Trojans in Stable Chaotic Motion

The orbits of 13 Trojan asteroids have been calculated numerically in the model of the outer solar system for a time interval of 100 million years. For these asteroids Milani et al. (1997) determined Lyapunov times less than 100 000 years and introduced the notion "asteroids in stable chaotic motion". We studied the dynamical behavior of these Trojan asteroids (except the asteroid Thersites which escaped after 26 million years) within 11 time intervals - i.e. subintervals of the whole time - by means of: (1) a numerical frequency analysis (2) the root mean square (r.m.s.) of the orbital elements and (3) the proper elements. For each time interval we compared the root mean squares of the orbital elements (a, e and i) with the corresponding proper element. It turned out that the variations of the proper elements e_p in the different time intervals are correlated with the corresponding r.m.s.(e); this is not the case for sin I_p with r.m.s.(i). This revised version was published online in July 2006 with corrections to the Cover Date.     

In memoriam .....

Ohne Zusammenfassung     

Lake Shala: Water chemistry,mineralogy and geochemistry of sediments in an Ethiopian Rift lake

Lake Shala, the deepest lake in the internal Galla lakes basin of the Ethiopian Rift, fills a depression in Pleistocene volcanic rocks. Its sodium-bicarbonate (-chloride) water (salinity 16 g/l) is remarkably low in earth alkalines and sulphate. Stratification is indicated by different ion concentrations in the surface and bottom waters and by a thermocline in a water depth of 50–70 m. Hot soda springs emerging on the shores of Lakes Shala and Langano are believed to be derived from a hot saline underground reservoir recharged by meteoric waters. The ion composition of the hot spring waters is uniform and matches that in the Galla Lakes except for total salinities. Anomalous heavy metal concentrations are lacking in lake and hot spring waters. Sediments of Lake Shala belong to an extremely fine-grained group of deposits. They are poorly sorted and the lateral distribution of the grain sizes does not follow the normal scheme for aquatic depositional environments. A belt, 50–100 m below lake level containing the finest-grained sediments, separates the shallow periphery of the lake bottom from the deep center, both characterized by coarser-grained deposits. The sediments consist of a large portion of glassy components. A poorly cristallized smectite is most abundant in the clay mineral group. The components of the sand fraction are quartz, feldspar, glass particles and occasionally calcite. Nickel, cobalt and lead are depleted in the Lake Shala sediments compared with the averages of shales. Iron, manganese and zinc are relatively high. Silver, cadmium and some of the rare earth elements are enriched by factors of > 5.     

Precambrian sedimentary carbonates: carbon and oxygen isotope geochemistry and implications for the terrestrial oxygen budget

Carbon isotope values of 260 Precambrian limestones and dolomites (most of them being substantially unaltered) have yielded an overall mean of $\text{δ}{}^{13}\text{C}\text{= +0.4 ± 2.7‰}$ vs. PDB; the corresponding oxygen values average at $\text{δ}{}^{13}\text{O = +20.0 ± 4.2‰}$ vs. SMOW. Like the overall mean, the δ ¹³C values furnished by individual carbonate occurrences are, as a rule, fairly “modern” and almost constant as from the very beginning of the sedimentary record. A remarkable exception are the “heavy” dolomites of the Middle Precambrian Lomagundi Group, Rhodesia, with $\text{δ}{}^{13}\text{C}\text{= +9.4 ± 2.0‰}$ vs. PDB. As a result of our measurements, the sporadic occurrence in the geological past of anomalously heavy carbonates seems to be established.The approximate constancy around zero per mill of the δ ¹³C values of marine carbonates through geologic time would imply a corresponding constancy of the relative proportion of organic carbon in the total sedimentary carbon reservoir since about 3.3 · 10⁹ y ago (with C_org/C_total ? 0.2). Utilizing this ratio and current models for the accumulation of the sedimentary mass as a function of time, we get a reasonable approximation for the absolute quantity of organic carbon buried in sediments and, accordingly, of photosynthetic oxygen released. Within the constraints of our model (based on a terrestrial degassing constant λ = 1.16 · 10^?9 y^?1) close to 80% of the amount of oxygen contained in the present oxygen budget should have been released prior to 3 · 10⁹ y ago. Since geological evidence indicates an O₂-deficient environment during the Early and most parts of the Middle Precambrian, there is reason to believe that the distribution of this oxygen between the “bound” and the “molecular” reservoir was different from that of today (with effective O₂-consuming reactions bringing about an instantaneous transfer to the crust of any molecular oxygen released). Accordingly, the amount of C_org in the ancient sedimentary reservoir as derived from our isotope data is just a measure of the gross amount of photosynthetic oxygen produced, withholding any information as to how this oxygen was partitioned between the principal geochemical reservoirs. As a whole, the carbon isotope data accrued provide evidence of an extremely early origin of life on Earth since the impact of organic carbon on the geochemical carbon cycle can be traced back to almost 3.5 · 10⁹y.     

