Colonisation d'une population d'Oscillatoria rubescens (Cyanophyceae) par une bacterie epiphyte

The presence of unknown bacteria upon filaments of the CyanophyteOscillatoria rubescens was observed in the Lake Leman (Lake of Geneva) during Autumn 1985. Conditions of this colonization and the morphology of the association are described. These bacteria probably belong to theCaulobacter group. To explain this colonization we suggest that bacteria respond chemotactically to Cyanophyte exudates.

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Colonisation d'une population d'Oscillatoria rubescens (Cyanophyceae) par une bacterie epiphyte

     

Eutrophisation et faune benthique de trois lacs de montagne

Zoobenthic communities were used to indicate the trophic state of three mountain lakes of western Switzerland. In these lakes, the macrobenthos was constituted mainly by oligochaetes and chironomid larvae;Tubifex tubifex was the most abundant worm species. In Lakes Lioson and Hongrin, the abundance of zoobenthos did not change or increased slightly with depth andStylodrilus heringianus was present. In Lake Chavonnes, zoobenthos decreased strongly with depth and it consisted mainly ofTubifex tubifex. These data, compared with those from other oligotrophic lakes, indicated that Lake Chavonnes was eutrophic whereas Lakes Lioson and Hongrin were oligotrophic. However, Lake Hongrin—an artificial lake made in 1968—constituted a special case. Its water comes in part from mountain rivers and in part from Lake Geneva. Consequently, some benthic species of Lake Geneva have colonized Lake Hongrin and its water is warmer than in a typical mountain lake.      

Contribution à l'étude des litages pegmatitiques: l'exemple de la pegmatite de Covide (Nord-Portugal)

Résumé La pegmatite de Covide (Nord-Portugal) fournit un nouvel exemple de litage à quartz-albite-feldspath potassique. Son étude conduit à proposer un modèle de pétrogenèse magmatique faisant intervenir des processus de double diffusion dans un gradient de température. Ces processus provoquent la formation simultanée et périodique de niveaux millimétriques d'albite, en »planchers flottants« horizontaux. Chaque cellule, phénomène-unité du litage compris entre deux niveaux planchers successifs, évolue ensuite en système clos par cristallisation orientée centripète. L'organisation litée est acquise en milieu magmatique liquide.

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The Covide pegmatite, (Northern Portugal), exhibit a new example of K-feldspar, Albite, quartz layering. The present study leads to propose a magmatic petrogenetit model, with double diffusive processe in temperature gradient allowing the simultaneous growth of millimetric and horizontal »floating layers« of albite. Each cell, unit-phenomenon between two consecutive boundary layers, evolves in closed system with a centripetal, oriented crystallization. The layered organisation is obtaind in the magmatic liquid medium.

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Zusammenfassung Der Pegmetit aus Covide (Nordportugal) stellt ein neues Beispiel für Quarz-Albitfeldspat Wechsellagerung dar. Seine Untersuchung führt zu einem magmatischen Petrogenesemodell, bei dem Prozesse der Doppeldiffusion in einem Temperaturgefälle intervenieren. Diese Prozesse bewirken die gleichzeitige und periodische Bildung von millimeterdicken Albitflächen in horizontalen »schwimmenden Flächen«. Jede Zelle, die ein Phänomen der Einheit der Anlagerungsfläche zwischen zwei aufeinanderfolgenden Flächenlagen ist, entwickelt sich schlie\lich durch zentripetale Kristallisierung zu einem geschlossenen System. Die schichtweise Anordnung erfolgt im flüssigen magmatischen Milieu.

