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"With reference to the Bangladeshi community in Rome, this paper provides some answers to three key geographical questions: what is the migrants' regional pattern of origin in their home country; what are the mechanisms and routes of their migration to Italy; how are they spatially distributed in Rome?... Chain migration links specific origins in Bangladesh with spatial clusters and economic activities in Rome; the key here is the role of Bangladeshi community leaders in Rome who act both as migration sponsors and entrepreneurs." 相似文献
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Roger M. Key Christopher C. Johnson Matthew S.A. Horstwood Dan J. Lapworth Katherine V. Knights Simon J. Kemp Michael Watts Martin Gillespie Michael Adekanmi Tunde Arisekola 《Applied Geochemistry》2012
Sixteen hundred stream sediments (<150 μm fraction) collected during regional geochemical surveys in central and SW Nigeria have high median and maximum concentrations of Zr that exceed corresponding Zr concentrations found in stream sediments collected from elsewhere in the World with similar bedrock geology. X-ray diffraction studies on a sub-set of the analysed stream sediments showed that Zr is predominantly found in detrital zircon grains. However, the main proximal source rocks (Pan-African ‘Older Granites’ of Nigeria and their Proterozoic migmatitic gneiss country rocks) are not enriched in zircon (or Zr). Nevertheless, U–Pb LA-ICP-MS dating with cathodoluminescence imaging on detrital zircons, both from stream sediment samples and underlying Pan-African ‘Older Granites’ confirms a local bedrock source for the stream sediment zircons. A combination of tropical/chemical weathering and continuous physical weathering, both by ‘wet season’ flash flooding and ‘dry season’ unidirectional winds are interpreted to have effectively broken down bedrock silicate minerals and removed much of the resultant clay phases, thereby increasing the Zr contents in stream sediments. The strong correlation between winnowing index (Th/Al) and Zr concentration across the study area support this interpretation. Therefore, ‘anomalous’ high values of Zr, as well as other elements concentrated in resistant ‘heavy’ minerals in Nigeria’s streams may not reflect proximal bedrock concentrations of these elements. This conclusion has important implications for using stream sediment chemistry as an exploration tool in Nigeria for primary metal deposits associated with heavy minerals. 相似文献
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Heavy mineral deposits of placer origin are mostly found as beach or dune deposits. Suction cutter or bucket-wheel dredgers are used to exploit such deposits when water is present for the maintenance of artificial channels and dredge ponds. Production reconciliation of dredge mining is a complex process because slumping of material causes mixing of material to take place amongst resource blocks during mining operations. This paper reviews the applicability of sonar technology for the determination of material losses taking place in the dredge ponds, and also reviews the applicability of radio frequency identification (RFID) tags in conjunction with sonar technology as a cost effective method to enhance dredge mine production reconciliation in the heavy minerals mining industry. 相似文献
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Regions with Mediterranean climates are seasonally arid and provide a niche for ephemeral species which germinate following discrete and variable rainfall during summer. These species must be able to detect when conditions are suitable for completion of their life cycle. Common heliotrope (Heliotropium europaeum) is one such species. It is considered a weed in its naturalised habitat in southern Australia as it uses resources that could be used by ensuing crops, and is toxic to livestock.We examined common heliotrope's germination responses to temperature and water potential, the effect of simulated rainfall on seedling emergence and plant growth in lysimeters.Fresh seeds of common heliotrope have relatively high optimum temperatures and water potentials for germination (around 35 °C and 0 MPa). Germination percentage of seeds extracted from the soil seed bank varies seasonally. In the soil types and climate of the study area, 24.2 mm is the minimum amount of rainfall resulting in emergence. Evapotranspiration required to reach minimal reproductive output was 19.4 mm.Common heliotrope's germination requirements have been selected to ensure that it will only germinate after sufficient rainfall to allow reproductive output. This is the mechanism through which common heliotrope detects its temporal niche. 相似文献
