Einige Beobachtungen und Überlegungen zur Kluftentstehung in Sedimentgesteinen

Zusammenfassung Ruhig gelagerte Sedimentgesteine eignen sich im besonderen Maße für grundsätzliche Untersuchungen zur Kluftentstehung. Die Klüftung erreicht nämlich ihre größte Regelmäßigkeit und Vollendung in Gebieten mit nahezu horizontal liegenden Sedimentgesteinen, in denen andere tektonische Strukturen kaum oder überhaupt nicht auftreten (Secor, 1965: 634).In den untersuchten, annähernd horizontal gelagerten, geschichteten Sedimentgesteinen Südwestdeutschlands sind zwei Kluftscharen entwickelt, die normal zueinander sowie normal zu den Schichtflächen stehen. Die Kluftscharen sind hinsichtlich Streuung, Rauhigkeit, Kluftabstand und Durchtrennungsgrad ungleichwertig entwickelt. Diese ungleiche Entwicklung läßt sich sowohl in den kompetenten (Kalk-, Sandsteine) als auch in den inkompetenten (Mergel, Tonstein) Schichten beobachten.Mit Hilfe experimenteller und rechnerischer Untersuchungen wird versucht, eine mechanisch sinnvolle Erklärung des geologischen Sachverhaltes zu finden. Eine wesentliche Rolle spielt dabei die Tatsache, daß das Gebirge hinsichtlich der in ihm entwickelten Klüfte als ein inhomogener, geschichteter Körper anzusehen ist. Es läßt sich zeigen, daß die beobachteten natürlichen und künstlichen Kluftsysteme am besten erklärt werden können, wenn die größte Hauptnormalspannung als vertikal gerichtet angenommen wird. Entscheidend für die gerichtete Anlage der Kluftstrukturen sind (zum Teil nur geringfügige) Betragsdifferenzen der zwei horizontal gerichteten kleineren Normalspannungen.

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Smoothly layered sedimentary rocks are particularly suited for fundamental investigations on joint generation, because jointing reaches its ultimate regularity and perfection in areas of near-horizontal sedimentary strata where other types of structural deformation are scare or absent (Secor, 1965: 634).The near-horizontal, stratified sedimentary rocks of southwest-Germany are investigated where two joint sets are developed in a position approximately normal to each other and normal to the bedding planes. The joint sets are nonsimilar in variance of joint orientation, asperities, joint spacing, and degree of separation. This nonsimilar development of the joint sets can be observed in competent layers (limestones, sandstones) as well as in incompetent ones (marls, claystones).It is attempted to find a mechanically efficient interpretation of the observed joint system by means of arithmetical and experimental investigations. It is important hereby that due to its jointing the rock mass has the property of an inhomogeneous, layered body. It can be shown, that the natural joint systems are in agreement with the artificially produced fracture pattern only in the case, when the major principal stress is in vertical direction. Partly only small differences in magnitude between the horizontally directed intermediate and minor princjpal stresses are decisive for the systematic arrangement of the joints.

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Résumé Les roches sédimentaires en gisement calme se prêtent particulièrement bien à des recherches sur la genèse des diaclases. Ce système de joints atteint en effet sa «régularité et sa perfection les plus grandes dans les régions à roches sédimentaires horizontales ou presque, dans lesquelles d'autres structures tectoniques n'apparaissent pas ou guère» (Secor, 1965: 634).Dans les roches sédimentaires presqu'horizontales étudiées en Allemagne du Sud, deux groupes de diaclases parallèles se sont développés perpendiculairement l'un par rapport à l'autre ainsi que par rapport aux plans de stratification. Ces groupes de diaclases parallèles se sont développés inégalement quant à leur dispersion, leur rugosité, leur écartement et leur ouverture. Ce développement inégal peut s'observer à la fois dans les couches «compétentes» (calcaires, grès) et dans les couches «incompétentes» (marnes, argilites).A l'aide de calculs et de recherches expérimentales, on a tenté de trouver une explication mécaniquement plausible de ce comportement géologique. Le rôle essentiel joué dans ce cas est celui de la chaîne qui doit être considérée du point de vue des diaclases comme un corps stratifié non homogène. On peut montrer que les systèmes naturels et artificiels de diaclases observés peuvent le mieux s'expliquer si l'on accepte que la contrainte normale principale la plus forte est orientée verticalement. Des différences (en partie même faibles) entre les deux contraintes normales mineures orientées horizontalement sont décisives pour l'orientation structurale des systèmes de diaclases.

