Levels and temporal trends of PCDD/PCDFs and non-ortho PCBs in ringed seals from East Greenland     

Riget F  Vikelsøe J  Dietz R 《Marine pollution bulletin》2005,50(12):1523-1529

The levels and temporal trend of polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) and non-ortho substituted PCBs (c-PCBs, i.e. CB77, CB126 and CB169) were determined in ringed seal blubber from central East Greenland collected in 1986, 1994, 1999 and 2003, respectively. Since 1986 the concentrations of PCDDs, PCDFs and c-PCBs all show a decreasing trend. The annual decreases were estimated to 5.2% and 5.3% for pg/g WHO-TEQ ww of PCDD/Fs and c-PCB, respectively. The annual median concentrations of PCDDs ranged from 5.4 to 24.4 pg/g WHO-TEQ ww and those of PCDFs from 2.5 to 5.1 pg/g WHO-TEQ ww. Compared to PCDD/Fs concentrations in ringed seals from other Arctic areas the levels of PCDD/Fs found in 1986 were the highest recorded. The annual median concentrations of c-PCBs decreased 24.2 to 9.1 pg/g WHO-TEQ ww. The levels of c-PCBs observed in 1986 are similar to levels found in ringed seals from Svalbard in 1990 and from eastern Hudson Bay in 1989–1992. The dominant and most TEQ-contributing PCDD congener was 1,2,3,7,8-PeCDD. CB126 was the dominating and most TEQ-contributing c-PCB congener. The concentrations of PCDDs, PCDFs and c-PCBs were highly significantly inter-correlated. Principal component analysis of the PCDD/PCDF congeners and c-PCBs was performed to analyse the pattern of compounds during time.  相似文献   

Barringer Medal Citation for Michael R. Dence     

Robert S. Dietz 《Meteoritics & planetary science》1988,23(3):317-317

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ELGYGYTGYN CRATER,SIBERIA: PROBABLE SOURCE OF AUSTRALASIAN TEKTITE FIELD (and Bediasites from Popigai)     

Robert S. Dietz 《Meteoritics & planetary science》1977,12(2):145-157

Elgygytgyn crater (lat. 67–30 N, long. 172–00 E) in remote northeastern Siberia is proposed as the meteorite impact site from which the Australasian tektite strewnfield was splashed. The following points support this interpretation: 1, Elgygytgyn very likely is an impact crater and is of adequate size, 18 km across, to generate tektites; 2, the apex of the strewnfield points towards this crater; 3, the terrane is Mesozoic which fits the age of the tektite parental material from Sr/Rb data; 4, compositional and specific gravity lineations within the strewnfield are directed, in part, toward this crater; 5, the high velocity tektites, australites, are distal with respect to this crater while the low velocity tektites, splash forms and Muong Nong tektites, are proximal; 6, the loess deposits and mixed acid/basic rocks of the impact site provide a suitable subgraywacke-type source material; 7, the erosional state of Elgygytgyn suggests that its age may well be in accordance with that of the Australasian tektite event, i.e., 700,000 years.  相似文献   

Crystal structures of iron bearing tetrahedrite and tennantite at 25 and 250°C by means of Rietveld refinement of synchrotron data     

Karen Friese  Andrzej Grzechnik  Emil Makovicky  Tonči Balić-Žunić  Sven Karup-Møller 《Physics and Chemistry of Minerals》2008,35(8):455-465

Rietveld refinement of X-ray synchrotron data was performed for two synthetic tetrahedrite samples, with 0.61 and 1.83 Fe atoms, and two synthetic tennantite samples with 0.10 and 1.23 Fe atoms p.f.u. M₁₂(Sb,As)₄S₁₃. Measurements were performed at 25 and 250°C. For both the phases, increased Fe substitution is reflected in the increased tetrahedral ‘Cu1’–S distance (‘Cu1’ is a site of Fe substitution) and Cu2–S distances. Cu2 was refined as a split position; the Cu2–Cu2 split about the plane of the S1₂S2 triangle is about 0.56 and 0.65 Å for tetrahedrite and tennantite, respectively. Cu2–Cu2 distances in the structure cavity are 2.8–2.9 Å. Between 25 and 250°C, the lattice parameter a increased by 0.02–0.04 Å and the interatomic distances by 0.01 Å on an average. Thermal expansion coefficients of little-substituted samples are similar to those of unsubstituted samples, whereas thermal expansion appears to decrease with increasing substitution by Fe. The Cu2–Cu2 split increases at 250°C by about 0.1 Å for tetrahedrite and by more than 0.15 Å for tennantite but the cage expansion is minimal so that the Cu2–Cu2 distances in the cavity decrease with temperature. Difference Fourier maps indicate that there is little residual electron density left between the two Cu2 half-sites in tetrahedrite but this inter-site density is substantially higher in tennantite. It increases with temperature, especially in the little-substituted tennantite sample.  相似文献   

