Zur Genese neuentdeckter Vulkanschlote in der Umgebung des Donnersberges/Pfalz

Zusammenfassung Einige neuentdeckte Vulkanschlote im Rotliegenden der Umgebung des Donnersberges in der Saar-Nahe-Senke/Pfalz wurden hinsichtlich Aufbau, Alter und Genese untersucht. Es sind calderaartig abgesunkene Pyroclastica von Schlotbreccien zu unterscheiden, von denen Ablagerung aus einer Gas-Partikel-Suspension angenommen wird. Die Breccien setzen sich aus einer Mischung von Lapilli und z. T. aufgearbeitetem Nebengestein zusammen. In fast allen Schloten wurde die Aktivität durch das Eindringen von Schmelzen abgeschlossen.Als Beispiel wird der Schlot des Rödern beschrieben.

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In the Lower Permian of the Saar-Nahe-Senke (Palatinate, SW Germany) several recently discovered volcanic necks have been investigated. In this preliminary report structure, composition, and age relationship of the breccias are described as well as their mode of emplacement which is believed to be due partly to fluidization and partly to calderalike subsidence of subaerially deposited tephra. In some of the necks magmatic intrusion terminated the volcanic activity.The volcanic neck of the Rödern is described as an example.

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Résumé Plusieurs cheminées récemment découvertes dans le Permien inférieur des alentours du Donnersberg dans la Saar-Nahe-Senke (Palatinat, Sud-ouest d'Allemagne) ont été étudiées. Leur composition et les problèmes de l'âge et de la genèse sont discutés. Il faut distinguer des sédiments pyroclastiques des caldères d'effondrement et des brèches formées à l'intérieur des cheminées. Celles-ci se composent des lapillis et des fragments des roches encaissantes. Leur formation est le résultat d'un dégagement de gaz tenant en suspension des particules solides.La fin de l'activité est marquée en général par l'intrusion des laves. La cheminée du Rödern est décrite à titre d'exemple.

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Cladoceran stratigraphy in two shallow brackish lakes with special reference to changes in salinity, macrophyte abundance and fish predation

Sub-fossils of Cladocera and Foraminifera were used to reconstruct changes since 1870 in the trophic dynamics of two brackish lakes, Glombak and Han Vejle, located in the Vejlerne nature reserve, Denmark, a site of international conservation importance. After creation of the lakes in the mid-1870s following land reclamation, the two lakes have developed quite differently; today Glombak is turbid, while Han Vejle is clear. In both lakes, stratigraphic changes in the assemblages of foraminifers and cladocerans indicate an abrupt shift from marine to brackish conditions at the end of the 19th century, coinciding with land reclamation. However, the composition of the fossil invertebrate assemblages in the 20th century implies differences in the exposure to salinity, in fish predation and in habitat diversity. In Glombak, the cladoceran record suggests relatively saline conditions in the first quarter of the last century and high macrophyte abundance followed by lower salinities and subsequently a major decrease in macrophyte abundance and an increase in fish predation during the past ca. 40 years. By contrast, in Han Vejle low salinity, high abundance of macrophytes and only minor changes in fish predation seem to have prevailed throughout most of the 20th century. The results are consistent with recent contemporary data, the few historical records, as well as with trends in the records of diatoms and macrofossils. This study highlights the potential of using crustacean remains as indicators of long-term changes in the trophic dynamics of brackish lakes.     

Density control in Potamogeton perfoliatus L. and Potamogeton pectinatus L.

The effect of density on the growth, recruitment of new ramets, biomass allocation, rhizome spacer length and rhizome branching of the submersed macrophyte Potamogeton perfoliatus L. was experimentally evaluated in a mesocosm with three different initial shoot densities. The findings suggest that the number of primary shoots available in the beginning of the season can strongly influence patterns of growth and clonal reproduction. In contrast to many studies that found decreasing production parameters with plant density, ramet and biomass production of Potamogeton perfoliatus plants were highest at medium plant density. This is probably due to negative feed-back through crowding at high density and unclear negative effects at low density. Shoot allocation tended to increase with density, rhizome allocation tended to decrease with density and root biomass remained unchanged. At low density Potamogeton perfoliatus produced longer rhizome spacers and more branchings than at higher densities. Shorter rhizome spacers at high plant density probably restrict patch expansion and cause discrete patch shapes. A likely mechanism for plastic changes in clonal architecture is increasing competition for light at increasing density, but other density-dependent factors may also contribute.Investigations on propagule numbers of tubers and turions show that self-regulation of ramet number was associated with self-regulation of propagule number in both Potamogeton perfoliatus and Potamogeton pectinatus, with higher densities producing less propagules per plant.     