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In coastal rivers, tides can propagate for tens to hundreds of kilometres inland beyond the saltwater line. Yet the influence of tides on river–aquifer connectivity and solute transport in tidal freshwater zones (TFZs) is largely unknown. We estimate that along the TFZ of White Clay Creek (Delaware, USA), 11% of river water exchanges through tidal bank storage zones. Additional hyporheic processes such as flow through bedforms likely contribute even more exchange. The turnover length associated with tidal bank storage is 150 km, on the order of turnover lengths for all hyporheic exchange processes in non‐tidal rivers of similar size. Based on measurements at a transect of piezometers located 17 km from the coast, tides exchange 0.36 m3 of water across the banks and 0.86 m3 across the bed per unit river length. Exchange fluxes range from ?1.66 to 2.26 m day?1 across the bank and ?0.84 to 1.88 m day?1 across the bed. During rising tide, river water infiltrates into the riparian aquifer, and the downstream transport rate in the channel is low. During falling tide, stored groundwater is released to the river, and the downstream transport rate in the channel increases. Tidal bank storage zones may remove nutrients or other contaminants from river water and attenuate nutrient loads to coasts. Alternating expansion and contraction of aerobic zones in the riparian aquifer likely influence contaminant removal along flow paths. A clear need exists to understand contaminant removal and other ecosystem services in TFZs and adopt best management practices to promote these ecosystem services. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
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P. Breen L.A. Robinson S.I. Rogers A.M. Knights G. Piet T. Churilova P. Margonski N. Papadopoulou E. Akoglu A. Eriksson Z. Finenko V. Fleming-Lehtinen B. Galil F. Goodsir M. Goren O. Kryvenko J.M. Leppanen V. Markantonatou S. Moncheva T. Oguz L. Paltriguera K. Stefanova F. Timofte F. Thomsen 《Marine Policy》2012
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Three distinctive stratabound phyllosilicate zones are present at the margins of the syntectonic Mt Isa Cu orebodies and their host rock, the ‘silica‐dolomite’, in northwestern Queensland. The zones show close spatial relationships with adjacent stratiform Pb‐Zn ores within the overall host, the dolomitic Urquhart Shale. The Pb‐Zn orebodies may be either sedimentary‐exhalative or, as suggested recently, late diagenetic in origin, whereas the Cu ores were formed during the third regional deformation event. Talc‐stilpnomelane layers within the footwall of the silica‐dolomite, and biotite‐rich layers at the margins and adjacent to the silica‐dolomite are present in sideritic intervals within the dolomitic sequence. These Fe‐carbonate‐rich layers correlate with the Pb‐Zn orebodies, but have a much greater lateral extent. Chloritic layers occur along the hanging wall contacts of several Pb‐Zn orebodies with overlying silica‐dolomite lobes. Microstructural and petrographic studies suggest that the phyllosilicates grew at the silicification stage during early syn‐D3 alteration that also formed the silica‐dolomite and its Cu orebodies. The stratabound distribution of talc, stilpnomelane and biotite is explained by the chemical control of the sideritic beds on silicification. The origin of the Fe‐carbonates is not known and may be either sedimentary or diagenetic. The chlorite zones may have developed because of more extensive fluid‐rock interactions controlled by higher permeabilities along the brittle shale and ductile galena layering within the Pb‐Zn orebodies. 相似文献
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Dan J. Lapworth Katherine V. Knights Roger M. Key Christopher C. Johnson Emmanuel Ayoade Michael A. Adekanmi Tunde M. Arisekola Olugbenga A. Okunlola Birgitta Backman Mikael Eklund Paul A. Everett Robert T. Lister John Ridgway Michael J. Watts Simon J. Kemp Peter E.J. Pitfield 《Applied Geochemistry》2012
This paper provides an overview of regional geochemical mapping using stream sediments from central and south-western Nigeria. A total of 1569 stream sediment samples were collected and 54 major and trace elements determined by ICP-MS and Au, Pd and Pt by fire assay. Multivariate statistical techniques (e.g., correlation analysis and principal factor analysis) were used to explore the data, following appropriate data transformation, to understand the data structure, investigate underlying processes controlling spatial geochemical variability and identify element associations. Major geochemical variations are controlled by source geology and provenance, as well as chemical weathering and winnowing processes, more subtle variations are a result of land use and contamination from anthropogenic activity. 相似文献
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