Meteoritic highly siderophile element and Re‐Os isotope signatures of Archean spherule layers from the CT3 drill core,Barberton Greenstone Belt,South Africa     

Seda Ozdemir  Toni Schulz  David van Acken  Ambre Luguet  W. Uwe Reimold  Christian Koeberl 《Meteoritics & planetary science》2019,54(10):2203-2216

Archean spherule layers represent the only currently known remnants of the early impact record on Earth. Based on the lunar cratering record, the small number of spherule layers identified so far contrasts to the high impact flux that can be expected for the Earth at that time. The recent discovery of several Paleoarchean spherule layers in the BARB5 and CT3 drill cores from the Barberton area, South Africa, drastically increases the number of known Archean impact spherule layers and may provide a unique opportunity to improve our knowledge of the impact record on the early Earth. This study is focused on the spherule layers in the CT3 drill core from the northeastern Barberton Greenstone Belt. We present highly siderophile element (HSE: Re, Os, Ir, Pt, Ru, and Pd) concentrations and Re‐Os isotope signatures for spherule layer samples and their host rocks in order to unravel the potential presence of extraterrestrial fingerprints within them. Most spherule layer samples exhibit extreme enrichments in HSE concentrations of up to superchondritic abundances in conjunction with, in some cases, subchondritic present‐day ¹⁸⁷Os/¹⁸⁸Os isotope ratios. This indicates a significant meteoritic contribution to the spherule layers. In contrast to some of the data reported earlier for other Archean spherule layers from the Barberton area, the CT3 core is significantly overprinted by secondary events. However, HSE and Re‐Os isotope signatures presented in this study indicate chondritic admixtures of up to (and even above) 100% chondrite component in some of the analyzed spherule layers. There is no significant correlation between HSE abundances and respective spherule contents. Although strongly supporting the impact origin of these layers and the presence of significant meteoritic admixtures, peak HSE concentrations are difficult to explain without postdepositional enrichment processes.  相似文献   

Bosumtwi impact structure,Ghana: Evidence for fluidized emplacement of the ejecta     

David Baratoux  Cheikh Ahmadou Bamba Niang  Wolf Uwe Reimold  Marian Selorm Sapah  Mark Walter Jessell  Daniel Boamah  Gayane Faye  Sylvain Bouley  Olivier Vanderhaeghe 《Meteoritics & planetary science》2019,54(10):2541-2556

The about 10.5 km diameter Bosumtwi impact crater is one of the youngest large impact structures on Earth. The crater rim is readily noticed on topographic maps or in satellite imagery. It defines a circular basin filled by water (Lake Bosumtwi) and lacustrine sediments. The morphology of this impact structure is also characterized by a circular plateau extending beyond the rim and up to 9–10 km from the center of the crater (about 2 crater radii). This feature comprises a shallow ring depression, also described as an annular moat, and a subdued circular ridge at its outer edge. The origin of this outermost feature could so far not be elucidated based on remote sensing data only. Our approach combines detailed topographic analysis, including roughness mapping, with airborne radiometric surveys (mapping near‐surface K, Th, U concentrations) and field observations. This provides evidence that the moat and outer ring are features inherited from the impact event and represent the partially eroded ejecta layer of the Bosumtwi impact structure. The characteristics of the outer ridge indicate that ejecta emplacement was not purely ballistic but requires ejecta fluidization and surface flow. The setting of Bosumtwi ejecta can therefore be considered as a terrestrial analog for rampart craters, which are common on Mars and Venus, and also found on icy bodies of the outer solar system (e.g., Ganymede, Europa, Dione, Tethys, and Charon). Future studies at Bosumtwi may therefore help to elucidate the mechanism of formation of rampart craters.  相似文献   

A petrographical and geochemical study of quartzose nodules,country rocks,and dike rocks from the Upheaval Dome structure,Utah     

Christian KOEBERL  J. B. PLESCIA  Chris L. HAYWARD  Wolf Uwe REMOLD 《Meteoritics & planetary science》1999,34(6):861-868