Isotopic and hydrochemical studies of groundwater flow and salinity in the Southern Upper Rhine Graben

In order to determine the origin and the propagation mechanisms of highly concentrated chloride brines within the Quaternary aquifer system in the southern part of the Upper Rhine Graben, a combined isotope (H, O, C) and hydrochemical analysis was carried out. Groundwater recharge in this area is a complex system, consisting of local precipitation, river bank filtration, lateral flow from the Graben borders and, to a minor extent, an old Pleistocene component. In some areas, groundwater consists of up to 90% of recent bank filtrate, reaching depths down to at least 100 m. The isotopic and hydrochemical results show, that the elevated chloride concentrations in the Quaternary aquifer mainly result from leaky settling basins charged by the French potash mines until the mid 1970s. Input of natural brines coming from tertiary salt diapirs is of only minor importance. While infiltrating, the anthropogenic brines were strongly diluted by local river bank filtrate of the Rhine. Nevertheless, maximum chloride concentrations nowadays still reach some 10,000 mg/l at the base of the aquifer at a depth of more than 100 m below surface. The main volume of the brines is stored in the less permeable lower part of the quaternary sediments (Breisgau-Formation) whereas only a minor part is transported northwards with the rapid convective groundwater flow. Brines undergoing only dilution preserve their hydrochemical characteristics (NaCl-type). In contrast, brines recirculated from the Breisgau-Formation show a northwards increasing alteration through ion exchange processes. Potassium and sodium may be fixed in the fine grained aquifer material while calcium is set free into the groundwater. After a flow distance of about 12 km, complex hydraulic interactions between groundwater and surface waters lead to the rise of strongly diluted and hydrochemically altered brines with chloride contents up to maximum 700 mg/l. The presented case study is an example for a detailed analysis of a multi-component groundwater mixing system using combined isotope and hydrochemical methods. Furthermore, cation exchange is shown as a major process affecting the hydrochemical evolution of the young groundwater in the southern Upper Rhine Graben which is locally strongly polluted by chloride as a consequence of former potash mining.     

A progressive line simplification algorithm

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On the last explosions of carbonatite pipe G3b, Gross Brukkaros, Namibia

 Pipe G3b is part of the Upper Cretaceous carbonatitic Gross Brukkaros Volcanic Field in southern Namibia. The pipe represents the root zone of a diatreme and is located 2800 m west of the rim of Gross Brukkaros, a downsag caldera. The pipe is exposed approximately 550 m below the original Upper Cretaceous land surface. It cuts down into its own feeder dyke, 0.3 m thick. The pipe coalesced from two small pipes and in plan view is 19 m long and 12 m wide. It consists of fragmented Cambrian Nama quartzites and shales of the Fish River subgroup. Despite intensive brecciation, the stratigraphic sequence of the country rocks is almost preserved in the pipe. In addition, the feeder dyke became fragmented too and can be traced in a 2- to 3-m-wide zone full of carbonatite blocks along the southern margin of the pipe. The void space of the breccia is 30–50% in volume. Finally, after the disruption of country rocks and feeder dyke, a little carbonatite magma intruded some of the void space. The breccia of pipe G3b is considered to represent a root zone at the transition from the feeder dyke into a diatreme above. Formation of the breccia required a shock wave thought to have been associated with a last explosion of the diatreme immediately above the present level of exposure. The explosion can be shown to have been phreatomagmatic in origin. Received: 11 October 1996 / Accepted: 6 March 1997     

Volcanological features of a low-viscosity melt: the carbonatitic Gross Brukkaros Volcanic Field, Namibia

 In the Upper Cretaceous Gross Brukkaros Volcanic Field, southern Namibia, a radial dyke system surrounds a dome structure and its 74 closely related carbonatite diatremes. This paper focuses on volcanological features which seem to be typical for a low-viscosity melt in various settings such as dykes, sills and diatremes. The total or near absence of vesicles in carbonatite ash grains and lapilli inside the diatremes is evidence against explosive exsolution of volatile phases and in favour of a phreatomagmatic fragmentation mechanism and thus for a phreatomagmatic eruption mechanism of the carbonatite diatremes. Received: 15 August 1996 / Accepted: 13 January 1997