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Temperaturen und Drucke bei der regionalen Metamorphose: Prinzipielle und praktische Hinweise

The author's concept (1970, 1974) of evaluating metamorphic conditions is explained on the basis of most recent petrologic data. The major points treated are:

1. Instead of usual petrographic mapping, petrographic work in the field is aimed at specific “targets”, i.e., rock compositions. This is so because only specific rocks may give petrogenetically relevant information in the four metamorphic grades.
2. There are very many mineral reactions in metamorphism but only a few are petrogenetically significant. These are important to know, and they are graphically demonstrated. Any mineral assemblage that is formed by a significant mineral reaction must be verified as a paragenesis of mutually contacting minerals. Only such parageneses deserve to be mapped in the field as isograds or isoreactiongrads.
3. Crossing isograds or isoreactionsgrads provide data on temperature and pressure during metamorphism for that part of a metamorphic terrane where the crossing has been observed.
4. The sequence of isograds or isoreaction-grads may be a pressure indicator. Moreover, such a sequence provides geodynamic information whether a larger metamorphic area has been lifted up evenly or has been tilted while it was uplifted after metamorphism.

     

The genesis of the methane in Lake Kivu (Central Africa)

Large amounts of methane and carbon dioxide, among other gases, are dissolved in the deep water of Lake Kivu. There is no dispute about the primarily magmatic origin of the carbon dioxide, but models of the genesis of the methane have been contradictory up to now. They have been based on too few and partly too inaccurate data.On the basis of new measurements obtained from gas and sediment samples, some of the old concepts have been further developed to a new model. According to this model, the methane is generated mainly by bacteria from the organic carbon of the sediment. It probably also contains minor amounts of thermocatalytic methane.About 70% of the organic carbon of the upper sediment is derived from mainly magmatic carbon dioxide (old carbon), which enters the biozone of the lake from the deep water by eddy diffusion and is assimilated there. The remaining 30% comes from atmospheric carbon dioxide (young carbon) assimilated in the biozone. But because methane also migrates into the lake from deeper sediment, the¹⁴C-content in the methane dissolved in the lake water is not 30% modern but only ca. 10% modern.More isotopic measurements on plankton, methane, carbon dioxide and sediment samples are necessary to support this model.

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Zusammenfassung Im Tiefenwasser des Kivusees sind u. a. große Mengen an Methan und Kohlendioxid gelöst. Während über den hauptsächlich magmatischen Ursprung des Kohlendioxids weitgehend Einigkeit besteht, sind die bisherigen Modellvorstellungen zur Genese des Methans widersprüchlich. Sie beruhen auf zu wenigen und zum Teil zu ungenauen Meß-daten.Mit Hilfe neuer Meßergebnisse an Gas- und Sedimentproben des Kivusees wurden einige der alten Vorstellungen zu einem neuen Modell weiterentwickelt. Danach ist das Methan hauptsächlich bakteriell aus dem organischen Kohlenstoff des Sediments entstanden. Wahrscheinlich enthält es auch geringe Beimengungen thermokatalytischen Methans.Der organische Kohlenstoff des oberen Sediments stammt zu rd. 70% aus dem vorwiegend magmatischen Kohlendioxid (alter Kohlenstoff), das aus dem Tiefenwasser durch turbulenten Austausch in die Biozone des Sees gelangt und dort assimiliert wird. Die restlichen 30% stammen aus dem in der Biozone assimilierten atmosphärischen Kohlendioxid (junger Kohlenstoff). Weil jedoch auch Methan aus tieferen Sedimentschichten in den See wandert, beträgt der¹⁴C-Gehalt des im Seewasser gelösten Methans nicht 30% modern, sondern nur ca. 10% modern.Weitere Isotopenuntersuchungen an Plankton-, Methan-, Kohlendioxid- und Sedimentproben sind notwendig, um das Modell abzusichern.