影响移民流动性的地理门槛与移民政策①     

Martin van der VELDE  Ton van NAERSSEN 《地理科学进展》2017,36(12):1540-1551

本文旨在更好地解读国家政策与移民流动性之间的相互关系。基于门槛分析方法,我们建立了关于边界、流动性和移民的基础理论,并重点探讨了具有移民倾向的个体心理过程。人们往往根据以下3个地理门槛作出移民抉择：一是关注迁移、离开熟悉的地方、跨越国界的想法;二是关注目的地区位及其边界和“接壤”(Bordering);三是关注迁移路径或途径。本文的案例主要来自欧盟和东南亚国家,分别包括荷兰和德国之间的跨界现象,2008年经济危机后欧盟的人口流动,2015-2016年期间欧盟和土耳其间的难民流动,菲律宾劳工移民,以及印度尼西亚—马来西亚迁徙通道等。最后,本文讨论了流动性和边界政策及治理。  相似文献   

Current state of water quality indicators in urban streams in New Zealand     

Jennifer Gadd  Ton Snelder  Caroline Fraser  Amy Whitehead 《新西兰海洋与淡水研究杂志》2020,54(3):354-371

ABSTRACT

National assessments have highlighted that urban streams have some of the poorest water quality in New Zealand, yet there has been no analysis focussing on these streams. In this study we defined urban streams as >15% catchment urban land cover and used monitoring data to assess several indicators of water quality including dissolved metals, nutrients, bacteria and the macroinvertebrate community index. We investigated associations between the indicators and urban land cover, imperviousness and geographic location. Water quality was highly variable, within and between monitored sites; and invariably poor when compared to water quality criteria. Dissolved zinc was positively related to the proportion of urban land cover and imperviousness in the upstream catchment. There were significant differences in ammoniacal-N, nitrate-N, turbidity, MCI, dissolved zinc and E. coli between geographic locations. The analysis indicates that if urban development continues in its current form, increases in urban land cover around New Zealand can be expected to result in further declines in water quality and a reduced likelihood that water quality objectives will be achieved at impacted locations.  相似文献   

Multi-scale modeling for dynamics of structure-soil-structure interactions     

Claude Boutin  Jean Soubestre  Logan Schwan  Matt Dietz 《Acta Geophysica》2014,62(5):1005-1024

This paper presents two cases of multi-scale modeling in the context of dynamic structure soil-structure interaction. The first case concerns the behaviour of reinforced soils. It is shown that such system may involve both shear and bending effect at the leading order, which corresponds to a second gradient material. The second case addresses the seismic response of soils in presence of a densely urbanized city. It appears that the effect of resonance of the whole buildings in interaction actually modify the seismic response. In both cases the theoretical approach is completed by a validation through analogous samples tested on a shaking table.  相似文献   

Can bottom-up procedures improve the performance of stream classifications?     

Ton Snelder  José Barquín Ortiz  Doug Booker  Nicolas Lamouroux  Hervé Pella  Ude Shankar 《Aquatic Sciences - Research Across Boundaries》2012,74(1):45-59