Abstract— Upheaval Dome, in Canyonlands National Park, Utah, USA, is a unique structure on the Colorado Plateau. It has earlier been interpreted as an impact structure or as a pinched-off salt diapir. Some subrounded quartzose fragments were found in a ring depression near the eastern margin of the structure and, based on vesicularity and apparent flow structure, the fragments were interpreted by early researchers as “impactites.” Our petrographic studies show no indication of a high-temperature history and are in agreement with a slow, low-temperature formation of the quartz nodules. Composi***ionally, the lag deposit samples are almost pure SiO₂. They show no chemical similarity to any of the possible target rocks (e.g., Navajo Sandstone), from which they should have formed by melting if they were impactites. Instead, the samples have relatively high contents of elements that indicate fluid interaction (e.g., hydrothermal growth), such as As, Sb, Ba, and U, and show positive Ce anomalies. Thus, we interpret the “lag deposit samples” as normal low-temperature (hydrothermally-grown?) quartz that show no indication of being impact-derived. In addition, a petrographic and geochemical analysis of a series of dike samples yielded no evidence for shock metamorphism or a meteoritic component.  相似文献   

Food web manipulation by extreme enhancement of piscivory: an invertebrate predator compensates for the effects of planktivorous fish on a plankton community     

Jürgen Benndorf  Bjrn Wissel  Anne F. Sell  Uwe Hornig  Pia Ritter  Wiebke Bing 《Limnologica》2000,30(3)

By enhancing the stock of piscivorous fish in a whole-lake experiment in Gräfenhain (Germany) since 1981 to such an extent that almost all planktivorous fish were exterminated, we examined the concept of “over-biomanipulation”. This hypothesis predicts that (a) extremely strong piscivory will allow uncontrolled development of large invertebrate predators and (b) these invertebrates can exert the same strong predation pressure on large herbivorous zooplankton as planktivorous fish. The hypothesis is tested the first time by a cross-comparison of the long-term response of the plankton community structure in the experimental lake (Piscivore L.) with (1) the intermediate response in the same lake and with (2) that of the long-term state in a nearby reference lake (Planktivore L.) densely inhabited by planktivorous fish (Leucaspius delineatus, a small cyprinid). The intermediate (1989–1992) response in Piscivore L. revealed a strong increase of the abundance of the invertebrate predator Chaoborus flavicans. Large daphnids were able to coexist with C. flavicans so that edible phytoplankton were suppressed and water transparency increased.As part (a) of the hypothesis predicts, the long-term response in Piscivore L. was characterized by the immigration of the larger predator Chaoborus obscuripes which displaced the smaller C. flavicans completely. The results support also part (b) of the hypothesis of “over-biomanipulation”: C. obscuripes-dominated Piscivore L. showed not much difference in biomasses of daphnids and total and edible phytoplankton as well as Secchi depth compared with fish-dominated Planktivore Lake. On the other hand, C. obscuripes-dominated Piscivore L. was characterized by distinctly lower biomass of daphnids, mean body volume of all crustaceans and Secchi depth as well as by higher biomass of edible phytoplankton compared with C. flavicans-dominated Piscivore Lake. We conclude that long-lasting success of biomanipulation cannot be achieved by extremely high piscivory leading to the almost complete extermination of planktivorous fish.  相似文献   

Natura 2000 sites and fisheries in German offshore waters     

Pedersen  Soren Anker; Fock  Heino; Krause  Jochen; Pusch  Christian; Sell  Anne L.; Bottcher  Uwe; Rogers  Stuart I.; Skold  Mattias; Skov  Henrik; Podolska  Magdalena; Piet  Gerjan J.; Rice  Jake C. 《ICES Journal of Marine Science》2009,66(1):155-169

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Using Java for distributed computing in the Gaia satellite data processing     

William O’Mullane  Xavier Luri  Paul Parsons  Uwe Lammers  John Hoar  Jose Hernandez 《Experimental Astronomy》2011,31(2-3):243-258

In recent years Java has matured to a stable easy-to-use language with the flexibility of an interpreter (for reflection etc.) but the performance and type checking of a compiled language. When we started using Java for astronomical applications around 1999 they were the first of their kind in astronomy. Now a great deal of astronomy software is written in Java as are many business applications. We discuss the current environment and trends concerning the language and present an actual example of scientific use of Java for high-performance distributed computing: ESA’s mission Gaia. The Gaia scanning satellite will perform a galactic census of about 1,000 million objects in our galaxy. The Gaia community has chosen to write its processing software in Java. We explore the manifold reasons for choosing Java for this large science collaboration. Gaia processing is numerically complex but highly distributable, some parts being embarrassingly parallel. We describe the Gaia processing architecture and its realisation in Java. We delve into the astrometric solution which is the most advanced and most complex part of the processing. The Gaia simulator is also written in Java and is the most mature code in the system. This has been successfully running since about 2005 on the supercomputer “Marenostrum” in Barcelona. We relate experiences of using Java on a large shared machine. Finally we discuss Java, including some of its problems, for scientific computing.  相似文献   