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Résumé De grandes quantités de méthane et d'oxyde carbonique sont dissoutes dans les eaux profondes du Lac Kicu. Alors qu'on est en général d'accord sur l'origine surtout magmatique de l'oxyde carbonique, les modèles devant représenter la genése du méthane sont contradictoires. Ils reposent sur des données trop peu nombreuses et en partie trop inexactes.A l'aide de nouveaux résultats de mesures faites sur des échantillons de gaz et du sédiment, on a développé un nouveau modèle, à partir des anciennes représentations. D'après celui-ci, le méthane provient pour sa plus grande part du carbone organique du sédiment, transformé par des bactéries. Il contient probablement des traces d'un méthane de thermocatalyse.Le carbone organique du sédiment supérieur provient pour 70% de l'oxyde carbonique surtout magmatique (carbone »ancien«), des eaux profondes parvenu, par échanges turbulents, dans la biozone du lac, où l'oxyde carbonique est assimilé. Les 30% restant proviennent de l'oxyde carbonique atmosphérique (carbone »jeune«) assimilé dans la biozone. Le méthane des couches profondes du sédiment migrant dans le lac, la teneur en¹⁴C de méthane dissous dans les eaux du lac n'est pas de 30% modernes, mais de 10% modernes.D'autres recherches sur les isotopes d'échantillons du plancton, du méthane, de l'oxyde carbonique et du sédiment du Lac Kivu seront nécessaires pour confirmer ce modéle.

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Experimente zur Zeolithbildung durch hydrothermale Umwandlung

The formation of zeolites by hydrothermal alteration has been investigated by taking trass from the Laach volcanic area as a sample. Zeolites to be found are chabazite, phillipsite and analcime, all of which originated from the same phonolitic glass. This paper aims at explaining the formation of zeolites by means of experimental alteration of the pumice with various solutions. NaOH and KOH solutions were used in the experiments, these limited the formation conditions of chabazite, phillipsite, analcime in alkaline environments. Moreover, experiments were carried out with H₂O dist and with solutions that formed during the alteration of pumice by reacting with H₂O. These experiments were conducted to supply clues concerning the formation of zeolites in the Laach volcanic area. With NaOH solutions zeolites were formed from pumice within a temperature range of 70 to 250° C and a concentration range of 0.001 to 1.0 n; with KOH solutions they were formed in the same concentration range within a temperature range of 100 to 250° C. The formation of zeolites proved to depend much on temperature and concentration: At low temperatures high concentrations are necessary, higher temperatures need lower concentrations. With NaOH solutions the zeolites are formed in the succession, chabazite, phillipsite, analcime at increasing temperatures and increasing concentrations. With KOH solutions the same succession is to be found with rising temperature. With rising concentration, however, this succession is changed at higher temperatures: Phillipsite appears in place of analcime at high temperatures and concentrations. By the alteration of pumice with H₂O, zeolites are formed from 200° C onward. With solutions that had been formed during the alteration of pumice by reacting with H₂O, the minimum temperature for the formation of zeolites was 180° C. Concerning the genesis of zeolites in the Laach volcanic area, the experiments showed that the temperature for the formation of analcime was about 250° C; for the formation of chabazite and phillipsite it was between 150 and 190° C. The pH of the reacting solutions may have been between 7 and 8. Thus the experiments proved that from the same material different zeolites, chabazite, phillipsite, analcime may be formed by temperature change of the reacting solutions (H₂O to slightly alkaline solutions).     

