoic polymetamorphism. The interpretation of the lead isotope correlations as mantle-crust mixing lines needs a rather homogeneous pre-Hercynian mantle lead of the MORB-type, delivered to the crust probably in part by ascending volatile phases. Thus geodynamical models are supported which involve subduction of oceanic crust or mantle pluming during the early Paleozoic. In Part I of this report, the trends in Schwarzwald lead isotopes are discussed as secondary isochrons and as mixing lines. Constraints are derived for a pre-Hercynian mantlecrust interaction and for lead redistribution by the Hercynian basement activation.     

Tidal correction of hydrographic measurements

Summary A simple procedure is presented to eliminate the influence of tidal oscillation from hydrographic measurements. This is done with a transformation from a co-ordinate system fixed in space to a co-ordinate system fixed in time using results of a numerical tidal model. The procedure is applied to a hydrographic survey in the German Bight. The hydrographic structure transformed onto a co-ordinate system fixed in time is much more similar to the structure obtained by satellite pictures and shows dynamical processes much clearer than in a co-ordinate system fixed in space.

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Gezeitenkorrektur von hydrographischen Messungen

Zusammenfassung In dieser Arbeit wird ein Verfahren vorgestellt, das hydrographische Messungen vom Einfluß periodischer Variationen durch die Gezeiten bereinigt. Dies geschieht unter Benutzung von Ergebnissen eines numerischen Modells durch eine Transformation einer gemessenen hydrographischen Struktur von einem ortsfesten in ein zeitfestes System. Dieses Verfahren wird auf eine hydrographische Aufnahme der Deutschen Bucht angewandt. Die auf ein zeitfestes Koordinatensystem transformierte hydrographische Messung zeigt sehr viel Ähnlichkeit mit einer synoptischen Messung, die bei einem Satellitenüberflug gewonnen wurde. Bestimmte dynamische Prozesse in der Deutschen Bucht lassen sich zudem im zeitfesten Koordinatensystem besser erkennen als im ortsfesten Koordinatensystem.

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Correction de l'influence de la marée sur les mesures hydrologiques

Résumé L'article présente une méthode simple pour éliminer l'influence de la marée sur les mesures hydrologiques. Cela est obtenu par le passage d'un système de coordonnées fixe dans l'espace à un système de coordonnées fixe dans le temps utilisant les résultats d'un modèle numérique de marée. La méthode est appliquée à un levé hydrologique dans la Baie Allemande. La structure hydrologique obtenue dans un système de coordonnées fixe dans le temps est beaucoup plus proche de celle qui est obtenue par les images satellites et décrit le processus dynamique beaucoup plus clairement que dans un système de coordonnées fixe dans l'espace.

     

Geotechnical characterization and rockfall simulation of a slope: a practical case study from South Tyrol (Italy)

Rockfalls occur often along the state road SS241 Val d'Ega in the Val d'Ega Valley (South Tyrol, Italy). In order to protect the road and the traffic, stabilization works are necessary. Detailed geological and geomorphologic mapping along structural and geomechanic sections was carried out, and experimental rockfall trails were examined on the slope above the state road between km 3.550 and 3.830. Rockfall simulation, rock mass classification and an interpretation of field data, especially concerning joints, wrapped up the investigations. They helped find the specific need for stabilization work and its positioning along the slope. The stabilization works were completed on February 2002. Another important aspect concerns ongoing geotechnical investigations and stabilization works urgently realised during autumn 1999.

This paper is a practical approach in solving rockfall problems along public roads. It should be considered as a workbook for university personnel and also for professional geologists.     

Alfvén wave coupling in the auroral current circuit

Auroral phenomena are controlled by the geomagnetic field.Since the terrestrial field lines connect the auroral oval to the equatorial region at large distances, the collisionless plasma in this remote space environment can act as a power supply for the high-latitude upper atmosphere where auroral emissions take place. The coupling process is intimately linked to currents which flow across the local magnetic field direction both in the equatorial part and at the atmospheric end of the auroral field lines. These two auroral key regions are connected through currents flowing along the terrestrial field lines, thereby completing the auroral current circuit. Such field-aligned currents are carried by Alfvén waves, that is, magnetohydrodynamic shear waves, which are thus a means to exchange momentum and energybetween rather remote parts of the geomagnetically controlledspace environment. Auroral dynamics is further affected by a third key region in the auroral current circuit, namely the auroral acceleration region, where parallel electric fields accelerate particle to keV energies. This review focuses on key region coupling through Alfvén waves. Continuity requirements for currents and electric fields provide a convenient means to describe the interaction of Alfvén waves with different plasma regimes. Basic coupling aspects can be demonstrated with the help of a simplified model. Inhomogeneities and nonlinear feedback can lead to resonance effects and instabilities.     

Noble gas systematics of the Réunion mantle plume source and the origin of primordial noble gases in Earth’s mantle

New noble gas data of ultramafic xenoliths from Réunion Island, Indian Ocean, further constrain the characteristics of primordial and radiogenic noble gases in Earth’s mantle plume reservoirs. The mantle source excess of nucleogenic ²¹Ne is significantly higher than for the Hawaiian and Icelandic plume reservoirs, similar to excess of radiogenic ⁴He. ⁴⁰Ar/³⁶Ar of the Réunion mantle source can be constrained to range between 8000 and 12 000, significant ¹²⁹Xe and fission Xe excess are present. Regarding the relative contribution of primordial and radiogenic rare gas nuclides, the Réunion mantle source is intermediate between Loihi- and MORB-type reservoirs. This confirms the compositional diversity of plume sources recognized in other radioisotope systematics. Another major result of this study is the identification of the same basic primordial component previously found for the Hawaiian and Icelandic mantle plumes and the MORB reservoir. It is a hybrid of solar-type He and Ne, and ‘atmosphere-like’ or ‘planetary’ Ar, Kr, Xe (Science 288 (2000) 1036). ²⁰Ne/²²Ne ratios extend to maximum values close to 12.5 (Ne-B), which is the typical signature of solar neon implanted as solar corpuscular radiation. This suggests that Earth’s solar-type noble gas inventory was acquired by small (less than km-sized) precursor planetesimals that were irradiated by an active early sun in the accretion disk after nebular gas dissipation, or, alternatively, that planetesimals incorporated constituents irradiated in transparent regions of the solar nebula. Previously, such an early irradiation scenario was suggested for carbonaceous chondrites which follow common volatile depletion trends in the sequence CI–CM–CV–Earth. In turn, CV chondrites closely match Earth’s mantle composition in ²⁰Ne/²²Ne, ³⁶Ar/²²Ne and ³⁶Ar/³⁸Ar. This indicates that mantle Ar could well be a planetary component inherited from precursor planetesimals. However, a corresponding conclusion for mantle Kr and Xe is less convincing yet, but this may be just due to the lack of appropriate ‘meteoritic’ building blocks matching terrestrial composition. Alternatively, heavy noble gases in Earth’s mantle could be due to admixing of severely fractionated air, but this effect must have affected all mantle sources to a very similar extent, e.g. by global subduction before the last homogenization of the mantle reservoirs.