Top-down methods for defining stream classifications are based on a conceptual model or expert-defined rules, whereas bottom-up methods use biological training data and statistical modelling. We compared the performance of six classification methods for explaining the taxonomic composition of invertebrate and fish assemblages recorded at 327 and 511 sites, respectively, distributed throughout France. Classification 1 and 2 were top-down classifications; The European Water Framework System A (WFDa,) and the French Hydro-ecoregions (HER 2). Four bottom-up classification procedures of increasing complexity were defined based on 11 variables that included watershed characteristics describing climate, topography, and geology, and site characteristics including elevation, bed slope and temperature. Classification 3 was defined using matrix correlation (MC) to select a combination of variable categories that produced the best discrimination of the observed taxonomic composition. Classification 4 and 5 were defined by clustering the sites based on their taxonomic data and then using linear discriminant analysis (LDA) and Random forests (RF) to discriminate the clusters based on the environmental variables. Classification 6 was defined using generalized dissimilarity modelling (GDM). Our hypothesis was that the bottom-up classifications would perform better because they flexibly accommodate complex relationships between compositional and environmental variation. We tested the classifications using the classification strength statistic (CS). The RF-based classification fitted the taxonomic patterns better than GDM or LDA and these latter classifications generally fitted better than the MC, WFDa or HER classifications. Cross validation analysis showed that differences in predictive CS (i.e. the CS statistics produced from sites not used in defining the classifications) were often significant. However, these differences were generally small. Gains in predictive performance of classifications appear to be small relative to the increase in complexity in the manner in which environmental variables are combined to define classes.  相似文献   

Geographically distributed hybrid testing & collaboration between geotechnical centrifuge and structures laboratories     

Mobin Ojaghi  Ignacio Lamata Martínez  Matt S. Dietz  Martin S. Williams  Anthony Blakeborough  Adam J. Crewe  Colin A. Taylor  S. P. Gopal Madabhushi  Stuart K. Haigh 《地震工程与工程振动(英文版)》2018,17(1):53-71

Distributed Hybrid Testing (DHT) is an experimental technique designed to capitalise on advances in modern networking infrastructure to overcome traditional laboratory capacity limitations. By coupling the heterogeneous test apparatus and computational resources of geographically distributed laboratories, DHT provides the means to take on complex, multi-disciplinary challenges with new forms of communication and collaboration. To introduce the opportunity and practicability afforded by DHT, here an exemplar multi-site test is addressed in which a dedicated fibre network and suite of custom software is used to connect the geotechnical centrifuge at the University of Cambridge with a variety of structural dynamics loading apparatus at the University of Oxford and the University of Bristol. While centrifuge time-scaling prevents real-time rates of loading in this test, such experiments may be used to gain valuable insights into physical phenomena, test procedure and accuracy. These and other related experiments have led to the development of the real-time DHT technique and the creation of a flexible framework that aims to facilitate future distributed tests within the UK and beyond. As a further example, a real-time DHT experiment between structural labs using this framework for testing across the Internet is also presented.  相似文献   

Substructuring stability analysis in light of comprehensive transfer system dynamics     

Zhenyun Tang  Matt Dietz  Zhenbao Li 《Bulletin of Earthquake Engineering》2018,16(1):129-154

Real-time dynamic substructuring (RTDS) is an experimental technique that splits the structure under test into coupled parts that run in parallel. The structural component exhibiting unpredictable behaviour is tested in the laboratory while the remainder of the structure is modelled numerically. As the test proceeds, the dynamic force state at the physical–numerical interface is measured and a transfer system, usually a servo-hydraulic actuator or shaking table, is used to impose the commensurate response on the physical substructure. The integral dynamics of servo-hydraulic transfer systems can frustrate RTDS implementation by destabilising the system. Many have noted the deleterious stability implications of excessive phase lag in terms of a pure time-delay. However, because of the existence of magnitude variations and more complex phase characteristics, pure time-delay is too simple to represent the inherent nature of servo-hydraulic transfer systems. This paper considers RTDS stability in light of comprehensive transfer system dynamics. A transfer-function model of a servo-hydraulic transfer system is adopted and used to reflect the oversimplification of pure time-delay. The concept of gain margin is employed to reveal the drawbacks of the pure-delay based RTDS stability analyses. In order to overcome the drawbacks, a new method based on gain margin was developed. The comparative analyses demonstrate that the gain margin based method is tailored to predict the stability boundaries of a RTDS system incorporating comprehensive transfer system dynamics. The validity of the technique is verified experimentally through virtual and authentic RTDS system employing a shaking table. The performance of delay compensated shaking table RTDS is also assessed in perspective of stability.  相似文献