Impact of methane seeps on the local carbon‐isotope record: a case study from a Late Jurassic hemipelagic section

An Oxfordian (Late Jurassic) hemipelagic succession from Beauvoisin (SE France) contains a pronounced, short‐lived negative excursion in the bulk‐carbonate carbon‐isotope record, with an amplitude of 4‰. It was shown previously that the Beauvoisin paleoenvironment was impacted by hydrocarbon seepage. New isotopic data corroborate that methane was a significant constituent of these hydrocarbons. The negative excursion was caused by transient enhanced precipitation of ¹³C‐depleted carbonate, mediated by anaerobic oxidation of methane. Despite its local diagenetic origin, the Beauvoisin excursion is similar in shape and duration to globally recognized negative C‐isotope excursions that have been related to catastrophic, massive dissociation of methane hydrate. Shape and duration of negative excursions therefore cannot be used as an argument when determining their origin if they have not been shown to represent a global perturbation of the carbon cycle.     

GARSTEC—the Garching Stellar Evolution Code

We describe the Garching Stellar Evolution Code. General features, treatment of the microphysics, details of the numerical solution, handling and particularities are discussed. The standard solar model serves as the most basic benchmark to test the accurateness of the code and is presented, too.     

Human impact on Holocene sediment dynamics in the Eastern Mediterranean – the example of the Roman harbour of Ephesus

During the past millennia, many erosion and accumulation processes have been modified by anthropogenic impact. This holds especially true for the environs of ancient settlements and their harbours along the Mediterranean coasts. Our multi‐proxy investigations in the Roman harbour and the harbour canal of Ephesus (western Turkey) reveals that humans have significantly triggered soil erosion during the last three millennia. Since the eighth century bc , and especially since the Hellenistic period, a high sedimentation rate indicates fast alluviation and delta progradation of the Küçük Menderes. Deforestation, agriculture (especially ploughing) and grazing (especially goats) were the main reasons for erosion of the river catchment area. One consequence was significant siltation of the Hellenistic/Roman harbour basin. This sediment trap archives the human impact, which was strongly enhanced from Hellenistic/Roman to Byzantine times (second/first centuries bc to the sixth/seventh centuries ad ), evidenced by high sedimentation rates, raised values of heavy metal contaminations [lead (Pb), copper (Cu)], the occurrence of fruit tree pollen and of intestinal parasites. From the middle to the end of the first millennium ad , the influence of Ephesus declined, which resulted in a decrease of human impact. Studies of several ancient settlements around the Mediterranean Sea tell a comparable story. They also confirm that during their most flourishing periods the human impact totally overprinted the climatic one. To detect the latter, geo‐bio‐archives of relatively pristine areas have to be investigated in detail. Copyright © 2016 John Wiley & Sons, Ltd.     

Spatio-temporal patterns of throughfall and solute deposition in an open tropical rain forest

The brief interaction of precipitation with a forest canopy can create a high spatial variability of both throughfall and solute deposition. We hypothesized that (i) the variability in natural forest systems is high but depends on system-inherent stability, (ii) the spatial variability of solute deposition shows seasonal dynamics depending on the increase in rainfall frequency, and (iii) spatial patterns persist only in the short-term. The study area in the north-western Brazilian state of Rondônia is subject to a climate with a distinct wet and dry season. We collected rain and throughfall on an event basis during the early wet season (n = 14) and peak of the wet season (n = 14) and analyzed the samples for pH and concentrations of , Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, , and DOC. The coefficient of variation for throughfall based on both sampling intervals was 29%, which is at the lower end of values reported from other tropical forest sites, but which is higher than in most temperate forests. Coefficients of variation of solute deposition ranged from 29% to 52%. This heterogeneity of solute deposition is neither particularly high nor particularly low compared with a range of tropical and temperate forest ecosystems. We observed an increase in solute deposition variability with the progressing wet season, which was explained by a negative correlation between heterogeneity of solute deposition and antecedent dry period. The temporal stability of throughfall patterns was low during the early wet season, but gained in stability as the wet season progressed. We suggest that rapid plant growth at the beginning of the rainy season is responsible for the lower stability, whereas less vegetative activity during the later rainy season might favor the higher persistence of “hot” and “cold” spots of throughfall quantities. The relatively high stability of throughfall patterns during later stages of the wet season may influence processes at the forest floor and in the soil. Solute deposition patterns showed less clear trends but all patterns displayed a short-term stability only. The weak stability of those patterns is apt to impede the formation of solute deposition-induced biochemical microhabitats in the soil.