Neue Hilfsmittel für den geologisch-paläontologischen Unterricht

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Tektonische Untersuchungen an Westerwälder Vulkanen

Ohne Zusammenfassung     

Karrat Fjord (Greenland) tsunamigenic landslide of 17 June 2017: initial 3D observations

On 17 June 2017, a landslide-generated tsunami reached the village of Nuugaatsiaq, Greenland, leaving four persons missing and presumed dead. Here, we present a preliminary high-resolution analysis of the tsunamigenic landslide scar based on three-dimensional (3D) reconstructions of oblique aerial photographs taken during a post-failure reconnaissance helicopter overflight. Through a 3D quantitative comparison with pre-failure topography, we estimate that approximately 58 million m³ of rock and colluvium (talus) was mobilized during the landslide, 45 million m³ of which reached the fjord, resulting in a destructive tsunami. We classify this event as a “tsunamigenic extremely rapid rock avalanche,” which likely released along a pre-existing metamorphic fabric, bounded laterally by slope-scale faults. Further analysis is required to properly characterize this landslide and adjacent unstable slopes, and to better understand the tsunami generation.     

Elektronenstrahlmikroanalyse der graphit-troilitphase des cañon diablo-eisenmeteorits

Zusammenfassung Der vom American Meteorite Laboratory, Denver (Colorado), erworbene Eisenmeteorit Cañon Diablo mit drei sphärischen Graphit-Troiliteinschlüssen wurde von uns mit einer Elektronenstrahlmikrosonde JEOL JXA-3 A untersucht. Dabei wurde in den Graphit-Troiliteinschlüssen eine bisher noch nicht festgestellte Phase der Zusammensetzung (C_0,250,Ni_0,321,Fe_0,429)S_1,013 und (Co_0,144,Ni_0,391,Fe_0,465)S_1,016 aufgefunden. Diese Phase ist in ihrer Zusammensetzung von den bereits bekannten Mineralen Pentlandit (Fe,Ni)₉S₈ und Kobaltpentlandit (Co,Fe,Ni)₉S₈ deutlich verschieden. Sie bildet in der Troilit-Graphitmatrix Einsprenglinge von 3–6 m Durchmesser. Aus diesem Grunde war es bisher nicht möglich, ihre Kristallstruktur zu ermitteln.

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Electron-probe analysis of the graphite-troilite phase in the cañon diablo iron-meteorite

Summary The Cañon Diablo iron-meteorite, containing three spherical graphitetroilite inclusions has been investigated by means of an electronmicroprobe JEOL JXA-3 A. Within these graphite-troilite inclusions a phase, not known so far, has been noted, whose composition ranges from (Co_0.250,Ni_0.321,Fe_0.429)S_1.013 to (Co_0.144,Ni_0.391,Fe_0.465)S_1.016. This phase is decisively different from pentlandite (Fe,Ni)₉S₈ and Co-pentlandite (Co,Fe,Ni)₉S₈. It forms phenocrysts of 3–6 m in diameter. For this reason it was not possible so far to determine its crystal-structure.

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Mit 5 Abbildungen

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Herrn Prof. DDr.H. Wieseneder zum 65. Geburtstag gewidmet.     

The timing of sub-solidus hydrothermal alteration in the Central Zone, Limpopo Belt (South Africa): Constraints from titanite U–Pb geochronology and REE partitioning

In the Central Zone of the Limpopo Belt (South Africa), Palaeoproterozoic granulite-facies metamorphism was superimposed on an earlier Archaean orogenic history. Previously determined ages of  2030–2020 Ma obtained from high-temperature chronometers (zircon, garnet, monazite) are generally thought to provide the best estimate of the peak of Palaeoproterozoic granulite-facies metamorphism in the Central Zone, whereas ages as young as  2006 Ma from late melt patches suggest that temperatures remained above the wet solidus for an extended period. We present a new MC-ICP-MS ²⁰⁷Pb–²⁰⁶Pb age of 2030.9 ± 1.5 Ma for titanite found in amphibolite- to greenschist-facies alteration zones developed adjacent to quartz vein systems and related pegmatites that cut a strongly deformed Central Zone metabasite. This age could potentially date cooling of rocks at this locality to temperatures below the wet solidus. Alternatively, the titanite could be inherited from the metabasite host, and the age determined from it date the peak of metamorphism. Integration of the geochronology with LA-ICP-MS trace element data for minerals from the metabasite, the hydrothermal vein systems and comparable rocks elsewhere shows that the titanite formed during the amphibolite-facies hydrothermal alteration, not at the metamorphic peak or during the greenschist-facies phase of veining. This suggests that high-grade rocks in the Central Zone have cooled differentially through the wet solidus, and provides timing constraints on when Palaeoproterozoic reworking in the Central Zone began. This study illustrates the potential of combined geochronological and high-resolution geochemical studies to accurately match mineral ages to distinct crustal processes.     