Die tektonische Fazies als ein Mittel zur Darstellung der Tektonik auf Übersichtskarten

Zusammenfassung Es wird die Darstellung der Fazies in einer Auswahl auf tektonischen Karten angeregt. Die Auswahl sollte nach der tektonischen Bedeutung der Fazies erfolgen. Labiler Schelf = faltbare Schichten, mächtige Kalkklötze sind solche lithotektonischen Komplexe. Molasse und Flysch, die bestimmte Entwicklungsstadien der Geosynklinale wiederspiegeln, gehören hierher. Besonders wichtig erscheint die Heraushebung der langanhaltenden Räume mit Schwellenfazies (Geantiklinalen) und der Kordilleren-Zonen.

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It is proposed to represent selectively the facies on tectonic maps. Selection should be governed by the tectonic significance of the facies. Such lithotectonic complexes are labile shelfs = foldable layers and thick limestones blocks. Also molasse and flysch reflecting certain stages of development of the geosyncline belong to this complex. The distinction of areas with long time swell facies (geoanticlines) and of Cordilleran zones, seems to be of particular importance.

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Résumé L'auteur suggère de représenter en partie les faciès dans des cartes tectoniques. La sélection doit s'effectuer selon l'importance tectonique des faciès. Les plateformes continentales mobiles = couches pliables et de grands blocs calcaires constituent de tels complexes litho-tectoniques. De même en font partie la molasse et le flysch qui reflètent de certaines phases de développement des géosynclinaux. Il semble particulièrement important de mettre en valeur les régions à faciès de bombement (geanticlines) persistant et les zones de cordillères.

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Etwas abgeänderter Vortrag, gehalten Februar 1966 auf der Hauptversammlung der Geologischen Vereinigung in Wien.     

Detailed investigation of preserved maar structures by combined geophysical surveys

The detection of completely preserved maar structures is important not only for underground mapping but also for paleoclimate research because laminated maar lake sediments may contain a very detailed archive of climate history. Objective evidence for the existence of such structures can only be provided by geophysics and boreholes. The combination of gravity and magnetic ground surveys appears to be an excellent tool to detect and identify buried maar structures. Their prominent properties are an almost circular gravity minimum corresponding to a crater filled with limnic sediments of low density, and a magnetic anomaly caused by a pyroclastic or basaltic body in the diatreme which indicates the volcanic character. Seismic measurements provide the most detailed information about the internal structure of the maar sediments. Zones of low seismic reflectivity and very low density represent sediments of the late maar-lake period. The early lake period is indicated by debris flow deposits and turbidites represented by seismic reflectors. The seismic sections clearly reveal the bowl-like structure of the maar. Outside this bowl-like structure, there are only a few reflections, which represent the basement. Taking into account the shape of the gravity anomaly, seismic information allows geometrical modelling of the maar structure. Optimal drilling sites can be selected based on the results of geophysical surveying. Comparing the results of combined geophysical surveys above two maar structures of different ages yields a marked similarity in their geophysical pattern.Editorial responsibility: J McPhie     

Zum Abschied von Wolfgang Geiger

Ohne Zusammenfassung