Implementing the Gaia Astrometric Global Iterative Solution (AGIS) in Java     

William O’Mullane  Uwe Lammers  Lennart Lindegren  Jose Hernandez  David Hobbs 《Experimental Astronomy》2011,31(2-3):215-241

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The Second Arab Impact Cratering and Astrogeology Conference,Casablanca, 14–20 November 2011—A bridge between geoscientists and astronomers     

David BARATOUX  Wolf Uwe REIMOLD  Hassna CHENNAOUI‐AOUDJEHANE 《Meteoritics & planetary science》2012,47(6):1098-1103

Abstract– We present here a report following the Second Arab Impact Cratering and Astrogeology Conference that took place in Casablanca in November 2011. It includes a summary of the main results presented at this meeting and recent developments in impact science in the Arab world. Known and potential impact structures in this region of the world are presented on a map. The conference ended with a synopsis and a list of recommendations that are reported here. We conclude with some remarkable aspects of the postconference field trip.  相似文献   

Use of hierarchical cluster analysis to assess the representativeness of a baseline groundwater quality monitoring network: comparison of New Zealand’s national and regional groundwater monitoring programs   总被引：1，自引：0，他引：1  

Christopher J. Daughney  Matthias Raiber  Magali Moreau-Fournier  Uwe Morgenstern  Rob van der Raaij 《Hydrogeology Journal》2012,20(1):185-200

Baseline monitoring of groundwater quality aims to characterize the ambient condition of the resource and identify spatial or temporal trends. Sites comprising any baseline monitoring network must be selected to provide a representative perspective of groundwater quality across the aquifer(s) of interest. Hierarchical cluster analysis (HCA) has been used as a means of assessing the representativeness of a groundwater quality monitoring network, using example datasets from New Zealand. HCA allows New Zealand??s national and regional monitoring networks to be compared in terms of the number of water-quality categories identified in each network, the hydrochemistry at the centroids of these water-quality categories, the proportions of monitoring sites assigned to each water-quality category, and the range of concentrations for each analyte within each water-quality category. Through the HCA approach, the National Groundwater Monitoring Programme (117 sites) is shown to provide a highly representative perspective of groundwater quality across New Zealand, relative to the amalgamated regional monitoring networks operated by 15 different regional authorities (680 sites have sufficient data for inclusion in HCA). This methodology can be applied to evaluate the representativeness of any subset of monitoring sites taken from a larger network.  相似文献   

An integrated zircon geochronological and geochemical investigation into the Miocene plutonic evolution of the Cyclades, Aegean Sea, Greece: part 2—geochemistry   总被引：1，自引：0，他引：1  

Robert Bolhar  Uwe Ring  Anthony I. S. Kemp  Martin J. Whitehouse  Steve D. Weaver  Jon D. Woodhead  I. Tonguc Uysal  Rose Turnbull 《Contributions to Mineralogy and Petrology》2012,164(6):915-933

Zircons from 14 compositionally variable granitic rocks were examined in detail using CL image-guided micro-analysis to unravel the complex magmatic history above the southward retreating Hellenic subduction zone system in the Aegean Sea. Previously published U–Pb ages document an episodic crystallisation history from 17 to 11?Ma, with peraluminous (S-type) granitic rocks systematically older than closely associated metaluminous (I-type) granitic rocks. Zircon O- and Hf isotopic data, combined with trace element compositions, are highly variable within and between individual samples, indicative of open-system behaviour involving mantle-derived melts and evolved supracrustal sources. Pronounced compositional and thermal fluctuations highlight the role of magma mixing and mingling, in accord with field observations, and incremental emplacement of distinct melt batches coupled with variable degrees of crustal assimilation. In the course of partial fusion, more fertile supracrustal sources dominated in the earlier stages of Aegean Miocene magmatism, consistent with systematically older crystallisation ages of peraluminous granitic rocks. Differences between zircon saturation and crystallisation temperatures (deduced from zircon Ti concentrations), along with multimodal crystallisation age spectra for individual plutons, highlight the complex and highly variable physico-compositional and thermal evolution of silicic magma systems. The transfer of heat and juvenile melts from the mantle varied probably in response to episodic rollback of the subducting lithospheric slab, as suggested by punctuated crystallisation age spectra within and among individual granitic plutons.  相似文献