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Precise lead isotope measurements by the double spike technique: A reconsideration

A reappraisal of the “Double Spike Method” for correction of the mass discrimination in lead isotope analysis is presented in view of the possible efficiency of this method, especially as related to the improved performance of the new multi-collector mass spectrometers.First, the effects of mass discrimination during lead isotope analysis, using the silica-gel phosphoric acid method, are discussed in order to verify the validity of the linear discrimination law. In this discussion, we give evidence of a slight bias of the isotopic composition of the SRM 981 NBS lead standard, commonly used for mass discrimination calibrations.A rigorous way to deal with the propagation of the within-run statistical errors on the precision of the double spike correction is then presented. This allows the optimization of the parameters of the method (spike isotopic composition and precision, spiking proportion) and the calculation of the respective error associated with each corrected isotopic ratio. The perturbation of the double spike correction due to analytical contamination, which has caused previous failure in the application of the method, is avoided by preparation of the mixture between the spike and the sample at the very end of the chemical separation procedure.Using a single collector mass spectrometer, examples of application on lead standards and on a set of samples suggest a possible improvement in the precision by a factor of 3 compared to the usual method of fractionation normalization and confirm the usefulness of the method with the new generation of mass spectrometers.     

Rare gas systematics: formation of the atmosphere, evolution and structure of the Earth's mantle

To explain the rare gas content and isotopic composition measured in modern terrestrial materials we explore in this paper an Earth model based on four reservoirs: atmosphere, continental crust, upper mantle and lower mantle.This exploration employs three tools: mass balance equations, the concept of mean age of outgassing and the systematic use of all of the rare gases involving both absolute amount and isotopic composition.The results obtained are as follows: half of the Earth's mantle is 99% outgassed. Outgassing occurred in an early very intense stage within the first 50 Ma of Earth history and a slow continuous stage which continues to the present day. The mean age of the atmosphere is 4.4 Ga.Our model with four main reservoirs explains quantitatively both isotopic and chemical ratios, assuming that He migrates from the lower to the upper mantle whereas the heavy rare gases did not.Noble gas fluxes for He, Ar and Xe from different reservoirs have been estimated. The results constrain the K content in the earth to 278 ppm. Several geodynamic consequences are discussed.     

Nankai Trough and Zenisu Ridge: a deep-sea submersible survey

Eight submersible dives between 3000 and 4200 m water depth were made off southern Japan in the eastern Nankai subduction zone. Benthic communities associated with chemosynthetic processes were discovered along the 800 m wide active tectonic zone, at the toe of the accretionary prism. A benthic community was also discovered along a zone of active compression, at the foot of Zenisu Ridge, 30 km south of Nankai Trough. Temperature measurements within the sediments below the benthic communities confirm that upward motion of interstitial water occurs there. Studies of water samples indicate advection of methane and light hydrocarbons. Specimens of the benthic community have been shown to have included in their shells carbonate resulting from methane consumption. Thus the benthic communities are related to overpressure-driven fluid advection along tectonic zones with active surface deformation. A 300 m high active scarp at the toe of the accretionary prism is related to relative motion in a 280° direction which is close to the 305° average direction of subduction in this area. The dives establish further that compressive deformation is presently occurring at the foot of Zenisu Ridge. The previous interpretation of the Zenisu Ridge as a zone of recent north-south intraplate shortening, 40 km south of the Nankai Trench, is confirmed. We conclude that tectonic evolution might well lead to future detachment of the Zenisu Ridge and overthrusting of this large piece of oceanic crust over the continental margin. Such a process might be an efficient one to emplace ophiolites over continents.     

Geochemistry of the Tenryu Canyon deep-sea fan biological community (Kaiko)

The Tenryu Canyon deep-sea fan biological community is characterized by both reduced and oxidized sediments in the immediate vicinity of the pore water vents. The upper sediments in contact with the clams are reduced, the organic matter is enriched in sulfur, and inorganic sulfides (Fe, Cu, Zn) are forming. Towards the outer fringes of the communities the sediment is oxidized and metals generally associated with ferro-manganese oxides are concentrated. Several metals, Cd, Pb, Mo show distributions which are strongly influenced by the metabolism of the clam colony. Comparison of water and sediment geochemistry leads to the conclusion that there should be a downward flux of oxygenated seawater on the boundaries of the colony and an upward flux of chemically more reduced deep pore water at the location of the colony. Trace metals anomalies as well asδ¹⁵N anomalies of organic matter may be useful to prospect for extinct venting areas in ancient subduction zones.