Do shorebirds graze down zoobenthic biomass at the Banc d'Arguin tidal flats in Mauritania?

The 500 km² of tidal flats at the Banc d'Arguin, Mauritania, are a wintering site for over 2 million shorebirds, making this one of the areas with the highest density of shorebirds in the world. In earlier studies, it was found that these huge numbers of birds are dependent on a rather low biomass of benthic animals. To explain this paradoxical situation, we hypothesized that the earlier assessments of biomass in late winter and early spring do not reflect a year-round situation. Benthic biomass may be high when the birds arrive in autumn to be subsequently grazed down in the course of winter. To test this hypothesis, we have determined benthic biomass in September 1988. The result has been compared with a biomass value obtained in February–April 1986 with the same methods and at the same stations. These two biomass values do not differ significantly; so our hypothesis has to be rejected. It has to be admitted, however, that our experimental design cannot discern between differences due to the year of sampling and those due to the season.     

Cyclone Gonu storm surge in Oman

Super Cyclone Gonu is the strongest tropical cyclone on record in the Arabian Sea. Gonu caused coastal damage due to storm surge and storm wave impact as well as wadi flooding. High water marks, overland flow depths, and inundation distances were measured in the coastal flood zones along the Gulf of Oman from 1 to 4 August 2007. The high water marks peaked at Ras al-Hadd at the eastern tip of Oman exceeding 5 m. The storm surge of Gonu is modeled using the Advanced Circulation Model (ADCIRC). The multi-hazard aspect is analyzed by comparing observations from Cyclone Gonu with the 2004 Indian Ocean Tsunami.     

Extreme aridity prior to lake expansion deciphered from facies evolution in the Miocene Ili Basin,south‐east Kazakhstan

Central Asia witnessed progressive aridification during the Miocene, commonly related to mountain uplift, the Paratethys retreat and global climate cooling. However, the formation of Miocene lakes in Central Asia seems to oppose drier conditions, suggesting that the precise timing, extent and forcing of the aridification is still not well constrained. This study presents a facies model for the alluvial–lacustrine part of the Middle to Late Miocene of the Ili Basin, obtained from two successions. The model enables the semi‐quantitative assessment of regional water level and salinity, and characterizes the control of water level on evaporite formation and diagenesis. Both the proximal Kendyrlisai and the distal Aktau successions show an overall increase in water availability from dry mudflat deposits to lacustrine sedimentation with a transitional playa phase. Increasing evaporation rates outpaced the water supply and caused groundwater salinization. Subsequent lake expansion coincided with a basin‐wide desalinization and required a shift to a positive water budget. A climatic control of the hydrological evolution is inferred due to abrupt salinization and a minor tectonic influence. The long‐term water accumulation is probably related to the hydrological closure of the basin in the early Middle Miocene (15·3 Ma). Starting at 14·3 Ma, the step‐wise salinization occurred simultaneously with the global cooling of the Miocene Climate Transition. The Miocene Climate Transition led to extreme aridity in the Ili Basin, highlighted by the early diagenetic formation of displacive anhydrite in the basin centre. The expansion of the freshwater lake (12·7 to 11·5 Ma) was possibly promoted by lower evaporation rates due to decreasing air temperatures in the Ili Basin after the Miocene Climate Transition. The extreme aridity in the Ili Basin is interpreted as a continental counterpart to the Badenian Salinity Crisis in the Central Paratethys. This emphasizes the role of atmospheric forcing on evaporite sedimentation across Eurasia during the Middle Miocene.