Buchbesprechungen

Ohne Zusammenfassung     

Die lokale Beeinflussung eines stehenden Gewässers durch eine punktförmige Abwasserbelastung; ein direkter Nachweis der wachstumsbegrenzenden Wirkung des Phosphors im Vierwaldstättersee

Zusammenfassung Die Beeinflussung des Vierwaldst?ttersees (Horwer Bucht) durch den mit dem Abwasser von 10 000 Einwohnern belasteten Steinibach liess sich im Oberfl?chenwasser auf eine Distanz von mehr als 2 km verfolgen (Abb. 1a, 1b, 1c). Es zeigte sich, dass eine erh?hte Phosphorzufuhr eine verst?rkte biogene Nitratzehrung erm?glicht. Die Abh?ngigkeit der Pigmentkonzentration y von der Gesamtphosphorkonzentrationx liess sich durch die folgende Mitscherlich-Gleichung beschreiben: log(30-y)=log 30–0,012 (x−6) Damit wurde im Vierwaldst?ttersee erstmals unter v?llig natürlichen Bedingungen die Beeinflussung der Prim?rproduktion durch eine erh?hte Phosphorkonzentration nachgewiesen (Abb. 2). Die Erfolgsaussichten von Abwassersanierungsmassnahmen an Seen verschiedener Trophiegrade werden diskutiert.

---

Summary The influence of the ‘Steinibach” tributary, into which the wastewater of 10,000 inhabitants is discharged, on the Lake of Lucerne (Horwer Bucht) is detectable in the surface water over a distance of more than 2 km (Picture 1a, 1b, 1c). It was revealed that the increased phosphorus supply enables more intensive biological nitrate consumption. The dependence of the pigment concentration,y, on the total phosphorus concentration,x can be expressed by the following Mitscherlich equation: log (30−y)=log 30–0.012 (x−6). This was the first time that the influence of an increased phosphorus concentration on primary production was proved in the Lake of Lucerne under completely natural conditions (Picture 2). The possibilities of implementing successful water pollution control measures in lakes with various trophic levels are discussed.

     

The origin of the Nicaraguan depression

A conspicuous graben extends for 800 kilometers through El Salvador and western Nicaragua to the Caribbean Sea in northeastern Costa Rica. Like the smaller but structurally similar Semangko and Toba Depressions of northern Sumatra, the trough is clearly related to voluminous volcanic eruptions during Late Tertiary time. In the region around Lakes Managua and Nicaragua, where the depression is best defined and reaches its greatest dimension, a thick series of Tertiary sediments and volcanic rocks provides a means of interpreting the Cenozoic history of the region. Following a long period of intermittent volcanic activity and sedimentation, extensive sheets of andesitic and dacitic ignimbrites were erupted during Late Miocene time from fissure sources which appear to have been located near the now-subsided central portion of the graben. Near the coast, ignimbrites flowed across a flat lagunal shore overwhelming and burying the tropical vegetation and finally coming to rest in shallow water. Unusual textures and chaotic mixtures of pumice with sediments and silicified wood characterize the bases of many of the water-laid ignimbrites. Subsidence of the graben does not appear to have occurred concurrently with the ignimbrite eruptions but followed them closely near the end of the Miocene or the beginning of Pliocene time. Subsequent activity has been confined to relatively smaller eruptions from central vents near the boundary faults of the graben.     

Buchbesprechungen

Ohne Zusammenfassung     

Mean bulk densities of samples of dry atmospheric aerosol particles: A summary of measured data

Mean bulk densities of various samples of dry atmospheric aerosol particles sampled at different sites and during different seasons and weather situations range between 1.8 and more than 3 gm cm³.     

U-Pb and Rb-Sr dating of a polymetamorphic nappe terrain: The Caledonian Jotun nappe,southern Norway

The Caledonian Jotun nappe in the Tyin area of southern Norway has been investigated using U-Pb analysis of zircon and sphene and Rb-Sr measurements of minerals and whole rocks, with special reference to the reaction of the isotope systems to various kinds of metamorphic event. The nappe consists of Precambrian basement rocks and their cover, inversely thrust over the Baltic shield and its parautochthonous, presumably lower Paleozoic sediments during the Caledonian orogeny. While the nappe basement retained its pre-Caledonian structures, the nappe cover was penetratively deformed and metamorphosed to lower greenschist facies conditions.U-Pb analyses of zircon and sphene ofnappe basement rocks point to the crystallization of a syenitic to monzonitic magma at 1694± 20m.y., the intrusion of gabbros into the syenites and monzonites at 1252_?25⁺²⁸ m.y., and the metamorphism (upper greenschist-lower amphibolite facies) and deformation (gneissification and mylonitization) of the whole complex at 909± 16m.y. Although this latest event caused strong lead loss in all zircon populations, it had no influence on the Rb-Sr system on a whole rock scale. The Caledonian movements did not disturb the U-Pb system of zircon and sphene but strongly influenced the Rb-Sr system in certain minerals and zircon and/or its inclusions (K-feldspar and brown biotite partially, green biotite completely reset).In contrast to the nappe basement, zircons from thecover rocks show pronounced lead loss during the Caledonian metamorphism/deformation — U-Pb analysis give discordia lower intercept ages of 415± 21 m.y. and 395± 6m.y. Again, however, the Rb-Sr whole-rock system has not been reset in Caledonian time. Minerals from the same rocks which provided the zircons give Rb-Sr isochron ages of 390± 11m.y. and 386 ± 20 m.y.Comparison of U-Pb and Rb-Sr results from the nappe basement demonstrates that only the analysis of U-Pb systematics in zircon and especially sphene resolved the 900-m.y. event. Rb-Sr mineral data alone yield ambiguous results.The results from the nappe cover confirm that U-Pb analyses from penetratively deformed sediments are very useful in dating a metamorphic/tectonic event in lower greenschist facies conditions.From the measured cell dimensions of the zircon populations it is concluded that lead can be completely retained in zircon during recrystallization.